> -----Original Message-----
> From: Chuck Lewis [mailto:clewis@knology.net]
> Sent: Thursday, January 25, 2007 5:16 PM
> To: garyschafer@comcast.net; towerTALK@contesting.com
> Subject: Re: [TowerTalk] Flexible coupler assembly
>
>
> > Does a shock absorber in the mast like this really take any stress off
> the
> > rotor gears? The shock absorber will wind up allowing the antenna to
> twist
> > further than it would without the shock absorber and have much more
> > "travel"
> > when returning in the opposite direction thus increasing the force by
> > allowing more travel and the antenna to whip around more than without
> any
> > flexible joint.
>
> Yes, but:
>
> The donut helps because it spreads the impact over a longer time. Without
> the donut, the force is applied over a short time after the backlash in
> the
> gear train is taken up, and the gears absorb the total energy almost
> instantaneously. But the donut allows the same total energy to be applied
> over a much longer time, and the result is a much lower peak force on the
> system. Think of the donut as the physical equivalent of a capacitor. A
> capacitor smooths the sharp transients in a spikey waveform and lowers the
> peak voltage...the donut smooths the hard bangs in the rotor system and
> lowers the peak forces on the gears, etc.
>
> The issue of how another spring (donut) in the system affects the dynamics
> is really complex, and the donut could either help or hinder oscillations,
> depending on the mechanical resonances of the elements, mast flexion, etc.
> But in any case the above paragraph still applies.
>
Hi Chuck,
Yes I understand how the force is spread over time with the absorber. My
question is more concerned with how it will affect the forces when
oscillations are present.
We know that energy can not be created or destroyed it can only be stored.
So most of the energy that is absorbed by the donut will be given up in the
opposite direction to cause the antenna to rotate backwards from its initial
direction from when a wind gust moves it.
As it moves backwards the energy that was absorbed by the donut and the
energy absorbed by the mast and antenna itself will all add together to turn
the antenna in the opposite direction of the original rotation. If another
wind gust in the opposite direction, or on another vector from the first,
happens while things are unwinding the stored energy will add to the wind
gust energy and speed up the antenna rotation more than the wind alone would
and cause greater stress on the rotator than if the donut were not there.
The antenna would have a chance to move further with the donut than without
it. The antenna has a chance to pick up more momentum with the donut than
without it.
I know, energy can not be destroyed or created but it can be stored and
released at different times and if that release time coincides with a wind
gust then peak loads may be increased. I am not saying the donut is not a
good thing just some things to think about.
Another thought on the subject: It may be more convenient to mount the shock
absorber under the rotor between the tower and rotor.
73
Gary K4FMX
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