Industrial Gear Box

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Rolling bearings in

industrial gearboxes


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Copyright SKF 1997

The contents of this publication are the

copyright of the publisher and may not

be reproduced (even extracts) unless

permission is granted. Every care hasbeen taken to ensure the accuracy of

the information contained in this publi-

cation but no liability can be accepted

for any loss or damage whether direct,

indirect or consequential arising out of

the use of the information contained

herein.

Publication 4560 E

Printed in Denmark on environmentally

friendly, chlorine-free paper (Multiart

Silk) by Scanprint as


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1 Industrial gearboxes overview

2 Bearing types for industrial gearboxes

3 Design of bearing arrangements

4 Dimensioning the bearing arrangement

5 Lubrication and maintenance

6 Recommended fits

7 Mounting and dismounting bearings

8 Application examples

Rolling bearings in industrial gearboxes


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Rolling bearings inindustrial gearboxes

Handbook for the gearbox designer


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Foreword

This Handbook is intended to provide the gearbox designer

with the knowledge required to select bearings for gearboxes

and to correctly design gearbox bearing arrangements. Recom-

mendations are given based on experience gained by SKF

during decades of cooperation with gearbox manufacturers the

world over.

General information regarding the selection, calculation,

mounting and maintenance of ball and roller bearings is given

in the SKF General Catalogue. The questions arising from the

use of rolling bearings in industrial gearboxes are dealt with

here. Data from the General Catalogue are only repeated here

when it has been thought necessary for the sake of clarity.

The application examples described comprise proven

gearbox designs from major manufacturers which are worthy

of note.

Grateful thanks are extended to the companies concerned

for the provision of the detailed information about their prod-

ucts and the permission to publish.

3



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1 Industrial gearboxes overview............................... 9

Types of gearbox............................................................ 9

Geared transmissions....................................................10

Demands made on gearboxes ...................................... 14

Selecting the gears ........................................................14

Designing the casing ..................................................... 15

2 Bearing types for industrial gearboxes.................. 17

Deep groove ball bearings ............................................ 18

Angular contact ball bearings.......................................20

Cylindrical roller bearings............................................. 22

CARB roller bearings .................................................24

Spherical roller bearings ............................................... 26

Taper roller bearings...................................................... 28

Spherical roller thrust bearings.................................... 30

3 Design of bearing arrangements............................. 33

Shafts and gear wheels in spur gearboxes ................. 33

Shafts in bevel gearboxes............................................. 44

Shafts in worm gearboxes.............................................50

Shafts and gear wheels for planetary gearboxes........ 56

Made by SKF stands for excellence. It symbolises

our consistent endeavour to achieve total quality in

everything we do. For those who use our products,

Made by SKF implies three main benefits.

Reliability thanks to modern, efficient products, based

on our worldwide application know-how, optimised

materials, forward-looking designs and the most

advanced production techniques.

Cost effectiveness resulting from the favourable ratio

between our product quality plus service facilities, and

the purchase price of the product.

Market lead which you can achieve by taking

advantage of our products and services. Increased

operating time and reduced down-time, as well as

improved output and product quality are the key to

a successful partnership.

Contents


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5

4 Calculation of bearing arrangement ....................... 65

Bearing loads ................................................................. 65

Determination of external forces.................................. 66

Calculation of bearing loads......................................... 74

Dimensioning the bearing arrangement ...................... 76

5 Lubrication and maintenance.................................. 91Grease lubrication.......................................................... 92

Oil lubrication................................................................. 95

Maintenance ................................................................... 98

6 Recommended fits..................................................103

7 Mounting and dismounting bearings....................109

Adjustment of angular contact bearings.................... 109

8 Application examples............................................. 115



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6

SKF is an international industrial Group

operating in some 130 countries and is

world leader in bearings.

The company was founded in 1907

following the invention of the self-align-

ing ball bearing by Sven Wingquist and,

after only a few years, SKF began to

expand all over the world.

Today, SKF has some 43 000 em-

ployees and more than 80 manufactur-

ing facilities spread throughout the

world. An international sales network

includes a large number of sales com-

panies and some 20 000 distributors

and retailers. Worldwide availability of

SKF products is supported by a com-

prehensive technical advisory service.

The key to success has been a con-

sistent emphasis on maintaining the

highest quality of its products and

services. Continuous investment in

research and development has also

played a vital role, resulting in many

examples of epoch-making innovations.

The business of the Group consists

of bearings, seals, special steel and a

comprehensive range of other high-

tech industrial components. The ex-

perience gained in these various fields

provides SKF with the essential know-

ledge and expertise required in order

to provide the customers with the most

advanced engineering products and

efficient service.

The SKF Group a worldwide corporation

SKF manufacturesball bearings, rollerbearings and plainbearings. The smal-lest are just a few

millimetres (a frac-tion of an inch) indiameter, the largestseveral metres. Inorder to protect thebearings effectivelyagainst the ingressof contaminationand the escape oflubricant, SKF alsomanufactures oiland bearing seals.SKF's subsidiariesCR and RFT S.p.A.are among theworld's largest pro-ducers of seals.


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7

The SKF house colours are blue and red,but the thinking is green. The latest exampleis the new factory in Malaysia, where thebearing component cleaning process con-forms to the strictest ecological standards.Instead of trichloroethylene, a water-basedcleaning fluid is used in a closed system.The cleaning fluid is recycled in the factory'sown treatment plant.

SKF has developed the Channel concept infactories all over the world. This drasticallyreduces the lead time from raw material toend product as well as work in progressand finished goods in stock. The conceptenables faster and smoother information

flow, eliminates bottlenecks and bypassesunnecessary steps in production. TheChannel team members have the know-ledge and commitment needed to share theresponsibility for fulfilling objectives in areassuch as quality, delivery time, productionflow etc.

The SKF Engineering & Research Centreis situated just outside Utrecht in TheNetherlands. In an area of 17 000 squaremetres (185 000 sq.ft) some 150 scientists,engineers and support staff are engaged inthe further improvement of bearing perform-ance. They are developing technologiesaimed at achieving better materials, betterdesigns, better lubricants and better seals together leading to an even better unders-tanding of the operation of a bearing in itsapplication. This is also where the SKFNew Life Theory was evolved, enabling thedesign of bearings which are even morecompact and offer even longer operationallife.


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1 Industrial gearboxes overview

Types of gearbox . . . . . . . . . . 9

Geared transmission . . . . . . 10

Demands on gearboxes . . . 14

Selecting the gears . . . . . . . 14

Designing the casing . . . . . . 15


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9

Industrial gearboxes overview

Gearboxes are devices for the transmission or

translation of movement. In industry gearboxes

are used to transform the speeds and torques

produced by the prime mover in order that

they are appropriate to the machine which is to

be driven. The speeds and torques required by

the machine are dictated by its use. Prime

movers can generally only meet these require-

ments when combined with gears.

Types of gearbox

Gearboxes are characterised by having

at least three members: the power in-

put, power take-off and the casing. The

casing transmits the support moment

to the base.

In contrast, a coupling has only two

members: the power input and power

1 Industrial gearboxes overviewTypes of gearbox

P1

M1

PV

P2

M2

n2

n1

P2P1M2

Pv

M1 n1 n2

take-off. The coupling housing has no

part in the flow of force.

The symbols used for power

transmission by gearboxes and coup-

lings are shown in figs and .21

Fig 1 Fig 2GearTorqueM1

Rotational speedn1 n2>

PowerP1 = P2+ Pv

CouplingTorqueM1 = M2

Rotational speedn1 n2

PowerP1 = P2+ Pv

(with slip)


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Geared transmissions

Geared transmissions are the most

commonly used. They transmit power

without slip, have high operational re-

liability and long life, require little main-

tenance and are characterised by the

ability to accept overloading, small size

and high efficiency.

Spur gearsThe spur gear is the most well-known

and commonly used design of geared

transmission. The dimensioning and

manufacture of the gear wheels are

the easiest to control. Their kinematic

behaviour also forms the basis of plan-

etary gears. Spur gears are in rolling

contact and, irrespective of tooth type,

have parallel axes.

1 Industrial gearboxes overviewGeared transmissions

10

The main types ofpower transmis-sion equipmentare shown in thefollowing.In addition, thereare many com-binations, for ex-ample bevel/spur

gears, spur gearswith belt drive input,or variable tractiondrives combinedwith a planetarygear.

Types of gearbox

Fixed ratio transmissions,

shift transmission

Geared transmissions

Spur gears

Planetary gears

Bevel gears

Worm gears

Hypoid gears

Helical gears

Eccentric drives

Cyclo drives

Harmonic drives

Traction drives

Belt drives

Chain drives

Infinitely variable

transmissions

Mechanical transmissions

Belt drives

Roller drives

Ratchet gears

Hydraulic transmissions

Hydrostatic transmissions

Hydrodyanmic transmissions

Gear wheels with straight cut teeth

( fig a) are simple in design

and can be accurately produced.

The axial forces generated by in-

accuracies and deformations

(twisting) are negligible.

Gear wheels with helical teeth

( fig b) run more smoothly and

can carry heavier loads than those

with straight cut teeth. A more elab-orate bearing arrangement is re-

quired because of the axial forces.

The double helix or herringbone

( fig c) allows for large tooth

widths and can carry particularly

heavy loads. The axial forces cancel

each other out. Deviations in the

helix angle cause axial vibrations.

3

3

3


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Internal gearing ( fig d) has

greater load carrying capacity than

external because of the favourable

osculation, but is more difficult to

produce. The bearing arrangement

is more complicated. The most fre-

quent use is in planetary gears.

Bevel gearsThe common characteristic of this

type of rolling contact gearing is that

the axes of the wheels intersect each

other. There are three basic designs

categorised by the form of the flank.

With straight cut teeth ( fig a),

the mesh begins and ends across

the total tooth width. The noise pro-duced considerably limits the use-

fulness of straight cut bevel gears.

Bevel gears with helical teeth

( fig b) have straight flanks.

The teeth are usually ground and

the mesh is gradual. The total over-

lap is bigger and the noise behav-

iour better than with straight cut

teeth.

4

4

3 Bevel gears having spirally cut teeth

( fig c) with curved flanks have

clear advantages in respect of load

carrying capacity. Particularly those

with ground teeth are quieter than

the types described above. For

bevel gears which have to transmit

high power, the spiral bevel gears

are the most frequently used.

4

Spur gear unita) straight cut teethb) helical teethc) double helixd) internal gearing

11


Fig 3

Fig 4

a b c d

Bevel gear unita) straight cut teethb) helical teethc) spirally cut teeth

a b c


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Worm gearsThe worm and wheel axes cross each

other at a considerable distance and

usually at an angle < 90 ( fig ).

Worm gears are suitable for large

single stage speed reduction. Their

operation is quiet and vibration damp-

ing. The efficiency is lower than that ofcompeting bevel/spur and planetary

gears, because of the higher propor-

tion of sliding motion. To reduce the

friction, the use of synthetic lubricants

is favoured.

The most commonly used design is

the cylindrical worm paired with a glob-

oid wheel ( fig a). The cylindrical

worm can be hardened and ground

which improves load carrying capacity;

it is also freely adjustable in the axial

direction so that bearing arrangement

and mounting can be simplified. Two

other designs globoid worm with spur

wheel ( fig b) and globoid worm

with globoid wheel ( fig c) are

also used.

Depending on the flank form, the

worm types are classified as follows:

ZA worm: trapezoidal worm thread

in the axial cross section; ZN worm: trapezoidal worm thread

in the normal cross section;

ZK worm; trapezoidal tool (in normalcross section); ZI worm; evolvent thread in end face

cross section; ZC worm: concave worm flanks

6

6

6

6

Hypoid gearsThe pinion axis is displaced so that the

axes of this type of bevel gear do not

intersect but are crossed ( fig ).

The wheels of hypoid gears are usu-

ally spirally cut. The advantages of this

type of gear derive from the larger pin-

ion and thus the smaller circumferential

force for the same torque, as well as

from the axis displacement which often

allows the pinion to be supported at

both sides so that the bearing arrange-

ment is stiffer. The noise behaviour is

also improved by the sliding motion in

the longitudinal direction of the teeth.

However, the additional sliding motion

increases the friction, wear and risk

of smearing and requires the use ofhypoid oils with high additive content.

5

12

Fig 5Hypoid gear unit

Fig 6

a b c

Worm gear unita) cylindrical worm

with globoidwheel

b) globoid wormwith spur wheel

c) globoid wormwith globoidwheel


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H

S

P

Z

The ZI and ZC designs are the most

popular. The ZI worm can be very ac-

curately ground whilst the favourable

osculation conditions of the ZC worm

(concave worm, convex wheel) bring

load carrying advantages.

Planetary gearsFrom the point of view of the tooth

flanks, planetary gears are mostly spur

gears. In contrast to the spur gear units

so far described, the shafts of which

are supported in stationary casings,

the planetary gear unit has gear wheels

which circulate. They are also referred

to as epicyclic gears.

In the simplest design ( fig ),

which is that most commonly used in

industry, the sun wheel drives the plan-

etary wheels (when acting as a speed

reducer). These are supported in the

hollow wheel and drive the planetary

carrier from which the power is taken

off.

Planetary gears have the following

important advantages compared with

conventional spur gear units:

the volume, weight and centrifugal

mass are smaller; the rolling and sliding velocities in

the mesh are lower, so that noise is

reduced; some of the power is transmitted as

coupling power, so that efficiency is

higher.

These advantages have led to a

continuous increase in the economic

importance of planetary gear units in

spite of their disadvantages which

include more difficult inspection, main-

tenance and repairs.

7


Simple planetarygear unit (prin-ciple)Z sun wheel P planetary wheelH hollow wheelS planetary carrier

13

Fig 7


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Demands made ongearboxesThe most important demands which

must be fulfilled are:

there must be a sufficient safety

margin in respect of fatigue and/orrequisite life for all components so

that the torques and speeds can be

reliably transmitted; there must be sufficient cooling even

under maximum power transmission

conditions; noise emission should not exceed

the permitted limits.

In addition to these demands, special

requirements in respect of operation

and design are dictated by the various

applications. Some examples:

radial and/or axial forces on the in-

put and output shafts, e.g. for ex-

truders; external forces on the casing, e.g. in

mining; heavy impacts, torque peaks, e.g.

when driven by single cylinder com-

bustion engines or when driving

bucket excavators; vibrations, e.g. in wire drawing;

extreme environmental influencesin respect of temperature, dirt, dust,

water, e.g. in arctic or tropical open

cast mining and in continuous cast-

ing plant; seals subjected to pressure, e.g. in

submerged gearboxes of dredgers

or in mixing equipment in the chemi-

cal industry; reversing operation, e.g. for rolling

mills; return stop, e.g. for conveyors; operation with little or no clearance

and torsional stiffness, e.g. for posi-tioning antennae and for robots;

precision, e.g. for printing presses; lubrication with non-flammable lub-

ricants, e.g. in mining; minimum maintenance, e.g. in wind

power plant; arrangement, e.g. slip-on gears for

converters; accessibility of measuring points to

monitor lubrication, temperature,

vibrations or torque, e.g. for large

plastic extruders.

Selecting the gears

To avoid either under or over-dimen-

sioning a gear unit the load and the

load carrying capacity of the gear must

be able to be determined as accurately

and reliably as possible. The size is

correctly chosen when a comparisonof the load spectrum and the load

carrying capacity gives the desired

service life. The determination of the

load spectrum is a time-consuming

and costly exercise calling for con-

siderable measurements. Therefore,

dimensioning is usually based on the

rated torque of the driven machine,

i.e. the operating torque for the most

arduous work conditions. For a rolling

mill, for example, this is the maximum

continuous rolling torque (not the initial

entry). The actual loads are higher

because of additional external forces,

produced by accelerations and vibra-

tions, for example. When calculating

the load carrying capacity of the gear

wheels, these additional loads are

considered by an application factor

KA according to DIN 3990.

One standard work on the subject

lists the following criteria for evaluating

the load carrying capacity of gear whe-

els:

resistance to pitting (tooth flank

fatigue), root strength (tooth fracture from

fatigue), resistance to scuffing (hot tooth

flank welding), wear strength (slow wear of tooth

flanks), grey spot resistance (fatigue from

micro pores on the tooth flanks, and lubricant film formation.

The load carrying capacity which isused as the basis for dimensioning

gear wheels is determined in rig tests

under standard conditions (partly stand-

ardised: FZG test to DIN 51 354).

14

1 Industrial gearboxes overviewDemands made on gearboxes/Selecting the gears


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2 Bearing types for industrialgearboxes

Deep groove ball bearings . 18

Angular contact ball

bearings . . . . . . . . . . . . . . . . 20

Cylindrical roller bearings . . 22

CARB roller bearings . . . . 24

Spherical roller bearings . . . 26

Taper roller bearings . . . . . . 28

Spherical roller thrust

bearings . . . . . . . . . . . . . . . . 30


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For the support of the shafts and gear wheels of

industrial gearboxes, rolling bearings are used

almost exclusively. The exceptions are in some

specialised areas, such as turbo drives, where

hydrodynamic plain bearings are used.

There are many good reasons for this

dominance of rolling bearings:

good location with minimum radial

and axial play enables optimum

meshing to be achieved; high specific load carrying capacity

with low friction; wide range of internationally stand-

ardised products produced in high

volumes at reasonable prices and

having good availability; can be calculated using reliable load

carrying capacity values; little design work for the user; simple arrangement; axially compact so that short and

stiff shafts can be used; normal tolerances and surface fin-

ishes for shaft and housing seatings;

less sensitive to misalignment thanplain bearings;

ability of radial bearings to accept

axial loads; not influenced by direction of load or

rotation; low starting torque; no starting problems in intermittent

operation; relatively easy to lubricate; favourable behaviour under emer-

gency conditions; economic maintenance.

Almost all bearing types are used in

industrial gearboxes and almost all the

available sizes. In the majority of appli-

cations, standard catalogue bearings

can be used; any variants with respect

to clearance or cage design are also

generally common, so that the com-

prehensive range of SKF cataloguebearings for general engineering appli-

cations covers the needs of gearboxes

very well and enables the designer to

make an optimum selection. The most

important bearing types for gearboxes

are described in more detail in the

following.

Bearing types forindustrial gearboxes

17



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18

2 Bearing types for industrial gearboxesDeep groove ball bearings


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2 Bearing types for industrial gearboxesDeep groove ball bearings

Deep groove ballbearingsDeep groove ball bearings are the

most popular of all bearing types and

this also applies for gearboxes. The

most important characteristics which

make them so popular are

they are able to carry radial loads

as well as axial loads acting in both

directions; they are suitable for high and very

high speed operation as their friction

is low; they have practically no tendency to

smear, i.e. cold welding when the

balls are accelerated; they run quietly, particularly if they

are lightly preloaded by axial force; they are robust in operation and

require little maintenance; they are favourably priced.

The dominant role for deep groove ball

bearings is where shafts have to be

located axially and loads are relatively

light. This is the case in

spur gear units (drive shaft and hol-

low take-off shaft), multi-ratio gear units (switching spur

gear wheels), geared motors worm gear units (worm wheels), planetary gears (drive shaft, planet-

ary carrier) and coupling shafts.

These improvements also bring ad-

vantages when the bearings are used

in gearboxes. In particular the reduced

sensitivity to misalignment means that

there is no reduction in bearing life

under the slight misalignments of up to

approximately 3 minutes of arc which

are normally encountered. The im-proved surfaces reduce friction lead-

ing to lower running temperatures so

that lubrication conditions are im-

proved and bearing life extended.

19

Benefits offeredby SKF In recent years SKF has made a

number of improvements to deep

groove ball bearings which have

resulted in further performance

enhancements. The more import-

ant include

optimised raceway geometry

and finish, reducing friction, run-

ning noise and sensitivity to

misalignment; improved cages which are more

stable, thus increasing reliability

at high speeds; improved seals, thus enhancing

the sealing efficiency of sealed

bearings.


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20

2 Bearing types for industrial gearboxesAngular contact ball bearings


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Angular contact ballbearingsThe raceways of these bearings are

arranged at an angle to the bearing

axis (contact angle), so that they are

able to carry heavier axial loads than

deep groove ball bearings. Slidingmovements of the balls are superim-

posed on their rolling motion, so that

the single row bearings require ac-

curate adjustment or a minimum

axial load to function properly.

Angular contact ball bearings are

available in the following designs:

single row, single direction angular

contact ball bearings, double row, double direction and

paired single row angular contact

ball bearings and four-point contact ball bearings, i.e.

single row, double direction ball

bearings.

Single direction implies that axial

loads acting in one direction only can

be accommodated, whereas double

direction bearings (and paired single

direction bearings, depending on the

arrangement) can take axial loads

acting in both directions.

The single and double row angularcontact ball bearings are preferred

as locating bearings for worm shafts.

Four-point contact ball bearings are

used primarily as thrust bearings in

high speed spur gear units, where the

outer ring is radially free.

The improvements made by SKF

to single and double row angular

contact ball bearings, e.g. reinfor-

cement of the ball set (single row

BE design, double row A and E

designs) to give higher load carry-

ing capacity means that worm gear

units can transmit more power

and, at the same time, the reduc-

tion in friction means that bearing

temperature can be lowered. The

reduced tolerances for axial clear-

ance and for dimensional and run-

ning accuracy which are standard

for SKF single row angular contact

ball bearings for paired mounting

of the CB design, because of the

improved location and reduced

running noise, are advantageous

in low-noise worm gear units suchas those required for lifts and

escalators.

21

2 Bearing types for industrial gearboxesAngular contact ball bearings

Benefits offeredby SKF


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22

2 Bearing types for industrial gearboxesCylindrical roller bearings


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2 Bearing types for industrial gearboxesCylindrical roller bearings

Cylindrical rollerbearingsThe special properties of cylindrical

roller bearings make them a popular

choice for gearboxes and include:

high radial load carrying capacity; low friction the lowest of any roller

bearing under purely radial load; suitable for a wide range of operating

speeds, including very high speeds,

as the cage has the correct combina-

tion of roller guidance, strength and

sliding friction properties; ability to accommodate moderate

axial loads, when they are simulta-

neously under radial load, via the

slid-ing surfaces of the roller

end/flange contact, although the inc-

reased

friction means that lubrication and

cooling must be adapted to the

conditions; the ease with which lateral displace-

ment can take place within the bear-

ing makes them ideal as non-locat-

ing bearings; proven good performance under

external radial accelerations; most designs are separable so that

mounting and dismounting are

simple.

These characteristics make cylindrical

roller bearings ideal for the following

applications:

as the non-locating bearings of all

high-performance units; the NU

design with its flangeless inner ring

is perhaps the most used, but also

the NJ, NJG and NCF find applica-

tion; the rings of these bearings

need only be axially located at one

side, and by mounting the rings withrelative axial displacement the bear-

ings can accommodate lateral

displacement in both directions. in spur gear units, even where com-

bined radial and axial loads are pro-

duced by the helical teeth; the most

popular positions are those on the

intermediate shaft, as the axial forces

from the driven and driving wheels

generally act in opposite directions

so that the resultant axial load is

light.

Practically all improvements made

to cylindrical roller bearings by SKF

could be considered as tailored to

gearbox needs, so that they make

an appreciable contribution to in-

creased performance. The main

characteristics are

the reinforced roller complements

and opened flanges of the EC

design give increased radial and

axial load carrying capacity; the logarithmic roller profile en-

sures an optimum stress distribu-

tion over the whole roller length

so that edge stresses are avoid-

ed even under heavy loads and

the permissible misalignments; the refined raceway micro-geo-

metry reduces friction and im-proves lubricant film formation; newly developed cages ensure

proper bearing function over the

increased performance range;

the standard polyamide cages

(designation suffix P) of small

bearings have low friction, are

elastic and have good sliding

properties;

the steel window-type cages

(designation suffix J) which are

standard for medium-sized bear-

ings and can also be fitted to thesmaller sizes (to special order)

withstand high temperatures

and also medium to strong vib-

rations;

the machined brass cages (for

gearbox bearings preferably out-

er ring centred and in two parts,

designation suffix MA, or in one

piece, suffix MP or ML) are stan-

dard for large bearings and can

be fitted to other sizes to special

order; they can tolerate high

speeds and are resistant to vib-

rations and accelerations.

The range of cylindrical roller bearings

is large compared with other bearing

types. The various flange configurations

(NU, NJ, NUP, N and NCF designs)

make the bearings suitable for a multi-

tude of applications and the different

cage designs extend the usefulness of

these bearings.

23



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2 Bearing types for industrial gearboxesCARB roller bearings

24


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CARB roller bearingsCARB is a completely new type of

bearing: a Compact Aligning Roller

Bearing. This single row roller bearing,

developed by SKF, is characterised by

a combination of properties whichmake it interesting for a multitude of

applications:

the ability to compensate for angular

misalignments or initial errors of

alignment typical of spherical roller

bearings; the ability to take up axial displace-

ments in the bearing itself typical of

cylindrical roller bearings; the low cross section typical of

needle roller bearings; the high radial load carrying capacity

imparted by long sphered rollers; the low friction obtained from optim-

ally matched raceway profiles; the quietness of operation.

Because of its many advantages, the

CARB makes an ideal non-locating

bearing. The points in favour of its use

in industrial gearboxes include, in addi-

tion its compact design and high radial

load carrying capacity even when

misaligned, the potential for downsiz-ing or increasing operational reliability

or the power rating. The CARB is par-

ticularly suitable for the bearing

arrange-ments of

heavily loaded shafts in spur

gearboxes, pinion shafts in bevel gearboxes,

and planetary gears.

Two versions of CARB are available:

a bearing with cage and a full comple-ment bearing.

SKF has introduced a completely

new roller bearing, the CARB. It is

the only bearing available which

combines the advantages of three

different bearing types without, at

the same time, incorporating their

disadvantages. For gearbox ap-

plications, these advantages trans-

late into the following opportunit-

ies for enhanced performance.

Up to 30 % higher load carrying

capacity at the bearing position

combined with small radial

space requirements The low cross section allows

downsizing or increased per-

formance Compensation for errors of po-

sition and also form of bearingseatings in housings thus allow-

ing machining costs to be

reduced Both bearing rings can be

mounted with an interference fit

so that there will be no wear in

the bore and no additional axial

loads under conditions of axial

displacement Quiet running and little vibration


25

2 Bearing types for industrial gearboxesCARB roller bearings


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26

2 Bearing types for industrial gearboxesSpherical roller bearings


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Spherical roller bearingsThe self-aligning capability (also in

operation) of spherical roller bearings

makes their use advantageous where

shaft bending occurs or where there

are errors of alignment between shaftand housing (casing). They are there-

fore used in all cases where misalign-

ment of the bearing rings would pro-

duce inadmissible edge stresses if rigid

bearings were used. Additional import-

ant characteristics make the spherical

roller bearing a reliable all-rounder for

gearbox applications. These include

the high radial load carrying capacity

and the ability to accommodate axial

loads acting in both directions; the wide range of dimension series

and very wide range of sizes even very large sizes.

The many successful development

refinements and the improved charac-

teristics resulting from them explain

the popularity of spherical roller bear-

ings for gearboxes (particularly in spur,

bevel and planetary gear units).

The design and functional charac-

teristics substantiate the leading

position of SKF spherical roller

bearings:

long, symmetrical rollers give

very high load carrying capacity; the floating guide ring between

the rows of rollers ensures that

the rollers are properly guided

(without wobble) into the load-

ed zone and, in cases where

axial loads predominate, that the

load is correctly carried by the

rollers and symmetrically distrib-

uted over the roller length; the special form and optimum

surface finish of the raceways

minimise friction and operating

temperature enabling highspeed operation; the latest development the E

design has even higher load

carrying capacity as the bearing

section is more efficiently ex-

ploited; the position of the guide ring

above the pitch diameter in the

E design favours lubricant film

formation between the rollers

and guide ring; all SKF spherical roller bearings

are fitted with robust metalliccages which perform well even

under arduous conditions.

27

2 Bearing types for industrial gearboxesSpherical roller bearings



32/117

28

2 Bearing types for industrial gearboxesTaper roller bearings


33/117

Taper roller bearings

The tapered form of the raceways

makes these bearings eminently suit-

able for combined radial and axial

loads. There is a choice of contact

angles so that the appropriate bearing

for the particular combination of radialand axial loads can be found. The

necessity for functional reasons to use

two bearings adjusted against each

other enables the force distribution on

the rollers to be controlled so that maxi-

mum life can be obtained at the same

time as the stiffness and guidance of

gear shafts can be optimised. The

main gearbox applications are

spur gear units with helical teeth, bevel and bevel/spur units and worm gear units.

As taper roller bearings can support

very heavy loads, they are always

used when the load carrying capacity

of other bearings for combined load

conditions (deep groove and angular

contact ball bearings) is inadequate.

Because the raceways are at an

angle to the bearing axis, an internal

axial force is produced when the bear-

ing is radially loaded, which acts on

the housing via the outer ring andcan deform it. With larger units (from

approximately 90 mm shaft diameter)

and specifically high performance

requirements, the casing walls are

often not sufficiently stiff, so that the

use of double row or paired single row

taper roller bearings (or spherical roller

bearings) is recommended, because

the internal axial forces cancel out

each other and the casing walls will

not be deformed.

Paired single row taper roller bear-

ings in a face-to-face arrangement(designation suffix DF) are always used

when the preset axial play can be ex-

ploited and when adjustment during

mounting is to be avoided.

SKF taper roller bearings have a

number of advantages which make

them suitable for industrial gear-

boxes. These include

the ideal form and optimum

finish of the roller end/guide

flange contact enable hydrody-

namic lubrication to be achieved

and mixed lubrication conditions

avoided, so that the critical run-

ning-in process normally re-

quired when commissioning a

gearbox is not needed; the logarithmic raceway profiles

guarantee optimum stress dis-

tribution over the whole roller

length and prevent edge

stresses;

the improved surface topographyof the raceways enhances lubric-

ant film formation and reduces

bearing noise.

29

2 Bearing types for industrial gearboxesTaper roller bearings



34/117

machine, e.g. in extruder gearing and

water turbine gearboxes. The bearings

are used successfully as thrust bear-

ings for the pinion and worm shafts of

large and very heavily loaded bevel

and worm gear units.

Spherical roller thrustbearingsThe special feature of these bearings

is their self-aligning capability. This

means that their full load carryingcapacity can be utilised, in contrast to

the very stiff cylindrical roller thrust

bearings, even when the bearing

washers are slightly out of alignment

with each other. The even distribution

of load is still maintained when there

are small angular misalignments of the

seating surfaces. Such misalignments

would considerably shorten the life of

cylindrical roller thrust bearings.

Spherical roller thrust bearings are

used in gearboxes, particularly where

axial forces are produced by the driven

30

2 Bearing types for industrial gearboxesSpherical roller thrust bearings

SKF spherical roller thrust bearings

have particularly low friction thanks

to the special roller end/flange

contact geometry.


Marine gearboxwith sphericalroller bearings,cylindrical rollerbearings, four-point contact ballbearings andspherical rollerthrust bearings


35/117

31



36/117

3 Design of bearingarrangements

Shafts and gear wheels

in spur gearboxes . . . . . . . . 33

Shafts in bevel gearboxes . . 44

Shafts in worm gearboxes . 50

Shafts and gear wheels for

planetary gearboxes . . . . . . 56


37/117

It is quite possible that several different bearing

types are used in one gearbox, and where com-

bined gear units are concerned, there are sev-eral types of gearing. A stepwise approach is

therefore appropriate when selecting bearings,

taking each shaft in turn so that the different

conditions for the individual shafts and gear

wheels can be fully considered. The bearing

arrangements described in the following are

well proven and the conditions specific to a

certain shaft are covered. A presentation of the

most commonly used bearing series facilitates

the initial selection.

Design of bearingarrangements

Shafts and gear wheelsin spur gearboxesSpur gearboxes are generally used to

reduce speed. There are three main

types which differ in the way they are

mounted: stationary units (mounted on

the machine base), cartridge units

(mounted on the drive shaft of the

driven machine) and flanged units

(flanged to the casing of the prime

mover and/or driven machine).

The drive from the prime mover isvia a coupling or a belt. The drive is

transmitted to the driven machine via a

coupling, a quill shaft connection or via

a pinion.

Input shaftsThe input (drive) shafts have the high-

est speeds and lightest loads provided

no additional external loads have to be

considered, e.g. belt tension forces.

Vibrations and imbalance forces may

be produced by the prime mover. It is

also necessary to consider the prob-

33

3 Design of bearing arrangementsShafts and gear wheels in spur gearboxes


38/117

lems of high angular accelerations

when starting without load as well as

operation without load at maximum

speed in order to prevent bearing dam-

age caused by the rolling elements

sliding on the raceways. There is a

danger of this occurring when loads

are suddenly applied. The temperature

differences and the associated thermal

expansions in the radial and axial

directions are high for input shafts, as

the speed related large power loss and

relatively small masses as well as therelatively small surface of the pinion

shaft mean that there is insufficient

heat removal. The distance between

bearings is dictated by the casing and

the low torque often means that slim

shafts are used. This means that shaft

bending and bearing misalignment must

be taken into account, particularly if a

belt drive is used.

Two deep groove ball bearings

arranged for cross location ( fig )

provide a cost-favourable bearing ar-

rangement for moderate power require-

ments. Deep groove ball bearings aresuitable for high-speed operation. Be-

cause of the low friction, small quanti-

ties of oil are adequate for lubrication

and cooling so that the collected oil

splashed by the gear wheels dipping

into the oil bath is generally sufficient.

In order to prevent axial clamping of

the bearings being caused by thermal

expansion of the shaft there should be

sufficient axial clearance between the

outer ring and the cover.

For shaft diameters of up to approx-

imately 90 mm, two taper roller bear-

ings arranged face-to-face ( fig )

are advantageous both from technical

and cost considerations. The taper

roller bearings are adjusted against

each other via the cover so that they

will have zero clearance when at the

operating temperature or, for reasons

of stiffness, they may have a slight pre-

load. When determining the initial axial

clearance it is necessary not only to

consider the temperature differential

between shaft and casing but also thedeformation of the shaft and, above all,

the casing. The casings of larger units

are often not stiff enough with respect

to the axial forces (tooth force + in-ternal axial forces in the bearings). In

such cases bearing adjustment is dif-

2

1

Bearing arrange-ment for an inputshaft with twocross-locateddeep groove ballbearings

Bearing arrange-ment for an inputshaft with twocylindrical rollerbearings

Bearing arrange-ment for an inputshaft with twotaper roller bear-ings arrangedface-to-face

34



39/117

ficult and shaft guidance is not suffici-

ent-ly accurate. The taper roller bear-

ing arrangement shown is, therefore,

not always suitable.

Cylindrical roller bearings ( fig )

have a high radial stiffness and guide

the shaft very accurately without having

to be adjusted as taper roller bearings.Axial forces are transmitted via the

flanges and roller ends. Because this

causes more frictional heat, lubrication

and cooling must be particularly good.

In order to prevent axial clamping of

the bearings when thermal expansion

of the shaft takes place, there should

be adequate axial play between the

flanges.

The classical locating/non-locating

arrangement ( fig ) is more com-

plicated from a design point of view

than the cross-located arrangements

described above, as the inner and

outer rings must be axially located at

both sides. However, it has advant-

ages with regard to dimensioning as

the axial force is always taken up by a

given bearing in this case the spher-

ical roller bearing irrespective of the

direction of the load. Additionally,

displacement of the non-locating bear-

ing is always assured so that there is

no risk of axial clamping occurring

when the shaft expands.Two NU-design cylindrical roller bear-

ings as radial bearings together with a

four-point contact ball bearing as the

thrust bearing ( fig ) have proved

suitable for very high-speed operation

(up to n dm 1 000 000). For such

5

4

3

high-speed operation the bearings

must have

machined brass cages, centred in

the outer ring, increased internal clearance: C3 for

the cylindrical roller bearings and

C4 for the four-point contact ballbearing, and

seatings having increased accuracy

of form and position (IT4/2).

At high circumferential speeds the

bearings will reject normal oil supplies.

Therefore, it is necessary to inject oil

at high speed (v 15 m/s) into thegap between cage and inner ring. Oil

drainage facilities should be provided

at the injection side of the bearings.

35


Bearing arrange-ment for an input

shaft with twocylindrical rollerbearings as theradial bearingsand a four-pointcontact ball bear-ing as the thrustbearing

Classiclocating/non-locating bearingarrangement witha spherical rollerbearing and acylindrical rollerbearing


40/117

other bearing types or arrangements

which are less unfavourable in respect

of casing deformation.

In comparison with input shafts,

the axial loading of cylindrical roller

bearings used to support intermediate

shafts ( fig ) is less critical. The

axial forces at the gears act in oppos-ite directions and cancel each other

out, at least partially, so that the axial

load on the bearings is light. Also the

speeds are lower so that frictional

losses deriving from the axial load

remain small.

The high radial load carrying capa-

city of the cylindrical roller bearings is

an advantage as the intermediate shaft

bearings are heavily loaded. The choice

between caged or full complement

cylindrical roller bearings is determined

primarily by the factors load, speed,

lubrication conditions, friction and

cost.

Compared with the input shaft, there

is only a small temperature gradient

between the intermediate shaft and the

casing. This makes it possible to use

spherical roller bearings in a cross-

located arrangement as shown in

fig which is simple in design and

therefore cost-favourable.

There is a wide range of spherical

roller bearings available, particularlyfor medium and large shaft diameters,

and there is also a choice of several

cross sections for each diameter. It is

thus possible to easily find bearings

which can support the heavy loads

acting on the intermediate shaft but

8

7

Intermediate shaftsIntermediate shafts are the most heav-

ily loaded as they are subjected to the

forces from two gear meshes. The

speeds are moderate. The axial forces

on pinion and wheel oppose each

other when the direction of the teeth is

the same so that they partially balanceeach other. There are no additional

external forces but vibrations may be

transmitted from the input or output

shafts. As there is no torque acting at

the shaft ends, reasonably small dia-

meters can be used enabling a rela-

tively large bearing section to be util-

ised for the accommodation of the

high radial forces. Design limits for the

bearing outside diameter are set by

the distance between input and output

shafts.

When using taper roller bearings

( fig ) it should be remembered

that axial forces are produced even

though the load is purely radial. This

may lead to axial deformation of the

casing. These deformations occur

in the central, less stiff region of the

casing because of the position of the

intermediate shaft, and are larger

than for the input shaft. They lead to a

change in position of the shaft and can

therefore cause inadmissibly high mis-

alignment of the bearings and themesh.

Experience shows that the casing

deformations occurring in smaller units

with shaft diameters up to 90 mm are

generally within acceptable limits. For

larger units it is necessary to resort to

6

36

Bearing arrange-ment for an inter-mediate shaft withtwo taper rollerbearings arrangedface-to-face

Bearing arrange-ment for an inter-mediate shaft withtwo cylindricalroller bearings



41/117

which have outside diameters within

the limits set by the distance between

the shafts.

A locating/non-locating bearing

arrangement as per fig with a

spherical roller bearing at the locating

side and a CARB as the non-locating

bearing offers the possibility of reduc-ing the cross section of the non-locat-

ing bearing arrangement, because of

the high load carrying capacity of the

CARB, so that the available space can

be better exploited. In many applica-

tions there is a risk that the bearing

seating in the housing will be ham-

mered out so that an intermediate

sleeve must be incorporated. By using

a CARB bearing this is no longer a

problem as the outer ring is mounted

with an interference fit in the housing,

so that a sleeve is not needed.

9

37

Bearing arrange-ment for an inter-mediate shaft withtwo sphericalroller bearings

Bearing arrange-ment for an inter-mediate shaftwith one sphericalroller bearing(locating) andone CARB (non-locating bearing)


Fig 9


42/117

(EHD) lubrication, i.e. the formation

of a separating lubricant film between

rolling elements and raceways, cannot

be achieved. Operating bearings

under conditions of mixed friction or

boundary lubrication will result in wear

and shorter bearing life. Besides rota-

tional speed, operating temperatureand lubricant viscosity are the most

important factors determining EHD

lubrication.

There is a limit to how high the

viscosity of the oil can be because

consideration must be paid to the

high-speed gears and bearings in the

unit. Therefore, a cooling of the gear-

box in the region where the low-speed

bearings of the drive shaft are situated

is often the most effective means of

increasing bearing life. Suitable ad-

ditives in the oil can also contribute

to a reduction in wear.

Other factors influencing drive shaft

bearings depend on the gearbox design:

In stationary, base-mounted gear-

boxes, depending on the type of

power take-off, it is necessary to

consider the forces of the coupling,

the propeller shaft, a pinion or of the

directly coupled driven machine

(e.g. extruders).

Bearing arrange-ment for an outputshaft with two

spherical rollerbearings

Locating/non-locating bearingarrangement foran intermediateshaft with twomatched singlerow taper rollerbearings and one

cylindrical rollerbearing

The locating/non-locating arrange-

ment shown in fig can carry very

heavy radial as well as axial loads.

Two matched single row taper roller

bearings (DF execution) are used for

the locating arrangement. In contrast

to the cross-located bearing arrange-

ments shown in figs 2 and 6, the inter-nal axial forces of the taper roller bear-

ings compensate each other within the

bearing pair and do not deform the

casing. The intermediate ring supplied

with the bearing pair ensures that there

is a minimum axial clearance within the

bearings. This is adequate for temper-

ature differentials between shaft and

casing of up to 20 C. To avoid deform-

ation of the thin-walled inner ring as

the cover screws are tightened, the

length of the centring surface (spigot)

of the cover should be chosen to give

a preload of approximately 0,01 mm.

Drive (output) shaftsThe conditions for the drive shafts

are characterised by high torques

and low speeds. The torque calls for

a large shaft diameter so that the

requisite load carrying capacity can be

obtained even when using bearings

with low cross sections. There are

potential problems with lubrication of

the rolling contacts if, because of thelow speeds, elastohydrodynamic

10

38



43/117

The bearings in cartridge-type gear-

boxes are subjected to the reac-

tionary forces of the torque support.

Additional forces may also be pro-

duced as a result of casing deforma-

tion. The casings of flanged gearboxes

are bolted to the driven machine.The shafts are generally rigidly

coupled so that the double support

of the output shaft becomes a mul-

tiple support in practice. Centring

errors of the coupled components

produce additional forces in the

bearings so that narrower tolerances

for the centring should ensure the

accuracy of alignment of the bearing

arrangement.

The arrangement with spherical roller

bearings ( fig ) is especially suit-

able for applications where rough

operation, external additional forces,

misalignments and shock loads place

heightened demands on the bearings.

Axial shock loads are taken up by the

less sensitive raceways in the absence

of flanges on the rings.

For cartidge-type gearboxes, the

relatively large diameters of the hollow

shaft mean that bearings having low

cross section are suitable. Fig

shows a well-proven bearing arrange-ment incorporating full complement

12

11

cylindrical roller bearings of series

NCF 29 V. For lighter loads but with

similar diameters, deep groove ball

bearings of series 619 can be used

in the same arrangement. For heavier

loads as well as larger deformations,

but still with the same diameters and

arrangement, spherical roller bearingsof series 239 are appropriate. Deep

groove ball and spherical roller bear-

ings have cages and are thus less

susceptible to wear when inade-

quately lubricated than full comple-

ment bearings.

Intermediate gear wheelsAn internal bearing arrangement is

most suitable for intermediate gears

as it takes up the least space. Internal

bearing arrangements are character-

ised by rotating outer rings. Therefore,

there is rotating outer ring load and

stationary inner ring load. This means

that the outer rings should have inter-

ference fits and the seatings should be

very accurately machined in order to

keep the rotating inaccuracies which

cause increased friction and additional

forces on the bearing cage to a mini-

mum.

With opposing meshes the circum-

ferential forces are added, so that high

radial load carrying capacity is re-quired. The axial forces from the

39

Bearing arrange-ment for an outputshaft of a cart-ridge-type unitwith full comple-ment cylindrical

roller bearings ofseries NCF 29 V

Bearing arrange-ment for an inter-mediate gearwheel with twocylindrical roller

bearings of theNJ design



44/117

the inner ring) oil should be supplied at

the side. To prevent the supplied oil

from being rejected by the bearing, the

seal gap at the supply side should not

exceed 1 mm.

Shifting gear wheels

For reasons of space these gear whe-els are supported internally in a similar

manner to the intermediate gears. The

torque is transmitted in the engaged

condition so that the bearings are sub-

jected to the tooth forces. The inner

and outer rings rotate but the relative

speed is zero. Both rings have rotating

load but the rolling elements do not

roll. The continuous changes in load

under these stationary conditions

cause micro-sliding to take place at the

rolling element/raceway contacts. As

there is no relative rotation of the rings,

a washboarding type of wear will be

produced in the raceways. This wear

can be reduced by using highly viscous

lubricating oil containing anti-wear

additives.

Where the wheels have helical

teeth, the axial force produces a tilting

moment and consequently a rotating

tilting motion which leads to axial move-

ment in the rolling element/raceway

contacts. This increases wear. Ball

bearings, adjusted to zero clearance,behave favourably as the balls can

helical teeth oppose each other and

partially cancel each other producing a

tilting moment on the bearing which

can cause misalignment.

Two cylindrical roller bearings of the

NJ design provide the requisite high

radial load carrying capacity in a re-

stricted space as shown in fig . Thedesign of the associated components of

the arrangement is simple. The bearing

arrangement of helical intermediate

gear wheels must be checked for angu-

lar misalignment. An unfavourable com-

bination of wheel diameter, pitch and

distance between bearings can produce

inadmissible values of misalignment.

An extended support width (distance

between bearing pressure centres)

can be achieved using, for example,

angular contact ball bearings.

Taper roller bearings in a back-to-

back arrangement ( fig ) also inc-

rease the support width as well as

reducing the influence of the tilitng

moment on the misalignment if they

are adjusted to zero clearance, or a

light preload.

Straight cut gear wheels may be

supported by a single spherical roller

bearing ( fig ). The intermediate

gear wheels are thus free to align so

that a good mesh is achieved.

In order to be able to use standardbearings (without lubrication holes in

15

14

13

40


Bearing arrange-ment for an inter-mediate gearwheel with twotaper roller bear-

ings arrangedback-to-back

Bearing arrange-ment for an inter-mediate gearwheel with a

single sphericalroller bearing


45/117

also roll in the axial direction and be-

cause the movement is reduced by the

clearance-free adjustment. Wear is

always load-dependent so that bear-

ings under low specific loads wear

less. The washboarding effect is also

less prominent as engagement always

takes place at new positions so thatthe wear is evenly spread over the

raceway.

For the support of shifting wheels,

deep groove ball bearings have proved

to give good performance ( fig ).

Bearings with increased radial internal

clearance (C3) are used. The clear-

ance-free adjustment via the inner rings

produces a contact angle in the bear-

ings of approximately 15, so that the

support width of the bearings is ex-

tended. This reduces movement in the

relatively stationary bearings under

rotating load and thus reduces wear. In

addition, the clearance-free back-to-

back arrangement improves guidance

of the wheel.

Lubrication of the bearings from the

outside is difficult as all components of

the arrangement shaft, bearings and

wheel rotate and because the bear-

ings are partly covered e.g. by the

coupling. The most reliable method is

to supply oil internally through the

shaft.

16

41


Bearing arrange-ment for shiftinggear wheel withtwo deep grooveball bearings


46/117

each particular bearing position must

be considered. To make the situation

clearer in Tables to , the text has

been kept as short as possible.

42

Demands on the bearingsModern spur gears generally have

hardened gear wheels with ground

teeth. It is then possible to obtain high

performance with relatively little friction

and low noise. A prerequisite for this is

the use of high-performance bearings,

which should have the properties listedin Table .

In addition to these general require-

ments with respect to ball and roller

bearings for high-performance gear-

boxes, other demands deriving from

the specific operating conditions at

1

42


Demand Required bearing design feature

High load carrying capacity Optimised rolling element size and number.Logarithmic roller/raceway contact.Good lubricant film formation through low friction andlow raceway surface roughness.

High stiffness Optimised rolling element size and number.Logarithmic roller/raceway contact.

High dimensional and running accuracy Particularly the inner ring running accuracy shouldpreferably be to tolerance class P6 or better.

Low friction Low friction in roller end/flange contact for taper andcylindrical roller bearings.Low friction in roller/raceway contact.Lightweight precision cage.Low raceway surface roughness.

Low running noise High precision of all bearing components.

Specific operating conditions Requirements of bearings/steps toguarantee performance

High speed and thus high friction Use low-friction bearings.and high operating temperature Avoid over-dimensioning.

Ensure lubricant supply whenstarting up cold.Provide good cooling.

Large temperature differential when Check required bearing internal clearance; if necessarystarting up (slim input shaft heats up select bearings with C3 clearance.

more quickly than the better cooled Ensure axial displacement at non-locating bearing position.solid casing)

Vibration from drive; imbalance Use bearings with stable cages, e.g. cylindrical roller bearingsforces with steel window-type cages or outer ring centred machined

cages, or spherical roller bearings with steel window-type cages.

Idling under light load Check minimum load. Avoid over-dimensioning.Use bearings with small roller masses where possible.Do not use full complement cylindrical roller bearings.Choose bearing types less susceptible to smearing,e.g. spherical and taper roller bearings.

Demands onrolling bearingsfor spur gears

Demands on inputshaft bearings

Table 1

Table 2


47/117

Specific Requirements of bearings/stepsoperating conditions to guarantee performance

Heavy radial loads Use bearings with high load carrying capacity.

Low to Check lubricant film formation. If necessary increase viscosity ormoderate speeds improve cooling. Use lubricants with wear-reducing additives.

Bearing selectionThe following check list will be found

useful when selecting bearings in order

not to forget any important factors.

Adjusted basic rating life Axial load carrying capacity when

the flanges of cylindrical roller bear-ings are under load

Friction Stiffness Misalignment

Sufficient play to prevent inadmiss-

ible clamping when temperature

differentials are large Minimum load Static safety under peak loads

A preliminary choice can be made from

the bearing series shown in Table .5

Demands on inter-mediate shaftbearings

Demands onoutput shaftbearings

Bearing selection

43


Table 3

Specific Requirements of bearings/stepsoperating conditions to guarantee performance

Very low speeds When lubricant film formation inadequate, i.e. a viscosity ratio (actual torequired viscosity) < 1, use lubricants with suitable EP additives.When < 0,5 bearings with cages (not full complement bearings) must be used.When < 0,1 reduce the specific bearing load;

aim for s0 > 10.

Shock loads from power Use robust, self-aligning, spherical roller bearings.take-off;deformations

Operating conditions Bearing series normally usedInput shaft Intermediate Output Intermediate Shifting

shaft shaft gears gears

Light loads 62 63 619 60 618/C363 NJ 2 EC 160 62 619/C360

Moderate loads NJ 2 EC NJ 22 EC NCF 29 V NJ 2 EC 160/C3320 X 322 239 CC 320 X 60/C3222 E(CC) 222 E(CC)

Heavy loads 322 NJ 23 EC 230 CC NJ 3 EC 62/C3232 CC NJG 23 VH 303223 E(CC) 223 E(CC) 232 CC

322/DF 223 E(CC)

High speeds NU 2 ECMA/C3QJ 2 N2MA/C4

In addition to the bearing series listed above, a CARB can be used as the non-locating bearing forlocating/non-locating bearing arrangements

Table 4

Table 5


48/117

Shafts in bevelgearboxesBevel gears are generally speed

reduction gears. The high-speed drive

shaft is termed the pinion shaft and the

slow-speed driven shaft carries the

larger bevel gear wheel.The pinion shaft is driven by the

motor via a coupling, a spur gear or a

belt drive. The power take-off is either

via a coupling or with bevel/spur gears

via a pinion.

Pinion shaftsThe pinion is generally supported in an

overhung arrangement. In a few cases

the pinion is supported between the

bearings but it is difficult to design in a

bearing with sufficiently high load car-

rying capacity at the head. The over-

hung arrangement offers more space.

Two taper roller bearings in a back-

to-back arrangement as shown in

fig offer a cost-favourable and axi-

ally as well as radially stiff arrangement

for small to medium diameter shafts

(d < 90 mm). The bearings are adjusted

using a shim between the shaft shoul-

der and the inner ring of the bearing at

the input side. The adjustment is deter-

mined to give zero clearance when the

bearings are in operation and warmor, if required for stiffness reasons, a

slight axial prelod. When determining

the initial axial clearance the tempera-

ture differential between shaft and

casing must be considered as well as

the deformations of shaft and casing.

17

Oil should be supplied between the

two bearings. A baffle plate ensures

that both bearings are reliably supplied

with lubricant. The oil drain at the cover

side reduces the amount of lubricant

reaching the seal.

Bearing arrange-ment for a bevelpinion shaft withtwo taper rollerbearings arrangedback-to-back

44

3 Design of bearing arrangementsShafts in bevel gearboxes


49/117

For larger shafts, the requisite load

carrying capacity can be achieved using

a locating/non-locating bearing arrange-

ment as shown in fig . The locating

arrangement is at the drive side and

consists of two matched single row

taper roller bearings (DF execution).

The intermediate ring which is suppliedwith the bearing pair ensures that a

minimum axial clearance remains when

the bearings are mounted which can

cope with temperature differentials be-

tween shaft and casing of up to 20 C.

For greater temperature differentials

such as may occur, for example, in

operation when ambient temperatures

are very low, paired bearings with

larger axial clearance are required

(special execution). In order not to

deform the thin-walled intermediate

ring when tightening the cover screws,

the length of the centring flange (spigot)

on the cover should be such that a pre-

load corresponding to approximately

0,01 mm is obtained.

The matched taper roller bearings

operate as a double row bearing. As

the axial load from the pinion domin-

ates, one of the two bearings de-

pending on the direction of the load

is completely unloaded. Experience

shows that this is not a disadvantage

when there is little vibration.The non-locating bearing adjacent to

the bevel pinion may be either a spher-

ical roller bearing, a cylindrical roller

bearing or a CARB.

18

For one-piece casings, spherical rol-

ler bearings offer mounting advantages

and they are also relatively insensitive

to smearing when loads vary consider-

ably and there are long periods of

idling. If cylindrical roller bearings are

used, the requisite axial displacement

can always take place in the bearingitself so that the outer ring can have

an interference fit in the housing, and

radial guidance is enhanced. The same

is true of CARB ( fig ). At this

position the bearing will not only enable

the axial displacements to be easily

accommodated, it will also accept the

angular misalignments caused by the

off-centre point of action of the tooth

forces with no reduction in life.

Oil should be supplied to the two

taper roller bearings between the outer

rings. Experience shows that for small

and medium-sized gears (up to approx-

imately d = 150 mm) the non-locating

bearing can be adequately lubricated

by the oil returning from the locating

bearings. For larger gears, however, it

is necessary to arrange for a separate

oil supply to the non-locating bearing.

For spherical roller bearings, the oil

should be supplied via the lubrication

groove and holes in the outer ring for

the best results.

19

Bearing arrange-ment for a bevelpinion shaft withtwo matchedsingle row taper

roller bearingsarranged face-to-face (locatingposition) andone sphericalroller bearing(non-locatingposition)

Bearing arrange-ment for a bevel

pinion shaft withtwo single rowtaper roller bear-ings arrangedback-to-back(locating) andone CARB (non-locating bearing)

45


Fig 19


50/117

Although the bearing arrangement

shown in fig is similar to that in fig

, there are considerable functional

differences. All roller rows are always

under load irrespective of the direction

of the axial load. If the direction of axial

load is from the pinion tip to the drive

input, the taper roller bearing at thecover side with its radially free outer

ring will be axially loaded, and the op-

posing bearing will be radially loaded.

If the load direction is reversed then

the smaller axial load will act on the

inboard bearing which is also under

radial load. The taper roller bearing at

the cover side will then only be sub-

jected to a minimum axial load by the

springs. Because all roller rows are

always under load, this arrangement is

less sensitive to vibrations than that

shown in fig .

Mounting is more complex because

there is no intermediate ring between

the taper roller bearings which have to

be adjusted on mounting. The radially

free outer ring of the taper roller bear-

ing at the cover side is prevented from

turning by an O-ring.

A variant of this bearing arrange-

ment incorporates a spherical roller

thrust bearing which has a higher load

carrying capacity. It replaces the taper

roller bearing which only carries axialloads.

With respect to lubrication, the same

recommendations apply as for the

arrangement shown in fig .18

18

18

20

Output shaftsThe gear wheels are generally ar-

ranged between the bearings for

design reasons. This is also true for

the bevel/spur gearboxes.

For shaft diameters up to approx-

imately 90 mm, two taper roller bear-

ings mounted back-to-back ( fig )provide a technically advantageous

and cost-favourable arrangement. With

larger dimensions, the casings are

often inadequately stiff with regard to

the axial forces (tooth force + internalaxial force of the bearings). This makes

adjustment of the bearings difficult and

shaft guidance is generally not suffi-

ciently accurate. The bearing arrange-

ment with cross location is then not

altogether suitable.

The axial force from the gear wheel

always acts in one direction. As the

axial force from the pinion dominates,

it is possible that the direction of the

resultant axial force will change. This

must be taken into consideration when

adjusting the mesh.

When adjusting the taper roller bear-

ings, the shim at the gear wheel side

determines the position of the wheel in

the gearbox. The shim at the pinion

side is used to set the axial clearance

of the taper roller bearings.

Oil from the collecting pockets abovethe bearings runs down at the cover

side of each bearing. From there the

oil must pass through the bearing and

thus lubricate it. Oil retaining plates en-

sure that there is an adequate supply

of oil available even when starting up.

21

Bearing arrange-ment for a bevelwheel shaft withtwo taper rollerbearings arrangedface-to-face

Bearing arrange-ment for a bevelpinion shaft withone taper rollerbearing as a thrust

bearing and onetaper roller bear-ing as a radialbearing (locatingposition) and onecylindrical rollerbearing (non-locating position)

46



51/117

The locating/non-locating bearing

arrangement shown in fig has the

advantage, compared with that shown

in fig , that no bearing adjustment

is required. The bearings are also in-

sensitive to axial deformation of the

casing. This will only be subjected to

the tooth forces and not to the internalbearing forces, so that there will be

less deformation.

A double row angular contact ball

bearing is used as the locating bearing.

Alternatively, single row angular con-

tact ball bearings in matched sets

having the same diameters as the

double row bearing and being margin-

ally wider can be used for higher load

carrying capacity.

To determine the position of the

gear wheel in the gearbox and to adjust

the mesh, a split washer is inserted

between the bearing outer ring and

the retaining ring. When doing this the

bearing can remain on the shaft. A

cylindrical roller bearing of the NU

design is used as the non-locating

bearing at the other side where the

radial load is heavier.

The locating/non-locating bearing

arrangement shown in fig is similar

in design and function to that shown in

fig . At the locating side, two single

row taper roller bearings are arrangedface-to-face. Compared with the double

row angular contact ball bearing, the

taper roller bearings provide higher

load carrying capacity and greater

stiffness.

22

23

21

22

Adjustment of the bevel gear wheel

is simplified using a special (hook-

shaped) sleeve. In order to prevent the

thin-walled intermediate ring of the

paired taper roller bearings from being

deformed as the cover screws are

tightened, the length of the spigot in

the cover should be chosen to give apreload corresponding to approxim-

ately 0,01 mm.

Oil should be supplied to the taper

roller bearings via the lubrication

groove and holes in the intermediate

ring. To allow an even distribution over

the two bearings, an oil drain should

be provided at the cover side.

47

Bearing arrange-ment for a bevelwheel shaft with

two matchedsingle row taperroller bearings(locating position)and one cylindricalroller bearing(non-locatingposition)

Bearing arrange-ment for a bevelwheel shaft with

a double rowangular contactball bearing(locating position)and a cylindricalroller bearing(non-locatingposition)



52/117

Demands on the bearingsModern bevel gearboxes usually have

hardened gear wheels with ground

helical teeth. This enables high power

transmission to be achieved with little

friction and little noise generation. A

prerequisite for this good performance

is the use of high-performance ball androller bearings which should have the

properties listed in Table .

In addition to these general require-

ments for bearings for high-perform-

ance gearboxes, there are additional

requirements which are specific to

the actual bearing position.

6

Bearings for the pinion shaft

High radial and axial forces act simul-

taneously on the pinion shaft. There-

fore high radial load carrying capacity

is required of the non-locating bearing

and high axial load carrying capacity

of the locating bearing. Because of the

high speed, bearings having low frictionshould be used. These two require-

ments are in part contradictory.

Experience shows that pinion bear-

ings do not fail from fatigue but are en-

dangered by other influences. From

this it is possible to derive the require-

ments and actions listed in Table .7

Demands onrolling bearingsfor bevel gears

Demands on bevelpinion shaftbearings

48


Demand Required bearing design feature

High load carrying capacity Optimised rolling element size and number.Logarithmic roller/raceway contact.Good lubricant film formation through low friction andlow raceway surface roughness.

High stiffness Optimised rolling element size and number.Logarithmic roller/raceway contact.

High dimensional and running accuracy Particularly the inner ring running accuracy shouldpreferably be to tolerance class P6 or better.

Low friction Low friction in roller end/flange contact for taper roller bearings.

Low friction in roller/raceway contact.Low raceway surface roughness.

Low running noise High precision of all bearing components.

Most frequent reason for How to alleviate problem/demandspinion bearing damage on bearings

Lubrication breakdown Guarantee lubrication when starting up

in the cold state.

Overloading because of too When selecting bearing size, check the temperature differentialheavy a preload between shaft and casing. C3 internal clearance often required.

Inadequate lubricant film generation be- Use low friction bearings.cause of too high operating temperatures Avoid over-dimensioning.

Improve cooling.

Smearing on rollers and Avoid over-dimensioning.raceways caused by roller Spherical roller bearings are more favourable than cylindricalslip or sliding roller bearings in larger size range (d > 150 mm).

When using cylindrical roller bearings aim for small rollerdiameters; use a full complement bearing.

Wear caused by contaminants Avoid contaminating the gearbox during production,assembly and in operation.

Table 6

Table 7


53/117

To obtain good meshing it is neces-

sary among other things to have a

bearing arrangement with high radial

and axial stiffness. The locating bear-

ing should therefore have a large

contact angle and as small an axial

clearance as possible.

Bearings for the output shaft

These bearings are predominantly

radially loaded so that high radial load

carrying capacity is also required of

the locating bearing. Because of the

slow speeds the risks in respect of

thermal behaviour and over-dimension-

ing compared with the pinion shaft are

negligible. The requirements for axial

and radial stiffness, for minimum axial

clearance and for bearing accuracy

correspond to those for the pinion

shaft bearings.

Bearing selectionWhen selecting the bearings it is use-

ful to refer to the cheklist given below.

Adju

Industrial Gear Box

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