pic: Gerrish Gearbox MKII

[Madison]

Originally posted by Tytus Gerrish
If it is not clear how this device works i will make a more clear and understandable render with labels for now i tell you it has two inputs (the motors) and one output (the shaft on top)

After spending the last few hours reading a lot about differentials and their hellspawn, I'm pretty sure that what you're attempting here will not work.

The entire point of a differential is to vary the speed and torque of two output shafts . By reducing it to a single output with two inputs, you're not creating a differential anymore. After all, what's "different" about the output(s)? Nothing. It's a single output operating at single speed at any given time.

Now, though, I think I understand what you're trying to accomplish. You're using the concept of a differential in reverse to mate two motors, and then you need two of these modules on a robot to operate both sides independently.

Based on this the mathematical relationship at work in a differential with 1:1 gearing is input = (outputA + outputB) /2.

So, for example, let's say input is 10. Therefore, it's 2(10) = outputA + outputB. While driving in a straight line, outputA and outputB would be equal at 10 each -- or the same as the input. If there is resistance against one output, more power/speed/torque will go to the output with lesser resistance. This might end up being a 12 + 8 or a 3 + 17 or even a 20 + 0 scenario.

Your idea here reverses this concept. So, therefore, out equation would read more like this:

output = (inputA + inputB) / 2

So, again for example, let's look at the motor RPMs for the chiaphua and drill under no load.

output = (5500 + 19000) / 2
2(output) = 24500
output = 12,250

Simple enough, right?

Well, now let's look at this in terms of torque.

inputA = 2.2 Nm
inputB = 1.2 Nm

2(output) = 2.2 + 1.2 = 3.4
3.4/2 = output = 1.7 Nm.

So, as you can see, the output of your differential is actually less than what you'd get from a single motor.

I'm unsure if there any circumstances where this wouldn't be the case, but it doesn't appear that there are.

I'm also unsure how things would be affected if the gearing were something other than 1:1.

If I've made some grossly wrong assumption here, please correct me. Thanks.