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Transcript of Oil Inject
•
ject·on Assemb y
of essure J ints
Lecture y G Bergling in Moscow April 1977
\
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.
- 2 -
. ! ''-..
(
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
• •
•
- 3 -
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
- 4 -
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
• •
• •
- 5 -
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
- 8 -
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
• •
•
- 9 -
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
..... -.--
- 12 -
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
/
• •
I '
•
- 13 -
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.
- 14 -
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
• •
• •
- 15 -
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.
- 16 -
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
• •
•
- 17-
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
- 18 -
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
• •
• mH-I . \lJ
- 19 -
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
- 20 -
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
• •
• •
- 21 -
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
- 22 -
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.
•
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
- 23 -
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
- 24 -
-+--- - -- -----·· · . .. -·-- ---- - ·- ·--- ---- ---
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
•
• •
• • •
- 25 -
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
.,
- 26 -
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
'
• • •
•
- 27 -
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
'
- 28 -
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
• •
•·· •
- 29 -
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 .