In a message dated 06/11/05, jpopelish@........ writes:

> All this discussion of using parts from ball bearings as hinge bearings, 
> and it finally dawns on me that people are talking about everything except the 
> surfaces that are made precisely as low friction bearing surfaces.
> 
> I haven't yet gotten my seismometer completed, but here is how I built 
> my hinges.
> 
> I sawed a small (3/4 inch O.D.) ball bearing outer race in half with a 
> moto cut off wheel and used the two halves as one side of the each of 
> the hinge bearings.  I dimpled a piece of aluminum and epoxied a ball 
> from the same bearing into the dimple as the other bearing surface.

Hi John,

       The hinge angle is defined by the contact on the spherical ball and 
more especially by the centre of curvature of the ball. If you mount the ball on 
the arm, you will have a major reset up job any time that the arm is 
dismounted / moved.

> The lower bearing (in compression) is just the half outer race epoxied 
> to the head of a bolt with the groove facing out and the groove 
> vertical. The matching half is a bullet nosed piece of aluminum with 
> a dimple in the end and the ball in the dimple.

         It is much better to mount both balls on the fixed vertical support 
beam. The vertical axis is then defined by the centres of the balls. This 
keeps the side to side zero adjustment fairly constant.

    The ground radius of curvature of the track is not usually much larger 
than that of the ball. As the arm swings from side to side the ball will tend to 
ride up the fairly sharp curvature of the track. This will most probably tend 
to give a strong centring moment to the arm - it may well not swing freely or 
have a fixed period.  

> The upper bearing is made with the other half of the outer race 
> epoxied to a small slice of aluminum U channel so that it completes a 
> loop.  The part that holds the ball is slotted with the dimple in the 
> inside of the slot, so it can pull on the ball.  The channel slice has 
> a hole for a mounting bolt, opposite the half of the bearing race. 
> The groove is horizontal.

       With one groove horizontal and the other vertical, the ball bearings 
will be seeing surfaces with dramatically different rates of curvature. This is 
very likely to effect the balance setting, the freedom to swing and the 
period.

> This combination seems very stable but also self aligning to some extent to 
> allow for sloppy tolerances in the match between the boom and the base.

       I would expect it to be extremely stable, but the whole point about a 
seismometer arm is that it should swing completely freely and NOT be 
constrained by the curvature of the support! 

> This hinge makes use of the exquisitely precise, smooth and hard 
> surfaces intended to carry load in the original bearing.  I hope to 
> get around to testing the design soon, but life keeps getting in the way.

       I doubt if it will give you the results that you want, for the reasons 
that I have stated, but do try it out and let us know the results! Have a 'do 
it now campaign'? 

    You might just 'get away with it' if you used a very much smaller 
diameter balls in the ground outer bearing track and aligned both tracks horizontally?

> I don't have a web page set up yet, either, but if anyone is 
> interested, having trouble understanding my description and emails me 
> 

       If these are ordinary bearings and not stainless steel, you will need 
to put a drop of light oil on the contact point (clock oil?) and make an 
overlapped paper screen around the joint to prevent the oil from collecting dust.

       Good Luck!

       Chris Chapman
In a message=20=
dated 06/11/05, jpopelish@........ writes:



All this discussion of usin=
g parts from ball bearings as hinge bearings, and it finally dawns on me tha=
t people are talking about everything except the surfaces that are made prec=
isely as low friction bearing surfaces.



I haven't yet gotten my seismometer completed, but here is how I built=20

my hinges.



I sawed a small (3/4 inch O.D.) ball bearing outer race in half with a=20

moto cut off wheel and used the two halves as one side of the each of=20

the hinge bearings.  I dimpled a piece of aluminum and epoxied a ba=
ll=20

from the same bearing into the dimple as the other bearing surface.



Hi John,



       The hinge angle is defined by the c=
ontact on the spherical ball and more especially by the centre of curvature=20=
of the ball. If you mount the ball on the arm, you will have a major reset u=
p job any time that the arm is dismounted / moved.



The lower bearing (in compr=
ession) is just the half outer race epoxied=20

to the head of a bolt with the groove facing out and the groove=20

vertical. The matching half is a bullet nosed piece of aluminum with=20

a dimple in the end and the ball in the dimple.




         It is much better to=
 mount both balls on the fixed vertical support beam. The vertical axis is t=
hen defined by the centres of the balls. This keeps the side to side zero ad=
justment fairly constant.



    The ground radius of curvature of the track is not us=
ually much larger than that of the ball. As the arm swings from side to side=
 the ball will tend to ride up the fairly sharp curvature of the track. This=
 will most probably tend to give a strong centring moment to the arm - it ma=
y well not swing freely or have a fixed period.  



The upper bearing is made w=
ith the other half of the outer race=20

epoxied to a small slice of aluminum U channel so that it completes a=20

loop.  The part that holds the ball is slotted with the dimple in t=
he=20

inside of the slot, so it can pull on the ball.  The channel slice=20=
has=20

a hole for a mounting bolt, opposite the half of the bearing race.=20

The groove is horizontal.




       With one groove horizontal and t=
he other vertical, the ball bearings will be seeing surfaces with dramatical=
ly different rates of curvature. This is very likely to effect the balance s=
etting, the freedom to swing and the period.



This combination seems very=
 stable but also self aligning to some extent to allow for sloppy tolerances=
 in the match between the boom and the base.




       I would expect it to be extremel=
y stable, but the whole point about a seismometer arm is that it should swin=
g completely freely and NOT be constrained by the curvature of the support!=20



This hinge makes use of the=
 exquisitely precise, smooth and hard=20

surfaces intended to carry load in the original bearing.  I hope to=
=20

get around to testing the design soon, but life keeps getting in the way=
..




       I doubt if it will give you the=20=
results that you want, for the reasons that I have stated, but do try it=20=
out and let us know the results! Have a 'do it now campaign'? 



    You might just 'get away with it' if you used a ve=
ry much smaller diameter balls in the ground outer bearing track and aligned=
 both tracks horizontally?



I don't have a web page=20=
set up yet, either, but if anyone is=20

interested, having trouble understanding my description and emails me=20

with their address, I will take some photos and send them to you.



       If these are ordinary bearings and=20=
not stainless steel, you will need to put a drop of light oil on the contact=
 point (clock oil?) and make an overlapped paper screen around the joint to=20=
prevent the oil from collecting dust.



       Good Luck!



       Chris Chapman