Quote:
Originally Posted by Monochron
Maybe you could shine a little light on how center to center behaves with a chain that stretches over time. For instance, we have attempted center to center with success in the past, but we have no where near the tolerances the list as "needed". Would better tolerances cause less stretch in the chain? Basically, I'm still skeptical that you can run chain without ever planning to tension it. I have heard of it being done, I'm just not sure how.
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My personal experience with c-c designs is limited, and a lot of the info I have is from talking to friends on other teams.
The systems I felt comfortable running exact c-c were all reasonably low torque, in manipulators. They were also overpowered, so efficiency was not a major concern. This eliminated two of my biggest worries about c-c designs.
First concern: That chains/belts could slip, or "ratchet" as they stretched over time. Basically, if they chain is a little loose, and you apply too much torque to the system, the angle on the teeth in the sprocket will push the belt/chain away from the sprocket. If the belt is loose enough and/or you apply enough torque, the chain/belt will actually fully disengage from the sprocket, and the system will slip. I was OK with this in the applications I used c-c for, because we never expected to see large torques in the system, and if we ever stalled the system, it wouldn't be the end of the world if the belt/chain slipped. Of course, repeated slipping is bad for the life of the chain/belt, especially belts. As you're often going to be stalling your drivetrain, need it to have a lot of torque, and really, really don't want your DT to break, I don't like the idea of using exact c-c DTs.
Second concern: loss of efficiency. If you're tensioning super hard on a chain/belt, there's going to be more friction. If you have sliding bearing blocks, you can dial this tension in, but if it's an exact c-c system, what you see is what you get. I was OK with potentially having a lot of friction in the system because it was overpowered for what it needed to do.
If you're belt/chain is too loose, you run into concern one. If it's too tight, you run into concern two.
Maybe I'm misreading your questions, but c-c tolerances don't directly effect chain stretch. Sure, if your c-c distance is too big, your chain will likely stretch over time, but that won't necessarily be a bad thing if your system is overtensioned.
Basically, tighter tolerances get you closer to the goldilocks zone of between concern one and two. If your application is very demanding on both sides (like DTs), you will need better tolerances. If you're OK ratcheting sometimes or losing efficiency (like in some types of intakes, for example), a c-c solution may make sense.
I don't mean to blast c-c designs. If your team can pull them off for DTs, awesome! They can be much lighter, and certainly are more simple. When I built them, I really liked them. I just didn't trust 100 to be able to pull off a perfect c-c DT when I was on the team, and doubt that the risk/reward calculus makes sense for most teams in FRC.