In a message dated 10/11/02, CapAAVSO@....... writes:

> Hi PSN,
> Could someone please comment on how John Cole's use of a ball bearing at 
> the top suspension point is an advantage? Does this not introduce friction 
> where there wasn't any before? 
> 

Hi Cap,

       It depends on friction to 'stay put', but the resistance to motion is 
that of a rolling contact. With a spherical onto a near flat hard surface, 
both polished to optical smoothness, this force can be extremely small. 
Moreover, there is no spring effect due to a bending foil or wire and the 
centre of rotation is closely defined.
       It is of course possible to overload any rolling contact, but you can 
check up on rough figures for this by looking a rated loads for ball races. 
It can also be calculated. For a hard smooth surface, you can also stick a 
bit of a razor blade onto the top of a bolt, or you can lap and polish the 
surface of a triangular lathe tool bit - used of course! 

       Regards,

       Chris Chapman   
In a message dated 10/11/02, CapAAVSO@....... writes:



Hi PSN,

Could someone please comment on how John Cole's use of a ball bearing at the top suspension point is an advantage? Does this not introduce friction where there wasn't any before? 

Thanks, Cap 



Hi Cap,



       It depends on friction to 'stay put', but the resistance to motion is that of a rolling contact. With a spherical onto a near flat hard surface, both polished to optical smoothness, this force can be extremely small. Moreover, there is no spring effect due to a bending foil or wire and the centre of rotation is closely defined.

       It is of course possible to overload any rolling contact, but you can check up on rough figures for this by looking a rated loads for ball races. It can also be calculated. For a hard smooth surface, you can also stick a bit of a razor blade onto the top of a bolt, or you can lap and polish the surface of a triangular lathe tool bit - used of course! 



       Regards,



       Chris Chapman