Charles,

Earlier I had been trying to visualize such an arrangement and had come to 
the conclusion that I wasn't going to be able to find one, which then 
pushed me toward the idea of the driven pivot system.  Can you give me a 
bit more detail on the geometry you're thinking of.  I can fairly easily 
draw it up on the CAD program, which can be used to plot the locus of the 
pendulum with as much accuracy as you'd like.

When I tried using the description below, I couldn't construct anything 
that looked right.

Speaking of Rollamite pivots.  I am personally convinced that they appear 
to be quite attractive. One symmetrical design has very low (nearly zero) 
restoring force, and they are likely to suffer much less from the 
hysteresis effects you see in commercial crossed-foil bearings such as I 
believe were used in some high-end sensors (STS-1?).  In particular, when 
using the thinnest foil which can safely carry the expected load, the 
bending stresses in the foils are very low and the volume of material under 
stress is also tiny; both contributing to minimizing losses.  Like anything 
else, the proof is in the testing, but I think the chances of it working 
well are good enough to justify the time required.  The only down-sides 
that I can see is that they might be less stiff than other pivot designs 
relative to side loads and moments, and you would have to make sure that 
their environment was kept clean to avoid getting dust in the works.

Regards,
Brett

At 01:03 PM 2/17/2008 -0800, you wrote:
>Brett,
>
>There is another possibility rather than the moving pivot as you 
>describe.  Keeping in mind that the basic pendulum period is due to the 
>change in height of the bob during the swing that sets the period, then if 
>we flatten the swing, the period will increase.  Therefore starting with 
>the concept that the upper pivot, rather than the customary shape, a point 
>on a flat supporting surface, is a flat rolling on a curved surface.  If 
>this curved surface is such that the height of pendulum is constant over 
>the swing, then the period is infinite.  Obviously a bit much.  It also 
>has the problem that the surface is not round, but increasingly steep off 
>the center, a recipe for slipping.  So we marry that with the old 
>Rollamite bearings, to prevent side slip, and put on (immerse in?) lots of 
>lubricant to prevent stiction.  Of course this then comes back to the 
>current discussion about macroscopic metal hysteresis, but I suggest that 
>the Rollamite bearings in this case will be very fine wires just 
>sufficient to prevent side slip, not large springs supporting the mass of 
>the pendulum, so the macroscopic properties will not intrude.
>
>Regards,
>
>Charles R. Patton


__________________________________________________________

Public Seismic Network Mailing List (PSN-L)