Re: treadmil motor speed reduction analysis

> 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

Could you post this info for our general use? This sounds like
valuable information we can use for any of the surplus motors we use
down the road.

Wally






--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Leslie Watts"
<leswatts@a...> 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
> Engineering:
> http://www.lmwatts.com/shop.html
> Cnc surplus for sale:
> http://www.lmwatts.com/forsale.html
> Carved signs:
> http://www.lmwatts.com/signwp.html