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Will worm gear/worm back drive?

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swcadguy

Mechanical
Jun 19, 2008
4
US
I am designing a simple fixture and would like to use a worm and worm gear arrangement for rotatation . I need to ratate the load +/- 5 degrees from zero

1. Can I move the load by turning the hand knob on top?
2. Any chance the worm gear will back-drive the worm?

Any help on this would be greatly appreciated
 
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hi swcadguy

Short answers no you can't back drive the worm

regards

desertfox
 
It will backdrive if the lead angle of the worm is high enough. Keep the lead angle below the arctan of the friction coefficient. A 5-6 degree (friction coefficient about .1) lead angle or lower generally will prevent backdriving.
 
It definately won't back drive with a single start worm of 168:1 ratio.

If the load has to be raised vertically check out this link for some calculations:


Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

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If your system is on a vibrating system it may back drive no matter what the lead angle is.
 
I agree, israelkk has a great point.

For a good general discussion of worm drives, "Machinery design" by Deutschman et al has a nice chapter.


Cheers

Greg Locock

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
To all,

Thanks so much for the input. First time using this forum but it is awesome.
My fixture see`s no vibration. Only static load. The worm/worm gear I have picked pout has a lead angle of 14 1/2 degree ( 180:1 ratio, 48 pitch). Have not see any lesser L,A..

Anybody have experience with this smaller worm/worm gear arrangement?
I understand MOST worm gears will not back drive but some will.


Again, Thanks for all the help.
 
The worm has a lead angle of 1 degreee 35 minutes
The worm gear has a pressure angle of 14 1/2 degrees..

Thanks for your help
 
Rule of thumb is the worm will not backdrive if the ratio is 50:1 or higher. You should have no problem at 180:1. If your application is critical, you might consider a brake; they are readily available, and might offer more peace-of-mind.
Regards,
Lcubed
 
Lcubed,

I did the preiminary design of an optical scanner which had a 20" diameter worm gear. I cannot remember the gear ratio, but it was well over 50:1. I had to design an oversized worm to get the lead angle down to the point where it was self locking.

Rules of thumb are not 100% reliable.

JHG
 
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