pic: DeWalt Gearbox with two CIMs

[Lil' Lavery]

(the delete button seems to have vanished from me.... it's not where it was)

[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!

[Greg Needel]

Originally Posted by newton418

To specify my question, why not get the extra torque after running each CIM through their own DeWalt transmissions?

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.

[Cody Carey]

Originally Posted by Greg Needel

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)

I'm confused... I don't quite see where this paragraph fits in with proving that there will never be more torque if you put the motors through separate gearboxes. Maybe it's just me only having a highschool physics background, but are some of the words taken a bit out of context? maybe a diagram would help me? I understand the terminology, this paragraph just confuses me.


But then again, maybe it is because it is 2:30 in the morning.

[Greg Needel]

Originally Posted by Cody C

I'm confused... I don't quite see where this paragraph fits in with proving that there will never be more torque if you put the motors through separate gearboxes. Maybe it's just me only having a highschool physics background, but are some of the words taken a bit out of context? maybe a diagram would help me? I understand the terminology, this paragraph just confuses me.


But then again, maybe it is because it is 2:30 in the morning.

not a problem, I am only half awake myself. That paragraph is talking more about the design system then proving that there won't be more torque. But since you asked I will explain why you don't get more torque.

Torque from dc motors is theoretically limitless, the more voltage you put to the motor the higher the torque you can create. Now in a real world (FIRST included) motors can only take so much voltage before they burn up. So especially in FIRST where your voltage is limited so is your maximum torque. The best way that I can think of describing the scenario in question is by comparing it to water. If the gearbox is designed correctly to maximize torque without drawing more then 40 amps then you have 1 full water container. This container got it's water (torque) from both motors combined (the max before tripping breakers). Separating the motors is like pouring the water into 2 glasses, you still have the same amount of water it is just in a different place. Because torque is limited by voltage and amperage it doesn't matter how you split it, it will remain the same.

Assuming the same gear ratio in each gearbox consider the following

max torque out of 1 transmission with 4 motors
=
combined max torque of 2 transmissions with 2 motors each
=
combined max torque of 4 transmissions with 1 motor each.

Now there are other things to consider like gearbox inefficiencies and other losses but for all intensive purposes you can pass the water around all day but in the end you are still left with the same amount of water.

[MAteo9944]

Originally Posted by Greg Needel

Now there are other things to consider like gearbox inefficiencies and other losses but for all intensive purposes you can pass the water around all day but in the end you are still left with the same amount of water.

Another thing that you hinted at was that if you have 4 transmissions, if some of your wheels lose grip or leave the ground then you suddenly have less torque available. (If your wheels are leaving the ground during a pushing match you have other problems.)

[Greg Needel]

Originally Posted by MAteo9944

Another thing that you hinted at was that if you have 4 transmissions, if some of your wheels lose grip or leave the ground then you suddenly have less torque available. (If your wheels are leaving the ground during a pushing match you have other problems.)

True but I was thinking more like climbing a ramp, once your weight leaves the front wheels the weight transfers to your back wheels. If you are running a gearbox with all wheels connected your effective force is the same because your normal force and traction is constant. If you run 1 motor to each wheel (not connected) then your effective force is much less due to the loss of normal force over your front wheels. depending on the situation it could be as much as half the total power of the system.

[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.