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Will worm gear/worm back drive?

Will worm gear/worm back drive?

Will worm gear/worm back drive?

(OP)
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
 

RE: Will worm gear/worm back drive?

hi swcadguy

Short answers no you can't back drive the worm

regards

desertfox

RE: Will worm gear/worm back drive?

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.

RE: Will worm gear/worm back drive?


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
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756
 

RE: Will worm gear/worm back drive?

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

RE: Will worm gear/worm back drive?

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

RE: Will worm gear/worm back drive?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Will worm gear/worm back drive?

(OP)
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.

RE: Will worm gear/worm back drive?

Is that the Lead Angle or the Pressure Angle?

RE: Will worm gear/worm back drive?

(OP)
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

RE: Will worm gear/worm back drive?

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

RE: Will worm gear/worm back drive?

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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