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

> There are several materials that I would consider if I were using a ball 
> bearing for a pivot. Ultra High Density Polyethylene for one bearing 
> surface and a stainless steel, sapphire or ruby ball for bearing.
> 
> Using this chart on Coefficient for Static Friction 
> http://www.carbidedepot.com/formulas-frictioncoefficient.htm a sapphire or 
> tungsten carbide ball on a polyethylene seat look petty good.

Hi Gordon,

       Thanks for the interesting table. We don't need a very high frictional 
coefficient, but we do need the surfaces to be non welding, non reactive, 
very hard and to be able to take quite a high load elastically. For instance, if 
you use ordinary unlubricated ball bearings, you eventually end up with a 
'stuck' suspension and a very small pile of rust.

> The UHDPET would need to be pretty thin and backed by a rigid surface so 
> the ball didn't creep into it and make a round socket that increased the 
> friction. To over come the creep problem using a tungsten carbide tool 
> insert with a screw hole in it as a socket 
> http://www.jjjtrain.com/vms/Media/glossary_c/carbide.jpg and a sapphire 
> ball may make a better bearing but the sharp edge of the hole in the 
> tungsten carbide tool much be lapped smooth.

       This is a problem that 'you don't need to have'! You look for carbide 
triangular turning inserts without a central hole, with sides of about 0.3". 
These will polish up nicely with some diamond paste and cost maybe $3. 

       Stainless steel ball bearings seem to work very well. You can buy just 
the balls - you don't need to buy a bearing!

       Regards,

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



There are several materials=
 that I would consider if I were using a ball=20

bearing for a pivot. Ultra High Density Polyethylene for one bearing=20

surface and a stainless steel, sapphire or ruby ball for bearing.



Using this chart on Coefficient for Static Friction=20

http://www.carbidedepot.com/formulas-frictioncoefficient.htm a sapphire=20=
or=20

tungsten carbide ball on a polyethylene seat look petty good.



Hi Gordon,



       Thanks for the interesting table. W=
e don't need a very high frictional coefficient, but we do need the surfaces=
 to be non welding, non reactive, very hard and to be able to take quite a h=
igh load elastically. For instance, if you use ordinary unlubricated ball be=
arings, you eventually end up with a 'stuck' suspension and a very small pil=
e of rust.



The UHDPET would need to be=
 pretty thin and backed by a rigid surface so=20

the ball didn't creep into it and make a round socket that increased the=
=20

friction. To over come the creep problem using a tungsten carbide tool=20

insert with a screw hole in it as a socket=20

http://www.jjjtrain.com/vms/Media/glossary_c/carbide.jpg and a sapphire=20

ball may make a better bearing but the sharp edge of the hole in the=20

tungsten carbide tool much be lapped smooth.




       This is a problem that 'you don't n=
eed to have'! You look for carbide triangular turning inserts without a cent=
ral hole, with sides of about 0.3". These will polish up nicely with some di=
amond paste and cost maybe $3.=20



       Stainless steel ball bearings seem=20=
to work very well. You can buy just the balls - you don't need to buy a bear=
ing!



       Regards,



       Chris Chapman