pic: 20's IRI Carnage

[Travis Covington]

I think Cory forgot that we actually did break 2-4 of these on our practice bot this year in a similar manner. They were also from WCP but modified to be 1" OD. We had run the same size dog since 2007 without failures, so I am pretty confident that the changes implemented in the WCP ones, along with the additional CIM motor (we had 3 per gearbox) caused the failures for us.

In the past, we had either used a #3-48 screw (which failed a few times) or a 3/32" roll pin (which never failed). I believe the larger diameter hole of the #4-40, as well as the inherent stress risers that exist with there being a thread, combined with the additional torque of our 3 motor gearbox caused our failures. As some have mentioned, the cyclical/shock loads that this part sees are very high, and the relatively thin cross section at that threaded side of the dog does not provide much, if any, factor of safety.

We lucked out because we never had a failure on the competition bot, but did have drilled (non-threaded) dogs as backup. Our kids got plenty of practice and were able to swap a broken dog in under an hour if they had to. I am glad we never had to do that between matches though!

I think for the future we will use non-threaded dogs, as well as possibly increasing the thickness of that section of the dog at the expense of an ever so slightly wider gearbox.

Adam may be able to provide more insight in to their circumstances, but I think 973 may have just lucked out on not breaking their parts considering they had very similar loading characteristics to us. It may have had something to do with the CG of their robot versus ours, and the way it decelerates, but might also be because their dogs are the standard 1.125" OD (I believe?). We broke almost all of our dogs when stopping or changing directions, or when going fast over bumps/metal plates under the carpeting in our lab. Given the failure, those loading conditions make sense.

Hope this helps!

Edit: On second thought, 973's larger plastic wheels might have dampened some of the shock-loading as well. Who knows.