Gear failure index

8.0 Worm gears

Worm gears operate with a unidirectional sliding movement between the worm and the wheel teeth. The worm is usually hardened and requires a very high standard of surface finish because of its high sliding speed and relatively low film thickness compared to an equivalent plain bearing surface. The worm wheels are usually made of bronze although cast iron and non metallic materials can also be used. The worm and wheel need setting to give a clearance at the in-going side and thus to form a tapered load carrying oil film.

The worm wheel usually suffers more damage than the worm and this may be of two kinds scuffing and incipient seizure due to lack of lubrication, or surface pitting due to long operation at high load.

Click on photograph to enlarge
	Figure 8a A wormwheel which has operated with inadequate lubrication.
	Figure 8b A wormwheel showing fatigue pitting after extended service. Pitting tends to occur preferentially towards the trailing edge of the contact where the temperature is higher.
	Figure 8c Heat cracking of a worm due to operation with too low an oil film thickness. The damaged area follows the bedding pattern of the worm with the wheel and runs out towards the tips of the worm teeth.
	Figure 8d A heavily loaded case hardened worm may suffer flaking of the case particularly if it is thin.
Main Characteristics
Cause
Note	Worms can also suffer damage and with inadequate lubrication heat cracking of the worm surface can occur. With case hardened worms, flaking of the case can also sometimes occur under conditions of high loading.
Possible Confusion with Other Types of Damage
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