"Joe AutoDrill" posted
in rec.crafts.metalworking on Fri,
27 Feb 2009 10:45:02 -0500:

One of our customers with a special three spindle drilling head is having
some problems. They went out and bought a drill press that can not get the
RPM / thrust settings right for a particular drilling process they need to
run.

Rather than buy a new drill press, they bought some tooling that can handle
the higher feed rate per revolution.

The head is being used to drill 2 holes at 0.326" diameter and one hole at
0.218" diameter. The material is stainless steel and their machine makes a
lot of thrust. They can not reduce the thrust or increase the RPM unless
they buy a new drill press and of course, they are resisting that idea
because the drill press they would need to buy (probably a used Bridgeport
actually) costs much more than the head I sold them.

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.

Before anyone asks, we can't change the input/output RPM of the head to
compensate either. If I could simply have the inpututput at 1:2, that
would solve it, but there is no room for the gearing inside their footprint
to do that...

Random thoughts from the wild welcomed! We have two solutions as noted
above, but the customer would like to avoid both if possible...

#1 - Grease change - Use a grease that has extra pressure additives.
This will help your bearings survive the incipient thrust loads by
keeping the boundary layer intact.

#2 - Cheap and easy way...wave washers. Old trick used by electric
motor manufactures to solve thrust loads without going to an angular
contact bearing. You might have to experiment....and it depends on how
high your loads are if this will work.

If I knew what size bearings you are using I could offer more help.
If you know what the loads are (radial and axial) and speed I can run
some life calculations and give you an *expected* life.
Since I have no real idea how your application is set up and what type
of fits you are using in your housing and shaft I can only offer
*general* suggestions.

and yes...I am a bearing engineer.

The other alternative is to go to an Angular contact or if there is
enough room try using matched sets of ball bearings or even a small
tapered roller bearing.