Re: [CAD_CAM_EDM_DRO] thermal expansion

Greg,

Certainly stretched ballscrews can be used for temperature
compensation up to about 5 degrees c or so. I often laugh
at a certain poorly translated manufacturer's design guide
warning about "the burn of the end bearing" beyond 3 to
5 degrees stretch compensation.

To do so is very very expensive though, as you say.
Since ballscrew temperature rise of up to tens of
degrees is common an expensive secondary temp
control is often required.

I prefer a less costly method that gives us most (not all) of
the advantages of this type of mounting in a lighter smaller
machine that we can all afford.

Completely fixed end bearings can resist axial, radial, and bending loads.

We use this at one end.

Simple can resist radial only.

A large part of the fixed configuration's ability to increase critical
speeds
is it's resistance to BENDING loads.

If that bearing consists of a spaced DB pair free to slide in a housing
a little bit it will still resist bending loads! It similarly will still
improve
critical speed a lot. Slocum, NSK, and some others generally consider
this as "fixed" although it cannot accept axial loads. I call it fixed often
as well, and it can be admittedly confusing.

So the only problem left is possible buckling in one direction. We deal
with this in 3 ways:

1) keep the loads below the safe maximum column load as specified by
the manufacturer for fixed-simple. It is ok to have some column loads,
as in a jack, as long as they are safe values.

2) Slightly spring preload the screw in tension. In my case this is a few
hundred
pounds supplied by stacked Belleville washers. This is a bit different from
using only the stretched screw as a spring, because it allows for some axial
motion without huge changes in loading. It does not thermally compensate.
It does prevent compressive loads when they are less than the spring force.

3) Limit the axial bearing sliding motion with a stop. If there were a wild
over rev
or crash this pins the end to limit the amount of possible Euler buckling.
This might not
save the screw, but it will limit "whipping" to a value less than violent
destruction of
many parts.

So we end up with a system that has critical speeds between fixed-simple and
fixed-fixed. It does not thermally compensate, but it allows for temperature
changes
without large bearing forces developing, so it's safe to do things like have
a steel
screw on an aluminum structure even though there is a 2:1 difference in
thermal expansion.
In normal operation there is little or no compression loading. There may be
more in
a crash, but it is limited.

Best of all it is easy to make and economical.
And it is in fact done just this way in countless spindles,
shafts, and leadscrews... and has been for decades.

So I decided to (re)post this information, because when I
say "read the book" I forget that it costs prob over a
hundred dollars now!

Les

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus cnc for sale:
http://www.alltel.net/~leswatts/forsale.html

----- Original Message -----
From: "Greg Jackson" <greg@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, January 28, 2003 7:42 AM
Subject: RE: [CAD_CAM_EDM_DRO] thermal expansion


> An axial float is certainly necessary on a lightweight machine, but
> fixed/fixed mounts are common on more rigid machines. This is not my
> personal experience, but that of my neighbor who is a former engineering
> manager for Giddings & Lewis. Their standard machine stretched the ball
> screws, but then their machines were commonly over 100,000 lbs and
millions
> of dollars. Sometimes they used hollow ballscrews and flooded the
> ballscrews themselves, They always used linear feedback, not depending on
> the screw for precision, just motion.
>
> Stretching even a 3" diameter ball screw is not too hard when the cross
> sectional area of the iron frame supporting the end points is 12 sq feet
or
> more.
>
> GTJ
>
> -----Original Message-----
> From: Les Watts [mailto:leswatts@...]
> Sent: Tuesday, January 28, 2003 6:15 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] thermal expansion
>
> The real issue is certainly ballscrew heating on the
> larger gantry machines. We had a pretty good thread
> a while back, so I won't rehash it. It can't really be
> ignored on longer screws. The bottom line is:
> with fixed- fixed end bearings one end must be able
> to float axially a little. A duplex pair of bearings with
> a push fit in their housing is what I use. It is very
> lightly spring preloaded. The forces generated
> by a tensioned fixed mounting are absolutely huge
> even with a few degrees temperature rise. Kind of like
> ice forming in rock cracks and splitting them...
>
> We have some numbers on the phenomenon in the
> thread.
>
> Les
>
>
> Leslie Watts
> L M Watts Furniture
> Tiger, Georgia USA
> http://www.alltel.net/~leswatts/wattsfurniturewp.html
> engineering page:
> http://www.alltel.net/~leswatts/shop.html
> Surplus cnc for sale:
> http://www.alltel.net/~leswatts/forsale.html
>
>
>
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