Do You Think We Would Stall?

[bardd]

1. As a whole your idea sounds good. it should work well. One thing that troubles me is the wheel setup. How will you connect the sprockets to the center wheel? Both on the same side? One on each? If you put them on the same size, I can almost guarantee the chains will get on top of one another and get stuck. We lost our entire 2012 season that way. Your best bet would probably be to put them in different sides, which might interfere with the gearbox... But if you plan correctly it would work.
   
2. I have no experience with #25 chains, but many teams in Israel use them for drives, and I haven't heard of a chain breaking yet (I've made some research when we thought of moving to #25 chains). They're weaker but will probably hold. If you're not sure, hang one from the ceiling and have a... "big" person hang off it, see if it breaks (with a helmet and safety goggles, just in case).

[Joe G.]

Regarding #25 chain, use the largest sprockets you reasonably can, and come up with a bulletproof tensioning system. Large sprockets reduce the load passed through the chain, and makes the chain more tolerant to stretch and misalignment. We had a real bad experience running a #25 chain drive with 16t sprockets. The weight and space reduction can be worth it, but you have to be more careful than with #35.

[Tristan Lall]

With those ratios, you're going to spend most of your time in high gear (if your driver knows what's good for them...), and use low for getting out of defence or pushing objects. And with that top speed, you won't stall, but you will pull significant current as you depart from a standing stop.

If you can shift fast, you can mitigate this by starting in low, and then transitioning to high quickly—it might be worthwhile to automate this.

Another solution is to add motors—because that spreads the load across more circuit breakers and more motors, for better overall performance. But that would require re-engineering the Super Shifter a little bit. (I've done it before; it's not a huge deal, especially if you're starting now.)

As for the chain, if you engineer it correctly—as Joe described—you can make it work. Understand the relationship between tension in the chain and torque on the shaft, and you'll get a good idea if you're exceeding the manufacturer's specifications. (Remember to consider the case of sudden reversing loads.)

[Craig Roys]

I can't speak to the stall question, but #25 chain is sufficient provided you follow a few simple rules:

1. Make sure you have at least 180 degrees of wrap on each sprocket.
2. Make sure your sprockets are properly aligned.
3. Make sure the chain is properly tensioned.
4. Make sure you buy quality chain - McMaster chain has been good to us. One year we bought less expensive chain from another supplier and had nothing but trouble with it. We will always use McMaster for chain now.

This year we had a high traction robot using #25 chain and had no issues. We've pushed robots sideways across the field. During our entire season, I only remember 3 robots who brought us to a stalemate while trying to push: 25, 68, and 296. There may have been others, but that's all I can recall. The moral of the story is despite doing lots of pushing against teams like these we had no issues with the #25 chain.

[AlexH]

i'd use 25h instead of regular 25 chain. its pretty much exclusively sold by scooter and gokart venders.

http://www.partsforscooters.com/25H_roller_chain

http://www.amazon.com/Mini-Pocket-Bi...dp/B0059ZPHEU/

25h is rated at 1750lbs while regular 25 is rated at 875lbs. 35 is rated at 2100lbs.

[RoboTigers1796]

Thank you all for the great input and suggestions. They are truly valued.
Tristan, we were definitely going with the pneumatic shifting option so I think that would be an Exciting project for the programming team.
As to bards concerns here is my super detailed cad model I just made . Sorry if I slow some of your computers down trying to view it.
Orange = dead axle
Red=live hex
Green =chain runs to wheels
Everything else should hopefully be self explanatory with the above description of the system.
Tensioners would be added where needed.

[Jeffy]

Do not gear low gear that slowly, you will regret it.

[mman1506]

That spread between gears seems bit too large, you want to keep it around 2-3 X spread for optimal all around usage. Your at a 4 X spread

[cbale2000]

Originally Posted by RoboTigers1796

Thank you all for the great input and suggestions. They are truly valued.
Tristan, we were definitely going with the pneumatic shifting option so I think that would be an Exciting project for the programming team.
As to bards concerns here is my super detailed cad model I just made . Sorry if I slow some of your computers down trying to view it.

...

Tensioners would be added where needed.

*picture*

Looks good to me. Though ideally if you plan your wheel spacing correctly you shouldn't need chain tensioners.

Originally Posted by Jeffy

Do not gear low gear that slowly, you will regret it.

That depends on the application, really. In 2007 we played as a primarily defensive robot and had a similarly low gear. We were able to simultaneously push 3 robots sideways on several occasions. For this years game, however, I would agree it's probably a bit excessive, but again, depends on what you need it for.

[MichaelBick]

Originally Posted by cbale2000

Looks good to me. Though ideally if you plan your wheel spacing correctly you shouldn't need chain tensioners.



That depends on the application, really. In 2007 we played as a primarily defensive robot and had a similarly low gear. We were able to simultaneously push 3 robots sideways on several occasions. For this years game, however, I would agree it's probably a bit excessive, but again, depends on what you need it for.

I would bet that your low gear was traction limited, so you could have geared higher without losing pushing force.

[Mark Sheridan]

Originally Posted by MICHAELABICK

I would bet that your low gear was traction limited, so you could have geared higher without losing pushing force.

I think their gearing is fine. Assuming a traction coefficient of 1.3 the current draw would just under 120 amps total with their gearing. This would give them plenty of safety factor just in case they ended up on the heavy side and having current drawn from another mechanism. I think for teams that are trying shifters for the first time, its better to start on the slow side. Having a extra traction limited low gear will give them one less thing to worry about.

I would recommend using the 22 tooth sprockets at least with that much torque if you want to try #25 chain.

This is very close to 987's gearing according the behind the bumper video from FRCtop25.