Picking Out Roller Chain
 

Can someone tell me what the calculations are for determining the choice for roller chain? I am guessing the terms you use are the maximum working load and average tensile strength but how do you calculate what that is going to be in the robot.

Re: Picking Out Roller Chain
 

Adam,

In its simplest form you need to calculate the torque at each sprocket and the find the resultant force on the chain. For example if you have a torque at the sprocket of 2000 inch-lbs and have a 4 inch pitch diameter sprocket, the force at the pitch line on the chain is 2000/2 = 1000 lbs. Since we like to have a safety factor of 2-3 than you would select a chain size with a breaking load in the 2000-3000 range minimum. #35 falls into this range. The load needs to be calculated from the direction that is applying the greatest force. For the drive, the wheels generally slip before the maximum torque of the motors is achieved. A lift mechanism may get jammed or try to lift an immovable object, the torque in this case is from the motor side.

Word of caution, sprocket teeth can also break. Try to use a sprocket with as many teeth as possible, the load is shared by each tooth so the more the merrier.

Re: Picking Out Roller Chain
 

What is the differnce between double pitch and single pitch roller chain?

Re: Picking Out Roller Chain
 

Double pitch roller chain has a pitch twice as large as single pitch equivlant chain. A double pitch chain should have the same strength as it's ansi equivlant, but weigh less because there are fewer rollers (note that a chain's pitch is the distance between rivets).

Thats the best I can do without a picture, you might try searching chain pitch on google.

Greg



(edit: for readability)

Re: Picking Out Roller Chain
 

According to Mcmaster:
Never mind I just answered my own question. Now what will happen if you constantly run the chain over the work load but under the break load?
Thanks for the help.

Re: Picking Out Roller Chain
 

Essentially, it's longevity will decrease, and it will eventually break. Chain (like any component) has a finite service life. Often times, chain isn't designed with sudden stops and changes in direction in mind. On the last leg of your drive train when you're connecting sprockets to wheels, just play it safe and go with #35. :)

Matt

Re: Picking Out Roller Chain
 

All right that is not a problem. Since the chain I am looking for only is sold at #40 and above. Though I wonder if using it as a tank tread will change anything. It is called ANSI attachment chain.http://www.mcmaster.com/

Re: Picking Out Roller Chain
 

I guess I'd like to see what you're going to do with it... those links are so darn expensive! In addition, #40 chain is one heavy hunk of metal. However, I'm curious to see what you've got in mind.

Matt

Re: Picking Out Roller Chain
 

I read a book that suggested using the attachment/conveyor chain as a tank tread. More specifically I want to use the chain with the flat attachments. I thought that by bolting on neoprene rubber inbetween two chains it would make a nice (abeit expensive and maybee heavy) tank tread. At it's cheapest it would be 7.00 a foot which would cheap enough for me. Mcmaster also is not know for being cheap.

Re: Picking Out Roller Chain
 

We used attachment chain from Diamond Chain for our 2002 Robot. We riveted steel cleats to the chain, which were narrow enough to allow the drive to turn, but dug into the carpet to provide pushing force.

FIRST outlawed metal cleats in 2003, so I can see why you are thinking about putting rubber cleats on the attachment chain. We actually tried doing this in 2003, with urethane cleats. We aligned our wheels, kept the chain unbelievalby tight, and we STILL threw our chain off. The urethane does no allow for turning as well as metal cleats do, since they were wider and stuck to the carpet better than metal.

I am not saying that this rubber cleated chain drive system cannot be done, but I am saying that it will be risky and tough to make it work. Rubber likes to stick to carpet and chain likes to fall off of sprockets at the slightest side force, as it's moving... so be careful in how you design your tensioning system and wheel alignment system. Don't skimp on those two features. Ensure that your wheels get aligned and create a separate tensioning device to get maximum tension on the chain.

Good luck,
Andy B.

Re: Picking Out Roller Chain
 

I agree with Matt's recommendation to use #35. Some people think its overkill but I've seen the lighter chains break under impact often enough to not want to use them.

One additional (and I think very important) advantage to #35 chain is that it is less misalignment-sensitive than the smaller chains (1/4" pitch or 8mm pitch). This alone is a great reason (imo) to use #35.

Ken

Preseason Rant: Don't Go with #25 Chain in the last leg of your drive train!
 

And though this is sort of off topic, since the question has already been answered, I'd still just like to throw in my four cents about #25 chain ahead of time.

Someone is going to say this. It'll be during week 2 of the build season, and a rookie team will ask for advice on chain in these forums. 40 people will reply, many of them like the above.

The truth is that there have been instances where #25 chain has broke on some teams' robots on the last leg of the drive train. Since the forces are relatively the same for about every team (and the only way to reduce this is by having poor traction or big sprockets) it's definitely in the best interest to not cut weight in something as critical as your drive train.

In addition, #25 chain will stretch significantly more over the course of the competition, and misalignment is not fun, as Ken has said. Let me be clear: chain stretching isn't a fun problem! Nobody gives high fives or a does a fancy FIRST dance because of chain stretching.

And numerically... I don't understand how or why it happens either, but it does. It's probably a dynamics forces thing. Every year there are teams that have #25 chain break on them. It's an extra couple pounds to ensure NO worries about this problem occur. It's well worth the weight to save on what could be competition altering failures.

So... there I go being blunt with a person that doesn't exist... :) If you disagree... I'm jealous of the 2 lbs you're saving this year.

My Advice:
#25 chain on the last leg of your drive train - Don't Go There.

My 4 cents,

Matt

Re: Picking Out Roller Chain
 

Would idler sprockets be ok in tensioning the drivetrain? Also would using a high durometer material lower the traction of the belting?

Re: Picking Out Roller Chain
 

OK Matt, I'll bite! We have been using #25 chain for many years on our drive system. Ask anyone if we ever broke down because a chain broke. Last year we even used #25 chain for our wings and that never broke even though I thought I had designed it "on the edge."

Here is my real point: Noteworthy engineering is about making the proper trade offs in an intelligent way to maximize the efficiency of your design. To flat out say that one should never use #25 chain on your final drive is not appropriate. It is no different than saying that you should only use steel for your frame because aluminum bends too easily.
If you are careful to design your system such that it will not see significant dynamic loads, you can use #25 chain and not ever see a problem. Without getting into too many details, that loading depends on your gear ratio, coefficient of friction and a few other things.
Besides, in some cases, you prefer to have this "mechanical fuse" as your controled breakage point. As an example, depending on your design, you may prefer to fix a broken (or stretched) chain rather than a transtorque coupler or a keyway.

To answer Adam's original question about what happens if you run chain above it max working load but below it break load - it stretches gradually.

Raul

Re: Picking Out Roller Chain
 

I am telling you now that #25 will and does break. Last year we continuously broke our chains. In one match we broke 3/4. I was told in 9 previous years we had never broken 1 chain. The difference is we went from 4 motor drive to 6 motor drive. The additional HP was too much for the chains. This year #35 will be our chain of choice.