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.
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.
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.
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
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.
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.
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
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).
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.
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.
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/robotics/images/2004Pictures/2004_29electronics.JPG
Just another alright shot of the drive system.
http://www.valleytech.k12.ma.us/robotics/images/2004Pictures/2004_211Bot.JPG
Cant find any others but feel free to take a look.
http://www.valleytech.k12.ma.us/robotics/robotics_photos2004.htm
or
http://www.valleytech.k12.ma.us/robotics/robotics_photos2004UCFRegional.htm
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.
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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.
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Proper alignment! is crucial for long lasting and efficent chain implementation. Any misalignment is bad engineering practice.
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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.
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Design chain for short spans long spans are hard to tension properly and will put unecssary strain on chains.
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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.
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.
[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.
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.
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?
DANGER: Shameless Plug!
Click here for the link to the thread with my center distance calculator
[left]Matt[/left]
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.
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.
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.