14.5 Tooth temperature

The tooth temperature (bulk temperature) ΘM that is relevant to the analysis of flash temperature and film thickness is the bulk temperature of the surfaces of the gear teeth just before they engage. The gear tooth bulk temperature is an important component of the total temperature that occurs during engagement of the gear teeth, which consists of the bulk temperature plus the instantaneous flash teruperature rise, i.e.:

ΘB = ΘM + Θfl

It is the total contact temperature,ΘB , which controls the scoring (scuffing) mode of gear tooth failure. Besides being an important contributor to the gear tooth total temperature, the bulk temperature controls the operating viscosity of the lubricant which is entrained into the gear tooth contact. The entrained lubricant is in thermal equilibrium with the surfaces ot the gear teeth and its viscosity determines the thickness of the EHD oil film. lt is therefore imperative that an accurate value of gear bulk temperature be used as input to Scoring.

In some cases, the equilibrium gear bulk temperature may be significantly higher than the temperature of the oil supplied to the gear mesh. For example, reference tested high-speed, single-helical gears typical of gears used in the turbo-machinery of the petro-chemical industry. With oil nozzles supplying lubricant to the outgoing side of the gear mesh, the temperature of the pinion teeth was 180 deg. F (76 deg. F rise over the inlet oil temperature) at a pitch line velocity of vtr = 20,000 fpm, and 275 deg. F (171 deg. F rise) at vtr = 40,000 fpm. For the mating gear the temperature was 138 deg. F (34 deg. F rise) at vtr = 20,000 fpm, and 208 deg. F (104 deg. F rise) at vtr 40,000 fpm.

This example indicates that the bulk temperature of ultra-high-speed gears may be significantly higher than the temperature of the oil supply (171 deg. F rise at vtr = 40,000 fpm) and that the pinion can be very much hotter than the gear (67 deg. F difference at vtr = 40,000 fpm).