In a message dated 15/06/2007, Paulc@........ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
When constructing my first experimen=
tal=20
Lehman units I used a piece of steel channel iron mounted in a vice at the=
=20
vertical member.
The first bearing surface was the he=
ad of=20
# 5 bolt polished, and mounted into a holed drilled into the=20
vertical member. The arm had a similar bolt mounted in the=20=
end=20
of an aluminum tube. This bolt had a hole in the center of the head and=20
a hard stainless ball glued into it.
Hi Paul,
You have two viable choices for a counterface f=
or=20
your SS ball bearing - another piece of very hard SS - like a bit of a=20
blade off a SS craft knife - you can get break off blades - or a triangular=20
tungsten carbide lathe tool tip - the plain ones are not expensive. I polish=
=20
mine with diamond paste. Use Devweld 531, Devcon Plastic Welder, Loctit=
e=20
Multibond, Permabond Quickbond or any other two component acrylic adhesive,=20=
NOT=20
an Epoxy. The acrylic glues have exceptional strength and are=20
'tough'.
<=
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
The suspension was a .020 piano wire=
and=20
the mass was a 5 Lbs roll of copper wire slide over the end of the aluminu=
m=20
pipe.
The top of the suspension wire was s=
imply=20
drooped over the top of the vertical member and "C" clamped to =
it=20
in order to made the pendulum as close as possible to=20
horizontal.
You need to control any tendency of the arm to=20
rotate about it's long axis. The easy way to do this is to put a 6" crossbar=
=20
just behind the mass and have a V suspension to the top bearing. 7 strand SS=
=20
fishing trace is great for this and it comes with crimp tube fittings. 30 to=
50=20
lbs line is OK.
You are better with a solid chunk of met=
al=20
for the mass, like a slice off a 3" to 4" OD brass rod.
You may find a mass of ~ 1 kg is easier, for bo=
th=20
the suspensions.
You need to provide a fixed / clamped top hinge=
=20
point.
<=
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
By moving the pivots laterally I cou=
ld see=20
the effect, and cause the mass to stop where ever I wanted within=20
reason.
OK comes the problem. After pu=
tting=20
guitar tensioner on the top of the vertical member, so I could adjust the=20
pendulum level, I could no longer cause the mass to stop=20
anywhere.
It seemed that no combination of lev=
el the=20
pendulum, changing the mass position moving the bottom pivot would allow t=
he=20
pendulum to stop.
Finally I realized that the adjustme=
nt=20
apparatus had moved the top wire suspension point back slightly so that it=
was=20
a bit behind the vertical member.
No longer directly vertical of the b=
ottom=20
pivot.
I find that I need to have the top p=
ivot=20
just slightly forward of the bottom pivot to get any pendulum I build to=20
balance.
Is there some rule of thumb... =
It=20
seems the closer to 90 degrees the angle between pendulum and vertical bec=
omes=20
the easier the pendulum will shift off balance.
Currently my bottom pivot is crossed=
hard=20
cylinders, and I want to do the top this way, but I would like to know mor=
e=20
about the "Pivotal Relationship" before I construct the=20
bearing.
Tungsten Carbide drills are not expensiv=
e=20
and the shafts make very good bearings. You can also buy tungsten carbide ne=
edle=20
rollers. You can also use shoulder bolts type 435 from McMaster Carr.
If you use a guitar tensioner, lead the wire=20
through a hole in a horizontal bolt and two washers and clamp them to=20
define the top flex point of the wire accurately - you need precisely define=
d=20
clamp edges. I 'dish' the washers slightly with a conical punch on a wo=
od=20
block.
If the length between the bearing and the balan=
ce=20
point of the arm close to the mass is L, the period T =3D 2 x Pi x Sqrt( L /=
(g x=20
sinA)), where A is the angle between the true vertical and the line joining=20=
the=20
centres of rotation of the top and bottom bearings. It is typically ~1/3 deg=
ree,=20
maybe less - a very small angle. If you have a ball or a vertical rolle=
r on=20
the vertical support column, the centre of rotation is the centre of the bal=
l or=20
the roller. If your ball or vertical roller is on the end of the arm --->=
=20
change it! g =3D 9.81 if L is in metres, g =3D 32.2 if L is in=20
feet.
Make the seismometer base a bit longer than the=
arm=20
and make the crossbar at the support end about 1/2 the base length --->=20
make a single unit supporting everything on the three screws. If you=20=
try=20
to use separate units, as described on psn, you will get never ending=20
problems. Set the height / position of the mass, taking account of the=20
sensor and damping systems. You set up the suspension angles with=20
three levelling screws on the base of the seismometer. Set up the cross=
=20
balance first, then set up the period and re-trim the cross balance as=20
necessary. Then set the damping to ~0.7 critical. If you push the arm 10mm t=
o=20
one side and release it, it should swing ~1/2 mm past the balance point.
Hope that this helps!
Regards,
Chris Chapman
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