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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|