Hi. What is the optimal length for the hinge? The ones i've seen on this 
list all have very short hinges. Would there be a benfit to using perhaps a 
one centimeter gap between the frame and the boom or pendulum? The upper and 
lower hinges can both be in tension, or both in compression or one of each. 
i don't thing longer/wider gapped hinges will work in compression. Come to 
think of it, i haven't won any prizes for thinking. geofff<><
----- Original Message ----- 
From: "Charles R. Patton" 
To: 
Sent: Monday, November 07, 2005 11:32 AM
Subject: Re: low friction hinge discusson


> John hit upon the very question I have had since the beginning of this 
> discussion.  I love the discussion and the simple tests for getting to the 
> low friction combinations, but once the hinge friction (which multi-hour 
> swinging certainly qualifies) is significantly below the level you'll use 
> for damping even the longest period pendulum, then I would think other 
> considerations are more important such as the stability and robustness of 
> the hinge against large side forces (read big quake such as those in 
> California are prone to) displacing the hinge point, changing the 
> geometry, and hence the functionality of the pendulum.  On a pure geometry 
> basis, the hinge point is the means by which the frame moves, moving the 
> pivot point, leaving the bob weight stationary (on a instantaneous basis). 
> Insufficient side friction, and this goes badly awry.  Which is why I 
> always perk up when the discussions hinges (pun intended) around the 
> Rollamite, crossed leaf hinges, etc.  These have very large stability (are 
> rigid) against side forces.  I was especially impressed with the crossed 
> wire discussion a few days ago.  The thing that always bothered me about 
> all those flexible hinge types is discerning the actually hinge rotation 
> point/tragetory.  Brett Norden has done excellent work figuring out some 
> of them.  I've been too lazy to project the consequences into the 
> seismometer suspensions.  The question is that if the point of rotation 
> travels, then does it do so in such a way to lead to stability or 
> un-stability (longer period) swings, i.e., what is the longest period (or 
> largest swing) possible before it might go unstable?  Rollamites are 
> probably subject to another problem which is dust collection, but the 
> crossed wire 'Rollamite" version would be almost immune, again why I 
> thought that was an interesting suspension.  The  downside of that 
> suspension would seem to be the orthoganol unstability, i.e., the hinge is 
> relatively rigid in the plane of the rotation, but the orthogonal axis is 
> another hinge with a different period potentially  making the seismometer 
> sensitive to another axis (if using some versions of coil/magnet motion 
> sensors.)
> Oh yes, one other point (pun intended) about points on surfaces, it would 
> seem to me that mixing material is a good idea.  I belive I've heard that 
> generally sliding surface bearings are better if the materials are 
> different.  I'm not a tribologist, but I'm sure this extends to the 
> application of the ball point rolling on a surface, type bearing, too.
>
> Just some idle musings.  Regards,
> Charles Patton
>
>
>
> John Popelish wrote:
>
>> .....  It will absorb a little energy as the boom swings, but a lot less 
>> than I will have to absorb, elsewhere, to achieve the desired damping.
>>
>
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