Note - this was posted to the PSN list at the same time as to Randall - =
my PSN post bounced back because of email address issues.  Bob

Randall,

Thank you for raising the issue of mechanical properties again.  I would =
like to add to the argument that better mechanical design is important =
also.  The last years of my career were spent designing laser pattern =
generators used to make "masks" for the semiconductor industry and now I =
edit the Horological Science newsletter for readers interested in the =
science of timekeeping.  Both endeavors involve electronic and =
mechanical design.  It is interesting to me to see how many newcomers to =
both endeavors want to "solve the problem" with sensor/electromechanical =
solutions.  Education on good mechanical design for precision mechanisms =
seems to be limited to a very few schools and to specialist symposiums.  =
My "clock nut" friends are more likely to look for historical solutions =
than newly minted engineers even in the day and age of Google which has =
made the information easier to find.

My experience with materials is not at the level of your dislocation =
level of material defects.  I have heard it said that non doped single =
crystal silicon is the most perfect material known to man.  A diamond =
lattice, with essentially zero impurities (as measured by its electrical =
properties) and almost no dislocations (in as pulled, ingot form).  The =
classic (though dated) reference is "Silicon as a mechanical material"  =
>http://inst.cs.berkeley.edu/~n245/fa01/PETERSEN.PDF  Silicon can be =
chemically etched or diamond machined to almost any shape.  Is there a =
use in your applications?

Assembling a "stable structure" is an imposible task, but it is possible =
to do better with good design.  A good overview of the required design =
and assembly techniques is covered in R. V. Jones "Instruments and =
Experiences" - fasteners though holes in one object attaching it to =
another object is not stable.  Tutorial sessions at precision mechanism =
symposuiums provide additional material. A Google search for "design =
guidelines for thermal stability" yields a bit of useful information. =
Most if not all materials undergoing a temperature cycle do not return =
to their initial dimensions!  Some materials do better than others.  Bob =
Matthys, in his book "Accurate Clock Pendulums" (Oxford University =
Press) evaluated many materials using the period of a pendulum as the =
measurement means (useful to better than .01ppm) - The best material he =
found was "aluminum silicon bronze" - e.g. it came back to its initial =
dimensions better than other materials.  Clearly a limited experiment, =
but a place where "amateurs" can contribute.

Much of the concern in seismometers seems to be the about the =
non-perfect nature of springs (or other mass suspension pivots, etc.).  =
I have attempted to suspend a pendulum so that it has the period vs =
amplitude predicted by theory.  It will not surprise you that springs, =
knife edges, rollers, etc give less than perfect results.  The best I =
have done is to use porous graphite air bearings.  =
http://www.newwayairbearings.com/  By best, I mean that the period vs =
amplitude error ("circular error" in clock vernacular) was the best.  An =
image of the pendulum and a bit of the results are at =
http://www.hsn161.com/HSN/airclock2.html  I asked Drew Devitt the =
president of New Way Bearings if he could supply a spherical air bearing =
to support a Foucault pendulum - he suggested a spherical segment =
supported on three pads with spherical seats.  It is not clear that this =
form of suspension is of any use for a seismometer, but perhaps one of =
the long period pendulum configurations could be implemented with air =
bearings.

Bob Holmstr=F6m
Editor Horological Science Newsletter
www.hsn161.com=

My =
experience with materials is not at the level of your dislocation level =
of material defects.  I have heard it said that non doped single =
crystal silicon is the most perfect material known to man.  A =
diamond lattice, with essentially zero impurities (as measured by its =
electrical properties) and almost no dislocations (in as pulled, ingot =
form).  The classic (though dated) reference is "Silicon as a =
mechanical material"  >http://inst.c=
s.berkeley.edu/~n245/fa01/PETERSEN.PDF  Silicon can be =
chemically etched or diamond machined to almost any shape.  Is =
there a use in your applications?

Assembling a "stable structure" =
is an imposible task, but it is possible to do better with good design. =
 A good overview of the required design and assembly techniques is =
covered in R. V. Jones "Instruments and Experiences" - fasteners though =
holes in one object attaching it to another object is not stable. =
 Tutorial sessions at precision mechanism symposuiums provide =
additional material. A Google search for "design guidelines for thermal =
stability" yields a bit of useful information. Most if not all materials =
undergoing a temperature cycle do not return to their initial =
dimensions!  Some materials do better than others.  Bob =
Matthys, in his book "Accurate Clock Pendulums" (Oxford University =
Press) evaluated many materials using the period of a pendulum as the =
measurement means (useful to better than .01ppm) - The best material he =
found was "aluminum silicon bronze" - e.g. it came back to its initial =
dimensions better than other materials.  Clearly a limited =
experiment, but a place where "amateurs" can contribute.

Much of =
the concern in seismometers seems to be the about the non-perfect nature =
of springs (or other mass suspension pivots, etc.).  I have =
attempted to suspend a pendulum so that it has the period vs amplitude =
predicted by theory.  It will not surprise you that springs, knife =
edges, rollers, etc give less than perfect results.  The best I =
have done is to use porous graphite air bearings.  http://www.newwayairbearings.co=
m/  By best, I mean that the period vs amplitude error =
("circular error" in clock vernacular) was the best.  An image of =
the pendulum and a bit of the results are at http://www.hsn161.com/HS=
N/airclock2.html  I asked Drew Devitt the president of New =
Way Bearings if he could supply a spherical air bearing to support a =
Foucault pendulum - he suggested a spherical segment supported on three =
pads with spherical seats.  It is not clear that this form of =
suspension is of any use for a seismometer, but perhaps one of the long =
period pendulum configurations could be implemented with air =
bearings.

Bob Holmstr=F6m
Editor Horological Science =
Newsletter
www.hsn161.com=