Help with Gearboxes

[D. Gregory]

I am in charge of my team's engineeing section and I am contemplating of machining 2 gearboxes (1 per side) to connect 2 motors to one output essentially driving each side with 2 motors. Keep in mind that this will be designed entirely by students but will be machined by professionals. Can you guys give me some tips and suggestions as to the feasibility of this (I know its possible, but is it over the head of a student) and ways I can go about doing this. Thank you very much.

[Suneet]

On my team, two groups of people came up with hypothetical gearbox designs. I head one of the groups, and I feel it has a better design than the other (shift gears on the fly). This is my third year doing robotics, so I did a full hypothetical model in Inventor (sans a motor mount, and other smaller details). We'll have to see the game and kit to procede further. To use four motors, I planned to make a separate box for each wheel, two motors per side. (inspired by team 60 ) Bit it depends on the game...

I'm no expert, but instead of putting the output of two motors on one shaft, it might be eaiser to make 4 gearboxes and have two motors per side. That's what I hope to do. But if you want just two boxes, I think it's better to use chain instead of gears, cause they're a bit more... forgiving than gears.

Also, there's tons of boxes in the whitepapers. Take a look:

http://www.chiefdelphi.com/forums/pa... SC&sort=date

[D. Gregory]

Wouldn't weight be a problem with 4 boxes? We aren't really looking to closely at shifting due to the complexity. Also, if you have a different box for each wheel wouldn't you get 4 slightly different speeds from your wheels. Even with mathematically correct gear ratios, you will obviously get slightly different speeds from different motors.

[Cory]

when thinking about shifting on the fly, keep in mind, that for all practical purposes, this is only accomplised by using a Pneumatic. This is the bad part. I too like team 60's four gearboxes, but that is the problem. They have four gearboxes. If you have four gearboxes, shifting each one, you have just used four pneumatics, which leaves you with only one for use anywhere else on the robot. For many teams this year, only having one pneumatic usable, they would not have been able to release the goals, like 60.

Anyways, I think it is a great idea if you dont have any other use for those pneumatics.

Cory

[Suneet]

Weight? Well, you can use some pretty thin metal (.25 in Al 2024-T4 for me) and with lean construction the weight should be reasonable. But yeah, one box per side probably would be lighter.

when thinking about shifting on the fly, keep in mind, that for all practical purposes, this is only accomplised by using a Pneumatic. This is the bad part. I too like team 60's four gearboxes, but that is the problem. They have four gearboxes. If you have four gearboxes, shifting each one, you have just used four pneumatics, which leaves you with only one for use anywhere else on the robot. For many teams this year, only having one pneumatic usable, they would not have been able to release the goals, like 60.

60 doesn't really shift on the fly, but I've seen their boxes up close at a workshop. Wonderfully simple+reliable mechinism, you've got to hand it to them... Actually, someone proposed running levers to the boxes so all could run off one cylinder. I think this would be way too complex, but if you need the pneumatics for something else, I have some ideas with springs, motors ,and cables...

[IVIaxor]

It you want to put two motor into one gearbox what you need to do is gear them so at the point where they combine they will freespin at the same RPM. That way the motor curves will match up for each motor, if you do not do that one motor will always be running with poor efficiency. That can cause both the tripping of breakers and drain your battery quickly.

[EricS-Team180]

...have you checked out DLAVERYs "Dual-Motor, Dual-Speed Drive Transmission Design" in the technical white papers? It may be useful.

[Al Skierkiewicz]

D.
I have seen some two motor designs. It is pretty simple to have both motors gear-coupled to a common driven gear. (i.e. when viewed from the shaft end you would see three gears, two small and one large)If you are attempting to have the motors turn on in a single and/or dual drive then it is recommended that you make provisions to take one motor mechanically offline when not used. I would not recommend that you feed two motors from one speed controller either as this produces some unusual loads for the controller. One controller feeding each motor should work OK. (BTW if you are going to use this approach be mindful that the braking function of the speed controllers need to be set the same and you may find that using braking at all is a problem when hard coupled) Keep in mind that if the motors are not closely matched or compensated for in software, the slower motor will provide some loading on the faster motor. In general this will not be a problem but off the top of my head the current in such a case would seem to be a little higher than the sum of the two motor currents when operated as single units.
Happy New Year

[D. Gregory]

Here is an idea I had for a non-shifting two motor gearbox. Can you guys and gals take a look at it and let me know how I can improve it. Keep in mind it is just a concept and is extremely rough with no actual measurements, just a concept. It put it as an attachment. Thank you very much for your help.

[D. Gregory]

Erg, here is the attachment. Edit post didn't work

[Madison]

Originally posted by D. Gregory
Here is an idea I had for a non-shifting two motor gearbox. Can you guys and gals take a look at it and let me know how I can improve it. Keep in mind it is just a concept and is extremely rough with no actual measurements, just a concept. It put it as an attachment. Thank you very much for your help.

In concept, that is what a non-shifting coupled motor transmission would look like. It's just a matter of determining gear ratios to minimize losses and designing a framework to house all of it.

The only part that I would show concern over is the need for chain at all. It is redundant to change the motors' output RPM through a gearbox and then connect each gearbox via chain. Instead, you may want to investigate, as has been mentioned, coupling each gearbox to a single output shaft. As an alternative to gears, you can use chain and sprockets to alter the RPM as needed and, again, couple the output to a single shaft. Gears would be much easier to deal with, however, as they won't change properties as much as chain might (i.e. stretching.)

You can eliminate chain from your design altogether by attaching a gearbox and motor to each driven wheel, even. Or, similarly, you can make driveshafts ( ) There are many possibilities for driving the robots and they each offer advantages and disadvantages.

You're on the road to designing a basic gearbox. Remember, though, that harnessing that power may not be the best solution for this year's game. Always design with the game in mind, as an amazing drivetrain is certainly something to be proud, but it may not serve your need to effectively compete.

[Al Skierkiewicz]

As stated above, you are on the right track. What you are looking at is something that is pretty heavy and the first question is "how is it going to steer?" With chain feeding both front and rear you can't turn either set of wheels. If you are going for tank drive, then the side friction of the wheels while turning skyrockets the motor current.

[D. Gregory]

Would there be friction if we went for a spin or straight setup. The left side and the right side go in opposite directions to rotate us. They go in the same direction to move us foward and backward. Obviously, this would kill are ability to do gradual turns because as you said this would cause one side of the robot to drag putting strain on the motors. Can you give me some other steering alternatives or different gearbox setups? Thank you very much.

[Al Skierkiewicz]

What several teams have started doing is either buying or designing/building their own wheels that grab in one direction and slip in the others. (We called ours BUPOD but I don't remember what the letters stand for at the moment. They were essentially small wheels mounted at the outer circumference of a larger wheel and at right angles to the driven axis. When driving the smaller wheels didn't move but when turning they did.)
Another alternative is to only drive one set of wheels and use casters (shopping cart) wheels for the other set.
Third and most difficult is to design a drive pair where the motors rotate as a module with the wheel and therefore drive and steer.
Good Luck,
Only two more days to go!

BUPOD=Biased Uni Planar Omni Directional.
Biased meaning they roll in only one direction.

[FotoPlasma]

Originally posted by Al Skierkiewicz
They were essentially small wheels mounted at the outer circumference of a larger wheel and at right angles to the driven axis. When driving the smaller wheels didn't move but when turning they did.

This sounds like the most popular thoughts of a holonomic system, despite the fact that the amount of controllable degrees of freedom is not equal the the total amount of degrees of freedom (seeing as how you can be forced to strafe, and it's not controlled, as explained in a second). I believe I know what you're saying, and this sketch might be close.

It seems to me that if you were to integrate these omni-wheels (as they're commonly referred to) into a 4-wheel-drive (two sets of two chain-driven wheels), without the wheel assemblies themselves turning (thinking of the Wildstang 2002 robot), then you can easilly fall victim to being pushed perpendicular to your wheelbase, whereas a team without such a system would be much less susceptible to such a maneuver.

However, such a situation makes more than a few assumptions about next season's game, so much of this post can be disregarded.