Oil Inject

26
ject·on Assemb y of essure J ints Lecture y G Bergling in Moscow April 1977

description

Método de inyección de aceite para montaje de rodamientos.

Transcript of Oil Inject

Page 1: Oil Inject

ject·on Assemb y

of essure J ints

Lecture y G Bergling in Moscow April 1977

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OIL INJECTION ASSEMBLY OF PRESSURE JOINTS

Lecture by G. Bergling in Moscow

April 1977

SOME COMMENTS ON THE THEORETICAL PREREQUISITES FOR THE DESIGN OF THE OK COUPLING ----------------------------------------------------------------------------------------------------As is now probably well known, the assembling or dismantling of

pressure joints by the SKF oil injection method implies that oil

(or some other medium) is injected into the joint under great

pressure so that an oil film is formed to separate the contact

surfaces. Thus, the force required for the relative displacement

of the components of the joint i s reduced to a fraction of what

would otherwise be needed . The elimination of the direct contact

between the surfaces duri1g assembly also prevents damage to

these surfaces and no residual frictional forces arise in the

joint. Such forces may result in misalignment between the

components of the joint and this is a risk that applies when

assembling, for instance, shrinkage joints by means of heating.

Consequently the oil injection method has greatly improved

conditions for the industrial application of pressure joints.

It simplifies the mounting and dismantling procedures and even

large joints, which previously could only be shrunk on after

heating, can now be assembled without resorting to such a

procedure. Furthermore, the problems previously involved in

dismantling large joints can easily be solved by using the oil injection method. An example from heavy industry of a joint

assembled by the oil injection method is the universal coupling

for a rolling mill, see Fig. 1. This joint is subject to a great torque and therefore has a heavy interference fit. The

coupling head has to be dismantled for . each change of worn out rolls. This happens fairly often, hence the need for a speedy method

of dismantling that does not damage the contact surfaces. The

oil injection method is of predominant importance for applications such as this.

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. ! ''-..

(

I 1 1 11 I

: i ::: : I I I ll I 1 1 1111 :: ::: : I I I I I Ill I .

·.·.·.·.··i··!·'····················r: )l, ! ! : 1 I ! I J l 11 1 1 1 11 I •

+-l • . • -11 • 1- -i" J 11 • -l . I. -· . - . -1 I: :::: : : ,, : : Jl i•u I 1 1: I I

·.·.·.·-·r r··.·.·.·.·.·.·.·.-.7 1 !1 1 ! ·

1 : : : : : i : : ./ :: : : :: : : .

1 1 1 I I J I I ; ' :I : I 1 1 I I

l \ 11( ---~ - -·- - .. _ ... ~-- - ___ ,. , __ _

Fig. 1

For the mounting operation a hydraulic jack is used to push the

coupling head up the slight external taper of the intermediate sleeve which is located on the roll neck. Oil is injected through

two ducts, one situated roughly in the middle of the joint and

the other at the small end of the taper, where the pressure is

greatesto When the desired axial drive up has been achieved

the oil pressure is released. The oil between the mating surfaces

is then automatically forced back through the supply ducts and

is drained off. In this way metallic contact is obtained, together with the desired friction between coupling and shaft.

When dismantling, the joint is released automatically as oil is injected between the tapered contact surfaces. The ·mounting tool

is then applied as a brake to prevent the coupling sliding off too

rapidly.

The oil injection method is particularly suitable for assembling

joints with slightly tapered mating surfaces. The inner and outer

components of the joint can be appropriately tapered for this

purpose. However, if motivated by design or manufacturing reasons,

a tapered intermediate sleeve can be used instead, as in Fig. 1.

For joints with cylindrical mating surfaces, the oil injection

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method is used for dismantling only, while assembly is generally ~ffected by shrinkage after heating. The method is also

frequently used for adjusting the position of shrunk-on machine components when, for instance, they have b~come misaligned on the shaft in consequence of uneven distribution of the axial friction forces during cooling.

TORQUE TRANSMISSION CAPACITY OF OK COUPLINGS

Before discussing this question, it may be of interest to study the torque transmitting capacity of the chief rival of the

,·-.·

shrinkage joint, namely, the simple keyed joint. Most designers, especially those of ·the old school, consider the key and keyway to be

reliable and strong. However, if we compare it with the shrinkage ~

joint, it is easy to find unmistakeable drawbacks.

l = f.s-2od ' .

LL_·_· __ 2-:2,zd

Fig.2

Fig. 2a shows a s haft with keyway and Fig. 2b a cast hub of

conventional dimensions. In these keyed joints the entire torque

. i

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has to be transmitted from shaft to hub via the small area bounded

by the length of the key and the distance between points A and 8

in Fig. 2a. This figure also gives an indication of the pressure

distribution, with a very heavy stress concentration at the

fillet B, which is where shaft fractures frequently occur.

Another questionable point is how much of the key length actually

participates in the torque transmission. The stress concentration

at 8 can be expressed with the aid of a notch fatigue factor

p- 2 . 5 relating to the maximum s hear stress.

'(' max - 2.5 .

where M is the torque V

and d is the shaft diameter.

M V

This means that the shaft with the keyway could be replaced by a

plain shaft with a diameter of 0.75 d. Furthermore, if the

manufacturing costs and the manual work required to assemble a

conventional keyed joint are taken into account, it is surprising

that these joints are used at all in modern designs.

The reason why friction couplings are not used to a greater extent,

instead of keyed joints with flanges, is quite often that designers

feel doubtful about their reliability. It must be admitted that

the drawings look rather "bare" with the cylindrical shafts

inserted in the couplings. Conservative engineers often say,

"Don't give me any of this rubbish! I want to see straight­

forward bolts fitted into proper flanges, so that I can be sure

the thing rotates." But they are not afraid of driving a car,

despite the fact that they then really have to rely on the friction

between the tyres and the road surface when travelling at, say,

100 km per hour in the dark and among other vehicles being driven

at the s ame speed. The driver knows the quality of one of the

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friction surfaces, the tyres , but very little is known about t heir

condition at that particula r moment . The driver may well not know

anything about the other friction surface, the road surface. Its

momentary friction properties are affected by the weather and can

vary enormously. Moreover, in this gamble with friction the

drivers involved may possess varying degrees of s kill and

responsibility. Accidents do happen •

The braking systems for trains are based entire ly on friction

between the brake s hoes and the wheels, and between the wheels

and the rails. Here too, the weather and dirt conditions play an

important part .

In my opinion, it is much safer to discuss friction between

surfaces of a quality that can be controlled in production and

which have a high degree of protection against wear and

contamination after assembly. I then have the mating surfaces of

OK couplings foremost in mind.

I will not go into details about the calculation method for

determining the torque t ransmission capacity of oi l injection

couplings. This information is given in the SKF leaflet TSP 6022

"Shaft couplings type OK-HB". Generally, oil injection couplings

of, for instance, type OK- HB may be said to be ver y reliable

devices for the t r ansmiss i on of torsional moments . The outer

sleeve is made of a material having strength propert ies that

are far superior to those of the shaft, which is usually made.

of ordinary machine steel . For this reason the shaft is generally

the component that gives way first, if the torque reaches on

inadmissibly high level.

Generally, the yield s t rength of the outer sleeve material is

decisive for the magnitude of the permissible interference

and hence for the coupling dimensions os well . Since it is

possible to calculate the stresses in a pressure joint with a

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to be of little consequence. It has also been shown that, even

with very accurately machined surfaces, only a very small area

of the nominal surface is in direct contact . . This area, which

is formed by local, plastic deformation, over which :adhesion

occurs, is proportional to the external load . . _Consequently, the

rise in the total frictional force in a pressure joint, when the

interference is increased, can be explained by the fact that the

part of the whole pressure surface in effective contact increases,

whilst the friction per unit of the effective -contact area remains

constant.

The intensity of the molecular adhesion in these effective contact

areas, which mainly determines the friction, is influenced

decisively by the very thin film which in practice always covers

the surface of metals. Oxide, for instance, forms on the majority

of metals immediately after machining ._ The surfaces are also

usually affected by absorbed molecular layers _of gases, moisture

and, especially, lubr icant; this cannot be prevented without

recourse to special measures.

These substances generally result in a reduction in the friction

between the surfaces. Whilst, for instance, in the case of

absolutely clean surfaces the coefficient of friction is of the

order of 5 to 100 due to "cold welding", in the presence of oxide

it is only 0.3 to 0.8, and in the presence of lubricant it is

further reduced to 0.1 to 0.3.

Thus it can be foreseen that the chemical purity of ' the mating

surfaces at the time of mounting is of great importance. A

perfectly clean surface is very difficult to obtain, but degreasing

of the surface with lime and water has proved to be an effective

method. If, after the cleaning proces~, the oil-injection coupling

is mounted with a pressure medium consisting of ~lycerine mixed

with 10 to 25% water·, a high coefficient of friction is obtained;

for practical calculations it can be taken to be 0.18. If a high

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coefficient of friction is required, we usually recommend t his

method. Another method, giving about the same coefficient of

f riction, is to thoroughly degrease the mating surfaces, in

the manner previously described, and to assemble the joint by

shrinkage, after heating to a suitable temperature in an electric

oven . It is also possible to obtain coefficients of fricti on as

high as 0 .5 to 0 .6 by coating one of the mating surfaces, before

assembly, with oil containing very fine emery. However , this

method is not recommended for general use as the dismantling

operation can be difficult. With oil injection joints and with

shrinkage joints assembled in the normal way without being

thoroughly degreased, a coefficient of friction of 0.12 to 0 .15

can be used for calculation pu rposes. SKF's OK couplings are

calculated for a coefficient of friction of 0.14, in accordance

with the r ules of the classification societies.

Investigations have shown that the coefficient of friction is

virtually una ffected by the magnitude of the surface pressure in

the contact zone of oil injection joints. There is no appreciable

difference between pressure joints made of mat erials such as the

various carbon steels, mild s t eel and tough-hardened steel, nor .

does the machining method appear to have any noticeable effect on

the magnitude of the friction. Approximately t he same friction

values are obtained with fi nish-turned and fine-ground surfaces.

The coating of one of the mating surfaces with phosphate does

not have any influence on the starting friction in an oil injection

joint either. However, oil is retained i n this phosphate co~ting

and this combination prevents s mearing of the contact surfaces .

For this reason the tapered mating surface of the inner sleeve of

the OK- HB couplings supplied by SKF has such a phosphate coating.

The oil injection method can also be employed for joints with

cast steel, cast iron or spheroidal graphite cast iron components.

However, it is necessary to e nsure that t he material is pressure­

tight so that the requisite oil pressure can be built up. If cast

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..... -.--

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was redesigned and now all couplings are supplied with integral

tool, see Fig. 6. Each coupling consists of a thin inner sleeve,

made of high-tens ile material with a yield point o'f 400 - 450

N/mm2, and a thick outer sleeve made of tough- hardened material

with a yi eld point of 640 N/mm2. The mating · surfaces have a

taper of 1 in 50 in small couplings for up to 90 mm shafting and

1 in 80 for the larger sizes coupling.

1 2 2

-t--11------ ·--- - ·- - --·-·--+- - - --

. · ·· ' .. .

Fig .6

The bote diameter of the inner sleeve is somewhat larger than

the diameter of the shaft. This means that the coupling can

easily be pushed on the shaft, if the latter is ma~ufactured

to the recommended h7 tolerance.

One injector is used to supply oil under pressure to small

couplings for shafting less than 170 mm in diameter. Larger

couplings for up to 490 mm shafting are equipped with two oil

supply ducts and hand- operated injectors are. suitable for

/

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cou·plings in this s ize range . We recom~end the use of a

pneumatically oper ated pump for larger sizes of coupling.

The couplings for 500 mm and larger shafting have three holes

for oil injection, Fig . 7, and the pneumatically operated

pump is then connected to the middle hole •

I t ·- ·--·-

' ; ;: . '

Fig. 7

Hand-operated injectors are connected to the outer holes for

the primary purpose of reducing the edge stresses. The

pneumatically operated pump must be capable of supplying oil

at a pressure of at least 250 N/mm2, in order to be able to

provide a sufficient quantity of oil when the pressure between

the contact surfaces is at a maximum level. The oil in the

hydraulic unit for driving up the outer sleeve attains a

maximum pressure of 35 N/mm2 and therefore a hand-operated or

pneumatically operated low- pressure pump can be employed.

The use of a pneumatically operated pump is recommended for

couplings for shafting larger than 500 mm in diameter as otherwise

the mounting operation will take an unnecessarily long time.

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The quantity of oil supplied to the hydraulic unit is ·roughly

0.15 x d 3 dm3 , where d =the shaft diameter in dm. The a a hydraulic unit is employed to overcome the axial force arising

from the oil pressure between the tapered mating surfaces.

The friction force is very light; when the surfaces are

completely separated by the oil film, the coefficient of friction

is approximately 0.005, or even less if the surfaces are of

good quality with regard to both macroscopic and microscopic

irregularities.

VARIOUS INDUSTRIAL APPLICATIONS OF THE OIL INJECTION METHOD -------~-------------------------------------------------------------------------------------------------------------~

ROLLING BEARINGS

The oil injection method is perhaps most widely used for the

mounting a~d dismounting of rolling bearings and has revolutionized

application techniques where large bearings are concerned.

Previously, for instance, thin sleeves with an external taper were

a prerequisite for the dismounting of large bearings without

damaging the components. Nowadays these sleeves are disappearing

more and more and the bearings are being fitted direct on the shaft,

either on a tapered seating, as in Fig. 8, or on a cylindrical

seating, as in Fig. 9, and are mounted and dismounted by the oil

injection method. Th~ oil distribution groove is positioned at a

distance from the bearing face that ·corresponds to a third of the

width of the bearing, see figures below.

" ;j . .

. : ~ :

Fig.8 Fig.9

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Fig. 10

Considerable force is required to release a large withdrawal

sleeve (the thin sleeve between the outboard bearing and the

shaft in Fig. 10) solely by means of a withdrawal nut. Since

in many applications, such as in rolling mills, it is essential

that dismounting is effected rapidly, large withdrawal sleeves

are manufactured with ducts for mounting and dismounting by means

of the oil injection method. A withdrawal sleeve of this kind is

illustrated in Fig. 10. Provision is made for two oil injectors

to be used, one being connected to the oil distribution groove

in the bore of the sleeve, the other to the corresponding groove

in the outer surface of the sleeve. The inboard bearing is

fitted direct on a tapered journal, which is equipped with a

groove so that the oil injection method can be used to mount and dismount this bearing.

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PROPELLER MOUNTING

Generally propellers are driven up a tapered . journal equipped

with a keyway and are held in place by a nut, Fig. 11. The

torque is transmitted by friction between the surfaces in contact

and the key serves as a safety measure, should friction be

inadequate . By using the oil injection method it is possible

to obtain such a high degree of interference that the key can be

omitted without any ris k, Fig. 12 . The mounting procedure

is also simplified.

Fig.11

Fig. 12

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The propeller is driven up a given distance on its seating,

the distance being selected to give a sufficient degree of

interference to ensure that the torque from the propeller

shaft can be safely transmitted. During the drive-up, friction

is reduced by injecting pressurised oil between the contacting

surfaces, see Fig. 13. The drive-up force is achieved by a hydraulic tool which is brought to bear on the propeller nut •

When oil is pumped into the tool, an annular piston presses

the propeller up on its seating. If the propeller is to be

removed, oil is injected between the contacting surfaces of the propeller hub and the journal. The propeller ·then slides

off its seating and the hydraulic tool is used as a brake

to prevent the sliding speed becoming excessive.

SKF supplies the hydraulic tool as well as the requisite pump

equipment and accessories. This method of mounting carries

the approval of the major classification societies and is

being increasingly used on large vessels •

Fig.l3

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MOUNTING RAILWAY VEHICLE WHEELS

The wheels of railway vehicles are generally shrunk on cylincrical

seatings on the.wheel axle to give a high degree of interference.

Mounting takes place following heating and. is a· simple procedure.

Dismounting, on the other hand, may be problematic~! • . When the .

SKF oil injection method became known, great interest. was shown

in using the method . to dismount railway vehicle wheels and trials

were conducted which showed that it became much easier to free

the wheels, see Fig. 14 .

Today the method has won acceptance throughout Sweden and

dismounting is carried out without any problems, particularly

since mechanically driven pumps were introduced instead of hand­

operated ones. Mechanically operated pumps maintain the oil film :

better than hand-operated ones. This is · particularly important

where components are mounted on cylindrical . seatings.

DISMOUNTING RAILWAY AXLEBOX BEARINGS

In modern arrangements, spherical roller bearings for railway

axleboxes are mounted direct on a cylindrical journal, Fig. 15.

Fig.l4 Fig.l5

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Mounting is simple, since it is only riecessary to heat the bearing~ but dismounting can be difficult in cases where the drilling o~ an oil injection duct in the axle is not permitted. To get round

this problem SKF has designed a tool, fig. 16. Its principle

of operation is that oil is pressed between the inner ring of the bearing and journal from the side, thus bringing about · a reduction in the force required to dismount the bearing. With the aid of a screw, acting centrally against the journal, the bearing may then be easily removed.

The main component of the withdrawal tool consists of a cover, made in two parts. This cover is centred on the outer ring of the bearing and encloses the bearing and the injection ring,

which is centred on the journal • . The injection ring is equipped with an annular piston which presses the ring against the inner

ring of the bearing when oil is injected yia -· a nipple~

- ----------·------+·-------+

Fig. 16

'I I

I

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Sealing between the injection ring, the bearing inner. ring and

the journal is effected by means of an 0-ring. This has a hole drilled through it and a narrow tube is inserted in this

hole, see enlarged view in Fig. 16. 0~1 is injected through

this tube to t~e mating surfaces of the be~ring inner ~ing

and the journa~. When these surfaces have been separated by

the oil, which should be a thick cylinder oil,_ only light

force need be applied to remove the bearing without damaging

the axle journal, Fig. 17.

Fig. 17

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BORDERLINE CASE

How far can you go with the oil injection method? Is there a

maximum surface pressure where it ceases to function? we· hove

.not found this yet. Let us take an example that we were dubious

about, though it all turned out well in the end. For a nuclear

power station, a coupling flange, see Fig. 18, hod to be

mounted in a cold condition on a tapered shaft seating •

I

1--··+--- --·--1

I ,f -;······-~v . ..-.•,•.•,·.···'·" , ... ·

~~·Ji-' --+H \ 1 I 11

'L -l}.·.·~ .. ,, .. ·,• .. .v.•u. :~-~

I i

I

Fig. 18

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A very heavy interference fit, corresponding to 0 .34% of the

shaft diameter , wa~ required . Heating wa s out of the question

as the s pecified interference would necessitate a temperature

that was too high for the high- tensile 1 ~ickel-chromium-molybdenum

ste~l which had a yie ld point of 720 N/mm2• The torque was so

great that~ coefficient of friction of 0.18 was required, although

the su rface press ure in the contact zone was as hig h as 280 N/mm2•

(Th is compares with t he approx. 130 N/mm2 of the OK coupling).

Consequently, oil cou ld not be used as t he press ure medium and

glycerine was selected instead. The surfaces were thoroughly

degreased in the manner described earlier. An additional supply

duct and distribution groove were provided near the end of the

shaft in order to ens ure that the pressure medium really reached

this zone. During trial mounting, without this additional supply

of glycerine, the coupling jammed. This was because the edge

pressure , ar1s~ng from the heavy-duty flange , in combinat ion with

the high surface pressure encountered towards the end of the

drive-u p, prevented the supply of pressure mediumo When t he

effect of the edge stresses was eliminated, it proved easy to

drive- up and di smantle the flange using the glycerine.

SKF STERN TUBE BEARING ARRANGEMENT FOR SUPERTONNAGE

To conclude t his lecture , a fi lm , dealing wi th the new SKF bearing

arrangement f or the propeller s hafting of ships in the supertonnage

class, was screened. This bearing arrangement has · featured in

articles in several trade and scientific journals and is now well

known. The application is indeed unique , as t his is the first time

the propeller shafting of a vessel of th is class has been supported

by rolling bearings. At the same time it represent s one of the

most sophisticated applications of the oil injection method. The

ship was delivered in December 1973 by Kockums Me kaniska Verks tad

of Sweden and is a 255 000 tdw su pertankera Fig ures 19 and 20

illustrate the way in which the spherical roller bearing

240/900 CABK30/C3 is moun ted on an adapter s leeve in t he stern.

The propeller s haft is 864 mm in diameter.

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1.

2. 3 . • 4. 5 . 6. 7. 8 . 9.

10.

11.

• 12 .

13.

Fig. 19

Hydrau l ic nut for propell er m ounting

Duct for pressurised oi l feed Air vent duct

Pressure tank f or lubricating oi l Air vent

Circu lation pump for lubricating oi l

Cock f6r sampl ing lubricat ing oi l Lubrica ting o il tank

D ra inage pipe for sea w ater

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Pressu rised air lead for in f lation of sea ls

Oil container for damping d evice

Monitoring inst rument MEPA 2 1 A

9 8 7 6 11

Bulkhead which is sealed when the aftermost seal is to be inspetted or replaced

10

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-+--- - -- -----·· · . .. -·-- ---- - ·- ·--- ---- ---

Fig.20

1. Adapter sleeve

2. Hydraulic nut 3. Oil supply duct

4. Stern eye 5. Outer sleeve with internal taper 6. Inner sleeve with external taper 7. Oil supply duct to contact surfaces

of sleeves 8. Hydraulic jack 9. Duct for oil supply to hydraulic

jack 10. Seal, aft, make Simplex

11. Seal, fore, m ake Huhn 12. Inflatab le rubber sea ls, type Huhn

Pneumostop 13. Seal sleeve 14. Bronze bush 15. Piston for damping device

16. Throttling valve 17. Overflow valve

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The bearing arrangement and mounting procedure were shown in the

film and are described in a brochure that is available in English.

I shall only comment br iefly on the design and mention the

experience gained with this arrangement. Although this does not

concern the oil injection method, it may well prove to be of

interest to an audience such as this, with a maritime background.

The performance of the bearing arrangement has been excellent

throughout more than three years of service. How do we know this,

apart from the fact that it has not given any trouble? Well, for

one thing, by measuring the bearing temperature. Fig . 21 shows

that this follows the temperature of the sea water. Therefore

the temperature differential between the bearing and the sea

water is of major interest.

60 .----·----,--.----.-·--r----.----,---,-- .,.- -.---.--.---.---, t I • _______ _ oc J------t ------ ----- -,,_ ------ ----- ~

_,....... . I .. , --·" ...... so ~ T "~:- - / ~+""~---+-.-___ -_-_+-_-___ -__ +-__ -_ --~

40 . --- -~ -·- - · ·---- -!--+ - -+-----+-+---+---+- "a. -v.......,

30 --:tl .. ..:c-~ - · .- · "'-'' c-. -+-V'-r_,-+-~ ..... --.. +---+-- +---+---i

- "' - · 1\. • · , __ _ / · 1 ',1\.

20 -~-· .. - - - - .. -··· . - - - ---+--+--+-~. ±-r--- .. -_+-1---·"--1----i

r- ·-... .• . --r--- r- --1--1-··---t--- --- - - - - . -- -10 ---.

I

0+--+i ---+--+--~~-~-r--r--r~--~~--+-~ 14/3 16 18 20 22 24 26 28 30 1/4 3 5 7 9 11

· Fig . 21

Dote

Tunnel shaft hearing

Stern tube bearing

Water

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.,

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How lar ge should this differential be? It is possible nowadays

to calculate this by computer. The unbroken line in Fig. 22

represents the theoretical way in which t he temperature

differential s hould vary according to the different water

temperatures. The difference· is greater at low water temperatures .

since t he viscosity of the oi l is then higher.

The tri angles on the graph represent temperatures actually measured

during a voyage and it can be seen that these correspond very

closely to the theoretical values. The maximum discrepancy does not exceed 2°C.

~ :::::..:-_ ~ ~ ... ~

. ... .... ~

I - · - - - -.;;;;.,.,:

~

25

20

I 15 ·-· --·-~-1------- - r--·-

10 -- ~-

.. _ .. ___ r--- ---___ _ ..

-----r-·

5 c----· ---r- - ·-·- - -I

I 0 5 10 15 20 25 ' 30 35

t °C

Difference At ' °C between bearing and water temperatures, at different sea water temperatures , t °C

Fig.22

'

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It is thus possible to determine accurately the bearing temperature

and thi~, as well as the temperature distribution in the stern

section, is very important information to know for a bearing ·

arrangement of the size in question with effective cooling of

both shaft and housing section. The theoretical temperature

distribution in the stern section is indicated by the isotherms in Fig. 23 •

-- . - ----+--- - - - -------

Fig.23

There must always be a slight play between the bearing outer ring

and the housing to ensure axial freedom of movement for the

bearing. It is therefore necessary to be able to calculate

· accurately the actual play under operating conditions. The

temperature of the bearing outer ring is higher than the average

temperature of the stern section - as is illustrated by Fig. 23 -

and thus the play obtained between the outer ring and its housing

when the bearing was mounted will diminish in service. Once again

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a computer was used to determine the way in which the temperature

affects the bore diameter of the stern tube. Using the so-called

finite e lement method, the stern section was divided into small

elements , as shown in Fig. 24, and temperatures were given to

these elements, with the aid of the calculated isotherms.

The calculations permit the correct radial internal clearance

of the bearing and the correct play for the outer ring in its

hou s ing to be obtained by adjustments made during the mounting

operation. However, the design of the arrangement allows for

subsequent adjustment to be carried out, should this prove

necessary.

Fig.24

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The design that has been described is interesting in that it

demonstrates that very complex questions regarding bearing

arrangements can be resolved with the aid of sophisticated

theory and calculation techniques. This was not possible earlier

and the necessary answers had to be obtained by trial and error,

a costly and time-consuming method. ihe design also shows the

way in which the SKF oil injection method is applied throughout;

indeed, this method made the 9esign of the bearing arrangement

feasible. It might be mentioned that mounting techniques were

put to a severe test when the aftermost seal started to leak to

such an extent that a repair was called for. With a conventional

plain bearing arrangement, this would have been a ·very complicated

and costly procedure, but it was possible for us to. dismount the

bearing and seals, vulcanize new seals and complete remounting

during the prescribed time limit of two days - all this while

the ship was at sea. Everything went according to plan and no

freight revenue was lost as the schedule was maintained.

We are eagerly awaiting installation of further bearing arrangements

of this kind, but unfortunately the bottom has fallen out of the

market with the shipping crisis. We did have, for instance, on

order for the propeller bearing arrangements for six 500 000 tdw

tankers, but the prospective owners cancelled the orders for

these vessels .

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