Re: Bearings vs Bearing Blocks
 

100 ratcheted in 2012 when we were climbing the bridge. 6" wheels with 16T sprockets. There were more than a few stupid things going on with that drive, but chain alignment wasn't one of them. The chains were properly tensioned too.

I guess I've been too conservative in my comments. I never intended to scare people off of c-c chain runs completely. I just want to caution teams against betting your season on something that has the potential to fail miserably if you don't know what you're doing.

Re: Bearings vs Bearing Blocks
 

How did it ratchet if the chains were properly tensioned? What defines "tensioned"?

Re: Bearings vs Bearing Blocks
 

I'm assuming the issue here was too small of a sprocket.

Re: Bearings vs Bearing Blocks
 

Tension is the force applied to a member in the direction that will cause it to expand. In this case, it refers to the force applied by the sprockets to the chain when there is no work being done (usually no motion at all). Chain tension is rarely measured directly, but estimated by pushing transverse on the midddle of the chain run and watching the "rebound". Just like the string on a musical instrument, a chain under more tension will be harder to displace, will return more quickly to the center, and will oscillate more quickly. Usually we just make sure it feels right; most of us grew up with bicycles and know what a properly tensioned chain feels like. Note that the required at-rest tension does depend on the load tension, and also the length of the runs. If you want to see how well you are tensioned, the real test is to put the chain under peak load and watch the loose side of the chain (the return to the drive gearbox). If its motions don't make you worry about the chain ratcheting or jumping off, you should be good.

Re: Bearings vs Bearing Blocks
 

To echo other comments, I too have seen ratcheting multiple times, twice on my high school team's early drive trains. We were using the 2004 and 2005 kitbot back then and I believe the issue was that we mounted our gearboxes on sliding rails in order to allow us to adjust tension. Unfortunately the gearboxes would slip during matches leaving slightly looser chain. As a result we ratched in multiple matches.

On our 2015 robot we have a chain run of around 3 feet with 12 tooth sprockets on either side. It encounters no issues unless the load of the elevator increases dramatically. The chain is tensioned so as to give a nice twang if plucked but it still ratches when under a lot of load. I can check how well aligned the sprockets are. I feel like the answer is "well aligned" but I have no idea of the tolerance off the top of my head.

Re: Bearings vs Bearing Blocks
 

I was actually asking how one would measure tension. Specifically, if the chain is ratcheting but it properly tensioned, does that mean that the shafts of the sprockets are bending, or just that the chain is stretching to the point where it becomes undertensioned, or something else? Or is "tensioned" for one team different from another?

Re: Bearings vs Bearing Blocks
 

You can measure tension either by the displacement under a known deflection force, or the frequency of the "twang". If you apply a known force F at the mid-point of a span of chain of length l, and it deflects by a distance h, the tension in the chain can be figured out with a force diagram. My quick scratches tell me that if h<<l *, the tension T can be calculated as:

• T = F l / 4h

As I indicated above, at a minimum, the stationary tension in the chain should be more than half the tension differential needed to turn the sprockets at peak load; otherwise the back side will go completely slack under load.

* - and if it isn't, the chain is definitely too loose!

Re: Bearings vs Bearing Blocks
 

After thinking about this a bit... If you are ratcheting a chain drive, you have a design issue. The issue might be using too light of a chain to meet another design goal such as weight. Engineering is about trade offs after all. A couple of chain drives designs to think about that don't ratchet despite the high loads:

Bicycles especially full suspension bicycles. The slack side has a relatively light tension from the rear derailleur. Relatively light side loading will move the chain from sprocket to sprocket.

Motorcycles: You actually set a chain deflection so at rest the chain has no tension. This is needed because the C-C distance changes with suspension travel.

Re: Bearings vs Bearing Blocks
 

Chain that is ratcheting is almost always not properly tensioned. If not, you either don't have enough wrap around the sprocket (a good rule of thumb is 120 degrees), or your sprocket / shaft is deflecting significantly. Both of these should be obvious. It's also possible that your sprockets are just way too small - a 16T #25 chain sprocket going to a 6" wheel is pushing it.

Haven't used chain more than a handful of times in the last few years, but the last exact C-C run we did was very much properly aligned, but the span was so long that the chain stretched enough to start ratcheting. I agree with you that alignment is far more critical than tension and that chain in general is pretty forgiving, but I've just seen a few cases of failure in what looks on the surface like a good exact c-c chain setup. Maybe it's just crummy chain, or the runs are ambitiously long.

I've heard of teams that do c-c that just replace entire chain runs if the chain gets too loose. Other than cost and having to keep track of old vs new chain, I don't see much wrong with that approach.

While I've never done it, I've heard c-c for 35 chain is much easier. I imagine the chain's much greater load capacity reduces the effect of wear, not to mention the smaller number of links for the same length. But I really think 35 chain is overkill in most applications, you're just throwing weight away.

Re: Bearings vs Bearing Blocks
 

Vex and Andymark have it for $1/ft, so it's a very valid approach price wise.

Re: Bearings vs Bearing Blocks
 

My previous team has done that. It is pretty easy to tell an old chain loop from a new one. The side deflection on a used chain is significantly more than on a new one.