PSN-L Email List Message

Subject: Re: low friction hinge discusson
From: "Charles R. Patton" charles.r.patton@........
Date: Mon, 07 Nov 2005 08:32:13 -0800

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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