Re: Angular Contact Bearings and Ballscrews question

--- In CAD_CAM_EDM_DRO@y..., "kaynrc" <tom.kay@n...> wrote:

> Hello
>
> I am starting to look for angular contact bearings to intsall in

one

> end of the leadscrews for a mill. I seem to recall that the screw

is

> firmly held at one end, while it can "float" in the other end,
> meaning that with expansion due to temperature, one end of the
> leadscrews sit in a radial bearing.

This is called the Fixed-Simple method of securing a ballscrew, and
yes, you are correct, it is done so for thermal expansion... be sure
the floating end is allowed to float, i.e. don't secure the outer
diameter in it's housing with snap-rings and such. In high speed
applications, it is recommended to use angular contact bearings on
both end, with one set of the angular bearings floating in it's
housing... as the angular contact set does two things, it handles the
axial forces applied to the screw and it gives rigidity to the shaft
to reduce the whipping action as the screw approachs critical
speeds.... that would be the Fixed-Fixed method.

>
> So, are there such beasts as double angular contact bearings? Or is
> it common practice to buy 2 for each screw, and mount them

backwards

> to take up thrust forces in both directions?

If you buy two, be sure to mount them "back to back" as this method
give the most rigidity to prevent whipping. Mounting the "front to
front" is never advised in a ball screw application.

>If they are sold in
> double row, do they make them small enough for a .631 screw, with a
> base diameter of about 0.500"?

Yes both INA, SKF and others sell double row angular contact ball
bearings... a single piece unit "ball selected" for preload at the
factory...not sure about SKF but INA is expensive... check out
INA.com and under products look at the AXIAL radial contact ball
bearings... axial is the key word here because ball screws appliy
most of their force in the axial direction, because radially they
only need to support the weight of the screw. The typical bearing
made these days are metric, it will be hard to find inch size
bearings. Check their website for aditional information....
www.ina.com and www.skf.com

>
> Would tapered roller bearings work just as well? Or are they too

long

> and bulky or something?

A poor choice. Runout may be a problem, as a screw rotates, if the
screw moves axially because of runout in the bearings, you have lost
all your accuracy... Good angular contact ball bearing have very
little axial runout.

>
> If anyone has cnc'd a mill and run into this, could you inform how
> you handled the bearings?
>
> Thanks a bunch, Tom.

Also, NOOK, NSK, and others amke or sell ball screw end bearing
assemblies....

Mike C