Wear mechanisms in oscillating bearings

Wear, 75 (1982)

WEAR

369 - 387

MECHANISMS

Y. BERTHIER

369

IN OSCILLATING

BEARINGS*

and D. PLAY

Laboratoire de M&unique des Contacts, Lyon, 20 avenue Albert Einstein, 69621

Znstitut National des Sciences Villeurbanne (France)

Applique’es

de

(Received August 29,1981)

Summary Surface examination of dry oscillating bearings reveals three different contact zones and indicates that various frictional behaviours take place during the process. The relation between wear and applied conditions such as load and amplitude of oscillation is obscured by the complex behaviour of most frictional materials. Studies of initial destruction, transfer and elimination are easier with materials that show simple behaviour. Friction and wear are governed by load-carrying third bodies transported in the contact. Transport was confirmed during observation through a hollow glass ring rubbing on a chalk sector. Wear debris motion explains the manner in which geometry modification occurs with time. Circumferential debris transport due to ring motion is modified at the centre of the sector where transverse flow takes place. The initial O-shaped sector can change into an w configuration. Thus the calculation of wear or of eliminated material must allow for the new w shape. The new shape also controls bearing clearance. The effects of load and oscillating motion on wear parameters are presented. More than one wear mechanism can exist in a single contact.

1. Introduction Dry bearing life is known to be limited by wear. Recent work has shown that wear can be affected by the load-carrying capacity of transported wear debris or third body (TB) [ 1,2] . Load-carrying capacity is in turn governed by mechanical parameters such as contact geometry, degrees of freedom and kinematics [3]. Thus a mechanical analysis of wear must be conducted together with physical studies in order to identify the nature of TBs and ultimately to understand how the difference in velocity between the two rubbing surfaces is adapted within the TB [ 41.

*Paper presented at the International Conference on Wear of Materials 1981, San Francisco, CA, U.S.A., March 30 - April 1, 1981. 0043-1648/82/0000-0000/$02.50

@ Elsevier Sequoia/Printed

in The Netherlands

(a)

(b) Fig, 1. TB morphology in ring and sector tests (90” polytetrafluoroethylene-polyimide sector;steel ring, 45 mm in diameter;angular displacement, 6” ; load, 8040 N): (a) counter” face (left, contact centre;right, contact exit);(b) sector centre.

Among various parameters, the effect of the nature of motion has been noted in earlier studies which have shown, for example, differences both in surface morphology and wear rate [3, 51 between identical test bearings submitted to continuous and alternating motion. Various TB morphologies appear in oscillating contacts (Fig. 1). The purpose of this paper is to describe the wear process that occurs under alternating motion and to relate it to the transport or load-carrying capacity concept defined earlier. It is hoped ultimately that this work will serve to formulate better material designs for dry bearings operating under cyclic motion.

2. Apparatus

2.1. Test specimen As in a previous study [ 6 ] , chalk and glass were used as the wear materials because chalk generates thick visible traces or TBs in wear experiments and also because TB generation can be observed directly through the transparent glass. This is a valid test couple material because as shown in

371

earlier work the conclusions were extrapolated with success to the analysis of data obtained with common industrial dry friction materials [3] . A hollow glass ring of outer diameter 150 mm is used in the observation studies. The outer surface is frosted with carborundum abrasives applied manually in such a way that the disk roughness is originally isotropic. The running-in effects on isotropy are small so that directional roughness has been ignored. Aluminium disks were also manufactured for special tests. The 160” sector is made of two symmetrical halves for ease of fabrication. The sector width is 25 mm. Both halves are made from a homogeneous mixture of chalk powder, water and binder. The chalk particle diameter varies from 2 to 20 pm. The hardness is equal to 3 HM. Specimen manufacture requires a special development program. A minimum percentage of water is needed to avoid cracks during drying. A dispersive agent (1%) and a Rhodopas A 10 binder (8%) are added to ensure cohesion. Chalk sector characteristics are as follows: relative density, 1.5; Young’s modulus, 18 GPa; ultimate compressive stress, 2 MPa. After they are moulded, the two symmetrical halves are glued to a metallic support and the final external shape obtained by separate machining. The final circular shape of the sector is imposed by mounting the specimen in the test machine and by substituting for the glass cylinder a special grooved aluminium disk of the same diameter used as a milling cutter. 2.2. Apparatus Figure 2 shows the apparatus. A ring 1 mounted on a main shaft rubs against the sectors 2, 3 fixed in a cradle 4. The cradle forms the lower arm of a parallelogram 4 - 7 which applies a vertical load. Knife edges 8 and 9 are mounted on the bottom and top link respectively to ensure frictionless alignment between sector and ring. The top of the apparatus includes a guiding column and a hydraulic jack. The problems linked to the modification of load with vertical displacement due to wear are avoided by imposing a constant pressure in the hydraulic jack independently of the ram position. Sinusoidal oscillating motion of the glass ring is obtained through a crank on

(4 Fig. 2. (a) Ring and sector

(b) geometry;

(b) test apparatus.

Fig. 3. General

view of the test apparatus.

the main shaft. The angular amplitude 01 can be varied. In order to limit increases in contact temperature the frequency is limited to 20 cycles min-’ . A dynamometer 10 which holds the upper bar of the parallelogram measures friction torque. The sector can move vertically through u and horizontally sideways through h. Electronic transducers 11, 12 measure the displacements u and h during a test. The displacement h can be controlled by stops 13,14. The torsional rigidity of the dyn~ome~r is high and prevents a third degree of freedom. A stationary mirror 15, inclined at 45” to the vertical axis and held on the axis of the ring 1, serves to enable the contact area to be observed. The image can be observed directly or with a television camera 16. Figure 3 shows a general view of the apparatus. 2.3. Measurements Simultaneous measurements of the friction torque and the horizontal and vertical sector displacements are performed. The coefficient of friction is deduced from classical formulae while wear rate evaluation necessitates further calculations. The wear process will be described in detail later but can be understood readily by looking at Fig. 10. The determination of wear rates in the sector-ring geometry is not easy for the following reasons. Figure 4 gives two characteristic shapes obtained during tests. The original 0 shape in Fig. 4(a) is changed into an w confi~ration in Fig. 4(b) and at least three parameters are needed to characterize that shape, the sector thickness V at the contact centre and horizontal and vertical displacement variations h and u respectively which take place during one oscillation. In conclusion, two different geometric modifications are brought about by wear. The first is the loss in sector thickness discussed above. The second

373

(b)

(4

Fig. 4. Geometric modifications due to wear: (a) 0 shape; (b) w shape.

results from the side opening wear which causes the 0 shape to go to the o shape. The central zone of the w is generated through debris transport. In the next analysis, we assume that the radii R of curvature of the w geometry (Fig. 4) are equal to the ring radius. In both shapes, wear is defined as the material removed from the sector. As a first approximation it will be assumed that material density is constant: wear calculations are then based on volume changes. In the 0 configuration, wear rate is given directly by the variation in sector thickness. In the o geometry, the situation must be analysed in detail. Wear rates are measured by comparing two successive geometries defined at time I and I - 1. Time increments I - (I - 1) of 30 min are chosen. The surfaces in Fig. 5 are defined at time I. The surface g(l)/Z (Fig. 5(b)) is bounded by the two circles centred in Or(I) and O,(I) and two radial lines which mark a sector end angle 0 and an angular abscissa y, which measured experimentally is always close to 85”. Elementary geometric calculations give

k?(I) -= 1

h cos(6 + $)

(1)

cos $

P

‘W (a) Fig. 5. Wear rate determination: wear.

(b) (a) displacement during half an oscillation; (b) vertical

374

where 3.=@-

h sin(U -g) cos [

’ t

and y ! h1 The variation in area g/l with time can therefore be known variations in h and u are monitored during the entire test: k =

arctan

if the

Surface @(1)/Z is swept by the vertical displacement of the ring during the 30 min increments and thus is bounded by two circles centred on Or (I - 1) and O1 (1). Equation (1) serves to calculate the rate $(1)/l in which u= 0 h = V(I - 1) - V(I)

As $/I and p/l are surface changes during the time increment, volume wear rate U(I) per unit time can be shown to be

the total

In this relation, d(I) corresponds to the contribution brought about by the “contact opening wear” rate while a(l) corresponds to the rate of change of sector thickness. The expression contact opening wear was chosen as it seems to describe adequately the modification from the original 0 geometry to the final w geometry. When in eqn. (1) the horizontal displacement h is equal to zero, the w configuration degenerates into the 0 shape and U(I) is equal to @(I). Figure 6 gives typical wear measurements which are continuously recorded, Variations in u and la are smooth even though they appear otherwise in the figure. This is due to the fact that the vertical (or distance). sciiles are extended while the horizontal (or time) scales are compressed. Wear results (Fig. 7) are obtained after digitalizing the signal and treatment with a microcomputer. They present identical fluctuations for the same reasons, so that a mean value can be considered after the non-steady state period. Reproducibility is difficult to estimate because the non-steady state conditions at the beginning of tests can vary and thus entirely modify the results. This point will be discussed later. However, for the same non-steady state conditions, final results are within 10%. Long-time tests can be broken down in series of shorter tests without modifying the final results.

375

I 2 (a)

10

I I 20

/

I

tlme(hwrx)

2

I

10

*

I I

20 tlme(hws)

@I

2

10

I 1I I

‘,

20 timebxs)

(c)

Fig. 6. Wear measurements with time ((w= 10” ; load, 425 N): (a) variation in V; (b) variation in u; (c) variation in h.

2

(4

10

20

time (h.1

(b)

Fig. 7. Wear results ((w= 10” ; load, 425 N): ( a ) variation in g; (b) variation in G.

2.4. Test program and procedure The following conditions were applied: loads, 280, 425 and 650 N; angular displacements, 1.5”) 2.5”, lo”, 30”) 40” and 60” ; angular frequency, 0.33 Hz. A complete test matrix was not, however, performed. Further visualization studies were made on a limited number of cases. Test specimens are mounted and milled as described earlier. The alternating drive is set in motion and the load is applied.

376

3. Results

3.1. Visualization Systematic examination of specimens was performed after each test. TB photographs are shown in Fig. 8. Just after the test, the contact surface of the sector (Fig. S(a)) exhibits smooth zones at its extremities which are separated by a large grooved central zone. The grooves are parallel to the motion and their depth is of the order of 0.6 mm. The length of the central zone depends on applied conditions and increases with angular displacement. Irregularities in the transfer film on the ring correspond to those noted on the sector (Fig. 8(b)), i.e. thick transfer on the ring corresponds to a loss of material on the sector. Thus a raise on the ring will cause a groove on the sector, which will obviously align in the direction of motion. Grooves can be of different depths. The deepest grooves are found in the central zone of the sector. Transfer film thickness varies continuously from the end to the centre of the ring. A maximum value of 0.6 mm is measured. Transfer films adhere strongly to the counterface while TBs which lie on the sector can be eliminated easily by blowing, thus eliminating sector grooves and uncovering craters also located at the sector centre (Fig. 8(c)).

_“._” ,.,_ A

c

s

-.-_-

c*

Ia

-3

------

L -

-__.i_

__L__

,

>,

(a)

0)

Fig. 8. TB morphology (cx = 30” ; load, 280 N);(a) on the ring; (c) sector after TB elimination.

sector after the test; (b) transfer film

Crater depth is greater than the depth of the grooves. Finally, large edge cracks appear on both sides in the central zone of the sector while only a few small edge cracks exist closer to the extremities. Debris removal takes place principally at the contact centre. Consequently, as because of geometry changes it seems reasonable to consider that debris are formed at the ends of the sector, debris movement must be assumed as debris removal out of the contact takes place principally at the centre. TB motion is shown in Fig. 9. Television pictures are stored in a magnetic recorder and photographs are taken where desired with the recorder stopped. The particle motion can be followed step by step. For simplicity, the five views which are given at different intervals correspond to the same half sector (see Fig. 5(a)). They show the following. (1) Debris are drawn from the end to the centre of the sector (Fig. 8(b)). (2) Debris are collected during travel so that large aggregates move (Fig. S(c)). This occurs at quasi-regular intervals. (3) Debris and particles are accumulated at the centre of the sector as oscillating motion implies symmetrical particle transport. Consequently, debris and particle removal take place axially along the centre of the sector (Fig. 8(c)). The removal is also intermittent. (4) Large cracks are formed on the lateral faces of the sector in the central region (Fig. 8(c)). These occur when a great amount of particles separates the test specimen in the central zone of the contact. In conclusion, visu~ization shows that the o shape contact is formed by three zones (Fig. 10). The ends of the sector are identical and symmetrical as alternating motion is imposed. Circumferential debris transport toward the centre is due to the ring motion while transverse debris removal at contact centre is imposed by the oscillation and mass conservation laws. Craters exist under the thick debris film or TB. These observations are also true when the O-shape contact geometry exists. In this case, the TB thickness at the sector centre is smaller. 3.2. Wear results (non-steady state) Various parameters (u, h, V, g) can vary with time as the geometry changes. In addition, some minor modifications at the be~nning of the test modify all the results. However, an easy classification consists of separating the part of the test which corresponds to a constant shape geometry and is referred to as the steady state period from the part when geometry modification occurs. The latter part will be called the non-steady state period (Table 1). For the reasons mentioned earlier, the non-steady state period is characterized by many events which must be added to the information given by visualization. Trends are shown in Table 1. Non-steady state periods can be separated in two domains: the start of TB formation and the stabilization of the vertical and horizontal displacements u and h respectively. The distinction is elementary but permits an overall view of the following phenomena. (1) We discuss first the beginning of TB formation. On the ring, a very thin transfer film is formed at the beginning of the test. At the same time,

378

(a)

(b)

(d)

(e)

Fig. 9. TB transport.

debris transport occurs on the sector owing to abrasion. In normal circumstances, a continuous TB is formed while under starvation conditions a disrupted film appears. These instabilities are accompanied by severe vibrations and will not be analysed here. However, these situations are uncommon

379

Fig. 10. Wear zones in the contact. TABLE Schematic

1 view of ring and sector

changes

with time

Sector

Permanent shape

but can produce the total destruction of the hollow ring. A solution to avoid starvation is to introduce in the contact an external flow of debris which is thus added to the wear mass transport. After that period, wear debris are

380

collected in the central zone and form a hill which corresponds to the vertical displacement u and a negative early value of V. The initial 0 shape is opened or widened and the horizontal displacement h can be measured. Geometry modification is achieved. (2) We now discuss h and u stabilization. After a time which depends on test conditions, debris removal is accompanied by the lateral fragile ruptures noted earlier. At the same time, geometry modifications subsequent to rupture lead to a sharp increase in the parameter V. Generally, the parameters h and u decrease after rupture. In some cases rupture does not take place and the parameters h and u then tend asymptotically to limiting values. Figure 11 shows the shape of parameter variations for a load of 425 N and different angular displacements (Y. The end of the non-steady state period as defined earlier occurs when h and u are constant. The results show the following.

A

l(mm)

/

,i

(a)

(b)

(d) Fig. 11. Non-steady state wear measurements (load, 425 N; -,/I--,u;-----, (a)cu=1.5”;(b)cr=l0”;(c)cr=30”;(d)a=60”.

V):

381

(1) For small angular displacements, u becomes constant simultaneously with h and u, i.e. wear stops. In this condition, the ring rolls without sliding. (2) For intermediate angular displacements, after reaching a negative minimum, V increases si~ifi~~~y.The parameters h and u reach a maximum and no apparent relation exists between the two. It should be noted that u takes longer to reach its limit than h. (3) For large angular displacements, if h increases, lateral cracks increase in size and appear earlier. Here again V will rise significantly. The non-steady state period is shortened (Fig. 11(d)). 3.3. Wear results (steady state conditions) Table 2 presents the test results. Two series of numbers are given for the condition in which instabilities appeared in the non-steady state period. For each test condition, the friction coefficient f and vertical and horizontal displacements u and h respectively are noted. As the sector thickness V decreases quasi-linearly, a mean value of V is defined; g represents the contact opening wear volume (Fig. 5) which quantifies the importance of the w shape with reference to the 0 shape. U gives the total wear rate volume per unit time. The results show the following. (1) Friction coefficients decrease with increases in load and are almost independent of the full angular displacements in these tests. (2) The total volume wear fi increases with load and angular displacement. Its variation is followed by the rate V of sector thickness variation. For severe test conditions, V increases more rapidly as rupture in the central zone reduces the sector width. (3) The contact opening wear volume g increases with load at small angular displacements Q:while it decreases at large values of (Y. (4) At low loads, these tendencies are reversed. It should be noted, for intermediate loads (Fig. 12), that large variations in g give a linear relation for the volume wear rate. At low angular amplitudes, the contact opening wear volume is high and steady state wear rate is zero. (5) No obvious tendencies or relations appear between steady state values of u and h; u is close to 0.05 mm while h can reach 0.5 mm.

4. Discussion Three topics will be considered: (1) the non-steady state period; (2) the fatigue process; (3) roughness effects. The non-steady state process, which is characterized by the change from the 0 shape to the o shape and the variation in the displacements h and u, is not specific to chalk bearing wear but has been noted in other studies performed with an industrial material (poly~~afluoroe~ylene fibre-based dry bearings). In these earlier studies [4, 51, the determination of wear rates from geometry considerations proved to be difficult for reasons which were not elucidated then. The identification of the changes in geometries is thus

40 60

1.5 2.5 10 30

Cd@

ff

0.18

1 0.6

0.62 0.07 0.69 0.09

0.55 0.02

0.08

0.62 0.04

0.31

0.28 0.31

2175 1260

360

0.07

1620

510 974

106

h-l)

h-")

180

(“mm3

ymm

0.02 0.04 0.03 0.07

ymm)

0.65 0.15 0.56 0.04

0.87 0.67 0.65 0.60

f

fmm3)

f

2mm)

425N

280N

Ymm)

Vaiuesof the pammeters under the foilowing loads

Results in steady state conditions

TABLE 2

0.06 0.11

0.47 0.08 0.05 0.12

h (mm)

340 280

760 210 135 310

g (mm3)

0.81 1.08

0 0.06 0.18 0.44

h-l)

ri (mm

1380 1860

0 120 330 720

h-l)

u (mm)

0.59 0.04

0.55 0.05 0.55 0.13

0.57 0.06

f

650N

0.1

0.18 0.07

0.11

h (mm)

-.

220

425 321

290

g (mm3)

e

4.17

0.61 1.36

0.08

h-')

(mm

7200

1200 2400

150

h-l)

383

2 500 3 r !Y t; '1 5 100 "

---

10

a(f)

(a)

Fig. 12. Wear result variations (load, 425 N): (a) variation in g; (b) variation in c.

useful in interpreting earlier results. It is clear now that we have to differentiate between wear due to contact opening and normal linear wear. Further, we have to relate contact opening wear to machine rigidity which has been shown recently to govern total wear [ 71. Further, the results presented here show that the wear rate, or more specifically wear debris removal, is largely controlled by lateral rupture. This suggests that wear could be significantly reduced if these ruptures were eliminated. Indeed, the introduction of side or lateral constraints (Fig. 13(a)) completely alters the non-steady state regime and again wear is seen to decrease (Fig. 13(b)). Side cracks do not appear to be caused, as believed earlier, by misalignment but by the consequences of the transport process, both longitudinal and transverse. It is interesting to note that lateral constraints are currently used industrially in carbon bearings [ 81.

1

(a)

(hours)

1

(hours)

(b)

Fig. 13. Effect of lateral constraints on wear results: (a) schematic view of the apparatus; (b) modifications in the results (-, without constraints; - * --, with constraints).

0)

Fig. 14. Fatigue process: (a) developed section of the sector; (b) hammer contact machine; (c) craters on the test block.

Craters were seen to exist in all tests beneath the TB. They do not appear at the beginning of the test and thus cyclic motion and Ioad have to be applied to produce them. Chalk spalls can either stay and move in the craters or adhere to the ring and groove the chalk sector because of their greater hardness. After TB elimination, non-symmet~c crater shapes are noted in the sliding region (or ends of the sector) while symmetry is observed at the centre where the ring rolls. These observations suggest that a fatigue process occurs and produces wear debris in order to build the TB continuously in the central zone of the sector. Some fatigue experiments have been performed (Fig. 14) on a hammer contact machine [9] which presses two flat surfaces together under a cyclic load. These surfaces can be separated by a TB of chalk powder. The sinusoid~ly loaded hammer is obtained by a rotating mass at mean pressures between 17 and 5.6 MPa. The choice of these mean pressures is somewhat arbitrary because TBs possess load-carrying

385

(a)

(b) Fig. 15. Effect of roughness on wear result: (a) metallic V-shaped circular rack; (b) glass ring.

capacities [ 11 and maximum pressures are unknown in this case. Results show, however, that craters only appear when the TB is present. So the craters observed on the sectors can be attributed to a fatigue process and thus different wear processes occur in a single dry friction contact as shown elsewhere with a pin and disk machine [lo]. The second obvious wear mechanism, i.e. abrasion, depends on roughness. To examine this aspect, the glass ring (peak-to-valley height RT = 10 pm) was replaced by a special aluminium disk transversely machined with V-shaped grooves (Rr = 600 pm) and of the same outer diameter as the glass ring (Fig. 15). Tests were performed at 425 N. When compared with those obtained with the glass specimen, results show that different non-steady state periods are followed by essentially the same variation in ? and the wear rate C?. In other words, when the voids between asperities, which are known as the Abbott volume [ 11, are filled, identical results can be expected. This is supported by Fig. 15 where transfer is seen to cover the initial geometry.

5. Conclusion The results presented bring further evidence to the belief that wear is not solely a material property but is also dependent on the mechanical environment (geometry, kinematics, loads etc.). Further, the results show

386

that wear measurements in dry bearings are complicated and simple techniques are not always meaningful. Indeed, contact shape modifications have been shown to be at least as important as the amount of material eliminated whilst producing these modifications. By reviewing earlier results obtained on industrial dry bearing materials, it appears that the time to reach the equilibrium shape is long and can often be equal to the total test duration (150 h) for these materials. Hence, performance evaluation is not easy. It must also be noted that the process of reaching the equilibrium configuration, which is conditioned by the mechanical environment, must be completely distinguished from running-in. The two phenomena are radically different. This is particularly apparent with small amplitude cyclic tests run on the disk and sector machine where geometry adaptation occurs to substitute rolling for sliding friction. These tests have also confirmed that more than one wear mechanism can coexist in a single contact.

Acknowledgments The research presented was performed under Research Convention 78.7.2662 of the Delegation G&r&ale A la Recherche Scientifique et Technique. The authors wish to thank M. Godet for helpful discussions during the writing of the manuscript.

Nomenclature g

B h

wear volume produced by contact opening horizontal volume wear rate horizontal displacement during one half-oscillation sector width ring radius total volume wear rate vertical displacement during one half-oscillation sector thickness at the contact centre rate of vertical displacement ring angular displacement angular coordinates

References 1 D. Play and M. Godet, Self protection of high wear materials, ASLE Trans., 22 (1) (1979) 56 - 64. 2 D. Play, Les films intercalaires ou troisiemes corps en tribologie: application a l’btude des paliers sets, Th&e Faculte’ des Sciences, Lyon, 1974. 3 M. Godet, D. Play and D. Berthe, An attempt to provide a unified treatment of tribology through load carrying capacity, transport and continuum mechanics, J. Lubr. Technol., 102 (2) (1980) 153 - 164.

387 4 J. K. Lancaster, D. Play and M, Godet, Third body formation and the wear of PTFE fibre-based dry bearings, J. Lubr. Teehnol., 102 (2) (1980) 236 - 246. 5 D. Play, Simulating contact conditions in dry bearings, Tribal. Znt., 11 (3) (1978) 193 - 196. 6 D. Play and M. Godet, Visuahsation of chalk wear. In D. Dowson, M. Godet and C. M. Taylor (eds.), Wear of Non-metallic Materials, Mechanical Engineering Publishers, London, 1978, pp. 221 - 229. 7 I. Kohen, D. Play and M. Godet, Effect of machine rigidity or degrees of freedom on the load-carrying capacity of wear debris, Wear, 61 (2) (1980) 381 - 384. 8 R. R. Paxton and A. A. Scilingo, Carbon-graphite bearings for high loads, Lubr. Eng., 25 (6) (June 1969) 246 - 252. 9 P. Guiraldenq, L. Vincent and B. Coquillet, Tentative de simulation au moyen d’une machine de fatigue, Rev. GAMI, (281) (May 1973) 12 - 19. 10 D. Play and M. Godet, Coexistence of different wear mechanisms in a single contact,

Wear, 42 (1977) 3.97 - 198.