Worm gearing strength is limited by various conditions, namely:
• Pitting formation on gear tooth sides
• Tooth side wear
• Fatigue break at tooth root
• Worm deflection
• Oil heating

These conditions must be reflected when choosing the worm and gear materials. Hardened and ground worm mating with tin or phosphorus bronze gear is mostly selected.

Material of worm gear

The basic material is bronze, less frequently being cast iron or brass. Plastic gears are used for lower powers (to absorb shocks and provide lower noise) and non-power gearing. Bronze gears are manufactured as composite for economical reasons (a bronze rim put on a steel or cast-iron wheel). Centrifugal casting is advisable.
The optimum options are tin bronze with high Sn content of 10-12% (excellent friction properties, high resistance to seizure and good running-in), but are too expensive. Their application can only be justified in loaded transmissions and with sliding speed exceeding 10 m/s. Even in these cases, however, there is an effort to replace them by Sn-Ni bronze and other alloys. Bronze with lower Sn content (5-6%) can be used for speed v = 4-10 m/s.
Less expensive bronze free from tin, e.g. aluminum or lead bronze, or brass are suitable for speeds v<4 m/s. They are relatively hard and strong, but are less resistant to seizure and are not so good for running-in. The mating worm, therefore, must have high surface hardness (HRC>45). In gearing with a greater worm gear, a combination of bronze worm and cast-iron gear can be used for economical reasons.
Grey cast iron mated with a steel worm can be used for low outputs, quiet load and low peripheral speed up to 2 m/s.
 

Material of worm

Carbon or alloyed steel which allows surface heat hardening (hardening to HRC 45-50, cementing and hardening to HRC 56-62, and nitriding) is used for worms. Tooth sides are ground and/or polished. When nitrided, the material need not be ground and polished only. Heat-treated or normalized steel worms are only used for lower outputs and lower peripheral speeds.