pic: New serve mk.1-2517

129dba3b5eb19c4d7c92413f7b4b01cc_l.jpg

http://imgur.com/a/H6Qcm-did not up load very well. Our new swerve module super small super light and less moving parts then a standard swerve module. Offers super low ground clearance and a low center of gravity.(did not upload well.)

That’s a really neat swerve drive. I don’t think anyone’s done something like this before.

A few questions/comments-

I’m assuming the really big green thing that goes in the middle of the wheels is a bearings. If not, what is it? If so, why is it so big?

Those bevel gears- where are they from? Are they strong enough? They’re at the end of the reduction so they’re going to try really hard to get away from each other. The little one is going to push up so you’ll probably want a thrust bearing/washer there too.

The green angle to support the sides has the mounting holes too close to the edge. Making it a little bit wider around the bolt holes is cheap strength.

How is torque transferred between the bevel gear and the wheel?

What type of wheels are those?

Overall, a pretty cool iteration on swerve.

It’s pretty neat to see these offseason swerve designs come back. There’s usually some pretty cool ones like this!

The green thing is a spacer that lets us translate the power from the wheel with the bevel to the wheel with out. It has 12 threaded holes in it so that we can offset the bolts so the load from each one of the wheels so there is a lot less shear force(each wheel has 6 holes.)

6 holes on the gear.

The bevels are from sdtp. We are not using the stock materials and will custom make them our selves if the ones are not strongthat we can buy.
We are looking at making the bigger of the gears out of a titanium alloy most likely Ti-6Al-4V Trading a week of my summer to clean there shop in trade for the machine time and resources to make 20ish on there 5 axis cnc. We do not want 2 of the same material gears so we don’t have to grease them so carpet cant get stuck in the grease.

Custom wheel 2.78 in diameter if i remember.

That is a very nice deal.

I’m having trouble picturing this so I can’t be sure, but doesn’t this just mean each bolt is loaded in single shear instead of double shear? I think 12 bolts going through both wheels would be stronger since the bolts are in double shear. Both are probably overkill.

We do not want 2 of the same material gears so we don’t have to grease them so carpet cant get stuck in the grease.

Why does having two different materials negate the need for grease? Why does having two of the same material necessitate it?

I suck at explaining.

The spacer in the middle will have 12 holes threaded in to it. each wheel will have 6 bolt holes they will be threaded in to the middle spacer every other hole so that we don’t have large amount of shear force going through the green spacer.
The wheel is really small and trying to get room for 12 bolts is both supper heavy and over kill for a 2.78 inch wheel. We were trying to have no nuts/bolt heads on the out side of the wheel so that we can get the walls of the moduler as close as possible.

Having the 2 different materials will cause much less wear. Just like running 2 steel gears on each other will cause rapid wear. Having the titanium gears running on a brass alloy gears will let us put very little grease on it with very little wear. That way the carpet fibbers will not get cough in the gear grease causing us not to bind our gears like last season.

Are you sure you want to be loading screws in shear at all? Why not replace half of them with spring pins?

actually the wheels are custum 2.75 inch, and the bevel is from SDP/SI part:
A 1B 3-Y32016B
A 1B 3-Y32064
the grean spacer is threadded on the outer holes, six bolts from the right wheel, six though the left
(im the CAD lead, i would know) :slight_smile:

also the brass on titanium is to make the brass take all the wear, so we dont need to replace the titanium

All the wear, as in “you better replace 'em often”. Brass isn’t known for wearing well–in fact, it’s better known for deforming when harder materials apply force to it (to the point where when a hard material is getting beat up, and brass is suggested as a replacement, engineers start asking “Are you sure about that?” and “Is there another material that will work?”). Try bronze instead–it’s a little harder, and to some extent can be made somewhat self lubricating (bronze bushings, NOT brass bushings). Titanium is already really hard, harder than most FRC gears are, which will tend to increase the wear on the brass gears even more than, say, aluminum/aluminum interactions or steel/steel interactions.

Regarding the screws: You’re loading the screws in shear. (Quick check: Do you know what shear stress is, in broad terms? If not, we’ll be happy to explain.) What that means is that the screws are going to want to “cut” themselves apart, probably at the spacer. The tendency is actually less if there are multiple locations for the shear stress to act at, hence the suggestions to run the bolts through, giving double shear instead of single shear. If you’re concerned about clearance, there are ways to deal with that, including sinking the holes for the screw heads and nuts so the heads drop right out of the plane of the side plates.

Why would you want titanium? It’s a worse material for gears than steel. The weight savings would be well under a tenth of a pound. The “good stuff”, grade 5 titanium, isn’t cheap either.

If wear is so bad that it is an issue for an FRC robot, then you’ll experience wear on both gears, regardless of material.

You will need grease regardless of the material you make your gears out of. If you run without enough grease, you’ll wreck the gears in no time.

TBH, I haven’t done the math, but a cantilevered 32 DP brass bevel gear at the output of the transmission, where the torque is the highest doesn’t seem like a good idea.

If you’re doing this on CNC anyway, I would get rid of the green thing and try something like the vex pro versakey pattern, or something that’s similar to the dog clutches used in FRC gearboxes.

http://imgur.com/6orZXtp - The green spacer 12 holes.

http://imgur.com/TskU7rT - wheel with 6 holes.

We will probably change it over to 6 single bolts going all the way through there was a really good reason we did not do this in the first place and now i cant remember why.

For gears steel on steel i think is what we will run for the initial prototype types.

I thought screws were meant to be in shear load? 10-32 screws have ratings in the hundreds of pounds range IIRC.

It depends on the screw and arrangement. Generally, an individual screw is meant to handle tension loads. However, in the right arrangement, they can handle pretty high-shear loads.

I broke out my machine-design textbook–it’s really handy for a lot of things with robotics, including a general screw-bolt-nut overview. (Also one of the few chapters I didn’t get into in college.) The answer there is that generally, screws are for tensile loads, but in structure use, they’re often used to resist shear. BUT, those shear loads are taken when the screw is already in tension. The suggestion from the book is to use dowel pins in machine-type shear situations to assist the screws.

I’ve seen what sometimes happens when a lone screw takes a shear load. It’s not fun to hear if it’s a good-sized screw. Come to think of it, I don’t know if we ever did find that one screw/bolt head…

Load rating =/= intended usage. Screws will misalign little by little when used as the sole locating feature.

Would a machine design textbook be a good buy/read for a prospective ME student with little to do this summer? I’ve heard a lot about this mythical textbook from mentors and on CD, and it sounds like it contains a lot of “real” engineering information. Any suggestions on where to buy it, or what edition (assuming there are multiple editions)?

The “old one” I have is Kent’s Mechanical Engineers Handbook. The copy gifted to me by my first boss is the 12th Edition (1950). It is a good reference, but not an exiting read cover to cover. It has a section on gearing stress and fasteners and many other mechanical things.

A “more modern” design book would be Mechanical Engineering Design by Shigley and Mischke. Those two have a ton of editions of mechanical design. Thanks to indecisive profs I have both the 4th and 6th editions. They are now on the 9th edition according to Amazon. Again, not a cover to cover read, but this book has a section on gearing stress and fasteners.

I caution you that these are 3rd or 4th year collegiate level texts. There are a lot of support classes that will help these equations make a lot more sense (statics, strength of materials, etc). Reading them cold could be very difficult.

-matto-

A former mentor gave this version to our team a few years ago. I have no idea how it compares to others, but it’s been handy as a reference and learning resource for new designers. All of the math is understandable with the equivalent of Calc BC and Physics C Mechanics knowledge.

Aren’t all of the extremely common dead axle (sprocket/pulley bolted directly to the wheel) drives in FRC putting 6 bolts in shear on each wheel? (we found out what happens when you accidently forget 3 of the bolts…ignore the EI metal :P)](https://fbcdn-sphotos-e-a.akamaihd.net/hphotos-ak-xfp1/t1.0-9/1173628_10152234535693559_823704788_n.jpg)