Will worm gear/worm back drive?

[swcadguy]

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

 

[desertfox]

hi swcadguy

Short answers no you can't back drive the worm

regards

desertfox

 

[BobM3]

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.

 

[SincoTC]

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:

http://www.eng-tips.com/viewthread.cfm?qid=125486&page=8

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

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
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[israelkk]

If your system is on a vibrating system it may back drive no matter what the lead angle is.

 

[geoffthehammer]

israelKK is quite right - as I know from experience. I learnt my lesson!

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[swcadguy]

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.

 

[BobM3]

Is that the Lead Angle or the Pressure Angle?

 

[swcadguy]

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

 

[Lcubed]

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

 

[drawoh]

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