Proceedings of SerbiaTrib '13

holes (pits) emerge in the shape of shell on toothflank. Pit peak always points in the slidingdirection. This damage occurs through a cyclicfatigue due to repeated elastic and plasticdeformations of the surface. The holes occur onlyafter a sufficiently large number of overrollings(from ca. 5x10 4 load cycles). If only initial pitting ispresent, the situation is not dangerous. Destructivepitting destroys the flank and causes failure due tonoise and fatigue. The pitting occurred on wheelsmade from materials S4, S5 and S6 by an outputtorque of 16 and 32 Nm after the trial time periodof 120 h to 240 h.Figure 6 shows initial and destructive pitting ontooth flank of wheel. In trials with material S1 –without additional treatment, initial pitting ocurredunder input speed n 1 = 5000 min -1 and outputtorque of 20 Nm. In trials with material S5 – sinterhardening,destructive pitting occurred under inputspeed n 1 = 5000 min -1 and output torque of 20 Nmand mineral oil.6.3. ScuffingThere is a difference between cold and warmscuffing. Both damage types are caused by the lackof lubricant in contact between teeth. Cold scuffingis relatively rarely seen. It occurs mainly at lowspeed (< 4 m/s) and between teeth that are havingrelatively high hardness and rough quality ofcontact surfaces. Warm scuffing occurs due to greatpressure and high sliding velocity between toothflanks. Under such a load, combined effects occurwhich lead to the increase in temperature thatdisrupts the lubricant film between tooth flanks,making the contact between tooth flanks direct anddry. This can cause a short local welding of theflanks which damages both flanks. Warm scuffingis characterized by strip-shaped bands in thedirection of the tooth height, and with the strongestexpression in the tooth addendum and tooth root.Scuffing on high-speed gears increases thetemperature and tooth forces, eventually leading toshaft fracture due to high damage on tooth flanks.Initial pitting: without additional treatmentT 2 = 20 Nm; t = 120 h; n 1 = 5000 min -1Lubricant: synthetic oilVariant with case hardeningT 2 = 20 Nm; t = 160 h; n 1 = 5000 min -1Lubricant: synthetic oilDestructive pitting: sinter-hardening variantT 2 = 20 Nm; t = 120 h; n 1 = 5000 min -1Lubricant: mineral oilFigure 6. Pitting on wheel tooth surface [6]Material with 2% copper additionT 2 = 28 Nm; t = 160 h n 1 = 5000 min -1Lubricant: synthetic oilFigure 7. Scuffing on tooth flanks of wheel for differentmaterial variants [6]13 th International Conference on Tribology – Serbiatrib’13 325