Small Banebots Motor Issue - Lessened torque and smoke

[sanddrag]

We are having a serious issue with the small bane bots motor in the 64:1 gearbox in the kit. This exact problem has also been reported by three other teams I have talked to. The scenario was lifting a rather light weight arm that was not terribly long. There was additional reduction after the gearbox, in the range from 2:1 to 9:1. Calculations show that even in the 2:1 scenario (my team), the drive should have powered the arm rapidly with more than enough torque available. The arm was unable to lift, and the motor started smoking as soon as it enountered any sort of real load. This problem was encountered the very first time the motor was tested in a load condition. This is the same problem three other teams have reported to me. The motor never once tripped a breaker (20A on my team), which, if it was stalling (spec'd) stall current about 37 amps isn't it?) it really should have. And it should trip a breaker before it ever starts smoking.

So, I don't know what's going on here. Post if it happened to you, and post if you know why.

[MrForbes]

how is the tranmission shaft supported? I assume you have a chain drive, is there a well mounted bearing block on the other side of the sprocket? thinking maybe the transmission could be binding due the output shaft deflecting.

We ran our 125:1 36mm gearmotor a little bit with a counterbalanced (gas spring) arm/claw/ringer on it, and had no problems, motor got slightly warmer than room temp after running it for a few minutes up and down. roughly 5 lbs total arm weight, 8:1 chain reduction, no bearing block.

[eugenebrooks]

Sanddrag,

We are using these motors and gearboxes on our arm. The gearboxes are 64:1 and there is an additional 40:1 worm gear setup being driven by the banebots gearmotor. The motor emits smoke very quickly if operated at partial load in a manner that stalls the motor. By quickly, I mean in a few seconds.

Our arm is run with the motor at full power to get it where it needs to be as quickly as possible and the motor them is then shut completely off. This can be done using feedback from a pot. You want to gear your arm so that the motor can be run at full speed and with an anti-back drive setup so that you don't need partial power to balance a load, or your motor will very quickly resemble burnt toast on the inside.

The FP motors have more power and are more robust, but will also emit their magic smoke if you abuse them. The FP motors can easily be adapted to the banebots gear box. If you do this, you can make a spacer that allows the fan in the motor to draw air through the motor as it runs. You may, or may not, need the FP motors elsewhere on your robot.

Before anyone writes anything disparaging about the banebots motors, I would like to remind everyone that all the motors that we have had in the KOP over the years have had their limitations and it has always been our job to sort out those limitations. We use the motors in a manner that keeps their limitations in mind. The banebots motors are perfectly useful, within their limitations. Running the banebots motors at stall, even at partial power, is operating them outside of their envelope. Running the banebots gearboxes with any radial or axial loading on the shaft is also operating them outside of their envelope. If you do either of these things, the motor and/or gear box won't last long.

Eugene

Originally Posted by sanddrag

We are having a serious issue with the small bane bots motor in the 64:1 gearbox in the kit. This exact problem has also been reported by three other teams I have talked to. The scenario was lifting a rather light weight arm that was not terribly long. There was additional reduction after the gearbox, in the range from 2:1 to 9:1. Calculations show that even in the 2:1 scenario (my team), the drive should have powered the arm rapidly with more than enough torque available. The arm was unable to lift, and the motor started smoking as soon as it enountered any sort of real load. This problem was encountered the very first time the motor was tested in a load condition. This is the same problem three other teams have reported to me. The motor never once tripped a breaker (20A on my team), which, if it was stalling (spec'd) stall current about 37 amps isn't it?) it really should have. And it should trip a breaker before it ever starts smoking.

So, I don't know what's going on here. Post if it happened to you, and post if you know why.

[Justin M.]

Stall current is 42A, spec'd stall torque is 158 in-lbs, but the safe operating range is 100 in-lbs max. So yes, if you were running these on 20A breakers they should be tripping before these motors stall.

However, the Banebots gearboxes HATE side-loading on the shaft, which could be causing the motors to smoke. Try using the bearing block - also, I'd recommend using two of the 64:1 36mm gearboxes to the lift it to double the safe torque operating range to 200 in-lbs., before additional reduction with sprockets.

[Joe Johnson]

Originally Posted by Justin M.

Stall current is 42A, spec'd stall torque is 158 in-lbs, but the safe operating range is 100 in-lbs max. So yes, if you were running these on 20A breakers they should be tripping before these motors stall.

snip

This is not true. The 20A Breakers are time dependent. 20A is their NOMINAL value. They should be able to carry that load for infinite time. They will trip at a value above this -- tripping faster the higher the current is above tghe 20A rating.

The Banebots motor can pretty easily over heat. And you don't even have to stall them.

For these motors (and the FP too for that matter), I strongly recommend that you design the worst case loading to be something like 20-25% of the stall torque.

Joe J.

[tgraham_533]

I am working with Team 533 and was using the Banebot motor as well. We designed in the 25% torque load area and had not trouble with moving an arm over varying load conditions. However, with the motor stalled and/or held in a fixed position the motor began to smoke almost immediately. After inspecting the motor under these conditions we found the case to be cool but the shaft of the motor to be extremely hot. It appears that the motor has not thermal path from the winds to the case. Therefore, if the motor is under load, but not moving the winds just heat up. We are working changing to a different motor. That's what we found, hope it helps.

[MrForbes]

That does help, thank you.

I wonder if the programming team can add a safety routine to shut off the motor if it is being commanded to drive, but there is no change in signal from the potentiometer on that mechanism.

[Al Skierkiewicz]

The snap action breakers are thermal devices capable of up to 600% over current for a few seconds before they trip (They can actually handle 200% of rated current for 10-15 seconds). It is conceivable that a 20 amp breaker will not trip at the stall current of this motor unless it is sustained. As with all motors of this type, there is a very high thermal resistance from the armature to the outside world. The only thing conducting heat is the shaft and bearings.
The breakers are sized to match with the wire size to prevent fires on the robot. A dead short on any branch circuit will trip these breakers as they are expected to. The stall current in your little motor will rapidly raise the temperature of the windings causing the insulation on the internal wires to fail which in turn causes shorts between windings. The smoke is the insulation burning as the wires short. When this occurs, the current skyrockets and the power falls while the motor is in self destruct. It is conceivable that the motor will still turn while drawing more than rated stall current with shorted windings.
Please remember that in raising an arm, there is a change in force/load. The greatest demand (load) on the motor will occur when the arm is passing through the horizontal plane. I am sure that Dr. Joe can explain this better than I can.

[MrForbes]

Originally Posted by Al Skierkiewicz

Please remember that in raising an arm, there is a change in force/load. The greatest demand (load) on the motor will occur when the arm is passing through the horizontal plane. I am sure that Dr. Joe can explain this better than I can.

This also depends on the configuration of any counterweights or springs or other devices that help support the arm (besides the motor).

For example, the gas spring on the 1726 robot arm is set up right now in a way that it takes more motor torque to lower the arm from the full up position, than it does to move it at any other position.

[Cuog]

Team 422 also had issues with the little banebots motor giving up the smoke.

The scenario:
We had an arm that weighed 5 lbs and we estimate its center of mass to be a little over a meter out. It was connected to the banebots 64:1 motor with an additional 9:1 sprocket reduction. We powered it up and it had plenty of torque, it was so fast and strong that it threw the tubes(not our intentions) but when we tried to hold the arm steady at a given point, by providing enough power to the motor to hold it against the gravity of the unloaded arm, we had it holding there for about 30 seconds and out pops the magic smoke.

[savage301]

We also had these issues. We think the problem is that a cooling fan is attached to the output shaft inside the motor casing. When you try and lift an arm, even with reduction, you can stall the motor. You still are supplying current and heating the wire coil, but its not being cooled. And here comes that magic smoke. We ended up using the Keyang motors to drive our arm. They work very well, plus they can't be back driven.