Wear, 150 (1991) 67-77 67

Friction and wear tests for elastomers

N. G. Payne and R. G. Bayer International Business Machines Corporation, Endicott, NY (U.U.)

(Received October 23, 1990; revised April 10, 1991; accepted May 2, 1991)

Abstract

A common use of elastomers in business machines is as rollers, controlling the motion of such items as checks, paper forms and printer ribbons. In these applications both the friction and the wear resistance characteristics of the elastomer are of major significance. In this paper, two tests, developed to perform such characterizations against paper and ribbon surfaces, are described. Two case histories in which these test methods were effectively employed in resolving design problems are discussed. In addition, data for several elastomers are presented and compared.

1. Introduction

Elastomers are frequently used to drive and stop ribbons, paper sheets and documents in business machines. Many things must be considered when selecting an elastomer for these applications. For example, the compatibility with ink and the marking characteristics of the elastomer are often concerns. However, the friction between the elastomer and the paper or ribbon surface, as well as the wear resistance of the elastomer, are generally the key concerns. The latter concern results from the slippage that frequently occurs between the two surfaces in these applications.

The genera1 wear and frictional characteristics of paper and inked ribbon are similar, as are the wear scars produced. To evaluate these characteristics [l], two tests have been developed and used to evaluate a number of materials. These test methods are presented in this paper and case histories in which the tests were utilized are discussed. In addition, data for several elastomers are given.

2. Apparatus and test method

2.1. Sample preparation The friction and wear tests utilize a common specimen design. A sample is

prepared by affixing the elastomer to an aluminum hub 2.54 mm wide by 15.8 mm in diameter with a mounting hole in the center (Fig. 1). The method of attaching the elastomer to the hub is optional, depending on the application and methods available or feasible. They may be bonded, cast or vulcanized. Again, depending on the application of the elastomer, the sample may be machined dr used as is, e.g. with molded surface, to perform the tests.

A large amount of sliding is needed to produce the wear necessary for evaluation. This is generated by using the Roshon [2] drum tester.

0043-1648/91/$3.50 0 1991 - Elsevier Sequoia, Lausanne

(a) @I Cc) Fig. 1. (a) The aluminum hub; (b) the hub with elastomer bonding; (c) typical wear scar produced in the test. The sliding direction is perpendicular to the hub axis. H is the maximum wear depth.

VIEW ELASTOMERIC B SPECIMEN

(NON-ROTATING)

Fig. 2. Drum tester A is an overview of the Roshon drum tester; B is the sliding interface. In the test the elastomeric roller is fixed so that it does not rotate.

This apparatus consists of a large circular drum (Fig. 2) and is described in ASTM Standard G 56-77 131. The drum is 1.2 m in diameter and 20 cm wide. Paper or inked ribbon is mounted on the drum periphery and is held taut by the tensioning device within the drum. The elastomer wear specimen is mounted on a cantilevered beam to which strain gauges are attached. The specimen is attached to the beam in a manner to prevent rotation, and the beam is attached to a movable table beneath the drum. The wear specimen is brought into contact with the paper or inked ribbon at a known load, which is monitored and adjusted by the use of the strain gauge attached to the beam. After the load has been adjusted, the drum rotation is started and the sliding occurs. The table is also motorized and, as the drum rotates, the table moves the wear sample across the face of the drum. The combined motions result in a helical

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path on the surface of the drum. This continually presents a new ribbon surface to the sample. At the completion of the test, the specimen is removed and the scar is examined and measured. Figure 1 shows the typical wear scar geometry produced in this test.

H, the depth of wear, is generally determined by comparing profilometer traces taken before and after the tests. If the wear is quite large, an optical comparator or shadow graph may be used in a similar manner.

The wear resistance of the specific elastomer is measured in terms of a wear coefficient K defined by the following equations [4]

where V is the volume of wear, P the load and S the distance of sliding. Therefore

K= (~IQ~HW

Put

where r (cm) is the radius of the roller, W (cm) is the width of the roller, H (cm) is the wear depth, P (g) is the load, v (cm s-) is the drum velocity and t (s) is the test time. The higher the value of k, the poorer is the wear performance.

The following sets of test parameters were selected as standard conditions for this test: for the inked fabric ribbon, a load of 1 N, a drum speed of 3000 mm s-l, a specimen feed rate of 0.25 mm s-l and a test duration of 30 min; for the paper, a load of 1 N, a drum speed of 363 mm s-, a specimen feed rate of 0.25 mm s-l and a test duration of 45 min. For typical elastomers, these parameters produced a wear scar that could be conveniently and accurately measured, without showing any evidence of thermal degradation. Since the coefficients of friction against inked ribbon are generally much lower than against paper, the tests with ribbon could be run at higher speeds without any adverse effects.

2.2. Friction test To determine the frictional properties of the elastomer, a reciprocating ball-plane

tester was used (Fig. 3) [S]. The apparatus consisted of a variable-speed reciprocating table to which the paper or inked ribbon was fixed. The elastomeric specimen was attached to a strain-gauged beam above the table in such a manner that it would not

/ STRAIN GAGED BEAM

/ I

ELASTOMERIC SPECIMEN (NON-ROTATING) PAPER/RIBBON

TABLE I

4 *

Fig. 3. Boden-Leben ball-plane tester. The paper or ribbon is tied to the reciprocating table. The roller is mounted so that it does not rotate.

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rotate. The test specimen was brought into contact with the paper or ribbon and the table was put into motion. The strain gauges allowed the monitoring and recording of the normal and frictional forces on a strip recorder. The following were the test parameters used to characterize the elastomer: loads of 0.5, 1 and 2 N, a stroke length of 13 mm, an average speed (60 Hz) of 25.4 mm mini and measurement over 3 cycles on the same surfaces. In this case the same values are used for both ribbon and paper. These values were selected for the following reasons.

The primary concern with friction in these applications is associated with the onset of slip and sliding under conditions of low relative velocity. Consequently, it was desirable to select test parameters that would allow the determination of a static coefficient of friction and a dynamic coefficient of friction for low sliding velocities. Initially, tests were done at the motion extremes allowed by the test apparatus and instrumentation for accurate measurement of the coefficient of friction. These resulted in similar behaviors and values and it was decided to use only a single stroke length and speed as a standard test.

Since loading conditions vary in roller applications, it was desirable to measure friction for several different loads. The three loads selected covered the range of loading conditions, i.e load per unit length, typical of these applications.

The frictional behavior of the elastomer was described in terms of an average value of the coefficient of friction, which is the ratio of the frictional force to the normal force. The average value is determined by averaging measurements of the coefficients at individual points in the three test cycles at the three different loads. An average value established in this manner has been found to be the most useful in engineering applications. However, the test procedure does allow for identification of possible load dependences, as well as the differentiation of kinetic and static coefficients of friction and the identification of stick-slip behavior.

2.3. Paper and ribbon specimens While the tests may be performed with any ribbon or paper, one specific paper

and one specific ribbon have been used as a basis to perform general comparisons and evaluations of elastomers. The paper was a bond paper. The ribbon was a general- purpose printer ribbon, which consisted of a nylon fabric impregnated with ink. Both materials may be described as being moderately abrasive, e.g., an abrasivity level of the order of 10e6 [l]. Tests with other paper and ribbon have been performed.

It has to be recognized that the values obtained with these tests are related to the specific paper and ribbon used in the evaluation. Different values will be obtained with different ribbons and papers. Since the abrasivity of paper and ribbon can vary with the relative humidity, the tests were generally conducted under the following conditions: 20-22 C and 35-45% relative humidity [l].

3. Case histories

The manner in which these two tests have been used to address engineering problems is illustrated by two case histories. In the first, a stuffer box drive roll, the concern is with friction and wear against a fabric printer ribbon. In the second, the concern is friction and wear against paper in a check sorter application.

3.1. Stuffer box One test application involved the selection of an elastomer for the drive roll of

a printer ribbon stuffer box (Fig. 4). In this application, both the friction between

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/CRITICAL REGION

Fig. 4. Illustration of the action of a harmonic stuffer box for ribbon. Additional information regarding the stuffer box may be obtained in ref. 6.

the roll and the ribbon and the wear resistance of the roll are of concern. High speed motion picture studies of the stacking process have shown that irregular stacking occurs when the ribbon roll cannot force the ribbon between the contact area of the roll and the ribbon on the stack. Prior to the ribbon passing this critical point, the ribbon profile is as illustrated in Fig. 4.

During this period the following inequality must hold if proper stacking is to be achieved

/_ulu--pAo-M>O (3)

where p is the coefficient of friction between the ribbon and the roller, A the area of contact between the ribbon and the roller, u the pressure between the ribbon and the roller, p the coefficient of friction between two pieces of ribbon, A the area of the ribbon surfaces that must slip against one another while the new fold is being slipped between the roller and the stack, (+ the pressure between the ribbon layer and M a force associated with the unrestricted movement of the fold. This expression is equivalent to the following

y>-$$+-&J CL

(4)

As an approximation, it is reasonable to assume that (T and d are approximately the same, and that M is smaller in comparison with the other resistance forces involved. Therefore,

Examination of the high speed film indicates that a significant amount of slip occurs in the stack, involving a total surface area of the ribbon greater than the contact area of the roller. Based on these visual observations, A/A is estimated to be approxi-

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mately 2. Hence, it can be seen that a requirement on the coefficient of friction between the roller and the ribbon is that it be at least twice that of the coefficient of friction between ribbon surfaces.

The coefficient of friction between ribbon surfaces was measured using the same apparatus as used for determining the coefficient of friction for the elastomer. However, the slider was modified for these tests. In this case, the slider consisted of a ribbon specimen wrapped around the elastomer specimen. Several tests were performed at different loads, and a nominal value of 0.2 was obtained for the coefficient of friction between ribbon surfaces. Therefore it is indicated that a minimum value of coefficient of friction for the roller for proper stacking is 0.40.

Experimentally, the coefficients of friction for a large number of materials were determined utilizing the friction test procedure described earlier. In this case, the ribbon used in the test was that used in the application. The results for several of the materials are listed in Table 1.

It should be noted that the tests were done at several different loads. However, none of these indicated a significant load dependence for these materials over the range tested. The values shown are average values obtained for the test at different loads.

Stuffing tests were performed on several of these materials. In these evaluations the single-component urethanes generally showed poor stuffing behavior, while EPT/ IIR (epichlorohydrin-butyl blend) and urethane B showed excellent stuffing charac- teristics. As can be seen by examining Table 1, the data are in excellent agreement with the theoretical estimate of a minimum value of 0.4 needed for proper stuffing.

High speed motion picture studies of the stuffing action also indicated that, while control of the ribbon is being transferred to and maintained by one of the rollers, the other roller continually slides against the ribbon stack. Because of the pressure in the ribbon stack, this rubbing against the ribbon results in wear of the roller. Consequently, it was essential to rank the candidate materials in terms of their resistance to abrasive wear by the ribbon.

To determine the abrasion resistance of the rollers, the wear test procedure was used to determine ribbon abrasiveness. Again, the actual ribbon was used in the test.

Utilizing wear data obtained in this manner and a linear wear law (eqn. (l)), e.g. wear proportional to the load and distance of sliding, to describe the abrasive wear situation, estimates of the field life may be obtained [l, 41.

On the assumption that the radius of the roller is uniformly reduced by an amount h as a result of wear, the volume of wear is

TABLE 1

Friction of ribbon drive roll material

Material Coefficient of friction

186 EPT/IIR (epichlorohydrin-butyl blend) 0.50 Single-component urethane, 50 Shore A durometer 0.36 Single-component urethane, 60 Shore A durometer 0.33 Single-component urethane, 70 Shore A durometer 0.37 Single-component urethane, 80 Shore A durometer 0.38 Two-component urethane, 80 Shore A durometer 0.32 Two-part cast urethane (A) 0.40 Two-part cast urethane (B) 0.45

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V=h-trWD (6)

where W is the width of the roller and D the nominal diameter. This assumes that h is much less than D/2. The sliding distance S associated with that wear would be the product of the surface speed v of the roller and the amount of time for which slipping occurs. If T is the total amount of time that the machine is on and /3 is the fraction of time over which slipping occurs, then

S=@T (7)

Substituting these into eqn. (l), the following equation results

h= PKPvT rWD (8)

K, the abrasive wear coefficient for the elastomer-ribbon combination, was determined from drum wear test data.

Measurements of the force exerted by the ribbon stack have indicated it to be in the range of 0.15 N, which would be distributed over three rolls. Therefore P was approximately 0.05 N. High speed picture analysis indicated that approximately half the operating time involves slipping against the stack. Hence, /3 is approximately 0.5. Analysis of the ribbon drive mechanism showed that the maximum wear that the roller can experience and still function was approximately half the thickness of the rubber, i.e. 1.3 mm. Wear beyond this would result in too large a clearance between the roller, and the traction would be lost.

The values of D and W for this application were 16.4 mm and 2.54 mm respectively. Wear tests on EPT/IIR result in a K value of 2.8X lo- mm2 N-l. Since the ink in the ribbon is the source of the abrasivity, K is lower for used ribbons [l]. Typically, the effective value of K over the life of the ribbon is half the value determined on an unused ribbon. Since in this application the ribbon is used repeatedly, this value was reduced to 4~ 10e5 mmm2 N-. Utilizing a normal stuffing speed of 15 cm s-l and eqn. (8), these values resulted in theoretical life of 225 h. Actual stuffer box tests with this ribbon indicated typical life to be in the range 100-300 h.

In addition to the concern with friction and wear, there were concerns with the ink compatibility and adhesion, as well as processing issues. These laboratory tests were used effectively to screen a large variety of materials, coupled with the additional concerns. Eventually urethane B was chosen and was estimated to have a lifetime of lo6 h. Field data have supported this selection.

3.2. Check sorter feed roll This situation is illustrated in Fig. 5. In this application the feed roll is used both

to stop and to start the check. High accelerations are involved, and slip can occur if the friction is not high enough. High speed photography indicated that the tendency for slipping is greater during acceleration rather than deceleration. This slip, together with microslip that may be associated with deformation, exposes the surface of the roller to wear. For each application it was therefore desirable to select materials on the basis of their coefficient of friction against paper and their resistance to abrasive wear by paper. The two tests discussed earlier provided a convenient way of screening materials for this application. The standard paper was used in this case.

Engineering studies of this application indicated that the elastomer was exposed to another mode of wear or damage, which necessitated the development of a third laboratory test. Examination of used rollers showed evidence of abrasive wear but, in

I I

Fig. 5. Illustration of the wear relation in the check sorter.

I 4 m 1 Fig. 6. Configuration used in the Boden-Leben tester to compare the fatigue wear properties of the materials.

addition, they also showed evidence of tearing, with relatively large catastrophic loss from the surface. This was diagnosed as a fatigue problem associated with the edge of the check engaging the roller during the deceleration portion of the cycle (see Fig. 5(a)). This mode of wear or damage was not simulated by the drum test used to determine resistance to abrasive wear.

As a way of simulating this mode, a steel platen was developed with a series of parallel, widely spaced grooves on its surface. The platen was placed in the ball-plane apparatus as the lower specimen and the roller pressed against it, as in the friction test. Test parameters were 4.5 N and 3 Hz with a 0.63 cm stroke. The degree of tearing after 10 h was used as a measure of resistance to this mode of wear. The test configuration is shown in Fig. 6.

In developing this test for tearing, several experiments were performed to select the parameters to ensure a reasonable test time, and that damage typical of the application was produced. Surface examination was used to verify this latter point.

These three tests were used to evaluate a variety of materials. The original material used in this application, a 50 Shore A durometer NBR rubber, was also tested and used as a reference. An improvement of ten in the life was desired.

7.5

TABLE 2

Friction and wear properties of feed roll material

NBR Millable gum 50 Shore A urethane

Modified Hardened NBR EPDM 60 Shore A 60 Shore A

Abrasive wear (depth-wear drum test) (mm) Coefficient of friction

0.066 0.053 0.058 0.051

1.22 1.03 0.860 0.720

Tear resistance Poor Good Good Very good

The results of these tests for three candidate materials are presented in Table 2.

While not exhibiting the best values in any one category, the millable gum urethane material has superior performance in all categories. Since all three aspects are important in the application, this material was selected as the best candidate to replace the original material.

In addition to the material consideration addressed by this type of testing, influences of other design parameters were addressed. In particular, the influence of elastomer thickness and roller diameter on life were considered. For fatigue, wear life is typically inversely proportional to some power n of the contact stress. Studies have shown that n can range from 2 to 20 [7-91. In the case of a more flexible coating mounted on a stiffer substrate the contact stress decreases as the thickness of the coating increases [lo, 111.

Consequently, the contact stress in the elastomer will tend to decrease as the thickness of the elastomer increases. In addition, the stress should also decrease as the diameter of the roller increases. For abrasive wear the principal relationship should be the same as those given by eqn. (8). It can be seen that increased thickness will also increase life in terms of abrasion. A similar increase is indicated for increases in diameter. However, since the slip velocity would tend to increase with increasing diameter, the effects tended to offset one another. Consequently, an increase in the thickness of the elastomer as well as in the diameter of the roller was recommended. Subsequent machine tests and field performance, incorporating both the recommended material and the changes in dimension, showed significantly improved life for these rollers.

4. Discussion

These two tests have been used not only to select an elastomer for a given application but also to investigate the influence of various processing parameters and compatibility with specific inks.

Process variables, such as cure time, temperature and rate, have a direct effect on the physical properties of the cured elastomer. The state of cure or optimum cure must be refined for the most important properties. The best level of cure for one property may not be best for the other properties. Consequently, friction and wear tests are needed to optimize processing aspects for these types of application.

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There are other aspects as well. The mixing or compounding may alter aging characteristics and need to be evaluated in a similar manner. Another concern is machining. To hold close tolerances, it is sometimes necessary to grind or machine the completed part to size. This is true with cylindrical rollers, which must be held to diameter and concentricity. This removes the skin or surface material, which may have slightly different characteristics from the material beneath the surface.

The effect of fillers on performance is also an element that is important. Additives or fillers have the ability to alter the physical properties of elastomers. For example, carbon blacks have the effect of stiffening or reinforcing. Silicas and fine blacks give better tear and abrasion strength.

These types of concern, as well as a variety of engineering situations, have called for a large number of evaluations. In Table 3 some results of these studies, which utilize the standard paper and ribbon, are presented.

Some additional data are also given for other papers and ribbons. The special ribbons and paper indicated in the table were unique for a specific application.

From examination of Table 3 it can be seen that the abrasive wear coefficients have similar ranges for ribbon and paper. Also, the wear resistance of the elastomer can vary over several orders of magnitude (see samples 6 and 15, and samples 8 and 14) and can vary with the abrasive media used (see samples 7 and 12).

The examples of the millable gum urethanes (see samples 10 and 11) also show the influence of process variables. The sensitivity to ink on the wear of the elastomer is illustrated by the data for EC0 (sample 12) and the millable gum urethane (sample 7 ). EC0 shows a variation of a factor of 28 while the millable gum urethane 2 factor of less than 2. In general, it has been found that the millable gum urethanes have superior wear resistance.

There is considerable difference between the frictional behaviors of ribbon and paper. Generally the coefficients of friction against ribbon range between 0.2 and 0.5,

TABLE 3

Wear and friction coefficients for several materials

Sam- Material

pie

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15

Wear (mm3 mm- N- ) Friction coefficient

Standard Standard Special Standard Standard ribbon paper ribbon ribbon paper

One-part cast urethane 50 dur One-part cast urethane 60 dur One-part cast urethane 70 dur One-part cast urethane 80 dur Two-part cast urethane Millable gum urethane 70 dur Millable gum urethane 50 dur Two-part cast urethane 70 dur Two-part cast urethane 50 dur Millable gum urethane (process 1) Millable gum urethane (process 2) EC0 (epichlorohydrin) process A EC0 (epichlorohydrin) process B Neoprene Butyl 186 EPT/IIR

2.9~ lo- 6.0~ 1O-9 5.2~ lo-

3.6 x lO-9 2.7 x IO- 6.0~ lo- 1.2x 10-s

3.0~ lo- 2.7 x lo- 4.0 x 10-s

4.7x10- 1.7x 10-7 1.6x10-+

7.4 x 10-x 2.8~10-~

0.36 0.33 0.37 0.38 0.32 0.98 0.45 1.12

0.86 0.72

0.51 1.18 0.46 0.91 0.50 0.47 0.49

1.00 0.50

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and against paper between 0.5 and greater than 1.0. Again, material variances and process variations are seen in the data. Also, similar friction does not imply similar wear (see samples 12 and 13), nor does similar wear imply similar friction (see samples 5 and 10).

It should be noted that the data given generally represent the average of several tests. Standard deviations were in the range lo%-20%.

5. Conclusion

The two test methods discussed have proved useful in resolving friction and wear problems with elastomers. They provided a means of ranking materials and investigating the effects of various processing parameters. In addition, they have provided the data used to predict field performance.

References

1 R. G. Bayer, Wear by paper and ribbon, Wear, 49 (1978) 147. 2 D. D. Roshon, Testing machine for evaluation of wear by paper, Weor, 30 (1974) 93-103. 3 ASTM Standard G 56-77, 1977. 4 E. Rabinowicz, Fricfion and Wear of Materials, Wiley, New York, 1965, pp. 167-180. 5 Friction and Wear Devices, American Society of Lubrication Engineers, Chicago, IL, 1976,

pp. 84-85. 6 E. F. Helinski, IBM J. Res. Dev., 23 (4) (1979) 411-415. 7 A. Schallamach, Relevant advances in knowledge of rubber, friction and tyre wear, Rubber

Chem. Technol., 41 (1968) 209. 8 I. V. Kraghelsky and E. F. Nepomnyashchi, Fatigue wear under elastic contact conditions,

Wear, 8 (1965) 8, 303, 9 V. K. Jain and S. Bahadur, Experimental verification of fatigue wear equation. In S. K.

Rhee, A. W. Ruff and K. C. Ludema (eds.), Proc. ht. Conf: on Wear of Materials, American Society of Mechanical Engineers, New York, 1981, p. 700.

10 D. Barovich, S. C. Kingsley and T. C. Ku, Stresses on a thin strip or slab with different elastic properties from that of the substrate due to elliptically distributed load, ht. 1. Eng. Sci., 2 (1964) 253-268.

11 T. C. Ku, S. C. Kingsley and J. H. Ramsey, Stresses in a thin slab with different elastic properties from that of the substrate due to distributed normal and shearing forces on the surface of the slab, ht. J. Eng. Sci., 3 (1965) 93-107.