Re: [CAD_CAM_EDM_DRO] Re: no thrust bearing - simple design ?

In a message dated 4/18/2003 10:40:33 PM Central Standard Time,
mmueller@... writes:

> How does one attach a leadscrew/threaded rod to a thrust bearing? Does one
> just make sure that the diameters are a tight fit, or are there screws to
> tighten or something?
>

Andrew:

I have built a dozen application-specific machines using various lead-screws,
most with stepper-motor/computer control. One DOES need some kind of
"thrust-bearing" on each such lead-screw, of course, but it need not
necessarily be an actual "thrust bearing" Attaching a thrust-bearing would
be difficult, even to describe, and anyway, one would need a "regular axial
ball-bearing" plus the T-bearing, were an actual T-bearing used.

For us ignoramous types, the "angular contact bearing" is a ball-bearing that
LOOKS exactly like an ordinary ball-bearing on the outside, but, as the name
indicates, the races are made in such a way that the force across the balls
is on an ANGLE. Thus, those do double-duty as a bearing, axial support, AND
"thrust". But generally, these are used in opposing pairs on ONE end of the
lead-screw, or, a "dual-row angular contact" type bearing might be used; I
understand these are standard for machine-spindle bearings. Then, a plain
ball-bearing (or TWO!) is used on the other end. On lead-screws larger than,
oh, I guess 1/4" diameter in general, there is often or usually a precision
threaded spot for a hex-nut (or ring-nut or whatever) used to lock on the
bearings which might be only an interference-fit. If they are a press-fit,
such a lock-nut might not be necessary if the linear thrust is not as great
as it likely is on such as a vertical mill or CNC lathe, etc.

BSA publishes some useful catalog-information along with their
products-for-sale giving the reader professional information as to how to
mount lead-screws which they manufacture. I built a rather large CNC X-Y
Table for a woodworking firm in ENGLAND (and had to go install it, once
delivered! NEVER again! Whew!) and had worries about "sag" and "whip" and
all that of those 10'+ screws, and got an education from BSA's catalog-info!
I used DUAL angular-contact bearings with spacers between them on the
"powered ends", the spacers being 4X the thickness of the bearings, and dual
"regular" B-bearings on the other ends. This would allow the 10' long screws
(1.5" O.D. screws!) to turn at up to perhaps 400 RPM ("rapid traverse")
without "whipping". Now, however, were I ever to be deprived of rationality
and be coaxed into building another such monster, I would consider FIXING the
screws, and rotating the dual-preloaded ball-nuts instead. This would mean a
bit more difficult machining to mount and rotate those parts, but may, in the
end, be a better idea. Also, it would mean the "X stepper" would have to be
mounted on the MOVING "Y carriage" as well as the "Y stepper" and lead-screw.
But, hey, that's invention, isn't it!

Jan Rowland


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