Robo-Geriatrics

[Wayne TenBrink]

Here in Michigan we get a lot of playing time. Our robot has been through 72 official matches so far (3 districts and MSC), a bunch of practice matches, and hours of operation during build. Now we are on our way to St. Louis.

The robot has a few battle scars, and has experienced its share of broken parts due to random impacts and normal excesses. At MSC, however, we experienced our first age/fatigue related failures of the year. In the span of a few matches where nothing drastic happened, we lost a KoP wheel and the output shaft on a PG-71 gearmotor. Also, one of our alliance partners lost the output shaft on one of their Super-Shifter transmissions. The wheel was located at a corner, where side loads are highest, and the spokes fractured. The gearmotor shaft broke at machined step (a stress riser) where it mates to the planet gear carrier plate. We used it to drive a wedge-type bridge tipper arm assembly, and it sees some shock load every time we hit the bridge.

This stuff seems to start happening this time of the season - Sometimes on purchased COTS items and sometimes on parts we make ourselves ("Breakaway" was good for that). I'm not complaining about any particular parts suppliers - fatigue happens as stress cycles add up, and life is exponentially/inversely related to stress. Teams tend to expose parts to more stress than they were designed for. We try to keep an eye on things, but our inspections and preventative maintenance aren't exactly up to aircraft standards.

What sort of fatigue failures have you seen? What parts? How are you stressing the parts, and how many cycles did the parts see? Perhaps we can learn from each others' experience.