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Thanks for the photos Ed.  That does help evaluate the “force grooves” or “lands and grooves” that we have been talking about.  I am revising my theory so that if the locking pin magically disappears from a stretched coupler, the knuckle will open.    I think what I concluded about the knuckle shifting function between a rotating element when open or closed, but not stretched; and a straight-line link when closed and stretched is true, but how it becomes a straight-line link is different from what I had assumed. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p> I thought it became a straight-line link by pulling directly into the lands and grooves engagement more or less perpendicular to it.  This would relieve both the locking pin and the knuckle pin from any loading. <o:p></o:p> However, I see that the lands and grooves are nowhere near perpendicular to the line of pull.  In fact they appear to be about 45 degrees to it.  So in effect, the lands and grooves surfaces and the locking pin surface become opposing sides of a “V” groove or cavity.  And the corresponding features of the knuckle tang wedge into that tapered cavity.  So the lands and grooves pulling surfaces and the knuckle pin surface share the pulling load that wants to wedge them apart.   <o:p></o:p> This still leaves the knuckle pin un-involved with the loading, but not the locking pin.  I think that Cordon may have proposed this functionality when he said that he thought the knuckle loaded against the locking pin and placed its reaction force against the force ridges.  I could not quite see that because I was thinking that the reaction force would need to be rotational.  But it is not a rotational reaction force.  It is a reaction force from one side of a wedge to the other. <o:p></o:p> The lands and grooves do not really look robust enough to be taking the full pulling force.  But together, top and bottom, they look about equal to the strength of the locking pin in its backing pocket, which would make sense if they were opposite sides of a wedge cavity.    I would say that if the lands in the lands and grooves feature ever broke off from overloading, the coupler would revert back to the elementary Janney design as the reaction load from the locking pin shifted to the knuckle pin.  I would also speculate that the lands and groove feature was added somewhere along the evolution of the automatic coupler to help the survival of the knuckle pin.    So just as the knuckle pivot pin could be missing while the coupler still functions, so too could the lands and grooves.  But one or the other needs to be there.
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