Okay, I have been trying to use 1114’s method of not using chain tensioners. In their “Drivetrain Design” presentation it states if you use the biggest sprockets possible that you do not need to tension the chains IF the distance between the the centers of the two wheels is a multiple of the chain pitch. BUT, I have gone online and used some chain distance calculators (http://www.botlanta.org/converters/dale-calc/sprocket.html and http://www.rbracing-rsr.com/chainlength.html) and when I put in a multiple of .375 for a center to center distance, it does not come up with an even number of chain links as 1114’s presentation stated. Is this correct? Should I use what the calculators are giving me or what 1114 stated in their presentation? I emailed Karthik already and he forwarded the message to their design lead and I haven’t got a response yet. I am very confused on which center to center distance to use. The calculators or 1114’s concept of multiples of .375. Well If anyone can help, Thanks in advance!
OK, let’s imagine we have a chain that has a pitch of 0.375 inch. That means each link is .375 apart, as are teeth on a sprocket…but there are even and odd links (or ‘innie’ and ‘outie’ links). So really, a full “link” is 0.750. Get that?
So let’s say you want an Integer (whole, not fractional) number of links. If we have a sufficiently large sprocket, we can say that:
- The number of links on 1/2 of the sprocket is an integer (one lnk per tooth, right?)
- If we imagine a point at the very top of the sprocket, it should coincide with the center of the sprocket. So for any chain going from top to top of two sprockets, if the centers are an Integer number of links apart, there must be an Integer number of links between the tops. And the bottoms.
- If we have an integer for the sprocket half and an integer between the tops and bottoms, that all adds up to an Integer. See it?
I went to the first calculator and used 2 sprockets of 40 teeth each and a center distance of 37.5 inches. Using the above, I would expect 2 * 20 links on the sprockets and 100 links top and bottom, a total of 240 links…and that is what the calculator says it is. So the calculator seems to be working for me…
In addition to what DonRotolo said, the guideline you’re referring to doesn’t work unless the two sprockets have the same number of teeth, like in a wheel to wheel connection. Are you looking at a chain that goes from a smaller sprocket on a gearbox to a larger sprocket on a wheel? If so, you can still use your center distance calculator to figure out a suitable center distance that gives a chain length that is a multiple of 0.750".
Oh okay so it works for same sprocket size. I was looking at from gearbox (15t) to wheel (32t).
I get what you mean though. So if I were to use the .750 for “pitch” in the calculator the number of links needed would be accurate If I am using #35 chain if I am trying to not use tensioners?
Yes I am trying to figure out how not to use tensioners at all on the robot so I will need to figure out the correct center to center Distances for gearbox to wheel, gearbox to wheel and also wheel to wheel. I am just confused on the whole concept really. It was only a couple bullet points in 1114’s presentation.
We don’t tension chains either. We use the center to center calculator in inventor to give us a starting point, then as the chains stretch, we put spacers underneath our gearbox to tighten the chains. We played > 35 matches this year on the competition bot, and we never had to adjust, but after a weekend of driver practice, it helps to put some spacers under the gearbox. Try to keep the chain runs as short as possible.
Oh okay, Do you have any suggestions on how to do this concept well?
Here is a tool from Dr. Joe that you could use. We used it for our arm actuators* and did not experience any problems**. The interface is not beginner friendly though, and unless you figure how to use the goal seek feature in excel, you will be playing a game of guess and check to get a good distance.
Alternatively, you can use this tool from Paul Copioli. It is much more user friendly, but isn’t as powerful as Dr. Joe’s tool. Also, the only person who has confirmed it actually works (as far as I know) is Paul Copioli himself, but I personally would be inclined to trust him.
Or, if you are too macho for these wimpy excel sheets, Paul Copioli provides the actual formula he uses in the his calculator:
of Links = 2CD+ N/2 +n/2 + (N-n) / (2PI)]2 / CD = 62.563
Round to nearest # of even links L = 62 links
Recalculate CD in inches using following formula:
Pitch/8*{2L – N – n + SQRT(2L – N – n )2 - 0.81*(N - n)2]}
CD = 4.429” … but we are not done.
Add 0.012” for #35 Chain
Add 0.018” for 25 Chain CD = 4.447”
Source: http://www.fightingpi.org/Resources/Business/MSC%20Seminars/2013%20Files/2013%20Copioli%20MSC.pptx
Not so macho now, huh 
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*given that the c-c distance was only ~4in and we were using it at 8:00PM on bag and tag day, so we weren’t that picky.
** except for that 1 time where a bolt head got into the path of the chain and the master link eventually snapped, but I doubt that this was a problem with our c-c distance.
OK, 15t and 32t, no problem.
If we assume a “long” chain, the wrap on each sprocket is about 50%, and the formula still works.
Let’s assume a ‘short’ chain though, c-c around 4" maybe?
15t gets 7 links, not 7.5, because it doesn’t wrap all the way around. YOU can calculate the exact number. If you don’t know the geometry, try doing it graphically - draw it to scale on paper.
32t gets just a hair more than 16, call it 17.
7+17=24, a nice even number.
If we pick a distance as a multiple of 3.75, it would almost be perfect, except the chain is at a slight angle, so the chain needs to be a hair longer…or the c-c needs to be a bit shorter… than normal c-c. So if I pick 3.75", I’d go with 3.7 and accept a little looseness or 3.65 and struggle installing the chain.
Again, YOU can do the math to figure exactly how much longer 10 links will be at an angle of (need to figure that too…)
The whole point is, if you can do the math, it’s just geometry and trigonmetry. If you can’t, or need help, draw it out to scale on paper. (a 15t sprocket can be depicted with a (15*.375)/Pi diameter circle). Measure the angles or calculate them, both are perfectly acceptable methods. And, in your head, know the answer will be just a bit short of (N*.375), so if your method gives such a result, you can know you are spot on.
Team 1640 uses this calculator by our head mentor, which automatically finds the actual correct wrap around each sprocket (or pulley), and finds the closest correct (integer link) distance for any given input. Just put in your 2 tooth counts and a ballpark of what you want the C-C distance to be, and it’ll spit back the true answer.
It’s been great for us, though being swerve drive we don’t use long chain runs very often. We’ve never had a C-C chain issue on our swerve modules in 4 years, and we’ve just shy of 90 matches this year to date. It also works perfectly for timing belts, which we have used over long distances.
So Paul suggests spacing out sprockets farther apart than exact C-C? Doesn’t that really increase the bearing load?
Not knowing the context in which he said that, I would assume at least part of this is to take up mechanical slop in the average team’s system.
If you go exact c-c, you’re actually going to be under that number by whatever slop exist in all the holes and shafts in the system.
In his spreadsheet calculator, he recommends adding .018" for #35 chain and .012" for 25 chain.
From my experience, the actual calculated C-C distances are exactly where you want the chain to be, but due to slop between the bearing and the frame, the hex shaft and the bearing, and the hex shaft and the hex bore in the sprocket, you can have a decent amount of slop that reduces the effective C-C distance.
You could always consider using belts. Much less headache that way, and they’re more efficient/make less noise, to boot! 
My only real issue with belts is that they’re thick. They take up a lot of valuable horizontal space on the robot that can be used for mounting electronics and such. Also, Vex doesn’t (at least yet) have GT2 profiles for VersaPulleys (I’m absolutely addicted to the VersaHub). We could probably get them manufactured, but I’d rather use our machining resources for other stuff.
If anything, c-c distance is much more critical with belts than it is with chain.
Yes, but they’re far more forgiving of minor alignment slop, and you don’t have to worry about integer numbers of links, and they don’t stretch.
I’ve done four drives with belt in my time in FRC, and I think four or five with chain. I’ve seen at least an order of magnitude more problems with the latter.
I disagree with all three of your claims.
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You can run chain considerably looser than exact C-C (because of alignment slop), and it will work, though it will be noisy/sloppy. It will continue to transmit torque until it is loose enough for the chain to slide off of a tooth, which requires the chain to be quite loose. Belts will slip if there is slop or if the center to center is to small, and snap if they are too tight. Chain is much more tolerant of C-C misalignment.
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I don’t understand this comment. Belts require an integer number of grooves/teeth, and chain does too. I don’t see how this can be an advantage of either.
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Belts stretch, but admittedly, less than chains.
I have been in charge of drive chain on the past 6 robots I’ve been involved with. We have never had a single issue. We’ve never broken a chain, as they’re rated for considerably larger loads than belts, we never thrown a chain on a competition robot, as we always cover them and ensure sprockets are aligned, and we’ve never had any other failures. We haven’t always had access to great machining resources, and we’ve never had a problem.
I have been in charge of one belt drive, so I realize my experience with them is limited, but they’re thicker, they slip, and we had a belt snap. So we’re going back to chains.
On the original topic, I find that putting in a floating idler sprocket, like this (http://www.team228.org/gallery/106/slideshow/connecticut-regional_88998-37b1e.jpg), is the way to go for chain. If you want it tighter, move it closer to the smaller sprocket, and if you want it looser, move it away. It’s easy, cheap, light and adjustable.
I have never, ever, ever seen a HTD belt come off of a sprocket, no matter how hastily or imprecisely-assembled. I’ve used quite a few HTD belts. I’ve seen sprockets buck chain regularly for my entire experience in FRC. So I think we’ll just have to disagree here.
- I don’t understand this comment. Belts require an integer number of grooves/teeth, and chain does too. I don’t see how this can be an advantage of either.
You’re correct here, I meant to edit that out of my post but you responded first.
- Belts stretch, but admittedly, less than chains.
Not by any appreciable amount in any drive I’ve worked with.
I have been in charge of drive chain on the past 6 robots I’ve been involved with. We have never had a single issue. We’ve never broken a chain, as they’re rated for considerably larger loads than belts, we never thrown a chain on a competition robot, as we always cover them and ensure sprockets are aligned, and we’ve never had any other failures. We haven’t always had access to great machining resources, and we’ve never had a problem.
I have been in charge of one belt drive, so I realize my experience with them is limited, but they’re thicker, they slip, and we had a belt snap. So we’re going back to chains.
On the original topic, I find that putting in a floating idler sprocket, like this (http://www.team228.org/gallery/106/slideshow/connecticut-regional_88998-37b1e.jpg), is the way to go for chain. If you want it tighter, move it closer to the smaller sprocket, and if you want it looser, move it away. It’s easy, cheap, light and adjustable.
It seems we’ve had different experiences. It’d be nice to know what we’ve been doing differently, since I’ve never ratcheted a belt nor have had one snap in competition, but have thrown plenty of chains (due to misalignment, bad masterlinks, and other causes).
Chains take 2 minutes to fix, unless your design has them hidden away stupidly.
Even an easily accessible belt requires significant work to replace, in comparison. WCD makes this easier, but still a major PITA.
TL;DR, I would rather repair the same problem in 2 minutes 5 times, than a 10 minute fix once.