pic: Gerrish Gearbox MKII

8f44b13105092ff6afb2633f4315c307_l.jpg

I will be So disapointed if nowone comments on this

Hmm, looks interesting. Perhaps some 2D orthographic views though? I can’t quite make out exactly what is going on…

Ortho-What now?

The standard in presenting mechanical parts. 2D Top, Front, and Side views…

A very interesting concept indeed. There is a lego differential that looks exactly like that, just minus the motors on each side and the final drive gear on top. But since that is a differential, it would compensate for the rpm differences in the motors. I’m just not sure how well it would work with a 14,000 rpm difference.

Thanks

But, Where Am I going to find a 14,000 rpm diffrence?

*Originally posted by Tytus Gerrish *
**Thanks

But, Where Am I going to find a 14,000 rpm diffrence? **

He’s referring to the difference between the Atwood’s speed and the drill’s. The Atwood is supposed to be ~5000 rpm, and the drill is supposed to be ~19000 rpm, hence the 14000 rpm difference.

…I’m not sure I understand how this would work.

Shouldn’t a differential have two outputs? I’ll admit, though, I’m really not clear on how they work.

But, again, since the motors would each have varying RPMs. . . well, how does it work? The Drill output is 19,470 RPM or something. The Chiaphua is 5,500 RPM. They’re each connected to a unique shaft and miter gear? Then those two shafts input into a single miter gear there at the bottom? That does nothing to compensate for the difference. Then, I guess, one of those original two miter gears is attached to the larger miter gear, with then gears up to the output?

I don’t know that I follow what this is meant to do at all.

Perhaps an explanation coupled with a few less fancy drawings is in order?

I did some tests with this theory out of legos.

On one side of the differential I geared a Lego motor up (40 tooth on motor shaft meshed with 8 tooth gear going to differential). On the other side I geared a Lego motor down (8 tooth gear meshed with 40 tooth gear going to differential).

The differential spun at the speed and torque of the geared up motor. The geared down motor did not have any effect on until I held the differential until the geared up motor stalled. That is when the geared down motor kicked in and spun the differential with the speed and torque of the geared down motor.

So from my findings it seems that this does not work, unless anyone can prove it correct.

I studied it for a few minutes… and I think it’s more for a pretty picture with 3DMax than anything else.

Well, I guess differentials don’t work in reverse then where there are two motors and one output instead of one motor and two outputs. It was an interesting concept to toy with though. I wonder if there would be better results if the speed difference was not so much.

The differential spun at the speed and torque of the geared up motor. The geared down motor did not have any effect on until I held the differential until the geared up motor stalled. That is when the geared down motor kicked in and spun the differential with the speed and torque of the geared down motor.

Yeah it really doesn’t work that way. It is just the way that a differntial is designed. That same problem can be rather quite problemsome backwards also if you loose all traction in one tire. Btw After watching a few episodes of junkyard wars can someone tell me what it means to lock a differntial??? From the way it sounds it might just fix the above problem though it would probably defeat the purpose of it.

*Originally posted by Adam Y. *
**Yeah it really doesn’t work that way. It is just the way that a differntial is designed. That same problem can be rather quite problemsome backwards also if you loose all traction in one tire. Btw After watching a few episodes of junkyard wars can someone tell me what it means to lock a differntial??? From the way it sounds it might just fix the above problem though it would probably defeat the purpose of it. **

Locking a differential is accomplished by physically connecting the two, otherwise separate output shafts so they act as one. In situations where an open-differential would result in getting stuck. . .where one wheel is sleep and the other is getting no torque, locking the differential would make the rear axle act as a solid piece, giving torque to the wheel that’s not stuck.

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)

*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.

OK take all that stuff And Turn it around . Look! this thing didnt come off of a truck! It Will never have to do what a diffrental on an automobile does, IN FACT it does the exact oppsite. the Cg model works, the Lego model works, the popcicle stick model works, AND THE PRODUCTION MODEL WILL WORK!

*Originally posted by Tytus Gerrish *
**OK take all that stuff And Turn it around . Look! this thing didnt come off of a truck! It Will never have to do what a diffrental on an automobile does, IN FACT it does the exact oppsite. the Cg model works, the Lego model works, the popcicle stick model works, AND THE PRODUCTION MODEL WILL WORK! **

If you’ll read the explanation I provided at the end of my post, you’ll see that, assuming I’ve done this correctly, the math shows that what you’re doing is actually less powerful than using a single motor.

It will work, yes, but it will be completely unnecessary and act as a burden! Something working on a computer is meaningless if you’re not using some hefty software to run your analysis. It looks like you’re using 3D Studio MAX. That’s not in the same ballpark by any stretch.

The Lego version will work, but again, you’re actually outputting less torque than you’re inputting on a single side. The second motor/finger/whichever is dragging on the system.

There’s no need to scream. I’m sorry I wasted my time trying to explain what’s actually happening here. I’ll remember that for the next time.

what made you come up with the idea

*Originally posted by M. Krass *
**If I’ve made some grossly wrong assumption here, please correct me. Thanks. **

You’re on the right track, but you’ve made a few wrong assumptions. I’ve done some work (and a lot of math) thinking of a way to use a differential in reverse as a gearbox.

In the end, I came up with several conclusions:

  1. A differential is merely a planetary gearbox, designed differently.
  2. A differential is less efficient due to the bevel gears needed.

With a differential, the housing is the input, with the two sides being the outputs. A differential allows either the two sides to have the same speed and torque. It also allows torque and speed to shift to one output for the sake of turning. One of the outputs could have twice the input speed, while the other has none.

Unfortunately, with such a design, as Paul Copioli said, the power of the second (“control”) motor is basically lost.

In using a differential as a gearbox, you basically put yourself back at the Thunderchicken’s problems, because it behaves like a planetary transmission. You cannot have two motors with dissimilar torque and speed characteristics hooked up without experiencing motor fighting. The motor with higher torque will stall the other motor and will win every time. The result is that your robot won’t go anywhere. The only way to make such a design work is using worm gears. I then determined that the Thunderchicken’s way of using the worm gears would be more efficient.

If you have two motors with similar torque and speed characteristics (or geared to match), you can, essentially, create a continuously variable gearbox similar to the Thunderchicken’s CCT. Using one motor to spin the housing, and the other as a control on one of the side outputs, you can simulate a load on that output. Thus, it will behave normally as a differential, achieving a variable gear ratio anywhere from 1:1 to 1:2. The maximum torque is that of a single motor, while the maximum achievable speed is twice that of a single motor, without all the quirks.

So, for my reference, which of my assumptions were incorrect?

I’ve said exactly what you have, but without drawing the parallel to a planetary gearbox.