Servo Motor Gearing

[Ed Williams]

I'm wanting to use these servo motors on my 19x40 router table. My X&Y have ball screws that are 1 turn per inch. What gearing would you guys recomend for these motors.
Thanks

Static Torque: 190 oz-in
No Load Speed: 4504 RPM
Max Bus Voltage: 42 VDC
Max Continuous Current: 3.4 Amps
Peak Stall Current: 6.7 Amps
Shaft Diameter: 0.25"
Encoder: HP 1024 PPR Optical Disc Encoder

[NC Cams]

At 1:1, you'd have a theoretical jog speed of ~4500 inches/min assuming a light driven system

At 2:2 you'd be at 2250 and at 4:1 it would be ~1125, again at a LIGHT driven system weight.

In the above "light" means that the motor is capable of moving the mass and the mass is not sufficient to stall the motors. Keep in mind that as you gear down, you multiply the torque that's available by the gear ration. Thus, you'd have 190 in-oz at 1:1, 380 @ 2:1 and 760 @ 4:1 and so on....

This assumes that the motors have the power to move the system at the geared down speed - the caveat here becomes "how much weight will you try to move?" which isn't part of your input data. The mass being moved, in concert with any friction that has to be overcome, will ultimately determine how much you can move and how fast and at what gearing you'll need to achieve so as to move things as desired.

NOw, although the motor might turn at at no load speed of 4504rpm, that doesn't mean much when it is loaded. You'd have to find the motor's speed/torque curve to determine the TRUE speed attainable under the actual load conditions.

[Ed Williams]

Thanks NC Cam but here my problem. I got these motors on FleaBay so this is the only info that I have. My ball screws for x & y are 1 inch with 1 inch per turn. My z is a 5/8 inch ball with 6 tpi. My table is built out of 3/4 inch aluminum with thk rails. Would any of the experts help me with and educated guess on gearing. And would 24v 10amp trans with 50000uF 90V cap run this.
Thanks

Globe PN (custom): 537A-214
Frame Size: Nema 23 (non-square mounts though)
Static Torque: 190 oz-in
No Load Speed: 4504 RPM
Max Bus Voltage: 42 VDC
Max Continuous Current: 3.4 Amps
Peak Stall Current: 6.7 Amps
Shaft Diameter: 0.25"
Encoder: HP 1024 PPR Optical Disc Encoder

[unterhaus]

I wonder if "Static Torque" means that if you keep it at that level, you'll soon see smoke, and the whole machine will be static? As NC Cams says, it's a matter of how much weight and what acceleration you are shooting for. It looks to me that your rapids are plenty fast enough, so you should probably go for as big a gear ratio as you can. There is a practical limit, even 2:1 means a very large and very small pulley used together.

Are you planning on using Geckos?

It would be nice to know the allowable peak current and allowable peak torque. It looks like your transformer should work, assuming you aren't maxing out the torque on two or more motors at once.

[Ed Williams]

I was going to use Gecko 320's. And I was thinking of using a 6 to 1 since my z is 6tpi. What do you think.

[NC Cams]

Here are some realities that you have to deal with when it comes to gearing (simplified):

The torque of a typical DC motor is MAX at stall and MIN at no load rpm. Torque at any RPM inbetween is a function of the current flow per turns of wire on the armature.

Thus the more load you apply, the more current you will draw. The current versus load curve determines now much torque you can generate by the motor and thus the speed at which it will run at any particular load.

Thus, unless you have the torque curve of the motor, your "guesstimate" of gearing is as good or bad as anyone's as far as calculating what gearing to use. Thus, you seemn to be relegated to the SWAG method for "starter gearing/oops try something else" as opposed to truly calculating a gearing.

With all due respect, unless you know the drag and the system inertia involved that you have to overcome as well as the torque curve of the motor, I don't see how to figure out gearing (asside from cut and try), especially when "I dunno" is the shape of the motor torque curve, the inertia factor and/or the friction factor.

The other option would be to luck into someone having prior applications experience with an equivalent motor...

[Ed Williams]

I found this on the motor would this help. It is the same motor spec the windings are for 30v mine is 42v.
Thanks
Max rated Torque = 24 oz,in.
Theoretical stall Torque = 190 oz,in.

[NC Cams]

Effectively, you're working with the basic application of the F=MA equation.

At this point, you have the instantaneous F available from the motor. You still don't have M (mass of system) and/or A (intended acceleration).

You also need to know the frictional resistance as the friction in concert with the mass have to be overcome before motion will/can occur.

Effectively, you're going to use gearing to take the F you have and multply the speed or torque of the motor via the gearing to generate the desired A given the M of your system (wherein neither are known or at least provided)..

[Ed Williams]

I'm using IKO LRW20 Linear Slide Rail Guide for X & Y with KURODA-RAA Ball Screw's with 1 tpi. It's made out of 3/4 in aluminum. I'm moving 40 in for x 19 in for y. My z is moving a 2 1/2 hp router. Surely someone has something close to this that would know these other factors that I don't to come up with gearing.
Thanks

[Ed Williams]

Since I've never got a educated guess I'm going 10 to 1.

[pstockley]

What are you using as control software?
What is the maximum speed you require?
What is the maximum resolution you require?

I think you will have a hard time getting a 10:1 reduction unless you use a worm and wheel. With timing belts its hard to get more than about 4:1. Otherwise you end up with really big pulleys.

[NC Cams]

Keep this trick in mind:

If you keep the total tooth count of the sprocets constant, you can change ratios extensively and NOT change belt pitch lenght.

Example:

if you use a 100 tooth driven and a 10 tooth drive pulley total tooth count is 110.

At 55 x 55 you have 1:1.

at 20 x 90 you have 4.5:1

at 30 x 80 you have 2.667:1

At 22 x 88 you'd have 4:1

But you won't be able to get to 2:1 with this tooth count (you'd need to go to 111 total tooth count or 37 x 74)

Be sure you check sprocket availability because any/all tooth count sprockets are NOT always available for a particular belt pitch.

Also keep in mind that you need to have enough tooth engagement/belt wrap on the small pulley. Too little belt engagement will result in slipped cogs....