Hi all,

Heres a quite different but very interesting and potentially
useful surplus item that could have best use in a hanging
mass (horizontal), i.e., a "S-G"; or, as part of a pivot
hinge for a vertical seismometer.  A strong incentive is
that the item is likely obtained just about everywhere world
wide at little cost...except for the rolling pivot rod axis used.
They can be found normally as obsolete, as factory rejects,
removed from equipment or even with damaged flat surfaces
from read heads.

This subject is in regard to discrete Hard Drive (HD) disks
(sputtered or electroplated; with a mirror like finish, and
NOT the older iron oxide reddish orange color disks) as part
of a seismometer pivot hinge.  See "normal" hard drive pictures
and misc., details at:
http://www.storagereview.com/guide2000/ref/hdd/op/ index.html

Specifically, its the inner inside circular but flat mounting
hole/ring that is part of the pivot.  The inner hole/ring on disks
I've seen is about 0.980" in diameter.  For a horizontal (S-G),
a straight rod rests on the separated disks inner ring surfaces;
and on the straight pivot rod is a shaft coupler with two
setscrews.  One setscrew is used to afix to the straight axis
pivot rod, and the other selected length sawed off head, screw,
is used to attach a boom/mass.  One could also use two
shaft couplers and space them with a bracket; to which a boom
is attached; or any number of ways of mechanical attachment
or improvements thereof.

For a vertical its much the same type of hinge layout with a
spring...somewhat like the Georgia Tech vertical, (it replaces
the flexure pivot/hinge), see:
http://quake.eas.gatech.edu/Instruments/LPVERTO.htm
or, any number of varietys of mechanical layout, including a
traditional mast type vertical.

The actual "desktop edge" horizontal test model was made out
of common material, and was only made to initially test its
usefulness.  For this model, the straight rod is 1/4" diameter
304 grade stainless steel, which was wiped and briefly lightly
sanded to get a clean surface.  Its length straightness was
only tested to roll straight down a flat incline surface for a
short distance; without deviating in its travel course.  Most of
the pivot/hinge support material is 3/4" thick MDF (Micro
Density Fiberboard), which is only glued directly onth the HD
disk with the brand "Titebond III Wood Glue".  The glue itself
is waterproof and is usually strong enough to where with any
wood, or MDF, will usually be well enough bonded to where
the wood or fiber will end up being torn at a different location
than at the joint.  Its not a "real" super glue, but good enough
for the test purpose; and is fairly commonly available in the
USA.  The boom was roughly 14" in length of aluminum
spacers and with a brass tube.  The mass is brass and
weighs about 2.25 pounds.  The two disks used were less
than 0.0315" thick for the contact surface...perhaps ~ 0.020"
wide at the rod contact area.  See:
http://www.geocities.com/meredithlamb/HDpivot.html

The two disks and MDF parts were aligned/glued together on
a surface plate for the bottom disk contact points and were
upright squared for contact with acouple "1-2-3" (1" x 2" x 3")
machinist blocks on the surface plate.  Alignment is critical.
One might be able to use plate glass as the "surface plate",
and use a carpenders square with clamps or similar devices
for doing the same alignment procedure.

Initial oscillation duration time tests were quite simple...but
obviously not lab quality.  The mass was displaced 1/16",
without dampening and a visual oscillation check was done
over time.  The time result was about 2 and 1/2 hours of
oscillation...really exceptionally long for a somewhat rough
setup: and was quite encouraging to say the least, for its
amazing lack of much friction at all, a most critical part of
any seismometer hinge/pivot.  A recorded duration would be
much better; but it would have to be done for each pivot made.
Of course a table top model isn't a very good place to test
such a model; as its subject to tilt from house hold movement
activity.  As a rough comparison, I've made/used acouple one
thousandth inch thick X 1/4" wide brass flexure hinges on the
same size mass in a hanging mass horizontal, and got about
30 minutes of free oscillation with a initial displacement of
the mass of 1/4".

The period of the described setup is about 1 second.  However
with a displacement sensor; the actual recorded seismic
signal phases can indeed be very much longer.  Its a often
overlooked aspect of hanging mass or S-G seismometers
with a sensitive sensor.  Another positive aspect is that they
require alot less base setscrew adjustments over time than
other types of horizontals...as the mass is absolutely a
zeroing reference to the earths gravity.  One light displacement
sensor is described at:
http://jclahr.com/science/psn/chapman/photo_detect/index.html

The specific hinge type is similar to a "crossed rod" hinge
described by Chris Chapman on a past PSN email some time
back.  This is not a direct model of a crossed rod hinge as such;
as the rod rolls on curved (relative to the direction of sensing)
flat surfaces on the inner disk ring.  It is possible that with
enough mass weight the straight rod could actually be bending
and rolling on the disks inner circle rounded chamfer.  The
curved rolling surface is most ideal for a "S-G", at least in its
direction of oscillation (sensing direction), as its curved zeroing
placement is more or less absolute once adjusted for...less
any tilt from ground movement or the the normal causes
experienced by any seismometer.

A real problem with this type of hinge is that the rod axis can
be displaced laterally to the ground reference accidentally.  One
idea might be to use opposing or same pole ring magnets
mounted both on the rod and other frame mounted magnets in
a opposing (North to North, or South to South pole) magnetic
field to "fix" its tendency to wander over time.  I've not tried
such yet; but it sees like a fix, so too speak.  The pivot rod
would be ideally non-magnetic as also with any shaft couplers
used.

It is suggested that the spacing between any two disks be
more than 3/4" if possible....especially on a vertical.  The
wider the spacing the more stable it should become.  I
accidentally knocked the mass on its axis rod side (S-G), and
it actually rocked back and forth with a 3/4" spacing for about
half a minute.

Such a hinge/pivot can actually have acouple varietys of
mechanical setups.  Outside of the description above, its
quite possible to mount the disks on a square hollow boom,
and have the straight rod affixed to two separate support
masts...I would think it might then oscillate longer due to
the diameter of the inside ring to the pivot straight rod.
Going further in this possible approach; once could have a
"Steel Works brand, 1" aluminum tube (square)", of a
chosen length, plug the bottom end, partially fill it with
shotgun lead shot pellets (the mass), and afix any
dampening copper or aluminum to the arrangement.  The
square tube would be oviously quite rigid which is desireable
and more easier to add component on their flat surfaces.  The
"Steel Works" brand is found in most USA hardware or home
improvement outlets like Home Depot or Lowes, at a
reasonable price.

These hard drive disks can obviously be found in defective
computer hard drives, and if, one can tear them apart
successfully.  For another source, they be found fairly
frequently at (getting rare) electronic surplus stores, in
various sizes from ~1.5 to 3.5" in diameter.  One needs
two disks, or; its also possible to saw up one disk and use
the two halves as part of the hinge.  Their disk thickness
can vary of course; I've seen 0.0315: and 0.050" thicknesses.
The actual rod contact surface is less than the disk thickness
with the machining chamfers done.  Over the years, I've seen
seen these items sell for between 10 and 25 cents each...a
really cheap price for a part of a potentially really effective
ground displacement sensing seismometer pivot/hinge.

Unfortunately, I haven't been able to discern the actual
plating material on the hard drives disks or their thicknesses.
Most of the disks are a rigid aluminum alloy; with the
plating adding alittle stiffness.  I think the hard metal
material could be a nickel metal plate, based solely from
the greyish mirror finsish reflection color tint.  The actual
computer useage magnetic recording medium is probably
measured in millionths of a inch thick; but I don't think
this applys to the metal thickness on the aluminum disk
itself.  This is some thin copper plating inbetween the
aluminum and the possible nickel plating.  If a reader has
more knowledge of the plating layers, I would be interested
to read of such.  They are high precision machined pieces,
and as a whole are guite rigid for their thickness.  I would
suggest using either new drills or carbide drills to work
any mounting holes, or other higher quality sharp tools..
as too much pressure with a worn drill can distort/bulge out
the drilling area surface of the disk.

Their is other possible seismometer uses for hard drive disks.
They do have a thin copper plating that is accessible via
careful sanding off of the top tough metal layer; which makes
its possible for use as a capacitance plate sensor, using
normal lead solder for lead wire attachment.  One would need
a 200 watt high heat soldering iron because of the aluminum
heat sink mass cooling; but I've done that before successfully.
Although small diameter, they be stacked for thickness and
then used with various length metal spacers/standoffs
inbetween for use as a S-G (horizontal sensor) frame with
having enough plates available.  One could also use
threaded rods and nuts as that would probably be a more
convenient and more accurate adjusted leveling spacing in
relation to the base plate used.  There is probably many
more good uses.

Take care, Meredith Lamb
Hi all,
 
Heres a quite different but very interesting and potentially
useful surplus item that could have best use in a hanging
mass (horizontal), i.e., a "S-G"; or, as part of a pivot
hinge for a vertical seismometer.  A strong incentive is
that the item is likely obtained just about everywhere world
wide at little cost...except for the rolling pivot rod axis used.
They can be found normally as obsolete, as factory rejects,
removed from equipment or even with damaged flat surfaces
from read heads.
 
This subject is in regard to discrete Hard Drive (HD) disks
(sputtered or electroplated; with a mirror like finish, and
NOT the older iron oxide reddish orange color disks) as part
of a seismometer pivot hinge.  See "normal" hard drive pictures
and misc., details at:
http://www.storagereview.com/guide2000/ref/hdd/op/ index.html
 
Specifically, its the inner inside circular but flat mounting
hole/ring that is part of the pivot.  The inner hole/ring on disks
I've seen is about 0.980" in diameter.  For a horizontal (S-G), 
a straight rod rests on the separated disks inner ring surfaces;
and on the straight pivot rod is a shaft coupler with two
setscrews.  One setscrew is used to afix to the straight axis
pivot rod, and the other selected length sawed off head, screw,
is used to attach a boom/mass.  One could also use two 
shaft couplers and space them with a bracket; to which a boom
is attached; or any number of ways of mechanical attachment
or improvements thereof. 
 
For a vertical its much the same type of hinge layout with a
spring...somewhat like the Georgia Tech vertical, (it replaces
the flexure pivot/hinge), see:
http://quake.eas.gatech.edu/Instruments/LPVERTO.htm
or, any number of varietys of mechanical layout, including a
traditional mast type vertical.
 
The actual "desktop edge" horizontal test model was made out
of common material, and was only made to initially test its
usefulness.  For this model, the straight rod is 1/4" diameter
304 grade stainless steel, which was wiped and briefly lightly
sanded to get a clean surface.  Its length straightness was
only tested to roll straight down a flat incline surface for a 
short distance; without deviating in its travel course.  Most of
the pivot/hinge support material is 3/4" thick MDF (Micro
Density Fiberboard), which is only glued directly onth the HD
disk with the brand "Titebond III Wood Glue".  The glue itself
is waterproof and is usually strong enough to where with any 
wood, or MDF, will usually be well enough bonded to where
the wood or fiber will end up being torn at a different location 
than at the joint.  Its not a "real" super glue, but good enough
for the test purpose; and is fairly commonly available in the
USA.  The boom was roughly 14" in length of aluminum
spacers and with a brass tube.  The mass is brass and 
weighs about 2.25 pounds.  The two disks used were less
than 0.0315" thick for the contact surface...perhaps ~ 0.020"
wide at the rod contact area.  See:
http://www.geocities.com/meredithlamb/HDpivot.html
 
The two disks and MDF parts were aligned/glued together on
a surface plate for the bottom disk contact points and were
upright squared for contact with acouple "1-2-3" (1" x 2" x 3")
machinist blocks on the surface plate.  Alignment is critical.
One might be able to use plate glass as the "surface plate",
and use a carpenders square with clamps or similar devices
for doing the same alignment procedure.
 
Initial oscillation duration time tests were quite simple...but
obviously not lab quality.  The mass was displaced 1/16",
without dampening and a visual oscillation check was done
over time.  The time result was about 2 and 1/2 hours of
oscillation...really exceptionally long for a somewhat rough
setup: and was quite encouraging to say the least, for its
amazing lack of much friction at all, a most critical part of
any seismometer hinge/pivot.  A recorded duration would be
much better; but it would have to be done for each pivot made.
Of course a table top model isn't a very good place to test
such a model; as its subject to tilt from house hold movement
activity.  As a rough comparison, I've made/used acouple one
thousandth inch thick X 1/4" wide brass flexure hinges on the
same size mass in a hanging mass horizontal, and got about
30 minutes of free oscillation with a initial displacement of
the mass of 1/4".
 
The period of the described setup is about 1 second.  However
with a displacement sensor; the actual recorded seismic
signal phases can indeed be very much longer.  Its a often
overlooked aspect of hanging mass or S-G seismometers
with a sensitive sensor.  Another positive aspect is that they 
require alot less base setscrew adjustments over time than
other types of horizontals...as the mass is absolutely a
zeroing reference to the earths gravity.  One light displacement
sensor is described at:
http://jclahr.com/science/psn/chapman/photo_detect/index.html
 
The specific hinge type is similar to a "crossed rod" hinge
described by Chris Chapman on a past PSN email some time
back.  This is not a direct model of a crossed rod hinge as such;
as the rod rolls on curved (relative to the direction of sensing)
flat surfaces on the inner disk ring.  It is possible that with
enough mass weight the straight rod could actually be bending
and rolling on the disks inner circle rounded chamfer.  The
curved rolling surface is most ideal for a "S-G", at least in its
direction of oscillation (sensing direction), as its curved zeroing
placement is more or less absolute once adjusted for...less
any tilt from ground movement or the the normal causes
experienced by any seismometer.
 
A real problem with this type of hinge is that the rod axis can
be displaced laterally to the ground reference accidentally.  One
idea might be to use opposing or same pole ring magnets
mounted both on the rod and other frame mounted magnets in
a opposing (North to North, or South to South pole) magnetic
field to "fix" its tendency to wander over time.  I've not tried
such yet; but it sees like a fix, so too speak.  The pivot rod
would be ideally non-magnetic as also with any shaft couplers
used.
 
It is suggested that the spacing between any two disks be
more than 3/4" if possible....especially on a vertical.  The 
wider the spacing the more stable it should become.  I
accidentally knocked the mass on its axis rod side (S-G), and
it actually rocked back and forth with a 3/4" spacing for about
half a minute.
 
Such a hinge/pivot can actually have acouple varietys of
mechanical setups.  Outside of the description above, its
quite possible to mount the disks on a square hollow boom,
and have the straight rod affixed to two separate support
masts...I would think it might then oscillate longer due to
the diameter of the inside ring to the pivot straight rod.
Going further in this possible approach; once could have a
"Steel Works brand, 1" aluminum tube (square)", of a 
chosen length, plug the bottom end, partially fill it with
shotgun lead shot pellets (the mass), and afix any
dampening copper or aluminum to the arrangement.  The
square tube would be oviously quite rigid which is desireable
and more easier to add component on their flat surfaces.  The
"Steel Works" brand is found in most USA hardware or home
improvement outlets like Home Depot or Lowes, at a
reasonable price.
 
These hard drive disks can obviously be found in defective
computer hard drives, and if, one can tear them apart
successfully.  For another source, they be found fairly
frequently at (getting rare) electronic surplus stores, in
various sizes from ~1.5 to 3.5" in diameter.  One needs
two disks, or; its also possible to saw up one disk and use
the two halves as part of the hinge.  Their disk thickness
can vary of course; I've seen 0.0315: and 0.050" thicknesses.
The actual rod contact surface is less than the disk thickness
with the machining chamfers done.  Over the years, I've seen
seen these items sell for between 10 and 25 cents each...a
really cheap price for a part of a potentially really effective
ground displacement sensing seismometer pivot/hinge.
 
Unfortunately, I haven't been able to discern the actual
plating material on the hard drives disks or their thicknesses.
Most of the disks are a rigid aluminum alloy; with the 
plating adding alittle stiffness.  I think the hard metal 
material could be a nickel metal plate, based solely from
the greyish mirror finsish reflection color tint.  The actual
computer useage magnetic recording medium is probably
measured in millionths of a inch thick; but I don't think
this applys to the metal thickness on the aluminum disk
itself.  This is some thin copper plating inbetween the 
aluminum and the possible nickel plating.  If a reader has
more knowledge of the plating layers, I would be interested
to read of such.  They are high precision machined pieces, 
and as a whole are guite rigid for their thickness.  I would
suggest using either new drills or carbide drills to work
any mounting holes, or other higher quality sharp tools..
as too much pressure with a worn drill can distort/bulge out
the drilling area surface of the disk.
 
Their is other possible seismometer uses for hard drive disks.
They do have a thin copper plating that is accessible via
careful sanding off of the top tough metal layer; which makes
its possible for use as a capacitance plate sensor, using
normal lead solder for lead wire attachment.  One would need
a 200 watt high heat soldering iron because of the aluminum
heat sink mass cooling; but I've done that before successfully.
Although small diameter, they be stacked for thickness and
then used with various length metal spacers/standoffs 
inbetween for use as a S-G (horizontal sensor) frame with
having enough plates available.  One could also use
threaded rods and nuts as that would probably be a more
convenient and more accurate adjusted leveling spacing in
relation to the base plate used.  There is probably many
more good uses.
 
Take care, Meredith Lamb