Rolling contact wear of polymers: A preliminary study

Wear, 132 (1989)

183

183 - 191

ROLLING CONTACT WEAR OF POLYMERS: A PRELIMINARY STUDY C. C. LAWRENCE

and T. A. STOLARSKI,

Brunei University, Department Middlesex lJB8 3PH (U.K.) (Received

August

17, 1988;

of Mechanical revised January

Engineering, 23, 1989;

Uxbridge,

accepted

February

15, 1989)

Summary The wear of three different engineering polymers under rolling contact conditions has been investigated. A subsequent analysis of the results has been carried out to assess the suitability of these polymers for their application in rolling contact bearings.

1. Introduction Polymeric engineering materials subjected to strong mechanical and environmental excitation show - as many other materials - gradual degradation of their performance including eventual failure. If the changes in properties are mostly due to chemical reactions we can speak of corrosion or of radiative degradation. The term fatigue is used if deterioration in material properties is caused by the repeated cyclic or random application of mechanical stresses. The volume fatigue of polymeric engineering materials has received increased attention in the last 40 years. Review articles on this complex subject have appeared from time to time [l - 31. In addition, a large number of research papers have been published. It is a general observation, in the case of volume fatigue, that failure in repeated loading occurs at load levels which are lower than the stresses sustained under the conditions of static loading (creep) or of monotonously increasing deformation (drawing). The lower the stress level to which a material is subjected, the larger the number N of load cycles which are sustained. It is now commonly accepted that three principal mechanisms may contribute to fatigue failure: thermal softening, excessive creep or flow and/or the initiation and propagation of fatigue cracks. The problem of surface fatigue of polymeric engineering materials, induced, for instance, by rolling contact, has received very little attention, although in recent years polymers 0043-1648/89/$3.50

@ Elsevier Sequoia/Printed

in The Netherlands

184

have been successfully used as materials for rolling contact bearing ~-.~~~mc~~rl.s. Normal practice is to use a polymer for the inner and outer race’ wh&. rciai!ling steel balls or rollers. This type of rolling contact bearing is generally used in appiications characterized by light loads, low speeds and inadequate lubricatiort. There are many advantages to be gained by replacing traditional bearing materials with polymers. Polymers, especially modern engineering polymers. with desirable properties, are often available at a much lower price than traditional materials. In addition, they frequently eliminate the need and therefore the cost of finishing and other surface treatments since moulded or extruded products come finished to dimensions ready for assembly. Vouided plastic components of rolling contact bearings can be produced at a i,i’ry low unit cost. The main objective of the study [4] reported in this paper was the assessment of the suitability of some engineering polymers for rolling contact bearing components. 2. Test procedure

and results

2.1.

The test apparatus The test apparatus used for the’rolling contact wear study was a rolling four-ball machine made by Cameron-Plint Tribology Ltd. In normal configuration, the top ball is located in a chuck carried by a vertically mounted spindle and is rotated in a loaded contact with three other balls which are free to rotate in a specially designed race. In the study reported here, the top ball was replaced by a cone made of polymer and placed in contact with three steel balls of 12.7 mm diameter each. Figure 1 shows, in a schematic /

/-

8

5 , ,/-

El representation Fig. 1. Schematic 4, loading lever; 5, driving motor;

of the test rig: 1, specimen; 2, steel ball; 3, spindle; 6, heated plate; 7, loading piston; 8, belt drive.

185

way, the test apparatus. The lower part of the apparatus carries the loading device and the assembly also incorporates an electric heater. The load is applied by means of a dead-weight. 2.2. The test materials The materials chosen for the preliminary evaluation of their behaviour in rolling contact were as follows. (1) Nylon 6.6. (2) Acetal (Hostaform C2521). (3) Modified acetal (Hostaform C9021K). They were kindly supplied by Hoechst Plastics. Each of the materials was machined into the cone-shaped specimens of equal size and geometry shown in Fig. 2.

Fig. 2. Geometry

and dimensions

of polymer

specimen.

Nylon 6.6 is a popular polymer and has already been used successfully in gears, cams, pipe fittings and a number of domestic appliances. Some of the mechanical properties of Nylon 6.6 are given in Table 1. Acetal resins have better dimensional stability than Nylon 6.6, mainly because of their low moisture absorption under varying humidity conditions. Acetals are very versatile polymers which have found widespread applications in the automotive industry. The modified acetal (C9021K) used in the tests reported here has improved sliding properties because of an additive in the form of specially prepared chalk. Some of the properties of both types of acetal tested are summarized in Table 2.

TABLE

1

Mechanical

properties

of Nylon

6.6

Tensile

strength

(MPa)

Modulus (MPa) Compressive strength Modulus Flexural

(MPa) strength

Modulus

(MPa)

82.7 2450 (MPa)

(MPa)

87.4 1750 91.2 2450

186 TABLE

2

Mechanical

properties

of acetals

Yield stress (MPa) Flexural stress at 3.5% strain (MPa) Flexural strength (MPa) Elastic modulus (MPa) Ball indentation hardness, 30 s value (MPa)

62 69 94 2750 1 44

ii 2

0 !!li ?Yari

I 15

2.3. Test procedure 2.3.1. Preliminary tests Before embarking on the main series of tests it was necessary to decide upon the test conditions and parameters as well as the method of assessing the wear of the material. This was done by means of carrying out some preliminary tests involving the following. (1) Applying a load to the stationary polymer cone in contact with three steel balls in order to determine the onset of gross plastic deformation. (2) Running short tests under the load determined in the way described in (1) to find-but the speed at which there were no signs of polymer melting due to excessive heat generation. These two preliminary tests were carried out to determine the value of PV at which long duration tests could be run. A specimen to be tested was placed into the machine and subjected to different loads ranging from 38 to 154 N. The specimen was then removed and inspected in order to determine whether permanent deformation had taken place. It was found that at a load of 154 N the nylon specimen appeared to have only a slight indentation on its surface. Therefore the acceptable working range of load was 38 - 154 N. In order to determine the maximum permissible rotational speed the following procedure was adopted. The specimen was cleaned with a solvent, Genklene, and weighed on a precision balance. The steel balls and raceway were also cleaned to ensure there was no dirt or grit particles which might lead to abrasive wear, or any traces of oil or grease which might act as a lubricant. Then, the specimen was tested under a load of 38 N at 400 rev. min-’ for 0.5 h under dry conditions and at room temperature. At the end of the test, the specimen was carefully inspected for signs of melting or gross plastic deformation and then weighed again to find out whether any material had been removed from the surface. Since, after this test, there had been no signs of melting or plastic flow as well as no significant weight loss from any of the specimens tested, it was decided to repeat the tests with an increased load of 77 N. Exactly the same procedure was followed and, after 0.5 h, a slight but measurable loss in weight was recorded. The results, in terms of material loss, are given in Table 3.

187 TABLE

3

Weight losses of specimensa

Material

Weight before test (g)

Weight after test (g)

Acetal Nylon Modified

3.1114 2.3406 3.1330

3.1112 2.3403 3.1325

acetal

aThe accuracy

of weighing

was +O.OOOl g.

2.3.2. Main tests As a result of the preliminary tests the general test procedure, given below, was decided upon. (1) Each polymer specimen to be cleaned with Genklene and weighed prior to testing. (2) The raceway and steel balls to be cleaned in a similar way. (3) Each test to be carried out under the load of 77 N (corresponding hertzian stress, 34 MPa) and the speed of polymer cone of 400 rev. min-’ (corresponding rolling velocity of lower balls is 248 rev mine1 and associated spin angular velocity, for a given geometry, is equal to 9.14 s--l). (4) Tests were to be carried out under dry and lubricated conditions. Shell Vitrea Oil 100 was used as the lubricant. In addition to weighing, the specimen surface was inspected under an optical microscope and scanning electron microscope and photographs were taken in order to identify the prevailing mode of wear. The first test, under dry conditions, lasting 3 h, showed that the Nylon specimens underwent excessive wear and it was resolved to test Nylon in 2 h intervals. Lubricated tests were carried out as planned. A completely different behaviour was exhibited by the acetal specimens. Their lubricated wear after 3 h of tests was negligible and the machining marks could still be discerned on the surface. It was, therefore, decided to increase the load to 154 N (corresponding hertzian stress, 68 MPa). 2.4. Results 2.4.1. Dry conditions Test results obtained under dry conditions are shown in Fig. 3 where the weight loss of material is plotted against the number of load cycles. First, a significant weight loss incurred by the Nylon should be noted. Damage in the form of deep pits and cracks could be seen on the surface of the wear track. Photographs taken with a scanning electron microscope show this more clearly (Fig. 4). It can be seen from these micrographs that cracking and flaking of the surface material has occurred. These features point to the fatigue nature of wear as the load on the contact was cyclic.

Fig. 3. Material loss in grammes as a function of number of load cycles. Test conditions: speed of the spindle, 400 rev mini’; load at the end of loading lever, 10 N; dry conditions; 0, Nylon 6.6; 0, acetal; 0, modified acetal.

(a)

(b)

Fig. 4. Scanning electron micrograph of the contact zone on the surface of a Nylon cone after 3 h testing at 400 rev min-‘, 10 N load applied at the end of loading lever and no lubrication.

The weight losses of the acetals (Fig. 3) were much less than that of the Nylon. On inspection, the surface of the contact track had a shiny appearance and no damage, such as that seen in Nylon, could be detected. The

189

photographs taken with the scanning electron microscope give a more detailed picture. Figure 5(a) shows the contact track which is smooth but no serious damage can be seen. Figure 5(b) shows, at a greater magnification, the same area. Cracks of a characteristic shape are clearly seen and could be the result of the spin of the three steel balls in contact with the conical polymer specimen. It is seen that the rate of weight loss at the beginning of the test is slightly slower than towards the end. This is especially true in the case of modified acetal. Moreover, the weight loss of modified acetal was less than that of acetal. Additional tests with Nylon were carried out in order to establish the time required to produce any visible surface damage. It was found that, under the test conditions adopted, damage with fatigue characteristics probably occurs after 1.5 - 2 h. 2.4.2. Lubricated conditions Results of lubricated tests are shown in Fig. 6. The general observation is that the overall performance of the polymers tested was improved by using the lubricant. In the case of the acetals, no weight losses were recorded after 3 h of testing. Consequently, it was decided to increase the load applied from 77 to 154 N. Under these conditions there was some very small weight loss but its magnitude was of the order of weighing accuracy. On inspection, the surface of the contact track has the same shiny appearance as that in the case of the dry tests. This dramatic improvement in performance of the acetals can be attributed to the elastohydrodynamic effects in the contact area. This supposition is quite plausible if the test conditions and mechanical properties of the polymers tested are taken into account. The Nylon showed improved performance when lubricated (Fig. 6); however, after 3 hours, damage of a fatigue nature had occurred. When tested for periods of 1 and 2 h there appeared to be no damage after 1 h but after 2 h some damage, although less developed than that occurring after 3 h, was detected. This is shown in Fig. 7 in the form of a scanning electron micrograph.

(a) Fig. 5. Scanning acetal cone after

(b) electron micrograph 9 h of testing under

of the contact zone on the surface of an ordinary the conditions as specified in Fig. 4 legend.

yj*1_-__L_i_l__i-.i

12

1LL

216

_ .-.. L_d_ 288

360

dl

432

504

516

J

--_1

MB

720

load cycles

Fig. 6. Material loss in grammes as a function of number of load cycles. Test conditions: speed of the spindle, 400 rev min ’ ; load at the end of loading lever, 10 N (for Nylon) and 20 N (for acetals); lubricated conditions; @, Nylon 6.6; 0, acetal; u, modified acetal.

(a)

(b)

Fig. 7. Scanning electron micrograph of the contact zone on the surface of the Nylon 10 N load applied at the end of loading lever. cone after 3 h testing at 400 rev min--‘, Lubricated conditions.

3. Concluding

remarks

The results presented, although of preliminary nature, clearly show that typical surface fatigue damage can occur in polymers during rolling con-

191

tact. This is especially true for Nylon 6.6. It was found also that the wear rate tended to increase after a certain amount of damage had occurred. Under both dry and lubricated conditions there was practically no damage during the first 1.5 h of testing. However, the damage had become widespread after 2 - 3 h of testing, in particular, under dry conditions. The acetals performed much better than Nylon. No serious damage could be detected in the contact area after tests carried out without lubrication. The modified acetal was shown to be even better than ordinary acetal. These results are rather encouraging although further tests are clearly required to validate them. The addition of a lubricant. improved the performance of all the polymers tested. For acetals the improvement was so substantial that an increased load had to be used to induce any damage. This improvement can be explained in terms of an elastohydrodynamic film generated in the contact area. It may be concluded that some engineering polymers have potential for use in rolling contact applications provided that the load and speed are carefully selected. Finally, it is rather surprising to find that the polymers tested under the conditions of rollling contact failed in a way typical for surface fatigue usually observed in metallic contacts.

References 1 E. H. Andrews, in W. Brown (ed.), Fatigue in Wiley, New York, 1969. 2 J. A. Manson and R. W. Hertzberg, CRC Critical 3 R. W. Hertzberg, Deformation and Fracture Wiley, New York, 1976. 4 C. C. Lawrence, Friction and wear of polymers as part of the B.Sc. degree, Dept. of Mechanical

Polymers,

Testing

of Polymers,

Vol. 4,

Reviews in Macromol. Science, 1973. Mechanics of Engineering Materials,

in rolling contact, A report submitted Engineering, Brunei University, 1988.