In a message dated 24/04/2010, tchannel@............ writes:
Thanks
_http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.html_
(http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.html) Chris this is a helpful site. My question is similar to the
illustration Figure 7 (a)
In this illustration the top pivot and bottom pivot are vertical on the
same line. (is this how I should align the pivots?) My Lehman and others
have the ability to move this top pivot behind or in front of this line,
before anything else is done. I am not saying I should, but I can make such an
adjustment.
Hi Ted,
I deliberately did not refer to Fig 7a. This shows NEUTRAL stability.
I use something equivalent to Fig 8a, with Alpha set to only about 0.3
degree - the top flexure is displaced from the vertical towards the mass end by
about 2.5 mm for a 40 cm vertical position difference between the
suspension points.
There is a definite advantage in making the boom lie parallel to the
frame, with the damping and sensor systems all aligned parallel, but then
tilting the whole assembly up at the mass end to actually set the period. The
setup is then separate sequential operations which are not interdependent.
I deliberately avoided using a nipple for the top suspension. They are
difficult to make and they don't actually CLAMP the wire! There must be NO
'free play'. Also, I use finer wire, about 12 thou OD. I use two bronze
disks, about 0.15" thick. They are dished on the inside faces by about 15
thou to leave a 1 mm wide rim on the periphery. The wire is clamped between
these rims on opposite sides of the disk. The hole for the clamp bolt is
offset from the disk centre by 1/8" to allow for adjustment. The disks can then
be rotated as a pair to offset the clamp position exactly. The wire goes
through a hole in the clamp bolt, which is aligned and glued to the upright
column. See
_http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg_
(http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg)
>> In setting up the frame I can align these two pivots as shown in
this illustration, in line...... The most obvious answer is to line both
pivots up on the same vertical line, as illustrated. BEFORE attempting to set
the period, using the front leg.
If you set up the system to give neutral stability with the frame
horizontal, you may not be able to lower the mass end of the frame enough to
give the period that you want, before it hits the floor! Note that the three
adjustment bolts use a wavy spring washer and a locknut to prevent any
rotation after they have been adjusted. This seems to prevent any rocking in
the threads.
Crossed foil and Cardan suspensions do need to be accurately aligned
vertically.
Note also that I have used a V wire top suspension made from 30 lb SS
fishing trace with crimped loop ends. This prevents the arm from rocking
about it's longitudinal axis. It proved to be very difficult to balance the
drag from the damper accurately enough using a single wire. If the arm can
rotate about it's long axis, you get spurious signals generated, especially
since this motion is not usually damped.
I tested my system by setting it up to give a stable period of 60
seconds. It behaved perfectly. I then reduced the period to about 25 seconds
for normal operation. You are limited in the maximum period that you can set
by the extreme sensitivity to tiny tilt drifts as the ground heaves
naturally with changes in air pressure, temperature and rainfall. The shorter arm
Sprengnethers were usually run at 15 seconds period to limit their natural
tilt drift, although they were specified to be used at periods up to 30
seconds.
I hope that this all makes sense.
Regards,
Chris Chapman
In a message dated 24/04/2010, tchannel@............ writes:
Thanks http://www.geophys.uni-s=
tuttgart.de/oldwww/seismometry/man_html/node15.html =20
Chris this is a helpful site. My question is similar to the illustr=
ation=20
Figure 7 (a)
In this illustration the top pivot and bottom pivot are vertical on=
the=20
same line. (is this how I should align the pivots?)&=
nbsp;=20
My Lehman and others have the ability to move this top pivot behind or=
in=20
front of this line, before anything else is done. I am not saying=
I=20
should, but I can make such an adjustment.
Hi Ted,
I deliberately did not refer to Fig 7a. This=
shows=20
NEUTRAL stability. I use something equivalent to Fig 8a, with Alpha set to=
only=20
about 0.3 degree - the top flexure is displaced from the vertical towards=
the=20
mass end by about 2.5 mm for a 40 cm vertical position difference between=
the=20
suspension points.
There is a definite advantage in making=
the=20
boom lie parallel to the frame, with the damping and sensor systems all al=
igned=20
parallel, but then tilting the whole assembly up at the mass end to actual=
ly set=20
the period. The setup is then separate sequential operations which are not=
=20
interdependent.
I deliberately avoided using a nipple for the=
top=20
suspension. They are difficult to make and they don't actually CLAMP the=
wire!=20
There must be NO 'free play'. Also, I use finer wire, about 12 thou=
OD. I=20
use two bronze disks, about 0.15" thick. They are dished on the inside fac=
es by=20
about 15 thou to leave a 1 mm wide rim on the periphery. The wire is clamp=
ed=20
between these rims on opposite sides of the disk. The hole for the cl=
amp=20
bolt is offset from the disk centre by 1/8" to allow for adjustment. =
The=20
disks can then be rotated as a pair to offset the clamp position exactly.=
The=20
wire goes through a hole in the clamp bolt, which is aligned and glued to=
the=20
upright column. See http://jclahr.com/science/psn/chapman=
/2008%20lehman/lehman_prototype.jpg
>> In setting up the frame I=
can align=20
these two pivots as shown in this illustration, in line...... The mo=
st=20
obvious answer is to line both pivots up on the same vertical line, as=20
illustrated. BEFORE attempting to set the period, using the front=20
leg.
If you set up the system to give=
neutral=20
stability with the frame horizontal, you may not be able to lower the mass=
end=20
of the frame enough to give the period that you want, before it hits the=
floor!=20
Note that the three adjustment bolts use a wavy spring washer and a=
=20
locknut to prevent any rotation after they have been adjusted. This seems=
to=20
prevent any rocking in the threads.
Crossed foil and Cardan suspensions do need=
to be=20
accurately aligned vertically.
Note also that I have used a V wir=
e top=20
suspension made from 30 lb SS fishing trace with crimped loop ends. This=
=20
prevents the arm from rocking about it's longitudinal axis. It proved to=
be very=20
difficult to balance the drag from the damper accurately enough using=
a=20
single wire. If the arm can rotate about it's long axis, you get spurious=
=20
signals generated, especially since this motion is not usually=20
damped.
I tested my system by setting it=
up to give a=20
stable period of 60 seconds. It behaved perfectly. I then reduced the peri=
od to=20
about 25 seconds for normal operation. You are limited in the maximum peri=
od=20
that you can set by the extreme sensitivity to tiny tilt drifts as the gro=
und=20
heaves naturally with changes in air pressure, temperature and rainfall.=
The=20
shorter arm Sprengnethers were usually run at 15 seconds period to limit=
their=20
natural tilt drift, although they were specified to be used at periods up=
to 30=20
seconds.
I hope that this all makes sense.
Regards,
Chris Chapman