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@contesting.com wrote:
Message: 1
Date: Wed, 15 Jul 2015 11:24:01 -0400
From: Hans Hammarquist <hanslg@aol.com>
To: towertalk@contesting.com
Subject: [TowerTalk] Fwd: FW: Fwd: Worm Gears
Message-ID: <14e9250bb76-6d23-10cef@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
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