Just wondering what everyones experience has been with the use of #25 chain between the middle and outer wheels of a 6 wheel robot (ie 2 loops of #25 chain on each side of the robot). I’m hearing different stories and have seen broken robot #25 chains being changed on the field right in the intermission between finals rounds. I also hear it will stretch substantially more than #35 chain. I know 1310 has used it in past but was using #35 last year.
Also, anyone confirm that #35 chain in the above configuration doesn’t stretch much at all, any slack accumulated appeared to us to be caused by wear on the aluminum sprockets (ie the slack remained even after replacing the chain with brand new chain).
We did this for the first time last year. Chain stretch was pretty dramatic, more than our tensioner designs could handle, though this was just as much a function of bad tensioner design, as it was increased stretch from the #25. I think we ended up having to pull a link about 2 or 3 times per chain over the course of the season.
Additionally, we made the mistake of using #25 chain without precision turned spacers. We used spacers cut from sheet stock which got the dimensions “about right,” to a degree which would have been acceptable for #35. “About right” wasn’t enough, and we threw a lot of chain due to sprockets not quite mounted in the same plane. Finally, we used smaller sprockets than we needed to, which increased load felt by the chain, and reduced the slack needed to throw it. We never had a break or throw which looked consistent with overloading the chain, and it should theoretically never happen with the loads experienced in a typical FRC drivetrain, but I’m sure this intensified the other two problems.
I know of many success stories as well, and I’m pretty confident that we could have made it work with better tensioners, bigger sprockets, and more precise spacers. But the experience left a pretty bad taste in my team’s mouth about the whole thing, and we’re likely going to be using belts instead next season. More upfront design work required, but much lower maintenance. There are advantages to #25 chain; that’s why a lot of very good teams use it. But proceed with caution.
3756 used #25 chain last year without issue. It does stretch quite a bit, meaning you have to either remove links or have some sort of other tensioning system to adjust every once in a while.
I personally love working with #25 chain… it feels almost like working with jewelry compared to #35. As mentioned by Joe though, it’s quite a bit more picky in the way of sprocket alignment. We used 1/6" plastic spacers (from McMaster IIRC) to line them up, and they never popped except for when one of our sprockets came lose.
We use the #35 chain and sprockets that we have sitting around from the KoP for prototyping and practice bots, but I don’t see us ever leaving #25 and 1/16" spacers for competition. (Edit: We had undersized aluminum spacers, leaving space for adjustments with the small plastic ones).
Thanks for the feedback. Lots of (critical) gotcha’s to watch out for.
Just in case I am missing something, what is the offsetting benefit of using #25 if there was so much stretching and retensioning required etc. Is it primarily weight savings (with sprocket weight savings) ?
While I can’t recall exactly where I read it, I remember seeing somewhere that #35 chain is about 2.5 times the weight of 25, so the weight difference is significant. For the small amount of time it takes up front to line up the chains properly, and re-tension them a couple times throughout the season, the weight you save is definitely worth it. It also looks fantastic and is much nicer to work with, but those aren’t exactly competitive advantages.
I highly recommend using multiple small plastic spacers if you join the 25 side. Makes lining up the sprockets a breeze.
Team 33 uses #25 chain almost exclusively on our robots. It is strong enough for pretty much any FRC application. Like many have said sprocket alignment is very important. Also, in our experience the chain doesn’t actually stretch - the slackening is from the sprockets wearing in. Generally if you have a way to tention the chain about a half inch (such as a sliding bearing block) that is enough to take care of any chain stretching. Alternatively, we actually ran with loose chain in 2012 (chain was not pulled tight by a tentioner) and never threw a chain. I think a lot of the problems people have is because of misalignment: if you have confidence that you can align the sprockets well then I would give it a try.
25 Chain is more than enough for drivetrain applications, assuming you have it aligned correctly.
To align sprockets, get a nice flat edge and span the distance between the sprockets. You should quickly see if they are aligned.
If you’re direct driving the center wheel, use 25 chain for the front and back. Even a break won’t put you out of business. If you are are not direct driving, use a double sprocket. Run one chain to the wheel in front of the transmission, and one behind.
The key to a successful drivetrain is NEVER reling on a single component, because it will break. From experience. (See Michigan State Champs 2011 where we sat motionless for 2 matches).
This past year, we used #25 chain to link the motors to the ball shooter wheel. We found that some of the sprockets that appeared to have irregular/non-standard tooth pitch. The large (28-32 tooth range) and small (10-16 tooth) sprockets were aligned so that they were co-planar. When rotated slowly, by hand, the chain can be seen to “climb” on top of one of the teeth of the large sprocket. This would always occur at the same spot on the sprocket regardless of which part of the chain was wrapped around it. We did not get significant wear on the sprockets or chain stretch. All the sprockets we had trouble with were stamped from sheet aluminum. On some of them, we noticed teeth that had “notches” in them, when new. It looked like the tooth pattern was “wrapped” around a circle that was slightly too large. We had to file the notch away to make the chain seat properly around that tooth.
I posted about this problem on a thread about #25 chain earlier this year. I seem to recall that some other teams ha no problems and some had similar problems and that it may be dependent on the supplier or even the batch of the sprockets.
Phil, we’ve seen the same thing. We generally just throw the ones like that away. They’re so cheap it’s not worth sending them back. On high torque low speed applications you can sometimes wait for them to break in, but on our shooter it was scary when they started jumping at 5k rpm.
There are several other threads where this has been discussed which you can read if you do a search but it basically boils down to several items already mentioned: #25:
is strong enough for the drive train
is lighter
is more fickle about alignment
#35:
is heavier
more tolerant of misalignment
easier to work with.
My preference for drive is to use #35 and for mechanisms to use #25 in most cases. I don’t like to have to worry about throwing a chain or even having to do real maintainance on the drivetrain which is what pushes me in this direction. Also I’m more comfortable with Al sprockets under that kind of wear at the #35 size. I know both work, but to me the trade-offs lean towards #35.
In the past 4 years I have been involved with the team, 610 has used 25 chain on our drivetrain 3 times. We only popped a chain off once. Alignment of the sprockets is crucial, but not really that hard if you design it in. Strength does not seem to be an issue, but larger sprockets lead to lower tension in the chain if you are concerned. It’s hard to say, but the “stretching” one sees with 25 chain may actually be wear in the aluminum sprockets. In recent years, I have noticed the sprockets from AndyMark are made out of thinner sheet metal without any chamfer on the teeth. It looks like they are just cut on a waterjet. I’m sure this reduces their cost of manufacturing, but I liked the thicker ones with chamfers to guide the chain, and I suspect they would not wear as quickly since the base of the teeth was wider.
The real benefits to 25 chain are lighter weight and smaller size. Also, if you get the “dark soul” chain tool, making chains is really easy, with no more master links! It’s actually much easier to work with than 35 chain.
Depending on the application, I would be hesitant to use 25 chain on arm joints and other heavily loaded mechanisms. Our flipper arms this year were powered by two 25 chains, and they broke a couple times. I think the shock of driving over the barrier or pulling the bridge down was more than it could handle over time.
With everyone saying its not the stretching but its the sprockets. Is there anyway to stop that? Would making steel sprickets be beneficial and still save weight?
We are thinking of switching but I gotta make sure before I use them. We will most likely use them on our prototype drive train this year.
During my time on 1503, we ran #35 chain in the drivetrain. While #25 is plenty strong enough, the biggest concern was sprocket allignment. Rather than spend hours machining spacers to 0.0005" tolerances, we felt our “quality” maching time could be better used for the parts that really needed it.
Personally, I’d rather spend the extra time and run #25 chain all around. The weight savings is huge.
We generally use #25 for mechanisms and #35 for the drive-train.
According to AM #35 chain is .25lb/ft and #25 is .10lb/ft. In a drive train that used 16ft of chain (average for us I think) #25 series would save 2.4lbs vs #35.
This weight difference could be reduced by using smaller sprockets and no tensioner with #35 chain. We’ve used half-links to tension our #35 chain drives during initial assembly, never touched them after that, and never thrown a chain in the 3 years I’ve been coaching.
In my 12 years, I’ve run 25 chain on 9 robots (1 other was #35, 2 others were belts) and never threw a chain. You definitely don’t need 0.0005" tolerances to get the alignment right. What you do need is careful design, and a little love at final assembly to ensure everything fits as designed.
I understand the “bullet-proof persona” that comes along with #35 chain, but I think any team is capable of using 25 if they want to. It just comes down to taking the time and effort in areas where you usually may move fast. Theres no need to machine spacers of +/- 0.001" of each other when you can buy shim stock and punch out spacers in whatever size you may need.
I know our final assembly of our drives with chains was carefully done over a couple days to ensure everything fit as intended. This is at the point where all of the pieces of the drive are DONE: machined, polished, cleaned, anodized. It’s often tempting to rush and throw every piece together in an hour. I think our track record shows taking your time at this step is time well spent.
One reason we actually switched to belts was to make assembly a bit faster, which absolutely did decrease the time required at that step.
Honestly, in our experience (a short run in a gearbox and a short run for a big arm), alignment was important but not absolutely critical or anything. I mean, for teams without a lathe that use the Kitbot and extruded parts, 25 might be a challenge, but if you have any reasonable manufacturing capability it’s more than doable.
Generally chain “stretching” is every little join on every link wearing.
The reason you see more stretch on #25 is not because it is weaker but is because you have more links per inch to wear down.
Did you order extras? I have been working with small teams so we have been ordering just enough + 1 spare so when both sprockets come in with notches in them, we are stuck. I will consider different sources in the future but the steel ones from Grainger get pretty heavy when the tooth count gets high. Does anyone have any suggestions for sources of better quality 25 sprockets other than AM?
Nick and Rob
It was great to see your most eleagant robot in the flesh in St. Louis and to be able to speak to some of your students. It was one of our favorites this year. I think your school is just a few km of where we used to live in TO. See you next time.
AM is the only person in the market that sells sprockets in 25 in aluminum. Maybe some other supplier might, WCP will in the distant future
But for now, you could send them back? We’ve called AM a few times for random misc. problems, they’ve been so kind to ship out replacements ASAP.
Instead of buying them you could router/laser/waterjet them from .118 or .125 plate and then chamfer them yourself? Do you have access to a CNC machine or do you have sponsors that own them?
