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Unread 03-05-2012, 01:55
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Re: Dual CIMs and #25 Chain

It sounds like you have only one chain running from your 2 cim gearbox to the center wheel, and then other chains run from that center wheel to the front and back correct?

If so I think that is why you are seeing issues, with only 16 tooth sprockets your chain tension will be extremely high, and you have only one strand of chain to deal with it. If you were to run chain from the motor gearbox to the front, and another to the back, with another to the center wheel then you will have 3 chains to transmit the total output of the motor to the wheels, which will (in the case with all wheels having the same traction) reduce the tension in the chain to 1/3 of what it is now.

Also as a reliability concern, I always run separate chains to the front and rear wheels from the transmission, If one chain fails for whatever reason you still have a somewhat functioning drive train. It takes a double chain failure to stop that side.

I did a bit of math, and assuming the following
2 CIM motors in stall, 16 tooth sprocket, and 12.75:1 gear ratio on your tough boxes, you are applying 848 pounds of force to the chain and the sprocket teeth. Diamond brand roller chain in #25 size is rated to have a tensile strength of 875lbs, so it really is not much of a surprise that the chains failed at the master link, which is typically weaker than the chain it holds together. If you change to a 32 tooth sprocket the chain would only experience 427 pounds of force, which is about a 2x factor of safety.
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Justin Stiltner
Lead Robot Inspector, VCU Regional
Unmanned Systems Lab, Virginia Tech
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