Typically two stage worm drives cannot be easily backdriven.
But I had two occasions (with the very hefty Boston gearbox that drives my
Big Bertha) where the force from 90+ mph winds either "backdrove" my 2-stage
worm drive, or the force allowed the gears to strip past one another.
There are ways to calculate the various torques. My box, for instance, has
over 68,000 in lbs of starting torque. You can find the mathematics in the
Boston Gearbox manual on Gear boxes.
If you look closely at the OSHA statements for 2+ stage worm-drives they
clearly explain that people cannot work under equipment suspended by a
gearbox. That's because they can be backdriven or they can fail. Not
common, but my guess is the statement has saved a few lives !
73, Scott W3TX
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