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[quote user="tree68"] [quote user="Bucyrus"] This is precisely were the deeper mysteries lie.  I have to think more about it to put my thoughts into words.  But I make an assumption that if the locking pin suddenly, magically disappeared in a loaded, stretched coupler, the knuckle would not open. [/quote] Each time I break the locomotive away from the cars for our usual runaround, I (not so mysteriously) make the locking pin "disappear" by lifting it.  The coupler opens.  That's how they work.  If the joint is stretched, I may have trouble lifting the pin due to friction on the pin, in which case the coupler doesn't open.  But if we take your hypothetical magical disappearance, where friction isn't a factor, my bet is on the coupler opening. [/quote] But in my hypothetical example, friction is a factor. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p> Sure when you lift the locking pin, the knuckle will open, but you can’t lift the pin when the coupler is stretched, and my example of the pin magically disappearing is predicated on the coupler being stretched.  There is to practical way to demonstrate it, so that is why I use the term, magic.   <o:p></o:p> Certainly there must be some load on the locking pin when the coupler is stretched.  Otherwise you could pull the pin when the coupler is stretched.  But my point is that the pulling force does not place a dynamic load on the locking pin.  It only loads the locking pin with a relatively small force as the pulling force is initiated.  But then the force ridges take up the load and lock the knuckle against rotation by their friction.  At that point, the small force on the locking pin becomes a static force that persists as long as the slack is stretched.  When the slack runs in and then runs back out, the whole process starts again.   So the locking pin is needed only to initiate the lock against knuckle rotation only until the slack stretches.  From that point, the locking pin is irrelevant as long as the slack is stretched.  If it disappeared, the knuckle would not open.
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