pic: DeWalt Gearbox with two CIMs

[newton418]

Originally Posted by newton418

To specify my question, why not get the extra torque after running each CIM through their own DeWalt transmissions? Though I have to say, if it only weights 2.5 lbs without the motors, that would be a good reason. If you build it make sure to post pictures and results!

Originally Posted by Greg Needel

Running each cim into it's own transmission would not give you extra torque. In most if not all cases the total torque is the same. Since each motor has a maximum amount of power it can generate at 12 volts within the given amperage requirements all you would be doing by making separate gearboxes is spreading out the load. In this situation the only reason I would consider doing that is if I was concerned with the torque of 2 cims breaking the deWalt transmission (yes I am thinking that).

From my experience in drive train design I will always advise that people go with a central gearbox and power transmitted through chain or another means. This is due to your overall friction force (another discussion) but basically if you spread out the load to all of your wheels each wheel can take the total torque produced in the situation where another set is off the ground (assuming the mass is over the wheels touching the ground) that is where the normal force/ weight of the robot plays in (yet another discussion) either way keeping your motors together will save you weight in the long run and give you the nice ability to put the torque everywhere you want it to be in the correct situations.

What I meant by "the extra torque" is the extra torque you are getting from the second motor. Basically, you have two different scenarios:

(1) You have a transmission with a given ratio, and you have two identical motors inputting power (as in your design), then you get a specific max output torque from the transmission. This can then be connected to a drive train (preferably where all wheels are connected, so you power all wheels), determining the max output torque on the wheel axels.

(2) Now consider taking the same motors and the same transmission, and having one motor input power into one transmission. If you take two of these single motor-transmission assemblies and connect the output shaft of each transmission to a drive train (where all wheels are connected), you end up with the same max output torque on the wheel axels.

So my question is, what is the advantage to the former setup (besides potentially saving weight)?

[Greg Needel]

Originally Posted by newton418

So my question is, what is the advantage to the former setup (besides potentially saving weight)?

well there are a few, but by no means would they outrank a centralized gearbox in my book.

weight distribution - with each motor (cim) weighing about 5 lbs you might want an even weight throughout the chassis as apposed to a centralized one.

swap ability - for some reason if a motor burns instead of taking a transmission apart disabling your drive train (with a centralized tranny) you can just quick replace the single module that is broken.

Robot layout - sometimes it may be easier to mount 1 single motor and tranny then a full gearbox. The example that comes to mind for me is a planetary set where they are really compact.

I am sure there are more....the main thing to remember is that there is no "one and only solution" and it is all about evaluating your specific situation, and making the best decisions you can. Every team has their own reason for doing things, all of which are valid.