Re: Encoders from Wirz

TADGUNINC@... wrote:

> From: TADGUNINC@...
>
> Jon,
> I keep hearing about 150 oz. in. being the min torque for servo motors, is
> that continuos or peak? Also what RPM should I be looking for? What about
> gear drive motors directly drivin the ball screw? And lastly are servo motors
> different from D/C motors?

150 oz-in is probably somewhere in the middle. I get a bit more peak on mine, and
I'm fairly on the weak side of things, but it is enough for me. So, maybe, for
Bridgeport-size vertical mills, 150 oz-in continuous would be the minimum.
My motors have a continuous torque of 70 oz-in, and I'm using a peak of
225 Oz-in. Running that through a 2.5:1 belt reduction and a .2" lead screw,
I can deliver about 1100 Lbs of linear force to the table.

Everything depends on what you will be doing, machine size, etc. If you are
providing the X drive on a 25 Hp horizontal mill, you need a much bigger servo
than on a Sherline. You need to figure what is the worst case cutting force, and
then how much of that will need to be resisted by the servo.


What about gear drive motors directly drivin the ball screw?

I wouldn't use any geared motors for positioning, unless they were specifically
designed for servo use. There are companies that make planetary gear and
harmonic speed reducers that are zero backlash, but most gear motors are not
of this type.

And lastly are servo motors different from D/C motors?

Yes, definitely. Some motors produced for ordinary applications work quite
well for servo work, some don't. Servo is too broad a category, anyway.
But, for machine tool positioning, there are several things that are important.

Some servo motors emphasize extremely low rotational inertia. Very important
for moving light loads quickly, not very important when we're moving a machine
table.

But, the rest of the characteristics that make a servo motor good are important.
So, you need a motor with very low torque ripple (mostly due to the design of
the armature windings) and low magnetic cogging (mostly due to the design
of the armature slots). For machine tool applications, you also want a motor
optimized for fairly low speed operation, and sufficient torque to move your
worst case load. You also want minimum stick-slip friction, as this will cause
jerky motion at low speeds.



This is an addendum to my previous question, Servo Systems has a
geared head
servo motor with encoder, 24vdc. the torque spec. versus rpm;
RPM TORQUE
285 30
233 125
150 265
THIS IS THE RANGE OF THE MOTOR, the encoder is 100cpr, 1250 cpr
after the
gear reduction, would this motor be practical for a table X/Y drive?

Well, I'd avoid any gear motors. Again, without knowing the size of your
machine, I can't get very specific. Even worse, the encoder on the motor side
of the gear reduction means the gear backlash is between the encoder and the
leadscrew. I don't know whether 1250 cpr is 'Counts/Rev' or 'Cycles/Rev'.
The difference is 4:1. 1250 Cycles/Rev would be quite good, if not for the
backlash. 1250 Counts/Rev would be a bit coarse. Depending on what
the maximum permissible current is, and the maximum permissible voltage,
This motor sounds a bit slow, unless you really turn the voltage up.
285 RPM into a .2" lead screw would give 57 In/Min feedrate. If the motor
could handle 48 V, you'd get close to 120 IPM, which would make a
pretty acceptable rapid feed. But, note, under load, the RPM drops
almost to HALF! This is not in the linear region of this motor! So, maybe
rating it at 100 or 125 oz-in is more reasonable.

Anyway, now for about 4 reasons, I would not use this motor. Note that
for a bridgeport-size machine, suitable motors should weigh about 8-12
Lbs. each. They use some super-advanced rare-earth magnet motors on
the Bridgeport Ez-trak machines that are much lighter and smaller, but these
are very exotic and expensive motors. You won't find these at the local
surplus store.

Jon