RE: [CAD_CAM_EDM_DRO] Preloading ball screws

Hi Bill

Interesting problem...and I don't think the issue is covered
as much as it should be.

I agree completely with Carlos in that the inability of the threaded
portion to backdrive would cause it to tighten to the highest
pitch portion of the screw and stay there, causing massive loads
on the bearings when they later went over a lower pitch area.

So you really need a stiff compact axial spring force between the nuts
that doesn't exceed about 5% of max dynamic load in areas of
maximum d(pitch)/dlength.

What about stacked Belville washers? I use them for rotary bearing
preloading commonly. Big spring constant, compact geometry.
I realize it would be some surgery to do that, but if the threaded
block were split and fastened with Belvilles on both faces of one
section (for bidirectional loading) it would take almost no extra
room.

Just something to think about.

Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia USA
(706) 212-0242
http://www.lmwatts.com
Engineering page:
http://www.lmwatts.com/shop.html
CNC surplus for sale:
http://www.lmwatts.com/forsale.html
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http://www.lmwatts.com/signwp.html




-----Original Message-----
From: Carlos Guillermo [mailto:carlos@...]
Sent: Saturday, September 27, 2003 2:33 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Preloading ball screws


Hi Bill -

I see a problem with that arrangement. The spring-loaded ballnut
would not be backdriveable in its mounting threads, so although it
would tighten up the nuts against each other on the areas of the
screw with lower pitch, it would not allow the nuts to open up on
areas of the screw with higher pitch. You would not have constant
preload, just a system that eliminated backlash, but then caused
potentially excessive ball contact forces when the screw pitch
increased.

Regards,

Carlos Guillermo
VERVE Engineering & Design

-----Original Message-----
From: Bill Kichman [mailto:billkichman@...]
Sent: Friday, September 26, 2003 7:21 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Preloading ball screws

I am sure this has been covered ad nauseum, but here goes...I have
2
ballnuts threaded into a steel block which is attached to my table
for my
ballscrew conversion...I had been depending on rigid backlash
adjustment by
simply screwing one of the nuts tightly against the other, and
tightening
both setscrews. As I understand it, a spring type arrangement is
more
advantageous due to wear issues which act to change the fixed
preload over
time.

My question is this....as I have no room whatsoever on at least
one of the
axes for adding anything to the periphery of the nut, and the
threads
already are there, would it be reasonable to attach a strong
spring such
that it would act to tighten the threaded ball nut's threads at
all times?
That way, the spring wouldn't have to be nearly as strong as an
installation
without the threads installed and a heavy spring arrrangement
in-line with
the nut. Am I seeing this clearly? The spring could partially
wrap around
the nut and keep a constant preload.
Has anybody done this?
R. William Kichman, P.E.
Kichman Engineering Associates
103 Old Furnace Road
Cornwall, PA 17016-0643
tel/fax 717/270-0714