pic: Better Sprocket Assembly with AndyMark Parts

[Rob Stehlik]

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[CalTran]

How did the wheels actually fare in season? Did they hold up well? Ever shear a roll pin? What about replacing the wheels? Did you just have some extra wheels with this assembly? If you ran out of extras during a competition, how fast would you say that this assembly could be put together?

[DampRobot]

Yeah, I'll echo what CalTrain is saying here. Seems like an interesting idea, but it looks like it has assembly/disassembly disadvantages. Although I don't like dead axle wheels, if I did, I'd probably just use the regular #10 screws that the wheels are designed for. If nothing else, they're easier to assembly, and you can replace them a lot more easily if they break. I've also never had a problem with a poorly radially aligned sprocket like you mentioned, so for me, it looks a bit like a solution in search of a problem.

On a separate note, those sprockets look poorly sized. With #25 chain, I would suspect that the smallest plate sprocket (32 teeth on AndyMark) would work just fine with 6" wheels. We ran roughtop 6" performance hex wheel with 16 tooth hex sprockets last year, and only ratcheted climbing the bridge. 60 tooth, or whatever you have here, looks like overkill.

[Mr. Lim]

This small detail was a big improvement for us.

For anyone who has actually built a dead-axle drivetrain with two sprockets bolted to a wheel, and designed it with no need for chain tensioners (perfect link spacing), concentricity is a challenge. A small lack of concentricity becomes a significant issue, and it gets amplified the farther the sprocket gets from the wheel.

Screws aren't really meant to take shear force, and everything is a clearance hole, so there is some play in where the screw sits within the hole. Even if we "cranked it down" and set it up perfectly initially, we had concerns things could slide around, or even loosen/wear/slide over time. Robots smash into things, and are constantly going forwards and backwards at full power. All those forces are taken up by the clearance between the screws and holes, impacting the side of a screw that isn't ideal for transmitting shear force in the first place.

The roll pins solved all of these problems. The screws were relegated to simply holding the wheel, sprockets and spacers together - exactly what screws should be used for.

No we never had to replace a wheel, chain, sprocket, shaft-bolt, roll pin or service our drivetrain at all, other than pulling bits of carpet out of it, and inflating our tires. We played 46 official matches last year, an off-season event, and countless demos, and not so much as having to tighten a fastener.

...and that's largely thanks to little details like this that were implemented.

Yes we had spare assemblies made in case of a failure.

Yes, if needed, even I could manage to hammer a few roll pins in - probably in less time it would take for you to find a screwdriver for the crazy Canadian Robertson #10 screws we have up here.

[JesseK]

We pressed the two bearings into the wheels, then put a long 3/8" bolt through the bearings. Then we hand-pressed a bearing into the outer sprocket of the assembly and assembled the wheel as shown. Once the bolts were properly tightened (note -- we didn't use the thread-making screws) we removed the 3/8" bolt et voila! a perfectly-aligned sprocket-wheel pairing. Even if you stick with your roll pin method, I'd recommend this method as a general concentric-assembly method.

The whole 'piloting' problem would be solved if AM/IFI would make their holes 0.192 rather than > 0.2". We water cut some holes on our shooter wheel at 0.192 and it was a perfect slip fit for #10 bolts with no wiggle on the assembled parts.

[Gary Dillard]

Originally Posted by Mr. Lim

and designed it with no need for chain tensioners (perfect link spacing)

Could you please supply the manufacturer and part number of your zero tolerance perfect fit bearings in the joints, the zero tolerance machine you use to put all the holes in the drive train, and the zero tolerance, zero stretch (infinite yield?) roller chain you used to obtain and maintain the "perfect" link spacing throughout the competition?

Back to the OP, I think the pin and screw combination is a great idea.

[Jon Stratis]

We've use double-sprocket dead-axle drive trains in 5 of our 6 years, and every time we've only used the bolts it was designed for, along with appropriate spacers. We have never had an issue with this set up. Never sheered a bolt, never twisted it when it shouldn't have. This certainly looks like a great solution... but it's a solution to a problem we haven't seen. Is it common for teams to sheer bolts in this setup?

[Alan Anderson]

Originally Posted by Jon Stratis

Is it common for teams to sheer bolts in this setup?

I can't say I've ever seen one of these bolts actually sheared off, but there have been plenty of times in my experience where the shear forces were enough for a sprocket to get eccentric.

[akoscielski3]

Originally Posted by Alan Anderson

I can't say I've ever seen one of these bolts actually sheared off, but there have been plenty of times in my experience where the shear forces were enough for a sprocket to get eccentric.

This year at Smoky Mountains we sheared our bolts off. I noticed it when we were moving the wheels and the other wheels werent moving too. The middle wheel with the two sprockets was just floating there. We removed them with grade 8 bolts and never had another problem. Please note we took 3 of the bolts out and only had 3 bolts holding on each sprocket, not a good idea, i dont know who decided that.

[Brandon Holley]

We are firmly in the live axle camp these days, as well as the timing belt camp so this is less of an issue for us. However, when were doing chain and sprocket with dead axle setups, eccentric sprockets WERE an issue for us as well.

I dont think we ever had issues with shearing bolts, but getting a nicely concentric wheel/sprocket combo is easier said than done. I think this is a good solution to the problem. For any one who uses a non-spring loaded tensioner setup (ie: a bolt pulling or pushing on the wheel assembly) think of the inefficiencies you could be experiencing with every rotation. To get a chain nice and tensioned all the time, you could be causing some serious binding when the sprocket passes its furthest-from-center point.


In regards to the sprockets slowly slipping over time, that is most definitely happening with plastic components involved in the wheel assembly. If you are using plastic wheels, or plastic hubs and you tighten them down nice and tight, over the course of the season the plastic material will creep. This could cause eccentricities down the road even with a concentric starting setup.

-Brando

[Rob Stehlik]

Originally Posted by DampRobot

Yeah, I'll echo what CalTrain is saying here. Seems like an interesting idea, but it looks like it has assembly/disassembly disadvantages. Although I don't like dead axle wheels, if I did, I'd probably just use the regular #10 screws that the wheels are designed for. If nothing else, they're easier to assembly, and you can replace them a lot more easily if they break.

True, this is slightly more difficult to assemble. It's not for everyone. But we liked how it kept things concentric and there was no chance of sheared bolts. The picture might be a little misleading since it shows the HiGrip wheel while we actually used 6" pneumatic wheels.

Originally Posted by DampRobot

On a separate note, those sprockets look poorly sized. With #25 chain, I would suspect that the smallest plate sprocket (32 teeth on AndyMark) would work just fine with 6" wheels. We ran roughtop 6" performance hex wheel with 16 tooth hex sprockets last year, and only ratcheted climbing the bridge. 60 tooth, or whatever you have here, looks like overkill.

Allow me to elaborate: We used the Cimple boxes last year because they were readily available. The student who designed the drivetrain was targeting a speed of about 9 ft/s. He found that for short distances, acceleration is more important than top speed. This dictated the rather large 48 tooth sprocket on the wheels to get the required reduction, since the Cimple box is only single stage.

[JJackson]

We only had 3 bolts per wheel and never had an issue with them. However we use grade 8 flat head cap screws.

[Billfred]

Originally Posted by Jon Stratis

We've use double-sprocket dead-axle drive trains in 5 of our 6 years, and every time we've only used the bolts it was designed for, along with appropriate spacers. We have never had an issue with this set up. Never sheered a bolt, never twisted it when it shouldn't have. This certainly looks like a great solution... but it's a solution to a problem we haven't seen. Is it common for teams to sheer bolts in this setup?

1618 in 2007 had this issue; after one good hard match we had five of the six bolts sheared off in our re-skinned kit wheels. The time of repair meant we had to send a human player to our next match (we lost it).

However, one incident in seven seasons of this style of bolt pattern is pretty respectable in my opinion.

[Phyrxes]

Our problems with wheels are generally a student sourced assembly oversight which inevitably causes a screw to work its way lose and then promptly hit the frame and cause all manner of havoc. Anybody have any quick solutions to this, theoretically they are using nylon lock nuts but if someone has a "student proof" solution I'm all ears.