[TowerTalk] Worm Gears

[Patrick Greenlee]

Thank you so much for telling the truth and shunning the devil.  Now to 
check the 1/2 HP motors RPM, worm drive ratio, lubricant, etc to see if 
my soon to be delivered tower has its motors-winches-lubricant done right.

Thanks Howard.

Patrick   NJ5G

On 6/30/2015 12:55 PM, Howard Hoyt wrote:
> 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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