Swerve drive questions
 

Hello!

Over the summer, and pretty much since the offseason began, I've been looking at various swerve drives created by different teams, trying to figure out how it all works. Unfortunately, I'm a bit stumped.

I've been looking at 1640's swerve, as the documentation on their wiki is wonderful. The first question I have is about the thrust bearings: I get the general idea that they're used to support a load in the direction of the shaft, but I don't understand what they do in their swerve drive. Do they help to turn the axles when there's an axial load being beared, or is it something else completely? Also, when looking at the documentation for assembly (http://wiki.team1640.com/images/4/49/Pivot_Asm_6c.pdf) I noticed on page 8 that on the transfer shaft, it doesn't seem that anything is actually attached by a key or anything. Is that by design, (and does the thrust bearing come into place then?) and if so, then does the transfer shaft just turn because of the friction between the miter gear and the shaft, or am I missing something?

Those are the questions I have right now, and the reason why I'm wondering so much about the thrust bearing is that I haven't really seen that on any of the other swerve drives I've looked at.

Just some questions from a programmer-turned-engineer. I'd like to do swerve drive, if only to have something more challenging, yet doable, for my programmers to work on this year. Any help is appreciated, and thanks in advance!

Re: Swerve drive questions
 

Torque is transferred from the shaft to the bevel gear through a key, which is not related to the thrust bearing.

When bevel gears turn, they try to slide down the shafts to get away from each other. It is important to design around this load to reduce friction and to keep the gears from pushing on things that are delicate.

Thrust bearings are used to reduce friction between two faces that can be rotating at different speeds that are being pushed together. If there wasn't a thrust bearing, and you pushed these faces together (like from the thrust loads from the bevel gears), friction would slow them down, like a brake on a bike. Unlike normal ball bearings, which deal with loads on the curved inner and outer races, thrust bearings deal with loads in the direction of the shaft on the flat surfaces. The rolling balls in the middle let the two washers rotate at different speeds, even when they are being pushed together with a lot of force. They're similar to bearings on lazy susans and microwave oven turntables; they still spin freely when you push down very hard on them.

http://en.wikipedia.org/wiki/Thrust_..._din711_ex.png

You can see another example of thrust bearings in 1640's module rotation. They use a thrust bearing between the top rotating surface of the swerve module and the fixed frame of the robot. There is the weight of the robot pushing down on the top of the swerve module, and the thrust bearing here allows the module to turn very smoothly and with low friction even though there is a ton of force pushing the two together.


The purpose of the thrust bearings in the picture you see are to deal with the thrust load of the bevel gear. If there were no thrust bearings, the bevel gear would sit against the inner race of the normal 3/8" bearing in the module. If moderate thrust loads are applied to the inner race of the bearing, the bearing can get destroyed, so they want to transfer this load to something else, like the outer race of the bearing, which, through its flange, transfers it to the side of the module. In order to do this, they have to deal with the gear, which is spinning, and the outer race of the bearing, which isn't, so they use a thrust bearing.

Teams have used swerve without thrust bearings here, but I have no idea if it worked well for them or not. The thrust bearings are cheap ($2.47), so it makes sense to put them in.

Ok. The reason I had mentioned the thing about the bevel gear on the transfer shaft is that in the documentation, there was no mention of needing a key (page 8), yet when assembling the drive shaft, the document mentioned a key was needed, yet not shown (page 4). I wasn't sure if there was a mistake in the documentation, or if it was deliberate.

Anyway, you've certainly answered my question regarding the thrust bearings, so thanks a lot!

Re: Swerve drive questions
 

That's an old version of our swerve module. I think 2011. Many things have changed. We now use timing belts on the first stage of reduction and # 25 chain and sprockets on the 2nd wheel reduction. The wheel cage is narrower and the transfer axle is shorter. The transfer axle Now is turned from .5 7075 AL The center Is left at .5 and constrains the sprocket and bevel gear. The thrust bearing washer for the bevel gear now sits on the outer flange of the flanged ball bearing and the sprocket is held in position with a nylon washer between the sprocket hub and the flange bearing. The transfer shaft can not move and we don't need set screws. both the bevel gear and sprocket have key ways broached in them and we cut a key slots in the transfer axle. The vertical drive shaft is keyed and we broach a key in the bevel gear. The bevel gear and axle are assembled with I believe 609 retaining compound. These changes came from experience with the past modules. Set SCREWS are best avoided. The 7075 transfer shaft has held up well. Normal ball bearings do not like axial loads and we use the ball thrust bearings to keep the axial load from the bevel gears off the ball bearings.

I've gathered that set screws aren't good after looking at the resources that your team provided.

Another question: if the transfer shaft was a hexagonal shaft, would it be possible to use a thrust bearing? I would guess that you would have to make a sort of adapter, to go from hex to circular, but I'm not sure if it'd work.

Re: Swerve drive questions
 

My team hasn't had any problems using standard thrust bearings that fit around the hex profile.

Re: Swerve drive questions
 

The vexpro miter gears are larger than what we use. We use 20 degree, 16 pitch,16 teeth Martin gears. We also use 3/8" needle bearings for the vertical drive shaft therefore must use a round shaft. Using hex miter gears would complicate several things in our design.

Re: Swerve drive questions
 

That's good to know. Thanks

Re: Swerve drive questions
 

The Vexpro miter gears have a nifty detail if you look closely....a small boss on the back side that is designed to work directly with a bronze thrust bushing.

They are a very nice choice for co-axial swerve design.

Re: Swerve drive questions
 

The max velocity of the vertical shaft can be > 1200 RPM and while the load is with in tolerance of bronze thrust washers, the ball thrust washers are lower friction. We have yet to see any signs of wear with the ball thrust bearings. Our design pushed us to needle bearings for the vertical drive shaft. They also have provided a low cost durable solution. My only complaint with the needle bearings Is the shaft. Because the needles run on the shaft, the shaft needs to have good surface hardness. I would prefer a case hardened precision shaft. How do you cut a key way in a case hardened shaft? You don't. You can grind one. To avoid the time and hassle we use a 4140/4142 steel shaft. Rockwell B90. After a season there is some wear but very tolerable. We have used Igus bushings for many apps and they have worked very well If you follow Igus design guide lines. The hex shaft on the vex miter gears do not fit our current design and I would not change for them.

Re: Swerve drive questions
 

For the shafts you might try ceramic coated aluminium shafts from McMaster they have a surface hardness much higher than the steel ones you are currently using and being aluminium are lighter and even easier to machine.

As for the thrust bearings since the inner race of the ball bearing, the miter gear and the shaft should all be spinning at the same speed I don't see where friction comes into play. I was wondering if anyone knows a way to calculate the force along the axle that would be applied to the inner race of the ball bearing by the back of the miter gear if the thrust bearing were not there, and if this would be enough to damage the ball bearing. I understand that with the thrust bearings being as inexpensive as they are it can't hurt to have it, but I was curious if calculations could be done to prove its worth.

Re: Swerve drive questions
 

My team used a modified 1640 design this year.

They use a pair of needle bearings when the bottom needle bearing sits in the co-axle. The thrust bearing is there to stop the top of the miter gears from contacting bottom of the co-axil during operation and to insure that the miter has a clean contact with the outer miter gear. -top miter

The bottom miter gear and its thrust bearing is there to deal with the axial loads that are produced by the miter gears. Thus increasing mechanical efficacy and allowing the module to run for long periods of time with out damage. (long ie a season)

The idea of the ceramic shafts do not sounds like the best idea hey seem like they might shatter under heavy impact. my two cents.

Re: Swerve drive questions
 

Those are 6061 shafts. They might not hold up to high loads. Plus, you would have to key them yourself.

Re: Swerve drive questions
 

I understand the need for the top thrust bearing as the needle bearing has no inner race to ride on, but for the lower miter gears to be useful it would have to be redistributing a force which would otherwise break the inner race out of the flanged ball bearing in the side plate. I doubt the axial loads produced by the miter gears are great enough to do that, but I really have no feel for either what will break the ball bearing or how much force the miter gear would be applying to it, hence calculations.

As for the shaft, the coating looks similar to anodizing and has no chance of shattering, it will bend with the shaft, can be filed off etc. These (in 1/2 in dia) are the go to shafts on MOE and are used in everything from drive shafts to intake rollers I don't think they would have a strength problem and they already need to key the shafts they buy now.

Re: Swerve drive questions
 

I stand corrected, McMaster recommends not using bearings where the balls/rollers contact the surface of the ceramic coated shafts.