Quote:
Originally Posted by Monochron
You're saying that ratcheting isn't a big issue even with "loose" chain as long as your lateral alignment is good?
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That would be an overstatement. Consider the chain on the sprocket being driven by the chain. The chain is exerting a torque
T on the sprocket. This is exerted as a force at the pitch radius
r of the sprocket. The chain is exerting a net force along its length of
T/r. This means that the tension on the "pull" side of the chain is
T/r greater than the tension on the back side. If the "at rest" tension in the chain is less that
T/2r, the tension on the back side when applying torque is less than zero (assuming the instantaneous chain stretch is linear). That's an almost sure recipe for ratcheting unless the chain is otherwise constrained to stay on the sprocket. If the sprockets aren't essentially in the same plane, it's a certain recipe for derailment.
Edit/added:
Quote:
Originally Posted by Cory
I don't know what you'd have to even be doing to see chain ratchet on a FRC drive.
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We had it happen this year, not on a drive, but on our lift. We mounted hex shaft bearings in holes bored directly in our lift frame (with a spade bit, no less), but those did not give us any trouble. In order to accommodate chain stretch, we did not make a full loop of the chain, but secured both ends to the back of the lift plate using end point mounts copied in concept from a garage door opener. We used thumb screws, and drilled holes through the thumb tabs an appropriate size for the pin in a master link. We then secured each end of the chain to these screws with a master link. The thumb screws passed through a hole, and were secured on the back side with a nylock nut. I know they were originally built that way; I inspected them myself. However, it appears that on one of our tournament repairs, the lift was put back together with regular hex nuts (in the interest of speed), which loosened, which caused ratcheting on one side and derailment on the other. (One was apparently lined up better.) We then re-tensioned and used lock nuts again, and haven't had the problem since. That reminds me - it's time to check the tension on that demo robot again.