SURFACE ROUGHNESS PARAMETERS EVALUATION IN ROLLING/SLIDING WEAR TESTS

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1 28 THE NNLS OF DUNĂRE DE JOS UNIVERSITY OF GLŢI SURFCE ROUGHNESS PRMETERS EVLUTION IN ROLLING/SLIDING WER TESTS Minodora RÎPĂ University Dunarea de Jos of Galati, ROMNI BSTRCT Rolling/sliding wear tests were performed on an msler test rig. Normal load (expressed as the Hertzian pressure) and the roll-slide ratio have been considered to have the most important influence in simulating gear functioning on two-rollers machines. The surface topography changes are studied using a comparison between unrun and worn digital profiles, some statistical parameters variation and the bearing area curves analysis. Keywords: surface roughness, roughness parameters, bbott curve, sliding/rolling wear tests 1. INTRODUCTION The contact phenomena between rough surfaces are the subject of considerable interest and intensive research. The rolling/sliding line contact, characteristic for spur gears, has been simulated on SE sets on two rollers machines by several researchers [1, 2, 4, 5, 8, 12, 14, 15, 16, 17, 18], many of them taking into account the microgeometry and the topography changes of the technical surfaces during the wear process, under dry or lubricated conditions [1, 9, 10, 11, 13, 14, 19]. Traditional roughness parameters as R a (rithmetic verage Rougness Height) and R q (Root Mean Square verage Rougness Height) do not provide any information about the shape, slopes and sizes of the asperities or about the frequencies of their occurrence [3]. It is possible to have the same R a and R q values for surfaces of widely different materials and working conditions. Worn surface profiles often reveal both periodic and random components and they could not be revealed by a single parameter description of the surface roughness. ny of traditional parameters such as R a, R q, R z (Ten Point Roughness Height), R t (Maximum Roughness Height or total roughness) gives the unit of size, but it is better to use the second, the third and the fourth central moments of the amplitude distribution function for having the shape. These moments are the values of R q, R sk (Skewness) and R ku (Kurtosis) parameters. The shape is adequately represented by R sk and R ku, the unit of size by R a or R q. Based on the bearing area curve (bbott curve), the effects of wear and running-in process can be evaluated or simulated and the functional roughness parameters can be defined, especially in 3D studies [7]. This paper presents a part of the studies concerning the surface topography changes in rolling/sliding wear tests. Results of tests done under the normal load (expressed as the H ertzian pressure) and the roll-slide ratio as commanding parameters are presented. These two parameters have been considered to have the most important influence in simulating gear functioning on two-rollers machines. 2. EQUIPMENT ND TESTS n msler test rig was adapted for SE test sets, adjusted and completed for a wear testing program. The test rig has been also provided with a torque transducer (type T30FN200 -Hottinger Baldwin ) for measuring the friction torque and the rotational speed, a force transducer and a data acquisition system (DMCPlus-Hottinger Baldwin ). Three values for rotational speed are available on the rig. The tests presented in this paper were performed for 525 rev/min. The rollers were loaded in a nominal line contact using a hydraulic loading system (load range: kn), at three values, producing Hertzian pressure comparable to those in the already studied gears, i.e.

2 THE NNLS OF DUNĂRE DE JOS UNIVERSITY OF GLŢI 29 p 1 =550 MPa, p 2 =650 MPa, p 3 =750 MPa. The rollers used for this study were made of carbon steel 1C45 (similar to 1045 SE steel grade), through-hardened at HB=2500 MPa. Their surfaces were obtained by turning. Because the rotational speed of each one of the two rollers in running is the same, three different diameter combinations enssure the roll-slide ratio, calculated as s=(v 1 -v 2 )/(v 1 +v 2 ). On the gear meshing these values correspond to the pure rolling s=0, around the pitch point zone, and to the rolling/sliding (double and single pair of teeth engagement zone, having s=0.225 and s=0.075). Table 1 presents the rollers codes and the commanding parameters used for these tests. The paper presents the results for the rollers having a larger diameter in the tested pairs. Table 1. Rollers characterisation roller diameter Hertzian pressure s [mm] [MPa] p 1 = 550 B p 2 = 650 C p 3 = 750 D 80 0 p 2 = 650 For all tested pairs the running time was 10 5 cycles, corresponding to the early-stage wear, when the most significant roughness changes occur. Lubrication is achieved by splashing and the lubricant is paraffinic mineral base-oil (GM lubricant no. 4 EP). 3. SURFCE PROFILOMETRY MESUREMENTS The roughness data acquisitions have been performed before and after each test, with a Surtronic3+ digital profilometer, and processed with TalyProfile (both Taylor Hobson ). For some rollers, measurements by relocation technique have been made. Vickers diamond pyramid has been used for achieving the relocation of the profile Digital Profiles Profile data were acquired from three positions on each roller, before and after running and the most relevant profiles are presented. Figure 1a presents the relevant profile before testing for the rollers, B and C. Profiles of worn surfaces of same rollers, B, C are presented in Figures 1b, 1c, 1d, respectively. a) Un-run profile, roller, B and C b) Worn profile, roller c) Worn profile, roller B d) Worn profile, roller C e) Worn profile, roller B Fig. 1. Profiles of the investigated rollers

3 30 THE NNLS OF DUNĂRE DE JOS UNIVERSITY OF GLŢI For both tests at p 1 and p 2 (i.e. rollers and B), more than half of total roughness ( R t ) was worn, the worn profiles having flatted tops and deep valleys. s for the roller C, tested at p 3, it was noticed that a layer having a thickness greater than R t was completely worn and the rubbing process takes place between new surfaces. The difference between the unrun surface and the working smooth surface can be better noticed in the relocated profile of roller C (F igure 2a and b). Near p 3 =750 MPa of the Hertzian pressure the wear rate is much more important as compared to the other pressures. Figure 1f presents the profile of worn surface for roller D ( s=0), i.e. for pure rolling relative motion. The influence on the topography is smaller than the one noticed for s=0.225, at the same loading (Fig. 1c), even the decreasing of parameters values is greater than expected Discussion on Roughness Parameters The discussion on roughness parameters is based on [7, 21]. Table 2 presents the roughness parameters for the tested rollers, both for unrun and worn surfaces. Figures 3a and 3b allow comparing, in percentages, the variation of parameters as a function of load and roll-slide ratio. One may notice from Table 2 and Figure 3a, a quite low influence of loading from 550 to 650 MPa (a pressure variation of 18%). For both loads, R a, R q, R z, R t parameters diminish with %. For a load p 3 =750 MPa, these parameters have values diminished with more than 95% as compared to those characterising the unrun profile, due to the generation of a new working surface. It seems that p 3 is a loading pressure that does not offer optimal conditions for running-in, giving a higher probability to initiate adhesive wear. Table 2. Roughness parameters values Roller R a R q R z R t R sk R ku [ m] [ m] [ m] [ m] [ m] [ m] unrun worn B unrun B worn C unrun C worn D unrun D worn The very low R sk values characterising the unrun surfaces indicates a symmetrical amplitude distribution. For the worn surfaces of all rollers but excepting roller C, R sk takes low negative values, indicating that the amplitude distribution has a longer tail at the lower side of the mean line [7]. few peaks flattening are especially wanted for the lubricant retention. The new working surface of roller C has peaks and valleys and the amplitude distribution has a longer tail at the upper side of the mean line. a) New profile, roller B b) Relocated worn profile, roller B Fig. 2. a) Roughness parameters variation b) vs. Hertzian pressure Fig. 3. b) Roughness parameters variation vs. roll-slide ratio

4 THE NNLS OF DUNĂRE DE JOS UNIVERSITY OF GLŢI 31 When R sk tends to zero, R ku is a more useful parameter, as it can distinguish between predominantly bumpy [21] or spiky surfaces. The kurtosis is often measured relatively to value 3, since it corresponds to the R ku of a Gaussian distribution. For the studied profiles, values smaller than 3 (corresponding to the R ku of a Gaussian distribution) show that there is a well spread distribution within the total height of the profile. The increase of R ku value, for the same surface, could be a sign of a significant surface wear. Similar to R sk, R ku is also sensitive to the outliers and spikes. Using a combination of R sk and R ku parameters, it may be possible to identify the surfaces that will behave better, especially those with flat top and deep valleys. Skewness and kurtosis are more sensitive to changes in profile shape than R a and R q. This makes them capable of analysing wear experiments [21] Bearing rea Curves nalysis The bbott curve is usually used to study and quantify tribological phenomena such as lubricant influence, bearing materials or surface topography [6, 20]. nalysing the bbott curves (F ig. 4) and comparing the values for Ht p 80/100 (Table 3), the increasing of retaining lubricant capacity by smoothening the profile asperities is confirmed. The parameter Ht p, the bearing height, is the height between two points on the bearing ratio curve at specified values for t p. Parameter t p % gives the bearing ratio at a given depth. Table 3: bbott curve parameters roller t p 5 [%] t p 80 [%] Ht p 80/100 [%] B C D B C D unrun surface worn surface Table 3 shows t p values normalised in accordance with the maximum depth of each profile. For the worn profiles of rollers and B (Fig. 4d and 4e) the bbott curves have the relevant three t p zones recommended for studying: 0 5%; 5% 80% and 80% 100% [6]. The high value of Ht p 80/100 confirms favourable changes of the amplitude distribution, due to smoothening of the peaks. a) Unrun roller b) Unrun roller B c) Unrun roller C d) Worn roller e) Worn roller B f) Worn roller C Fig. 4. bbott curves for unrun and, respectively, worn rollers, B and C.

5 32 THE NNLS OF DUNĂRE DE JOS UNIVERSITY OF GLŢI 4. CONCLUSIONS In rolling/sliding wear tests, studies on the surface profilometry can provide useful information about the load-carrying capacity and some functional properties concerning lubricant retention. The analysis of some amplitude parameters shows a quite low influence of load increasing, from 550 MPa to 650 MPa and significant changes for p 3 =750 MPa. Due to the higher load, the layer having a thickness greater than R t was completely worn and the new working surface is much smoother than desired, existing the risk of scuffing occurrence. nalysing R sk parameter and the bbott curves confirm favourable changes of amplitude distribution due to smoothening of the peaks and the increasing of the retaining lubricant capacity. REFERENCES 1. kbarzadeh S., Khonsari M. M., 2008, Thermo-elastohydrodynamic analysis of spur gears with consideration of surface roughness, Tribol. Lett, vol. 32, pp kbarzadeh S., Khonsari M.M., 2011, Experimental and theoretical investigation of running-in, Tribology International, 44 (2), pp Bhushan B., 1996, Tribology and Mechanics of Magnetic Storage Devices, 2nd edition, Ed. Springer. 4. Castro J., Seabra J., 2007, Coefficient of friction in mixed film lubrication: gears versus twin-discs, Proc. Inst. Mech. Engrs Part J: J. of Eng. Tribology, vol. 221(3), pp Chou C., Lin J., 2002, new approach to the effect of ep additive and surface roughness on the pitting fatigue of a line contact system, J. of Tribology, vol. 124(2), pp Dai J.Y. et al., 2011, Characteristic of Laser Texturing Surface ided with bbott Curve, dvanced Materials Research, , pp Dong W.P. et al., 1994, Comprehensive study of parameters for characterising three-dimensional surface topography. III: Parameters for characterising amplitude and some functional properties. Wear, 178, 1-2, pp Dormois H., 2008, Frottement dans les contacts EHD de grandes dimensions, rôle du pivotement, Thèse INS Lyon. 9. Effner U., Woydt M., 1998, Importance of machining on tribology of lubricated slip-rolling contacts of Si3N4, SiC, Si3N4,- TiN and ZrO2. Wear, 216, pp Evans H.P., Snidle R.W., 2009, The future of engineering tribology in concentrated contacts, Proc. Inst. Mech. Engrs Part C: J. of Mech. Eng. Science, 223(12), pp Foucher, D., 1981, Parametres mecaniques du rodage dans le contact elastohydrodinamique, These doc.-ing., INS Lyon. 12. Govindarajan N., Gnanamoorthy R., 2007, Rolling/sliding contact fatigue life prediction of sintered and hardened steels, Wear 262 (1-2), pp Jacod B., Venner C.H., Lugt P.M., 2004, Influence of longitudinal roughness on friction in EHL contacts. Trans. SME J. Tribol., 126, pp Johnson K.L. Spence D.I., 1991, Determination of gear tooth friction by disc machine, Tribology International, 24 (5), pp Kleemola J., Lehtovaara., 2007, Experimental evaluation of friction between contacting discs for the simulation of gear contact, Tribo test, vol. 13(1), pp Kleemola J., Lehtovaara., 2009, Experimental simulation of gear contact along the line of action, Tribology International, 42 (10), pp Mang T., Bobzin K., Bartels T., 2010, Industrial Tribology: Tribosystems, Wear and Surface Engineering, Lubrication, Ed. Wiley-VCH. 18. Olver., V., 2005, The Mechanism of Rolling Contact Fatigue: n Update, Proc. Inst. Mech. Engrs Part J: J. Engineering Tribology, 219 (5), pp Prakash J., Czichos H., 1983, Influence of Surface Roughness and its Orientation on Partial EHD Lubrication of Rollers, J. of Lubr. Tech., 105, pp Steiner L. et al., 2010, Modelling of unlubricated oscillating sliding wear of DLC-coatings considering surface topography, oxidation and graphitisation, Wear, 268, 9-10, pp Stout K.J. et al., 1977, nalytical Techniques in Surface Topography and their pplication to a Running-in Experiment, Wear, 43, pp