Re: leadscrew bearings / machine design

Jon, Balendo, and all:

This is exactly the information I am after. Apparantly being a half-
wit is indeed a burden in life, as my father often scolded me :}
Despite what are likely clear descriptions, I've got to locate some
stackup drawing to get this down.

Since one of my screws will exceed 48", I've already decided to build
stability into my design and to use RIGID or FIXED supports. I will
study your responses in detail and ad infinitum. I will generate
some sketches of what I think I'm being told, and see if I can upload
them for confirmation.

Now, if I can just remember how many cherries go in a Manhattan!

Cheers, and thanks to all.

Ron

--- In CAD_CAM_EDM_DRO@egroups.com, ballendo@y... wrote:
> 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.)