Re: age old question

I really hate it when only torque is mentioned for a motor.:-)

What gets things done is power, as in Watts. Power is torque times
RPM. Specifically, Watts = RPM times in-oz / 1351.

One horse-power is 746 Watts. One HP is the ability to lift (or
shove) 550 lbs to the tune of 720 IPM.

Servos are not step motors. Step motors are high torque, low RPM
motors. Servos are the opposite, high RPM, low torque motors.

They need reduction gearing to match a load. The servo must be
turning 80% of no-load speed when your mechanism is moving at its max
design speed.

Small NEMA-23 servos can provide more power than a NEMA-42 motor;
250W versus 150W. The thing is the small NEMA-23 servo develops that
power at 4,800 RPM (80% of a 6,000 RPM no-load).

You want your big machine to motor along at 180 IPM on a 5 TPI
leadscrew; that is 900 RPM on the screw. The reduction must be
4,800/900 or about 5.33:1.

The NEMA-23 servo motor will have about 70 in-oz of continuous rated
torque. The screw will see 5.33 times that or a respectable 370 in-oz
and will produce 730 lbs of "push".

Being a servo, it will provide 5 times as much torque and "push" for
short periods of time (<1 sec), 350 in-oz and 3,660 lbs respectively.

Reduction gearing is of paramount importance with servos. They are
not steppers, don't treat them as if they were. You do, you will burn
down your motor.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> a57chevytruckguy wrote:
>
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...>
wrote:
> >
> >
> >>a57chevytruckguy wrote:
> >>
> >>
> >>
> >>>The size I am looking for is around 20in/lb or close
> >>>
> >>>
> >>>
> >>>
> >>Now, is that peak or continuous? I have a line on some small
> >>
> >>
> >motors
> >
> >
> >>that can
> >>deliver 357 Oz-In peak, or 22 Lb-In, and 71.5 Oz-in continuous,
or
> >>
> >>
> >4.5
> >
> >
> >>Lb-In.
> >>
> >>Would those work?
> >>
> >>Jon
> >>
> >>
> >
> >I am looking for 20lb/in continuous I am retro-ing a bridgeport
> >
> >
> >
> OK, my motors are DEFINITELY too small for that. Unless you get
lucky on
> eBay, I think you will have to spend some money on motors that
size.
> The treadmill
> motors likely are big enough, but there's been some question about
> smoothness
> and reversing on these motors. Also, are they ball bearing?
Constant
> stops and
> reversals, as well as mostly low-speed operation is hell on sleeve
bearings.
>
> C&H is a great place, but they know what they have, and are NOT
cheap.
>
> Jon