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I have wondered the same thing.
This is the conclusion I came up with.
If you want the two motors to run in unison; thus acting like a single super motor then yes they have to be geared so that their free wheeling speeds match closely.
However If you want to have one motor be the loaded motor in the low RPM output range and the other motor be the loaded motor in the high RPM output range then no. The only concerns in this setup are. First that the motor that is gear for the high range doesn't stall out in the low RPM range I.E. don't power it until it comes with in its own range. The other concern is the motor that is geared for the low range is uncoupled at the high RPM range so that it doesn't explode from over revving.
The next question one might ask is how is this accomplished?
First you need two motors, Motor A will become the motor for the low range and Motor B will become the motor for the High RPM range.
Next you gear the motor for they desired ranges and couple them to the output shaft.
Next you'll need to attach an optical encoder to the output shaft so that you know what the output shafts RPM is. This reading will govern the power for the high range motor (Motor B) turning it off at low RPM and for the low range motor (Motor A) disengaging it form the system at high RPM.
Here's the hard part. You need a device that disengages the low range motor (Motor A) from the system when the output RPM goes too high and the motor is in danger of exceeding it's RPM max. Possible devices are a clutch either frictional over positive engagement or a device that I call a "captured ratchet clutch" meaning that the only time that the motor is coupled with the system is when the motor is powered.
The captured ratchet clutch or CRC has an Input plate and an output plate. The input plate has ratchet teeth on the face of it these teeth engage similar depressions in a receiving plate that is held to the input plate by a spring. So that when torque is applied the ratchet teeth on the input plate try to ratchet over to the next hole. While in the process of doing this it forces the receiving plate to push out however this action of pushing out the receiving plate causes the receiving plate to engage with the output plate using dog teeth and receiving holes on the output plate. However the distance between the receiving plate and the output plate is less than what is necessary for the ratchet teeth to disengage and move over to the next hole but that same distance is just enough so that the receiving plate doesn't engage the output plate when no torque is being applied.
The previous explanation is best understood through a diagram but I don't currently have time to draw one. I will try to post a follow up with a diagram included.
An example of a combined low and high drive system is this. There is this grinder that we use at school and because of the motors gearing it doesn't have enough stall torque to start the grinding wheel spinning so we have to use either our hand or the object the we need to grind to give the grinder a kick start.
This is a fairly difficult concept that needs some exploration. I'm not sure how advantageous this system is compared to a same speed two motor system. But I think that it deserves some exploration.
Just food for thought, anyway you'll be better off just gearing two motors the so that the speeds match.
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