15-09-2015 21:04

asid61

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.

15-09-2015 21:14

orangemoore

Re: pic: WCD 3 CIM PTO Ball Shifter Render

I think it is a 6 cim drivetrain. I think it's hidden behind the rest of the gear box.

15-09-2015 21:20

asid61

Re: pic: WCD 3 CIM PTO Ball Shifter Render

I continue to amaze myself with my idiocy. Whoops.
Thank you.
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?

15-09-2015 22:07

GeeTwo

Re: pic: WCD 3 CIM PTO Ball Shifter Render

Yes, the gearbox shifts, ball shifters based on the subject line, though the mechanism isn't stated. Low gear is around 7:1 (see first post), so with three CIMs per side, you can easily get up to enough speed for the shift, which can then take you to 20 fps.

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:
Yes, and that's why. The reason PTOs have been so useful in the past is that the tightest resource limitation was motor count. This promoted using the same motors for multiple functions. Now, the biggest limitation is the current draw, as in order to keep the motors running, you can't let the voltage drop. Unless the PTO, including the switching hardware and shafts/chains to get the power to where you need it are significantly lighter than the motor and controller, there isn't as much benefit.

15-09-2015 22:15

R.C.

Re: pic: WCD 3 CIM PTO Ball Shifter Render

I'm unsure how PTO's are less useful, as they just power another system. I would say 3 CIM drives will be less useful with the brownouts.

15-09-2015 22:21

asid61

Re: pic: WCD 3 CIM PTO Ball Shifter Render

I'm aware that it shifts, however, the acceleration/current may still be too high for safety even if you start from low gear. Probably need to test that to be sure, because I have no idea, honestly. You'd also need to implement autoshifting for that to work.
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).

16-09-2015 03:24

Bryce2471

Re: pic: WCD 3 CIM PTO Ball Shifter Render

The slowest the gearbox can be configured without significant plate changes is 7.1 ft/s in low and 17.9 ft/s in high (80% of free speed)
This is all interesting conversation, and thank you all for your replies. However, it would be particularly useful if someone could post a personal testament to how the roboRio responds to having a 6 CIM drive train. This way I could walk the new CAD student through the process of reevaluating the gear ratios with some real evidence. Unless that happens, this project will continue to be a test of the new control system's outer limits, as it was originally intended to be.
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.

16-09-2015 08:35

Knufire

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

16-09-2015 15:39

Bryce2471

Re: pic: WCD 3 CIM PTO Ball Shifter Render

Thank you for posting that. I don't know how I missed it.

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:

16-09-2015 16:55

Chris is me

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.

16-09-2015 17:20

Knufire

Re: pic: WCD 3 CIM PTO Ball Shifter Render

The newest rev of the ball shifter shaft has the hex pinned instead of pressed.

16-09-2015 17:41

Aren_Hill

Re: pic: WCD 3 CIM PTO Ball Shifter Render

Pinned AND Pressed.

-Aren

16-09-2015 18:25

Bryce2471

Re: pic: WCD 3 CIM PTO Ball Shifter Render

Thank you two for clarifying. However, I am not planning on using the vex shifter shaft.
Thank you bringing up the question of CIM support. I think your concern is stemming from the fact that there are several aspects of this gearbox that are not easily visible from this angle.
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.
For this reason, I have never been a fan of the design of the ball shifter shaft from vex. In this design there are two custom simplified one piece shifter shafts.
This is a cool idea in my opinion, Even though it would cause a few problems. The only issues I see with it right now are that it would make gearbox assembly difficult, and cause there to be an unused gap in the gearbox unless the first stage was cantilevered.

16-09-2015 23:32

Knufire

Re: pic: WCD 3 CIM PTO Ball Shifter Render

Whoops, that was actually the wrong paper. Here's the complete testing they did: http://www.chiefdelphi.com/media/papers/3071

17-09-2015 10:24

Chris is me

Re: pic: WCD 3 CIM PTO Ball Shifter Render

These features should really help. You're probably okay since I don't think there are any weird loading conditions that would torque the CIMs away from the center of the gearbox. I'd probably leave the material on, but that's just me being overly cautious.

Any chance you could post a quick screenshot / view of one of these? I'd love to take a look. Might end up doing something similar if it isn't too difficult to make. Certainly easier than doing design gymnastics to make the COTS part work the way I want it to.

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.

Yeah, the first stage would need to be cantilevered. The idea here being that the extra space taken up between the gears by the coupler isn't such a big deal if it's going through a plate in that area anyway.

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).

17-09-2015 16:54

Bryce2471

Re: pic: WCD 3 CIM PTO Ball Shifter Render

Yes. I'll probably just post another render with only the shifting components visible.
Their shifter shaft definitely does the job they want it to, but I still think it could have been designed in a way that woud have made their gearbox more versatile and the shifting components more useful. The way they have it set up makes two stage drive systems impractical. Wheather it's in custom gearboxes or their COTS box.
I agree with this, but another option you should consider is using belts in the first stage to relocate the CIMs. This would turn out similar to 192's gearbox, but you could possibly get the pneumatic cylinder out of the belly pan as well.