Charles R. Patton wrote:
> 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.

That's the way it looks to me, as long as the friction of the hinge is 
proportional to velocity, not position (fluid friction, not lumpiness 
of the force versus position).

> 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.

A good reason to minimize boom mass, at least on the hinge end.  And 
it makes those units with huge, massive basses light up question marks 
in my brain.  Why do they make the moving part of the device so massive?

 > 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,

Yes, that dreaded lumpiness.

> 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.)

The biggest weakness of the ball and half race hinge I mentioned is 
that the ball might slide along the race instead of rolling, changing 
the side tilt of the hinge line, and so, shift the stable position of 
the mass.  Here in Virginia, I doubt that it will ever experience a 
large enough vibration to cause that, but in California, it probably 
isn't stable enough.

> 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.

Iron and gold are known for not sticking together.

>  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

Thanks for those.

> 
> 
> 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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