pic: Carnage from Girls Generation

We competed at Girls Generation in Portland today. It was…Interesting.

During our practice match, we got hit, and one of our axles sheared at the e-clip grove. We ran 2 matches only do auton and climb, before we got our backup axles to fix it.

Then in our last qualifier, one of the AM kit wheels broke. We were 1 seed, and fixed that during lunch.

Then we broke another AM kit wheel during our 2nd match of eliminations. We didn’t have time to fix it this time, so we ran the next 3 matches with a wheel that was missing about 2/5 of it. We ran the robot at full capabilities, and we eventually won, thanks to teams 2471 and 2811.

Afterwards, we realized one of the plates that held our shooter cover on had snapped as well.

So all in all, we broke an axle, 2 wheels, and our shooter in the span of 6 hours, and 13 matches.

Districts are going to be interesting next year if this happens again.

Are these the ones that they updated a few weeks ago?

nope they are the 2013 KOP wheels. the new ones have a much meatier hub

I don’t think that Andymark has begun to sell the version 2 wheels yet

I’m sure the OP has already figured this out, but for those newer members of the forums, there’s a reason why the shaft snapped at a snap ring groove. Snap ring grooves (with their sharp corners) make what’s called stress risers in the shaft. The shaft’s much weaker where it has a smaller diameter (at the grove itself), and the sharp corners in the groove are great places for cracks to start.

For this reason, a lot of teams avoid snap ring grooves altogether, or at least stay away from them between places on a shaft where torque is being transmitted. I’m not sure exactly what the shaft that snapped was being asked to do, but I suspect that using spacers instead of snap rings to locate components on the shaft (with snap rings near the shaft’s ends or screws tapped into the shaft’s ends to retain the shaft in place) would have made for a bit stronger solution.

They’re selling them, but they apparently won’t ship until November.


So this was our center drive wheel axle. The snap was at the bearing in our wcd, and snapped when another robot lifted us and we dropped. Next year we are replacing the clips with spacers to fix the shaft issue. As for the wheels we are moving to versa wheels, so that shouldn’t be an issue.

This explanation is a good start, but not quite the whole picture. The implication you’ve made is that a snap ring anywhere on a shaft will weaken it, and that teams have good reason to avoid all snap ring use because of this. I don’t want people to get the wrong idea here - in general, snap rings on the ends of a shaft will work just fine.

The shaft sheared cleanly right at the groove. This is almost certainly because torque was being transmitted through shaft components on either side of the shaft. (I feel like loading from other sources would result in a different looking failure?) The torsional load goes right through the section of the shaft with the snap ring groove, and the sharp corners the groove makes are stress risers - cracks are prone to form there under load leading to failure.

If torque is not being transmitted through a snap ring groove, this problem doesn’t happen. No one sees numerous west coast drive axles failing because of snap rings on the ends. The takeaway lesson here is to not place stress risers on shafts between loads - not to avoid snap rings altogether.

(Aside: While the smaller diameter of the grooved area does make it weaker than other areas of the same shaft, the stress risers were probably the main reason this failed the way it did. The differences in maximum torque between a 1/2" round section and a 1/2" hex section are not that great…)

Full disclosure: Running on six hours of sleep and studying for a test, maybe none of this is correct / makes sense…

Actually it broke not in rotation, but when the dropped and hit the ground. But torque was being put through the groove, and we will be avoiding that next year. Also that shaft is steel, not aluminium, and the groove was made too deep, which didnt help.

Watched this happen… wow. That was quite impressive. Way to hang tough and go on to win!

I didn’t see when the shaft broke, but I saw both halves afterwards, and in this case torque was not the factor, it was bending stress from an impact on the cantilevered wheel - on the fracture face you could clearly see where the break originated.

To better clarify, I believe this snap ring groove was just outside the frame, between the bearing and the wheel on a WCD setup. It is a 1/2" hex shaft, and all the grooves had E-Rings. At a glance, the ring grooves seemed quite deep - I didn’t measure, but the remaining diameter was about 3/8". After looking up E-ring groove tables, they weren’t far off on proper depth, as the groove diameter on a 1/2" shaft e-ring groove should be .396"

As Chris mentioned you have to consider what the stresses are in a shaft where you intend to put a retaining ring - in this case, it was in the middle of the shaft where both significant torsional loads (from driving the wheel) and shear loads (from the cantilevered WCD wheel) existed. As DampRobot mentioned, sharp corners are the worst for stress raisers. The larger the radius at the bottom of a step or groove the better, but we tend not to do this when machining grooves. Also, the smaller the groove depth, the better. I looked up stress concentration factors for these e-clip grooves, and its somewhere around Kt=8+ for bending, and Kts=4.5+ for torsional stresses, which are pretty severe and make it clear why the shaft broke here. (This means the maximum stress from bending forces due to the stress raiser will be ~8x what the average stress is in the normal section of shaft.)

An alternative to an e-clip is a standard retaining ring - they can be more difficult to work with without the proper pliers, but they require a slightly narrower and much shallower groove, leading to much lower stress raisers, in this case the stress concentration factors would be around Kt=3 for bending and Kts<2.5 for torsion (you may also note the thrust ratings for the retaining rings are much higher than comparable e-clips, despite the shallower grooves). Its also not quite standard to work with hex shaft (good luck finding a table for it), but I bet you could even cut shallow of their recommended groove depth for a 1/2" shaft, as the corners of the hex add effective depth for engaging the retaining ring.

Also a very important point that Chris tried to clarify: any type of retaining ring on the end of a shaft is basically a non-issue, as no torsional or bending stresses pass through that section of shaft, and the retaining ring only sees thrust loading along the axis, which is exactly what it is designed for. So in this case, they could have used standard retaining rings at the mid-points on the shaft for significantly lower stress concentrations, and still used deep e-clip grooves at the ends to make changing wheels easier. Alternatively, they could go to shaft collars or properly sized spacers between the wheel and bearing to avoid stress risers entirely (just don’t expect tremendous thrust load resistance from a shaft collar alone, and realize thrust loads with a spacer will be carried through to the retaining rings on the ends of the shaftl).

If you want to check stress concentration factors yourself, check out Figure A-15-16 and A-15-17 on pages 1031-1032.

BTW - Great job dealing with unexpected damage and playing through without getting discouraged. Its always good to hope for the best, but design and plan for the worst, and never give up - its always better to put as much effort into a quick fix with the time and resources available than to give up when something unfortunate happens.

After looking at that one groove, it was actually machined too deeply. Normally we turn the shaft down to .44 for the groove, but that was was closer to 3/8th. Also you are right that was the eclip that held the bearing in on the wheel side of the WCD. The prototype we are building right now uses spacers with eclips on the end, which should make this less of a problem.

And I am so amazed at everyone on our team. With the broken axle, we had it fixed in about 10 minutes, once we got the spare. We ran the first 2 matches only auton and climbing, because we had to run to the school to get the spare. As for the broken wheel during the last 3 matches, Our driver made it look like nothing was wrong, which was great for a sophomore that had just joined the team.

As for next year, we are going to make sure replacing most parts can happen in 10 minutes, because it seems like there could be times were we only have that long between matches.

When writing my post I didn’t realize that the grooves were E-clip grooves, which makes the smaller diameter of the shaft a greater issue, nor did I realize the failure occurred as a result of bending forces. My bad.

One thing to consider - do you need an e-clip to retain the bearing? On our set up, we just have the wheel itself physically constrain the bearing - there’s no space between the edge of our wheel hub and the inner bearing race. Sure, a bearing could possibly wiggle loose during a wheel change, but that’s not a hard fix anyway.

The design was using eclips this year. Next year it will use spacers. No torque will be going though grooves.