Proceedings of SerbiaTrib '13

(friction) efforts through resistive converter straingauges,assembled on the elastic blade (5). The useof a pair of converters strain-gauges connectedwithin the circuits of two strain-gauges bridges,offers us the possibility to make simultaneousmeasurements, while separately, gives us thepossibility to measure the normal and frictionforces. We apply the normal load to the elasticblade, through a calibrated spring system. Theinstallation allows us to register the friction forceon an X-Y recorder. We control the tests’ durationthrough an alarm clock and we measure the contacttemperature with the help of a miniaturethermocouple, connected to a millivoltmetercalibrated in 0 C.I used the uni-directional testing because thepurpose of investigations was the study of metallicsurface wear. We perform the tests, based onHooke's law, at normal loadings of 10; 20; 30; 40and 50 N, loadings which are adequate to somecontact pressures all calculated considering theelastic contact hypothesis, that is: 16.3; 23.5; 28.2;32.6 and 36.4 MPa (for Nylonplast AVE polyamidewith 30% glass fibres) respectively, we use slidingspeeds, adequate to the diameter of the plasticcomposite sample, which are: 0.1856; 0.2785;0.3713; 0.4641; 0.5570; 1.114 and 1.5357 m/s, andwhich resulted as a consequence of electric motor’sspeed and the belt pulleys’ primitive diameters.As we know [21], we may characterize amaterial’s wearing coefficient (percentage) bywearing factor k. Archard’s relation defines thisfactor:V u kNvt(1)where: V u – the wear’s material volume; N - the testload; v - the sliding speed; t - the test period; k –volumetric wearing factor.By dividing both of this relation’s terms (4) bynominal contact area A, we obtain:Which means that:V u/ A kNvt / A(2)h u* k pvt(3)where: h u - wear’s material depth; p - the pressureon the nominal contact area and k * is the linearwearing factor. Relation (6) expresses a general lawof the wear as a function of the contact pressure pand the length of the wearing path, so that L f vt.We could then write:respectively:k V / Nvt V / NL(4)uuf*k hu/ pvt hu/ pL(5)fConsidering the large area of the load (N) orpressure (p) and the relative speed values usedduring tests in order to evaluate the wearingreaction of the metallic counter-pieces amid thefrictional couples, we use comparative wearcoefficients K and K * , defined by:and:K V / L kN (cm 3 / cm) (6)K*uf* h / L k p (cm / cm) (7)ufWe consider these wearing coefficients withrespect to the period in which the frictional couplefunctions at different sliding speeds, under certainloading conditions (contact pressure).The main objectives of these tests are thedetermination of the volume of material removedby wearing, the mean depth of the wearied layers,the frictional factors and coefficients, for differentloading conditions.Coefficients k and k * are coefficients of the wearprocess, while the comparative factors K and K * arecoefficients of this process’s consequences, that is,the amount of resulted wear and reported to thelength of the friction pathway. They can bequalitatively expressed in units of wear volume ona measure of the length of the friction pathway (cm 3/ cm), as wear’s depth on a measure of the length ofthe friction pathway (cm / cm) or as wear’s weighton a measure of the length of the sliding frictionpathway (mg / cm). Coefficients K and K * have nomathematical implication (can not simplify).Using the procedure described in [22], at the endwe obtain the mean depth (8) and the volume ofworn metallic material (9):and:Vu2l/ 8r10,527NE1 E2LE1E2h (8)n i1Siqi0.351E1 E2Nlm/ E1E2(9)where l m is the mean width of the wear imprint.Practically, we have to measure the width ofwear imprints in three points established before,computing then the mean value of this width. Withthis value we can obtain the volume of wornmetallic material V u and the removed layer’s meandepth h mu .We study the wearing of the friction couple’smetallic component on linear contact Timkenmachinery, see Fig. 1. Almost all tests are madewithout lubricating the frictional surfaces, but thereare also tests with micro-lubricating.60 13 th International Conference on Tribology – Serbiatrib’13