Re: spinning ball nuts (heat and physics)

Hello,

Don't neglect expansion in length due to frictional heating. For the
smaller screw to work reliably, you need to have one end "live",
using belleville disc springs or die springs or similar. These will
ensure a constant "stretch" of the screw as it heats up and lengthens.

I ust posted all the numbers for this on the cnczone site, so here
I'll just summarise:

1018 Steel CTE Approx. 6.4x10-6 100" screw= .006 lengthening (and
shortening!) PER 10 degrees F change...

So unless your shop/garage stays the same temp ALL the time, you
better take this into account. And even if it does, the nut spinning
is gonna heat things up anyway...

Hope this helps,

Ballendo

P.S. CTE= Coefficient of Thermal Expansion, given per inch, per
degree F (Data from the Matweb materials database)

P.P.S. Related note: 6061-t6 aluminum CTE is 13.1x10-6 (Do you really
think your machine is holding plus/minus .001"?!? <G>)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Douglas Pollard"
<dougpol1@o...> wrote:

> Seems to me that you can get away with the small diameter lead

screw. The

> nut will always be pulling against one end or the other and it

will never

> be pushing against the screw unless the end that it is pulling

against

> stretches. I would mount the motor next to the nut and drive the

nut with a

> gear tooth belt. The screw may sag a little but that should not

matter as it

> won't be very heavy for the nut to lift as it travels along.
> Doug
> ----- Original Message -----
> From: <JanRwl@A...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Saturday, February 07, 2004 3:17 AM
> Subject: Re: [CAD_CAM_EDM_DRO] spinning ball nuts
>
>
> In a message dated 2/7/2004 12:36:09 AM Central Standard Time,
> plastiguy@a... writes:
> Besides the added design challenge of having the servo motor move

with the

> gantry that it drives, what snafus might I encounter with this

approach?

> That "design challenge" will indeed be the "SNAFU"! I still think

you will

> need AT LEAST a 1.25" O.D. screw! I built such a machine in '86
> (126"-"X-screws") and I DID use 1.5"O.D. x ½" pitch ball-screws.

Expensive,

> yes, but it
> worked!
>
> The preloaded ("dual") ball-nuts were quite large over the ball-

return

> tubes,
> etc., and it WOULD have been a challenge to rotate the nuts

instead! You'd

> also need DUAL thrust-bearings in addition to the axial bearings,

or, just a

> pair of angular-contact ball-bearings at each end (facing opposite

ways).

>
> Lossa luck! Jan Rowland
>
>
> [Non-text portions of this message have been removed]
>
>
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