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Why Worms?

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#1 imjeffp


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Posted 30 January 2011 - 11:49 AM

Out of curiosity, why are mounts built using worm gears instead of spur (?) gears?

#2 Midnight Dan

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Posted 30 January 2011 - 12:26 PM

Well, to start with, you need a very high gear ratio to make the driven gear turn very slowly. You can achieve this with a series of spur gears or a single worm gear. Whenever you have a series of gears, you introduce a lot more backlash so the worm gear will produce less backlash for the given reduction ratio.


#3 burb scope

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Posted 30 January 2011 - 12:32 PM

A large gear is "flatter" at the circumference than a small gear. The "long" worm has more teeth engaging the large gear at any point in time. That is why the worm arrangement is smoother and backlash free compared to the 1 or 2 tooth contact of a spur gear arrangement.

#4 scope dog

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Posted 30 January 2011 - 05:38 PM

Spur gears are used in the motor gear box. Both types only have a few teeth that make contact. However the worm is self braking and will not race away, the spur gear will. But the spur gear is very efficient, I have seen HP spur gears, but placing weight on a small motor shaft could blow the gear off. If you stop the motor it could spin backwards unless a stepper motor is used.

#5 Kfrank


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Posted 30 January 2011 - 05:56 PM

"And the worms crawl in
And the worms crawl out
The worms play pinochle on your snout"

Sorry, just couldn't resist!

#6 frolinmod


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Posted 30 January 2011 - 06:21 PM

The Paramount series are still using worms, but at least they're using belts instead of spur gears to drive the worms. You'd think we'd see more harmonic drive and direct drive mounts than we do.

#7 Peter in Reno

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Posted 30 January 2011 - 06:27 PM

I think direct drive without gears would be ideal like ASA mounts. No more backlash. Direct drive motors will have to be very slow and beefy to handle the torque to move the mount. The concept sounds very simple.



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