[TowerTalk] $30 Worm Drive Winch

[Jim Lux]

At 10:40 AM 12/10/2003 -0600, EDM1 wrote:
>I am also looking for a Winch. My application is raising and lowering a 40 
>Foot Rohn 25 with a hinged base (The kind with the hinge base sitting on 
>the ground)
>What keeps this worm type from falling or free wheeling?


Friction.. worm gear drives are characterized by very high gear ratios 
(30,40,50, even 100:1 are common), and real poor mechanical efficiency, 
because most of the input force goes to overcoming friction.  They are 
basically a lead screw (or jack screw) wrapped around a circle.  Say it 
takes 10 ftlb to turn the worm gear, with no load on the pinion (and, 
usually, that's symmetric)... Assume it has 100:1 gear ratio and you've got 
a 6" diameter cable drum (0.25 ft radius) and your tower weighs 500 
pounds.  The torque on the drum, from the tower, is 500*.25 or 125 ft 
lb.  Through the 100:1 gearing, that's 1.25 ft lb at the worm gear handle, 
so you don't have to turn all that hard.  However, that same 1.25 ftlb 
isn't enough to overcome the friction in the mechanism (which was 10ftlb)...

If you're really a glutton for analysis, consider the worm gear as a fairly 
shallow inclined plane.  It holds for the same reason that a door stop or 
other wedge holds.

By the way, if you've got a system where you expect significant "back 
drive" force on a worm gear system, better make sure that the shaft can 
take the load.  Folks using electric window motors for robotic actuators 
have found this out the hard way... They're not designed for much backdrive 
torque (because windows aren't very heavy (they have spring loads to hold 
them up), so you either put a slip clutch in, a breakaway pin, or wind up 
with failed gear trains.


>Any other suggestions?
>Thanks,
>Dave
>
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