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Unread 05-01-2016, 08:46
BoilerMentor BoilerMentor is offline
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FRC #1747
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Re: Sonic Shifter - recent feedback?

Quote:
Originally Posted by aldaeron View Post
I want to point out for folks who don't spend a lot of time looking at gearboxes that this is a very aggressive gearing with one of the highest CoF wheel tread materials. My quick calcs say this generates ~747 pounds of force at the wheel patch with 6 CIMs, well over the traction limit of 185 for a maximum weight robot in 2014. Meaning it is an extreme case and may not be the best one to guide the decision of an average team.
The decision was based not on pushing power, but a time to distance calculation that considered a two speed transmission. That pair of ratios minimized field crossing timed on a tool that was written up in Excel taking the two speed gearbox into account. People perceived 1747's robot as being tippy that year, specifically because of the brutal acceleration this setup generated. It was the one thing we didn't take into account and actually down-regulated our peak acceleration. No doubt the robot was quick across the field though. I personally think time to distance is an important consideration most teams neglect, but there are certainly some games where it's not a useful piece of data i.e. 2015.

Quote:
Originally Posted by aldaeron View Post
Overall this is great feedback. I have been wondering how the 3rd stage on the 3 CIM works at high loads. I always thought the sheet metal with standoffs 3rd stage looked a bit rickety. I am curious if you ever subbed out the 7075 aluminum gears for 4140 steel gears in the third stage of the 3-CIM shifter? Sounds like it was a misalignment issue more than a material strength issue. Did you ever try stacking on a second Vex third-stage plate to add stiffness to the third stage bearing? This would pickup more of the bearing race and possibly prevent angular deflection.
We ended up machining a replacement for the shaft with a 1/2" round to sit in a bearing in a plate opposite the face of the plastic housing. The 3rd stage mounting plate wasn't used. The hole pattern was transferred onto our drive train plate. We haven't had a problem since implementing the modified shafts, which lends a great deal of credence to the theory that it was deflection in the cantilevered shaft that caused the failure.

Quote:
Originally Posted by aldaeron View Post
I am assuming the output of the gearbox was direct driving your center wheel in a tank drive. How do you think a 2 stage 3-CIM shifter would work if offset from the wheel axles and with a #35 chain reduction between the gearbox and wheel axles in lieu of the Vex 3rd stage?
The axle was supported with a bearing on the opposite side of the drive pod from the gearbox, so as long as the mounting of the gearbox was adequate the bearing in the face of the gearbox and the outer bearing we added should have picked up the load. I did forget to mention that we machined custom shafts from the get-go for the output of the gearbox. We needed a longer output shaft to span our drive pod. These shafts went to 4140 steel, instead of aluminum. Also, out of center might be a concern with a custom machined hex shaft. I verified they were centered prior to installation.

Quote:
Originally Posted by aldaeron View Post
Was there a particular reason for adding this "stop on a dime" feature? Did it work as you had hoped? Would you recommend it? It does seem like a recipe for gear teeth shearing.
The automatic shifting code was very simple. Outside of handling for turning cases, there were two shift thresholds based on gearbox encoder feedback. I do not remember what drove the specific value used for down shift, but I think it was based on the highest speed that allowed enough separation between values to prevent oscillation between gears. The reason that the downshift occurred before full stop was in consideration of a number of logical conclusions about robot-robot interaction. The downshift feature did have one drawback. We had to have a specific case to handle direction changes, because the robot would end up on its back otherwise.

Just to clarify, my responses haven't been about "winning and argument" I'm just try to establish that we covered all of our bases as far as a well put-together investigation of the cause of the failure. It was actually a great exercise to be able to work through with my students. You can imagine in a competition setting there's a great deal of anguish caused by a failure of this magnitude in the student's eyes. It's a great feeling for them to be able to say, "This was not a flaw in our design, it was a failure of an input part. Here's why:"

-Charlie
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2008-2011 FIRST Team 1747 Harrison Boiler Robotics-College Mentor
2012 FIRST Team 4272 Maverick Boiler Robotics-Founding College Mentor
2013-Present FIRST Team 1747 Harrison Boiler Robotics-Engineering Mentor
2015-Present Ri3D Team Indiana - Mechanical and Fabrication