In a message dated 13/07/2009, paleoartifact@......... writes:
Hi All,
Both _http://www.myeclectic.info/SeismoPivots/seismopivots.htm _
(http://www.myeclectic.info/SeismoPivots/seismopivots.htm%20) and
_http://myeclectic.info/SeismoPivots/seismopivots.htm_
(http://myeclectic.info/SeismoPivots/seismopivots.htm) both seem to work on my Internet Explorer
BACKGROUND PIVOT: Chris Chapman's newer horizontal designs with his lower
pivot being a boom end flat that is pivoting on a mast mounted ball
bearing seems to be a very ideal, reliable and much more stable "standard pivot"
for those of us endeavoring to make a commonly used home brew horizontal
seismometer. It was studied in 2008 by him, Charles Patton and Brett
Nordgren and briefly described by the web paper:
See Lehman photo
_http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg_
(http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg)
This uses a 16 thou thick SS strip cut from a scalpel blade glued to
the end of the arm, rolling on a 1/2" SS ball on the upright. This has a
MUCH better stability than a ball rolling on a plane.
Also note the 30 lb 7 core V SS fishing trace suspension, which
prevents the arm from rotating about it's long axis. I tested this suspension OK
for stable periods of up to 60 seconds. Note also that the frame level
screws use a wavy washer and a locknut to prevent rotation as the frame expands
or contracts when the temperature changes. This seemed to improve the long
term drift stability quite a bit. SS dome nuts make a very good rolling
contact on the lower ends of levelling screws.
I find that 1/16" copper is preferable to Aluminum for the damping
plate. Aluminum is slightly paramagnetic and the edges of an Al plate need to
overlap the damping magnets at all times. If you don't do this, the arm
will skate sideways to the stop if the edge of the Al plate comes close to the
edges of the magnets.
A square section coil is attached to the underside of the Al mounting
block. This photo was taken before the sensor cable was installed along the
arm.
The question that comes up from all this is whether such a lower pivot/s
could be ~ reliably used in a home brew short period vertical spring
seismometer? I note no reference to such a trial; but that doesn't exclude that
someones hasn't already experimented with such?
This excludes known boom mounted balls on a mast flat. The size/weight of
the seismometer mass in mind is similar to that of the AS1.
This was effectively done with cylinders rolling on a plane with the
EAI vertical seismometer, now discontinued. I still have a copy of the
manual.
You need to space the two balls fairly well apart on the upright to
prevent them from slipping due to horizontal seismic motions. It is also
desirable to design the arm so that there is no vertical force on the rolling
contacts. You attach the spring just behind the centre of mass and the
weight of the arm is balanced by the weight of the mass about this attachment
point.
Alternatively, you can fit an I bar across the vertical and make a C
fitting to go around it, attached to the end of the arm. You can then use
two piano wires under tension from the ends of the C to the enclosed I.
I would also assume that it is likely necessary to have any such balls
mounted on the side of a mechanical support/s to fix the spring opposition.
It would necessitate two such pivots for maintaining vertical positional
reliability...i.e.; the "boom" would need to be "T" shaped to constrain its
otherwise undesirable lateral movement with only one pivot.
Regards,
Chris Chapman
In a message dated 13/07/2009, paleoartifact@......... writes:
Hi All,
Both http://www.myeclectic.info/SeismoPivots/seismopivots.htm=
and http://myecl=
ectic.info/SeismoPivots/seismopivots.htm both
seem to work on my Internet Explorer
BACKGROUND PIVOT: Chris Chapman's newer horizontal designs wi=
th his
lower pivot being a boom end flat that is pivoting on a mast mounte=
d ball
bearing seems to be a very ideal, reliable and much more stable "standar=
d
pivot" for those of us endeavoring to make a commonly
used home brew horizontal seismometer. It was studied in 2008=
by
him, Charles Patton and Brett Nordgren and briefly described by the=
web
paper:
This uses a 16 thou thick SS strip cut from=
a
scalpel blade glued to the end of the arm, rolling on a 1/2" SS ball=
on the
upright. This has a MUCH better stability than a ball rolling on a
plane.
Also note the 30 lb 7 core V SS fishing trace=
suspension, which prevents the arm from rotating about it's long axis. I=
tested
this suspension OK for stable periods of up to 60 seconds. Note also that=
the
frame level screws use a wavy washer and a locknut to prevent rotatio=
n as
the frame expands or contracts when the temperature changes. This seemed=
to
improve the long term drift stability quite a bit. SS dome nuts make a ver=
y good
rolling contact on the lower ends of levelling screws.
I find that 1/16" copper is preferable to Alu=
minum
for the damping plate. Aluminum is slightly paramagnetic and the edges of=
an Al
plate need to overlap the damping magnets at all times. If you don't do th=
is,
the arm will skate sideways to the stop if the edge of the Al plate comes=
close
to the edges of the magnets.
A square section coil is attached to the unde=
rside
of the Al mounting block. This photo was taken before the sensor cable was=
installed along the arm.
The question that comes up from all this is whether such a lowe=
r
pivot/s could be ~ reliably used in a home brew short peri=
od
vertical spring seismometer? I note no reference to such a tri=
al;
but that doesn't exclude that someones hasn't already
experimented with such?
This excludes known boom mounted balls on a mast flat. Th=
e
size/weight of the seismometer mass in mind is similar to that of th=
e
AS1.
This was effectively done with cyl=
inders
rolling on a plane with the EAI vertical seismometer, now discontinued. I=
still
have a copy of the manual.
You need to space the two balls fa=
irly
well apart on the upright to prevent them from slipping due to horizo=
ntal
seismic motions. It is also desirable to design the arm so that there is=
no
vertical force on the rolling contacts. You attach the spring just=
behind
the centre of mass and the weight of the arm is balanced by the weight of=
the
mass about this attachment point.
Alternatively, you can fit an I ba=
r across
the vertical and make a C fitting to go around it, attached to the end of=
the
arm. You can then use two piano wires under tension from the ends of the=
C to
the enclosed I.
I would also assume that it is likely necessary to=
have
any such balls mounted on the side of a mechanical support/s to f=
ix the
spring opposition.
It would necessitate two such pivots for maintaining vertica=
l
positional reliability...i.e.; the "boom" would need to be "T" sh=
aped
to constrain its otherwise undesirable lateral movement with only one=
pivot.
Regards,
Chris Chapman