Could someone explain to me how a chain tensioner works? I understand how to link two wheels to one motor with a chain, and I know that a chain tensioner is used to make sure the chain doesn’t slip, but I don’t understand exactly what a chain tensioner is and how it accomplishes this.
Thanks,
~Gabriel
On last year’s robot, we had our motor munts on these plates so that we could slide them back and forth to increase the distance between our two sprockets for each wheel, increasing the overal tension of the chain system. This can be done be putting slots (parrallel to the chain) into a plate of metal and mounting the motor, a gerbox, and the sprocket mounting devices on that plate. You would have a bolt and washers, all the usual to tighten down the plate. To move the plate to adjuct the tension, you would just loosen the bolts and slide the plate then tighten the bolts down again.
When I hear the phrase “chain tensioner” I think of two things. One is a threaded device used with two sprockets, one of which is moveable. The tensioner is inserted between the two sprockets and adjusted until the chain is tight, then the hardware on the moveable sprocket is tightened to hold it in place. The tensioner is then removed. The second is an additional sprocket that is moveable but is not used to couple power out of the chain. This sprocket is commonly called an idler and is used to take up any slack in the chain. The sprocket is positioned so that the reguired tension is placed on the chain and then the mounting hardware is tightened to hold it in place. Frequently idlers are placed in the long path of a chain where the sprockets are far apart. By pushing in on the chain, the sprocket(s) help hold the chain in place on the drive and driven sprockets and prevent movement in the chain that would allow it to come off one of the sprockets. Please remember that the chain tension is directly translated to the bearings supporting the shafts of the sprockets. If you tighten the chain too much, bearing failure will occur.
FIRST gave us chain tensioners (idler gears) with the new chain drives they included in the kit this year.
Important to note also that chain DOES stretch, expecially the #25 (1/4" pitch) chain common to light sprockets with a high load. You can’t always “set it and forget it” - you probably need to include adjustment in the sprockets or idler to take up this change in length of the chain over time. The total amount of adjustment you’ll need is one half inch for #25 chain (one complete link, an inside and an outside) - more than that and you add or remove links to make up the difference.
Also don’t forget that you can also use a ‘chain guide’ type of system to do the same thing. Last year (At Buckeye) our robot was hitting things so hard the 80/20 frame was twisting and it would cause the chain to loosen then derail from the sprockets, then the frame would come back and we were stuck. We added delrin plates to the sides of the sprockets that formed a v-shape that forced the chain on the sprocket if it became loose or tried to derail. These solved the problem for Geat Lakes and Nationals and we never had a chain problem again.
The kit also supplies what i believe are called “half links.” I was counting on these to give us enough ability to change the chain length. Someone correct me if I will need more fine tuning than a half link.
I wouldn’t bet anything on using half-links. Although there is a sprocket center distance equation, that was posted by Matt Adams, will help you acheive a better placement of your sprockets, your chain will eventually strech. My experience with chain is that you will have very little luck with longer lengths, over two feet, because the chain will strech without a method of tensioning. A tensioner can be rather bulky and complex. Slotted plates are simple and quite effective to help keep your chain on track.
Could you guys elaborate on what you mean by slotted delrin plates? I know that delrin is some kind of metal-like plastic with very little friction. Do you just use delrin plates to hold the chain in place? If so, do you need to cover the whole chain or just the sprockets? What is the slotted part?
Thanks,
~Gabriel
what theyre saying is if you mount your sprockets and shafts in a slotted piece of material, you can slide them in the slot, creating more tension. Or, you can mount an idler sprocket in a set of vertical slots, allowing you to tension it by pushing the idler up, rather than pushing the drive sprockets out.
Cory
Something our teams have done in the past is use an adjustable piece of delrin or HDPE as a tensioner. The simpliest of these was just a piece of round stock with a hole drilled through, off center. The chain runs over this piece, and by rotating it you can adjust the tension (since the hole is off center you’ve got this cam sort of motion going on). Sure you’ve just added some losses into the system, but it doesn’t get much simpler than a piece of round stock cut to length, 3 holes (2 chassis pieces and a delrin) and a fastner. We’ve used a few variations on this over the years, but that was the one I liked the best. You have to keep an eye on it in case the tension from the chain causes the piece to rotate the opposite way and loosen the chain, but that’s a simple fix.
If you have a fixed-wheelbase 4 wheel drive system, the delrin post works fine. There are also spring-loaded tensioners available.
If you have a simple 2 wheel drive using the drill motors and sprockets, mounting the entiire drill motor/tranny’small gear on a plate works great. You can either slot the holes in the plates or stack washers under the plate to raise it higher. I like the latter because it can’t slip and you don’t have to cut/file slots. It does require removing the chain to add washers unless you cut out a slot in the washers.
Use #35 chain and keep it short to minimize the need to retension. Avoid half links.
Any reason for avoiding them? It wont be too hard to have a slightly adjustable wheelbase in our design, but i thought it would be simpler to take just put in or remove a half link.
How long will this stretching take? Will the majority occur during practice so we can have the robot properly trimmed before the competition?
Max,
You are going to want to check it regularly and make adjustment as needed. For short runs the off center drilled delrin post is the easiest to adjust, for longer runs slotted base seems to work well. In both cases, you have to be sure that the hardware is locked down. If it loosens in a match, the chain flys off.
Half-links (single pitch offset links) tend to be weak. There are double-pitch offset links that are stronger. See http://www.diamondchain.com/techfaqs.asp items 17 and 18
If you are using #35 chain and do a good bit of practice, a lot of the stretching should happen before you get to the regionals. However, use a good chain lube periodically and watch the tensioning. Use of a very small sprocket close to a large one (a la 10:45) will require close attention to prevent the chain skipping off. See item #2 in the Diamond tech facts.
Also see this collection of facts, drawings and application notes: drivesinc.com