Re: [CAD_CAM_EDM_DRO] treadmil motor speed reduction analysis

I don't know Ron, in my experience, most servos on gantries that I use
(~300w - 600w) run best at about 12:1, 1.25" pinion, giving ~3
revolutions per inch, give or take. acceleration is reasonable, and
rapids are good. While theoretically, I should be matching motors to
each project, from a logistical standpoint, I use about 3 different
servos, and maintain service spares fo my customers, if they're within a
3 or 4 hours flying time, I fly out and replace them (typical pilot, any
excuse to go flying). The sad reality is that many of the machines I
service would be scrapped otherwise. At least this way, my customers get
several years life out of a 15 year old machine, that they are familiar
with and have figured out the problem areas with.

erie

R Rogers wrote:

>Having a bit of trouble understanding why so much emphasis is being placed on rotor inertia. That only effects acceleration and deceleration, correct? The treadmill motor and a Nema 42 servo having basically the same armature dimensions, how much difference could there be? Would it even be worth considering since these motors are at top speed under load in less than a second if geared properly? A 1-2 horsepower DC motor basically disregards armature weight when the voltage and current is applied. If I'm missing something, please explain.
>
>A note on ratios in relation to machine load. Driving the knee on a Bridgeport which is similar in task to accelerating a large gantry, I gear them at 25 revs of the servo to traverse 1 inch of the Z axis. The X and Y are 12.5 revs to traverse 1 inch. Every rev of Dave's proposed pinion gear will represent 4 inches of traverse. So to arrive at the same cutting forces and speed one would have to mutiply the 25:1 by 4. Making it 100:1 ratio from the servo to the pinion gear. Obviously a gantry machine wont require the cutting forces that a Bridgeport encounters so the ratio could be reduced by whatever force and speed is desired. 10:1 would not make the servo very happy. Maybe I'm way off on this??
>
>Ron
>
>
>
>Leslie Watts <leswatts@...> wrote:
>Hello all,
>
>As promised I did an analysis of a hypothetical cnc router
>with belt or rack drive using treadmill motors.
>
>I had to do some eductated guesses...because there are few
>specs on them. These are the cheap 1-2 hp 100vdc units
>at surplus center and others.
>
>First, I am going to seriously derate these things because they are
>"treadmill" duty. I don't believe the HP ratings anyway. Let's derate
>them 50%.
>
>Second I have to get some idea of the rotor moment of inertia. None have
>such specs, but I know a 3/4 hp square frame typical PMDC motor comes
>out at about 0.4 oz*in*sec^2. I will use that figure. This is a very
>high inertia compared to a typical industrial servo and will be one of the
>biggest factors limiting performance.
>
>Here is the router specs:
>
>100 kG moving mass
>125N (25 lb) cutting force
>50 N (10 lb) friction force
>100 mm/sec (4 inches/sec) max speed
>.25g (2.45 M/sec^2) max acceleration
>50mm (2 in) rack pinion/timing pulley diameter
>
>Ok.
>First the moment of inertia at the pulley/pinion.
>That is simply Mr^2= 100*(.025M^2)= 8.8 oz*in*sec^2
>
>A 1:1 load to motor inertia match occurs with SQRT( 8.8/.4)=
>4.69 gear reduction ratio. Lets make it an even 5:1.
>
>Starting with that I went to motioneering and
>checked it with real motors.
>
>I needed 600 in oz peak at only 171 rpm.
>Peak input power was only 90 watts.
>But....this is a no go because I have derated the motor.
>I don't think they can put out much more than 400 in oz peak
>and much less continuous.
>
>Let's go with 10:1 gear ratio.
>Peak input power was 170 watts.
>but required inertia for 1:1 was .12 oz*in*se^2
>...4 times lower than our motor.
>But inertia match does not have to be exact
>... 0.25 to 4 is generally ok.
>At .25 we have about 375 oz*in required.
>Looks close!
>
>Now 20:1
>270 Peak input power
>Still about 375 in* oz
>But even at an inertia ratio of .25 we need
>about 0.1 in*oz*sec^2. So there is no solution.
>The treadmill's high inertia has killed it.
>
>
>OK...done I think! With the derated motor we just
>get by with a 10:1 gear reduction. anything much higher
>or lower gets worse.
>
>Max rpm is 382 so we can go much much faster than design
>top speed if we want.
>
>We are right at the comfortable motor current limit.
>
>The motor will only need about 20V for design speed but
>LOTS of current. This depends on the torque constant
>but might be in the 10 amp range.
>
>If the Pinion was one inch diameter rather than two,
>5:1 would be optimum and so on.
>
>Understand the results might be VERY different for
>industrial servos and steppers...this is only for the
>treadmill motors.
>
>As a last bit...if any one has some of these motors
>and a reasonable power supply I can show you how to
>measure moment of inertia etc so we won't be doing
>educated guesses.
>I would like to know!
>
>Les
>
>
>
>Leslie M.Watts
>L M Watts Furniture
>Tiger Georgia
>(706) 212-0242
>
>Main page:
>http://www.lmwatts.com
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