Offsetting shifter piston?

[asid61]

I'm in the middle of making a small gearbox that utilizes the Vex ballshifter shaft. However, the cims are the first stage of gearing and go straight to the shifting shaft, and as a result the cims are so close to the shifting shaft that I can't fit the regular Vex shifting piston inline with the shifting shaft. Heck, I can't even fit a thin long piston there.
I'm faced with two options: Place the shifting piston behind the cims (~4.5" away from the shifting shaft) or to put the piston to one side and connect it by a bar to the shifting shaft. Is it dangerous to offset the piston to one side and use a metal bar to connect the piston to the shifting shaft?

[Mike Marandola]

Originally Posted by asid61

I'm in the middle of making a small gearbox that utilizes the Vex ballshifter shaft. However, the cims are the first stage of gearing and go straight to the shifting shaft, and as a result the cims are so close to the shifting shaft that I can't fit the regular Vex shifting piston inline with the shifting shaft. Heck, I can't even fit a thin long piston there.
I'm faced with two options: Place the shifting piston behind the cims (~4.5" away from the shifting shaft) or to put the piston to one side and connect it by a bar to the shifting shaft. Is it dangerous to offset the piston to one side and use a metal bar to connect the piston to the shifting shaft?

How much offset would be needed?

Or even better, a CAD drawing.

[Knufire]

5188 coupled a large bore cylinder off center to a ball shifter for our winch gearbox last year. What we found was that pulling so far off center created a moment and would cause the winch to occasionally jam and not disengage. This is a much different load scenario, so take it with a grain of salt. Our theory is that the only reason the system worked was due to the 2" bore piston used, simply brute forcing the mechanism to shift despite pulling so far off center.

[asid61]

Originally Posted by Mike Marandola

How much offset would be needed?

Or even better, a CAD drawing.

Around 0.75" of offset. It's not too much I think.
CAD will be released in due time...
EDIT: more like 1.25".

[Mike Marandola]

Originally Posted by asid61

Around 0.75" of offset. It's not too much I think.
CAD will be released in due time...
EDIT: more like 1.25".

That might work. Attaching the bar to the pancake cylinder will be easy since it has threads. You will just need to make the little bar as stiff as possible and try to eliminate most, if not all of the play at the spot where the bar attaches to the bearing on the shifting plunger.

[asid61]

Originally Posted by Mike Marandola

That might work. Attaching the bar to the pancake cylinder will be easy since it has threads. You will just need to make the little bar as stiff as possible and try to eliminate most, if not all of the play at the spot where the bar attaches to the bearing on the shifting plunger.

Okay, thank you. If it doesn't work out I can alwys switch to a different configuration.

[Lil' Lavery]

[grandpa]
Back in my days, just about everyone's shiftin design incorporated offset pistons (team 45 being the notable exception). Granted, none of you whippersnappers use crash shifting designs these days.
[/grandpa]

[FrankJ]

Pneumatic cylinders do not like moment (offset) loads. You can get away with it with short strokes & relatively light loading. Or you can add a rod & bushing that takes the moment letting the cylinder see only the axial load.

[asid61]

Originally Posted by FrankJ

Pneumatic cylinders do not like moment (offset) loads. You can get away with it with short strokes & relatively light loading. Or you can add a rod & bushing that takes the moment letting the cylinder see only the axial load.

What do you mean bt adding a rob and bushing? Just to let them flex and spread out the moment?

[EricH]

Originally Posted by Lil' Lavery

[grandpa]
Back in my days, just about everyone's shiftin design incorporated offset pistons (team 45 being the notable exception). Granted, none of you whippersnappers use crash shifting designs these days.
[/grandpa]

[second grandpa]And that extended to the KOP shifters. You younguns think you've got it made with that single-speed gearbox and 6 CIMS? Back in my day, we got SHIFTING gearboxes in the Kit! And they shifted offset![/second grandpa]

And yes, those drill motor transmissions had to be shifted offset if you were going to shift. As I recall, my high school team ran a slider bar under ours to engage the shift ring and connected a short-throw cylinder to the other end.


This might be a good time to grab any engineering mentors you have and run through some numbers on buckling loads. I don't think it'll be too big of an issue, given a nice short distance and nice "light" load, but it's an opportunity to learn.