A voice from the past has arisen;  hopefully, it can be of some
benefit to you guys.  I recently happened to stumble across my name in
your seismicnet-psnl site--- in a Charles Patton message back in March. 
It, and several other messages on this site, have brought the memories
flooding back.

   As you might gather, I am semi-retired, doing consulting work only. 
However, my past experience--- especially at Sandia Labs (including
design of unattended seismic stations), Rolamite/Foothill Inc.
("inventor-engineer" was the most fun position of my career), and
Sparton Southwest (geophones and pressure instrumentation)--- might
qualify me to make a few comments on flexural suspension systems.

   I've long been a "champion" of flexures--- a paradox of simplicity,
and likely the least well known, yet most relied upon, mechanical
element.  Consider a few "elegant" (i.e., simple but highly effective)
examples--- steel tape measures (where both the "blade" and retracting
element are flexures), spring washers (common lock washer to
Belleville), leaf springs (including the one in that stapler on your
desk), paper clips (common and "binder"), and 3-ring binders (ever take
one apart to what creates the action?).  A few comments which might be
of benefit:

1.  Yes, spring metal flexures should definitely be the suspension of
choice in seismic instruments (rather than bearings, hinges, knife
edges, etc.).  They can provide zero friction, insensitivity to dirt,
zero lubrication, zero "dead zone", negligible hysteresis, and
indefinite life with no degradation.  They can also provide the
centering force for the "zero position" and, if desired, an electrical
path without dangling wires.

2.  I agree with the consensus that the rolamite, which includes rollers
as well as the flexural element, is NOT the suspension of choice. 
Basically, the rolling action, although very low friction, is
susceptible to minor variations on the contacting surfaaces
(contaminants, scratches, etc.) which become critical on the typically
tiny motions of seismometer pivots.  Alas, once again, the knock on
rolamite of being "the second-best answer to anything" has arisen! 
Incidentally, although I have some rolamite-related patents, I am not
the inventor of the basic device; that was the late Donald Wilkes.

3.  The "crossed-flexure" design certainly has a lot going for it, since
it can provide a very stable suspension as well as an "exact" center of
rotation.  For those designing devices which will go into production,
the Bendix Free-Flex flexural pivot is an excellent choice--- I see from
other messages that Lucas is now the manufacturer and/or distributor. 
However, for those building one-of-a-kind devices, a less expensive but
more time-consuming solution is to do it yourself.  I'm not familiar
with the "Cardan hinge", but your descriptions indicate to me that it is
also a crossed-flexure, utilizing a more complex flexure (band) which
provides simpler assembly.

4.  One possible exception to the choice of a crossed-flexure is the
"inverted pendulum", which can provide a long-period instrument in a
very compact package.  (I've had lab setups of 20-second periods.)  Yes,
the crossed-flexure could work here also, but it would be difficult to
do the final, precise "free-length" adjustment which is normally
required.

5.  As you can see from the above items, I generally agree with the
inputs and conclusions of B. Nordgren and S-T Morrissey--- "taut" vs.
"taught" not withstanding!  :)  Keep up the good work and analyses,
guys.

6.  The material from which flexures are made can be important.  I used
the term "spring metal flexure" above because this is normally what you
want.  Do NOT be tempted by the ease of plastics;  you'll eventually be
disappointed by hysteresis, "cold flow", and other degradations.  To a
lesser extent, soft metals (such as brass shim stock) have similar
problems, and are relatively easily damaged.  Spring steels are fine if
corrosion is controlled.  For flexure applications that "work hard"
(large deflections and/or large forces), beryllium copper is typically
the best choice, due to a high material effectiveness ratio--- allowable
stress divided by elastic modulus--- and good fatigue life.  However,
this is normally unimportant for the minute motions of seismic devices. 
BeCu also has the advantage of being readily formed via photochemical
machining (etching) into intricate two-dimensional shapes.

7.  For anyone wanting to pursue the design of flexures in more detail,
there is an unfortunate lack of textbooks, or even technical papers.  By
far the best book I've encountered (chock full of formuli, graphs, and
tables) on the subject is Elastic Elements of Instruments by L. E.
Andreeva.  It was originally published in Russian, but was translated
into English by the Israelis (IPST Cat. No. 2152).  It can probably be
found only in top-notch technical libraries.  I do have a small
collection of technical papers, including those published when rolamite
was "hot".

   For anyone who feels that further input from me might be of some
value, I'll plan on monitoring this site for a few weeks.  After that,
contact me via my E-mail address.  Think flexures!

	---Dean E. Gladow---

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