Hi Charles,
You got it!
Actually; Brett and Chris had a PSN email on the subject back in 2005, where Chris well defined such; and he also used
a pulley/s as a explanation example:
For those even vaguely interested; the double/grove/belt pulley shown is presently still on sale for $5 each. I can only guess the quanity they
have is quite limited. They do look precision made, and the inner belt surfaces look ultra smooth and of course the belt grooves would
handily contain and constrain the wire/s lateral and vertical setting used. Of course one can guess that there is a great variety of other
pulleys available that could do the same pivot purpose. I only have one such pulley actually; but it should be alot of fun to get more and actually
set up a horizontal sensing unit. It will be a natural short period, but with a positional sensor, it should obtain alot of the longer period
signals. Using pulleys is presently not a "proven" model approach....but at least too me, it looks quite feasible and much more mechanically
easier to machine and employ.
Am pretty sure their description of the material (stainless steel pulley), is not correct; I think its actually a nickel/chrome plated
alloy of aluminum, as it exhibits a eddy current result with magnets and it doesn't have the hand weight feel of real stainless steel.
Regardless, the pulley/s could greatly simplify a wire pivot setup, as at least the top 2 pulleys, would only need wire holes through
the outer top side/s and perhaps another drilled and tapped hole to secure the two ends of the wires for each loop. The bottom 2 pulleys
where the suspended/attached mass is; "could be" just a single grove pulley/s on either side of the attached rolling pivot mass.
Admittedly; there could be problem/s with using pulleys, but most likely with strict alignment of the pairs used. Getting the wire loop
lengths the same will probably be the most tedious adjustment. Of course with whatever pulley is actually used, you do need a
non-magnetic pulley/s around the bottom two attached to the mass.
Of course I don't have even near the expertise of any of you; but Brett's result looks the same as my very old flimsy table top test of years
back; i.e., ultra low friction, far and away the best of any pivot I ever tested. There I used 2 cyclinders. Low friction literally means a more
accurate representation of the seismic signals induced on the enertia seismic mass. Chris mentioned also, zero net torque! This
is of course also called a "Figure 8" suspension he has mentioned many times.
WONDERFUL on the collaboration effort with the pivots! That should be invaluable to everyone and a giant leap over the
older pivots traditionally used!
Take care, Meredith
On Tue, Sep 9, 2008 at 10:36 PM, Charles Patton
<charles.r.patton@........> wrote:
It might make a good crossed wire roll pivot - if it has about three brothers. Crossed wire/band pivots are similar to a Rollamite except with space between the rolls so they're not running in contact. In previous Brett Nordgren tests these types of pivots had the lowest friction.
Brett, Chris Chapman and I are putting the finishing touches on a web site page with exact formulas in spreadsheet form for several pivots so amateurs can experiment with more insight into what is happening with a given geometry, i.e., finding the locus for the pivot along with some practical comments about their use.
Formulas/spreadsheets for:
+ Crossed wire/band rolls (such as separated Rollamite cylinders)
+ Crossed wire/band suspensions (such as used in LIGO vibration isolation systems)
+ ball on plate (the typical pivot of choice for many Lehmans) (a bit of advance warning -- this is one of the worst)
+ plate on ball (much better than ball on plate, and same complexity of construction.)
Regards,
Charles Patton
![http= ://gelindgren.googlepages.com/080806.073329.ch1.psn.gif](3D"http://gelindgren.googlepages.com/080806.073329.ch1.psn.gif"){kind=link}
![http://www= ..blue-eagle-technologies.com/images/ch1.gif](3D"http://www.blue-eagle-technologies.com/images/ch1.gif"){kind=link}