In a message dated 13/05/2009, rog@.......... writes:
Chris,
Do you have any close up pictures of your upper and lower pivots?
Rob
Hi Rob,
Sorry, but I don't have any other photos.
_http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg_
(http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg)
The bottom bearing is a 1/2" OD SS ball bearing crimped in a 1/2" hole
drilled into the upright bar. The arm is 1/2" HT30 Aluminum alloy, turned
flat on the end. A rectangular SS faceplate about 10mm x 8mm and 16 thou
thick is stuck onto this using 2 component acrylic adhesive Holdtite ST3295.
The faceplate was cut from a Swann Morton medical scalpel blade using a 1"
carbide disk. Other brands of 2 component acrylic adhesive are available.
Unlike the brittle epoxies, acrylic adhesives are slightly flexible, very
tough and stick the softer metals quite well.
The top suspension is a short 8 thou OD nickel plated piano wire
clamped between two bronze disks with an eccentric centre hole. The inside faces
of these disks are both recessed about 15 thou so that only the outside
1.5mm rings grip the wire. The wire is threaded through a hole in the centre
bolt. Both eccentric disks can be rotated to vary the position of the wire
clamp.
Behind the disks is the 'wire wind on' SS bolt for trimming the
position of the arm. It is held in position by friction at the base using a
spring washer and a SS nut. The wire wraps into the bolt thread, so that the
wind-on position is constant. The bolt is drilled near the top. The wire is
threaded through this hole and is clamped by the top nut. I used a taper
reamer to put a small flat on the tip of the first thread in this nut, so that
the wire is clamped between this flat and the V of the bolt thread. To trim
the position of the mass, you slack off the bolt clamping the bronze
washers and adjust the 'wind on' bolt. Then you clamp the bronze washers again.
The V suspension is 30lb 7 core SS fishing trace with crimped end
loops fitting in V section rings on the 1/2" OD extension rods bolted to the
square mounting plate. The top fitting is a 1" and a 1.5" OD SS mudguard
washers stuck together with acrylic adhesive. The 1.5" disk has a V section
edge to hold the trace wire. I turned this groove, but you can use a
triangular needle file. The washer is 1/16" thick. The 4 mm SS wire clamp bolt is
suitably drilled, the wire is threaded through and clamped between two flat
SS washers. This V suspension prevents the mass from rotating about it's
long axis due to any off centre forces from the damping blade.
The mass should be prevented from rotating / oscillating around the
long axis of the arm. This motion may not be damped on amateur seismometers
and can give rise to serious resonances at a few Hz, particularly with
single wire top suspensions. Unless the axis of the damping force intersects the
perpendicular from the centre of mass to the swing axis, any earth motion
will try to rotate the mass as well as deflecting it from side to side.
This null is difficult to achieve in practice. You can use a V wire or a twin
tube suspension. You can also design the magnetic damping to control both
rotational and translational motions of the mass - in two directions at
right angles.
You need to make the arm and the mass from non magnetic materials. Never
mount a magnet on the arm. Never use a ball rolling on a plane - they slip
too easily. Choose the position of the top support so that the bottom
'bearing' has an almost zero vertical loading.
Designing out problems / unwanted responses is something of an art!
I hope that this helps?
Regards,
Chris Chapman
In a message dated 13/05/2009, rog@.......... writes:
Chris,
Do you have any close up pictures of your upper and=
lower
pivots?
Rob
Hi Rob,
Sorry, but I don't have any other photos.
The bottom bearing is a 1/2" OD SS ball beari=
ng
crimped in a 1/2" hole drilled into the upright bar. The arm is 1/2" HT30=
Aluminum alloy, turned flat on the end. A rectangular SS faceplate ab=
out
10mm x 8mm and 16 thou thick is stuck onto this using 2 component acr=
ylic
adhesive Holdtite ST3295. The faceplate was cut from a Swann Morton medica=
l
scalpel blade using a 1" carbide disk. Other brands of 2 component acrylic=
adhesive are available. Unlike the brittle epoxies, acrylic adhesives are=
slightly flexible, very tough and stick the softer metals quite well.
The top suspension is a short 8 thou OD=
nickel
plated piano wire clamped between two bronze disks with an eccentric centr=
e
hole. The inside faces of these disks are both recessed about 15 thou so=
that
only the outside 1.5mm rings grip the wire. The wire is threaded through=
a
hole in the centre bolt. Both eccentric disks can be rotated to vary=
the
position of the wire clamp.
Behind the disks is the 'wire wind on' SS bol=
t for
trimming the position of the arm. It is held in position by friction at th=
e base
using a spring washer and a SS nut. The wire wraps into the bolt thre=
ad, so
that the wind-on position is constant. The bolt is drilled near the=
top. The wire is threaded through this hole and is clamped by the top=
nut. I used a taper reamer to put a small flat on the tip of the=
first
thread in this nut, so that the wire is clamped between this flat and the=
V of
the bolt thread. To trim the position of the mass, you slack off the=
bolt
clamping the bronze washers and adjust the 'wind on' bolt. Then you clamp=
the
bronze washers again.
The V suspension is 30lb 7 core SS fishing tr=
ace
with crimped end loops fitting in V section rings on the 1/=
2"
OD extension rods bolted to the square mounting plate. The top=
fitting is a 1" and a 1.5" OD SS mudguard washers stuck together with=
acrylic adhesive. The 1.5" disk has a V section edge to hold the trac=
e
wire. I turned this groove, but you can use a triangular needle file. =
;The
washer is 1/16" thick. The 4 mm SS wire clamp bolt is suitably drilled, th=
e wire
is threaded through and clamped between two flat SS washers. Thi=
s V
suspension prevents the mass from rotating about it's long axis due to any=
off
centre forces from the damping blade.
The mass should be prevented from rotating /=
oscillating around the long axis of the arm. This motion may not be damped=
on
amateur seismometers and can give rise to serious resonances at a few Hz,=
particularly with single wire top suspensions. Unless the axis of the damp=
ing
force intersects the perpendicular from the centre of mass to the swing ax=
is,
any earth motion will try to rotate the mass as well as deflecting it from=
side
to side. This null is difficult to achieve in practice. You can use a V wi=
re or
a twin tube suspension. You can also design the magnetic damping to=
control
both rotational and translational motions of the mass - in two directions=
at
right angles.
You need to make the arm and the mass from no=
n
magnetic materials. Never mount a magnet on the arm. Never use a ball=
rolling on a plane - they slip too easily. Choose the position of the top=
support so that the bottom 'bearing' has an almost zero vertical
loading.
Designing out problems / unwanted responses=
is
something of an art!
I hope that this helps?
Regards,
Chris Chapman
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