pic: WCD 3 CIM PTO Ball Shifter Render

[Bryce2471]

loading photo ...

[asid61]

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.

[orangemoore]

Originally Posted by asid61

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.

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

[asid61]

Originally Posted by orangemoore

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

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?

[GeeTwo]

Originally Posted by asid61

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?

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:

Originally Posted by R.C.

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.

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.

[R.C.]

Originally Posted by GeeTwo

Given the voltage brownout levels, I now expect that PTOs are much less useful than they may have been in the past.

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.

[asid61]

Originally Posted by GeeTwo

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.

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).

[Bryce2471]

Originally Posted by asid61

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.

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)

Originally Posted by asid61

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?

Originally Posted by GeeTwo

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.

Originally Posted by R.C.

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.

Originally Posted by asid61

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).

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.

[Knufire]

Instead of anecdotal evidence, how about some objective data?

http://www.chiefdelphi.com/media/papers/3067

[Bryce2471]

Originally Posted by Knufire

Instead of anecdotal evidence, how about some objective data?

http://www.chiefdelphi.com/media/papers/3067

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:

We only did this specific test with the 4 CIM configuration because with 6 CIMs or 4+2 we just spun the wheels.

[Chris is me]

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.

[Knufire]

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

[Aren_Hill]

Originally Posted by Knufire

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

Pinned AND Pressed.

-Aren

[Bryce2471]

Originally Posted by Knufire

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

Originally Posted by Aren_Hill

Pinned AND Pressed.

-Aren

Thank you two for clarifying. However, I am not planning on using the vex shifter shaft.

Originally Posted by Chris is me

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.

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.

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.

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.

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.

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.

[Knufire]

Originally Posted by Knufire

Instead of anecdotal evidence, how about some objective data?

http://www.chiefdelphi.com/media/papers/3067

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