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A new concept in friction testing
PolymerTesting 1 (1980) 9-25
A NEW
CONCEPT
IN FRICTION
TESTING
D. I. JAMES and W. G. NEWF_~
Rubber and Plastics Research Association o[ Great Britain, Shawbury, Shewsbury, Salop SY4 4 NR, UK
SUMMARY
Methods of measuring [fiction are reviewed and their shortcomings discussed. The advantages of using the wide range of test speeds, environmental conditions and recording facilities available on a tensile test machine are indicated and a new apparatus designed to fit a tensile tester is described. The design concepts are outlined in some detail and the factors limiting stick-slip motion are indicated. The apparatus can be used either for studies of materials or as an aid to product selection. Test results on a polyurethane rubber, covering a range of velocities and temperatures, illustrate the former, and tests on windscreen wiper blades under both wet and dry conditions illustrate the latter. Although primarily designed for testing the friction of polymers the apparatus is of universal application and can be used for measuring, against a chosen substrate, the coeJTlcients of friction of metals, fabrics, paper, glass, leather, or any other material capable of being mounted in a vertical plane. It is not suitable for particulate matter such as sand, but could be used, for example, for studying the effect of floor finishes on a range of flooring materials. 1.
INTRODUCTION
Friction is so much a part of everyday life that its importance and influence are often overlooked. Frequently the requirements are either that friction should be adequate or minimal, and manufacturers often rely on their experience of known materials to enable them to market a satisfactory product. Rarely is friction the only parameter governing the choice of material and the constraints placed on other requirements apparently leave little option but to accept the friction value as it is. The discerning manufacturer will recognise, 9 Polymer Testing 0142-9418/80/0001-0009/$02.25 1980 Printed in Northern Ireland
© Applied Science Publishers Ltd, England,
10
D . I . JAMES, W. G. N E W E L L
however, that an increased understanding of friction enables a proper choice to be made from modern materials and formulations, resulting in a better and more competitive product. For example, although electrical properties govern the choice of material for cable covering, De Coste ~ has pointed out that it is friction and tensile strength which govern the ease of pulling cables through long lengths of ducting. A reduction in surface friction reduces the limiting value of tensile strength needed in any given situation, or alternatively increases the number of cables which can be pulled into a given duct. There has long been a need for a simple and reliable test for assessing surface friction and the RAPRA/Daventest apparatus has been developed to meet this need. It is the result of many years experience in measuring the friction of polymers but is of universal application, except for particulate material such as soil and gravel and heterogeneous surfaces, such as a cobbled road, where the phases are coarser than the full range of travel allowed for in the instrument. 1.1. Background In any method of measuring friction there are two contacting surfaces, a method of creating relative motion between them and a means of measuring or indicating the resistance to motion. Static friction is a special case and may be likened to the force necessary to shear an adhesive joint. In many cases, particularly with rubbers, it is very sensitive to the previous history of the surfaces, the time they have been in contact and the rate at which the shear force is applied. 2-7 Unless all these factors can be controlled it is best not to quote 'static' friction but friction at an extremely low velocity, with an indication as to whether friction can be expected to rise or fall as velocity increases. Although frictional resistance is related to the normal load applied there is not always direct proportionality between these parameters. Nevertheless, it is customary to refer to a coefficient of friction, the ratio of frictional force to normal force, in accordance with practice established over the past three centuries. It is, perhaps, unfortunate that some rough and ready rules established by Amontons8 and Coulomb, 9 at about the time when Lavoisier was disproving the plogiston theory, have been blessed with the name 'Laws of Friction'. Schoolboys ever since, whilst giving the expected assent to these laws, have established to their own satisfaction that they do not hold for many things they have been asked to test, but especially rubber. Many workers x°-18 have given a more formal dissent, but perhaps the most succinct statement of all was given by Derieux 12 in 1934. In just a single page he demolished all preconceived ideas about the friction of rubber. It is surprising, therefore, that even today the old laws of friction are still quoted 19"2° as articles
A NEW CONCEPT IN FRICTION TESTING
11
believed by the faithflfl. As Braun and Brungraber have put it 21 'With regard to measurement of friction, there still persist some o u t m o d e d preoccupations which certainly had their justification in the past, but which need to be carefully reviewed and corrected'. T h e friction of any given material depends on the nature of the other contacting surface and varies with many factors such as temperature, velocity, surface finish, time of contact, etc. 22 In any test programme it is necessary either to specify single point test conditions very closely or to carry out measurements over a suitable range of conditions. The versatility of a universal tensile machine to which a climatic cabinet can be fitted makes it an ideal instrument for this purpose and the R A P R A / D a v e n t e s t apparatus makes use of the wide velocity range available. When testing polymers, some caution in the choice of test speed is needed, for it is difficult to obtain meaningflfl friction results at high velocities with bad thermal conductors because p o o r heat dissipation leads to marked temperature changes at the surface. U n d e r these conditions neither the test conditions nor the results can be quoted with any accuracy. T o avoid this difficulty it is advisable to work at low velocities when testing the friction of thermal insulators. Predictions of friction values at high velocities may then be made by using a relationship between velocity and temperature 23 which, it has been established, holds for many rubbery materials. Experiments of this type demand an apparatus with a ratio of highest to lowest velocity of about 1000 to 1, where the surfaces are enclosed in an environmental test chamber. However, some caution is needed in interpreting the data, as is discussed later.
2.
REQUIREMENTS
T h e requirements of any friction test apparatus may be summarised as follows: 1. 2. 3. 4. 5. 6. 7. 8.
The centre of the friction-measuring load cell must lie in the same plane as the two friction surfaces. It should be possible to apply a wide range of normal loads. The instrument must be capable of measuring a wide range of friction forces. It must be possible to test over a wide range of velocities including very low velocities. T h e instrument should maintain the chosen test velocity within closely defined limits. The apparatus should be rigid enough to minimise stick-slip. It should be possible to test over a wide temperature range. Conditions must be reproducible so that repeat tests give similar test
12
D.I. JAMES, W. {3. NEWELL
traces. (These may not always be identical as the act of testing may alter a surface sufficiently to change the friction on repeat testing.) . It is convenient if the apparatus will accept surfaces of widely different thicknesses and shapes. 3.
OTHER TEST APPARATUS
Commercial friction testing devices may be broadly classified under the following headings: 1. 2. 3. 4. 5.
Inclined plane. Towed sled with (a) constant towing force; (b) constant velocity drive. Pendulum-type testers which are especially suited to measuring the 'skid resistance' of roads or 'slip index' of installed floors or sports surfaces. Articulated strut devices, mostly used for floors. Specialist apparatus.
The inclined plane and towed sled are very well-known methods 24"2s and commercial units are available. Other constant velocity methods are the Horizontal Pull Slipmeter26"27 and the Topaka Tester. 28 The best-known pendulum tester is the British Road Research Laboratory Portable Skid Tester 29 developed from the earlier Sigler Tester a° but mention should also be made of the apparatus described in Federal Test Method No. 7121. 31"32 The most widely used strut device is the James Machine33 (not to be confused with the R A B R M research apparatus described by D. I. James 34) but it is probable that the newer NBS Brungraber test35 will achieve equal importance in the USA. Numerous specialist pieces of equipment have been tried, for example those described by Westover and Vroom 36 and P r e i s s , 37 o r more recently those described by Reed and Mahon 38 and Majcherczyk.39 In a somewhat controversial paper Bikerman4° describes several other test methods, but it is doubtful if all the equipment is commercially available. The principles governing friction measurement can be best understood by considering the two dassical methods, namely the inclined plane and the towed sled.
3.1. Inclined plane A moveable sled of known mass covered in one of the materials under test rests on a plane surface covered in the other test material. To measure static friction the plane is raised until the sled just slides down the slope, when the tangent of the angle of inclination gives the coefficient of static friction. The method is necessarily approximate as it is difficult to judge when motion begins. For this reason it has sometimes been recommended~ that the sled be
A NEW CONCEPT IN FRICTION TESTING
13
given an impulse to start the motion. The friction measured is then kinetic friction at the velocity of travel. T h e stability of motion down an inclined plane depends on friction increasing as velocity increases. U n d e r these conditions the stable velocity achieved depends entirely on the friction/velocity characteristics of the surfaces and it is not possible to test at a stated velocity. If, on the other hand, friction decreases as velocity increases then runaway conditions exist and test results are extremely erratic. It is important to realise that the results obtained are not applicable to conditions which are markedly different from the test conditions. Tests are usually carried out at room temperature only and the test velocity cannot be controlled or defined, but can sometimes be measured. 12 Additionally, the normal force is not constant but decreases as the inclination of the plane increases. 3.2. Towed sled In those cases where the sled is towed by a constant force (e.g. a hanging weight) some of the same criticisms apply and stable motion is possible only under circumstances when friction increases as velocity increases. Using a m o t o r it is possible to tow the sled at a nominally constant velocity. If friction increases as velocity increases then steady readings will occur, but the reverse conditions are unstable leading to stick-slip. 41 The magnitude of the stick-slip vibrations depends on the spring constants and damping of the system, and in those cases where a cord or wire is used to tow the sled the stick-slip will be largely uncontrollable. If, in order to use the sled on a tensile tester, the cord is used to translate a vertical into a horizontal motion, 2S this will introduce an error into the measuring system, as the load cell will measure additionally any friction in the pulley system. T o minimise stick-slip 41 it is necessary to make the system as stiff as possible, avoiding the use of cord or wire to tow the sled. It follows, therefore, that if a vertical tensile tester is to be used the system needs to be mounted directly in the machine with the friction surfaces in a vertical plane. This is the principle on which the R A P R A / D a v e n t e s t t apparatus was designed. 4.
BASIC DESIGN OF NEW APPARATUS
T h e apparatus is represented diagrammatically in Fig. 1. The U-shaped arm A is firmly bolted to the crosshead B of the tensile tester while the sample plate C carrying one of the friction surfaces D is suspended from the load cell E which is mounted on the frame F of the tensile tester. The other friction surface G is mounted on the detachable block H fixed to the U-shaped arm A. t A commercial version of this apparatus is available from Davenport (London) Limited, Tewin Road, Welwyn Garden City, Hertfordshire AL7 1AQ.
14
D.I. JAMES, W. G. NEWELL
IL~c0 [E
Fig. I.
l
Diagram showing the component parts of the RAPRA/Daventest friction apparatus.
A NEW CONCEFr IN FRICTION TESTING
15
Weight W hanging from the cranked lever arm J, which is pivoted about bearings at K, enables a normal force to be applied to the friction surfaces through the two freely rotating bearings L. As the crosshead B moves down at one of the velocities available on the tensile test machine, the friction force between surfaces D and G is transmitted to the load cell through the joints M, N and the connecting rod R. Friction force is then indicated on the chart recorder in use with the tensile machine. 4.1. Important details When friction testing is carried out with the friction surfaces mounted in a horizontal plane a weight can be used directly to apply the normal force between the surfaces. With vertically mounted samples this is not possible and a choice has to be made between a calibrated spring, a hydraulic system or a weight applied via a pivoted cranked lever. There are errors inherent in all these methods, but the basic simplicity of the weight system is attractive. This was the method chosen for the RAPRA/Daventest apparatus and it was recognised from the outset that very free running bearings would be needed for the cranked lever assembly in order to minimise error at low values of applied force. If, additionally, the normal force applied to the sample is to be independent of the position of the crank then the two arms must be orthogonal. This is easily seen by reference to Fig. 2.
Wa cos 0 = Nb cos Only if O= ~b is N independent of the position of the cranked arm. Hence the two arms a and b must be orthogonal.
W Fig. 2.
Relationship between the weight applied and the normal force ael~ng on the sample.
16
D.I. JAMES, W. G. NEWELL
Fig. 3.
Arrangement for testing the residual friction in the rollers.
A NEW CONCEPT IN FRICTION TESTING
17
Obviously, the normal force N is Wa/b, where a and b are the effective lengths of the lever arms. Additionally, the lever assembly must be balanced so that when no weight W is applied, the normal force is zero. The rolling contact between bearings L and sample carrier C leads to a residual friction error which is picked up by the load cell and recorded. It is possible to measure the residual friction by mounting a block T with two ground surfaces instead of sample carrier C, and a bearing V, similar to L, instead of sample carrier H. This arrangement is illustrated in Fig. 3. As the crosshead moves down the measured friction is then 1.5x the value of the residual friction encountered in normal testing. It has been found that high quality, unoiled, clean bearings give a residual friction which is barely detectable. One of the conditions listed previously is that the contacting friction surfaces G and D must contain the line of action of the load cell E. This implies that the centre of gravity of the sample carrier assembly must lie in the surface of sample D, in order that this surface will hang vertically below load cell E. To achieve this the sample carrier C is of U-shaped horizontal section as illustrated in Fig. 4. The connecting rod R is fastened to a bridge joining the two arms of the U, as shown in Fig 1. Samples of different thicknesses are accommodated by using suitable packing plates P behind either sample G or sample D as appropriate. Non-uniformity in sample thickness is allowed for by pivoting sample G about a vertical axis (not shown in the simplified diagram labelled Fig. 1). The carrier C is so designed that if sample D is steel then the surface hangs exactly in the line of action of the load cell. If sample D is some other material, the error brought about by the change in mass is small, but the carrier will then hang so that the two test surfaces are slightly separated and a small normal force is needed to bring them together. At large loads this small force may be neglected, but at light loads this residual force to bring about contact can be measured. The small displacement is taken up by the joints N and M. The upper joint N should have universal freedom, but it is not immediately obvious that the lower joint M should have only one direction of rotation with its axis parallel to that of the bearings L. This avoids sample D twisting during testing because of uneven friction across the contacting surfaces.
5.
EFFECTOF VELOCITYAND TEMPERATUREON THE FRICTIONOF POLYMERS
The friction of all polymers depends on velocity but the influence of velocity cannot be isolated from that of temperature for two reasons. First, because
18
D.I. JAMES, W. G. NEWEJ~
I
Load cell line of action
/P
c Plan view Fig. 4.
Plan view of a sample holder. D, one of the friction surfaces; P, packing piece to bring the sample surface into alignment with the load cell.
polymers are bad thermal conductors, any heat developed at the interface it likely to cause local temperature changes---an effect which is bound to be more marked at high velocities. Secondly, in those cases where friction is dominated by a hysteresis loss mechanism the position of the loss peaks is a function of both velocity and temperature, an increase in velocity being equivalent to a drop in temperature. Grosch23 has published master curves for several rubbers showing the coefficients of friction at room temperature over a very wide range of velocities, predicted from data obtained at a number of temperatures and a limited range of velocities. The general form of these curves is illustrated in Fig. 5. However, the values of friction observed experimentally at high velocities will differ from those predicted theoretically because of the temperature rise which is induced by local heating. Thus although the room temperature master curve applies at low velocities, if the equilibrium temperature reached
19
A NEW CONCEPT IN FRICrION TESTING I
I
I
I
* ¢-
o .~_ 2.0
I/
rised nllturlil rubber
/ / .2 1.0 0 ~/// 0
\ Acrylonitrile' ~ e n e rubbe
I
--4
I
I,
I
0
4
8
LOGvelocity Fig. 5.
Master curves for coefficient of friction. Values at high velocity deduced theoretically. (From data published by Grosch. 23)
at high velocities is t °C, then it is the master curves at t °C which should be used at high velocities. In general this has the effect of broadening the friction/log velocity peak, as shown in Fig. 6, illustrating that at high velocities the real value of the friction of acrylonitrile-butadiene rubber against glass is likely to be a good deal higher than that predicted from the 20 °C data shown in Fig. 5. This shows the value of carrying out friction tests under conditions as close as possible to those found in service. For this reason the RAPRA/Daventest equipment has been designed to fit into a climatic cabinet so that testing can be carried out at any of the velocities available on the tensile test machine over a temperature range of - 4 0 ° C to +150°C. i
I
I
I
I 4
I 8
=L
"~ 2.0 ~1.0 o
7J''/ i --4
I 0
Log velocity Fig. 6.
Master curves for coefficient of friction. , D e d u c e d from data at low velocities; - - -, likely experimental curve. (From data published by Grosch. 23)
20
D.I. JAMES, W. G. NEWIELL
In order to obtain consistent results agreeing with those found in service it has been found advisable to separate the surfaces on the return stroke using an automatic mechanism which can be mechanically, electrically, or pneumatically operated (omitted from the simplified diagram shown in Fig. 1). This has been found to give more consistent results than those obtained with a reciprocating contact. 6.
APPLICATIONS AND TYPICAL RESULTS
Friction tests conveniently fall into two groups: 1. 2.
Studies of materials under a wide range of test conditions as an aid to design. Comparative tests on manufactured articles as an aid to product selection.
Although friction is rarely the only property being considered it is inappropriate to discuss here the wider implications of material or product selection, and friction alone will be considered. The general principles applying to the study of any material will be illustrated by considering tests carried out on a polyurethane rubber, one of a class of rubbers important for their outstanding wear characteristics. Product selection will be illustrated by reference to tests on windscreen wiper blades under dry and wet conditions. 6.1. Condition of surfaces Because friction varies with surface condition it is customary to remove mould release agents, grease and other contaminants, by grinding away the original surface. As testing proceeds the ground surface becomes more polished and wear products contaminate the surface. This leads to a dilemma-should the surface be ground before each test run or should testing be continued until equilibrium conditions are reached. Both approaches have been used but we prefer running until equilibrium is reached, as this more nearly represents service conditions. This takes about fifty test runs during which time friction gradually changes as indicated in Fig. 7. Once the surfaces have been run-in, agreement between successive test runs at any velocity is extremely good and, by way of illustration, the machine can be set so that the recorder pen follows exactly the same trace as the previous run. 6.2. Tests on a polyurethane rubber Although the abrasion resistance of polyurethane rubbers has been extensively studied their frictional properties are less well known. A few results were given by Westover and Vroom 36 and by Preiss 37 and the results given here
A NEW CONCEPT IN FRICrION
21
TESTING
1.e I
J
c 1.6 o o
o "~ 1.4 e
o 1.2
1.0
1
0
2
10
I
30
I
40
I
50
Number of runs
Fig. 7. Effect of running-in on the coefficientof friction of a polyurethane rubber.
were obtained with one of a n u m b e r of polyurethane rubbers examined during recent development work at R A P R A . Friction against steel was measured at velocities of 0-02, 2-0 and 50 cm/min at a number of discrete temperatures between 20 °C and 100 °C. The results are presented in Fig. 8 where coefficient of friction is plotted against temperature for each of the three test velocities. It is at once obvious that a single reading obtained, at one velocity, say, from an inclined plane at 20 °C could give a very misleading impression of the coefficient of friction to be expected at an entirely different velocity and temperature. It can be seen that at all temperatures friction increases as velocity increases, an effect which is particularly noticeable at room temperature. Comparison with Fig. 5 shows that the velocity range under consideration lies to the left of the peak at all the temperatures used. Construction of a full master curve covering twelve decades of velocity involves a considerable experimental effort. This type of plot is of great theoretical importance, but as has been explained, the predicted high velocity values are in error and in design work it has generally been found that a limited presentation such as that given in Fig. 8 is more satisfactory.
22
D.I. JAMES, W. G. NEWELL
1.8 1.6
1.4 -~ 1.2 0
g ~.0 0 0
0.8 0.6 50 ore/rain 0.4 ' 2.0 cm/mln 0.02 crn/mln 0.2 0
Fig. 8.
i 10
I
I
I
20
30
40
I
I
I
50 60 70 Temperature "C
I
I
|
80
90
100
Coefficient of friction of a polyurethane rubber against steel, measured at different temperatures and velocities, plotted against temperature.
6.3. Tests on wiper blades Four sets of blades labelled A, B, C and D were examined. One of each pair was kept in the original packet and the other taken out and stored in the laboratory atmosphere for 1 month before testing. Each blade was cut into four equal pieces which were then fitted one above the other in a specially constructed holder replacing block H in Fig. 1. A piece of 32 oz glass forming the other friction surface was mounted in holder C. Tests on cars indicated that the spring force fell from about 350 g for a new car down to about 125 g after some years of use. On this basis a normal force of 290 g was taken as being realistic and this was used throughout all the tests. Since in practice the velocity of traverse varies considerably, 5 cm/min and 50 cm/min were taken as representing a reasonable range. The blades were passed down the glass three times at 50 cm/min and three times at 5 cm/min while continuous records of friction were taken. These tests were then repeated under wet conditions, a fine spray being used to wet thoroughly the blade and the glass before and during testing. While it is clear that friction under dry conditions should be as low as possible, it is not at all obvious what represents a good performance when wet. Since an efficient blade is intended to remove most of the water from the windscreen it would seem reasonable that the nearer the ratio of wet friction to dry friction approaches unity, the more efficient the wiper blade.
23
A NEW CONCEPT IN FRICTION TESTING TABLE 1 COEFFICIENTSOF FRICTIONOF WIPERBLADESUNDERTHE CONDITIONSSTATED
New blade Sample A B C D
Velocity (cm/min)
Dry
Wet
5 50 5 50 5 50 5 50
0.52 0.53 0-53 0-50 0.47 0"54 0-72 0.65
0.43 0-44 0.62 0.50 0.54 0-59 0-62 0"57
Wet~Dry 0.83 0-83 1.17 1.00 1.15 1.09 0-86 0.88
Stored blade Dry
Wet
0.70 0.70 0-83 0.99 0.90 1.03 0-81 0.83
0-64 0.67 0.75 0-75 0-71 0.76 0.65 0.65
Wet~Dry 0-91 0.96 0.90 0.76 0.79 0.74 0.80 0-78
Mean figures for wet and dry friction, at both velocities, for both new and stored blades are presented in Table 1, together with the calculated ratio of wet to dry friction in each case. Figures for the ratio of wet to dry friction which are greater than 1 (Samples B and C) probably indicate that the low initial friction encountered with these blades is due to a surface contaminant which is quickly removed under wet conditions. The very high figures for the corresponding stored blades, with wet to dry ratios less than unity, confirm that the Iow initial friction values are transitory and these blades would quickly give trouble. Of the remaining two blades, sample A is probably superior since it not only has lower initial friction but this advantage is retained on storage, when additionally, the wet to dry friction ratio is close to unity.
7.
CONCLUSION
The two sets of experiments described illustrate the versatility of the new apparatus and indicate its usefulness both in design work and in component selection. It is clear from Fig. 8 that single value friction tests are inadequate and can give totally misleading results. The RAPRA/Daventest apparatus has been designed to accept a variety of materials and components and enables friction to be measured accurately over a wide range of experimental conditions. REFERENCES 1. 2.
DE C o s ~ , J. B. (1969). Friction of vinyl chloride plastics. SPE Journal, 25, 67-71. BAR'rm~v, G. M. (1954). Theory of dry friction of rubber. Dokl. Akad. Nauk. SSSR, 96 (6), 1161--4.
24 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
D.I. JAMES, W. G. NEWELL RA~, S. B. and LAva,'error, V. V. (1956). Evidence for static friction of uulubricated rubber. Dokl. Akad. Nauk. SSSR, 10g (3), 461-4; RABRM Translation N o . 591, Sept. 1957. B A R ~ V , G. M. and LAVR~a~mv, V. V. The nature of static friction in elastomers. Rubber Chem. Technol., 34, 1961, 461-5. From Vys. Soed, 2 (2), 1960, 238-42. LA~v, V. V. Static friction and the law of rubber friction. Rubber Chem. Technol., 36, 1963, 365-76. From Plaste Kautchuk, 9 (6), 1962, 282-7. ROBF_~TS,A. D. and THOM~, A. G. (1976). Static friction of smooth clean vulcanised rubber. NR Technol., 7, Part 2, 38-42. ROBF~TS,A. D. (1979). Looking at rubber adhesion. Rubber Chem. Technol., 52,, 23-42. AMO~rroNs, M. (1699). De la r6sistance caus~es dans les machines. Histoire de l'Acad~mie Royale des Sdences avec les M~moires de Math~matique et de Physique, Paris, p. 206. COULOMb, C. A. (1785). M~moires de Mathdmatique et de Physique de l'Acaddmie Royale des Sciences, Paris, 10, 161. HUNTER,R. B. (1930). A method of measuring frictional coefficients of walk-way materials. 3.. Res. National Bureau Standards, 5, 329-47. ROTH,F. L., DmSCOLL, R. L. and HOLT, W. L. (1942). Frictional properties of rubber. 3". Res. National Bureau Standards, 28, 439-62. DERmUX, J. B. (1934). The coefficient of friction of rubber. 3. Elisha Mitchell Sci. Soc., 50, 53--5. Rubber Chem. Technol., 8, 1935, 441~-2. HURRY, J. A. and PROCK J. D. (1953). Coefficients of friction of rubber samples. India Rubber World, 128 (5), 619-22. S ~ c r t , A. (1952), The load dependence of rubber friction. Proc. Phys. Soc. B, 65, 657--61. ~ c ~ , A. (1953). The velocity and temperature dependence of rubber friction. Proc. Phys. Soc. B, 66, 386-92. BIANCA, D. (1967). Sliding friction and abrasion of elastomers~ Rubber World, 1 5 6 (6), 65-7. Ksmsra, K. T. Sliding friction of elastomers, Paper A3, 6th Intern. Conf. Fluid Sealing, Munich, February 1973. LANCASTER, J. K. (1973). Basic mechanisms of friction and wear of polymers. Plastics Polymers, 297-306. LuIymN, J. A. J. and ProD, H. A. (1973). Slip resistance of soling materials and top-pieces. Satra Information Publication I.P. 141. PoorlY, R. W. (1978). Measurement of frictional properties of footwear sole and heel materials. Walkway Surfaces: Measurement of Slip Resistance. ASTM STP, 649, 11-20. BRAUn, R. and BRtr~ORABER, R. J. (1978). A comparison of two slip-resistance testers./bid, 49-59. JAMES, D. I. (1973). Variation in the friction of rubbers and plastics. R A P R A Members Journal, 1 (7), 170-5. GROSCH, K. A. (1963). The relation between the friction and visco-elastic properties of rubber. Proc. Roy. Soc. A., 274 (1356), 21-39. BS 3424, 1961 (revised 1973). Methods of test for coated fabrics. Method 1 2 Determination of surface drag. ASTM D-1894-63. Standard method of test for coefficients of friction of plastic film. latvar~, C. H. (1967). A new slipmeter for evaluating walkway slipperiness. Materials Res. Standards, 7 (12), 535-42. RoumsoN, W. H. and KOPF, R. E. (1969). Evaluation of the Horizontal Pull Slipmeter. Materials Res. Standards, 9 (7), 22-4. WUaJAMS,W. D., SMrrR, J. A. and DRAUGELXS,F. J. (1972). TOPAKA, A new device and method for measuring slip resistance of polished floors. Soap~Cosmetics/Chemical Specialties, July. Gn.~s, C. G., SABEY, B. E. and CARDEW, K. H. F. Development and performance of the portable skid-resistance tester. Road Research Technical Paper No. 66 (1964); ASTM STP 326 (1962). SI~tJ~, P. A., GEm, M. N. and BooteE, T. H. (1948). Measurement of the slipperiness of walkway surfaces. & Res. National Bureau Standards, 40, 339-46.
A NEW CONCEPT IN FRICTION TESTING 31. 32. 33. 34.
35. 36. 37. 38. 39. 40. 41.
25
Federal Test Method No. 7121, Standard No. 501(a). Dynamic coefficient of friction. 15th June 1966. JABLONSKY,R. D. (1978). Standardization of test measurement of floor slipperiness. Walkway Suryaces: Measurement of Slip Resistance. ASTM STP 649, 21-39. Jinx.s, S. V. (1944). What is a safe floor finish? Soap Sanitary Chemicals, 20, 111-15. JAMES,D. I. (1961). Measurement of friction between rubber like polymers and steel. J. Sci. Inst., 38 (7), 294-9. BRUNGRABER,R. J. (1977). A new portable tester for the evaluation of the slip-resistance of walkway surfaces. National Bureau of Standards Technical Note 953. WESTOVER, R. F. and VROOM, W. I. (1963). A variable speed frictionmeter for plastics, rubbers, metals and other materials. SPE Journal, 19 (10), 1093-9 (Adopted by ASTM D-3028-72, reapproved 1978). PREXSS,D. M. (1965). Coefficient of friction of elastomers. Rubber Journal, 14"! (10), 42-51. REED, M. E. and MAHON, R. D. (1978). Description of the National Institute for Occupational Safety and Health (NIOSH) Universal Friction Testing Machine (UFTM). Walkway Surfaces: Measurement of Slip Resistance. ASTM STP 649, 60-70. MAJC~RCZYK, R. (1978). A different approach to measuring pedestrian friction: The CEBTP skid meter./bid, 88-99. BIKERMAN, J. J. (1974). Sliding friction of polymers. J. Macromol. Sci.--Revs. Macromol. Chem., CII (1), 1-44. RA~INOWITZ, E. (1958). The intrinsic variables affecting the stick-slip process. Proc. Phys. Soc., 71, Part 4 (460), 668-75.
A NEW
CONCEPT
IN FRICTION
TESTING
D. I. JAMES and W. G. NEWF_~
Rubber and Plastics Research Association o[ Great Britain, Shawbury, Shewsbury, Salop SY4 4 NR, UK
SUMMARY
Methods of measuring [fiction are reviewed and their shortcomings discussed. The advantages of using the wide range of test speeds, environmental conditions and recording facilities available on a tensile test machine are indicated and a new apparatus designed to fit a tensile tester is described. The design concepts are outlined in some detail and the factors limiting stick-slip motion are indicated. The apparatus can be used either for studies of materials or as an aid to product selection. Test results on a polyurethane rubber, covering a range of velocities and temperatures, illustrate the former, and tests on windscreen wiper blades under both wet and dry conditions illustrate the latter. Although primarily designed for testing the friction of polymers the apparatus is of universal application and can be used for measuring, against a chosen substrate, the coeJTlcients of friction of metals, fabrics, paper, glass, leather, or any other material capable of being mounted in a vertical plane. It is not suitable for particulate matter such as sand, but could be used, for example, for studying the effect of floor finishes on a range of flooring materials. 1.
INTRODUCTION
Friction is so much a part of everyday life that its importance and influence are often overlooked. Frequently the requirements are either that friction should be adequate or minimal, and manufacturers often rely on their experience of known materials to enable them to market a satisfactory product. Rarely is friction the only parameter governing the choice of material and the constraints placed on other requirements apparently leave little option but to accept the friction value as it is. The discerning manufacturer will recognise, 9 Polymer Testing 0142-9418/80/0001-0009/$02.25 1980 Printed in Northern Ireland
© Applied Science Publishers Ltd, England,
10
D . I . JAMES, W. G. N E W E L L
however, that an increased understanding of friction enables a proper choice to be made from modern materials and formulations, resulting in a better and more competitive product. For example, although electrical properties govern the choice of material for cable covering, De Coste ~ has pointed out that it is friction and tensile strength which govern the ease of pulling cables through long lengths of ducting. A reduction in surface friction reduces the limiting value of tensile strength needed in any given situation, or alternatively increases the number of cables which can be pulled into a given duct. There has long been a need for a simple and reliable test for assessing surface friction and the RAPRA/Daventest apparatus has been developed to meet this need. It is the result of many years experience in measuring the friction of polymers but is of universal application, except for particulate material such as soil and gravel and heterogeneous surfaces, such as a cobbled road, where the phases are coarser than the full range of travel allowed for in the instrument. 1.1. Background In any method of measuring friction there are two contacting surfaces, a method of creating relative motion between them and a means of measuring or indicating the resistance to motion. Static friction is a special case and may be likened to the force necessary to shear an adhesive joint. In many cases, particularly with rubbers, it is very sensitive to the previous history of the surfaces, the time they have been in contact and the rate at which the shear force is applied. 2-7 Unless all these factors can be controlled it is best not to quote 'static' friction but friction at an extremely low velocity, with an indication as to whether friction can be expected to rise or fall as velocity increases. Although frictional resistance is related to the normal load applied there is not always direct proportionality between these parameters. Nevertheless, it is customary to refer to a coefficient of friction, the ratio of frictional force to normal force, in accordance with practice established over the past three centuries. It is, perhaps, unfortunate that some rough and ready rules established by Amontons8 and Coulomb, 9 at about the time when Lavoisier was disproving the plogiston theory, have been blessed with the name 'Laws of Friction'. Schoolboys ever since, whilst giving the expected assent to these laws, have established to their own satisfaction that they do not hold for many things they have been asked to test, but especially rubber. Many workers x°-18 have given a more formal dissent, but perhaps the most succinct statement of all was given by Derieux 12 in 1934. In just a single page he demolished all preconceived ideas about the friction of rubber. It is surprising, therefore, that even today the old laws of friction are still quoted 19"2° as articles
A NEW CONCEPT IN FRICTION TESTING
11
believed by the faithflfl. As Braun and Brungraber have put it 21 'With regard to measurement of friction, there still persist some o u t m o d e d preoccupations which certainly had their justification in the past, but which need to be carefully reviewed and corrected'. T h e friction of any given material depends on the nature of the other contacting surface and varies with many factors such as temperature, velocity, surface finish, time of contact, etc. 22 In any test programme it is necessary either to specify single point test conditions very closely or to carry out measurements over a suitable range of conditions. The versatility of a universal tensile machine to which a climatic cabinet can be fitted makes it an ideal instrument for this purpose and the R A P R A / D a v e n t e s t apparatus makes use of the wide velocity range available. When testing polymers, some caution in the choice of test speed is needed, for it is difficult to obtain meaningflfl friction results at high velocities with bad thermal conductors because p o o r heat dissipation leads to marked temperature changes at the surface. U n d e r these conditions neither the test conditions nor the results can be quoted with any accuracy. T o avoid this difficulty it is advisable to work at low velocities when testing the friction of thermal insulators. Predictions of friction values at high velocities may then be made by using a relationship between velocity and temperature 23 which, it has been established, holds for many rubbery materials. Experiments of this type demand an apparatus with a ratio of highest to lowest velocity of about 1000 to 1, where the surfaces are enclosed in an environmental test chamber. However, some caution is needed in interpreting the data, as is discussed later.
2.
REQUIREMENTS
T h e requirements of any friction test apparatus may be summarised as follows: 1. 2. 3. 4. 5. 6. 7. 8.
The centre of the friction-measuring load cell must lie in the same plane as the two friction surfaces. It should be possible to apply a wide range of normal loads. The instrument must be capable of measuring a wide range of friction forces. It must be possible to test over a wide range of velocities including very low velocities. T h e instrument should maintain the chosen test velocity within closely defined limits. The apparatus should be rigid enough to minimise stick-slip. It should be possible to test over a wide temperature range. Conditions must be reproducible so that repeat tests give similar test
12
D.I. JAMES, W. {3. NEWELL
traces. (These may not always be identical as the act of testing may alter a surface sufficiently to change the friction on repeat testing.) . It is convenient if the apparatus will accept surfaces of widely different thicknesses and shapes. 3.
OTHER TEST APPARATUS
Commercial friction testing devices may be broadly classified under the following headings: 1. 2. 3. 4. 5.
Inclined plane. Towed sled with (a) constant towing force; (b) constant velocity drive. Pendulum-type testers which are especially suited to measuring the 'skid resistance' of roads or 'slip index' of installed floors or sports surfaces. Articulated strut devices, mostly used for floors. Specialist apparatus.
The inclined plane and towed sled are very well-known methods 24"2s and commercial units are available. Other constant velocity methods are the Horizontal Pull Slipmeter26"27 and the Topaka Tester. 28 The best-known pendulum tester is the British Road Research Laboratory Portable Skid Tester 29 developed from the earlier Sigler Tester a° but mention should also be made of the apparatus described in Federal Test Method No. 7121. 31"32 The most widely used strut device is the James Machine33 (not to be confused with the R A B R M research apparatus described by D. I. James 34) but it is probable that the newer NBS Brungraber test35 will achieve equal importance in the USA. Numerous specialist pieces of equipment have been tried, for example those described by Westover and Vroom 36 and P r e i s s , 37 o r more recently those described by Reed and Mahon 38 and Majcherczyk.39 In a somewhat controversial paper Bikerman4° describes several other test methods, but it is doubtful if all the equipment is commercially available. The principles governing friction measurement can be best understood by considering the two dassical methods, namely the inclined plane and the towed sled.
3.1. Inclined plane A moveable sled of known mass covered in one of the materials under test rests on a plane surface covered in the other test material. To measure static friction the plane is raised until the sled just slides down the slope, when the tangent of the angle of inclination gives the coefficient of static friction. The method is necessarily approximate as it is difficult to judge when motion begins. For this reason it has sometimes been recommended~ that the sled be
A NEW CONCEPT IN FRICTION TESTING
13
given an impulse to start the motion. The friction measured is then kinetic friction at the velocity of travel. T h e stability of motion down an inclined plane depends on friction increasing as velocity increases. U n d e r these conditions the stable velocity achieved depends entirely on the friction/velocity characteristics of the surfaces and it is not possible to test at a stated velocity. If, on the other hand, friction decreases as velocity increases then runaway conditions exist and test results are extremely erratic. It is important to realise that the results obtained are not applicable to conditions which are markedly different from the test conditions. Tests are usually carried out at room temperature only and the test velocity cannot be controlled or defined, but can sometimes be measured. 12 Additionally, the normal force is not constant but decreases as the inclination of the plane increases. 3.2. Towed sled In those cases where the sled is towed by a constant force (e.g. a hanging weight) some of the same criticisms apply and stable motion is possible only under circumstances when friction increases as velocity increases. Using a m o t o r it is possible to tow the sled at a nominally constant velocity. If friction increases as velocity increases then steady readings will occur, but the reverse conditions are unstable leading to stick-slip. 41 The magnitude of the stick-slip vibrations depends on the spring constants and damping of the system, and in those cases where a cord or wire is used to tow the sled the stick-slip will be largely uncontrollable. If, in order to use the sled on a tensile tester, the cord is used to translate a vertical into a horizontal motion, 2S this will introduce an error into the measuring system, as the load cell will measure additionally any friction in the pulley system. T o minimise stick-slip 41 it is necessary to make the system as stiff as possible, avoiding the use of cord or wire to tow the sled. It follows, therefore, that if a vertical tensile tester is to be used the system needs to be mounted directly in the machine with the friction surfaces in a vertical plane. This is the principle on which the R A P R A / D a v e n t e s t t apparatus was designed. 4.
BASIC DESIGN OF NEW APPARATUS
T h e apparatus is represented diagrammatically in Fig. 1. The U-shaped arm A is firmly bolted to the crosshead B of the tensile tester while the sample plate C carrying one of the friction surfaces D is suspended from the load cell E which is mounted on the frame F of the tensile tester. The other friction surface G is mounted on the detachable block H fixed to the U-shaped arm A. t A commercial version of this apparatus is available from Davenport (London) Limited, Tewin Road, Welwyn Garden City, Hertfordshire AL7 1AQ.
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D.I. JAMES, W. G. NEWELL
IL~c0 [E
Fig. I.
l
Diagram showing the component parts of the RAPRA/Daventest friction apparatus.
A NEW CONCEFr IN FRICTION TESTING
15
Weight W hanging from the cranked lever arm J, which is pivoted about bearings at K, enables a normal force to be applied to the friction surfaces through the two freely rotating bearings L. As the crosshead B moves down at one of the velocities available on the tensile test machine, the friction force between surfaces D and G is transmitted to the load cell through the joints M, N and the connecting rod R. Friction force is then indicated on the chart recorder in use with the tensile machine. 4.1. Important details When friction testing is carried out with the friction surfaces mounted in a horizontal plane a weight can be used directly to apply the normal force between the surfaces. With vertically mounted samples this is not possible and a choice has to be made between a calibrated spring, a hydraulic system or a weight applied via a pivoted cranked lever. There are errors inherent in all these methods, but the basic simplicity of the weight system is attractive. This was the method chosen for the RAPRA/Daventest apparatus and it was recognised from the outset that very free running bearings would be needed for the cranked lever assembly in order to minimise error at low values of applied force. If, additionally, the normal force applied to the sample is to be independent of the position of the crank then the two arms must be orthogonal. This is easily seen by reference to Fig. 2.
Wa cos 0 = Nb cos Only if O= ~b is N independent of the position of the cranked arm. Hence the two arms a and b must be orthogonal.
W Fig. 2.
Relationship between the weight applied and the normal force ael~ng on the sample.
16
D.I. JAMES, W. G. NEWELL
Fig. 3.
Arrangement for testing the residual friction in the rollers.
A NEW CONCEPT IN FRICTION TESTING
17
Obviously, the normal force N is Wa/b, where a and b are the effective lengths of the lever arms. Additionally, the lever assembly must be balanced so that when no weight W is applied, the normal force is zero. The rolling contact between bearings L and sample carrier C leads to a residual friction error which is picked up by the load cell and recorded. It is possible to measure the residual friction by mounting a block T with two ground surfaces instead of sample carrier C, and a bearing V, similar to L, instead of sample carrier H. This arrangement is illustrated in Fig. 3. As the crosshead moves down the measured friction is then 1.5x the value of the residual friction encountered in normal testing. It has been found that high quality, unoiled, clean bearings give a residual friction which is barely detectable. One of the conditions listed previously is that the contacting friction surfaces G and D must contain the line of action of the load cell E. This implies that the centre of gravity of the sample carrier assembly must lie in the surface of sample D, in order that this surface will hang vertically below load cell E. To achieve this the sample carrier C is of U-shaped horizontal section as illustrated in Fig. 4. The connecting rod R is fastened to a bridge joining the two arms of the U, as shown in Fig 1. Samples of different thicknesses are accommodated by using suitable packing plates P behind either sample G or sample D as appropriate. Non-uniformity in sample thickness is allowed for by pivoting sample G about a vertical axis (not shown in the simplified diagram labelled Fig. 1). The carrier C is so designed that if sample D is steel then the surface hangs exactly in the line of action of the load cell. If sample D is some other material, the error brought about by the change in mass is small, but the carrier will then hang so that the two test surfaces are slightly separated and a small normal force is needed to bring them together. At large loads this small force may be neglected, but at light loads this residual force to bring about contact can be measured. The small displacement is taken up by the joints N and M. The upper joint N should have universal freedom, but it is not immediately obvious that the lower joint M should have only one direction of rotation with its axis parallel to that of the bearings L. This avoids sample D twisting during testing because of uneven friction across the contacting surfaces.
5.
EFFECTOF VELOCITYAND TEMPERATUREON THE FRICTIONOF POLYMERS
The friction of all polymers depends on velocity but the influence of velocity cannot be isolated from that of temperature for two reasons. First, because
18
D.I. JAMES, W. G. NEWEJ~
I
Load cell line of action
/P
c Plan view Fig. 4.
Plan view of a sample holder. D, one of the friction surfaces; P, packing piece to bring the sample surface into alignment with the load cell.
polymers are bad thermal conductors, any heat developed at the interface it likely to cause local temperature changes---an effect which is bound to be more marked at high velocities. Secondly, in those cases where friction is dominated by a hysteresis loss mechanism the position of the loss peaks is a function of both velocity and temperature, an increase in velocity being equivalent to a drop in temperature. Grosch23 has published master curves for several rubbers showing the coefficients of friction at room temperature over a very wide range of velocities, predicted from data obtained at a number of temperatures and a limited range of velocities. The general form of these curves is illustrated in Fig. 5. However, the values of friction observed experimentally at high velocities will differ from those predicted theoretically because of the temperature rise which is induced by local heating. Thus although the room temperature master curve applies at low velocities, if the equilibrium temperature reached
19
A NEW CONCEPT IN FRICrION TESTING I
I
I
I
* ¢-
o .~_ 2.0
I/
rised nllturlil rubber
/ / .2 1.0 0 ~/// 0
\ Acrylonitrile' ~ e n e rubbe
I
--4
I
I,
I
0
4
8
LOGvelocity Fig. 5.
Master curves for coefficient of friction. Values at high velocity deduced theoretically. (From data published by Grosch. 23)
at high velocities is t °C, then it is the master curves at t °C which should be used at high velocities. In general this has the effect of broadening the friction/log velocity peak, as shown in Fig. 6, illustrating that at high velocities the real value of the friction of acrylonitrile-butadiene rubber against glass is likely to be a good deal higher than that predicted from the 20 °C data shown in Fig. 5. This shows the value of carrying out friction tests under conditions as close as possible to those found in service. For this reason the RAPRA/Daventest equipment has been designed to fit into a climatic cabinet so that testing can be carried out at any of the velocities available on the tensile test machine over a temperature range of - 4 0 ° C to +150°C. i
I
I
I
I 4
I 8
=L
"~ 2.0 ~1.0 o
7J''/ i --4
I 0
Log velocity Fig. 6.
Master curves for coefficient of friction. , D e d u c e d from data at low velocities; - - -, likely experimental curve. (From data published by Grosch. 23)
20
D.I. JAMES, W. G. NEWIELL
In order to obtain consistent results agreeing with those found in service it has been found advisable to separate the surfaces on the return stroke using an automatic mechanism which can be mechanically, electrically, or pneumatically operated (omitted from the simplified diagram shown in Fig. 1). This has been found to give more consistent results than those obtained with a reciprocating contact. 6.
APPLICATIONS AND TYPICAL RESULTS
Friction tests conveniently fall into two groups: 1. 2.
Studies of materials under a wide range of test conditions as an aid to design. Comparative tests on manufactured articles as an aid to product selection.
Although friction is rarely the only property being considered it is inappropriate to discuss here the wider implications of material or product selection, and friction alone will be considered. The general principles applying to the study of any material will be illustrated by considering tests carried out on a polyurethane rubber, one of a class of rubbers important for their outstanding wear characteristics. Product selection will be illustrated by reference to tests on windscreen wiper blades under dry and wet conditions. 6.1. Condition of surfaces Because friction varies with surface condition it is customary to remove mould release agents, grease and other contaminants, by grinding away the original surface. As testing proceeds the ground surface becomes more polished and wear products contaminate the surface. This leads to a dilemma-should the surface be ground before each test run or should testing be continued until equilibrium conditions are reached. Both approaches have been used but we prefer running until equilibrium is reached, as this more nearly represents service conditions. This takes about fifty test runs during which time friction gradually changes as indicated in Fig. 7. Once the surfaces have been run-in, agreement between successive test runs at any velocity is extremely good and, by way of illustration, the machine can be set so that the recorder pen follows exactly the same trace as the previous run. 6.2. Tests on a polyurethane rubber Although the abrasion resistance of polyurethane rubbers has been extensively studied their frictional properties are less well known. A few results were given by Westover and Vroom 36 and by Preiss 37 and the results given here
A NEW CONCEPT IN FRICrION
21
TESTING
1.e I
J
c 1.6 o o
o "~ 1.4 e
o 1.2
1.0
1
0
2
10
I
30
I
40
I
50
Number of runs
Fig. 7. Effect of running-in on the coefficientof friction of a polyurethane rubber.
were obtained with one of a n u m b e r of polyurethane rubbers examined during recent development work at R A P R A . Friction against steel was measured at velocities of 0-02, 2-0 and 50 cm/min at a number of discrete temperatures between 20 °C and 100 °C. The results are presented in Fig. 8 where coefficient of friction is plotted against temperature for each of the three test velocities. It is at once obvious that a single reading obtained, at one velocity, say, from an inclined plane at 20 °C could give a very misleading impression of the coefficient of friction to be expected at an entirely different velocity and temperature. It can be seen that at all temperatures friction increases as velocity increases, an effect which is particularly noticeable at room temperature. Comparison with Fig. 5 shows that the velocity range under consideration lies to the left of the peak at all the temperatures used. Construction of a full master curve covering twelve decades of velocity involves a considerable experimental effort. This type of plot is of great theoretical importance, but as has been explained, the predicted high velocity values are in error and in design work it has generally been found that a limited presentation such as that given in Fig. 8 is more satisfactory.
22
D.I. JAMES, W. G. NEWELL
1.8 1.6
1.4 -~ 1.2 0
g ~.0 0 0
0.8 0.6 50 ore/rain 0.4 ' 2.0 cm/mln 0.02 crn/mln 0.2 0
Fig. 8.
i 10
I
I
I
20
30
40
I
I
I
50 60 70 Temperature "C
I
I
|
80
90
100
Coefficient of friction of a polyurethane rubber against steel, measured at different temperatures and velocities, plotted against temperature.
6.3. Tests on wiper blades Four sets of blades labelled A, B, C and D were examined. One of each pair was kept in the original packet and the other taken out and stored in the laboratory atmosphere for 1 month before testing. Each blade was cut into four equal pieces which were then fitted one above the other in a specially constructed holder replacing block H in Fig. 1. A piece of 32 oz glass forming the other friction surface was mounted in holder C. Tests on cars indicated that the spring force fell from about 350 g for a new car down to about 125 g after some years of use. On this basis a normal force of 290 g was taken as being realistic and this was used throughout all the tests. Since in practice the velocity of traverse varies considerably, 5 cm/min and 50 cm/min were taken as representing a reasonable range. The blades were passed down the glass three times at 50 cm/min and three times at 5 cm/min while continuous records of friction were taken. These tests were then repeated under wet conditions, a fine spray being used to wet thoroughly the blade and the glass before and during testing. While it is clear that friction under dry conditions should be as low as possible, it is not at all obvious what represents a good performance when wet. Since an efficient blade is intended to remove most of the water from the windscreen it would seem reasonable that the nearer the ratio of wet friction to dry friction approaches unity, the more efficient the wiper blade.
23
A NEW CONCEPT IN FRICTION TESTING TABLE 1 COEFFICIENTSOF FRICTIONOF WIPERBLADESUNDERTHE CONDITIONSSTATED
New blade Sample A B C D
Velocity (cm/min)
Dry
Wet
5 50 5 50 5 50 5 50
0.52 0.53 0-53 0-50 0.47 0"54 0-72 0.65
0.43 0-44 0.62 0.50 0.54 0-59 0-62 0"57
Wet~Dry 0.83 0-83 1.17 1.00 1.15 1.09 0-86 0.88
Stored blade Dry
Wet
0.70 0.70 0-83 0.99 0.90 1.03 0-81 0.83
0-64 0.67 0.75 0-75 0-71 0.76 0.65 0.65
Wet~Dry 0-91 0.96 0.90 0.76 0.79 0.74 0.80 0-78
Mean figures for wet and dry friction, at both velocities, for both new and stored blades are presented in Table 1, together with the calculated ratio of wet to dry friction in each case. Figures for the ratio of wet to dry friction which are greater than 1 (Samples B and C) probably indicate that the low initial friction encountered with these blades is due to a surface contaminant which is quickly removed under wet conditions. The very high figures for the corresponding stored blades, with wet to dry ratios less than unity, confirm that the Iow initial friction values are transitory and these blades would quickly give trouble. Of the remaining two blades, sample A is probably superior since it not only has lower initial friction but this advantage is retained on storage, when additionally, the wet to dry friction ratio is close to unity.
7.
CONCLUSION
The two sets of experiments described illustrate the versatility of the new apparatus and indicate its usefulness both in design work and in component selection. It is clear from Fig. 8 that single value friction tests are inadequate and can give totally misleading results. The RAPRA/Daventest apparatus has been designed to accept a variety of materials and components and enables friction to be measured accurately over a wide range of experimental conditions. REFERENCES 1. 2.
DE C o s ~ , J. B. (1969). Friction of vinyl chloride plastics. SPE Journal, 25, 67-71. BAR'rm~v, G. M. (1954). Theory of dry friction of rubber. Dokl. Akad. Nauk. SSSR, 96 (6), 1161--4.
24 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
D.I. JAMES, W. G. NEWELL RA~, S. B. and LAva,'error, V. V. (1956). Evidence for static friction of uulubricated rubber. Dokl. Akad. Nauk. SSSR, 10g (3), 461-4; RABRM Translation N o . 591, Sept. 1957. B A R ~ V , G. M. and LAVR~a~mv, V. V. The nature of static friction in elastomers. Rubber Chem. Technol., 34, 1961, 461-5. From Vys. Soed, 2 (2), 1960, 238-42. LA~v, V. V. Static friction and the law of rubber friction. Rubber Chem. Technol., 36, 1963, 365-76. From Plaste Kautchuk, 9 (6), 1962, 282-7. ROBF_~TS,A. D. and THOM~, A. G. (1976). Static friction of smooth clean vulcanised rubber. NR Technol., 7, Part 2, 38-42. ROBF~TS,A. D. (1979). Looking at rubber adhesion. Rubber Chem. Technol., 52,, 23-42. AMO~rroNs, M. (1699). De la r6sistance caus~es dans les machines. Histoire de l'Acad~mie Royale des Sdences avec les M~moires de Math~matique et de Physique, Paris, p. 206. COULOMb, C. A. (1785). M~moires de Mathdmatique et de Physique de l'Acaddmie Royale des Sciences, Paris, 10, 161. HUNTER,R. B. (1930). A method of measuring frictional coefficients of walk-way materials. 3.. Res. National Bureau Standards, 5, 329-47. ROTH,F. L., DmSCOLL, R. L. and HOLT, W. L. (1942). Frictional properties of rubber. 3". Res. National Bureau Standards, 28, 439-62. DERmUX, J. B. (1934). The coefficient of friction of rubber. 3. Elisha Mitchell Sci. Soc., 50, 53--5. Rubber Chem. Technol., 8, 1935, 441~-2. HURRY, J. A. and PROCK J. D. (1953). Coefficients of friction of rubber samples. India Rubber World, 128 (5), 619-22. S ~ c r t , A. (1952), The load dependence of rubber friction. Proc. Phys. Soc. B, 65, 657--61. ~ c ~ , A. (1953). The velocity and temperature dependence of rubber friction. Proc. Phys. Soc. B, 66, 386-92. BIANCA, D. (1967). Sliding friction and abrasion of elastomers~ Rubber World, 1 5 6 (6), 65-7. Ksmsra, K. T. Sliding friction of elastomers, Paper A3, 6th Intern. Conf. Fluid Sealing, Munich, February 1973. LANCASTER, J. K. (1973). Basic mechanisms of friction and wear of polymers. Plastics Polymers, 297-306. LuIymN, J. A. J. and ProD, H. A. (1973). Slip resistance of soling materials and top-pieces. Satra Information Publication I.P. 141. PoorlY, R. W. (1978). Measurement of frictional properties of footwear sole and heel materials. Walkway Surfaces: Measurement of Slip Resistance. ASTM STP, 649, 11-20. BRAUn, R. and BRtr~ORABER, R. J. (1978). A comparison of two slip-resistance testers./bid, 49-59. JAMES, D. I. (1973). Variation in the friction of rubbers and plastics. R A P R A Members Journal, 1 (7), 170-5. GROSCH, K. A. (1963). The relation between the friction and visco-elastic properties of rubber. Proc. Roy. Soc. A., 274 (1356), 21-39. BS 3424, 1961 (revised 1973). Methods of test for coated fabrics. Method 1 2 Determination of surface drag. ASTM D-1894-63. Standard method of test for coefficients of friction of plastic film. latvar~, C. H. (1967). A new slipmeter for evaluating walkway slipperiness. Materials Res. Standards, 7 (12), 535-42. RoumsoN, W. H. and KOPF, R. E. (1969). Evaluation of the Horizontal Pull Slipmeter. Materials Res. Standards, 9 (7), 22-4. WUaJAMS,W. D., SMrrR, J. A. and DRAUGELXS,F. J. (1972). TOPAKA, A new device and method for measuring slip resistance of polished floors. Soap~Cosmetics/Chemical Specialties, July. Gn.~s, C. G., SABEY, B. E. and CARDEW, K. H. F. Development and performance of the portable skid-resistance tester. Road Research Technical Paper No. 66 (1964); ASTM STP 326 (1962). SI~tJ~, P. A., GEm, M. N. and BooteE, T. H. (1948). Measurement of the slipperiness of walkway surfaces. & Res. National Bureau Standards, 40, 339-46.
A NEW CONCEPT IN FRICTION TESTING 31. 32. 33. 34.
35. 36. 37. 38. 39. 40. 41.
25
Federal Test Method No. 7121, Standard No. 501(a). Dynamic coefficient of friction. 15th June 1966. JABLONSKY,R. D. (1978). Standardization of test measurement of floor slipperiness. Walkway Suryaces: Measurement of Slip Resistance. ASTM STP 649, 21-39. Jinx.s, S. V. (1944). What is a safe floor finish? Soap Sanitary Chemicals, 20, 111-15. JAMES,D. I. (1961). Measurement of friction between rubber like polymers and steel. J. Sci. Inst., 38 (7), 294-9. BRUNGRABER,R. J. (1977). A new portable tester for the evaluation of the slip-resistance of walkway surfaces. National Bureau of Standards Technical Note 953. WESTOVER, R. F. and VROOM, W. I. (1963). A variable speed frictionmeter for plastics, rubbers, metals and other materials. SPE Journal, 19 (10), 1093-9 (Adopted by ASTM D-3028-72, reapproved 1978). PREXSS,D. M. (1965). Coefficient of friction of elastomers. Rubber Journal, 14"! (10), 42-51. REED, M. E. and MAHON, R. D. (1978). Description of the National Institute for Occupational Safety and Health (NIOSH) Universal Friction Testing Machine (UFTM). Walkway Surfaces: Measurement of Slip Resistance. ASTM STP 649, 60-70. MAJC~RCZYK, R. (1978). A different approach to measuring pedestrian friction: The CEBTP skid meter./bid, 88-99. BIKERMAN, J. J. (1974). Sliding friction of polymers. J. Macromol. Sci.--Revs. Macromol. Chem., CII (1), 1-44. RA~INOWITZ, E. (1958). The intrinsic variables affecting the stick-slip process. Proc. Phys. Soc., 71, Part 4 (460), 668-75.