Wear mechanisms in unlubricated chains

Wear mechanisms in unlubricated chains

S.J. Radcliffe*

This paper describes the results of wear tests on unlubricated articulating chains. Experiments with a martensitic stainless steel chain showed that under certain conditions of load, motion and environment, unexpected wear patterns could develop, with high and low wear alternately on adjacent oins and bushes. Examination of specimens tested in different atmospheres showed that the same fundamental wear mode, severe adhesive wear, was involved throughout. The variation in wear rate, both environmental and geometric, was caused by wear debris acting as a solid lubricant of variable efficacy. A second, surface hardened chain material was also tested. Whilst this wore relatively evenly at a very low rate indeed, it also suffered from progressive surface spalling and this feature would probably be life-limiting

The Central Electricity Generating Board currently operates in England and Wales some 18 gas-cooled nuclear reactors for the generation of electricity. The nuclear reactors are controlled by means of rods of neutron absorbing material which can be inserted into the reactor core, thus regulating the thermal output . Each control rod is suspended from its own actuating mechanism by a steel rope or chain, and fine control of the reactor involves repeated movements of a few millimetres or centimetres of individual rods. In addi- tion to fine control, these rods are also used to shut-down the reactor. For the second generation Advanced Gas- cooled Reactor (AGR) it is the small control movements which constitute the greater part of the cumulative move- ment.

As a result of the various operational requirements and physical constraints, the AGR control rod actuators are complex mechanisms containing bearings, gears, clutches and brakes, and thus there are many features of tribolog- ical interest. In order to ensure that the mechanisms have the required reliability, a substantial amount of development work has been carried out.

In the course of prototype tests on one particular design of actuator, a curious wear phenomenon was observed in the rod suspension chain. Alternate pins and bushes from a particular section of this chain showed a very large amount of wear, corresponding to a specific wear rate of about 1042 m 3 N -1 m -1 . The less worn components had wear rates 30 times lower, about 3 × 10 -14 m 3 N -1 m - l . The pins and bushes were made from a hardened martensitic stainless steel (En 57), which is generally a good tribo- logical material for unlubricated sliding in gas-cooled reactor environments. However, the wear rate of the more worn components was typical of that expected for severe adhesive wear of an austenitic stainless steel. Austenitic steels are noted for their poor behaviour in unlubricated large amplitude sliding as adhesion combined with work-hardening leads to severe pick-up and transfer of material.

This paper is concerned with an investigation of the wear phenomena, and the achievement of an acceptable solution prior to plant operation.

*CEGB, Research Division, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire, UK, GLI 3 9PB

Fig I A and B: upper and lower pins from prototype test. C and D: upper and lower pms from BNL Test 3 showing similar behaviour to prototype

Initial observations

Investigation showed that in the course of tests the affect- ed section of chain had passed repeatedly over a pair of guide rollers which articulated the joints through an angle of -+ 26 ° under a tensile load of 2700 N. Typical worn pins are shown in Fig 1 (A) and (B), and the notat ion "upper"

0 3 0 1 - 6 7 9 X / 8 1 / 0 5 0 2 6 3 - 0 7 $02.00 © 1981 IPC Business Press TRIBOLOGY international October 1981 263

RadcEffe - Wear mechanisms in untubr icated chains

and "lower" is explained in Fig 2 (A). Since the chain hangs vertically, half of the pins (and bushes) are loaded on their upper surfaces whilst the rest are loaded on their lower° Each type experiences, eventually, the same loading and articulation, but the wear rate of the "upper" pins was about 30 times greater than the "lower". Fig 2 "illustrates the model which was proposed to explain this effect. It was hypothesised that wear debris from the upper pins and bushes would fall away from the contact zone, whereas that from the lower would tend as a consequence of the conformal pin-bush geometry to be trapped in the interface. Thus tlte upper components may experience more direct metallic contact, whilst this will be mediated by debris for the lower case. The effect of prolonged rubbing will depend on the environment. The debris can either remain predominantly metallic or become oxidized, and may become finely divided before eventually escaping axially. There is also a possibility that debris could aggregate sufficiently to take up the clearance (of 0.23 ram), but jamming would only be likely if the aggregates were relatively strong. It is reasonable to postulate that the presence of debris in whatever form would reduce both friction and wear. Friction coutd be reduced either by larger and stronger pamc_,es acting as rolling elements or, perhaps more likely by oxide debris providing a low shear-strength interface and reducing adhesive interactions. Adhesive wear would similarly be reduced by debris, though abrasive wear might be enhanced. One possible result would be that upper pins and bushes might show characteristics of adhesive wear, while tI~e lower might resemble mild wear. Certainly the measurements of specific wear rate were not inconsistent with this very simple model. However, in order to invest- igate the matter further it was decided to mount a laboratory-scale experiment simulating the "reactor" conditions used in the full scale prototype test as far as was practical.

Experimental details

The philosophy behind the laboratory experiments was to use real chain specimens with a similar ioading and cyclic articu!ation to that expected in service.

Rig design

Within the above constraints, it was decided to accelerate the test timescale by increasing the frequency and elimi- nating the dwell periods which were present in the prototype tests. It was also decided to aim for test conditions at least as arduous as those in the reactor in order to in- still confidence in any alternative materials tested. A comparison of the conditions with those in the reactor is given in Table I.

As described in the next section, an ambie~It pressure COt envirog_ment was provided for the main tests. Gas 1%essure is not usually regarded as particularly important in tiffs type of experiment, though it can influence wear transitions even at moderate temperatures ~ o However, tb_e pres.- ent series of tests were intended mabhy to improve the funo damental understanding, and the final test of any design changes wo~ld be made in the "prototype" test facility at the correct reactor conditions. Temperature is simitariy expected to influence wear transitions, b~.t in this case the discrepancy between test and appiication was ~ess marked and might therefore be less significant.

Since some resuIts for the correct envirov.ment were a?- ready avallabk, it was co~sidered titat the test cor[dJtions could be proved adequate if results agreed both q'aantitat- ively and visually with the prototype tests.

Table 1 shows that it was possible to acquire data i~a the tes rig about 50 times faster than in the much more complex prototype tests, whilst only increasing the actv.at frequency by a factor of 10. Sliding speeds were still tow enough ~o avoid inertial or heating effects. The test rig is shown b.~ Fig 3, and the simple dead-weight loading system is clearly visible. Articulation was provided by ar~ eccentric, driven by a geared motor, with sliding guides to provide the correct chakn motion.

Experimental programme

in a!l, six experLments were performed; the first, in air, served mainly to check the apparatus but also revealed that control of the atmosphere was essential. (Detai!S wi11 be given in the next section.)

L "Upper" p~n I [ "Outer" I~nk and b u s r q j J

i..~..-"1 nneb" !tnk

"Lower'" p L ~ ' ~ and bush L ~ _

a

~nd bush Deb¢L~

snd bush ~ * - ~ ' ~ b ! Debr~

Fig 2 (a) Simplified representations o f a chain. {b ) DixtE- bution o f wear debris: section of outer link and parts QF two inner links

Table ! Comparison of conditions in these tests with those in prototype reactor tests

Prototype These tests Reactor Atmosphere CO2 CO2 (Tests 3--6) CO2 +1% CO Pressure 30 x 105 N m -2 t x 105 N m -2 30 x 10 s N m -2

Temperature ~ 70 ° C 20 ° C --~ 70°C

Cycling f requency Average 30 30 cycles/rain - cycles/Peak 3 cycles/min

Tension 2700 N 3000 N 2700 N

Angular amplitude +- 26 ° +- 26 ° +- 26 °

Total cycles 8 x 104 Up to 3 x 105 < 8 X 10 4

264 T R t B O L O G Y internat ional October 1981

Radcl i f fe - Wear mechanisms in un lubr ica ted chains

For the second test, a " ten t" of reinforced polythene sheet- ing was constructed around the apparatus, and sealed to the floor with pvc tape. The tent was then f'dled with carbon dioxide from a bulk supply and finally sealed after starting the motor , so some contamination with air was unavoidable. For subsequent experiments the test was purged for a period before starting thus improving the C02 purity. The results, given later, were similar to those in the "full scale" test in a controlled COz atmosphere confirming that the environmental control, though simple, was satisfactory.

The first four tests were performed using the original type of chain. For the final two tests an alternative design was available. The dimensions and materials were similar, but the pins and bushes had been Tufftrided*. This is a salt-bath diffusion treatment forming hard carbides and nitrides in the surface layers.

Specimen examination

After each test, the chain was dismantled to give four pin and four bush specimens. Bushes were cut in half for in- spection but only the pins were systematically measured by profilometry. A pin-holder with a horizontal "chuck" was mounted on a Talysurf long-traverse table, and a series of eight or nine axial traverses were made at azimuthal inter- vals of 30 ° . Where the pins passed through the side plates there was no wear, and measurements here provided a reliable datum level. In some cases, the wear depth approached 2 ram, well beyond the range even of the "low magni- fication" Talysurf pickups. An extra-long extension arm was constructed, enabling the standard magnifications to be divided by a factor of 25. On the standard chain specimens the wear scars were very rough and ball-ended styli of radius 0.5 or 1 mm were used. In some circumstances, such coarse styli can seriously distort wear measurements on rough surfaces 2 . ttowever, microscopic examination and fine stylus traces were used to confirm the measurements.

Results

The results are summarised in Table 2 and shown graph- ically in Fig 4; measurements from the full-scale prototype tests are included in the last column for comparison.

Typical Test 1 upper and lower pins are shown in Fig 5(a) and (b) which show that the wear was relatively slight. The wear debris was finely divided and reddish brown in colour, and therefore probably consisted mostly of F% 03. This contrasts with the pro to type test (Fig I (A) and (B)) where the debris had been coarse, black, and largely metallic, and thus it was not surprising that the wear rates were significantly different. However, one encouraging feature of Test 1 was that there was more wear on the upper pin than the lower, as expected from the simple geometric model.

Microscopic examination of the pin and bush surface revealed several interesting features. The worn surface was covered with roughly circular pits typically 0.5 mm wide and 1 2 mm apart, generally reminiscent of fretting pits. Secondly, there were many grooves about 0.5 mm wide, 2 3 mm long (occasionally 5 mm) and about 50 pm deep. These were orientated parallel to the sliding direction; the

*Tufftride ® is the registered trademark of Degussa AG, marketed in the UK by the Durferrit Division of Degussa Limited

Fig 3 BNL test rig

U;tcer D ~ wea r r ~ L,twer D,P ¢~ear

"0 --

¢: !C i

f ]St '

L t 10":

h

, <4.',

1

q

t Tdtftmde d CI~C P

L L_

~NL test number

E

10 E

(b

E b

- 12"

O

E - 1,2 cn

PrototyPe test

Fig 4 Results o f wear tests

sliding stroke was 3.8 nun (peak to peak). These appeared to be fairly typical indicators of severe adhesive wear. Further adhesive wear features were found only on the pins in the form of small lumps at the end of some grooves typically standing about 100 ~tm above the surface. These lumps appeared to be formed from heavily deformed material machined from the grooves and perhaps transferred from the bush. Both the grooves and the lumps were more

TRIBOLOGY international October 1981 265

Radcl i f fe - Wear mechanisms in un lubr fca ted chains

prominent on "upper" components. Both bushes carried considerable amounts of compacted and adherent brown debris, and in the upper bushes this had also transferred around to the unloaded region.

As a result of the change to a CO2 environment, in Test 2 both specimen appearance and the specific wear rates were closer to those obtained in the prototype tests. Whilst the overall wear rate was somewhat lower, probably reflecting the more oxidising environment owing to air contamination, the ratio of wear rates for the upper and lower pins was 17. This compares quite well (given the differences between the tests) with the Value of 27 obtained in the prototype tests. The wear debris was course and black, and appeared to be predominantly metallic. However it adhered to the pin somewhat more than in the prototype tests. (Figs I(A), I(B), 5(C) and 5(D)) and this may have indicated the presence of oxides in the debris. The main features of both upper and lower components under low power microscopy were ploughed grooves, very similar to those found in Test 1.

In the purer CO2 environment of Tests 3 and 4, the wear rates of all components were, as expected, significantly higher and the surface, particularly of upper components, had a much more shiny appearance (Fig 1 (C) and (D)). As before, the dominant features were grooves showing that severe adhesive wear was the main metal removal mechanism for both upper and lower components. Whilst the difference in rate between upper and lower pins was not so marked as before, the wear rate of the upper pins was actually greater than that in the prototype test, showing that the test facility could achieve more arduous conditions than might be expected in the reactor.

Fig 5 A and B: upper and lower pins from BNL Test 1. C and D: upper and lower pins from BNL Test 2

Fig 6 Section of Tufftrided chain (unetehed). A: Oxide layer, B: Diffusion layer

Table 2 Percentage of nominat bearing area spa~led on p~n;

Upper Lower

Test 5 {21 h) 2~½ 1 Test 6 1t67 h) 25 8

Tests 5 and 6 used a section of Tufftrided chain, under similar conditions to those in Tests 3 and 4. Fig 4 shows that in each test, upper and lower pins wore by a very similar amount, but in each case there was more wear on the lower, in contrast to the earlier observations. However, whilst earlier experiments had suggested ~hat the wear was generaUy at least roughty proportional to the sliding distance, there was nearly an order of magnitude difference between the wear rates in the two tests. The raw data in Table 1 helps to provide an exNanation. A!though Test 6 lasted for eight times as long as Test 5, the measured wear depth was essentially the same at about t0/~m. E×~rr, ination of a metallographicaUy-prepared section showed that the unworn Tufftrided material had a compound structure. The outer layer, about 11 #m thick, was featureless both ii~ optical aM scanning electron microscopes, whilst an inner layer about 45 #m thick showed the interlocking-needle structure typical of nitrided and carburised materials (Fig 6). Analysis of the material by electron microprobe az~d an X-ray dispersive analyser on the scanning microscope showed that the outer layer was a pure c_kromium oxide, presumab!y Cra 03. The wear rates are discussed further in the next section, but other observations of the Tufftrided specimens suggested that wear might not be the life-!imiting feature of the material. Spa~ing was observed on ali core- ponents; the spails were about 50 ~m deep and thus evidently corresponded to loss of the whole of the Tuff- trided layer. The percentage of the nominal bearing area spalled on pins is shown in Table 2. Thus the spal!ing rate was rougbJy linear with duty. The spalling appeared to start with the development of normal cracks through the coating, followed by severe plastic deformation of the substrate leading to a "rippled" surface, and finally Ioss of sections of the hard material.

266 TRIBOLOGY international October 1981

Discussion

Because of the great differences between the performance of the two types of chains tested, these will be discussed separately.

Standard (original) chain

Wear modes

Initial explanation of the different behaviour of the components included the suggestion that the upper were experiencing severe wear whereas the lower were experiencing mild wear conditions. In fact in all the CO2 tests the only wear mode found was severe adhesive wear, shown by the remains of strong metallic junctions. There was no evidence of mild wear, which involves only the removal of established oxide layers on oxidised surfaces at a rate eventually in balance with their rate of formation.

In the air tests, pits were noted which resembled fretting pits and whilst the amplitude was larger than conventionally associated with fretting, a considerable amount of fine Fez 03 similar in appearance to fretting debris was present. The mechanism involved in the formation of fretting pits is not understood, but it is reasonable to assume that some circulation or aggregation of debris is involved, and it may be that the same process was able to occur under this constrained geometry.

Geometric effects

One of tile most interesting observations in the experiment on the standard chain was the alternating pattern of wear. The ratio of wear rates between upper and lower pins varied between 3 and 20, compared with 30 in the prototype test. From the observation of the way debris transferred around into the unloaded regions of upper pins but not of lower ones, there can be little doubt that the influence of gravity on the debris was most important. The contrast seen in Fig 1 between upper and lower pin surfaces was also striking. Quite apart from the different amount of wear, upper pins were bright and shiny whereas lower pins were dull and had considerable amounts of debris compacted into the surface roughness. This will have reduced the wear by reducing the proportion of asperity contacts resulting in the formation of strong, adhesive bonds.

Environmental effects

Even though the environment was not controlled particularly stringently, it had a considerable effect on the wear rate and on the ratio of upper:lower pin wear. In general the total wear was inversely related to the oxidising potential of the environment, though the maximum ratio in fact occurred in an intermediate atmosphere. The prototype test also fits in this series; whilst the COz was as pure (if not more pure) as that in Tests 3 and 4, the higher temperatures will have increased the oxidising potential, and as expected the total wear was less than in the latter tests. The overall results were consistent with the wear model already advanced, with severe wear being greatest in the less oxidising environment because interfacial contamination will reduce or prevent the formation of adhesive junctions.

Summary

It is instructive to compare the observations with the proposed wear model. The volume of a typical "groove" irre- spective of the test conditions was in the range 0.05 to

Radcliffe - Wear mechanisms in unlubricated chains

0.1 mm 3 , a figure which compares well with the dimension of the deformed particles observed at the end of some grooves. Postulating a typical particle size of O. 1 mm 3 and using contact theory, it can be inferred that each particle arose from a junction which is likely to have been supporting the whole applied load. Thus an upper rate of particle formation would be about one per cycle. The measured wear rates of lower pins in air (ie a highly con- taminated situation) corresponds to about one particle in 400 cycles, whilst for upper pins in clean COz the wear was equivalent to the formation of one particle in about four cycles. Thus the observed surface topography, measured wear rates, and geometric and environmental effects can be plausibly related by assuming a severe adhesive wear model.

Tufftrided chain Wear

The explanation of the variation of wear rate with sliding distance in terms of a two-layer structure has been discussed in the previous section. In contrast to the standard chain results, the lower pins showed greater wear than the upper, ie the presence of debris appeared to increase the wear. Metallographical sections of the chain showed that the chromium oxide layer could break into hard, angular fragments which would probably act as abrasive particles. Some of the specimens showed regular circumferential scoring marks typical of abraded surfaces, and it is suggested that the basic wear mode for the Tufftrided chain was three body abrasive wear.

Some interesting calculations are possible of relative wear rates of the oxide and the inner layer by assuming that the wear rate of each was linear with sliding distance, and knowing by direct measurement the approximate oxide layer thickness. Results for "upper" and "lower" pins were averaged in order to improve the statistics. Estimated wear rates for the oxide and inner phase were 2 x 10 - ' s and 4 x 10 -17 m 3 N -1 m -1 respectively. From more detailed considerations (Appendix 1), it may be shown that a lower bound for the oxide wear rate is probably within 10% of the former figure, though no upper bound can be given. For the inner layer, an upper bound would be about 8 x l 0 -17 m a N -1 m -1 , though no lower bound can be given. Comparison of these figures with published 2 and unpublished data for similar environments suggests that the Tufftrided layer has one of the lowest wear-rates known in large-amplitude sliding, being comparable with the best superalloys and better than any cermets.

Spalling

Failure of the Tufftrided layer by spalling appeared to present more of a problem than wear in the conventional sense. Within the experimental limitations, the spalled area was proportional to the number of cycles, with about three times as much spalling on upper pins. Since spalling appeared to be initiated by surface cracking, tensile stresses in the surface layers could be responsible. One possibility is that frictional stresses were higher in upper pins, perhaps because even abrasive debris could act as a lubricant.

Use of Tufftrided chain

The very low wear rate of the Tufftrided material makes it suitable for use in this particular application, although the


Radc/iffe - Wear mechanisms in unlubricated chains

occurrence of spalling obviously raises some questions. However, in terms of number of cycles, Test 6 lasted at least three times as long as the calculated maintenance interval. Whilst a quarter of the nominal bearing area of upper pins was lost in this test, there was no evidence on any of the specbnens of catastrophic failure of the hard layer. ]n the circumstances, the material could be recommended for reactor duty, though the final a~urance of its suitability vail be provided by full scale tests.

Conclusions

The investigations showed that debris entrainment can have a considerable effect on wear rates in conformal geometries, particu!arly under severe adhesive wear conditions. Under the most favourable circumstances trapped debris could act as a solid lubricant and reduce wear rates by a factor of 30. For a material hardened by a diffusion treatment, where asperities have insufficient ductility to undergo adhesive behaviour the effect was less marked. The particular Tuff- trided material tested had a surface oxide layer 11/lm thick which resulted in a very non-linear wear depth with

siiding distance, but the wear rate of @e true diffusion layer was very tow indeed. However, under the ~retativety high test loads, cracks developed in these layers fotlowed by plastic deformation of the substrate leading to rippling of the surface and eventual sp~/lling of the layero Nevertheless the evidence suggests that adequate bearing area witl remain throughout the component life, justifying the expense of a full scale prototype test with the Tufftrided material.

Acknowledgement

This paper is published by permission of the Centra~ Ebc- tricity Generating Board.

References 1. Skinne~ J. The Wear o f a Chromium Carbide Hard Facing : The

Effect of High Gas Pressure on Weal Transitions. Trans. ASME Journal of Lubrication Technology, April 198i, ! 03{2), 228-235

2. Chivera T°C. and Radcliffe S.J. The Role oi" Surface ProY~ometry in Failure Diagnosis. Wear, 1979, 5 7 , 3 1 3 32i

Appendix 1

Detailed analysis of results for Tufftrided chain

The test results are shown in Table 3. Measurements of the oxide thickness by sectioning followed by optical and scanning electron microscopy suggested that the thickness was typically 10 -+ 1 #m. However, the result is sufficiently uncertain to permit accurate measurements of the wear rate of the inner layer, and in particular to differentiate between upper and lower pin performance. The average results for the two pins were therefore emp!0yed in the following calculations.

Let the tMckness of the oxide layer be t (assumed constant in both tests). Let numbers of cycles and wear depth be n and d, with subscripts indicating Tests 5 and 6. Assume the wear rate of both oxide and inner layers is linear with numbers of cycles, and is denoted by K ( = d / n ) . Subscripts i and o represent the two phases, inner and outer (or oxide). it is quite clear from the non-linearity of wear over the two tests, from sections of Test 6 pins, and from a comparison of the wear depth and the measured oxide thickness that in Test 6 the oxide phase was fully worn away. The same cannot be said for Test 5, and thus two cases must be considered.

(1) t < ds (2) d ~ > t > d 5

Consider case (1), where the transition between materials occurs in both Tests 5 and 6. Let the number of cycles to the transitions be n

Then

Table 3 Results of wear tests on the Tufftrided chain

Test No Cycfes Average wear,/~rn Upper pin Lower pin Both pins

5 3.9 x 104 9,5 12.3 10.9 6 3.tx10 s 11.8 12.7 12,25

O x i d e " leyer

%

®

~ld ~

.17 10 L _ _

1 0 7

" I n n e r " layer

L I I ~ I 1 0 8 10.9 11.0 111 1! 2

t , s m

I1 3 !I 4 11 5

Fig 7 ~'ear rate o f T u f f t r i a e d chain as a f u n c t i o n o f o x i d e th ickness

For the inner phase, ds - l

K i - n s - t 7

from Test 5 and

d 6 - t K i -

H 6 - n

from Test 6.

268 TRIBOLOGY international October 1981

Radcf i f fe - Wear mechanisms in un lubr ica ted chains

Both n and t may be eliminated from these equations to give

d 6 - d s K i -

H 6 - t l s

and hence the number of cycles to the transition can be calculated as a function o f t. Extrapolation of the wear data for the inner phase gives an estimate of the minimum possible value for t,

train = ds - k i n s

which for the data is lO.71#m. For this case where the oxide phase is assumed to have worn through completely in Test 5, no direct estimate of the wear rate Ko is possible. However, the requirement that the two tests follow the same curve puts a boundary on possible values of K0 and, for the second case, where the oxide film is assumed to survive into Test 6 the results are the other way round. From Test 5, the wear rate of the oxide is known exactly

ds KO ~ - - -

l l s

so the transition occurs at n cycles where

_ n s l l l

d s

giving, fromTest 6, d 6 - t

K i - n 6 - H

a result dependent on the assumed t.

In Fig 7, specific wear rates for both oxide and inner phases are plotted as a function of assumed oxide thickness, t. Curves are only plotted from t = 10.75/Ira upwards because of the implication of the results that t must have exceeded this figure.

A lower bound for the oxide wear rate can be given to an accuracy of about -+ 10%, since this is the accuracy of the wear depth and layer thickness measurement. The rate for the inner phase, which showed much less wear is corre- spondingly less certain. For an oxide thickness of 11/~m, the wear rate was about 4 x 10 -~v m 3 N -~ m - 1 , and is likely to have been more than twice as high as this. However because for Test 6 the possibility cannot be ruled out that the oxide thickness approached the total measured wear (12.35 ~m) no lower bound can be given to the inner phase wear rate.

Oil consumption cut

Production process developments and design changes have resulted in a significant reduction in oil consumption in the up-rated Leyland TL I 1 turbo- charged eleven litre diesel engine. A key aspect of this development is the change from a retaining cylinder liner with a high interference fit to a low interference fit supplemented by an adhesive.

The interference between the cast iron liner and cylinder block is, in fact, only half that normally necessary if solely relying upon an interference fit. Individual liners are now pushed in with a load of up to 3 tonnes. But removing them, tests have shown, requires 14 19 tonnes per bore on unused engines, and 10 tonnes after lengthy service.

Prior to tile re-design, liners were frozen and then hand-fitted into the cylinder blocks. Now installation is a semi-automatic process. The block arrives on station from the machine shop: a rotating brush which dispenses Genklene only on its downward stroke cleans out each bore, and by the time the last bore has been cleaned the first bore is dry enough for a coating of Loctite High Strength Retainer 640. This is applied as a fine mist by a spinning cup dispenser which descends into each bore and applies an even

coat of adhesive on its return, upward, travel. The treated block then moves to the next work station where the liners are pressed into position.

According to a geyland spokesman, this method of fitting the liners results in better oil consumption and crankcase blow-by. The improved control of bore geometry made possible by the lower interference fit allied to a re-design of the liner and detailed piston improvements, has enabled the ratio of oil used to fuel to be reduced to less than 0.5wt%. This process also allows the use of pre-finished liners which do not require subsequent honing.

Loctite UK, Watchmead, Welwyn Garden City, Hertfordshire, UK, AL7 1JB

Pump seal programme Premature seal failure, one of the most costly and persistent problem areas in process plant, is the subject of a com- prehensive three-year development programme to extend the state of the art of sealing technology. The clear objective is to develop hardware for a new generation of high-performance, long-life seals for rotary pumps.

As energy, raw materials, and labour costs rise, users demand greater efficiency from plant and components. Processing speeds, temperatures and pressures have been increased to meet

this demand, resulting in conditions in which the seal is the critical component. Results from a BHRA survey of rotary pump seals in process plant indicated that a large proportion of mechanical seals failed within six months of installation, while a recent report from ICI traced 32/~ of outages in petrochemical plant directly to mechanical seal failure. Figures as high as 70cA have been reported from oil refineries. The BHRA survey also showed, however, that a good mechanical seal can give excellent performance. Why is there such a variation, and why is the failure rate high? A greater understanding of the key factors determining component and system behaviour and life is obviously needed before more effective seals can be developed.

Tile BHRA/MTI heavy-duty extended life pump seals programme aims to improve mechanical seal life by systematically establishing a fuller understanding of these key factors and by applying this new knowledge to advance seal and seal chamber design. Plant operators, seal users, and equipment manufacturers are invited to join the consortium funding the project.

Further details from D.F. Wilcock, MTI, Latham, New York 12110, USA or R. Flitney, BHRA Fluid Engineer- ing, Cranfield, Bedford, UK, MK43 OAJ


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Wear mechanisms in unlubricated chains