pic: Gerrish Gearbox MK-III

[rbayer]

There's absolutely nothing wrong with a 4-bevel gear differential. Check out http://auto.howstuffworks.com/differential2.htm for an animation of what it would look like.

As for reducing friction, I don't think it will do that. What it will do, however, is distribute the force more evenly over the two gears, thus reducing the chance of tearing teeth off, etc. Then again, I don't really know all that much about differentials so I could very well be wrong on that last point.

--Rob

[sanddrag]

Oh my gosh. I feel so embarrased. Thanks for correcting me though Rob because I would not want a potentially good design to not be tried because soemthing I said. Tytus, it looks like I'm the one that needs some help. I am so sorry for not believing your idea would work and not listening when you said it would. I never really took as much time to sit down and think about it as you did. I guess when I get so caught up in all these advanced mechanical things I forget the basic priciples and need to revisit the Legos but never do.

Anyway, I'm really sorry for everything I said and the 4 gear bevel gear differential does work like you said it would. I think your drawing is really great and you should keep up the good work.

And with that, I stand corrected.

And for all of you who follow my postings often, these past few are just due to some pre-school (pre as in before, not preschool) anxiety. You do not have to be concerned with the truth to all of my past and future postings about various mechanical things.

[sevisehda]

My only question is has a reverse differtial ever been tested? On paper or monitor it looks good. But in the real world how will the motors react? Will the load be divided between the 2 evenly? How will the motors react as they throttle up and down. I'm not saying it won't work I'm just saying this is something you've got to build and test to check feasability.

[Anthony Towne]

I see where you are going and I have to admit that it looks like it will work. It took me a sec to realize that the 2 back helical gears aren't attached to the input shafts, they are just attached to each other through the cage structure and rotate on bearings. This isn't a new idea, however. All it is is a differential working backwards. Teams in years past have tried this. Many teams don't do this because it requires 2 motors for each set of wheels, and most teams use 2 sets of wheels, so the robot gets heavy fast and run out of room in the drive area to fit 4 motors with everything else.

[Tytus Gerrish]

Who's tried it before?, Show me i could learn from them

[Anthony Towne]

there was a team at the UCF regional that had a differential in their drive system. I don't know their team #, I just know it wasn't a UTC team I know that doesn't really narrow it down). When I was involved back in High School (6-7 years or so), I saw them used at a few regionals, but all the team numbers have changed since then and I can't even remember what their original team numbers were. Sorry I can't help you with that further.

[P.J. Baker]

This looks like a pretty cool idea. Thanks for sharing. Here are my thoughts:

Without doing too much analysis, I think that this works as follows: The two motors adjust their speed until both are exerting the same amount of force on the two gears on the output section of the differential. The driven speed of the output will be the average of the two input shaft speeds. Essentially, it forces the motors to output the same torque, rather than the same speed – which is what happens with the more traditional dual motor designs. I don’t see any reason why it wouldn’t work in a robot.

However, it will suffer from the same problem as a car differential if one motor loses power – almost no torque will be transmitted through the drive train even if though there is still one good motor. The dead input shaft (unless it is somehow locked up) will be backdriven by the good input shaft, and the output of the differential into the treads or wheels of the robot will barely move.

I wouldn’t consider this to be a fatal flaw in the design, just one to be aware of. If one motor burns up or some how becomes mechanically or electrically disconnected from the transmission, the other will be just as useless.

Also, rather than matching free speeds between the two motors, I might suggest matching the stall torques. The differential design appears to have problems when motor A is supplying a lot more torque than motor B. Therefore, I would suggest gearing the motors to have similar torque performance rather than similar speed performance.

Good Luck,

P.J.

[Bill Beatty]

Sorry, the design does have a fatal flaw. The higher torque motor will overpower the lower torque motor.

I am not sure why all the concern of matching motor speeds. Just gear the two motors to output the approximate same speed and both will contribute very nicely.