The principle form of damage observed in worm gearing is surface deterioration on the flanks of the gear wheel teeth. Frequently the explanation for such phenomena is the transmission of a torque in excess of the capacity of the gearset. Torque determines the level of pressure exerted on the material surfaces. Exceedingly high pressure on a small wheel area leads to early pitting of the wheel teeth and/or cracking of the worm-thread surface. The load capacity, when related to wear, is based on the tooth flanks resistance to wear and pitting when they are subjected to the tooth flanks imposed loads. The German standard for Calculation of Cylindrical Worm Gear Capacity DIN #3996 Pt. 1., takes into consideration the following damage limitations, “wear, pitting, worm deflection, tooth breakage, and temperature.” AGMA, ISO, and similar standard groups have produced information on failures and usually classified them as being either wear, fatigue, plastic flow or breakage. Neither the AGMA or ISO document has dealt in any depth with worm gearing failures.
The common theme in many available rating formulae is the number of cycles the loaded gears can accept based on the fatigue capacity of the teeth, using an S/N curve, the stress/cycle curve. A feature of this curve is that it has a theoretical stress figure that can be applied ad infinitum without causing failure. This leveling out to a straight line (Fig. 6.4), (page 6–187) is termed the endurance limit.
Worm gears, under properly maintained circumstances, operate for many years problem free and can outlast the life of both the prime mover and the driven machine. Some gears are still in operation after two hundred years. Despite the well known facts about the importance of correct lubrication more than 90 percent of one manufacturers' reducers, returned for reconditioning, were found to have failed from inadequate lubrication.
