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[quote user="nbrodar"] Again...The pivot pin bears NO force when the knuckle is closed.  The knuckle continues to preform it's job even if the pin is missing.  Much to the dismay of trainman, when they open the knuckle and it falls out.  Be there. Done that.  Have the T-Shirt.Even when the knuckle is open the pin bears little force.  Take a look at the pic of the coupler without the knuckle.  Most of the wieght of the open knuckle is bourn by the "yoke" that the knuckle pivots in.Plastic pins are cheaper, but don't wear as well as the steel ones.  Most of the plastic pins I've seen are broken.Nick[/quote]Nick, I understand your point about the knuckle falling out when you pull the locking pin and the knuckle pin is missing.  And I hear what you are saying about the knuckle pin not bearing any load while the coupler is under pull.  However, I cannot see how the second point is possible.  If you pull on one end of the knuckle that is hinged by a pin about midway, and the opposite end of the knuckle is being prevented from rotating by the locking pin, how can there not be a reaction force transmitted back to the hinge pin?On one that I looked at, the knuckle was closed and locked, and the knuckle pin was loose with a very sloppy fit.  When I pulled on the locked knuckle, it came forward while rotating to the left toward the knuckle pin.  This lose action of the locked knuckle  bottomed out on the knuckle pin, thus making the pin tight with no slop.  Pulling on the knuckle obviously put pressure on the knuckle pin.I can see how it might be possible for the coupler to be locked and loaded with the knuckle pin missing, without the knuckle pulling out.  In the case of a pull-loaded, locked knuckle with the pivot pin missing, I would think that the knuckle rotation tendency might be stopped by either the wall of its coupler body behind the knuckle, or by the static face (or "thumb") of the mating coupler.  The knuckle would simply rotate a little further than it would if the pin were in place.  But if the pin is in place, I cannot see how it could escape being loaded in shear when the coupler is pulling a load.  The patent on the plastic pins says their functional benefit is that they bend under load, so they don't break as easy as steel pins.
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