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.
#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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
As a team that has successfully implemented 25 chain in numerous situations, including both drive-trains and serious arm mechanisms, we switched to #35 this year for our drive after snapping 25 a few times.
A few things I’ve seen in our own robot:
Alignment is key. A chassis/drive pod/drive assembly must maintain that alignment under all game conditions. This includes thrashing around on a defense. A chain that looks well-aligned statically may not be aligned at all when dynamic loading is considered.
Related: if you’re not using beveled sprockets, you’re going to have a bad time.
Shock loading is brutal this year. With dramatically inconsistent contact with traction surfaces during defense crossing wheels will spin up, then catch, then spin up again. This dynamic behavior can potentially overload chain.
If you use 25 chain, make sure you’re using 25 HD chain. Typical 25 chain is quite weak.
As mentioned we changed our drive-train to #35 this year. We are still using #25HD on our main shoulder joint though.
I wanted to comment on a couple of factors that influence chain selection that had yet to be mentioned. Others have already hit on items like larger wheel sizes, sprocket size, alignment, tension, etc.
I know that in at least one past year, 2791 has run their drive gearboxes at the corners rather than the middle of the robot. This can be great for making space for a mechanism low down in the middle of the robot, but it also means more tension on one of your chains. Instead of having 1 chain carrying torque to each corner from the center axle, you now have a chain from the gearbox corner carrying the torque for BOTH the center wheel and the far corner wheel. Not sure whether this is in play with this year’s bot, but I wanted to mention this.
In 2013 and 2014, Team 20 consciously used #35 chain to reduce part count. The machining resources were certainly more than capable of hitting the tolerances necessary to run #25 chain, but would still require a proper tensioning method, likely including cams and bearing blocks. Instead we chose to directly press our bearings into the 2 x 1 x 1/8" wall tubing and use dead spacing + 0.012" to take advantage of the extra lee-way afforded by #35 chain to reduce complexity and part count. In 2013, this was with 6" AM HiGrip wheels, and in 2014 this was with 4" colsons.
We threw one chain in 2013 at least in part due to a brain fart in design (my bad…) which led to using a half link to get the proper number of chain links.* But with a direct drive center wheel, even that wasn’t a big deal and was easily replaced between QF matches. No issues were noticed for 5 events thereafter.
*When doing center spacing for a chain application, design for a C-C distance with an EVEN integer number of links. An odd number means you will be using half-links.
First, as someone who’s admired Grasshopper Machines for 20+ years, I am going to tell you that you really need to give JamesCH95’s experience strong weight.
Almost all my experience with successful 25 chain implementations on drive trains comes before anyone used conveyor belt for tire tread and certainly before pneumatic tires with sprockets that bolt on were only a few clicks away. It may very well be the case that with these new fangled grippy tire options, young’ins these days just have to go with #35 chain for their own safety.
I didn’t see any reference between 25 and 25H in this thread. 25H is a far better solution without the weight of 35. It is still a bear to work with, but way better than standard 25. And yes, you have to run 25 tighter than you would think. 25 is very sensitive to alignment and tension. Somewhere I was told the “7 tooth minimum” rule. No idea if it is folklore or real but it works in general. Keep 7 teeth engaged at all times. This means no drive sprockets smaller than 14 teeth on a simple drive.
I build silly tiny electric cars and race them. I have run #25 chain for hundreds of hours with motors equivalent to 6+ CIMs to big (by FRC standards, 11") pneumatic tires. and it works just fine even under shock loads like getting put into the wall at 20mph. I even did some testing with 6.5hp brushless motors spinning a 20 tooth #25 sprocket and it worked fine for FRC time scales of less than a couple of hours of operation. With that said, I would swap over to #35 if at all possible. In my experience it is much more forgiving in fabrication tolerances and dealing with the unexpected situations like those that are common in FRC like getting repeatedly rammed, stalled motors, slight shifting of sprockets etc.