[TowerTalk] Worm Gears

[Howard Hoyt]

Hi all,

Great topic!  Each gear setup is designed for a specific load/relative 
surface velocity in order to give good service.  Spur gears can be run 
in the open at very low velocities with zero lubrication due to the 
mainly rolling, not sliding motion between two properly contoured 
gears.  Of course, neglecting bearing drag, spur gear systems have 
extremely low resistance to back driving at all, even in high ratios.  
If you are using a spur gear winch without the brake engaged, watch out 
for the drive handle when it starts to spin, it can be back-driven to 
extremely high rpms and cause you damage!

Due to the sliding motion of worm gear systems, they rely on lubrication 
to achieve good functionality.  For a reference relative to this 
discussion, you can see my white paper at:

http://zddplus.labecon.com/TechBrief11%20-%20Internal%20Combustion%20Engine%20Lubrication.pdf

Although the paper is largely about internal combustion engines, in 
particular study the Stribeck Curve on page 2.  The advantage which worm 
gears have over spur gears is specifically the high resistance to back 
driving due to the lead angle and contact area, which I'll explain in a 
bit.  When run above critical speed with adequate lubrication, worm 
gears will operate in a hydrodynamic lubrication regime and as such will 
be very low friction, with the remaining drag due solely to viscous 
shear of the oil film.  Running worm gears without lubrication, or at 
very low speeds with insufficient lubricant viscosity will shift the 
operating point of the gear system to the left on the Stribeck Curve, 
increasing asperity contact, friction and wear.  This is why antenna 
rotators which use worm gears powered by a relatively high rpm electric 
motor can see long service with little wear, the high surface speed 
shifts the lubrication mode as seen on the Stribeck Curve to the right 
into the hydrodynamic regime.  On the other hand, a worm gear hand winch 
with it's intermittent and very low speed would have to use extremely 
thick grease in order to achieve this condition under load.

The high resistance to back-driving which worm gear systems display is 
due to this differential friction characteristic of driven vs 
back-driven interacting with the lead angle.  The lead angle is 
determined by the pitch of the teeth and diameter of the worm, and the 
higher the lead angle the lower the friction and resistance to 
back-driving.  When driving the worm at high speed the system is in a 
low-friction hydrodynamic regime with attendant low wear.  When 
back-driving, there is very high pressure at zero velocity and resulting 
high friction which is an advantage in many applications and will cause 
no wear...until the combination of force and lead angle induce 
movement.  Then the wear can be extreme.

Take home message: worm drive systems do not have high friction when 
operated correctly, and lubricant viscosity is not a user choice: it is 
a design variable in any system, and the manufacturer's recommendations 
should be followed.

Howie - WA4PSC
www.proaudioeng.com