Amount of coupler purchase required on 1/2'' hex?
 

If one were to couple two 1/2'' aluminum hex shafts on a drive train with a hex sleeve, how small of a coupler would you be comfortable with? I am wondering if 3/8'' on each shaft would be sufficient.

Re: Amount of coupler purchase required on 1/2'' hex?
 

Well what kind of drive train (IE how much torque can the shaft expect to see)? And what kind of coupler?

Questions like these always need more information then just a can we do this phrase.

Re: Amount of coupler purchase required on 1/2'' hex?
 

Seconded what Mark said.

Think of the hex sleeve (coupler) as you would a hub for a wheel. How much engagement do you typically get on a wheel hub?

How are you supporting the two shafts? If not done properly, you could be inducing additional stress on the shafts and coupler than anticipated.

Re: Amount of coupler purchase required on 1/2'' hex?
 

I didn't think it was all that underspecified. The standard deviation on the amount of torque seen by FRC drive trains isn't all that big (essentially all of them are going to be on the order of 10 ftlb, unless you're running a ridiculously light robot or ridiculously big wheels). Say 160lb robot, wheel COF of 1, 4'' wheels. That gives a torque-at-shaft during wheel slippage of ~13 ftlb if you're running each side chained together, or ~6.5 ftlb if you're running each wheel off its own motor.

Coupler is, as mentioned, just a hex sleeve sitting on the ends of both shafts. Not sure how the specifics of the shaft support affect anything, so long as neither shaft is being supported by the coupler (which you clearly wouldn't do with a sleeve on a shaft taking a large load).

I know that for reference VexPro's gears all have 1/2'' of engagement on the shaft, but I have no idea how conservative that is.

Re: Amount of coupler purchase required on 1/2'' hex?
 

I can answer this in three ways
1) WAG: I'd be comfortable with engagement of at least 1D. So, for 1/2" hex, I'd have 1/2" engagement on each side.
2) Experience with what works: In our WCD style drive trains, we use VEXPro #25 hex sprockets. These are .361" thick, and we have not seen any signs of load related wear on either the sprockets or the axles. But, we haven't really disassembled and looked for wear, either.
3) Analysis: Haven't done it. Probably won't. It would take a good bit of thinking to decide how to come up with an answer based on analysis.

Re: Amount of coupler purchase required on 1/2'' hex?
 

We used 1/2in hex bar with 1/4-20 all thread in the ends to make our toro sticks. We threaded that piece into the end of a 1/2in hex output shaft on a Versaplanetary gearbox. We used a 2" vex spacer to lock the rotation of the toro sticks. Best guess, we had ~3/8 inch of engagement on the sticks. We modified the spacers to accept a hose clamp on the gearbox end so that they wouldn't go anywhere. If I remember correctly, on the second day of qualifications at PNW champs, we lost one of our toro sticks when one of the delrin spacers cracked, but otherwise they worked great, and they were cheap and easy to replace. YMMV

Re: Amount of coupler purchase required on 1/2'' hex?
 

Thanks.

The reason I ask is I'm playing around with some drive train designs and am wondering how feasible it would be to couple an output shaft from a gearbox and the center-wheel shaft of a 6WD inside a piece of 2''x1'' tubing. 1'' isn't all that much space to work with to start, and once you account for the bearing you're left with ~3/8'' of engagement on each shaft. I could use 2''x1.5'', if all else fails, but that's significantly heavier and takes up more space.

Re: Amount of coupler purchase required on 1/2'' hex?
 

What is the OD of the 'hex sleeve'? If it is thin walled I'd be worried. We run a Colson Live Hub in a similar application; albeit not drivetrain. Glad you are supporting the axles such that the coupling isn't taking any load other than torque, this can easily be overlooked. Why not just use a longer shaft?

I would say, based on your assumptions and some back of the envelope calculations, that 3/8" engagement would be ok. Would I feel super comfortable with it, probably not - especially something as critical as the drivetrain.

Re: Amount of coupler purchase required on 1/2'' hex?
 

Very interesting. I'm assuming this is a drive train with the wheel axle supported by bearings on both ends, not a WCD style drive train, where the center wheel axle IS the gearbox output shaft. Correct? Got a sketch?

Re: Amount of coupler purchase required on 1/2'' hex?
 

Yes, that is correct; it's very similar to the AM14U setup. I'll be posting a CAD when I finish toying around with it; it's also got a 3-CIM shifter gearbox with the CIMs floating over the wheel assembly (as opposed to over the center of the robot). The goal is to take up less space than a standard WCP 3-CIM shifter, and to allow for easy access to the belts and the wheels for maintenance (my primary gripe with west-coast style setups with belts is that if a belt fails, you've got no choice but to take the gearbox apart).

Re: Amount of coupler purchase required on 1/2'' hex?
 

Really need to see a detail of how the shaft is supported and where loads are applied before suggesting anything. Using a Colson Live Hub for this purpose is probably the best off the shelf solution you could rig up without a broach. Thick wall, solid, deep hex profile; all good things.

There are a lot of ways you could be putting a bending moment on the coupler so how the shaft is supported is critical.

Re: Amount of coupler purchase required on 1/2'' hex?
 

The output shaft of the gearbox is contained inside a piece of aluminum tubing (the chassis member to which the gearbox is attached). In the side of the tubing opposite the gearbox, coaxial to the output shaft, there is a hex bearing. The wheel shaft runs from that bearing through a bearing in a piece of aluminum sheet on the other side of the wheel.

Edit: Here's a picture of the CAD I'm currently toying around with; the plate on the far side of the wheel hasn't been designed yet, and thus is absent, but this should give you an idea of the setup. The coupler is currently inside a piece of 2''x1.5'' tube; I'm wondering if I can get away with 2''x1''.

Re: Amount of coupler purchase required on 1/2'' hex?
 

Assuming you can keep the gearbox and shaft concentric and that the wheel shaft is supported on both ends, I think you shouldn't have any weird forces on the coupler to worry about.

The biggest challenge would be making sure everything is lined up and concentric.

I'd have to do some math to see how the hex engagement would work out, but basically you want to engage as much of the wheel shaft as practical (at least 1/2"). A 1" coupler could fit in a 2x1 if you either omit a bearing (ehhh) or if you use a large ID bearing that goes around the coupler (better).

This is a bit of a tangent, but looking at your specific design, maybe you could build a gearbox that straddles your rectangular tube? Instead of driving a live shaft, have a gear off the gearbox drive another gear bolted to the wheel. I feel like this might achieve what you're trying to do (shifting gearbox with CIMs over drive wheels) a bit better than the coupler solution.

Re: Amount of coupler purchase required on 1/2'' hex?
 

That's a cool idea, but it'd be harder to manufacture and I want to be able to swap the gearboxes easily as well as the wheels.

Re: Amount of coupler purchase required on 1/2'' hex?
 

+1 to dead axles. There are a lot of elegant ways to install dead axles for easy maintenance. We've designed (with a lot of inspiration from others) a system that allows for a complete change of drive wheels on one side in under a minute.

You could attach a double wide pulley to the output shaft of the gearbox that would be used to drive two axles, and then another double wide pulley on the center axle to drive the other. All of this can be done with COTs. The trick to getting transmissions out is to leave access to it and space around it to get it out.