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Re: [TowerTalk] Flexible coupler assembly

To: "Chuck Lewis" <>, <>,<>
Subject: Re: [TowerTalk] Flexible coupler assembly
From: Jim Lux <>
Date: Thu, 25 Jan 2007 15:52:23 -0800
List-post: <>
At 02:15 PM 1/25/2007, Chuck Lewis wrote:

> > 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.

And, the saving grace in the case of the real device is that the 
donut isn't perfectly elastic.. it's lossy, so it's more like a RC 
low pass.. it absorbs some of the high frequency energy and 
dissipates it in the form of heat.

The real devices are available with a variety of properties, in the 
case of an automotive application probably toiled over for thousands 
of work hours to optimize it for the specific requirements.  For us, 
we sort of use what we can get.

The other advantage of the donut type devices is that they can 
tolerate a fairly substantial misalignment of the shafts (at the 
expense of increased loss from increased flexing of the elastomer, 
which affects its life).


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