RE: [CAD_CAM_EDM_DRO] Does this make sense? was Re:BentNookballscrews ?????

On Tue, 2003-08-12 at 15:13, Carlos Guillermo wrote:
> Kim -
>
> So then by your wheel bearing example, if I adjust my wheel
> bearings for 50 lbs of axial preload, then it would take an extra
> 50 lbs per wheel to roll the car?

By my calcs, it would take 500 lbs/31 inlb per lb = 16 in lb. The ratio
of force to torque for a 5 TPI ball screw is about 31 lbs / in lb.

> I'm not disputing that the drag
> increases, but it is certainly not 1-to-1, and you must take into
> account the moment arm length. You hit it on the head, though:
> The drag force is roughly PROPORTIONAL to the preload, and the
> proportion would be a function of the efficiency of the rolling
> elements.
>
> According to the formula that you read:
>
> Td = (Kp*P*L) / (2*3.14),
>
> where:
> Td = preload drag torque (Kgf-mm)
> Kp = preload torque coefficient = typically between 0.1 and 0.3,
> I'll use 0.2

The docs that I've read use a ball screw efficiency of 0.9 or 90%. Here
is the difference between our arguments.

torque = force/(2 pi x TPI x eff) = force/ (31.4 x .9) for a 5 TPI ball
screw.

The difference between our equations is that I am using 0.9 and you are
using 0.1 to 0.3 ???? We both agree there is drag due to the preload
and we are using the same equation, with different co efficients.


> P = preload force (Kgf)
> L = lead (mm)
>
> Then:
> For a 5mm lead screw with a 250 Kgf preload between ballnuts:
>
> Td = (0.2*250*5) / (2*3.14)
> = 39.8Kgf-mm
> = 0.390 N-m
> = 55.3 in-oz
> = 3.45 in-lb
>
> This torque would be a small fraction of the peak torque available
> from the motors of a medium-sized mill.

Depends on the motor. The Camtronics are rated at 600 oz in peak. I
call that 120 oz in continuous. 3.45 in lb = 55.2 oz in, which is 46%
of their continuous capacity.




> I'm done.
>
> Regards,
>
> Carlos Guillermo
> VERVE Engineering & Design
>
> -----Original Message-----
> From: Kim Lux [mailto:lux@...]
> Sent: Tuesday, August 12, 2003 1:32 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] Does this make sense? was
> Re:BentNookballscrews ?????
>
> Yes, I read them and I fully understand them.
>
> Did you ever replace the wheel bearings in a car ? The two
> bearings act
> the very same way as the ball nuts do on a ball screw. When you
> tighten
> the preload the drag increases. The amount the drag increase is
> proportional to the preload and in our case, with a 5 TPI ball
> screw,
> the torque required in inlbs is 1/31st of the preload in pounds.
>
> Have you guys ever actually worked with a ballscrew and ballnuts,
> applying preload ? I've set up 6 axes in the last few weeks.
>
> Kim
>
>
>
>
>
>
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