Re: Preloading ball screws

Bill, a good Antibacklash adjustment device for
2 Ballnuts on Ballscrews that have a posible
pitch error features a ridgid takeup and a spring
type preload.

The ridgid takeup allows the Nuts to be ajusted
apart from each other in a way that they are not
allowed to ever get closer to each other then what
they have been set at.
They are however free to move further apart as this
is nessesary to allow for the pitch error that may be
fond at some point along the working range of the
screw.
This point is what is used to determine the final
adjustment, the assembly should be able to move
troughout the working range without binding up.

The preload spring is ensuring the nuts are pushed
apart durring the parts of travel where they are
Not positioned at the tightest spot.
The amount of pressure on the spring is a bit of a
compromise, at a minimum the pressure should be adequate
to push the weight of the machine table + any possible
part mounted to it this will ensure backlash free positioning
is possible, at a maximum it would have to be less then
the load rating of the nut/screw getting too close to
this value would likely course excessive wear and lots
of drag because this will be the force between the components
that is active at all times.
You will not have to subtract this preload from the rating
of the nut/screw because it is not acumulative.
The spring force will likely be overcome by the force of the
cutter durring machining but will restore after the force is
dissipated.

A example of a low profile device i have found is
http://www.mmanrow.20m.com/anti_backlash_nuts.htm

I have build a slightly modified version of this design
that entails the basic principal of its functionality.
Did not like the setscrews used and changed the outer
dia. Works well now.
Hope this helps good luck.



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bill Kichman"
<billkichman@c...> wrote:

> 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