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[quote user="tree68"]Using Ed's illustration from earlier in the thread, I might buy somewhat into your premise, Bucyrus, but keeping the knuckle closed will require constant tension.  Without the locking pin in place, even momentary slack will cause the coupler to open when pressure is applied to the bearing face. [/quote] I agree that the instant the slack begins to run in, the knuckle bind-up will be lost, and the kuckle will open upon re-stretch if there is no locking pin in place. As I mentioned, I do not know whether my conclusion about the locking pin not being needed when the couplers are stretched is true or not.  However, there is something about what this thread has revealed so far that drives me to that conclusion.  That is the fact that the knuckle pin serves no load transmitting function, but rather is only there to keep the knuckle from falling out.  It would be logical to conclude that the pulling load tries to open the knuckle, the locking pin prevents that opening, and so the locking pin is bearing a dynamic force that rises as the pulling load rises.  However:  If you pull on the knuckle and that pull induces it to rotate, and the locking pin counteracts that rotation force, then there must be a reaction force back to the knuckle pivot pin.  And yet the knuckle pin carries no load.  The knuckle pins can be missing and the mated couplers will continue without them.  That is why I have to conclude that there is no active force on the locking pin.  If there were, it would cause a reaction force, and that force would need to borne by some other feature.
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