Extruder motor replacement

The second motor of my BitsFromBytes laser-cut extruder, a GM3, has just failed. I do have not a record of the time it has been operating but I estimate it is between 5 and 10 hours, running with M108 S120. The gearbox looks OK but the brushes of the motor are worn. Is this a reasonable life for the GM3 motor/gearbox? Any suggestions for a more robust replacement without building a different extruder?

I may be wrong but I don't think BfB use a GM3. I think they use an equivalent one that has a smaller shaft coupling.

My experience with GM3 is that the 6V motors (white end cap) have very thin brushes and last for tens of hours. The 12V motor (black end cap) has better brushes and lasts 100's of hours.

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[www.hydraraptor.blogspot.com]

I have been using the 6v version. Is there a supplier of the 12v version in England?

I don't think so. I had to phone Solarbotics in Canada to get the 12V version as they don't offer it as standard on their website.

I think they do sell the motors separately though and it is easy to just swap the motor.

Edited 1 time(s). Last edit at 04/08/2009 04:47AM by nophead.

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[www.hydraraptor.blogspot.com]

Is the Solarbotics RM2 the motor I am looking for? It does not mention 12 volts but is described as a more powerful motor for a GM3. Active-robots.com have it on their web site for sale in uk.

I am not sure. It looks the same but I didn't notice the 12V GM3 going any faster than the 6V GM3 run from 12V. You could ask Solarbotics, they seem pretty helpful.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> I am not sure. It looks the same but I didn't
> notice the 12V GM3 going any faster than the 6V
> GM3 run from 12V. You could ask Solarbotics, they
> seem pretty helpful.

It does go faster. I measured once and determined that it is about half again faster at 12v as the 6v motor is at 6v.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Yes, but is it faster than the 6V motor run at 12V? It seemed the same to me. The spec for RM2 says it it three times faster.

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[www.hydraraptor.blogspot.com]

LOL! It never occurred to me to run the 6v motor at 12v. It's service life was already ridiculously short without doing that.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Solarbotics tell me the RM2 is not the 12 version of the RM3.

To correct my original post, the second motor had not burnt out. It stalled with similar symptoms to the failed first motor but further investigation showed the problem was elsewhere. The HDPE had developed a kink between the extruder drive screw and the channel down to the heater barrel. It has happened twice more and this problem is perhaps a topic for a different thread.

Greetings all,

I think the motor for the solarbotics GM3 is the same size as those used in 1/32 scale slot cars (they agree to a couple thou when I measure them, and the shaft diameter is dead on.) Slot cars use 12 Volts (in the U.S. I don't even know if slot cars are much of interest elsewhere....)
I suspect that the torque/Amp is as good as the GM3 motor, but I haven't checked that. So, 1/32 scale slot-car motors seem like an option.

Possible improvement?
Slot car motors are often available with dual shafts. If one were to replace the GM3 motor with a dual shaft motor, one could put an encoder off the back shaft.
Given the gear ratio involved, one could measure velocity much easier from the motor shaft, and could do so with a very simple (read coarse) encoder disk -- half white and half dark. (Or four alternating light/dark quarters if you want to gild the lilly.)

I bought some dual-shaft motors for this purpose, but haven't tried them out yet.
(I have some distractions of late. We just {today} started a major remodeling project on our house.)

I got dual-shaft motors from www.brshobbies.com The exact part I bought is no longer stocked, but I emailed and got the equivalents. (And I got a reply within the hour.) Looks like his AUTOart #13300 Motor (or 13302) are both dual shaft. They're USD $5.95 +shipping.

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Larry,

We no longer carry the SCW1 motor but stock some AutoArt dual shaft motors. Here is the link: [www.brshobbies.com]

Best regards,
Brian
BRS Hobbies

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Larry Pfeffer,

My blog about building repstrap Cerberus:
[repstrap-cerberus.blogspot.com]

I have tried the RM2 motor and it is not a good replacement for the RM3 in the BitsForBytes extruder. It naturally runs much faster and, slowing it down to an appropriate speed, the torque is too low.

Have you tried adding a shaft encoder? Apparently you can run it slower, but keep the torque high? Or have I got that wrong?

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Reprapping blog and other rants: [renoirsrants.blogspot.com]
My Reprap: [sites.google.com]

@jonwise
I am new here, so I am not too familiar with specifics of the Reprap and its motors, but I do know a lot about motors in general (steppers and DC motors).
I don't understand that you say "It naturally runs much faster and, slowing it down to an appropriate speed, the torque is too low."

Torque delivered by a dc motor is linear to the current that you drive it with. If you experience the torque is too low at low speeds, you are not pushing enough current through the motor. This can have several causes.
My guess after reading your situation is the following: you drive the motor with a PWM signal. You regulate the speed of the motor with the PWM signal in a no-load situation. When the mechanical load of the system increases, the motor stops. The current is simply too low because the net voltage on the motor (%PWM*driver voltage) is low.

To get a constant speed you need a speed controller. And for that you need some kind of feedback. Widely used in the industry are incremental encoders.

Yep, this is the problem. The extruder load varies, depending on the shape of the filament, the temp, where the filament is melting, etc.
This means if we use PWM alone, the speed varies.

Adding an encoder allows the controller to run the motor (and therefore the filament) at a constant speed.

Stepper motors have also been used to drive the extruder - they have more control of the rate, and more torque.

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Reprapping blog and other rants: [renoirsrants.blogspot.com]
My Reprap: [sites.google.com]