#25 Chain
 

Hi Folks,

Typically we use belt for our drive train. This year with the bigger wheels we are decided that we wanted a stronger transfer method so we switched to #25 chain. We are running 8" AndyMark Pneumatic wheels, with a WCP Dual Speed Gearbox. Our low end is about 6 fps and the high is about 12 fps. We keep breaking chain. Is anyone else having problems with the large wheels breaking the chain? Some on the team want to switch to #35 chain, but that seems like overkill to me. I feel like we must be doing something wrong, not that the chain is too weak.

Thanks!

Re: #25 Chain
 

Alignment, alignment, alignment.

#25 chain needs to be aligned properly.

NO HALF LINKS!!!!

That is how we are using it this year and so far, so good.

BTW, #35 will work and is not overkill.

Re: #25 Chain
 

#35 Chain is definitely not overkill! CIA has used chain drives in the past and #35 is the way to go, especially if you get in a pusing contest with a defending robot. Also, how big are your sprockets? Using small sprockets increases the stress on your chain. Simply using bigger sprockets lightens the load on your chain.

Re: #25 Chain
 

The best way to fix #25 chain is to use #35 chain. People claim that if you use it correctly, #25 is perfectly fine. It's that first part that's hard. If you have the weight, I highly reccomend #35.


*Disclaimer: I hate #25 chain with a fiery passion because it cost us over 12 hours during RI3D this year. We would've been done a solid 12 hours earlier if we had used #35.

Re: #25 Chain
 

Yes, we made the switch to 35 about 3 weeks ago. While prototyping different size wheels, we kept breaking chain even without master links. Most of it was impact driven - as we hit obstacles, it shock loaded the chain. We've been driving a lot with 35 since and no problems - it's also more resilient to tension/slack.
At a scrimmage last week, we talked to another team that had switched. They had actually calculated the shock loading and it exceeded the 25 strength by a fair amount.

Re: #25 Chain
 

We started off running 9mm belts inside of our tubes running 8in pneumatic wheels which caused too much slippage so then we swapped to #25 chain like you and haven't had any problems since. The only time that one broke on us was when we had a power-pull get trapped in the drivetrain. Our drivetrain is even faster going at 20fps and it has been great. I'm not quite sure what is wrong with your setup but #25 chain should be able to handle the loads. Maybe a picture would help diagnose the problem.

Re: #25 Chain
 

We broke a #25 pitch chain this year for the first time.

Robot is geared for ~15ft/s, 8" pneumatic wheels, ~115lbs actual weight. We were coming over a defense dukes of hazard style and the driver slammed it in full reverse as the front wheels were landing and point loaded them, causing the break.

All of our chains were run chain in tube with dual bearing supports, however there was an issue found in the post mortem where aan additional 1/16th spacer got added, which caused 1/16th misalignment on an 8" CtC run.

Bot #2 (comp) was made with #35 pitch chain as an insurance policy since we had the weight allowance. We fixed bot #1 and left the #25 pitch on it. After a week zero event and ~5-10 additional hours of driver practice under extremely heavy defense, we haven't broken another #25pitch.

tl;dr - Make sure your sprockets are well support and well aligned and you shouldn't have an issue with #25 even in the most abusive drivetrain conditions. Want some extra insurance, run #35, but it still isn't an excuse to not pay attention to alignment and tension. It always pays to do the math though and compare it to the ultimate strength of your chain for edge cases.

Re: #25 Chain
 

At first blush, it seems that #25 chain should be able to provide more torque to the wheels than traction from the carpet (unless you are using teeny tiny sprockets) BUT, with drive systems, I don't mess around. There is nothing that lowers your draft potential as fast as driving in circles for a match because your robot threw a chain.

I recommend #35 chain unless you are completely painted into a corner and there is just no way to make it work.

YMMV.

Dr. Joe J.

Re: #25 Chain
 

Lots of things that could be going on here that could cause these issues, depending on a number of factors. What is the tooth count of your sprockets? How are you tensioning your chain? How long are your chain runs?

First things first, if you're breaking 25 chain in a drivetrain yes there's probably something you're doing "wrong" with it, but switching to 35 will give you a lot more fault tolerance so you don't necessarily have to solve that problem to keep going. In the interests of time and reliability, if you have the weight 35 chain might just be the easiest way to go.

I'm assuming you guys don't have any problems with alignment - that would be pretty easy to see.

If I had to wager a guess, if you're using 16T or 18T sprockets (the "standard" for 4" wheel WCDs for the past several seasons), but driving 8" wheels with them, this is probably part of the problem. Smaller sprockets with bigger wheels will be downstream from a larger reduction to get the same speed, so they are loaded with more torque. Shock loading is also more of a concern this year than other years. If you can make your sprockets larger, you will have less tensile force in your chain. In a WCD, you should be able to go up to at least 22T if I recall correctly.

Chain that isn't properly tensioned will have more problems - you're more likely to skip a tooth and shock loading will be greater, leading to a larger chance of chain failure. Chain on small sprockets connected to large wheels will stretch more quickly than chain on large sprockets or connected to smaller wheels, as a function of the increased load put on them. If you did exact center spacing with your chain to start, it might have stretched too much under load by now.

Re: #25 Chain
 

Search through the past Chief Delphi discussions about the relative merits of #25 and #35 chains and you will find plenty of facts, experiments, opinions and anecdotes.

One thing that should become apparent during your search is that your machine is a system made of many parts, that it is operating in varying environments, and that it is being asked (by its operators & software) to do more than one thing. That makes it easy to reach the conclusion that you have to ask the right question (one that takes into account all the important independent variables) before you can answer whether #25 or #35 is the right chain for your machine.

I don't say this to make the topic sound too hard. I say it to help you quickly reach a valid, complete answer. Answers to incomplete questions are incomplete answers, and letting incomplete answers churn your design is not good.

Bottom Line: As is often the case, the right place to start is by learning what has already been recorded; so that you can stand on the shoulders of the ones who have gone before you.

Blake

PS: Some engineers and shade-tree mechanics might just decide to do a quick experiment. That would be adding some dead weight and/or rotating mass to your robot to approximate the effects of switching to #35 chain+sprockets, and then (using the current #25) deciding if the robot is likely to perform well enough when it has to move the the more massive, and stronger, #35 chain+sprockets. If it does still perform well enough, switch to the stronger and "more forgiving" chain this season (before next season do more sophisticated experiments).

PPS: There is more than one type of #25 chain ..., and more than one type of #35 chain.

Re: #25 Chain
 

Sounds like you have a physics problem on your hands. We know torque = r x F, where r is the radius to the point where the force is applied, and F is the applied force.

No matter how you drive the wheels, if you desire a specific speed, the required torque applied to the wheel is a constant. Knowing this, we have a couple options to either reduce force on the chain or increase the maximum force the chain can withstand.

1. Increase the radius of the sprocket. This will reduce the force on the chain, and with a little algebra, you can calculate how much the force will be reduced if you so choose. Additionally, this will give more teeth for the chain to "bite" on, so the potential to put a ton of load on a single link pin is somewhat reduced

2. Upgrade to #35 chain. I say upgrade because if you have the weight for it, #35 chain is a fantastic solution for drivetrains. This will increase the maximum force the chain can withstand, and lessen the chance that the applied force on the chain will exceed the breaking point. This can also help you to lower your center of mass slightly, which, depending on the shape and height of your robot, could be a nice improvement.

3. Combine 1 and 2 by switching to #35 chain and increasing the size of your pulleys. Since you have to buy new sprockets anyway if you switch, why not get the largest radius that will work for you without creating clearance issues? You can decrease the force on the chain and at the same time increase the max allowable force for the chain.

You could also experiment with properly tensioned belts of appropriate width, spacing, sprocket size, etc, but I honestly do not know enough about belts off the top of my head to give a good suggestion for you.

Additionally, like other people have said, you will in any case do well to make sure that everything is properly aligned and tensioned without any half links. I know that when my team did some seriously tough drives during my high school years, we didn't even use master links, let alone half links. If you invest in a good chain tool, you can make complete loops of chain for your drive, and by removing master links you can remove another point of failure from your system, provided you properly use the tool to reassemble the chain.

EDIT: While I was typing this, some other people beat me to the punch. But nonetheless, if you can do it, #35 chain is awesome.

Re: #25 Chain
 

To be fair, your Ri3D robot had some massive, exposed chain runs that are exactly where using #25 chain would cause issues.

This is a good case example: #25 chain needs to be used with care and precision. If you aren't confident in your ability to provide that, or want to push your chain to its limits (small sprockets on big wheels, lots of torque or speed), you really should be using #35, just due to the fault tolerance.

This year lends itself well to very fault-tolerant, robust robots. While #25 can live up to that challenge, it requires a lot more care and effort to make it do so, especially after build season.

Re: #25 Chain
 

What is wrong with half links? Are they extremely weak?

Re: #25 Chain
 

I'd check your setup and your tension. Now I see all these people up here saying that it's an easy solution to switch to #35 chain and be done with it, but that's not fixing a fault of #25 chain, that's trying to cover up poor design and assembly, and in this endeavor, you should always work to have the best setup you can. Loads of highly successful teams use #25 chain in their drives and are likely using it more strenuously than you are. You can run #25 chain on tiny sprockets with sub-optimal tension and it's still going to run for the most part fine, as lone as your alignment is correct. Getting completely straight alignment of your sprockets is key. Use CAD to make sure you have correctly made spacers with little wiggle room to fit everything where it needs to be. Use the location of one sprocket in your CAD to drive the position of its partnering sprocket, and then design your spacers around that. After everything is aligned, make sure you have adequate tension. If you don't have tensioning blocks built in, you can go for other ways of tightening your chain (zip ties are a decent last resort to keeping tension in a system). If you're going C-C, use Paul Copioli's chain calculator in his Useful Calcs spreadsheet. You shouldn't just look at poor execution of #25 chain and think the solution is to move to #35.

I agree with Joe: I don't want an alliance partner that spins circles in matches because of lost chain. However I also don't want an alliance partner that covers up poor execution in design, because they will likely cover up poor execution in strategy and performance, which is the last thing I need in the finals.

Re: #25 Chain
 

We were having a lot of trouble with #25 chain during our Week Zero event in Merrimack, and we too have been thinking about making the change during some of our free time.

What we did to fix the issue for now is adding a 1/4" Lexan strip along the bottom of our chassis where the chain was hanging. We think it was hitting and catching on the defenses. After we did that, no more shearing. We also added some static tensioners, and are looking to put more long term dynamic tensioners on at some point.

Alignment is key for sure with #25 chain, or chain in general. Definately make sure you do your chain calculations right.

We have done #35 and #25 in the past on our drive trains, but none of the existing team has remembered a bot we made using belts. I do not know if that is a better idea, I know slippage is an issue with belts.