Re: [CAD_CAM_EDM_DRO] Re: re: New file uploaded to CAD_CAM_EDM_DRO

I think what I'd probably do to overcome the problem of heat expansion on these shafts is make the bearing mount on the end where the motor drive connects fixed so it CANNOT move axially. Then on the other end, I'd let the shaft float axially in the bearing mount. The amount of expected expansion can be calculated and allowed for.

I've never used the ball screws, but I gather from everything I've read there should be some kind of preload on the nut itself. So, with ball nut connected to a moving part via some sort of bracket, I see no reason to worry about expansion. The electrical system should be set up to ALWAYS "register" from some point near the fixed end of the shaft. That way any expansion would always be cumulative.

Let's consider how much expansion there's going to be:

With a coefficient of expansion of .00000633 inches per inch per degree F, and a 50 degree temperature rise, a 48" shaft will expand a total of .01519". This means a shaft starting out at 68 degrees would heat up to 118 degrees during operation. With a 25 dregree temperature rise, there would still be .0075" expansion.

This, of course assumes the shaft temperature rise was evenly distributed over the entire length of the shaft.

So the builder will have to consider how much temperature rise is permissible during operation AND allow for expansion OR provide a method of cooling the ball nut or screw...or both. Based on the amount of expected expansion over 48", I'm sure I'd try to get the unit to run as cool as possible, by keeping preload to a bare minumum.

Does this help?

Smoke


-----Original Message-----
From: ballendo@... <ballendo@...>
To: CAD_CAM_EDM_DRO@egroups.com <CAD_CAM_EDM_DRO@egroups.com>
Date: Friday, October 27, 2000 9:01 PM
Subject: [CAD_CAM_EDM_DRO] Re: re: New file uploaded to CAD_CAM_EDM_DRO


Smoke,

I realised you didn't FORGET when I read your earlier response.(just
AFTER I had posted mine) However...

Relying on the other end mount is something you don't usually want to
do.(even though, I myself described this earlier in this thread with
two plain radial bearings in SIMPLE mounts) With short screws or
light loads, it can be OK, but as a general practice for machine
tools' leadscrews... NO.

I realise you have much experience in production machinery and design
so I know you can point out cases where it IS done. So Can I :-)

The gotcha's are that any expansion(or contraction) of the shaft or
machine mounting surfaces(in relation to each other,or independantly)
will affect the "end-play" of the leadscrew shaft.

Careful engineering and precision machining can make this approach
work well in specific instances, but it should not be considered a
general solution, IMO.

From your earlier post, I gather that this is a dual row bearing. In
this case, you have drawn a VERY sturdy SIMPLE mount! The very
meaning of "fixed" is to "fix into position" WITHIN the single mount.


Ballendo

>No, I didn't forget. As I posted in an earlier reply...I'm assuming
>the opposide end of the shaft is fixed. If the same or similar
>mount is used on both ends of the shaft, the shaft would be trapped
>between both sets of bearings and would not be able to move
>lengthwise.
>Smoke

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