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