Re: [CAD_CAM_EDM_DRO] Z axis servo reduction gear

Hi Les
thanks for the reply, I checked the Nook catalog where it specs the nut as
being 36% efficient. I like worst case calculations too, so I think I'll go
with a drive ratio of 1 off the motor shaft to 1.5 on the screw shaft to
couple the motor to the screw. I think this will provide a safety margin.
What would be a good way of determining I'd overloaded the motor?
The only method I know is if it's too hot to touch, then it's overloaded. Are
there any other methods to determine if a motor is being overload.
Are there any design rules or criteria that I could access because I'm going
to have to do calculate the ratios for XY drives also.
cheers,
Denis

On Friday 27 March 2009 01:52:23 Leslie Newell wrote:
> Assuming the screw is 100% efficient the formula for thrust is:
> torque * (2 * pi) * TPI
> 2 * pi is near enough 6 for our purposes.
> So the thrust available at the motor's continuous rated torque is:
> 50 * 6 * 10 = 3000 oz or 187 lbs.
>
> However an Acme screw is only around 20 - 30% efficient. Assume worst
> case so the available thrust is more like 37 lbs.
>
> You can't safely use peak torque but it is usually safe to use 3 times
> continuous torque for short times so the peak available thrust is in the
> region of 110 lbs.
>
> So - theoretically direct drive should work. However a good zero
> backlash coupling will cost pretty much the same as a couple of pulleys
> and a toothed belt so I would probably go for a belt. That way you can
> change the ratio if for some reason you find it is under powered. Don't
> directly bolt the shaft to the motor. You must have some sort of
> flexible coupling otherwise you will probably overload the motor bearings.
>
> Les
>
> dcassyc1 wrote:
> > Hello,
> > I'm trying to decide whether I need a speed reduction on my Z axis router
> > carriage design. The Z axis is powered by a nema23 servo motor from
> > homeshopcnc, http://www.homeshopcnc.com/servo2.html
> > and is rated 50 oz in continuous and 350 oz in peak torque. The acme lead
> > screw is 10 tpi V thread and the nut is Delrin AF, the bearings are IGUS
> > dry lin on 3/4 inch shafts. I plan on using the homeshopcnc 3 hp spindle
> > at 21 pounds, so I think the carriage weight will come in at around 30
> > pounds total by the time the spindle is mounted in a clamp. Is it
> > possible to drive the lead screw directly off the motor with no reduction
> > or would I be better off coupling the servo to the screw through a
> > timing belt reduction setup to increase the torque and reduce the load on
> > the motor somewhat? If it's obvious that I need a reduction what sort of
> > ratio should I aim for? Thanks
> > Denis