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Originally Posted by Alpha Beta
Enjoyed reading through the blog immensely. I always love hearing about how teams make their early design decisions. I also found the driver selection process very interesting.
How did you eventually solve the pinion problem on the arm gear box?
PS. Thanks for inviting us along for the ride at Champs. We knew you had several very good options to choose from.
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Yes and no. The original arm gearbox design that was used in San Diego was mounted to just one side of the A-frame. There were some concerns about this at our low level design review, but we figured we'd fix it if it became a serious issue.
We broke out first pinion a few days before stop build. In the original arm gearbox design the pinions and motor mounting screws for that matter were not accessible. So in order to replace the pinion it required a lot of work. Our initial failure assessment was that the corner of the key way was lined up with the low point or "valley" of the gear profile, which focuses a lot of stress on an area that had very little material. Our fix was to make another pinion where the corner of the key ways lined up with the peak of the gear profile. This seemed to help. Before stop build we broke yet another pinion. We had no reason to believe the failure was any different from the first.
As we continued to practice leading up to the San Diego Regional we noticed that the side plates of the arm gearbox on the practice robot were starting to warp. This created other issues such as putting the chain out of tension, which caused the chain to skip, which made up lose our arm pre-sets. In addition we noticed that as the plate the motors mounted to warped it was changing the spacing between the pinion and the initial reduction gear. One pinion was being jammed into the gear while the other was being pulled away from it. One situation caused the gear to wear faster, the other situation put a lot of stress on the point of the gear tooth. We also noticed that we were breaking pinions more frequently the more the plates warped.
By the time we came to this conclusion we were about 3 or 4 days before the San Diego Regional. We decided to run the original gearbox for SD because the competition robot's gearbox didn't have a lot of time on it and the pinions were in pretty good shape before we put it in the bag. So really we had decided before the SD regional that the gearbox was going to be replaced.
The new gearbox design mounted to both sides of the A-Frame. We also added some counter bored standoffs to help hold the plates in alignment. In the event we still broke a pinion, we made the motor mounting screws easily accessible so we could pull the motor off, replace the pinion and remount the motor in a few minutes.
The gearbox redesign help a lot, we've never had to replace the pinions on the competition robot, however the practice robot still broke quite a few pinions. I think this is due to how much we were running the practice robot. The competition robot has under 5 hours of actual run time (from build season to post IRI). I figure the practice robot was doing about 10-15 hours of actual run time a week for about 9 weeks.