pic: Team 701's 3 CIM In-line 2 Speed Shifting Transmission Prototype



This is a protoype for the transmissions we may be using for our 2014 prototype robot and final robot during the build season. So far, we still need to tweak the gear spacing to make them mesh better. The plates are polycarbonate in this picture, but the final transmissions should be painted aluminum. The gears all came from AndyMark SuperShifters. The only parts we had to make ourselves were the plates.

I would worry about the gear tooth stresses in the CIM gear on the last CIM in line. It will be transmitting three times the torque it was designed to transmit.

Could you explain why that would happen?

he is referring to the cim gear closest to the shifting mechanism. not only does it carry the load of the cim its on but it will carry the load of the 2 cims farther out . the farthest cim will be at normal stress the second cim gear in the middle has the load of 2 cims on it and the cim gear closest to the pneumatic cylinder will have all the torque from all 3 cims passing thru it

All I see there is lots and lots of weight.

It looks really bad@@@, though! :smiley:

Are we talking about the torque of the motor itself while it runs, or are we talking about the displacement the motors themselves causing a misalignment of the angle between the axes of the gears? If we are talking about the motors bieing misaligned due to their weight, then yes, we understand that the CIMs on the plate are very heavy all together, and since the transmission will be mounted directly on our frame, there will be also stress on the frame as well. We plan to design a support system so the weight of the motors wil be distributed somewhere else other than the transmission itself, and the part of the frame it is attached to, so we do not run into probems with warped, broken, or damaged robot components.

I believe that he’s talking about the fact that the torque of 3 motors will be transferred through the sprocket on the first CIM motor in the chain (the leftmost one in the picture).

The teeth might not physically be able to handle the torque of 3 CIM motors, especially at stall and teeth might break off of the sprocket or other catastrophic failure might occur.

Even if the teeth do not fail due to static overload, the face pressures will be tripled, and the wear will be greatly accelerated, and I have seen CIM gears wear out quickly under heavy load, even when carrying the load of one CIM motor.

The most likely time for that gear to fail is under a rapidly reversing load. When you go from full forward to full reverse, there’s a brief moment where the back EMF of the motor effectively doubles the voltage across the motor and you can double the current in the motors. Or atleast you’ll be drawing as much current as 24V can pull from your battery and through your wires. Double the current is double the torque, so you’d be wise to atleast size the teeth for a static load of 6x the stall torque of a CIM.

I talked to my team to day and this makes complete sense now. We may have to opt for a stronger gear

I suggest going for a steel equivalent. Should work fine.

Since the Andymark CIM gear is 4140 steel already, not sure what you mean by this.

Finally found Joe Johnson’s old post on this subject:
http://www.chiefdelphi.com/forums/showpost.php?p=438674&postcount=9
He goes through the calculations for that particular gear, which happens to be rather similar to the size and load of the last CIM gear in the train above. The above gear has slightly more load and a slightly larger width, so it actually works out to about the same 11,000 psi stress that JJ calculates there, assuming 3x CIM stall torque. Even at 6x the stall torque, that’d only give you 22,000 psi stress, so you should be well within the tensile and fatigue strength of 4140 steel.

Joe’s calculation only suggests that you’re not likely to break off a tooth if you have your all those gears properly aligned. It’s still possible that you’ll wear it out rather faster than you’d really like.