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Matt_Kaplan1902 03-04-2004 22:19

Chain Drive Question
 
I'm trying to find out if we have our chain tensioned so it doesnt hang down would that make the chain stronger and less likely to break?

computhief263 03-04-2004 22:23

Re: Chain Drive Question
 
Quote:

Originally Posted by sigmakid108
I'm trying to find out if we have our chain tensioned so it doesnt hang down would that make the chain stronger and less likely to break?

I dont know about stronger, but definatly less likely to come off the sprocket. Just be carefull to make sure the sprockets are inline w/ each other.

coreyjon 03-04-2004 22:30

Re: Chain Drive Question
 
Quote:

Originally Posted by sigmakid108
I'm trying to find out if we have our chain tensioned so it doesnt hang down would that make the chain stronger and less likely to break?

It wouldnt cause the chains physical strength to increase, but more likely keep if from slipping off the sprocket. If your chain is too loose, you could try removing a full link, and replacing it with a half link, if its really loose, take out enough chain to fix the problem, you could just add a tension device, but this adds weight, and removing chains takes away weight :) . Generally speaking, you dont want your chain so tight that it puts alot of pressure on the axles, Try getting the chain as short as possible without trying to stretch the chain. Chain will stretch after you run it for a while, so most of the time after say the regional event is over, you can remove a half link or so to keep the chain tensioned.

Rob Colatutto 03-04-2004 22:31

Re: Chain Drive Question
 
Better tensioned chain will have less backlash when you are switching from driving forward to driving backwards. Wether its tensioned or untensioned you will get similar moments on the chain and run the same risk of snaping the chain in either case. Preventing the chain from hanging down is not something I'd suggest doing and as Tom said, it makes it easier for the chain to come off your sprockets. The precission you need in lining up sprockets also varries for different chain sizes as does the chains strength.

kacz100 03-04-2004 23:21

Re: Chain Drive Question
 
Just some friendly advice (for next time) go find the formula that will tell you the center to center distance of the sprockets so that the chain fits and no tensioner is needed

Lil' Lavery 03-04-2004 23:38

Re: Chain Drive Question
 
You might also want to make sure your sprockets are in line with eachother, so your chain isnt being bent. this will cause the chain to strecth further. Keep in mind that chains are one of the times where my "measure once, cut twice and claim the mistake was a lightening hole" philosophy doesnt work.

Kevin Sevcik 04-04-2004 01:03

Re: Chain Drive Question
 
One more note. If you do tension the chain, make sure to tension is properly. If the tensioner is spring loaded, you're fine. if it's not, make sure you don't tighten it too much. The chain should really have a small amount of slack in it at rest anyways. if you over tighten/tension the chain, it will start binding and wearing the rollers and sprockets too quickly.

Cory 04-04-2004 13:52

Re: Chain Drive Question
 
Team 258 made the mistake Kevin just described this weekend, and after tensioning it quite a bit, they sheared their drive shaft in half.

The only thing I like about the 80/20 we use is the ability to slide our wheels to compensate for chain slackness.

Cory

coreyjon 04-04-2004 14:25

Re: Chain Drive Question
 
last year we used 80/20, it was pretty nice, made it easy to tension the chains, and attach things like motor mounts and the such, We didn't use it this year, instead we went with 1x3 Al box channel, Hopefully next year the team will use 80/20, It looks good, and works well. What more could you ask for (except maybe for a cheaper price tag).

FotoPlasma 04-04-2004 14:34

Re: Chain Drive Question
 
Quote:

Originally Posted by Cory
Team 258 made the mistake Kevin just described this weekend, and after tensioning it quite a bit, they sheared their drive shaft in half.

The only thing I like about the 80/20 we use is the ability to slide our wheels to compensate for chain slackness.

Cory

Heh. Actually, we made some stupider mistakes than that. The tention on the chain was fine, but the output shafts from our drive gearboxes were cantilevered out into the open. Words of wisdom: never do this. Another problem we had was that the shaft was pinned off-center. If we hadn't had these other problems, we would have been fine.

Ryan M. 04-04-2004 19:19

Re: Chain Drive Question
 
Quote:

Originally Posted by coreyjon
last year we used 80/20, it was pretty nice...

80/20 worked well for us, also.

Just to be different, the advantage of tensioners is that they make the streching of the chain much less of a problem. :)

BillyGoats 04-04-2004 20:08

Re: Chain Drive Question
 
we have always used chain and never used a chain tensioner. This year we did not have one drive train problem... usually we have a chain fall off or something once. This year we used 1inch shafts (we kept bending the 1/2 inch shafts) we also used spacer on the shaft so that they can not walk off. below are some pictures.

This is an ok picture of our left side the drills and you can see the spacers.

http://www.valleytech.k12.ma.us/robo...lectronics.JPG

Just another alright shot of the drive system.

http://www.valleytech.k12.ma.us/robo...004_211Bot.JPG

Cant find any others but feel free to take a look.

http://www.valleytech.k12.ma.us/robo...photos2004.htm

or

http://www.valleytech.k12.ma.us/robo...CFRegional.htm

maclaren 05-04-2004 18:42

Re: Chain Drive Question
 
ALWAYS TENSION CHAINS!!!!!!!!!!!!!!!

The reason is that you will have better efficency, better durability, better drive train response and an overall better drive system.

I'm partial to tensioning mechanisms beacuse chains will loosen over time. An adjustible tensioner WILL save you time. Adjusting a tensioner is far faster then changing the length of a chain.

There are several guidelines when using chain.

1. Lubricate!! chain will always need lubrication especialy chain sizes of #35 and smaller since they don't have rollers in them. IE built in bearings because of their size.

2. Proper alignment! is crucial for long lasting and efficent chain implementation. Any misalignment is bad engineering practice.

3. Tension chains! loose chains snap and whip under varying loads which is exactly how they are run in FIRST. Plus they last longer when properly tensioned. Properly being the keyword. The addition of an adjutible tensioner is the ideal tensioning solution in the perspective of tensioning not necisarily from the perspective of weight.

4. Design chain for short spans long spans are hard to tension properly and will put unecssary strain on chains.

5. Proper axial support. NEVER, NEVER, NEVER EVER cantilever any axle if you can posibly help it always double bearing whenever possible place a bearing on each side of any force applied. In other cases beef up axle size, and place additional bear on the other side of the already existing bearing, and shorten axle length to reduce shear force.


FOLLOW GOOD ENGINEERING PRACTICES and every component will realize it's full potential and perform to the best of it's ability giving you the best robot possible.

Grommit 05-04-2004 19:36

Re: Chain Drive Question
 
We have been using chain drives for about 7 years and until this year have not ever used a tensioner. For drive shafts, we would just have someone pull as hard as possible on the wheel while tensioning it. This has never caused any problems for our drive shafts, chains or wheels, but we have a colorful history of chains coming off because they were loose.

This year we used a tensioner, not spring loaded, just a plastic adjustible runner, on our drive system, on the chain that allows it to swerve. The reason was not to protect the chain or make it easily tensioned. The real reason was that it was impossible, with our tiny sprockets, to get the wheels all aligned without tensioners.

Travis Covington 05-04-2004 20:37

Re: Chain Drive Question
 
Quote:

Originally Posted by Grommit
We have been using chain drives for about 7 years and until this year have not ever used a tensioner. For drive shafts, we would just have someone pull as hard as possible on the wheel while tensioning it. This has never caused any problems for our drive shafts, chains or wheels, but we have a colorful history of chains coming off because they were loose.

[speaking as a previous MVRT member]

You need to make sure people know that instead of an actual tensioning device, we had slotted wheel mounts. To make the chains "taught" we loosened the wheel mounting bolts, slid the wheel back until the chain was taught, and tightened the wheel mounting bolts. Before we made that nifty modification, in 98 and 99 we used teflon blocks that were slid under the runs of chain and held in by screws/velcro. Both ways seemed to work just fine.

[speaking as a current mentor]

The past 2 years on 968 we used slotted wheel mounts... which worked fine (simply because we used a bosch type frame) which enabled us to easily slide the wheel mounts on the slotted chassis material with t-nuts.

This year, we chose to weld the frame (to reduce weight) and decided to come up with a tensioning device. Strangely, we came up with something similar to 115, and used HDPE rollers of various sizes. When the chain initially stretches, we simply install a larger diameter roller and the chain is tight again. (this way isnt as elegant is I would have liked, but it was about as simple and as lightweight as we could think of at the time)

Note: All of these examples are #25 chain. #35 chain is much more forgiving and sometimes does not require tensioning.

Adam Y. 05-04-2004 21:12

Re: Chain Drive Question
 
1 Attachment(s)
Quote:

Just some friendly advice (for next time) go find the formula that will tell you the center to center distance of the sprockets so that the chain fits and no tensioner is needed
Err that is nice and all but I really doubt you would be able to get an answer that would really work nicely. You are probably still going to need tensioning. Anyway here goes the process.
Chain Length=L
Center Distance(Distance between sprokets center)=C
Pitch Diameter of Larger Sprocket=D
Pitch Diamter of Smaller Sprocket=d
Errr wow is that confusing or what. I hope someone can find a simpler form than what I just typed. Instead of typing it I scanned them. Unfortunately its sideways.

sanddrag 06-04-2004 02:46

Re: Chain Drive Question
 
Quote:

Originally Posted by Adam Y.
Err that is nice and all but I really doubt you would be able to get an answer that would really work nicely. You are probably still going to need tensioning. Anyway here goes the process.
Chain Length=L
Center Distance(Distance between sprokets center)=C
Pitch Diameter of Larger Sprocket=D
Pitch Diamter of Smaller Sprocket=d
Errr wow is that confusing or what. I hope someone can find a simpler form than what I just typed. Instead of typing it I scanned them. Unfortunately its sideways.

What type of units is L in? Where exactly do you measure the length of chain? Overall length or between pins or what? Or do you count the number of links?

Matt Adams 06-04-2004 03:49

Re: Chain Drive Question
 
DANGER: Shameless Plug!


Matt

Adam Y. 06-04-2004 09:27

Re: Chain Drive Question
 
Quote:

What type of units is L in? Where exactly do you measure the length of chain? Overall length or between pins or what? Or do you count the number of links?
Might as well borrow a bit more from my book.
Quote:

These equations will provide physical lengths. To determine the correct chain length and center distance, use this procedure:
1)Estimate what the center distance (C) should be.
2)Chain lengths are based on an even number of pitches. To determine the number of pitches, divide the chain lenth by the pitch length. If the number of pitches of the chain is not an even number, round up or down for an even number.
3)Multiply this result by pitch length to get the final physical length of the chain.
4)Plug the pitch length into Equations 19 and 20 to calculate the center distance between sprokets.
The book was a whee bit vague as to what units L was in. Im guessing that it should be whatever untis the pitch is in. Metric for metric chain and the same for American chain.

maclaren 06-04-2004 20:41

Re: Chain Drive Question
 
Oh yes I forgot to mention something FRICTION IS YOUR ENEMY. Gloified plastic chain gaurds are a large drain on efficeny. We used idler sprockets which have built in bearings. We took the idler sprockets and mounted them to a small piece of 80-20 and then bolted the slide to another piece of 80-20 and you simply slide the idler sprocket and then tighten the bolts.

It's simple and easy.

Astronouth7303 06-04-2004 20:44

Re: Chain Drive Question
 
Oh, right, that.
One word: WEIGHT.

Kevin Sevcik 06-04-2004 22:07

Re: Chain Drive Question
 
I'll toss in my two cents. In every chain length formula I've seen, L is is actually in number of pitches, as opposed to and measure of length. and the chain doesn't HAVE to be an even number of pitches, it's just good practice not to use offset links.

maclaren 07-04-2004 00:15

Re: Chain Drive Question
 
Quote:

Originally Posted by Astronouth7303
Oh, right, that.
One word: WEIGHT.

Yeah that's true weight is allows a problem in first however that is also why you follow good engineering practices.

Just for the person that posted the picture of their teams robot using short section of chains. AVOID CAST IRON LIKE THE PLAUGE and you will save at least 1-2 lbs per bearing.

The best way to remove weight is to go to aluminum axles and and fixed axles with live wheels. It takes a little effort but could give you at least 5-8 lbs back out of your drive train. Also the wheels will be easy to install, remove, and repair.

Dick Linn 07-04-2004 15:25

Re: Chain Drive Question
 
A couple of times, we've mounted each drill motor and small sprocket shaft on an aluminum plate. This plate has slotted holes and can be moved in relation to the drive wheel to tension the chain when necessary. We can also use washers between the plate and the chassis rails to get additional range (yeah, the slots weren't long enough...). Keep the chains lubed, snug and aligned and you'll have less chance of breaking a chain when it tries to climb up and over the sprocket teeth. Also avoid tiny sprockets.

Dan Richardson 08-04-2004 01:26

Re: Chain Drive Question
 
Another way to tension chain besides Slotted wheel mounts and tensioners is an adjustable gearbox.

This year we built our gear boxes on the frame with a sandwhich mount and shims. WHen the chain would get loose we could slide the gear box up or down ( depending on location of the 2nd sprocket ) and tension that way. It worked out really well for us. It ended up being much easier than sliding every wheel like we used to in old chain drive designs. Just loosen a few bolts, slide, tighten the bolts up again and walla Tight chains.

Its probably late to consider something like this but Its just an interesting idea that works really well.

Grommit 08-04-2004 02:18

Re: Chain Drive Question
 
Yes, Travis is quite right, I neglected to mention that our wheels have always been mounted such that they could slide for us to tension them. What I intended to explain was that spring tensioning is not the only solution, and that slots are a convenient alternative.

Also, note that Bosch extrusion particularly has very easy, built in slots that hold well (if you are using 30mm X 30mm, that is). There are T-Blocks available (at great expense :( ) that make mounts very solid.

Even if you do not wish to use Bosch, and you don't like your chassis to have a big aluminum plate base, you can still use slots. Take your welded chassis and bold on a plate, almost as a gusset, but with a big slot and two smaller ones on each side to hold the wheel and two mounting blocks. The slots on the side allow you to adjust the mounting block position. But it gets better: many teams have trouble tensioning this, because it takes two people: one to pull on the wheel and one to tighten, and it just isn't fun. The clever plan is to use a custom tool. Mount a block with a threaded hole in front of the mounting block slots. Use a larger hold for greater control and ease of use. Then take a long bolt and put it in the thread, so that when you tighten the bolt, it will push on the mounting block. Then you can even use a drill (on very low clutch setting, maybe as low as 11 or so) to tighten the mounting blocks for you, and they won't bounce back. VERY useful for quick tightening jobs.

ahecht 09-04-2004 13:55

Re: Chain Drive Question
 
I'll add in another plug for the adjustable sprocket-box that 190 has used the past few years:



The black line is the chain, the two larger sprockets are idlers, and the smaller sprocket is attached to the motor. On the bottom of the image, you can see the chain returning. By tightening or loosening the bolt, we can adjust the tension. Also, this design assures that we have a full 180 degrees of wrap around the motor's sprocket (this image isn's quite accurate).

The other nice thing is that by flipping this design around on the other side of the robot, and driving the lower part of the chain, we can have both motors driving in the same direction when we drive forward:

EDIT: Just to clarify, on each side, there are two identical lexan plates, one on either side of the sprocket. Therefore the adjustable sprocket is actually supported by two bolts, which need to be adjusted together.

Biff 09-04-2004 14:15

Re: Chain Drive Question
 
Quote:

Originally Posted by maclaren
Yeah that's true weight is allows a problem in first however that is also why you follow good engineering practices.

Just for the person that posted the picture of their teams robot using short section of chains. AVOID CAST IRON LIKE THE PLAUGE and you will save at least 1-2 lbs per bearing.

The best way to remove weight is to go to aluminum axles and and fixed axles with live wheels. It takes a little effort but could give you at least 5-8 lbs back out of your drive train. Also the wheels will be easy to install, remove, and repair.

Like that idea, we considered it but could not figure out a way to get the sprockets attached to the live wheels, Skyway had a bike wheel that was close but it had a coster break installed and was to large anyway. Had to go with the live axles. Last year we got cantiliverd (sp) axles bent, this year we over compensated. We just finished a design in inventor for a 45 tooth #35 chain Aluminum sprocket, With out the weld that holds the hub on inventor says weight is .317lbs or 5.072 oz our target was 6 oz or less.

ahecht 09-04-2004 14:24

Re: Chain Drive Question
 
Quote:

Originally Posted by Biff
Like that idea, we considered it but could not figure out a way to get the sprockets attached to the live wheels...

If you were using the kit Skyway wheels, MMH Ventures sold a hub kit that allowed you to attack sprockets to wheels. You can also make your own hub that clamps onto the wheel spokes, but that requires a bit of machining work.

Andrew 09-04-2004 14:29

Re: Chain Drive Question
 
Machinery's Handbook has all the chain formulas you could want, plus an explanation. Units are usually in chain pitches.

But...
Chains have tolerances associated with the links. As your chain gets longer, the tolerance stack up makes the chain center distance formula less accurate. Even for a moderate tolerance, the droop in a chain will be too much. Idler sprockets, tensioners, rubbing blocks become a must.

Chains also wear with use. So, even if you get the center distance correct in the first place, over time, you will start to see increasing loopiness.

In short center distances (say 2" to 6") you won't see these effects too drastically. But, in longer distances, the situation gets worse and worse.

With the push towards multi-drive motors in a drive system, we have gone with direct drive on all of our wheels over chain drive. We take a hit in the weight department, but we gain in the reliability department and in the manufacturing department.

Grommit 10-04-2004 00:45

Re: Chain Drive Question
 
Quote:

Originally Posted by ahecht
I'll add in another plug for the adjustable sprocket-box that 190 has used the past few years:

The black line is the chain, the two larger sprockets are idlers, and the smaller sprocket is attached to the motor. On the bottom of the image, you can see the chain returning. By tightening or loosening the bolt, we can adjust the tension. Also, this design assures that we have a full 180 degrees of wrap around the motor's sprocket (this image isn's quite accurate).

The other nice thing is that by flipping this design around on the other side of the robot, and driving the lower part of the chain, we can have both motors driving in the same direction when we drive forward:
EDIT: Just to clarify, on each side, there are two identical lexan plates, one on either side of the sprocket. Therefore the adjustable sprocket is actually supported by two bolts, which need to be adjusted together.


That's a great way to keep the chain tensioned! I was curious about your bolt: did you have trouble with the bolt loosening in the competition? Did you use some kind of solution inside to increase friction so the bolt wouldn't slip? Or was that not a problem anyways? Also, was the sprocket shaft on a sliding slot or was there some other way to move the sprocket along?

We might consider something along those lines next year, especially if we added another pair of wheels. It would avoid using three sprockets on a shaft, one for drive and one to each other wheel: we could drive off one of the other two without sacrificing the 180 degrees of chain contact.

Very well implemented idea, thanks for sharing it.

ahecht 12-04-2004 02:50

Re: Chain Drive Question
 
Quote:

Originally Posted by Grommit
That's a great way to keep the chain tensioned! I was curious about your bolt: did you have trouble with the bolt loosening in the competition? Did you use some kind of solution inside to increase friction so the bolt wouldn't slip? Or was that not a problem anyways? Also, was the sprocket shaft on a sliding slot or was there some other way to move the sprocket along?

We didn't have any trouble with the bolt loosening, but if it is a concern, you can coat the bolt with a very thin layer of locktite before inserting it (make sure the locktite is completely dry before inserting, as the solvent will melt the lexan). In 2k3 we had trouble with our chains popping off, but that was due to our flexible frame and flaws in our manually milled (not CNC) 80 tooth sprocket, not our chain tension.

The adjustable idler's shaft slid in a slot milled into the two lexan plates, relying on the tension in the chain to keep it pressed up againt the bolt. The fixed idler's shaft just fit into two holes.

Another thing to note is that if you don't need your motors spinning in the same direction (which we didn't this year because we could pneumatically lock the two sides of the driveline when we needed to go straight), it is possible to design the system without the stationary idler (thus having your chain make a 'z' shape).

Avarik 12-04-2004 03:11

Re: Chain Drive Question
 
Cheesy Poofs had a little rubber chain tensioner last year, though I have no clue what it was actually called (I believe they ordered it from McMaster?)

Basically you would squeeze it in and stick it in the chain. As the chain loosened, it expanded and tensioned it. I am not quite sure how well this worked, but I did hear them say that it never fell off their robot. Try asking them at nats what it was exactly.

Mr. Ivey 12-04-2004 10:44

Re: Chain Drive Question
 
I'm sure the others have given out the knowledge that I'm about to post, but I would like to just sum up my knowledge. It never hurts to have more info out there.

Chains are fun, till they pop off, then you have a problem, because you are dead in the water.

Tensioner
A tensioner will help you avoid the problem of a chain popping off. Reason, it adds tension to the line so there is no play in the line allowing the chain to stay securely on the sprocket. But you must have the tensioned in really really really good alignment with the other sprockets. If it's 1/2 an inch wide from the drive sprocket you have a big problem. Time for example lesson: Little Johnny V-Neun is riding his Schwin Apple Crate, and he shifts to 5 gear, and his chain pops off. Why, his derauler pushed the chain too far past the 5th sprocket. End of lesson. Moral if the tensioner is too far wide it will pull the chain right off your drive sprockets. That's the reasoning behind that point of alignment. Next is free spin of the tensioner. Remember this little guy has to spin with the chain, so you don't want to tack weld it to a shaft, which is obvious. This little guy needs to have something holding him in place to the shaft that it's on. You can use many "holding devices" like washer but steel ones don't allow much spin. You can use ball bearings on the sprocket and have something light weight holding them in which would be really cool but costly in weight and price. I recommend using something to the extent making a designed fault into this system, sorta. You really don't want to mess up your sprocket. So the intentional weak in my opinion would need to be the things holding the sprocket in line, but don't make them out of foam or Krispy Kreme Doughnuts. Use something like nylon washers with lots of lube between them, with a nylon spacer between the shaft and the sprocket. This will allow free spin and good strength holding the tensioner in space. No where do you want them, how many do you want? The best way I think to do a tensioner is using 2, one on each side of the chain, having them on some sort of screw so both sprockets are evenly set on the chain. Orentation of the tensioners yet again in my opinion is in the middle, having them on the outside of the chain, pinching it creating tension. Spreading the chain out is inviting something to get stuck in it. The biggest fault about a good tensioner is the torque that is put on it. If you over stress the shaft that the tensioner is mounted to you can cause trouble. By bending the tensioner too far, you can cause the chain to jump or other issues, like loosing tension and not going anywhere.

No Tensioner
This is the way I would honestly go, nothing to get in the way, or add weight, or break in my opinion. Just cut out a few links in the chain, and you have nothing to worry about, except if you don't leave enough play in the chain. If you don't leave enough play in the chain you can cause the system to bind, or you can torque the shaft that the sprockets are connected to and then you have even bigger problems. But you can reduce these problems by using mounts, so you don't couter leaver the drive shafts.

Just wanted to get what I know out there for those that asked for information. Hope it helps.
ivey

Brandon Holley 12-04-2004 14:21

Re: Chain Drive Question
 
Quote:

Originally Posted by kacz100
Just some friendly advice (for next time) go find the formula that will tell you the center to center distance of the sprockets so that the chain fits and no tensioner is needed


Unfortunately the world isnt perfect and that doesnt always work. Sometimes u may just be between a half and whole link ;)


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