Chain Tensioning
 

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/d.../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!

Re: Chain Tensioning
 

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:

1. The number of links on 1/2 of the sprocket is an integer (one lnk per tooth, right?)
2. 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.
3. 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....

Re: Chain Tensioning
 

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".

Re: Chain Tensioning
 

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?

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

Oh okay, Do you have any suggestions on how to do this concept well?

Re: Chain Tensioning
 

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:
Source: http://www.fightingpi.org/Resources/...oli%20MSC.pptx

Not so macho now, huh ;) .

*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.

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

So Paul suggests spacing out sprockets farther apart than exact C-C? Doesn't that really increase the bearing load?

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

You could always consider using belts. Much less headache that way, and they're more efficient/make less noise, to boot! ;)

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

If anything, c-c distance is much more critical with belts than it is with chain.

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

I disagree with all three of your claims.

1. 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.

2. 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.

3. 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/s...998-37b1e.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.

Re: Chain Tensioning
 

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.

You're correct here, I meant to edit that out of my post but you responded first.

Not by any appreciable amount in any drive I've worked with.

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).

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

Our team has never used belts in a drive system, but we've always used chains. Our experience is that chain issues are mainly caused by stuff getting stuck in the chains or the sprockets, or the sprockets being bent or misaligned (located incorrectly axially on the shaft, not C-C being wrong).

From my point of view, chains are little better for center to center, as they do actually work if they're too tight, or too loose. That said, it's not that difficult to get the center to center correct on either, so it's not that much of a benefit.

I think a disadvantage of belts is that it's tricky to get a center to center close to what you want. With half links on chains, you can get tons of different chain lengths, but it's difficult to find belts with strange numbers of teeth.

Chains are also absolutely terrifying at 5,000 rpm, and belts aren't great for higher torque.

Re: Chain Tensioning
 

FWIW, I'm pretty sure even if I were to weight failures by repair time my experienced rate of chain failures would be about ten times greater than belt failures. Maybe I've been (un?)lucky.

This has been my experience, as well, though I've had a few come off due to them becoming exceptionally loose if they weren't tight to begin with and had time to stretch.

This is also certainly true.

Re: Chain Tensioning
 

In our 7 years of being a team, we've never once broken a chain, in any application. I don't have comments on belts though, I've ever used them.

Re: Chain Tensioning
 

"Dead spaced" belts tend to align themselves on a pulley. Sprockets need to be aligned. The holes need to be cut accurately, but there's less opportunity for error in assembly and maintenance of belts than with chain.

Re: Chain Tensioning
 

Really, it depends on what kind of drivetrain you have. Our team had separate methods.

If the wheels didn't need to worry about being bent (as in, they can just be a straight line) we used the simple method of just getting the chain, wrapping it around the two sprockets, and seeing which link to break. We broke the right one, and used the master link to join them. Simple.

However, there may be a time when you can't make the chains go straight to the wheels. We had this issue in 2010, when we needed our robot to be able to traverse the bumps, but a straight link for the chains would grind on the bumps, causing damage to the field and the robot. To fix this issue, we basically had the chains go across the robot, parallel to the frame, before bending down to the sprockets. However, if we did this method, measuring the chain the old way would be incredibly difficult. That is why we made tensioning devices to help keep them taut. What we did is we made a mount to go under the robot, putting two of the hard white plastic spools on them. They were attached to the underside of our robot by two bolts (we always use extruded aluminum for our robots, we just put a couple of cut bolts into the groove for placement, held in place by lock nuts and washers). if we ever needed to replace chains, we simply released the tensioners by sliding them towards the center, loosening the tension so it was easy to remove the longer chain. When replacing it, we made the chain long enough where it would at least reach the two and be able to have enough give. We strung them over the tensioning spools, then slid them out until the chain was taut, and bolted it into place. The wheels were still able to turn with our direct drive, and the chains were well out of the way.

But, in all honesty, that's a very odd circumstance, like going over tall obstacles or having tiny wheels and big sprockets. The easiest thing to do is just dead reckon it by making the chain the right size and measuring the links needed. If anything, a slightly loose chain is better than an overly tight one in any case.

Re: Chain Tensioning
 

This summer we are experimenting with non-adjustable wheel positions, and chains inside our side rails.

The first rail we made had nominal C2C distances. While it is functional, the chains sag enough to touch the inside of the rail tube, making the drive assembly noisy when running. For the second tube, we took a WAG (embarrassing to admit, for engineers), and increased the C2C distance about .008". This resulted in saggy chains again. After that, we did what we should have done from the start, and actually measured the C2C distance of tensioned chains, instead of just guessing.

We made this test setup, first using the chain from our experimental drive train, with a nominal C2C distance of 10.75". Note that the sprockets, bearings and shafting are all 1/2" hex type, all from VEXpro. There are slip fit tolerances between these parts that require the C2C distance of the bearing pockets to be greater than nominal to achieve a tight chain. Also note that the neither the hex holes in the bearings nor the hex holes in the sprockets are truly concentric. This leads to visible wobble in the OD of the sprockets, when they spin. This causes the chain tension to vary, and is a source of vibration and cyclic fatigue to the overall drivetrain.



To test whether any C2C variations were related to the length of the chain, we also tested a nominal 4" C2C chain setup.



The chain tension was set by anchoring one of the bearing blocks, pulling the chain "finger tight" and locking down the other bearing block. We then spun the chain by hand to observe that it could run free, and hand checked the chain tension the same way we do in the pits, to be sure it was tensioned comparably to our competition drive trains. Then we removed the chains, sprockets, shafts, and upper bearings. The upper bearings have a tight slip fit. To remove them, it was necessary to insert the end of an axle shaft and wiggle it around, working the bearing loose. This left the two bearing blocks with the bearing pockets exposed.



We then used the milling machine edge finder and the dimensional readout to find the C2C distances (we actually measured to the left sides of both bearing pockets).

Here are the results:


We concluded that the increase in C2C distance was due to the tolerance stackup of the parts, and not tightly related to the length of the chains. We plan to incorporate a .019" to .020" delta to our nominal C2C distances in furute designs.

Re: Chain Tensioning
 

This is some awesome information. I'm having a difficult time telling from the picture, is that #35 chain or #25 chain? (My guess is #35). Would you be able to test if switching to the other size would result in any more or less delta? My instict says to add more for #25 chain (given the same C2C) because there are simply more chain links and the tolerance will stack up even more.

Also, I'd be curious on the additional Run-out the hex broaching causes to the bearings.

Re: Chain Tensioning
 

They actually did it with #25 chain.

Re: Chain Tensioning
 

As Matt said, this was done with #25 chain.

We believe the tolerance stack up is due to the following fits:
-sprocket bore to shaft
-bearing bore to shaft

The bearing fit into the bearing blocks was a tight slip/light press, with no observable play.

We tested two lengths of chain specifically to see if the chain itself was a contributor. If it were a major contributor, we would expect the delta values to be somewhat related to the lengths of the chain. Instead, the delta values for the two cases were nearly identical (.001" is probably within the margin of error of our measurements). We concluded that the chain was not a source of the delta.

Another possible contributor could be that the sprocket pitch diameters are undersized. It would take some thinking, and maybe a different test setup to check this.

Re: Chain Tensioning
 

Personally, regardless of belt or chain, I would use tensioners.
Cams are good, but they need to be cadded before you add them into a robot. You cannot just throw them on without considerable effort, depending on how your drivetrian looks.
Screw tensioners are really nice, as they use a solid bearing block and the tensioning is very fine, but they do add a pound or two to the drivetrain depending on their design. I think this would be my preferred method of tensioning chains.

Belts: Too wide for my liking. Width on a belt is several times more than the width on a chain, which decreases the amount of electronics space you have. Plus, if by some fluke a belt breaks, we would have to take apart a gearbox to access the pulleys. I would rather have 4 5-minute chain snaps during competition than 1 15-minute belt break.

Admittedly I hate (or better, despise) working with master links, but 221's chain attachment tool is great. Just make sure you design with enough space in the chassis to use it.

Re: Chain Tensioning
 

Tensioners on a belt are decidedly unnecessary, in my experience, and will do nothing but add additional friction.

Re: Chain Tensioning
 

Friction, but only if you use on-belt tensioners. Plus, that would be pretty low anyway depending on the setup.
I'm thinking more of cams or screw tesneioners. Even better would be to use 192 method that they used on the gearbox: screw holes that were slightly farther or closer to the other side of the belt, from -50 to +50 thousandths. It's a really clever system.

Re: Chain Tensioning
 

Has anyone used the nautilus cams from WCP? We are planning on incorporating them into one of our off-season drive bases this fall as the chain tensioner. I guess I don't understand what the big deal is about using tensioners anyway; why would that be less of a problem in a design than doing c2c calculations? I've worked with teams that have done both, with both chain and belt, and it seems to me that the advantage would tilt toward adjustability.

Re: Chain Tensioning
 

You need the bearing blocks for that to work. The cams are there to adjust the blocks. The advantage of chain tensioners is to keep the chain in proper tension at all times. Over time chain stretches especially under aggressive driving.

Re: Chain Tensioning
 

If you have a significant length of chain, and need to take some play out of it, a simple method of easily taking out slack is to wedge a loose plate sprocket between the chains as such. I suppose this could work for belts too.
The green sprocket is not fixed or touching anything but the chain, and remains in place with no need for support. Since it is not transferring any load either, it can be made of extremely pocketed aluminum and can be super light weight.

Re: Chain Tensioning
 

keep in mind that you have to stop it from falling over, especially under load.
It will auto-align, but having a small bearing block setup would be good.

Re: Chain Tensioning
 

Why would it fall over? The tension of the chain and the two existing fixed sprockets should keep the chain level.

Re: Chain Tensioning
 

Is this "broscience" or have you ever had an actual issue with floating sprockets.

Re: Chain Tensioning
 

Yeah, 100 did last season, and I just loved them. Basically, take anything that slides (on slots, as a bearing block, a VersaBlock, etc) that needs to be tensioned and put a cam next to it. Your bearing block/whatever will never slip, and the cams are real easy to design in and use.

I think of the tensioner vs c-c debate like this. If you want to put a chain/belt between two points in something that is getting milled (like a plate or piece of tubing) and you don't care a ton about slack in the system, go with exact c-c spacing, as it'll make your life a ton easier. If you're going between two points where it's difficult to get good tolerances (like from the bottom of the robot to the top of a big welded superstructure to drive an arm) or where it's critical you dial in the tension so it can handle lots of torque, go with sliding tensioners. It'll be much easier to dial in the exact tension you want, and you can soak up the tolerance stack up through the sliding tensioner system.

Re: Chain Tensioning
 

This is very, very sound advice.

It occurs to me that I should try using one of those cams on a piece of 80/20, to capitalize on its natural sliding capability.

Re: Chain Tensioning
 

You don't need to go exact c-c and have a bunch of slack though. Use the information provided in this thread to figure out the correct distances for a tensioned chain.

Re: Chain Tensioning
 

For tensioning chain, we had great luck with these this past year:

http://www.zoro.com/i/G0347697/?category=6001

Slipped off twice that I can remember and it was about a minute to get it back on and we added a little extra tension (they are adjustable) after to take care of it.

We were happy with them. They are also completely off the shelf so we'll be able to use them again.

Re: Chain Tensioning
 

True, but if you had a design that was intolerant to any slack, I feel most comfortable with a sliding tensioner in the system.

Re: Chain Tensioning
 

I've tried it a couple times just as a concept for chain tensioning during the season. The sprockets had a tendency to fall out. Just my experience.

Re: Chain Tensioning
 

We had a couple sprockets in our <shudders> crab drive, and those were in <again shudders> long runs of horizontal chain, none of which fell out*


* That drive had 99 billion problems, chain slack being one, but sprockets falling out was not

Re: Chain Tensioning
 

Same experience. Floating sprockets were one of the better/less painful tensioning methods 2220 used back when we used chain. I think we used them in one of the worst possible cases on our swerve drive (horizontal instead of vertical, lots of slack), but they still didn't give us any problems.

That being said, I vastly prefer trying to get as close to c-c as possible with minimal tensioning (on both chain and belt). Even if you still need a bit of tension, going as close to c-c (adjusted or exact) as you can means you can get away with a less beefy tensioning mechanism (in most cases).

Re: Chain Tensioning
 

If a floating idler sprocket is falling out of a chain run, the chain run is undertensioned. That's fixable just by moving the idler sprocket.

It's actually really important that tensioner sprockets "float". It is actually pretty hard to locate a single static position for a sprocket to engage both the top and bottom of a chain run, and this spot will change over time. In use, a "floating" sprocket may oscillate to maintain chain contact, but as long as the sprocket has reasonable contact with both sides of the chain it will stay roughly in the same place.

Rigidly mounting a "floating" tensioner sprocket is a very bad idea for this reason, plus it defeats the purpose of an idler sprocket tensioner - a simple solution to tensioning that can be adjusted later.

It's important when participating in a technical discussion to make your posts based on experience, or at the very least explain the logic of why you are making a recommendation. It's a mistake I've made before (probably still do occasionally, to be honest) and in the worst case it results in other teams making costly design decisions based on what is really just your conjecture. This is especially a big deal in communities of "smart" people who are usually correct on intuition alone - it's easy to become complacent and not give your ideas the scrutiny you should. So please, be clear with your rationale and tell people when you aren't speaking from experience.

Disclaimer: I have not used floating idler sprockets specifically on an FRC competition robot before, only on other projects using roller chain. YMMV.

Re: Chain Tensioning
 

Yeah, those sprockets never came out. And it makes sense that they should float. But I'm an EE so don't take this opinion as gospel.

Geez, 99 billion is a little dramatic - that drive had one, and only one, major problem. The turning motors were not geared properly plus they were window motors, my bad on both counts. We just didn't have the resources, time or money that year to re-design. Rick's drive was darn clever in many ways (for example the alignment block for the spur gears).

Have any CDers experienced significant chain "stretch" during a season? It is really bushing wear I reckon, the links do not actually stretch - I think. Is some chain better than others? Is it something you typically take into account? Is it a reason to use a tensioning device? Does 25 chain "stretch" more or less than 35 chain?

Re: Chain Tensioning
 

Not entirely sure what you mean by bushing wear, but in 2012 team 2220 had some major issues with our chain stretching in the finals at the Lake Superior regional-- which lead to us throwing a chain. That was why we added (hacked together, badly) tensioners for our next tournament. Come to think of it, it might not have been the chain itself stretching, but certainly sometime between ship and the end of Lake Superior the drive train wore in enough that our relatively tight chain was visibly slacking. We certainly blamed it on the chain at the time, but I'm not sure what else could have worn-- the bearings? The sprockets?

Regardless, a tensioner "fixed" the problem for North Star and MSHSL.

Re: Chain Tensioning
 

I have read that bushing-less chains "stretch" less than chains with bushings. The "stretch" comes from bushings or holes in the links wallowing out. It is particularly bad news if one uses aluminum sprockets since a worn chain puts more pressure (than normal) on the teeth under load.

For your team, it sounds like a tensioner was the solution. On the other hand we ran belts this year and broke quite a few (mostly due to under-sized pulleys). Those belts that did not break did not seem stretched leading me to believe the chain (in last years similar drive train) did stretch a little. Interesting problem...

Re: Chain Tensioning
 

Okay, that makes sense. Thank you for explaining that. I would hazard a guess that that was what happened.

Re: Chain Tensioning
 

The lovejoy tensioner that I linked to previously is made exactly for this. It rides with the chain and maintains tension. It's basically a captive idler on the end of an adjustable arm. They worked great for us at the 3 events we went to and assuming we can use them again and don't have to radically redesign the drivetrain for any reason, we will be. I'll try to get a video created the next time we are running the robot. It might be helpful for some teams. Fair warning, they are somewhat heavy being steel but the weight is down low and it really wasn't too bad. We didn't have any weight issues this year.