I need some live axil design tips!

This is our thrid year in FRC. In the past we have been using a dead axil system. This year we want to try a West Coast system. I have two questions about the live axil system.

  1. How do you keep the axil from moving out of the bearings? I first though of snap rings, but I am worried about them failing. Also we are going to try a timing belt drive system and I am not sure the clips would be precise enough.

  2. How do you attach the wheel to the axil? Again snap rings come to mind, but I have the same reservations here.

What methods do you suggest?

Thanks.

We’ve never done a specific WCD drive train, yet for the last two years we have done a live-axle setup. I can’t answer your question about keeping the axles in since we always direct-drove the wheels from the transmissions (linkage drive 1 year, mecanum the other year). However, to attach the wheels to the shafts we used AndyMark hubs.

Another option is to use hex shafts for the axles and then use wheels* that have a hex bore in them.

*I do not recommend using these wheels for an FRC competition. Perhaps they’re a good cheap alternative for the offseaon so you can get the overall drive train design correct, but niether the tread nor structure of the wheel will last more than a few matches during FRC competition.

  1. Typically, either shaft collars or snap rings are used. Shaft collars simply clamp onto a shaft to prevent motion, while snap rings are seated in a groove, which must be machined into the shaft. Snap rings are very precise, however, and much smaller compared to shaft collars.

  2. For securing the wheel’s linear position, the same methods are used. However, careful use of spacers can reduce the number of collars and snap rings needed.

For securing the wheel’s rotational position, a keyed or hex shaft is used, and the wheel is either mounted using a special hub that matches this shape, available from andymark, or broached, to have a hex hole or keyway cut in its central bore.

Actually we plan to use hex shaft, I failed to mention that in my post. The snap ring preforms without issue?

I’ve seen firsthand and heard of plenty of teams that have used snap rings in all kinds of live axles without issue. The main thing I’ve garnered from them is that you have to plan out your drive system carefully and with good tolerances to make sure everything’s tight.

Snap rings are quite useful in the right applications. As was mentioned before me though, you need to be careful with tolerances when using snap rings. I favor using spacers to keep the wheel seated linearly on the shaft. This way you can adjust to the exact spacing you need on the fly. It may be slightly heavier, but I enjoy the adjust-ability. I’ve made successful drive systems using both methods.

-Brando

We just use spacers over the hex shaft to keep the wheels in place and lock the ends of the shafts by tapping them and using hardened washers held in place by flanged head 10-32 or 1/4-20 screws. I always though simple was best.

a properly seated 7/16" snap ring can take a 3,000 lbf thrust load, if I remember correctly. The key here is properly seated. As long as you can cut the groove properly, ensure proper axial spacing so the snap ring groove isn’t interfered with, and don’t re-use snap rings, you shouldn’t experience any problems.

a little advice to you, DONT USE LIVE AXLE DIRECTLY OFF BANE BOTS P80’s the e rings inside the gearbox break all the time, then they have to be taken apart and fixed.

we typically use collars, but we taper the edge on a mill or with a grinder so that the edge of the collar does not rub against the bearing seal. key-stock or hex broached axles are necessary to drive the wheel. with live axles i also would recomed a larger diameter of axle, while 1/4 or 3/8 axle is fine for dead axle, 1/2 or 5/8 is better for live axle, good luck.

This sounds right for ring shear, but if you use a material with a lower yield strength than the retaining ring itself, you will likely see a lower thrust load capacity due to groove yield. With 7075 or something similar, you should still get ~2000 lbf of thrust load capacity. These theoretical calculations assume a perfectly round retaining ring with no seam, so real numbers will vary.

If this is your first experience with live axles and with a new drive arrangement, it might be a good idea to build it initially with spacers and collars. There is a very good chance you will make changes and adjustments as you go along, which is easily done with spacers, but not with ring grooves. Hex shafts are a bit expensive to remake too many times. You can always add grooves and rings and remove excess length when you have the arrangement perfected. Buy a stick of nylon tubing (Mcmaster) which fits your shaft, and you can quickly make custom spacers as needed.

These folks make nice shaft collars. I got some from MSC a long while back.

http://www.ruland.com/

If you need spacers, you might try making them from PEX tubing. It’s tough stuff and CHEAP (like 30 - 50 cents per foot) if you can find short, straight lengths. Try Lowes or Home Depot.

We direct drove 2 traction and 2 omni wheels off of P80s last year, but we had the shaft run through a bearing block first so the internal bearings of the gearbox wouldn’t have as much load.

I know for a fact that each of the crab drive modules on our robot this year had a snap ring (about 1/2" diameter I think, but don’t quote me) holding them to the shafts that essentially bore the weight of the entire robot, which i would think would fall around 25 lbs per snap ring or so when you consider that they don’t bear the weight of the wheels themselves and then the battery is added. So if you are concerned about their durability when bearing force, I would think it shouldn’t be a concern. The only times I know of this season when we broke or warped a snap ring was when we were taking them off or putting them back on. They were very reliable for our application, but I can’t say much about their application in live axle design.