Just adding my two cents, worm gears are very inefficient and produce a fair amount of heat. The heat is generated due to high sliding velocities characteristics of worm gears. The heat can quickly degrade the grease and also the high friction can will slowly abrade the worm gear.
Looking at the worm pinion from the pictures it looks like it has been worn thru abrasion not a brittle fracture of the teeth. The worm pinion gets thinned down to a point where there isn't enough material to support the load and then it strips the remaining portion of the pinion.
There are several options to address the abrasion, find a way to make these oil lubricated, which will help dissipate the heat. The parts look like there isn't any heat treatment to the pinion. Dependent on the base material used, a surface hardening ( induction) would make the surfaces harder to resist wear, however the teeth may get to brittle and fracture. Another option would be to nitride the parts, it creates a very hard surface that resist wear. Its a very thin layer so you maintain the ductility of the base material making it tough. The other option is PVD (physical vapor disposition) it puts a very hard, diamond like coating on the surface which increases hardness while also reducing friction. PVD doesn't care on what's its applied too, induction hardening and nitriding does care what steel alloy the pinion was made from. At work we use PVD on case carburized gears to fight wear, the sales guys always points out that it's used on NASCAR rear ends to increase wear resistance as well.
I also assume they chose a worm gear so the can set a position and hold it there. The friction aspect of worm gears also acts as a brake and doesn't allow the gear to back drive the pinion. If you redesigned something to remove this and incorporate all spur gears, the motor would then need a brake off some sort.
Last edited by critical05; 01-13-2013 at 12:33 PM.