If a King Pin, when looked at from the side, can be considered to have an "hour glass" shape that is more massive on the top than on the bottom, and where the "neck" of the hour glass is 2" in diameter by about 1.45" in height, I am curious about the height of the upper "heavier" part of this hour glass.
How tall is the upper part of your king pin... between the load bearing surface and the transition of the "hour glass" to the neck around which the jaws to your hitch clamp around?
I have one diagram that shows the height of the upper part king pin as 2.115", and yet I have a hitch that only has 1.56" (1 9/16") between the strike plate and the top of the jaws. Since the jaws clamp around the neck, and since the jaws close down to only a 2" opening, and since the upper part of the king pin is, according to the diagram, 2.875" in diameter (and 2" tall, as earlier stated), it appears that the jaws would bear all the vertical load of the king pin on the "shelf" created by the reduction in diameters at the neck, prior to the load bearing plate contacting the strike plate.
Probably every one of you will tell me that I am over thinking this, but I've never owned a fifth wheel trailer before, and my understanding of clearances has been so anemic, I've gone ahead purchased and rebuilt an old "practice" hitch just so that I could learn what I needed to know before ordering a new custom air ride hitch... and find that I still get puzzled about some details, mostly because I do not have immediate access to any trailer with a king pin.
But according to the diagram of standard king pin dimensions, compared to the dimensions of my fifth wheel hitch, it appears to me that the jaws will end up bearing all the vertical pin weight of any trailer with a king pin dimesioned as per the drawing... and I find that hard to believe possible. That's what the strike plate is for... to bear the weight. The jaws secure the pin, and handle the pull weight, not the vertical weight. Yet I only have 1.56 inches between the strike plate and the top of the jaws inside my practice hitch (PullRite Super 5th... old cable pull style).
Something isn't right. Even adding a 1/8" lube plate, which would increase the distance between strike plate and top of jaw to 1.69", that is still a good deal shy of 2.115" for the top half of that hour glass shaped king pin.
Maybe the king pin dimension diagram I have is in error? I assume that all king pins have a standard SAE specified dimension. Here are the dimensions of the king pin in this diagram:
Overall height: 4.13"
Overall width, determined by the upper portion of "hour glass": 2.875"
Upper portion height of "hour glass": 2.115" (Tthis is the crux of my question and poll... is the distance between the load bearing surface and the neck of your king pin really this tall?)
"Neck" height of king pin: 1.45" (This is the business portion of the king pin, around which the jaws clasp, and this height includes two RO.125 radii transisitions into the upper and lower halves of hour glass)
"Neck width of king pin" 2.000" (+/- 0.005 wear tolerance)
Lower portion height of "hour glass": 0.565" (This is the part of the pin that is captured below the jaws... the part that you look for to see that you are not high hitched)
Lower portion width of "hour glass": 2.812" (Slightly smaller diameter than upper portion of hour glass)
As you can see, it appears that the entire weight of any fifth wheel trailer with pin dimensions as specified above would ride on a bearing surface donut on top of the jaws with an annular ring width of only .4375"!!!!
(2.875 - 2.0) / 2 = .4375" annular ring land width riding on top of jaws, bearing all the pin weight, due to:
2.115 - 1.69 = .425" vertical gap clearance between load bearing plate of trailer pin box and strike plate of 5th wheel hitch.
Something isn't right. My hope is that many of you will report back that the upper portion of the "hour glass" of your king pins is really only 1.5" nominally.
Please let me know!
Thanks in advance for your help.
Edited by SKP, 25 September 2012 - 12:54 PM.