Re: [CAD_CAM_EDM_DRO] Re: Best source for Servo's ??

----- Original Message -----
From: "Mariss Freimanis" <mariss92705@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, December 17, 2004 11:12 PM
Subject: [CAD_CAM_EDM_DRO] 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
>
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