Re: leadscrew bearings / machine design

Jon:

I have uploaded a file called Bearing_stackup.bmp in the files
section. It contains my interpretation of your description for two
possible lead screw thrust bearing stackups. (sorry about the size)

Method A:
If this is correct, then I agree that the double counter bore would
prove to be a challenge for me to build. Especially since I planned
on enforcing concentriciy of the holes through the lead/guides
support plates by drilling them simultaneously. As I understand the
method A stackup, I'd have to drill the support plates and then
locate the hole center on the thrust plate to perform the counter
bore operation. Probably beyond my megar construction skills and shop
of tools consisting mostly of different size rocks :}

Also, because there is no direct load path from nut to the sholder on
the lead screw, I can easily see how the assembly would eat bearings
if not carefully adjusted. With no spacer, one bearing takes all of
the axial load...which depending on wether the screw is in tension or
compression. Correct?

Method B:
Now this looks pretty good to me. Easy to bore the clean hole, as you
mention. I would think that this arrangement could be improved by
inserting a spacer between the two outer races. With the proper
shiming and adjustment, the load path would be continious for both
races, and the axial load should be shared equally between the two
bearings.

What do you think? Are the stackups correct?

Once again, thanks to all who have addressed this issue.

Best regards,

Ron

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:
>
>
> r_fl_z@h... wrote:
>
> > Unfortunately, the manufacturers seem to show cartoons of common
> > bearing arrangements, but they ommit the entire stackup...showing
> > only the lead screw and the bearings. No retainment details are
> > provided.
>
> There are two pretty standard ways to do this, and they work with
radial
> or
> angular contact bearings, within the ratings of these units.
>
> If the threaded part of the leadscrew is larger in diameter than the
> ends, which
> is common for machine leadscrews, then you can rest one bearing
> against the turned flange this step provides. The outer housing can
> have a
> reduced diameter in the center of a bore that holds the bearing
outer
> races.
> This provides a surface to accomodate pressure from two bearing
outer
> races pressing inwards on that step. Finally, a fine threaded
collar
> can be
> provided to screw onto the end of the leadscrew to press the inner
race
> of the
> 2nd bearing toward the 1st bearing. This will allow you to preload
the
> bearing set until you get the desired end play - or wreck the
bearings.
> This is the scheme that is used on an ancient GE CNC axis drive that
> I adapted to my mill, and also how I did the same for my Z axis
drive.
> It is what I've seen on numerous other leadscrew applications.
>
> There is another way to do it, that may be simpler for certain
leadscrew
>
> configurations, especially with small diameter screws.
> You have to make a sleeve that attaches to the leadscrew such that
> it has a step of larger diameter in the center of the sleeve. Or,
the
> leadscrew could be made with this step in it. A bearing slides in
from
> each
> end to rest the inner race against the step. These bearings fit
into a
> housing made of such a length that the bearing outer races are about
> at the face of the housing. An end plate bolts over each bearing.
> By placing spacers over the outside of the most accessible bearing,
you
> can adjust preload, as the end plate presses on the spacer, which
> presses
> the outer race deeper into the bore, pressing the inner race
against the
> step
> in the collar.
>
> I prefer the threaded collar as it allows pretty fine adjustment of
the
> preload.
> It does require some tricky boring work to get a bore with a reduced
> diameter step in the middle. It makes preparing the end of the
> leadscrew
> very easy, however.
>
> Jon