[TowerTalk] Worm Gears

[Howard Hoyt]

The torque differential between driving the input and output of a worm 
gear drive (driving the worm, versus driving the worm gear) is largely 
dependent on the pitch angle.  You could call driving the worm gear 
"back-driving" the worm gear drive, since worm gear drives are almost 
exclusively designed to be worm driven.

One could design two different worm gear drives with the same ratio yet 
different pitch angles, and the resulting drives would have greatly 
varied resistance to back-driving.  In general the resistance to being 
back-driven is due to this pitch angle, which determines how much of the 
back-drive force is translated to axial thrust on the worm vs. rotation 
of the worm via inclined-plane slippage.  As with a helical gear drive, 
at a 45° pitch angle, 50% of the worm gear torque is directly translated 
into worm rotation, the other 50% into worm axial thrust.  It is this 
axial thrust component coupled with insufficient lubrication which can 
make a static worm gear drive difficult to back-drive.

As stated previously, if properly lubricated friction should not be a 
dominant factor.  This means the viscosity must increase in an inverse 
proportional manner to the rotational velocity.  Obviously this leads to 
the conclusion at zero velocity there is no lubricant viscosity high 
enough, which is why friction then often becomes a factor in 
back-driving a worm gear drive.  Worm gear drives, as is the case with 
helical gear drives, are not specified for life expectancy in this 
lubrication regime.

A good tool for inter-relating these variables is the MITCalc:
http://www.mitcalc.com/en/pr_wormgear.htm

Cheers & 73,

Howie - WA4PSC




On 7/15/2015 12:00 PM, towertalk-request at contesting.com wrote:
> Message: 1
> Date: Wed, 15 Jul 2015 11:24:01 -0400
> From: Hans Hammarquist <hanslg at aol.com>
> To: towertalk at contesting.com
> Subject: [TowerTalk] Fwd:  FW:  Fwd:  Worm Gears
> Message-ID: <14e9250bb76-6d23-10cef at webprd-a01.mail.aol.com>
> Content-Type: text/plain; charset=utf-8
>
> I don't remember the exact formulas and I am not sure if it is only valid for worm gears but it turns out then you calculate efficiency for gear boxes you have a different efficiency depending on which direction the power goes.
>
>
> It turns out that when the efficiency is 50% when down-shifting, the efficiency is 0 when up-shifting. I am sure you noticed that is very hard ( if not impossible) to turn the output axle on a worm gear. (It is ~50 years ago I had this class, 1966). I suspect the same is valid for any gear box but let somebody else answer that.
>
>
> 73 de,
>
>
> Hans - N2JFS
>