Re: Re: leadscrew bearings / machine design

Jon Elson writes:<snip>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.

Ron,

With one addition, what Jon has described above is what is known as a
"Rigid" end mount. The addition is: a sleeve of a PRECISE length to
allow the preload nut (on the leadscrew) to be tightened without
wrecking the bearings. It is often lost or omitted on re-builds!(It
is easier to "tweak" the nut a little more, than to grind a little
from the end of the sleeve -when the bearings wear)

A "Fixed" mount is similar, but the bearings are touching each other
(no space or spacer between). This has the effect of "fixing" the end
axially(thrust), but does not provide the "rigidity" to the leadscrew
of the spaced bearings. This should be obvious: simple mechanical
leverages at play.

A "simple" mount is a radial bearing alone. This will restrict axial
(thrust) loads only within the "clearance" of the bearing.(discussed
in an earlier post),typically .003-.010 . It also has even less
mechanical advantage (in preventing "whip" and "compression load
bending") than the Fixed mount.

Various combinations of these three types of end mounts are used (in
commercial applications) depending on cost and mechanical needs.

Most Leadscrew catalogs have a nomograph(chart) of the
length/diameter/distance/end mount relationship.

Lets go sideways a minute and talk about what we're trying to
accomplish with this choice:
1.Eliminate the EFFECT of resonant frequencies causing whipping of
the shaft. 2.Reduce or eliminate the effect of compression loading.
(Think of hanging a load from a thread, then pushing the load
"uphill" WITH the thread.)

For a given diameter, length, and rotational speed of leadscrew
(ballscrew too), there is a direct relationship between mount
rigidity and success dealing with these two items.

The most critical and precise applications use rigid mounts at both
ends.

The most common is a rigid or fixed at the motor end, with a simple
at the other end.

Hope this helps.

Ballendo

P.S.

>It does take some tricky boring... with a reduced
>diameter step in the middle. It makes preparing the end of the
>leadscrew very easy, however.
>Jon

You can use a straight bore with two(2!!) expansion rings for the
bearings to push against. You can also turn a sleeve and fix it into
the straight bore to give the "steps" needed.(pins, screws, glue,
etc.)