Ned Simmons posted
in
rec.crafts.metalworking on Fri, 27 Feb 2009 11:18:52 -0500:

On Fri, 27 Feb 2009 10:45:02 -0500, "Joe AutoDrill"
wrote:



The bearings inside the head do not last very long because they are being
pushed way past their thrust ratings. Because of the close C-C spacing,
there are no larger bearings or "stacked" bearing options available to me at
this time without actually building them a brand new drilling head. They
are open to this as the fault here is clear - they need the proper machine
to drive the head...

But... Before we go down either the new and more robust head route or new
machine to drive the head route... I'm wondering if there are any bearing
experts out here who can tell me whether there are any specialty bearings
that take higher thrust numbers or even vibration better than a typical ABEC
1 bearing. I was thinking ceramic, but I don't think they would take
vibration very well... We were thinking of going to higher ABEC numbers on
the bearings but I'm not sure that will help either s it shows higher RPM
ratings, but not much difference in thrust ratings.

Assuming you're using standard deep groove bearings (62xx, 63xx,
etc.), the most obvious solution is to replace them with angular
contact bearings (72xx, 73xx). Angular contact bearings are better
suited to handling combined axial and radial loads, especially where
thrust predominates. But, there are fewer choices of sizes and
sealing/shielding available.

If you can't easily make a size-for-size swap, specifying deep groove
bearings with a larger than normal internal clearance will give you
some additional thrust capacity.

That is not the case.
Radial internal clearance is for accommodating tighter fits on either
the housing and/or the shaft the bearings will be mounted in/on.
More clearance is not the solution.
If anything more clearance will lead to the balls in the bearing being
able to ride higher in the raceway and get onto the shoulder of the
raceway...this will induce skuffing and skidding and will cause a
premature failure.

The internal clearance is specified
independently of ABEC class, usually with a C plus a number. There is
no consistent nomenclature between mfrs, so speak to a supplier you
trust or directly to the mfr's tech support before concluding that one
mfr's C3 is looser than another's C0. SKF has always been helpful when
I've had a question.

Larger internal clearance in a deep groove bearing allows the balls to
ride higher on the race's walls. This will put the contact points
between the balls and races more in line with an axial load, which is
what an angular contact bearing is designed to do.

Don't expect a ball bearing to be able to do what an angular contact
bearing can do. while ball bearings may be able to handle a certain
amount of thrust the more clearance just allows the ball to get out of
the load zone and into areas where there will be problems.