From: "Gerhard Fiedler"

> Maybe most but not all... for example the gear motor I sent the data sheet
> link in a previous message has a worm drive, but it's not self-locking.
>
> I'm not sure how to see that easily if it's not specified.

Unfortunatly, there isn't a concrete rule of thumb. Theoretically, a worm
drive will not back drive if the friction angle is greater than the worm
lead angle. However, the actual surface finish (AGMA spec) and choice of
lubrication may reduce this significantly; among many other factors.
More importantly, vibrations may cause motion at the points of mesh
with further reduction in friction angle.

Now for the good news, generally speaking, if the worm lead angle is 5
degrees or less there is reasonable expectation of self-locking. Between
(5,11] degrees of worm lead there is a percentage of back drive. Over 11
degrees back driving can be expected.



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