Swerve Concept

[BBray_T1296]

Are you asking how to hold the crown gears in the module in place? or the spur gears between the motors.

When we did our car/crab drive in lunacy and breakaway we used one single brass pillow block to hold both crown gears and support the axles. of course, it looks like the large crown gear shares an axle with the wheel here, where ours had another stage lower down

For the spur gears (depending on live/dead axle) there is hardly any axial load (gravity isn't much) if you can machine groves for E-clips they would work just fine. Shaft collars (especially the thin and light VexPro ones) would work as well

EDIT: looking back at the pictures, PVC or Delrin (or anything really) spacers would hold the spur gears

[Jared]

What are you using for your bearings between the rotating module and the frame?

Also, if this were to be made, how would you make the .125" sheet metal part? Getting that to line up nicely for a gearbox is easier said than done.

As others have said, the normal gears and their shafts can be held in place with snap rings or spacers. It is also useful to turn down the end of the 1/2" hex shaft to 1/2" round and use a round bearing to keep the shaft from sliding out. The bevel gears will have thrust loads, which are along the axis of the shaft so it's important to be aware of this in your design. Team 1640 uses a thrust bearing probably similar to this one http://www.mcmaster.com/#6655k13/=smu6aw

I've also seen teams get away with using a thrust washer similar to this one
http://www.mcmaster.com/#5906k411/=smu6ry

Where do you plan on purchasing the bevel gears from? Also, where does the wheel go?

[jimbo493]

We will have 2 wheels on either side of the bottom gearbox with the bevel gears.

The module will be cut and bent at a laser cut place near our build space.

The bearings for rotation is a big thrust bearing on the part in the upper gearbox, and a beveled bearing(I think thats what its called, mcmastercarr isn't loading for whatever reason ATM). EDIT: its a roller bearing

Im not sure where we got the bebe gears from, we don't meet until the 7th, but ill ask then

[Greg Woelki]

Originally Posted by jimbo493

We will have 2 wheels on either side of the bottom gearbox with the bevel gears.

Why do you want wheels on each side? What reduction are you using for the BAG motor to pivot them?

Also what are the flanged plates made of? 0.125" aluminum?

[jimbo493]

2 Wheels allow for more traction and less stress on the bevel gears, plus its just helps with overall stability, we are using the new 4in vex wheels

We will probably use .125 in

[Greg Woelki]

Originally Posted by jimbo493

2 Wheels allow for more traction and less stress on the bevel gears, plus its just helps with overall stability, we are using the new 4in vex wheels

We will probably use .125 in

This one?

Well keep in mind that, while more wheels will keep the tread pattern from wearing down as quickly, doubling the number of wheels will give only a very minor improvement in traction since, while it is simplified, friction very closely equals N*mu. Having a wheel on each side will significantly increase the torque required for pivoting and it will probably take a minimum of a quarter of a second or so to rotate a module 180 degrees.

[Bryce Paputa]

Originally Posted by Greg Woelki

This one?

Well keep in mind that, while more wheels will keep the tread pattern fromthe wearing down as quickly, doubling the number of wheels will give only a very minor improvement in traction since, while it is simplified, friction very closely equals N*mu. Having a wheel on each side will significantly increase the torque required for pivoting and it will probably take a minimum of a quarter of a second or so to rotate a module 180 degrees.

On a deformable surface like carpet, the n mu approximation isn't a valid assumption, he'd pretty much need to test it himself with his wheel setup in order to figure out how friction would be effected. With a rough top tread I would think you would get more friction, but probably not with a smooth vex wheel, but this is just a guess. The increase in turning torque is correct though and if more surface area was beneficial I'd just have a single thicker wheel to take advantage of the low torque required to turn near the axis of rotation.

Also, getting the holes on the two 1/8th inch pieces of aluminium to line up is going to be very challenging, I would recommend making it out of a single piece of extrusion or two flat plates with standoffs.

[Greg Woelki]

Originally Posted by Bryce Paputa

On a deformable surface like carpet, the n mu approximation isn't a valid assumption, he'd pretty much need to test it himself with his wheel setup in order to figure out how friction would be effected. With a rough top tread I would think you would get more friction, but probably not with a smooth vex wheel, but this is just a guess.

Do you know of any quantitative data that teams have collected about this with different wheels/treads? I have been unable to find any.

[Bryce Paputa]

Originally Posted by Greg Woelki

Do you know of any quantitative data that teams have collected about this with different wheels/treads? I have been unable to find any.

Nope. Vex has some CoF's listed, but they say nothing about the testing method or how it's related to surface area. Later this summer I plan on doing a test to determine this with different wheels and to figure out which gives the best traction, but this probably won't be for a while.

[jimbo493]

We will see how accurate our vendor is, we may have to do standoffs, we'll see

As for the wheels, you bring up a good point, the 2 wheels will add a lot of friction. I don't foresee that it will be too much of an issue.

I think the best thing we can do is build one module like it is, then we'll have to tweak it quite a bit. What i like about it is at the form factor, its very small, and its all gear driven with no belts or chains which means less chance of things breaking.

Thinking about it more, It would also be pretty difficult to make this design with a single wheel. I still think we will have enough torque to turn the wheel with adequate speed. There really is only one way to find out..to test it.

[Tyler2517]

Some things to think about. Weight of a swerve drive is often very heavy trying to reduce the weight is a major chore.

Just to keep in mind. The closer your gears are to the final reduction of the drive train the more or a moment they will be taking. This can be come a huge problem when you take large amounts of pushing or something unexpected. The moment is directly related to your wheel size.

Also remember your whole robots weight will be carried through the upper gear box this can cause major problems for thin metal flexing and losing gear spacing causing them to skip/bind I like belts for this reason. The competition is not the most ex-stream things that will happen some one might drop it and one of the poor modules will take the load of a potently 150 pound robot falling from 2 feet. I like belts for this reason.

My team has been designing a swerve drive using the same duel wheel concept.
http://imgur.com/a/H6Qcm
The main goal over the old was was to reduce the size/ machining complexity/assembly complexity/points of failure/ and the biggest was increasing mechanical efficiency.

[jimbo493]

The original idea was to put holes in the 1in flanges pointing up/ down and bolting that into the frame with a sorta of square of 1x1, (ill post a pic later)

I do see how it could twist and bend, I have some ideas...

Im thinking we may stick with 2 wheels for now, I don't think its worth a total redesign of the bottom GB ATM, What id like to do is just get a prototype made to see how it it all goes together and see what has to change.

I might play around with a 1 wheel version if I get the time to. This is actually my first major CAD project, so I am pretty proud of it, but I see a lot of valid points that ill try to implement. I really appreciate the feedback.

PS:

This is the large tapered bearing I was talking about http://www.mcmaster.com/#5709k31/=smy6cy

[Tyler2517]

Try 3D printing it out to insure that everything works as expected. It is always easier to re design then spend a couple hundred hours of frustration trying to get a bad concept to work.

[AdamHeard]

Originally Posted by Greg Woelki

This one?

Well keep in mind that, while more wheels will keep the tread pattern from wearing down as quickly, doubling the number of wheels will give only a very minor improvement in traction since, while it is simplified, friction very closely equals N*mu. Having a wheel on each side will significantly increase the torque required for pivoting and it will probably take a minimum of a quarter of a second or so to rotate a module 180 degrees.

Dangerous statement here. I agree with everything you said EXCEPT the claim of .25 seconds to go 180*. That speed would actually be pretty quick for most swerves people are currently running.

[Chris is me]

Originally Posted by jimbo493

2 Wheels allow for more traction and less stress on the bevel gears, plus its just helps with overall stability, we are using the new 4in vex wheels

We will probably use .125 in

2 wheels doesn't inherently equal more traction. It would result in pretty much the same stress on the bevel gear as well - how do you figure that it would result in less?

Originally Posted by Bryce Paputa

With a rough top tread I would think you would get more friction, but probably not with a smooth vex wheel, but this is just a guess.

Please don't guess.

The edge case I know about that seems to affect pushing force based on wheel width is roughtop tread on 4" wheels. I remember reading data somewhere (but not experimenting myself, something to do in the fall) which showed a negligible difference in traction for 6" and 8" wheels of different widths as well as wedgetop tread. Specifically 4" roughtop tread wheels have noticeably better traction in a 2" wide configuration as opposed to 1" wide.

I have never seen any data on Colson wheels or Vex wheels with regard to width versus traction. Based on purely subjective experience, I think people are somewhat overstating the benefits of a wider wheel in terms of carpet traction. We would have to do testing to be sure though, perhaps in the fall.

The increase in turning torque is correct though and if more surface area was beneficial I'd just have a single thicker wheel to take advantage of the low torque required to turn near the axis of rotation.

Yes, definitely a factor here. Your turning torque is a function of the size of your moment arm when spinning the wheel, so the farther from the center the tread is in contact with the ground the greater the torque needed to turn the module.

Also, getting the holes on the two 1/8th inch pieces of aluminium to line up is going to be very challenging, I would recommend making it out of a single piece of extrusion or two flat plates with standoffs.

Alternately they could leave the bends in, with a bit more clearance away from each other, and use standoffs to actually hold the plates together to ensure alignment. This keeps the rigidity benefits of the bends.

OP: Have you thought about how to mount this to a frame? A popular idea on the west coast is to make a 2x1 frame and mount the gearbox to each side of the 2x1, using the existing frame as a big spacer. Also, what are your gear ratios?

Thinking about it more, It would also be pretty difficult to make this design with a single wheel. I still think we will have enough torque to turn the wheel with adequate speed. There really is only one way to find out..to test it.

Not at all, you can easily do some math to get an approximation of how much torque it takes to turn a wheel and whether or not your system has enough reduction to do so.