Re: Best source for Servo's ??

My 2 cents on the subject, no particular order:

1) Never mind torque, never mind RPM. Think Watts. Watts is power and
power is what gets things done.

2) Power is torque times RPM or linearly, force times velocity.
Specifically, Watts = RPM * in-oz / 1351 or linearly, Watts = IPM *
Lbs / 531

3) 1 Watt can push (or lift) 1 LB at 531 inches per minute or it can
push (or lift) 531 Lbs at 1 inch per minute. Mix or match; you see
the point.

4) 1 horse-power is 746 Watts.

5) Steppers or servos? A stepper (NEMA34) tops out at 200 to 250
Watts (1/4 to 1/3 HP) mechanical. A servo goes past 1,000 Watts
(Using 80VDC rated drives). Both are equally accurate and reliable as
positioning motors.

My recommendations? If you need 100W or less, think steppers. If you
need 200W or more, you have to go with servos. In between, either
will do.

6) How do you estimate how many Watts you need?

The best trained Olympic atheletes can generate a sustained 200 Watts
of power output. I'm talking about Marathon runners, long distance
swimmers or Tour de France bicyclists. This is documented in the lead-
up to the human-powered Gossamer Condor human powered flight across
the English Channel.

I have in my experience been on the shop floor more than a few times.
The machinists I remember didn't look like Olympians and they weren't
working up a sweat running manual Bridgeports. It's safe to say less
than 200W was being put into the hand-cranks of these machines.

Start by figuring what torque you need on the lead-screw. Measure the
diameter of the hand-crank of a manual machine. Use your experience
as a machinist of how much force you would apply to that handle
before you figured you are doing something wrong. 5 Lbs, 10 Lbs, or
both hands to get over 25 Lbs?

Multiply that number by 16 to get ounces, then multiply that by the
radius of the hand-wheel. The result is the torque in in-oz you apply
to the lead-screw.

If you apply 10 Lbs on a 5" diameter crank, you should get 400 in-oz
of torque.

Calculate the screw RPM for the speed you want. If you have a 5 TPI
screw and you want 120 IPM, you get 600 RPM (IPM * TPI = RPM).

Multiply screw RPM by in-oz, divide by 1351; that is the power you
need. Using the above, you get 177 Watts.

7) Steppers are better than servos for milling CNC applications.

Steppers have a speed-torque curve that makes it ideal; lots of
torque at low speeds where work gets done, miserable torque for
rapids where nothing is needed of it. Think of a stepper as a motor
with an infinitely variable automatic transmission gearbox. Trade in
torque for speed.

8) Gearing is very imortant. Steppers or servos, each has
their "sweet point" for best power delivery to the load. Steppers
should be run between 900 to 1500 RPM to deliver max power. Servos
should always be run near 80% of no-load RPM for same.

9) Never have a power supply voltage higher than what you need when
using a stepper. It is absolutely insane to run a 5A NEMA34 step
motor at 80VDC when the motor will never turn faster than 500 RPM.
All you get for your trouble is a very hot motor that could deliver
decent power at 2,000 RPM but it never gets run there. What a waste.
Use a lower voltage.

This particularly applies to people using rack and pinion
transmissions.

10) Don't replace a stepper with a servo of the same frame size and
expect things to work OK. They won't.

Steppers are high torque, low RPM motors. Servos are the opposite.
They have little torque but they develop that at high RPM. They need
reduction gearing to to match the load.

Think of being in a sports car with a manual transmission. The light
turns from red to green; which gear do you want to be in? 1st or 5th?
Same engine, same power.

Were it a desiel, (stepper) the gearing would be different than if it
were a turbocharged 4-valve, 9,000 RPM red-lined 4-banger (servo). If
transmissions matter so much on cars, they matter just as much on
your choice of motors. Steppers and servos are entirely different
animals and must be treated that way.

I could easily do (11) thru (99), there is so much territory to cover
but that's enough for now.

Mariss