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[TowerTalk] Worm Gears

To: TowerTalk@contesting.com
Subject: [TowerTalk] Worm Gears
From: Howard Hoyt <hhoyt@mebtel.net>
Date: Tue, 30 Jun 2015 13:55:12 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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

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