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pic: WCD 3 CIM PTO Ball Shifter Render
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Re: pic: WCD 3 CIM PTO Ball Shifter Render
Very nice. PTO/shifter for 2.2lbs is really good.
Is there any way to change the gearing to get a free speed under 18fps? Going above 20fps free speeds seems sketchy with 4 cims. |
Re: pic: WCD 3 CIM PTO Ball Shifter Render
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Thank you. :P In any case, you can't accelerate with 6 cims to above 20fps with the roboRIO anyway without a brownout, especially a low voltage, so is it possible to reduce the gearing for that? |
Re: pic: WCD 3 CIM PTO Ball Shifter Render
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Given the voltage brownout levels, I now expect that PTOs are much less useful than they may have been in the past. The first roboRIO year with bumpers, defense, or crazy acceleration requirements will likely be the proof/disproof on this point. The bottim line is that the additional weight for shifting may completely override the weight for separate motors, especially for lower-resource teams. Added: Quote:
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Re: pic: WCD 3 CIM PTO Ball Shifter Render
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The weight cost is on the order of a pound or two for a PTO if you go with the WCP PTO (plus whatever you're using for integration into the system). The alternative for endgame mechanisms is to use discrete motors or pneumatics, both of which usually cost more weight for similar outputs although they are easier and faster to integrate. Of course, the last endgame was in 2013, so maybe they're gone for good (and with them most of the use of PTOs). |
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Hopefully with this gearbox, we will be know what is too fast, so that when next season comes we will be able to pick appropriate speeds. |
Re: pic: WCD 3 CIM PTO Ball Shifter Render
Instead of anecdotal evidence, how about some objective data?
http://www.chiefdelphi.com/media/papers/3067 |
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The data presented in cyber blue's testing papers is concerning in some ways, but not in others. For instance, it is concerning that their test platform that is geared for 11 ft/s dropped it's voltage so close to the brownout point, but their testing also shows me that this gearbox will probable not have trouble with tripping the breaker, or causing a brownout while in low gear. From the thread you just linked: Quote:
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Re: pic: WCD 3 CIM PTO Ball Shifter Render
So back to the actual gearbox...
I think your CIMs aren't supported as well as they could be. It's pretty common in these kind of gearboxes to have a web that follows the diameter of the CIM in order to support the whole face. Right now the little sliver of metal supporting the middle of the CIM may not be substantial / rigid enough. The tiny amount of weight savings isn't really worth risking flex in that area of the gearbox. If that is a stock ballshifter shaft, I would be concerned about direct driving a wheel off of that shaft. It might not be a good idea to load the shaft that way. The hex end of it is just pressed in to the end of the shifter shaft, with about 3/4" of engagement if memory serves. I think this is one reason the COTS direct drive ballshifters have a third gear stage. This does indeed make fitting a ballshifter into a WCD gearbox in 2 stages quite difficult. I'm working on a similar gearbox for a similar application, and the constraint of using the ball shifter shaft without direct driving off of it is quite annoying. I don't know how feasible this is at all, but have you considered using it backwards? Having the CIMs drive a gear on the output of the shifter shaft, then putting two output gears on the wheel shaft? Might be worth a shot, particularly if you're into the latest WCD fad of hanging your drive motors over the wheels. Probably not a good idea, but something unique to look at. |
Re: pic: WCD 3 CIM PTO Ball Shifter Render
The newest rev of the ball shifter shaft has the hex pinned instead of pressed.
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Re: pic: WCD 3 CIM PTO Ball Shifter Render
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-Aren |
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1. The CIM is resting against 0.25" Al along its entire bottom edge. 2. nestled in between the CIM motors are standoffs that run back to support the pneumatic cylinder that runs the PTO. These standoffs will also help support the CIM motor from below. Quote:
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There's certainly nothing wrong with the Vex shaft design; it does what Vex is trying to accomplish and it works for their COTS product, but it definitely isn't tailored for WCD use. Quote:
I sketched this out last night and the biggest difficulty with it is clearance between the motors and the middle gears. You end up with a really big gearbox if you go 3 CIM unless you use idler gears (not necessarily out of the question). |
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