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[quote user="tangerine-jack"] Now this is getting interesting.  Thank you cordon and Bucyrus, you both give me much to work on.  Bucyrus, where did you get your information?  Is it from a tech journal or from your own research?  I would like to study it in greater detail.  I think I am beginning to understand now the forces acting upon a coupler, but I want to be sure what I know is correct.[/quote]I am not an expert on all facets of coupler design, and if what I said is incorrect, I would welcome any criticism or correction.  I do not have a technical publication that details the operation of the modern coupler, however, I do have diagrams of the initial Janney design.  Prior to this thread, I had not considered the function of the knuckle pin.  I simply took it for granted (falsely) that the knuckle pin was loaded by the pull through the coupler, as is the case with the original Janney coupler.  So my explanation is simply based on the logical conclusions that must follow from the fact that the knuckle pin bears no load of the locomotive tractive force.  With the original Janney coupler as well as several imitators, the tractive force was transmitted to the knuckle and tried to swing the knuckle open, by pivoting it on the knuckle pivot pin.  So the knuckle amounted to a simple lever with the knuckle engagement head at one end, a kind of tailpiece or "tang" at the other end, and a pin passing through the middle.  When the knuckle was closed, a locking pin prevented the knuckle tang from swinging in rotation about the knuckle pin, and thus prevented the knuckle from opening.  So with the original Janney, the pulling load transferred through the pivotal knuckle to the locking pin, which prevented the pivot.  And in reaction to that prevention of pivot by the locking pin, the knuckle transmitted force back to the knuckle pin.  So both the locking pin and the knuckle pivot pin shared in a force that was applied in shear to each pin.  This explanation is clearly indicated in the diagrams of the original Janney design.Somewhere along the way to today's coupler, there was a sea change in this knuckle functionality.  Apparently the locking pin does not engage the knuckle tang in a way that prevents the knuckle from pivoting, but rather, in a way that prevents the knuckle from being pulled directly out of the coupler body in a straight line that matches the train centerline.Picture the letter "C".  If you connect a load to one end of the "C", and pull on the other end, the "C" will simply transfer the load (in a north/south direction in relation to the letter "C") with no inducement to rotate.  This is the modern coupler.  The locking pin simply connects the load to one end of the "C", and the "C" of the mating coupler pulls on the other end.  But the "C" sill has to pivot in order to open for connection to another "C".  So add a pivot pin through the line of the "C" half way from one end to the other.  With the locking pin locked, the force runs through the "C" in a straight line.  Thus it places no load on the pivot pin providing that the pin fits loose enough to not get involved in transferring any of the straight line pull.  Now when you pull the locking pin, the "C" is pulled forward taking up the slop in the fit of the pivot pin.  When this slop is taken up, the "C" then pivots to open.  So the functionality of the knuckle changes from a straight-line connector to a pivotal member according to the engagement or disengagement of the locking pin.With the original Janney design, there is this same "C" and midway pivot pin.  However, instead of the locking pin resisting a straight line pull of the "C", it resists the rotation of the "C" on the pivot pin.  So the fundamental difference between the early Janney design and today's coupler is the relationship of the locking pin to the knuckle tang.
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