Aalto- DD geometry for production...

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Departme nt of E ngi neeri ng Desig n a nd Prod uctio n Opt imisat io n o f the roll geomet ry fo r pro duct io n co ndit io ns Thomas Widmaier DOCTORAL DISSERTATIONS

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ISBN 978-952-60-4878-9 ISBN 978-952-60-4879-6 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 ISSN 1799-4942 (pdf) Aalto University School of Engineering Department of Engineering Design and Production www.aalto.fi

BUSINESS + ECONOMY ART + DESIGN + ARCHITECTURE SCIENCE + TECHNOLOGY CROSSOVER DOCTORAL DISSERTATIONS

Aalto-D

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In coating and calendering processes the rolls give the produced paper its final surface structure and finish. Thus the geometry of the rolls has a strong effect on the paper, which was seen in the analysis results. Similar results were observed in the rolling process in the steel mills. A solution was found to reduce the observed geometry errors in the currently used rolls or to reduce their effects on the end-product. The developed methods could be used also as a part of the normal roll maintenance. The need to deepen the understanding about the roll behaviour in the production conditions led to development of a new roll modelling method. An ultrasonic measuring device was used for measuring the internal structure of a thermo roll and detailed roll models could be created. With the presented results it was shown that the effects of the geometry errors coul be reduced. This and the roll modelling method enable an optimisation of the roll geometry for the production conditions. Original cover photo by Jukka Pirttiniemi

Thom

as Widm

aier O

ptimisation of the roll geom

etry for production conditions A

alto U

nive

rsity

Department of Engineering Design and Production

Optimisation of the roll geometry for production conditions

Thomas Widmaier

DOCTORAL DISSERTATIONS

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Aalto University publication series DOCTORAL DISSERTATIONS 156/2012

Optimisation of the roll geometry for production conditions

Thomas Widmaier

A doctoral dissertation completed for the degree of Doctor of Science (Technology) to be defended, with the permission of the Aalto University School of Engineering, at a public examination held at the lecture hall 216 of the school on 30 November 2012 at 12.

Aalto University School of Engineering Department of Engineering Design and Production

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Supervising professor Professor Petri Kuosmanen Thesis advisor Professor Petri Kuosmanen Preliminary examiners Professor Erno Keskinen, Tampere University of Technology, Finland Associate Professor Irem Tumer, Oregon State University, USA Opponents Professor Erno Keskinen, Tampere University of Technology, Finland Professor Aki Mikkola, Lappeenranta University of Technology, Finland

Aalto University publication series DOCTORAL DISSERTATIONS 156/2012 © Thomas Widmaier ISBN 978-952-60-4878-9 (printed) ISBN 978-952-60-4879-6 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 (printed) ISSN 1799-4942 (pdf) http://urn.fi/URN:ISBN:978-952-60-4879-6 Unigrafia Oy Helsinki 2012 Finland Publication orders (printed book): [email protected] The dissertation can be read at http://lib.tkk.fi/Diss

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Abstract Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi

Author Thomas Widmaier Name of the doctoral dissertation Optimisation of the roll geometry for production conditions Publisher School of Engineering Unit Department of Engineering Design and Production

Series Aalto University publication series DOCTORAL DISSERTATIONS 156/2012

Field of research Paper machinery technology

Manuscript submitted 18 June 2012 Date of the defence 30 November 2012

Permission to publish granted (date) 24 September 2012 Language English

Monograph Article dissertation (summary + original articles)

Keywords paper machine, steel mill, roll, machining, roll geometry, geometry measuring device, modelling

ISBN (printed) 978-952-60-4878-9 ISBN (pdf) 978-952-60-4879-6

ISSN-L 1799-4934 ISSN (printed) 1799-4934 ISSN (pdf) 1799-4942

Location of publisher Espoo Location of printing Helsinki Year 2012

Pages 184 urn http://urn.fi/URN:ISBN:978-952-60-4879-6

Abstract In the coating and calendering processes in the paper industry the rolls are in direct contact

with the paper and give it the final surface structure and finish. The knowledge about the relation of the roll geometry to the paper quality and to the runnability can be used to improve the operation of the rolls and the machine. Similar methods can be applied to steel mill rolls even though the rolling process and the rolls are different. The importance of the roll geometry for the quality of the end-product was seen in the end-product analysis results, e.g., the geometry components of the backing roll in a coating station were identifiable in the paper analyses. Similar results were observed in the rolling process in the steel mills. The major difference between the rolls in the paper and steel industry was that the measured paper quality variations in the paper industry correlated with the static and dynamic geometry errors of the rolls, but in the steel industry the observed variations correlated with the positions of the key grooves in the bearing arrangement of the backup rolls.

A cost effective solution to the geometry related problems would be to find a way to reduce the observed geometry errors in the current rolls or to reduce their effects on the end-product. This was the main target of this research. Methods to achieve this were developed. After the introduction of a roll geometry measurement device, a compensation machining technology was developed and the static geometry of the filled calender rolls was improved. The development of the dynamic roll geometry measuring device for the geometry errors of the fast rotating rolls led to the the compensation method of the dynamic geometry errors of the backing rolls. After the development of an in-situ run-out measurement device for fast rotating rolls similar attempts were made to reduce the thermal run-out of thermo rolls of calenders. The drawbacks in the experiments demonstrated the need to deepen the understanding about the roll behaviour. An ultrasonic measuring device was used as a roll shell measuring device. With the measured information detailed roll models could be created, which was the secondary target of this research. In the steel industry the developed measuring and machining methods were used to compensate the spring of the bearing arrangement caused by the key groove. This reduced the variations of rolling force and of steel strip thickness.

With all the presented results it can be concluded that both targets of the research could be reached: effects of the geometry errors were reduced and detailed roll models were created. This enables an optimisation of the roll geometry in the production conditions.

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Tiivistelmä Aalto-yliopisto, PL 11000, 00076 Aalto www.aalto.fi

Tekijä Thomas Widmaier Väitöskirjan nimi Telojen geometrian optimointi tuotanto-olosuhteita varten Julkaisija Insinööritieteiden korkeakoulu Yksikkö Koneenrakennustekniikan laitos

Sarja Aalto University publication series DOCTORAL DISSERTATIONS 156/2012

Tutkimusala Paperikonetekniikka

Käsikirjoituksen pvm 18.06.2012 Väitöspäivä 30.11.2012

Julkaisuluvan myöntämispäivä 24.09.2012 Kieli Englanti

Monografia Yhdistelmäväitöskirja (yhteenveto-osa + erillisartikkelit)

Avainsanat paperikone, terästehdas, tela, valssi, työstö, telageometria, geometrian mittaus, mallinnus

ISBN (painettu) 978-952-60-4878-9 ISBN (pdf) 978-952-60-4879-6

ISSN-L 1799-4934 ISSN (painettu) 1799-4934 ISSN (pdf) 1799-4942

Julkaisupaikka Espoo Painopaikka Helsinki Vuosi 2012

Sivumäärä 184 urn http://urn.fi/URN:ISBN:978-952-60-4879-6

Tiivistelmä Paperiteollisuudessa telojen geometria vaikuttaa oleellisesti valmistettavan paperin laatuun

päällystys- ja kalanterointiprosessin aikana. Tietämystä telojen geometrian vaikutuksesta paperin laatuun ja paperikoneen ajettavuuteen voidaan käyttää telojen ja paperikoneen toiminnan parantamiseen. Samanlaisia menetelmiä voidaan käyttää terästehtaan valsseihin, vaikkakin valssausprosessi ja valssit poikkeavat paperikoneen teloista ja paperin valmistuksesta. Telageometrian vaikutus valmistettavan tuotteen laatuun todettiin paperianalyysien tuloksista. Esimerkiksi päälystysaseman vastatelan geometriakomponentit olivat identifioitavissa paperianalyyseissä. Samantyyppisiä tuloksia saatiin terästeollisuuden valssausprosesseista. Selkeä ero paperiteollisuuden telojen ja terästeollisuuden valssien välillä oli se, että paperiteollisuudessa mitatut vaihtelut korreloivat telojen staattisen ja dynaamisen geometrian kanssa ja terästeollisuudessa valssien laakerien kiilauran paikan kanssa.

Kustannustehokas ratkaisu näihin telojen geometriaan liittyviin ongelmiin olisi, jos pystyttäisiin kehittämään menetelmä, jolla havaittuja telojen ja valssien geometriavirheitä tai niiden vaikutuksia lopputuotteeseen pystyttäisiin pienentämään. Tämä oli tutkimuksen päätavoite. Tutkimuksen aikana kehitettiin menetelmiä useassa eri vaiheessa, joiden avulla tämä tavoite saavutettiin. Telahiomakoneisiin ja sorveihin soveltuvan telageometriamittalaitteen valmistumisen jälkeen pystyttiin kehittämään telatyöstön kompensointiteknologia, jolla kuitutelojen staattista geometriavirhettä voitiin pienentää. Telan dynaamisen geometrian mittalaitteen valmistumisen jälkeen nopeasti pyörivien telojen ajonaikainen geometria voitiin määrittää. Tämä johti päällystysaseman vastatelojen dynaamisten geometriavirheiden kompensointimenetelmän kehittämiseen. Kalanterin termotelojen ajonaikaisen lämpötaipuman kompensointimenetelmän kokeet eivät onnistuneet. Tämän seurauksena kävi selväksi, että tarvittiin lisää tietoa telojen käyttäytymisestä. Ultraäänimittalaitetta käytettiin telan vaipan mittaamiseen, mikä mahdollisti tarkkojen telamallien luomisen, joita voidaan käyttää telojen simulointiin. Tarkkojen telamallien luominen oli tämän tutkimuksen toinen päätavoite. Kehitetyillä mittaus- ja työstömenetelmillä saatiin kompensoitua terästeollisuuden tukivalssien laakeroinnin kiilaurasta johtuvaa joustoa. Kompensoinnin avulla valssausvoimavaihtelu ja teräsnauhan paksuusvaihtelu saatiin pienennettyä.

Tulosten perusteella voidaan perustellusti väittää, että työn tavoitteet saavutettiin: geometriavirheiden vaikutusta saatiin pienennettyä ja tarkkoja telamalleja pystyttiin luomaan. Tämä mahdollistaa telojen geometrian optimoinnin tuotanto-olosuhteita varten.

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Preface

The thesis was written in the Department of Engineering Design and Production in

2004-2012. I wish to thank Professor Petri Kuosmanen for the opportunity to produce

this thesis. He has ensured continuity and helped me to fund my research, which I am

very grateful for.

In addition, I am most indebted to my former and present colleagues and many others,

for creating a supportive and fun-to-work-with research environment. Especially I wish

to thank Esa Porkka for aiding me when I took my first steps in the world FEM and for

his help with FFT, Jukka Pirttiniemi for interesting and useful discussions and for

convincing me to take the ultrasonic measurement device with us, Jouni Pekkarinen for

helping me in the practical part of the research and for being a good colleague, Jari

Juhanko for being objective and for his useful tips during the final stages of the

dissertation process, Janne Haikio for being a reliable colleague and co-developer of

the machining methods, Tommi Uski for always being patient and helpful, but

especially for the reliable ultrasound measurement device, Pekka Väänänen for the help

with the original measurement method, Panu Kiviluoma for a thorough review of my

papers, Jari Toiva for his research work with the thermo rolls and for good discussions

and Mats Boman for various good ideas concerning the machining methods.

I also wish to thank all the numerous people from Finnish industry who have

contributed to this dissertation by giving their time and thought. I dedicate special

thanks to the people at the former Myllykoski Paper Mill, where this research started.

Several companies operating in Finland are gratefully acknowledged for funding (UPM-

Kymmene Oyj, Stora Enso Oyj, MetsäBoard, Metso Paper Oy, Vaahto Oyj, etc.), as well

as Tekes – The Finnish Funding Agency for Technology and Innovation, the Graduate

School of Papermaking Science and Technology (PaPSaT) and the former Department

of Mechanical Engineering at Helsinki University of Technology. The IT Center for

Science (CSC) is gratefully acknowledged for their help with the FEM software.

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Preface

Professor Irem Tumer from Oregon State University in the USA and Professor Erno

Keskinen from Tampere University of Technology are greatly acknowledged for pre-

examining this dissertation.

Last and but not least, I wish to thank my daughter Isa, my family, other relatives and

friends for their support for my work.

Espoo, October 2012

Thomas Widmaier

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9

Contents

Preface .......................................................................................................................................... 7

List of publications .................................................................................................................... 11

Author’s contribution ............................................................................................................... 11

List of abbreviations and symbols ........................................................................................... 13

1 Introduction ....................................................................................................................... 15

1.1 Research problem ............................................................................................. 17

1.2 Objective of the research .................................................................................. 17

1.3 Motivation and limitations of the research ...................................................... 18

1.4 Scientific contribution ...................................................................................... 18

2 Rolls in paper and steel industry ..................................................................................... 19

2.1 Roll geometry errors ......................................................................................... 19

2.1.1 Geometry definitions ................................................................................. 19

2.1.2 The error motion of rotational axis ........................................................... 23

2.1.3 Roll geometry measurements in the paper industry ................................. 26

2.2 Paper machine .................................................................................................. 27

2.2.1 Calenders used in the research .................................................................. 28

2.2.2 Coating station ...........................................................................................30

2.2.3 Yankee dryer .............................................................................................. 31

2.3 Steel mill ........................................................................................................... 31

2.3.1 Rolling mill stand ....................................................................................... 32

2.4 Machine tools for large rolls ............................................................................. 32

2.4.1 Grinding machines .................................................................................... 33

2.4.2 Lathes ......................................................................................................... 35

2.4.3 Machining accuracy of lathes and grinding machines .............................. 36

2.5 Rolls used in the research................................................................................. 41

2.5.1 Supercalender soft roll ............................................................................... 41

2.5.2 Backing rolls .............................................................................................. 41

2.5.3 Backup rolls ............................................................................................... 42

2.5.4 Thermo roll ................................................................................................ 42

3 Materials and methods ..................................................................................................... 45

3.1 Measurement methods and devices ................................................................. 45

3.1.1 Data analysis and filtering methods TSA & FFT ....................................... 45

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Contents

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3.1.2 Roll geometry measuring device................................................................ 46

3.1.3 In-situ run-out measurement device ......................................................... 51

3.1.4 Ultrasonic measurement device ................................................................ 51

3.2 3D machine control systems ............................................................................ 52

3.3 Compensation profiles for the 3D machining .................................................. 54

3.3.1 Compensation profile for static geometry errors ...................................... 56

3.3.2 Compensation profile for dynamic geometry errors ................................. 56

3.3.3 Compensation profile to compensate a process variable variation ........... 57

3.4 Measurement based roll modelling .................................................................. 57

3.4.1 Direct measurement based roll modelling ................................................ 57

3.4.2 Indirect measurement based roll modelling ............................................. 58

3.5 Uncertainty assessment of the guideway alignment system ............................ 61

4 Main results and discussion .............................................................................................. 63

4.1 Static geometry error compensation (Paper I) ................................................ 63

4.2 Dynamic geometry error compensation (Paper II & V) ................................... 64

4.3 Process variable variation compensation (Paper III & IV) .............................. 66

4.4 Measurement based roll modelling (Paper V, VI & VIII) ................................ 67

4.5 Uncertainty assessment (Paper VII) ................................................................ 68

5 Concluding remarks .......................................................................................................... 69

References .................................................................................................................................. 71

Publications ................................................................................................................................ 79

Paper I ......................................................................................................................... 81

Paper II ........................................................................................................................ 93

Paper III .................................................................................................................... 101

Paper IV ...................................................................................................................... 115

Paper V ...................................................................................................................... 125

Paper VI ..................................................................................................................... 139

Paper VII ....................................................................................................................157

Paper VIII .................................................................................................................. 165

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List of publications

This thesis consists of an overview and of the following papers, which are referred to in

the text by their Roman numerals.

I. Widmaier, T., Kuosmanen, P., Haikio, J. and Väänänen, P. 2004. A 3D turning system to reduce geometry errors of flexible rotors. Proceedings of the Estonian Academy of Sciences. Engineering. 10(4):251-260

II. Kuosmanen, P., Juhanko, J. and Widmaier, T. 2005. Verringern der Papierqualitätsschwakungen durch 3D-Schleifen der Walzen. Wochenblatt für Papierfabrikation. 133(11-12):674-682.

III. Widmaier T., Uusimäki J., Kuosmanen P., Juhanko J. and Väänänen P. 2007. Non-circular grinding of backup rolls to reduce rolling force variation, Estonian Journal of Engineering. 13(2):168-179

IV. Widmaier T., Salmela T., Kuosmanen P., Juhanko J., Kärhä P. and Uusimäki J. 2009. Reducing thickness variation of hot rolled steel strip by non-circular backup roll geometry. Ironmaking and steelmaking. 36(2):133-140

V. Juhanko, J., Porkka, E. and Widmaier, T. and Kuosmanen P. 2010. Dynamic geometry of a rotating cylinder with shell thickness variation. Estonian Journal of Engineering. 16(4):285–296

VI. Widmaier T. 2011. Thermal deflection of a finite element modelled chilled cast thermo roll. Journal of Structural Mechanics. 44(2):113-127

VII. Widmaier T., Kuosmanen, P. and Juhanko, J. 2011. Measurement of guideway alignment of an on-site grinding machine. Proceedings of the 56. Internationales Wissenschaftliches Kolloquium, Technische Universität Ilmenau, 12 – 16 September. 5 p.

VIII. Widmaier, T., Pirttiniemi, J., Kiviluoma, P., Porkka, E. and Kuosmanen, P. 2011. Modelling of a chilled cast thermo roll utilizing ultrasonic measurement. DAAAM International Scientific Book 2011, Vienna. pp. 347-364. ISSN 1726-9687, ISBN 978-3-901509-84-1.

Author’s contribution

The papers are independent research. The writing and the main part of research was

carried out by the author in Papers I and VI to VIII. In Paper III the experimental part

was carried out by Jari Uusimäki as a part of his master’s thesis, and in the paper IV the

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Author's contribution

12

experimental part was carried out by Toni Salmela as a part of his master’s thesis. The

author was a co-author in Papers II and V. In both papers the author was the co-

developer of the measurement and machining methods and took part in the

experimental and model creation parts. He also had the main responsibility in the

development of the used machine control systems including hardware and software.

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13

List of abbreviations and symbols

1G-3G Machining technology generations

3D Three-dimensional

BE Back wall echo in ultrasonic measurement

CD Cross direction of paper machine or produced paper

FE Finite element

FEA Finite element analysis

FEM Finite element method

FFT Fast Fourier transform

GUM Guide to the expression of uncertainty in measurement

HMI Human to machine interface

IF Interface echo in ultrasonic measurement

JCGM Joint Committee for Guidance in Metrology

LSC Least squares circle

MCC Minimum circumscribed circle

MD Machine direction of paper machine or produced paper

MG Machine glazed

MZC Minimum zone circles

NC Numerical control

PLC Programmable logic control

S1-S4 Four sensors of the four-point measurement device

TD Thickness direction of paper

TSA Time synchronous averaging

ZD Same as TD

3Dprof 3D compensation profile, a data matrix

CDprof CD (axial diameter variation) compensation profile, a data vector

E Elastic modulus of material (GPa)

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List of abbreviations and symbols

14

MDprof Profiles for MD (roundness or run-out) compensation, a data vector

Rk0, αk0 Distance and angle from origin to wheel contact position

Rl0, αl0 Distance and angle from origin to measurement position of the optical

micrometre

T Time of flight of ultrasound pulse (s)

a Acceleration (m/s2)

c, cl Speed of sound (m/s)

k Coverage factor for uncertainty calculation

r Radius

s Measured distance with ultrasound

t Tolerance zone

xk0, yk0 Contact position of the grinding wheel at the beginning

xk, yk Contact position of the grinding wheel

xl0, yl0 Measurement position of the optical micrometre at the beginning

xl, yl Measurement position of the optical micrometre

z Axial position along the roll

δ Roundness

θ Roll angle

ν Poisson’s ratio of material

ρ Density of material (kg/m3)

ω Angular velocity (rad/s)

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15

1 Introduction

The paper machine rolls make up the major part of a paper machine. Some estimates

state that over half of the manufacturing costs of a paper machine come from the rolls

(Kuosmanen, 2004). In the coating and calendering process the rolls are in direct

contact with the paper and give it the final surface structure and finish. Thus their

geometry has a strong effect on the produced paper quality. The knowledge about the

relation of the roll geometry to the paper quality and to the runnability of the machine

can be used to improve the operation of the rolls and the machine. Similar methods can

be applied to steel mill rolls even though the rolling process and the rolls are different.

The roll geometry is important for the quality of the end-product, which is seen in the

paper analyses results. The geometry components of the individual rolls in coating

(Juhanko, 2011; Kuosmanen, 2004; Juhanko, 1999) and in calendering sections

(Pirttiniemi et al., 2009; Pirttiniemi et al., 2010) are identifiable in the machine

directional (MD) paper analyses. In addition, the cross directional (CD) variation in the

roll geometry is visible in the paper analysis results (Kuosmanen, 2004).

Similar results are seen in the rolling process in the steel mills. Force variations

(Uusimäki, 2004; Kuosmanen et al., 2006) of the rolling mill stand and thickness

variations of the steel strip (Salmela, 2006; Widmaier et al., 2008) that synchronise to

the rolls in the stand, are observable in the measurements. The wave lengths of the

components synchronised to rolls in the starting section of the steel mill are elongated

during the rolling process (depending on the position where the analyses were taken),

and thus make their identification difficult. This is the reason why the last rolling mill

stand of a total of six stands of the finishing mill was used in the research. In the

studied cases the major difference in the results between the paper and the steel

industry is that the measured variations in the paper industry correlate with the static

and dynamic geometry errors of the rolls (Paper I; Kuosmanen, 2004; Paper II), but in

the steel industry cases they correlated with the positions of the key grooves in the

bearing arrangement of the backup rolls (Paper III; Paper IV).

A solution to this problem would be to replace all the problem causing rolls with rolls

with less static and dynamic geometry errors or with a better (keyless) bearing

arrangement. This would be a costly solution, but may be acceptable in the case of a

new paper mill or steel mill. In the case of older mills a cost effective solution would be

to find a way to reduce the observed geometry errors in the current rolls or their effects

on the end-product. Pullinen et al. (1997) studied the effects of a renewal of a backing

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Introduction

16

roll. This method was comparable to replacing the roll with a new one. The costs were

relative high (ca. 50 % of a new backing roll), but the paper quality improved. A more

cost effective method was found that can be used as a part of the normal roll

maintenance. After the introduction of a roll geometry measurement device for

grinding machines and lathes, a compensation machining technology was developed

(Haikio, 1997). With this technology the static geometry of the filled calender rolls was

improved (Paper I). After the development of the dynamic roll geometry measuring

device by Pullinen et al. (1997) and Juhanko (1999) the geometry errors of the fast

rotating rolls could be measured. This led to the development of the compensation

method of the dynamic geometry errors of the backing rolls (Kuosmanen, 2004; Paper

II).

After the development of an in-situ run-out measurement device for fast rotating rolls

(Kiviluoma, 2009) similar attempts were made to reduce the thermal run-out (mainly

bending) of the thermo rolls (Pirttiniemi et al., 2009), but they were not successful.

This unexpected outcome of the experiments with the thermo rolls demonstrated the

need to deepen the understanding of the roll behaviour in the operating conditions. A

previously developed ultrasonic measuring device (Uski, 1999) for inspecting the

coating defects of the coated rolls was used as a roll shell thickness measuring device.

With the measured shell thickness information detailed roll models could be created

(Paper V; Paper VI; Paper VIII). In the steel industry the developed measuring and

machining methods were used to compensate the spring of the bearing arrangement

caused by the key groove (Paper III; Paper IV). This compensation reduced both the

rolling force and steel strip thickness variations.

The presented methods in this thesis for geometry error compensation of static or

dynamic errors, or both, are currently used in the maintenance in many paper mills.

Their usage in the manufacturing is more limited, because the rolls need regular

maintenance, which is typically done in the roll shops of the paper mills, though

outsourcing is a growing trend and may change the current situation. If a roll

manufacturer uses compensation methods in the roll manufacturing then the customer

has to have a compensation system in his machine tools in the roll shop or outsource

the roll maintenance to a company with such machine tools. This requirement reduces

the customer interest in the error compensated rolls from the manufacturers. The

increased quality of new rolls has also reduced the need for error compensation.

During the last two decades the paper machine research group of the Department of

Engineering Design and Production at Aalto University (which was previously the

paper machine group of the Laboratory of Machine Design at the former Helsinki

University of Technology) many paper machine roll measuring methods and devices

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Introduction

17

were developed. In addition, new machining methods, especially for grinding and for

turning, were developed. Only few studies have published in this research area at other

universities. This is the reason for the large number of publications by the present and

former group members of the paper machine group and for the relative large amount of

publications from the roll and paper machine manufacturers in the references.

1.1 Research problem The geometry and the geometry changes of the rolls or roll bearings have many

influences on the end-product. Some of them are easily understandable, e.g., the

measured CD variation of the supercalender rolls is seen in the gloss and caliper

variation of the produced paper in the CD direction (Kuosmanen, 2004), the dynamic

deformations of the shell of the backing roll (including roundness changes and dynamic

bending) can be seen in the paper quality measurements (Juhanko, 2011; Kuosmanen,

2004; Juhanko, 1999) or that the force variation caused by the spring of the bearing

arrangement of the backup rolls in a rolling mill stand is seen as force variation and

thickness variation in the produced steel strip (Uusimäki, 2004; Kuosmanen et al.,

2006; Salmela, 2006).

Some phenomena are not so easily understandable. The deformations caused by the

temperature changes in the roll appear similar to the dynamic deformations caused by

the centripetal acceleration (“centrifugal force”). Still, the application of the same

compensation method as with the backing rolls to compensate the thermal deflection of

the thermo rolls was not successful. The hot chilled cast iron thermo roll of a calender

did not behave like the backing roll of the coating station (Pirttiniemi et al., 2009).

The main research problem is to find out how to reduce the end-product quality

problems caused by the static and dynamic geometry errors of the rolls. This includes

the bearings of the rolls if they have an influence on the roll geometry or on the end-

product. The secondary research problem is how to deepen the understanding about

the phenomena behind the geometry changes of rolls in the production conditions.

1.2 Objective of the research The primary objective of this research was to develop machining methods for the rolls

to increase the quality of the end-product, i.e., paper or steel strip by reducing the

problems caused by the static or dynamic geometry errors of the roll bearings or of the

rolls acting on the product. This required the ability to use the geometry information

measured from the rolls in the workshop or in the operating environment.

Measurements from the end-product also had to be available to verify the effects of the

methods used. The objective required some understanding of the phenomena causing

these geometry errors. To gain a deeper understanding of these phenomena, roll

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Introduction

18

modelling can be used for the simulation of the roll behaviour. This gave the second

objective of the research, which was to be able to create detailed roll models using

measurement data collected, e.g., by ultrasonic measurements.

1.3 Motivation and limitations of the research The motivation for this work was to develop methods for increasing the end-product

quality in the steel and paper industries. The efficiency (runnability) increase of the

machine was also a motivation, but it was not studied. At the same time this thesis

should give an overview of the research area. The problems that arise from the

properties of the end-product itself, i.e., the paper or the steel strip are not discussed in

this thesis.

1.4 Scientific contribution This work showed that the optimization of the roll geometry is a challenging task and

to obtain the optimal geometry the behaviour of the roll and the effect of the

parameters (rotational speed, temperature and its distribution, roll material and

structure, etc.) have on the roll must be known or measurable at least to some extent.

But this work also showed that if this information is available it is possible to machine

such geometry into the roll, which gives a better performance (paper quality,

runnability) in its production conditions. The Papers of this work presented methods to

reduce the roll geometry errors caused by different sources (Papers I-VI). This means

that the geometry can be optimized to reduce the some of the geometry errors, without

the need to renew the whole roll, e.g., if the static geometry was the main error then it

could be reduced (Paper I). The work also presented a roll modelling method by using

the information obtained from ultrasonic measurements (Papers V, VI & VIII), e.g., the

modelling of the internal structure of a chilled cast iron roll body (Papers VI & VIII).

No evidence has been found in the literature that this has been done before. Paper VII

gave an example of the error compensation of the guideway of a grinding machine and

the uncertainty assessment of the derived compensation equation.

The results presented in the Papers II and V were also presented in the dissertations

of Kuosmanen (2004) and Juhanko (2011). This is due to the fact that the Author and

both above-mentioned colleagues worked in the same research group and in the same

research projects. The Author's scientific contribution during that research was to

theoretically derive the procedure for the roll geometry compensation from the

measured geometry and run-out data and to take part in the scientific analyses of the

results. He was also responsible for the practical implementation of the compensation

methods in the used machine control systems of the roll lathes or roll grinders.

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2 Rolls in paper and steel industry

The appended Papers discuss different rolls and their behaviour in different operating

environments. These different operation environments are discussed here briefly. In

this discussion the term roll can include also the group of cylinders that are used in the

paper industry. The studied rolls are mainly from the paper industry, but two cases are

from the steel industry. Because the roll geometry plays an important role in this thesis,

it is discussed in the beginning of this chapter. In the second half of this chapter the

machine tools used during the research are briefly discussed. The machine tools

include the roll grinding machines and roll lathes. As the last part of this chapter the

studied rolls are presented.

2.1 Roll geometry errors The geometry errors of a roll are usually indicated by run-out, roundness or

straightness, which are measured by specific devices. Quantities that can be measured

in the roll shops are

� run-out (both static and dynamic run-out), � roundness profile and roundness, � axial diameter variation (axial diameter profile), � cylindricity, � conicity of the shell, � straightness, � error motion of rotational axis, � thickness of a roll shell, � unbalance, and � surface roughness. (Juhanko, 2011)

Unbalance and surface roughness are not geometrical quantities by definition.

Systematic surface roughness variation can be interpreted as geometrical error, if a

paper machine roll has been worn by, e.g., barring vibration to have visible n-lobe

geometry where the amplitude is very small; the surface has polished systematically

and only little changes in surface roughness can be measured (Toiva, 2006; Hermanski,

1996).

2.1.1 Geometry definitions

The standard ISO 1101 (2004) defines the form and location tolerance as well as some

measurement methods. The notation in this chapter is based on the standard and is

used throughout the thesis. A tolerance zone of a feature is either an area or a space

within or between two lines (circles) or areas (cylinders). For location tolerances, it is

necessary to define a datum indicating the exact location of the tolerance zone. A datum

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is a theoretically exact, geometrical feature (an axis, a plane, a straight line, etc.); A

datum can be based on one or several datum features. The toleranced feature may be of

any form, location or orientation within the tolerance zone, unless a more restrictive

indication is given. The tolerance value t is indicated in the same unit used for linear

measurements. If not otherwise specified, the tolerance applies to the whole length or

surface of the toleranced feature. The common geometrical tolerances for rolls are

presented in Table 1.

Table 1. Geometric tolerance definitions as defined by ISO 1101. (Juhanko, 2011)

Roundness Radial run-out

The tolerance zone in the measuring

plane perpendicular to the axis by two

concentric circles a distance t apart.

The tolerance zone is limited in the

measuring plane to the axis by two

concentric circles a distance t apart, the common centre of which lies on the datum axis. The

workpiece has to be rotated about the

datum axis. Cylindricity Conicity

The tolerance zone is limited by two

coaxial cylinders a distance t apart.

The tolerance zone is limited in the

measuring plane by two straight lines a distance t apart and

parallel to the datum.

Straightness Parallelism

The tolerance zone is limited in the measuring plane by two parallel straight lines

distance t apart.

The tolerance zone is limited in the

measuring plane by two straight lines a distance t apart and

parallel to the datum.

The axial diameter variation (axial diameter profile) is not defined in the ISO 1101,

but it is still the most common measurement of the large paper and steel industry rolls.

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It is a measurement of diameter deviation at several axial positions along the roll axis.

Normally, for manual or semi-automatic measurements saddle-type calipers (roll

mikes) are used. For manual measurements large scaled micrometres are also used.

The semi-automatic roll mikes can have automatic data transfer to measurement PC.

The first roll measurement devices installed on roll grinders or lathes had the same

working principle as the roll mikes. They were fastened on the machine in such a way

that they could be lowered onto the roll for the measurement. This measurement

method with two contacting measurement points for the diameter is also called a (roll)

profile measurement or a two-point method (compared to methods with more or less

measurement sensors and contacting points).

Figure 1. Saddle-type roll calipers. A) Swing arms. B) Fixed arms.1

The measurement profile of the axial diameter variation is often misinterpreted as a

straightness measurement. The difference in the measurements becomes clear if a bent

roll (“banana shaped”) with equal diameters in all cross sections is measured for

straightness and for diameter variation. The straightness profile will show something

between an almost straight line and a more or less curved profile depending on the

measurement angle of the roll. The axial diameter variation will show a straight line in

every roll angle.

For roundness the tolerance zone in a plane perpendicular to the axis of rotation is

defined by two concentric circles a distance δ apart. The roundness is defined using

concentric circles enclosing the circular profile and having the radial separation:

. (1)

The determination of roundness can be carried out with at least four methods

(Whitehouse, 1994; Dagnall, 1996), which are shown in Table 2. The methods produce

1 FMT Equipment Corporation, Roll Measurement, http://www.fmt-equipment.com/roll_measurement_and_tolerances3.htm

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different values for roundness. For numerical application either regression circle i.e.

least squares circle (LSC) or minimum zone circles (MZC) are preferred.

Table 2. Definition of the centre point of the reference circle to calculate roundness. (Whitehouse, 1994; Juhanko, 2011)

Regression circle (Gaussian straight circle)

Circle laid into the measured circular profile such that the sum of the squares of all profile deviations is a minimum. The centre is that of the least squares circle. (LSC=Least Square Circle)

Circular zone with minimum radial separation MZC

Concentric circles enclosing the circular profile and having the least radial separation. (MZC=Minimum Zone Circles)

Minimum circumscribed circle MCC

Smallest possible circle which can be fitted around the circular profile.

Maximum inscribed circle MIC

Largest possible circle which can be fitted within the circular profile.

These bounding references are studied by formulating their definitions

mathematically as optimization problems (Chetwynd & Phillipson, 1980). The

utilisation of least squares method in roundness evaluation is studied, e.g., by Tan et al.

(1992) and by Kim & Kim (1996). They present a mathematical method for estimating

the centre point that gives the minimum value for roundness.

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Because of the large scale of the paper or steel mill rolls, the measurement of their

roundness is difficult in the optimised laboratory conditions, and thus it is normally

done on the machine (grinder or lathe). There the one-point measurement method is

typically not sufficent, because the error motion of the rotational axis can be relatively

large. Commonly used measuring devices have 2 to 4 measuring sensors depending on

the measurement method. They are used to separate the error motion of the rotating

axis from the other geometrical profiles, such as the run-out or to measure the true

roundness (Väänänen, 1993).

2.1.2 The error motion of rotational axis

The error motion of a rotational axis is the unwanted movement of the rotational axis

of an object during rotation. An example is shown Figure 2.

Figure 2. An example of the error motion of a rotational axis. Here the movement path of the rotational axis forms a circle.

The source for error motion can be other geometry errors or the error motion can be

the source for other geometry errors. One possible source for the error motion comes

from the machining process. If the rotating object (rotor) is machined with different

machine tools or with different supports (the roll is first supported from a sliding

surface on the roll surface and then changed to the roll neck), then the rotational axis

can change and the machined surfaces have different rotational axes. Another common

cause for the error motion is the imperfect bearings of the machined object or of the

machine tool, but there are other causes as well. The error motion is seen in the radial

run-out profile if the object has a perfect roundness profile. In the measured radial run-

out profile of a real machined object both profiles, the error motion of the rotational

axis and the roundness profile, are present and a multi-probe measurement method

can be used to separate them from each other. In the case of Figure 2, the measured

radial run-out will have a sinusoidal profile with the wave length of the measured

circumference (if the roll has a perfect roundness profile).

1 2 3

4 5 6

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In large and long objects the path of the error motion often varies along the axial

positions. In these cases some of the measurement reports use the term ‘error motion

of rotational centre point’. This term specifies the error motion of the rotational axis in

the measured cross section. The large objects can deform themselves during the

rotation due to gravity, centripetal acceleration, thermal deformations, forces caused by

unbalances, etc. These deformations can alter the motion path of the rotational axis,

and change the originally in-tolerance motion into error motion. The magnitudes of

these deformations are in many cases related to the rotational speed and the rotation of

the object increases initial errors due to centrifugal forces.

The bending stiffness variation is caused by differences between the principal

moments of inertia of a cross section. The bending stiffness variation of a rotor under

influence of a force, a line load or a force field (machine tool, nip-load, gravity, etc.) can

cause an error in the motion of the rotational axis. For example, an oval centre hole of

the roll body, grooves in the shell (U.S.Pat. 5,940,969) or uneven material distribution

(Pullinen et al., 1997) can be the source for a bending stiffness variation. Figure 3

shows an example of a “roll” with very large bending stiffness variation. If the centre

point of the disk is followed during the rotation a trace like in Figure 4 (left hand side)

is observed. The centre point of disk seems to rotate twice per one revolution of the

“roll”.

Figure 3. The roll demonstration device demonstrates the bending of a roll axis during rotation due to gravity when the value of the bending stiffness has a clear direction dependent variation caused by the flat shaft. The white arrow shows the location of the possible radial run-out measurement in relation to the zero point (black dot with white circles) and the black pen draws the black trace into the disk when rotated.2

If the radial run-out of the disk in Figure 3 is measured, its profile will be as in Figure

4 (right hand side). If a roll is machined by turning or grinding, the error motion of the

rotational axis will cause a roundness error. The oval black trace in Figure 3

demonstrates this. It is drawn by the black pen in the roll demonstrator and it

simulates the machining tool. The same roundness error is also shown in Figure 5. In

2 Original photos by Santtu Teerihalmi

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real cases the roundness error is the sum of all the errors. This will be discussed in the

machining accuracy section.

Figure 4. Error motion of rotating axis and the radial run-out profile caused by it.

Figure 5. Roundness error of a machined cylindrical object caused by the error motion shown in previous figure.

As seen in Figure 4 (left hand side), the “rotating speed” of the rotational axis can be

higher than that of the rotor itself. This higher “rotating speed”, which is actually an

oscillation caused by the bending stiffness variation, can excite natural vibrations in the

rotor even though the rotating speed of the rotor is below its critical speed. The critical

speed is the speed where the natural frequency of the rotor and the rotating frequency

coincide. Natural vibrations of a rotor rotating at a speed below the critical speed are

called sub-harmonic resonances (“sub-critical [rotor speed] vibrations”) and natural

vibrations of a rotor with a rotating speed at or above the critical speed are called

harmonic resonances (“critical [rotor speed] vibrations”), but the frequency of the

excitation is always the natural frequency or its multiple. (Lee, 1993)

The vibrations of a rotor (roll) with asymmetric stiffness are called in the paper

industry as the “half-critical [rotor speed] vibration” (Paulapuro, 2000). There can be

Roundness 2t

Error motion

t

90° 180° 270° 360° Run

-out

t

-t

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other “sub-critical vibrations”, one-third, one-fourth, etc., depending on the bending

stiffness distribution of the rotor.

The presented profiles are only examples of the error motion of the rotational axis.

The resulting error motion is a combination of different causes and every roll has a

unique error motion path, which is also rotational speed dependent.

2.1.3 Roll geometry measurements in the paper industry

In the paper industry the terms CD and MD measurements are used for roll geometry

measurements. The names come from Cross (Machine) Direction and Machine

Direction. They are related to the principal directions of the produced paper (Figure 6),

which are the same as the operational directions of the paper machine. The names for

the principal directions are also used in the paper machine roll measurements.

Both measurement types (MD and CD) measure changes in the radial direction of a

roll. The CD measurement is performed in the axial direction of the roll (Figure 7). The

measurement result can include an axial diameter variation profile, a straightness

profile, the conicity, an alignment error, etc. The MD measurement is performed in the

circumferential direction of the roll (Figure 7). The MD measurement result can include

a roundness profile, a run-out profile and an error motion profile of the rotational axis,

etc.

Figure 6. The principal directions of the structure of paper: machine direction (MD), cross direction (CD) and thickness direction (TD or ZD). (Wilbrink, 2011)

Figure 7. The main measuring directions of a paper machine roll.

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A third roll geometry measurement type is the 3D measurement. It can be seen as a

combination of MD and CD measurements, and it is typically a helix measurement of

the whole perimeter of the roll. The 3D measurement does not belong to the standard

measurement set of a paper machine roll, but most of the measuring systems installed

on roll grinders or lathes can perform it. Some examples of MD, CD and 3D

measurements are given later in the section where the roll geometry measuring device

is discussed. The same roll measurement types exist also in the steel industry, but the

naming can be different.

2.2 Paper machine During the manufacturing process of paper, a fibre suspension is sprayed from a

nozzle onto a net, called a wire. Some water is drained on the rapidly traversing wire.

Shear forces in the area where the jet hits the wire ensure that the fibres are more

oriented in the direction of the paper machine (MD) than in the cross machine

direction (CD). Another result of the manufacturing process is that nearly no fibres are

directed in the through-thickness direction (ZD). After dewatering on the wire more

water is removed from the paper web in the press section. After the press section the

paper web is dried in a heated dryer. A schematic picture of the manufacturing process

is shown in Figure 8. (Stenberg, 2002)

Figure 8. Principle of a paper making. A fibre water suspension is first sprayed through a head box onto the wire. Some water is drained through the wire. The press section still reduces the moisture content. The final moisture content of the paper is obtained after drying. (Stenberg, 2002)

Modern paper machines are large machines with many sections (Figure 9). Rolls are

used in all the sections. Basically, the paper machine consists of following sections

starting from the wet end: head box and forming section (wire section), press section

and drying section. Most of the machines have at least one calendering section to

achieve a better paper gloss and printability. If coating is applied then a coating section

is included typically between calendering sections. The paper is stored and transported

in large paper rolls. The reeling and cutting to the width ordered by the customer is

performed in the slitting-winding section. (Paulapuro, 2000; Karlsson, 2000; Jokio,

1999)

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Figure 9. Modern paper machine for wood free uncoated paper.3

2.2.1 Calenders used in the research

Rolls from two types of calenders were studied: a soft calender and a supercalender. A

brief description of both calender types is given. In the terminology used in the paper

industry, the rolls in the calenders are called soft or hard rolls depending on the

hardness of the roll surface. The separation into hard and soft rolls is defined regarding

how the roll surface interacts with the paper in the calender. The surface of the hard

rolls has practically no deformation during the calendering. Only the paper is

deformed. The surfaces of the soft rolls are deformed during the operation. This

deformation is an important part of the soft calendering process. The soft calenders

produce paper with a constant density, but a varying caliper. The constant density gives

more uniform printing properties compared to a hard nip calender, which gives a

constant caliper of the paper. The roll hardness comes from the roll material itself,

from the treatment of the roll surface (for example hardening) or from the roll cover.

Hard roll surfaces are mainly cast iron or steel and if coated then the coating materials

are alumina, alumina and titanium, chromium oxides, tungsten carbides, etc. Soft roll

covers are often natural fibres (for example cotton) or polymers. The soft calenders,

supercalenders and the different roll types are described in detail in the book edited by

Jokio (1999).

Soft calender The heated roll studied, a thermo roll, was the upper roll of a soft calender. A soft

(nip) calender has at least one nip roll pair (Figure 10). One of the nip rolls is a soft

(covered) roll, often a deflection compensated roll. The deflection compensated rolls

are used to produce an even nip-load and there are several designs available4,5. The

3 Picture from: Sweitlik, F., Anttilainen, S., Kojo, T. and Vaittinen, H. 2011. Evolution of OptiConcept – Entering a New Era. Metso Technical Paper Series. Metso Paper Oy. 4 SymRoll family, Metso's deflection compensated roll solutions, Metso Paper, Inc. 07/2005. Available at www.metso.com/MP/marketing/Vault2MP.nsf/BYWID/WID-111019-2256E-13BF6/$File/32006_V2_EN.pdf?OpenElement

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other roll of the calender is typically a thermo roll.

Figure 10. Left: soft nip calender concept (Jokio, 1999). Right: a two roll soft calender used in the research6. Upper roll is a thermo roll and lower roll is a soft covered deflection compensated roll.

Supercalender The studied filled roll was a soft roll of a supercalender. A supercalender is multiroll

and multinip calender composed with alternating hard and soft rolls and it is normally

an offline calender (Figure 11). The top and bottom rolls are deflection compensated

rolls and the intermediate hard rolls are typically thermo rolls. The soft rolls allow

heavy linear loads to be used. Previously the soft rolls were filled rolls, but currently

polymer covered rolls are also used. The maintenance interval of the filled rolls is

relatively short compared to polymer rolls, but a better gloss of the paper can be

achieved with filled rolls more easily. Both types of rolls can be mixed in a single

supercalender. (Jokio, 1999)

Figure 11. A supercalender (Jokio, 1999).

5 Deflection compensating rolls by Voith – reliable technology for a market orientated quality of your paper and board grades, Voith Paper Limited. 2006-07. Available at http://voithpaper.com/applications/documents/document_files/707_e_pp_vpkr_walzen_en.pdf 6 Original photo by Jukka Pirttiniemi

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The large number of the rolls and the heavy linear load makes the supercalender

vulnerable to vibrations excited by different causes, such as the geometry errors in the

rolls. A vibration related phenomenon called barring occurs in the supercalenders and

in other calenders (Toiva, 2006; Wirtz, 2002; Hermanski, 1996; Shuffler, 1982).

2.2.2 Coating station

The backing rolls studied in this work were from blade coating stations. The function

of a backing roll is to guide the paper web through the coating station, and to give

backup for the coating process. The aim of coating of paper is to obtain a better surface

for printing and publishing. The common coating methods are blade coating, film

coating, air brush coating and spray coating. Blade coating is the most common coating

method. (Juhanko, 2011)

In blade coating the applicator roll transports the coating colour to the paper web

surface from the colour container. At the same time it presses it against the paper. The

metering blade removes the extra colour from the paper surface. The removed colour is

circulated back to the colour container, see Figure 12.

Commonly used pigments in the coating process are kaolin, calcium carbonates, and

plastic pigments. The binders are synthetic latexes and natural binders, such as,

starches. If the paper is coated on both sides then one side is coated first and the paper

is dried. The second side is coated separately after the drying. After the coating the

paper is calendered. (Jokio, 1999; Lehtinen, 2000)

Figure 12. A blade coating station of the coating machine: (1) metering blade, (2) support beam of the blade, (3) applicator roll, (4) backing roll, and (5) collectors of colour overflow from applicator and back circulation from blade (Lehtinen, 2000).

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2.2.3 Yankee dryer

The Yankee dryers are used in soft tissue machines, in board machines and for

machine glazed (MG) paper machines. The largest part of the drying section is the

Yankee dryer with the Yankee cylinder (Figure 13). The cylinder is used to transfer heat

to the paper web. The cylinder is heated by pressurized steam through internal piping.

Most of the drying is done by the drying hood above and around the cylinder. It uses

hot air blown onto the paper web for the drying. The tissue paper is creped with a

doctor blade (crepe blade). The crepe form of the paper is dependent on the blade and

its operating parameters such as contact angle, web take-off angle, etc. (Karlsson,

2000)

Figure 13. Tissue machine with a Yankee drying section. (Karlsson, 2000)

The large size of the Yankee cylinder is challenging for the maintenance (typical sizes:

length 4 to 6 m, diameter 3 to 6 m). It is often, if not directly impossible, but at least

very expensive to remove the cylinder from the drying section, and thus the

maintenance is preferably done in the machine, i.e., on-site. The maintenance includes

the polishing, regrinding, and coating of the cylinder surface. Polishing is performed

every time as the last procedure. Grinding is performed only if polishing does not give

the required surface quality fast enough and coating only if the old coating must be

renewed. The polishing can be done with a polishing belt of the grinding machine.

2.3 Steel mill Steel mills produce steel strips and sheets. Sometimes they are specialized in one or a

limited product range, for example special sheets. In a hot steel strip mill the strips are

produced from hot steel slabs by rolling them in one or several rolling mill stands until

the desired thickness is achieved. The cold strip steel mill uses pre-rolled strips to

produce even thinner or smoother strips. Typically, the strips are also cut to the form

ordered by the customer. Modern steel mills are large industrial plants with different

sections (Figure 14). The studied rolling mill stand was located in a hot strip steel mill.

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Figure 14. Hot strip steel mill, 1 furnace, 2 scale breaker, 3 vertical mill, 4 roughing mill stand, 5 coilbox, 6 end cutting, 7 finishing mill, 8 cooling showers, and 9 coiling.7

2.3.1 Rolling mill stand

There are several designs for the rolling mill stands (Figure 15). The studied mill

stand was of type c) with two working rolls and two backup rolls. It was the last stand in

the finishing mill. The type c) is the most common stand type (Ginzburg, 1987). The

main working parameters in the mill stand are rolling force, rolling speed and the

profile of the working rolls. Automated systems can be used to control the rolling

process, e.g., to adjust the rolling force during the process. (Ginzburg, 1998; Kugi et al.,

2000)

Figure 15. Different rolling mill stand designs. (Ginzburg, 1989)

The usage of the backup rolls and intermediate rolls allow the reduction of the

diameter of the working rolls. This increases the pressure acting on the strip by

reducing the target area of the rolling force. The larger sized backup rolls support the

thinner working rolls during the rolling process.

2.4 Machine tools for large rolls In the manufacturing of modern rolls for the paper industry or for the steel industry

different machine tools are used: lathes, milling tools, drilling machines, roll grinding

machines, etc. Only lathes and grinding machines were used during this research.

Grinding machines are used with all types of rolls during the manufacturing and the

maintenance as well. Lathes are used mainly during the manufacturing process. If they

are used in the maintenance then they are typically used for turning of the soft rolls

(filled rolls or polymer rolls) or for turning of the steel endplates of the rolls.

7 P. Lehtikangas, Rautaruukki Oyj Raahe mill steel strip layout, PowerPoint presentation.

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2.4.1 Grinding machines

The book from Lurie & Komissarzhevskaya (1987) says about grinding: “Grinding is a

productive method of machining which ensures a high surface finish, superb geometry

and dimensions of parts”. Made out of high precision components, a grinding machine

can produce even large scale parts with an accuracy close to 1 μm. Traditionally the

precision of the grinding machine was the sum of the precision of its parts, but today

most of the large roll grinding machines have an error compensation system in their

control systems.

The working principle of the grinding machines is to remove material from the

workpiece with the rotating grinding wheel. The abrasive grains of the wheel material

are interconnected by a bond substance. During grinding the wheel wears off and new

grains come into contact with the workpiece. Regular sharpening of the tool ensures a

correct tool profile and a clean cutting surface. Coolant usage can be required by the

cutting process and to remove the heat from the workpiece. The grinding process is

often instable and prone to vibration problems, because of the large rotating workpiece

and the rotating tool. Because of the large size of the machine and the workpiece,

deformations due to gravity and other forces can become relative large and set

demands on the stability and damping effects on the machine. The grinding process has

been modelled and simulated by Yuan et al. (Yuan et al., 2001; Yuan et al., 2002). The

simulation provides a view of the stability of the grinding process.

Figure 16. Modern roll grinding machine in a workshop of a paper mill.8

If high precision is required, then the roll is supported by shoes or fastened between

centres, but during the maintenance the rolls are often ground with their own bearings.

In these cases the amount of material ground from the roll should be kept small. The

grinding should be more like a finishing procedure. If the machine control has a

8 Original photo by RollResearch Int. Ltd.

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compensation system for the geometry errors, then it is possible to remove more

material.

Modern grinding machines can be equipped with automated (Figure 16), semi-

automated or manual roll geometry measuring devices. Some of the grinders have

additional devices for turning, for milling and for drilling. Other supporting devices,

such as roll or wheel balancing systems, automated roll alignment systems, exhausters,

etc. are common with these large scale grinding machines. The measurement and roll

databases can be connected to the network of the plant, e.g., for efficiency monitoring

purposes.

On-site grinding machines As discussed in the section of Yankee dryers, the large scale of the Yankee cylinders

requires often on-site grinding of the cylinder surface during maintenance. An on-site

grinding machine was developed during the research. The principle is shown in Figure

17. The harsh environment around the Yankee cylinder set special requirements for the

grinder and its measurement systems. The maintenance break is normally so short that

there is no time to wait for the cooling down of the cylinder, so the environment is hot

and probably moist.

Figure 17. Model of an on-site grinding machine located next to the surface of large cylinder.

The on-site grinding machine is brought by the maintenance team onto the site,

where the grinding machine has limited space around the cylinder (Figure 13; Paper

VII). The guideway of the machine must be supported somehow. Alternatives to

movable guideway support structures are the support structure of the smaller pressure

roll(s) or of the doctor blade. If the support of the pressure roll is used, then the

removal of the pressure roll might be necessary. After removing the blade, the support

of the doctor blade is often usable as such. The guideway of the grinding machine is

placed on top of the support. In both locations the grinding is performed from below

and the guideway must be tilted (Figure 17). Paper VII discusses an alignment system

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for a tilted guideway. If rolls or cylinders other than Yankee cylinders are ground on-

site, then the environment can be friendlier and the guideway tilting may not be

necessary.

In the modern machine tools (for example roll grinders and roll lathes), if the

geometry errors of the machined workpiece are systematic and measurable, they can be

compensated by the control system. It is even possible to measure the systematic errors

of the measuring system of the machine tool and compensate them, too. (Boman, 2004;

Widmaier et al., 2004; Haikio, 1997; Kotamäki, 1996)

The machining accuracy of modern roll grinders is good. For example, the accuracies

promised by the roll grinder manufacturers are typically ±1 μm for roundness,

±1 μm/m for straightness of generating line, and ±1 μm/m for axial diameter variation

tolerance. These tolerances are achievable only in a controlled workshop environment,

where the roll is supported by shoes from the roll neck, the grinder is in a perfect

condition, and is handled by skilled operators.9 These accuracies can be achieved when

using the error compensation system of the machine.

In on-site grinding machines the accuracy is worse than in the normal grinding

machines. The environment can be hot and moist, depending on the location of the roll

in the machine. This affects the accuracy negatively. Additionally, the grinding

direction is often not ideal and the alignment system needed for the guideway makes it

prone to random errors, such as operator errors.

In general, grinding is a machining method where knowledge and experience of the

operator plays a major role in the final quality of the ground workpiece. Another

important characteristic of the operator is patience. The phenomena during the

grinding process often develop slowly and sometimes the effects of the changes in the

grinding parameters become visible after a while.

2.4.2 Lathes

Roll lathes (Figure 18) and roll grinders are similar machines. The shape and the

operation principle of the tool is the clear difference. The non-rotating lathe tool is

moving in horizontal directions. If the tool is not changed or sharpened, the cutting

edge remains the same during the operation (some wear will occur). The working area,

where the material is removed, is small compared to a grinding wheel.

Coolant can be used, but natural cover materials of soft rolls might not tolerate the

oily additive mixed into coolant water and the water itself can make the fibres of the

cover material swell. A chip removal exhauster can be used instead. Often the air flow

9 RollResearch Int. Ltd. Acceptance test for CNC roll grinder (paper industry type), 2011.

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into the exhauster cools the cutting area or the tool sufficiently if cooling is needed.

Because the tool is not rotating the vibration excitation is smaller. This allows higher

rotating speeds to be used. The finding of the correct machining parameters is

generally easier compared with grinding.

Figure 18. Roll lathe in a roll shop of a paper mill.10

The machining accuracy of modern roll lathes is as good as of the roll grinders and

error compensation systems are available. The same workshop environment and

operator requirements apply as with the roll grinders. If the roll is rotated with its own

bearings as in Figure 18, the achieved accuracy is dependent on the accuracy of the

bearings and the machine control system of the lathe. Typically the accuracy is worse in

these cases.

2.4.3 Machining accuracy of lathes and grinding machines

Machining accuracy gives the accuracy of the manufactured workpiece. The accuracy

includes the dimensions, shape and surface quality of the workpiece, and it depends on

many different components and subsystems of the machining process and their

relations. Okafor & Ertecin (2000) list four sources for geometry errors. The first

source is the geometric inaccuracies, which are regarded as the machine tool errors,

which exist under cold start conditions. The second source are the thermally induced

errors, which are generated by environmental temperature changes and local sources of

heat from drive motors, friction in bearings, gear trains and other transmission devices

and heat from the cutting process. Load induced errors are the third error source. They

come from three different types of forces present during machining process: (1)

workpiece weight (2) forces resulting from cutting process, and (3) gravity forces

resulting from the displacement of masses of the machine components. They all cause

elastic strain on the machine tool structure. The last error source is the rest of the

errors, e.g., fixturing errors. Ramesh et al. (Part I & Part II, 2000) have classified the

error sources into following similar categories:

a) Geometric errors of machine components and structures

b) Kinematic errors

10 Original photo by RollResearch Int. Ltd.

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c) Errors induced by thermal distortions

d) Errors caused by cutting forces including

(i) by gravity loads,

(ii) by accelerating axes, and

(iii) by the cutting action itself

e) Material instability errors

f) Machine assembly-induced errors

g) Instrumentation errors

h) Tool wear

i) Fixturing errors and

j) Other sources of errors like servo errors of the machine (following errors and

interpolation algorithmic errors).

Väänänen (1993) has studied the parameters affecting the accuracy of a machined

workpiece. He presents the relations between the individual sources, parameters and

machine parts visually (Figure 19).

Figure 19. Influencing parameters on the accuracy of the machined workpiece. (Väänänen, 1993)

Generally, all the unwanted translations or rotations of the tool or of the workpiece,

that have a vector component in the machining direction (X direction in Figure 20),

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will be a potential cause for an error in the geometry of the workpiece. The component

of this unwanted movement in the machining direction can cause a diameter error in

the workpiece if it lasts at least one revolution of the workpiece. Otherwise the result

can be a roundness error or a straightness error.

Figure 20. Six degrees of freedom of a machine tool carriage system. The machining direction is the X direction.

Some examples are given from the paper machine roll environment. They are

classified into geometry errors caused by the machine tool, by the workpiece properties,

and vibration and chatter.

Geometry errors caused by the machine tool and by the workpiece The typical error sources in roll grinding machines and lathes are the errors in the

guideways, which produce unwanted diameter variation into the workpiece and the

spindle and bearing errors, which are the source for roundness errors. These errors are

usually relative static. If they can be measured, then modern control systems can

compensate these errors.

The properties of the roll itself are a potential error source. When the roll is rotated,

the variation in the bending stiffness is a cause for the error motion of the rotational

axis (Kuosmanen, 1992). When the roll is machined, this error motion can cause an

error in the roll geometry. This was discussed in Section 2.1.2.

Another error source is the bearings of the rolls. In many cases the rolls are machined

with their own bearings. This is true especially in the roll shops of the mills where the

rolls are ground or turned during the maintenance. The accuracy of the bearings is not

as good as the accuracy of the shoes or of the spindle of the machine tool. The error in

the bearings is copied to the roll geometry during machining (Juhanko, 1999; Kim,

1983).

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Unlike in the previous examples, the source for the geometry error can be

nonsynchronous to the roll rotation or it is not as static as the guideway error, and thus

difficult to compensate. Some examples are given.

Figure 21. Left: the rotation arrangement of a roll with a steady axis, e.g., deflection compensated roll. Right: the internal structure of the bearing of a deflection compensated roll11.

Not all the rolls are rotated directly by the roll spindle of the machine tool. This is due

to the design of the roll, for example in the case of the roll in Figure 21 only the roll

shell is rotating and the roll axis is steady. Often, the deflection compensated rolls

operate in this way. The roll shell is rotated with a belt from one end of the roll during

the machining. The driving pulley is located about at the same height as the roll itself.

The arrangement causes a fluctuating horizontal force to one end of the roll, because of

the friction and the stiffness changes in the belt and in the bearing. The sliding bearing

of the roll is relatively large (Figure 21, right hand side) and to operate correctly the

lubrication system of the roll must be turned on. In the roll shops it is not uncommon

that lubrication system is not present or it is not operating at the full hydraulic

pressure, because the rotational speeds in the grinding machine are much less than in

the paper machine and some oil is still present in the bearing. Without the lubrication

pressure, the play in the bearing can be large. The play in the bearing and the

fluctuating horizontal force can make the end of the roll wobble, and thus produce a

rotationally nonsynchronous error into the roll geometry.

Another example of errors that are difficult to compensate is the heat build-up in

some machine parts or in the workpiece. The heat build-up causes temperature

dependent geometry changes during the machining process (Figure 22). Typically, the

compensation of these kinds of errors is not possible, at least not without an online

measurement system.

11 Original picture from: Profiling Sym rolls for calendering applications. Metso Paper Oy, Roll Technology. http://www.metso.com/MP/Marketing/vault2mp.nsf/BYWID2/WID-111019-2256E-5D486/$File/32015_V1_EN.pdf?openElement

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Figure 22. A 3D measurement of a roll where the friction of sliding pads of the grinding machine warmed up the roll surface locally during grinding. After cooling down the local diameter minima are located at those cross sections that were warmed up during grinding.

Vibration and chatter Vibration is normally not seen as the cause for geometry errors of a workpiece. It is

more a surface quality problem of the workpiece. Also the tool wear and the useful life

time of the tool and the machine are often affected by the vibration forces. The

vibrations of a machine tool for large scale cylinders can be divided into two categories:

forced vibration and self-excited vibration (Lurie & Komissarzhevskaya, 1987). Forced

vibration has an external cause. The forced vibrations can be avoided or damped by

influencing the dynamic characteristics of the machine tool.

Figure 23. Chatter marks on the roll surface. (Juhanko et al., 2003)

Self-excited vibration, also called chatter, is caused by the machining system itself. If,

for example, the natural frequency of the workpiece is close to the rotating speed or its

multiple then the relatively small resonance vibration of the workpiece produces an

uneven load for the tool (Figure 23). There are many papers related to grinding chatter

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and vibrations. Han & Hoshi (1990) state that already from the start of grinding

vibrations exist in the frequency multiples of the grinding wheel speed. The effect of the

torsional vibrations on the birth and growth of the grinding vibrations has been studied

by Drew et al. (2001), Manan et al. (1999) and Drew et al. (1999). Hardwick & Benhafsi

(2004) have developed a non-contacting system to measure the frequency and depth of

the chatter marks. Ahn et al. (2001) studied the effects of stiffness and damping

coefficients of the spindle bearing on the dynamics of the cylindrical plunge grinding

process. Yuan et al. (2004) used simulations to study the time delay effects on chatter.

2.5 Rolls used in the research The rolls that were used in Papers of this thesis are briefly discussed here. More

details are in the appended Papers. The Yankee cylinder is not discussed here, because

no actual Yankee cylinder measurement or machining results are presented or

discussed in the appended Papers.

2.5.1 Supercalender soft roll

The supercalender soft roll used in Paper I was a filled roll (Figure 24). The body

length of the roll was 5600 mm and its diameter was 607 mm. The roll cover was made

from cotton sheets, which were stacked around the roll shaft and then pressed together

with the help of the endplates. The endplates were fastened with nuts at both axle ends.

(Jokio, 1999)

Figure 24. Filled roll. (Jokio, 1999)

2.5.2 Backing rolls

The backing roll in Paper II was made from steel sheets by sheet-bending and

welding. The length of the roll body was 5870 mm and the nominal diameter 965 mm

(Figure 25). The backing roll in Paper V was made in the same way from steel sheets.

The length of the body of the second roll was 8200 mm and the nominal diameter 1500

mm (or as used in the research, the diameter without the surface cover was 1450 mm).

Both rolls were also coated with rubber, but from the larger test roll the rubber was

removed before the testing. The initial rubber thickness in both rolls was 25 mm. Both

backing rolls were from blade coating stations.

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Figure 25. Backing roll of a coating station. The roll is coated with rubber. (Kuosmanen, 2004)

2.5.3 Backup rolls

Two backup rolls were studied in the Papers III and IV. They were the top and the

bottom backup rolls of the same finishing rolling mill stand. Their sizes and dimensions

were almost identical. The main dimensions are shown in Figure 26. The mass of the

rolls was ca. 60 000 kg. The roll body was solid (no centre hole), so there were no

dynamic geometry changes caused by the rotating speed, which would have an effect on

the rolled steel strip (max. strip speed in the rolling stand was 13.5 m/s, which is less

than 90 rpm of the backup roll).

Figure 26. Backup roll of the rolling mill stand. (Uusimäki, 2004)

There are two types of bearing arrangements for the backup rolls: key-type and

keyless. Both rolls had a key-type bearing arrangement.

2.5.4 Thermo roll

The studied thermo roll (Figure 27) was from a two roll online soft calender. It was

the upper roll in the calender (Figure 10). The roll was a chilled cast iron thermo roll

with 40 peripheral bores and was designed for heating oil temperatures up to 250 °C.

The lower roll was a deflection compensated roll with a soft roll cover, but it was not

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studied in the Papers of this research. The exact dimensions of the thermo roll are

given in Papers VI and VIII.

Figure 27. Chilled cast iron thermo roll with duo-pass heating channel arrangement. A section of the body has been cut to make the heating fluid channels visible.

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3 Materials and methods

3.1 Measurement methods and devices Different types of measurement devices were used during the research. In addition,

the methods used with these devices are briefly discussed. All the used measurement

devices, except the Tapio PMA12 paper machine analyser (U.S.Pat. 4,773,760), were

created by the research group, which the Author was a member of. The Tapio PMA

analyser was used in Papers II and V.

3.1.1 Data analysis and filtering methods TSA & FFT

The time synchronous averaging or TSA (McFadden & Toozhy, 2000; McFadden,

1987) is a signal processing technique which enables periodic signals to be extracted

from the measurement signals. It is typically used in the vibration analysis of rotating

components such as gearboxes to separate a single harmonic component from the

vibration of the complete system. The technique is based on a trigger signal which is

phase locked with an angular position of the rotating component (Juhanko, 2011;

Kiviluoma, 2009). This method is used for separating the rolling force and strip

thickness variation from the raw force and thickness signals measured from rolling mill

stand in Papers III and IV. At the same time resolution of the measurement signals is

enhanced by the averaging and the noise in the signal (Etchenique & Aliaga, 2004). In

Paper II it was used in the correlation analyses of the produced paper and the backing

roll geometry.

Figure 28. Time synchronous averaging applied to the rolling force measurement. Left: raw force measurement in time domain; right: TSA synchronized force profile of a roll.

The FFT belongs to the group of Fourier analyses and it is used in all the Papers of

this thesis (except Paper VII). In the analysis of measurement signals, especially of

rotating objects, it is often relevant to decompose a measurement signal into its

harmonic components. The studies of J. Fourier showed that general functions may be

12 See http://www.tapiotechnologies.fi/paper_machine_analyzer.html

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represented by sums of simpler trigonometric functions. This operation is called

Fourier analysis, and several mathematical methods of performing the analysis exist. In

this thesis the used Fourier analysis is calculated with the fast Fourier transform (FFT)

algorithm developed originally by Cooley & Tukey (1965). The FFT gives the harmonic

components of a measurement signal in the frequency domain in the form of complex

numbers. Their phase angles and absolute values represent the phases and amplitudes

of the individual harmonic components. The inverse FFT algorithm can be used to

compose the original measurement signal in the time domain from these complex

numbers. Filtering of some unwanted frequencies or components is straightforward.

The complex number representing the unwanted frequency or component is set to zero

before the inverse FFT (Mosier-Boss et al., 1995). An example of filtering is Paper IV.

Figure 29. Examples of run-out profiles and their harmonic components calculated with FFT algorithm.

In Figure 29 some examples of FFT usage are shown. The upper row graphs show

profiles from rotating objects, i.e., run-out profiles, and the lower row shows the

amplitudes of the harmonic components of the profiles calculated with the FFT

algorithm. Typically, the harmonic components are indicated as the multiples of the

frequency of the original signal, i.e., in this case they must be multiplied by the rotating

frequency of the measured object to obtain their actual frequencies. The number of the

component gives also the lobe number of the profile if it is a clearly dominant

frequency, e.g., Figure 29 graphs a), b) and c). In the analysed measurement signals of

this research the FFT algorithm is used both for identifying certain harmonic

components and for filtering purposes.

3.1.2 Roll geometry measuring device

All the used roll grinders and lathes had a four-point measuring device developed for

precise large-scale rotor measurements especially for paper machine or steel mill rolls.

1 2 3 4 5

harmonics

a

1 2 3 4 5

harmonics

a

1 2 3 4 5

harmonics

a

1 2 3 4 5

harmonics

a

2a d) 2a c) 2a b) 2a a)

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Some of the devices were manual and others fully automatic. For high speed roll

geometry measurements a laser based version was used. The devices were capable of

measuring the MD profiles (axial run-out profile, roundness profile and profile of the

error motion of the rotational axis, see Figure 30 and Figure 31), CD profiles (axial

diameter variation, see Figure 32), a 3D measurement profile (Figure 22) and some of

them were capable of measuring the absolute diameter of a roll.

Figure 30. Roll roundness profile and its harmonic components.

Figure 31. Error motion of the rotational axis (centre point) measured in a cross section of a roll.

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Figure 32. Axial diameter variation of a roll.

The four-point measuring method for the roundness profile and error motion profile

of the rotational centre point measurements used with these measurement devices was

developed by Väänänen (1993). The further development of the measuring device is

discussed by Mattila (2004). The four-point measuring method uses four sensors

(sensors S1-S4 in Figure 33) in a combination of the three-point method (sensors S1-

S3) and the two-point method (sensors S1 & S4). The four-point method combines the

methods into a more accurate method. The two-point method is also used for the axial

diameter variation measurement.

Several other roll measuring principles were also developed by other researchers or

research groups. For example, the V-block method was studied for the measurement of

the cylindricity of large scale cylinders by Adamczak et al. (2010) and Stepien et al.

(2011). Janusiewicz et al. (2011) have made analyses of the theoretical method of the

on-machine roundness measurements. Shimazutsu et al. (U.S.Pat 6,151,791) have

patented a roll profile measurement method that uses two or more displacement

detectors on a moving carriage. For roundness measurements there is the original

three-point method developed by Aoki & Ozono (1966) and Ozono (1974). There are

several other variants of multi-point methods in the literature, e.g., Gao et al. (1996)

present a method where one displacement sensor is replaced by an angle sensor at one

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of the remaining sensors. Nissinen (2010) has listed different measurement principles

in his study and Kotamäki (1996) discusses the roll geometry measurement together

with a roll geometry error compensation system for grinding.

Two variations of the four-point measuring device exist. They are presented in Figure

33. Type a) measuring device is capable of measuring all MD, CD and 3D profile types

discussed before. All the used measurement devices in this research were of type a).

The limitation of type b) is that it cannot be used for the measurement of the error

motion of the rotational axis. The probes of the typical four-point measuring device

operate in contact with the measured surface. In slowly rotating applications on the

lathe or grinding machine this gives the needed accuracy for the measurement. In fast

rotating applications such as balancing machines or dynamic roll geometry

measurement devices contacting probes cannot be used, because the measuring heads

do not follow the measured surface accurately. Non-contacting sensors can be used

instead. A laser probe based measuring device was developed by Juhanko and Pullinen

(Juhanko, 1999; Pullinen et al., 1997). The measurement algorithm is the same four-

point method as with the traditional roll geometry measurement devices, but with an

extra signal filtering system for the laser sensor signals.

Figure 33. a) A rigid four-point measurement device, and b) floating four-point measurement device (two sensors only). (Väänänen, 1993; Juhanko, 1999)

The 3D measurement profile (Figure 22) consists of several roundness measurements

along the roll axis. This measurement provides a quick overview of the whole geometry

of the roll. The disadvantage in the 3D measurement is that it takes a relative long time

to measure (depending on the number of the roundness profiles used) and that the

probes touch a large area of the roll surface, which sets a high demand for the

a) b)

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cleanliness of the roll surface. Single chips or grinding wheel grains on the roll surface

give a large error if they travel under the probes during the measurement.

Figure 34. Four-point roll measuring device types used during the research. Top row from left to right13: a manual roll measuring device installed on a roll lathe and a full automatic roll measuring device installed on a roll grinder. Bottom row: roll measuring device14 with laser sensors.

The roll measuring device types used in this research are shown in Figure 34 and they

were used in Papers I to V. They were all four-point measuring devices using the

measurement method developed by Väänänen (1993). This type was chosen because it

was available in all the machines. The accuracy of this measurement method has also

been compared with reference measurements performed by the Center of Metrology

and Accreditation of Finland (Mikes). Juhanko (2011) compared the laser based

measuring device and in the report by Pirttiniemi et al. (2009) the contact based

measuring device was compared with the results from Mikes.

13 Original photos by RollResearch Int. Ltd. 14 Original photo from the photo collection of paper machine group of Aalto University, Department of Engineering Design and Production, laser sensor photo by Jari Juhanko.

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3.1.3 In-situ run-out measurement device

The principle, development and application of the in-situ run-out measuring device

was described by Kiviluoma (2009), by Pirttiniemi et al. (2009; 2010) and by

Kiviluoma et al. (2011). It can be used to measure the radial run-out of rotating rolls,

e.g., of hot thermo rolls or of other fast rotating rolls, in the operating conditions in the

machine. The measurement is based on accelerometer located on a sliding pad. The pad

follows the surface contour of the roll and the displacement is obtained by a double

integration of the accelerometer signal over time. The device and the principle are

presented in Figure 35.

Figure 35. Left: in-situ run-out measuring device. Right: the measurement principle. (Kiviluoma, 2009)

The in-situ measurement device was used to measure the run-out of a thermo roll in

Paper VI.

3.1.4 Ultrasonic measurement device

The ultrasonic measuring device (Figure 36) used in Papers V, VI and VIII was

developed by Uski (1999). It was based on a system originally developed by Savolainen

(1996). The device used consists of an ultrasonic measurement probe and position

sensors. It is capable of measuring a map of the whole shell of a rotating cylinder (roll).

The shell thickness, run-out or coating defect maps are the measurement types

available. The resolution of the measurement grid is limited only by the amount of free

memory available on the measurement computer and the 32-bit operating system.

The ultrasonic shell thickness measurement is based on the measurement of the flight

time of an ultrasound pulse sent through the material (Figure 37). The correctness of

the measured distance is linearly dependent on the correctness of the used speed of

sound value. The measurement can fail if the absorption of the sound is too high in the

material or if there is no acoustic border at the back wall. This means that there is no

back wall echo to be measured. If the ultrasonic transmitter and receiver are in the

same sensor unit as in Figure 37, then the time of flight must be divided by two,

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because the ultrasound pulse travels twice through the material. Ultrasonic

measurement devices do this automatically, if the used sensor type is known.

Figure 36. Ultrasonic measurement system consists of an ultrasonic probe, a computer with ultrasonic measurement unit and two position encoders to map the data in circumferential and axial directions. (Uski, 1999)

Figure 37. Measurement principle of ultrasonic thickness measurement.

For the ultrasonic measurement of a fast rotating roll in Paper V a non-contacting

probe type was used. The ultrasound dampens fast in the air, so a water jet was used as

sound carrier between the sensor and the roll surface. The principle is described in

Paper V. Otherwise the principle of the thickness measurement is the same as above.

3.2 3D machine control systems Machine control systems are used for controlling the operation of a machine tool.

Modern control systems consist of several computers that have certain tasks, e.g., the

IF BE

Time of flight T Am

plitu

de

s = T·c

Sen

sor

Interface (IF) Back wall (BE)

Ultrasound pulse

Roll

Ultrasonic Probe

Rotary Encoder

Rotary Encoder

Carriage

Computer

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programmable logic control (PLC) performs simple control tasks and controls the

safety of the machine, numerical control (NC) controls the movements of the machine

axes, measurement computer the measurement tasks and the human to machine

interface (HMI) allows the human operator to operate the machine15. The 3D machine

control systems are normal control systems equipped with measurement capabilities to

measure the geometry errors of the workpiece and with special software. A typical 3D

machine control system is shown in Figure 38.

Figure 38. A machine control system of a 3D grinding machine.

Kuosmanen (2004) introduces a classification of the roll machine control technology

generations. This classification reflects the advances in the development of the

measurement and control systems technologies of the roll machine tools, i.e., for the

roll lathes and for the roll grinders.

The first generation (1G) technology can measure and compensate for variations in

the diameter of the roll in the axial direction. This error comes mainly from the

guideway straightness error of the machine tool.

The second generation (2G) technology includes, in addition to the first generation

technology, roundness measurement and compensation technology. Other

compensation forms are also possible, for example bending. By means of this

technology, rolls can be machined very accurately to a desired geometry.

15 See, e.g., Siemens: http://www.automation.siemens.com/doconweb/

C1 spindle Wheel

C spindle Roll rotation

Z axis Carriage

X axisCross axis

U axis Crowning

Drives

X1 axismeasuringarm

Measuring device

Numerical control (NC) & Programmable logic control (PLC)

Ethernet

I/O modules

Auxiliary device

Profibus

User interfaces (HMI)

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The third generation (3G) technology aims to optimise the roll geometry under

operating conditions on the production line. This is performed normally by measuring

the roll geometry in the operating conditions or in the simulated operating conditions,

e.g., on a balancing machine. The measurement results are then used to generate a

compensation profile to be used in the machining. The compensation profile is

normally an inversion of the measured geometry with some filtering applied.

Additionally to the classification by Kuosmanen, the third generation can be extended

and divided into two sub-classes. The first sub-class is the original method to

compensate for the unwanted roll geometry changes and to achieve the designed

geometry (typically cylindrical) in the operating conditions. In the second (new) sub-

class the roll geometry is optimized to compensate for the changes in one or more

process variables (for example force variation) or changes in the end-product

properties (for example steel strip thickness variation). Here the deviation from the

originally designed roll geometry is not relevant as long as it produces the desired

change in the process variable. Both third generation technologies are typically used in

the roll maintenance to correct the observed quality or runnability problems caused by

the rolls. This reduces the need to replace the rolls with newer ones. Kuosmanen has

named the third generation the predictive 3D machining.

The development of the third generation was possible because of the advances in the

measurement technologies and software. The machining technology had only minor

changes in the software from the second generation.

3.3 Compensation profiles for the 3D machining The first of the three discussed compensation profile types compensates static

geometry errors of the roll. The calculation is based on the measured static geometry

error profiles of the roll. The second compensation profile type corrects geometry

errors during the operation in the machine. It is based on the roundness profile or

radial run-out profile measurements in the operating speed range of the roll. The third

compensation profile type reduces the observed variation of a process variable during

the operation of the machine, i.e., force and end-product thickness variation. Here the

calculation is based on finite element (FE) analyses and the measured variation of the

variable to be compensated.

The compensation profiles are discussed here separately, but often they are combined

to compensate static and dynamic errors also. In some cases the term correction profile

can be used instead of the compensation profile. This is especially true for the static

compensation profile, where the machined geometry is corrected with the help of the

compensation profile. When the usage of the compensation profiles leads to roll

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geometry that is out of tolerance, the roll has the required geometry only when an

operational parameter or parameters have reached certain values, or the roll never has

the originally designed geometry, the term correction does not describe the meaning of

the profile correctly.

The name 3D for the compensation profile comes from the fact that there are 3

coordinate values for each point in the compensation data: axial position along the roll,

circumferential position (roll angle) and radial position (the offset for the tool). The

compensation profile can be displayed visually as a 3D topographic map.

When the compensation profile is calculated, its usage is the same with all the

compensation profile types. The compensation profile is fed to the machine control

(NC) part of the control system, where it is used during machining. If it is sent

combined with other possible compensation profiles or separately depends on the used

software of the control system, but common control system software versions have no

knowledge about the purpose of the used compensation profile, and so profiles are

separated only into MD profiles (e.g. for roundness or run-out compensation) and into

CD profiles (e.g. for axial diameter variation compensation). This allows other

compensation profiles (crowning, guideway correction profile of the machine tool,

chamfer profiles, etc.) to be included more easily.

Figure 39. Left: linear interpolation (yellow) between the MD profiles (red), right: spline interpolation (yellow) between the MD profiles (red). Spline interpolation produces smoother surfaces without discontinuities.

If the CD profiles are separated from the 3D compensation profile (3D profile), a

number of MD profiles must be created from the 3D compensation profile data and

they must fulfil the equation (2):

3Dprof(z, θ) = CDprof(z) + MDprof0(θ) + MDprof1(θ) + ...+ MDprofn(θ). (2)

All the profiles are sent to the control system. The memory capacity of the control

system limits the number of the profiles, and thus the control system must be able to

generate intermediate tool points between the profiles during machining. The method

chosen for intermediate points depends on the capabilities of the control system. Most

of the systems support linear interpolation and if the number of MD profiles is large,

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the linear interpolation produces a surface with sufficient accuracy. If a linear

interpolation is used, the machined surface can have discontinuities at the cross-

sections of the roundness profiles, especially if the number of the profiles is small or the

shapes of the profiles are dissimilar. A spline interpolation produces a surface without

discontinuities, but only advanced control systems have that function (Figure 39).

3.3.1 Compensation profile for static geometry errors

There is more than one method to produce the compensation profile for static

geometry errors. The simplest method is to use the result of the 3D measurement and

to invert it. Beside the disadvantage of the sensitivity of the measurement to dirt and its

long duration, the large amount of profile data requires a large amount of memory

capacity from the control system. To reduce the time and data needed, less MD

measurements can be used.

A more practical and faster method is described in Paper I. It uses only three

roundness measurements and one axial diameter variation measurement and

constructs a 3D geometry error profile of the roll from these measurements (similar to

Figure 39). To produce a smooth surface the Newton form of the interpolation

polynomial of second degree was used to calculate the intermediate roundness profiles

between the measured roundness profiles. The inversion of the constructed 3D

geometry error of the roll can then be used as a 3D compensation profile. It can be split

into a suitable amount of MD profiles and a CD profile as described before if necessary.

3.3.2 Compensation profile for dynamic geometry errors

The dynamic geometry errors are those errors that are caused by the change in the

roll operating conditions from the conditions present at workshop, for example

rotational speed (centripetal acceleration), temperature, etc. The compensation profile

must be measured in the operating conditions or in the simulated operating conditions,

e.g., on a balancing machine. If the operating conditions change during operation, no

optimal compensation is possible. In this case an average profile or a worst case profile

may be used instead. Typically, the compensated geometry is still better in the

operating conditions than the uncompensated geometry.

The measurement of the roll geometry in the operating condition can be made as a

run-out measurement (Kiviluoma, 2009). If a roundness profile is needed, the

operating conditions, i.e., rotating speed, can be simulated in a balancing machine or in

a dynamic roll geometry measuring device (Juhanko, 2011; Kuosmanen, 2004;

Kuosmanen et al., 1998; Pullinen et al., 1997). The usage of a compensation profile of

this kind is discussed in Paper II.

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3.3.3 Compensation profile to compensate a process variable variation

The profile generation is dependent on the variable to be compensated. If it is a force

variation in a nip, then a conversion factor or function is needed to convert the force

variation to a displacement of the roll surface. A similar procedure must be applied to

other variables. If the variable is not independent like the thickness variation caused by

the force variation, no additional conversion is needed. Papers III and IV discuss the

compensation profile usage to compensate the process variable variation.

3.4 Measurement based roll modelling Two types of roll modelling were used during the research. The first modelling

method can be called a direct modelling method. The measured data was fed to the roll

model as such. The second modelling method can be called indirect, because the

measurement data alone do not give usable results, e.g., in the case of a material with a

varying speed of sound. The data must be processed before it can be used. Roll

modelling was used in the Papers III and IV as well. The modelling did not use any

measurement based information from the rolls. It relied on the design information

(dimensions) of the backup rolls and their bearing arrangement only.

3.4.1 Direct measurement based roll modelling

Figure 40. Wall thickness map of the backing roll. (Forsberg, 2006)

This method is based on the ultrasonic thickness measurement of the roll shell. The

roll shell was made from homogenous steel sheets. The measured thickness data could

be fed to the roll shell model directly, only with clear measurement errors filtered out

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(Figure 41). The task after the measurement was to write a program to convert the

measured thicknesses in the database of the measurement software in a form to be

used by the modelling software, i.e. the Elmer FEA software16. Forsberg (2006)

discusses the conversion in his work. He used a MATLAB script to perform the

conversion.

Figure 41. FE roll model of the backing roll. (Forsberg, 2006)

Forsberg’s script generated the FE model of the roll (Figure 41). The model

dimensions were based on the roll geometry and ultrasonic thickness data. The created

model was stored in a file. From the file the model was used for further analyses. The

analyses are discussed in Paper V.

3.4.2 Indirect measurement based roll modelling

Figure 42. The volume fraction of white iron and grey iron in chilled cast iron test roll obtained by simulation and by measurement. (Jacot et al., 2000)

16 Elmer is freeware multi-physics simulation software by CSC, Finland. A description is downloadable at: http://www.csc.fi/english/pages/elmer/ElmerBrochure.pdf

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In the chilled cast iron thermo roll the surface is white iron and the inside is grey iron

(Figure 42). This is due to the casting process and is discussed in Papers VI and VIII.

Between the layers there is a transition zone (sometimes called the mottle iron layer)

where the volume fraction of white iron changes from majority to minority. The

material properties of these iron phases are different, for example, the elastic modulus

of white iron varies from 170 to 200 GPa and of grey iron from 95 to 130 GPa (Wirtz,

2002; Maijer, 1998, Zwart & Farrel, 1992; Brierly et al., 1977). In addition, the density

and the hardness of white iron are higher than that of grey iron. The hardness of white

iron is used in thermo rolls to obtain a hard and wear resistant surface for the roll

without coating.

The speed of sound in solid materials can be calculated with the equation (Kinsler et

al., 1999)

. (3)

The equation (3) shows that the speed of sound in the material is dependent on its

density, elastic modulus and the Poisson’s ratio. The Poisson’s ratio is the same for

white iron and grey but the elastic moduli and densities are not. Moreover, from the

literature it is known that the speed of sound value for white iron is higher than that for

grey iron. The derivation of the function to calculate the layer thickness of white iron is

based on the fact that when a layered chilled cast iron shell is measured with

ultrasound, its observed thickness is reduced by the white iron layer (Figure 43, below).

Figure 43. The simplified roll shell layer structure in the model (above) and normal layer structure of the thermo roll shell (below). (Paper VIII)

The ultrasound pulse travels faster in white iron than in grey iron, and thus the

observed distance appears shorter. The thickness reduction is directly related to the

thickness of white iron. This effect is used in the Papers VI and VIII to derive a function

Distance from the outer roll surface (depth) [mm] 185 198 30 0

White iron Mottle iron Grey iron

9 53

Modelled grey iron Modelled white iron

Measured thickness

Layer thickness Layer thickness

s = sg+sw

sg = tg·cg sw = tw·cw

sm = tm·cm

Shell thickness

Used indexes: m measurement g grey iron w white iron

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for the thickness calculation of the white iron layer. The layer structure is also

simplified to contain two layers only (Figure 43, above). The simplification is based on

the assumption that one half of the mottle iron layer behaves similar to white iron and

the other half similar to grey iron.

Figure 44. Wall thickness measurement of a chilled cast iron thermo roll.

With the derived function the measured shell thickness (Figure 44) can be converted

into a layer border model (Figure 45).

Figure 45. The model of layer border between white and grey iron calculated from the wall thickness data of the roll shell.

The indirect modelling method includes more calculation and in the case of the

thermo rolls in Papers VI and VIII the bores in the roll body set some challenges, as

well, because the ultrasound pulse does not travel through the bores. The ultrasound

measurement data needed to be filtered, because the echo of the sound from the back

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wall of a rotating roll is often distorted or lost. In the future some intelligent filtering

should be applied to the measurement data, because at present some of the filtering is

optimized specially for the used data. For the shape of the centrelines of the peripheral

bores direct modelling was used, although the ends of the bores were adjusted to

known locations. The measured centrelines of the bores were not corrected with the

calculated layer distribution of white iron, because some calculated examples showed

that the correction would produce only relatively small changes in the bore depths in

the shell of the roll model (< 10 %).

3.5 Uncertainty assessment of the guideway alignment system During the research an on-site grinding machine was developed (Figure 46). For the

alignment of the guideway of the machine a steel wire was chosen. A laser based

version was studied, but the harsh environment excluded its usage for the alignment.

An optical micrometre (Figure 47, right) was chosen for the position measurement of

the wire. For the location of the micrometre on the carriage only two possible locations

were available: on top and in the rear of the carriage (Figure 47, left). The rear location

was chosen after an error analysis. In the rear location the optical micrometre was also

considered to be more easily protected from the heat radiating from the hot cylinder

surface, the grinding dirt and the possible coolant fog.

Figure 46. Prototype of the on-site grinding machine.17

The guideway and the carriage moving on it could be tilted during operation (see

Sections 2.2.3 and 2.4.1). In this position the sag of the alignment wire produces an

error in its position measurement with the optical micrometre. For the error

compensation of the wire sag an equation was derived. The details of the derivation are

in Paper VII. An uncertainty analysis of the equation was performed. All the

uncertainties were calculated according to the Guide to the expression of uncertainty in

measurement (GUM). The principle is originally presented in GUM:1995. The Joint

Committee for Guidance in Metrology later published a revised version (JCGM, 2008).

17 Original photos by Vaahto Group Oy and RollResearch Int. Oy

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The expanded uncertainties were calculated with the help of the GUM Workbench

software18.

Figure 47. The chosen location on the carriage for the optical micrometre (left). The type of the optical micrometre19 is shown on the right.

The assessment of the guideway alignment system was only a part of the total

uncertainty assessment for the measurement system of the on-site grinding machine.

18 Metrodata GmbH, GUM Workbench Professional Version 2.4. Downloadable at http://www.metrodata.de 19 Picture from Keyence: Micrometers - Technical guide. Available at http://www.keyence.com/services/download.php?file=micrometer_tg_ka.pdf,

Wire location

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4 Main results and discussion

This chapter summarises the main results of the appended Papers and discusses them

in five sub-chapters, which include a discussion of their limitations. The Papers are

listed in Table 3 with their main issue and main target.

Table 3. The Papers and their field of research

Publication and roll type

Main issue Main target

I, supercalender filled roll

Measured static geometry errors

Improvement of static geometry by compensating diameter variation and roundness errors

II, backing roll of a coating station

Measured dynamic geometry errors

Improvement of dynamic geometry by compensating run-out and roundness errors

III, backup roll of a rolling mill stand

Measured rolling force variation traceable to deformation of the sleeve in the bearing arrangement

Compensation of the sleeve spring in the bearing arrangement by grinding

IV, backup roll of a rolling mill stand

Measured end-product thickness variation traceable to deformation of the sleeve in the bearing arrangement

Compensation of the sleeve spring in the bearing arrangement by grinding

V, backing roll of a coating station

Measured dynamic geometry errors

To understand and to simulate the behaviour of a backing roll with shell thickness variation

VI, chilled cast iron thermo roll of a soft calender

Measured thermal deformations, mainly bending

To understand and to simulate the thermal behaviour of a thermo roll

VII, Yankee cylinder

Uncertainty about the accuracy of the developed alignment method

Alignment of the guideway of an on-site grinding machine to prevent geometry errors

VIII, chilled cast iron thermo roll of a soft calender

Measured thermal deformations, mainly bending

To create a detailed thermo roll model to be used for simulations and FE analyses

4.1 Static geometry error compensation (Paper I) The developed method successfully reduced the static geometry errors of the filled roll

(Figure 48). The reduction of the geometry error was 55 % or more. The reduction of

the roundness errors of the roll was the least effective compensation. The roll was

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turned on its own bearings. The quality of the bearings of supercalender rolls in not so

relevant, because the rolls are more supported by the nip contact than by the bearings,

and thus no tight tolerances are needed in the bearings of these rolls and the play in the

bearings can be relative large. The problems in the bearings are seen when the

supercalender filled rolls are turned or ground rotating on their own bearings. The

geometry error of the rolls that were machined with their own bearings is typically

larger than that of the rolls supported by shoes from the neck. This is the probable

explanation for the less effective reduction of the error in the roundness profiles.

Figure 48. a) The 3D roll geometry error was 107 μm after conventional turning, b) and after turning with static error compensation the geometry error was 23 μm. (Paper I)

In the presented form in Paper I the method is limited to the roundness error and

axial diameter variation compensation. It cannot be used for correcting other errors,

such as the bending of the roll. The bending is observable in the first harmonic

component of the run-out measurement, and this component was always filtered out

from the used measurement results. The bending is normally not a problem in the

turning of the calender rolls. Nonsynchronous and non-static errors cannot be

corrected with the presented method. This is seen in the roundness error reduction as

discussed above. If equipped with an online measurement device this could be possible.

The effect on the paper quality has been verified only partly. Kuosmanen (2004) has

studied the effect of the reduction of the axial diameter variation of the supercalender

filled rolls. Smaller axial diameter variation in the rolls produced a smaller CD (Cross

Direction) variation in the gloss and caliper profile of the paper. An unverified

advantage was that the operators of the supercalender reported that sound level of

calender had decreased after the introduction of the static geometry error

compensation of their filled rolls. The results in Paper I are from one roll only, but the

method is used routinely in the industry in the roll maintenance.

4.2 Dynamic geometry error compensation (Paper II & V) The presented dynamic geometry error compensation method reduced the measured

run-out and roundness errors of the roll significantly at the operating speed (Figure

49). This is also measurable in the paper quality as an improvement (Paper II). The

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results from the simulation of the roll model were consistent with the measured

behaviour of the roll (Paper V).

Figure 49. Geometry of the backing roll after traditional grinding rotating with a slow speed of 50 m/min (a) and with production speed of 1120 m/min (b). Geometry after grinding with dynamic error compensation rotating with the slow speed (c) and with the production speed (d). (Paper II)

Figure 50. The first harmonic component of the run-out of the roll ground with traditional grinding and with dynamic geometry error compensation versus the running speed of the roll.

The dynamic geometry error compensation included the measured first harmonic

component of the run-out profiles in the calculation of the compensation profiles. This

enabled also the reduction of the dynamic bending of the roll during the operation

compared with the traditional machining. This is seen in Figure 50, where the first

harmonic components of the run-out are compared with respect to the running speed.

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The comparison shows also that the size of the bending of the roll is in a wide range

before the actual operating speed of 1120 m/min lower than with traditional grinding.

The main advantage of this compensation method is that a paper mill does not have

to replace their rolls with new ones. Backing rolls are expensive rolls to replace. This

method is currently used by some paper mills in their roll maintenance.

4.3 Process variable variation compensation (Paper III & IV) According to the measurements and FE modelling a cam shaped profiles were ground

to the backup rolls as a compensation of a process variable variation (Figure 51). The

measured variation in the rolling force could be traced to the key-type bearing

arrangement of the backup rolls (Paper III; Paper VI). The rolling force variation was

also the probable cause for the thickness variation of the steel strip. With this

compensation the variations of rolling force and strip thickness could be reduced. The

Paper III focused on the reduction of the rolling force variation. The compensation

profile for the rolling force variation was of the same magnitude in the top and bottom

backup rolls. The force variation synchronous to the bottom roll remained close to the

original level. This was very likely because of the mass of the other three rolls of the

stand rested on the bottom roll, but the compensation profile for the top and bottom

rolls was the same. Paper IV focused on the reduction of the rolling force and strip

thickness variation. The average strip thickness variation was reduced by more than

50 %. The reduction of the rolling force was further improved because of the

introduction of different sized compensation profiles for the top and bottom backup

rolls as described in Paper IV. The method was applied only to one of six rolling mill

stands in the finishing mill. Extending it to the other mill stands may improve the strip

quality further.

Figure 51. Cam profile ground to both backup rolls in Paper III and to the top backup roll in Paper IV.

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Main results and discussion

67

The method was tested on a limited amount of strip qualities and rolling forces at a

time. If the conditions are changed then the need for new compensation profiles should

be calculated and if necessary machined on the backup rolls. The main advantage of

this compensation usage is that it allows the usage of the cheaper key-type bearing

arrangements in the backup rolls. There is a keyless alternative on the market, but its

cost is higher than that of the key-type bearing.

4.4 Measurement based roll modelling (Paper V, VI & VIII) The ultrasonic measurement proved to be very useful in the model creation. The

ultrasonic measurement device was well suited for roll shell thickness measurements.

The created roll models were also consistent with previous knowledge about the rolls

and the simulated behaviour was as expected (Figure 52). This applies to both

modelling methods, direct and indirect.

Figure 52. Left: shell deformations caused by centrifugal forces. Right: shell deformations caused by thermal expansion.

The direct method is limited to roll models with homogenous shell materials.

Moreover, many of the roll cover materials make the measurement, if not directly

impossible, at least very difficult. In the test roll the rubber cover was removed before

the measurement. The indirect method can be applied to non-homogenous materials

also, but their acoustic properties must be similar in the different material layers. Roll

covers set limitations to those of the direct method. The material properties must be

known or some calculated thickness values must be verifiable otherwise, e.g., by

comparing them to known values.

The roll modelling is needed to widen the understanding of the phenomena behind

the dynamic geometry changes in the rolls. Especially the geometry changes caused by

thermal deformations can have different causes. The shell of the chilled cast iron

thermo rolls with its layered structure and with its peripheral bores makes the

prediction of the roll behaviour under thermal load difficult. The thermal deformations

are discussed in Papers VI and VIII. There are differences in the equations in the

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Main results and discussion

68

calculation of the layer thickness. In these Papers the differences originate from the

different boundary conditions. In Paper VI it is assumed that the speed of sound in the

iron layers are known (values found from the literature were used) and in Paper VIII

the known dimensions of the thermo roll were used to calculate the speed of sound

values in the layers. This is also the reason for the different scales in some of the

figures, e.g., layer thickness and bore depth figures although the measurements are

from the same roll.

4.5 Uncertainty assessment (Paper VII) The developed on-site grinding machine performed well and the roll manufacturing

and maintenance company was satisfied with its operation. The results from the

uncertainty assessment were in line with the observed performance of the machine.

The assessment result could be used when choosing the sensors for the system, because

in the assessment the accuracy requirements of the individual components are

presented.

The assessment was limited to the chosen method and excluded the testing of the

optical micrometre and assumed that the steel wire was ideal. Defects, density and

diameter changes of the wire were not discussed. To obtain an overall assessment of the

uncertainty of the measuring system of the on-site grinding machine, further tests

should be conducted.

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69

5 Concluding remarks

The present thesis and the appended papers give an overview of the roll geometry and

roll bearing related problems both in the paper industry and in the steel industry. Three

methods to compensate these problems were presented and tested. Some of these

methods have become widely used in the roll maintenance in Finland. Today all the

main roll grinding manufacturers and companies that offer retrofits for old grinding

machines have in their product range grinding machines that are equipped with their

own version of a control system capable of performing compensation grinding. The

Author believes that the work of the paper machine research group of the Department

of Engineering Design and Production at Aalto University (the Laboratory of Machine

Design at the former Helsinki University of Technology) has contributed to this

development.

The advances in the error compensation machining have always been preceded by

advances in the measurement technologies. Before static geometry error compensation

was possible the roll geometry error had to be measurable with a measuring device.

After the development of a measuring device for the dynamic geometry changes of

paper machine rolls it was possible to measure the shell deformations of a fast rotating

roll and to create compensation profiles for the deformations caused by the high

rotating speed. The creation of compensation profile for the thermal bending was pre-

dated by the development of the in-situ run-out measuring device even though the

compensation grinding itself was not successful.

The failure in the compensation profile usage for the thermal bending demonstrated

the need for a deeper understanding of the roll behaviour in production conditions. The

modelling was first tested on an easier case of modelling a backing roll of coating

station with the help of an ultrasonic shell thickness measurement. After that the more

complex case of a thermo roll was modelled, first without the heating fluid peripheral

bores, and then a more detailed model with the bores was made. Here the ultrasonic

measurement of the shell thickness and the derivation for the layer border calculation

were used to model the normally hidden structure of the white iron layer in the chilled

cast iron thermo roll. The purpose of the roll models is to gain a deeper understanding

of the roll behaviour and with this understanding to be able to create working

compensation profiles. This was not done in this thesis, but it will be on the focus of

studies in the future.

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Concluding remarks

70

The uncertainty calculation of the guideway alignment system shows an example in

assessing the developed measurement methods for their uncertainty. The

understanding of the measurement procedure in detail can help in improving the

methods in the future. It also helps to find out the most relevant factors affecting the

measurement accuracy.

This work summarizes the work in the roll research that has lasted for two decades. It

includes various research topics and different roll types. The research was carried out

both in the industrial and laboratory environment.

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71

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Publications

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9HSTFMG*aeihij+

ISBN 978-952-60-4878-9 ISBN 978-952-60-4879-6 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 ISSN 1799-4942 (pdf) Aalto University School of Engineering Department of Engineering Design and Production www.aalto.fi

BUSINESS + ECONOMY ART + DESIGN + ARCHITECTURE SCIENCE + TECHNOLOGY CROSSOVER DOCTORAL DISSERTATIONS

Aalto-D

D 15

6/2

012

In coating and calendering processes the rolls give the produced paper its final surface structure and finish. Thus the geometry of the rolls has a strong effect on the paper, which was seen in the analysis results. Similar results were observed in the rolling process in the steel mills. A solution was found to reduce the observed geometry errors in the currently used rolls or to reduce their effects on the end-product. The developed methods could be used also as a part of the normal roll maintenance. The need to deepen the understanding about the roll behaviour in the production conditions led to development of a new roll modelling method. An ultrasonic measuring device was used for measuring the internal structure of a thermo roll and detailed roll models could be created. With the presented results it was shown that the effects of the geometry errors coul be reduced. This and the roll modelling method enable an optimisation of the roll geometry for the production conditions. Original cover photo by Jukka Pirttiniemi

Thom

as Widm

aier O

ptimisation of the roll geom

etry for production conditions A

alto U

nive

rsity

Department of Engineering Design and Production

Optimisation of the roll geometry for production conditions

Thomas Widmaier

DOCTORAL DISSERTATIONS