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  #1  
Old Tue 06 November 2012, 11:55
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Would a size 23 motor work?

This morning I was checking the specs on Oriental Motor size 23 motors (compared to the size 34 motors that are normally used on a MechMate). The PK268-E2.0A motor caught my eye. It has a virtually flat torque curve out to 1,100 RPM of 240 oz*in. The much larger PK296-F4.5 motor has about the same torque at 1,000 RPM.

I started thinking.

Because of the physical dimensions, the PK268 motor could be geared 5:1, 6:1 or even 7.2:1 with a belt drive. The larger PK296 motor is limited to 4:1.

With a 1.25" spur gear, the PK268 motor would move 0.7854 inches per revolution when geared 5:1. Mach 3 can run comfortably at 45,000 steps per second, so 0.7854 / 2000 * 45,000 = 17.67" per second - faster than necessary for jogging.

Tongue is 75 lb*in, or about the same as the Alpha motors on the Shopbot PRS-Alpha.

The current drawn is 2.8 amps, and the maximum voltage is 60 VDC. That means that the inexpensive Gecko G540 could be used if the power supply were limited to 50 volts or less.

It's very possible that that motor with a 5:1 belt drive would perform very well on the MechMate at less cost than the larger size 34 motors.

(Edited: The very popular PK296A2A-SG7.2 only has 40 lb*in of torque.)

Last edited by Richards; Tue 06 November 2012 at 12:11..
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  #2  
Old Thu 08 November 2012, 06:13
Surfcnc
Just call me: Ross #74
 
Queensland
Australia
Hi Mike

Can you post the torque curve, the OM site requires a log on.

Thanks
Ross
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  #3  
Old Thu 08 November 2012, 06:56
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
From a purely thermodynamic viewpoint, we load the bigger bodied Nema 34 motors up to near their max temperature. Assuming a Nema 23 has similar efficiency, it won't be able to dissipate the same waste heat from the same volume because of its smaller surface area. One would have to put less load on a Nema 23 to keep it as cool as a Nema 34.
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Old Thu 08 November 2012, 09:12
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ross,

After looking at the charts again, I noticed that they listed the speed in kpps (thousands of pulses per second) instead of RPM, as they normally do with other motors. The step rate of 1,050 pps with a 1.8-degree motor is about 300 RPM, instead of the 1050 RPM that i thought the chart was listing.

The torque is almost flat at 240 oz*in (1.70 N-m) until the motor hits 1,050 pps, then the torque drops rapidly.

If I had that motor, I would test it against the other motors that I have to see how it performs with a Gecko-drive. Experience tells me that it would perform better than the PK268-02A motors that I used to use in photo equipment. Those motors could easily be driven to 1,500 RPM. Using a 6:1 belt reduction transmission, those motors were handling loads equal to the loads on a CNC machine. But, it would take some extensive testing before I could make a recommendation to use size 23 motors.

Gerald,

You're right about the fact that a larger motor can dissipate heat faster than a smaller motor, but I think that a size 23 motor mounted onto a 1/4" piece of aluminum (belt-drive transmission) would not overheat. I tested a PK268-02A motor mounted to a 7.2:1 belt drive that I used in a Shopbot demonstration. I also tested a PK296A2B-SG3.6 motor. Both motors were equally loaded. Both motors heated up to 55-degrees C and then stabilized at that temperature when measured with an infrared thermometer. More testing would be required to see what would happen if those motors were used on a CNC machine. I think that if the motors were run at the proper voltage and if the load driven was within torque range of the motors, that the current drawn would be within the operating range of the motor. That means, to me, that the temperature of the motor would be in the motors safe operating range. The motors would get hot, but not hotter than the temperature that they were designed to handle.

Again, it would take some serious testing to verify everything.

We all know that the size 34 motors work very well. I'm not suggesting that everyone change to the smaller motors. When we compare stepper motors to automobile engines, the size 34 motors would be like the large, low rpm, high torque motors used in the "muscle cars" in the 1960's that had a three or four speed transmission. The size 23 motors would be like the smaller high rpm, low torque motors used in race cars that have a five or six speed transmission. Either automobile motor, when used as designed, gave impressive performance. Stepper motors, when properly implemented, do the same thing.

The size 34 PK296-03AA motor (which is the motor used in the PK296A2A-SGxx motor/gearboxes) drops to 100 oz*in at 900 RPM. The size 23 PK268-03AA motor drops to 100 oz*in at 800 RPM. With a 1.5" pitch diameter spur gear and a 6:1 belt-drive, that is still 600 oz*in at 10" per second. A non-geared PK299-03AA motor has about 600 oz*in of holding torque and about 125 oz*in of torque at 800 RPM, so the PK268-03AA motor geared 6:1 would have four times more torque than the ungeared PK299-03AA motor. Several people have built MechMates using non-geared motors. My Shopbot Alpha was shipped with ungeared motors (which I later upgraded to belt-driven 3:1 motors and finally replaced with 7.2:1 geared motors). The Shopbot ran fine with ungeared motors - except for unacceptable "chatter". The geared motors mostly cured the "chatter" problem.

Most of the comments that I make are test-bench observations. The comments are meant to help people think "outside the box" and to realize that what is tried and true may be improved upon by trying something different.
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