Worm gear units.
I have a bit of a love-hate relationship with them. 
As a single-stage unit, they are normally available from a ratio 7 up to ratio 100. As a right-angled unit they are very versatile and like bevel helical offer an easy mounting arrangement using the hollow bore and a torque arm. 
They are easily mounted, light, cost-effective and offer a natural anti-run back, however the reason I like them creates a reason to hate them, that reason is efficiency. In the worst case, a ratio 100:1 can be as low as 30% efficient. I always try to design or select a system that is as efficient as possible, therefore sometimes I avoid worm gears. Occasionally this lack of efficiency plays into our hands and offers an advantage by not allowing it to be back-driven. For example in lifts, winches, skip drives etc it can act as a natural run back in addition to a safety brake, meaning the load does not move unless being driven by the motor. Often these loads are not full s1 duty therefore efficiency is not a big factor. 
The fact the worm gear can't be back-driven is advantageous on some incline conveyors and overrides the need for a backstop clutch. 
We sell lots of them and like most things have pros and cons so it is about marrying them to the right application.

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