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Personally, I like to overbuild everything. I don’t like things breaking, and the degree to which I overbuild is directly proportional to how critical it is for success. The drive train is probably the most critical part of the robot for success. I would never use less than #35 chain for a drive train. Then again, since we made the switch a couple of years ago I’ll jump through hoops to avoid using chain at all on the drive train - direct drive all the way!
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IMO, #25 chain is great for mechanisms but unless you know what you’re doing it’s a pain for drive systems. Some teams can make it work, but most underestimate the level of difficulty to get it right. Essentially #35 chain has the benefit of added tolerance for mistakes, so as others have rightly pointed out, the quick fix for #25 chain acting up, is to just use #35 chain.
That said, issues with chains breaking, skipping, and maintenance are the main reason we no longer use chains on our drive system. Geardrive ftw. :rolleyes:
Thanks for this thread. It’s very enlightening.
Does anyone have a reliable source for #25HD chain? It’s a bit harder to find, from my cursory looks (the vendors we use don’t sell it).
Thanks!
25 chain is very sensitive to alignment, if one sprocket is 1/8" further out than the other, it’ll bind up and have problems. Make sure you don’t over-tension your chain, as this also makes it more likely to snap. There should be a little bit of slack in your chains, but not enough for the chain to be noticeably floppy. Especially with a game like this, #35 chain is definitely not overkill, and it’s much less sensitive to alignment. We’re using 25 for mechanisms, but #35 on the drivetrain is definitely a good option.
I would argue that if you can make a drive unit rigid enough to not mis-align gears, and precisely enough to enable proper gear meshing, then you can make a drive that can easily handle no-maintenance chain or belts reliably.
And, yes, no-maintenance (aside from lubrication, which gears need too) chain drives exist.
VexPro only sells #25HD as far as I can tell, by deduction in that they only sell 25 HD master links and state that only HD master links work with their chain. Comparing it side-by-side with other 25 chain sources makes it even more obvious.
I wish they’d state it on their website though…
Edit: boy, Dr. Joe sure knows how to make an engineer blush…
We are setup with 22T sprockets on 8" pneumatic wheels geared at 8 and 18 ft/s off of 2, 3 CIM shifting gearboxes, and #25 completely doesn’t cut it.
We will be spending all of Thursday at week one tearing apart our drivetrain to replace all of the chain.
The spacing and tensioning and alignment of the chain was done absolutely perfectly, every chain had good tension without being too tight, it was by far the nicest drivetrain in team history, until we tried to go over defenses and chains started snapping.
Use #35 if at all possible, #25 really burned us this year.
I believe if you are running the 7.56" wheels in a total reduction that gets you 6 fps, I think that operates outside of the recommended range of #25 chain. We’ve been fine with it this year after beating up three different robots with it over the last 7 weeks but we are running a single speed north of 12 fps on 22T sprockets.
I’ve come to reiterate most of what has been said in this thread.
4901 ran #25 chain in tube with 17 tooth double sprockets for our drive system. If you were at Palmetto you saw first hand the time we had with it. We ended up taking the drive rails out 4 times.
What we learned is this, DON’T USE HALF LINKS. We ended up with a problem where we had to use half links to get our chain tension-ed without using a tension-er. We would break chain in 5 minutes after installing it again.
I will say if you use #25 chain is that making your chain runs continuous via Dark Soul tool or whatever other method you know of is a great idea. We managed to do so with the help of 1296 and never broke a chain again after that, of course we had to use our pre-planned tension-er slots after all.
Whaaaaaaat you don’t have a 221/DarkSoul tool?
25 is a total dream if you use it right, we used it wrong once after a couple former mentors tried to sell some fiction one year and of the 4 chain runs we had on that robot at Alamo in 2014, we threw all of them at least once. In 2014 because of snow we were allowed to bring in 45 pounds of withholding so we left the drivetrain out of the bag before Virginia and rebuilt the whole thing.
Every other time the chain has been properly tensioned and spaced and we have loved it.
We don’t have one. We just bought one Yesterday. I was only brought into the light about that tool this week.
The recommended working load of #25HD is around 90lbf. This is fine to exceed for shorter lifespans, but with the tiny sprockets teams are running this year… :eek:
In round numbers:
8in wheel diameter
2in chain wrap diameter
µF = 1.3
Robot weight = 140lbf
Assume:
All weight on one wheel (possible, if not likely, during defense crossing)
Drive is traction-limited (certainly the case at 6ft/s with 3 CIMs per side!)
Chain tension:
140lbf1.38in/2in = 730lbf
Breaking strength of 25 is around 780lbf, giving one a FoS of 1.1, and virtually guaranteeing yielding in the chain.
This case is quite conservative, but being conservative in drive-train stress considerations is probably a good idea.
For consideration: in our chain-snapping 2016 drive train the calculated tension was 550lbf. Changing to #35, with a breaking strength of about 1900lbf, did the trick. In prior years we’ve had chain tensions in the 150-200lbf range. Having a FoS of 3+ versus chain breaking strength is probably a good idea.
My philosophy, especially when it comes to drive trains, is, “do the math, then use the math to go for total overkill.”
The importance of robustness in a drive train cannot be overstated, literally–if your robot can’t drive around, you can’t compete***, no matter how awesome your upper mechanical systems happen to be.
***Except last year, maybe. But let’s not talk about last year, like, ever. Let’s file it away in the Mental Folder of Things that Do Not Actually Exist, like HIGHLANDER 2. Ahhhh, that’s better.
I think I recall a similar thread a couple of years ago which went in a different direction, with a lot of folks emphasizing the qualities of #25. My team has been using #25 for last several years with no problems, and we used it on our drive train this year. I guess we will see how well it holds up next week. Maybe I should bring sullied to do a “quick” swap in AZ if necessary.
Looking at this site here: http://www.partsforscooters.com/PFS-Chain-Chart
I was able to see a difference of thickness between #25 and #25H chain (roughly .24mm per plate, for overall thickness of about 1mm).
I am looking for a side-by-side tensile strength comparison to determine if it’s truly worth making a jump from mcmaster #25 to a #25h change. If I find one, I’ll post it here.
It is tough to find good manufacturer data for 25 series chain. Have a look at the table at the bottom of page 1 here.
The max allowable load column is synonymous with max recommended working load. Converted to lbf, it is 165 lbf for 25, 242 lbf for #25H. Average tensile strength is 992 lbf and 1323 lbf, respectively. These values are a little higher than I recall from past research, but at least gives you a side-by-side comparison.
We have used 25 chain successfully in drive and manipulator applications since our rookie year in 2004. I would definitely recommend #25H chain over 25 in all applications, the weight penalty is marginal for the added strength. I generally avoid #35 chain at all costs, due to weight, but if you are having failures with 25, I would say switching to 25H alone will likely be a marginal improvement - try to increase sprocket size as well. If you have the weight, consider #35 for the added reliability.
Drivetrain failures are are the worst possible failure point, as it leaves you useless on the field. Whenever possible, I encourage direct-driving at least one wheel, so that no chain is a single-point-of-failure to half your drive. If you must have chain directly from your gearbox, then I would consider #35, as failures there are unacceptable. This year, with 8WD using 8" pneumatic tires, we chose to direct drive one center wheel, use #35 to the other center wheel, and 25 to the outer corner wheels. The idea is a compromise between weight and reliability - no 25 chain failure will leave us unable to drive reasonably well, as it would only cut power transfer to one corner tire.
I think a huge part of the problem people are seeing is the dramatically higher forces involved with the larger tires paired with smaller sprockets. For years FRC drive trains have been evolving toward smaller wheels and less gearbox reduction, which pairs favorably with transitioning to 25 chain. In James’ example case, if you were instead running 4" wheels, the max possible chain tension would drop to 365 lbf, and SF would increase from 1.1 to 2.1. I agree that his scenario is conservative, but I think most teams (running live-axle WCD, at least) are using even smaller sprockets than he calculated. Consider that the largest hubbed 25 sprocket Vex sells is 22T, with a pitch diameter of 1.75".
Where did you find it? Everywhere I’ve looked is sold out.
A google search for a “221 chain tool” (that’s what we call it since all of ours are 221 branded) will lead you to 221’s page for the item which they no longer sell…
…but the page has a link for the tool sold elsewhere and is available from that seller.
We usually use #25 chain in our drive, but not this year. We decided not to push our luck with 8" wheels and a field that promises to provide more drive train abuse than usual.