Is a little pin the size of your fist able to pull all that weight on a trailer without breaking?
And has anyone ever seen one break?
It’s a question that’s puzzled me for some time now 
I have had a collar shatter being used over a 2’’ pin with a 3’’ fifth wheel, I have never heard of a pin breaking away- although any fault in making it may have it happen I guess
If its a snug fit then it will be very hard to apply enough force to break it.
A cylinder is one of the strongest shapes. (Put three empty toilet roll tubes together and you’ll be able to stand on them) I’ve seen pictures where the plate the pin attaches to has given way before any pin will.
What I have wondered is if the pins on heavy haulage rigs are any different?
Knew there was some science behind it, but still surprised with some of the drivers around that they never break. Some of they stuff they break takes skill.
The pin is attached to a thick plate with a LOT of bolts so it’s quite a strong setup.
Just look at wagons that have turned over. The unit’s chassis can be badly bent before the pin will give way.
Pins have been known to shear but I think it happened more in years gone by. They can also shear in an accident, obviously.
I think more extroidinary is how driveshafts dont snap like carrots , when you think of the forces they suffer when having to move a huge weight from a standing start , considering how small they are
Not seen or heard off the pin breaking, though I suppose it could happen, but I’ve seen a trailer were the pin had been ripped out off the rubbing plate, big mess. from what the wrecker driver said the pin was still in the 5th wheel along with a sizable chunk off the rubbing plate 
One of our old drivers a couple of Christmas’s ago flipped a trailer with two big forklift trucks on the cab stayed up but the trailer ripped the 5th wheel off, it was still attached to the trailer some way off.
He was cut up by two cars racing, luckily for him an off duty bobby witnessed it.
The highest shere force one can apply to a kingpin would be to steer sharply, whilst riding over an adverse camber.
Even then, chances are the pin would rip the teeth out of the 5th wheel, before the trailer parts company with the tractor - kingpin still attached.
Iron is a very tough metal, which is why we’re still using it after more than 2500 years. 
Not quite the same but 9 or 10 year ago I bobtailed from Newark Mastercare up to near Blackburn to pick up one of those crappy old grey boxes they use for shop deliveries, anyway, backed under, heard the pin click in & done tug test.
I shot forward leaving the 5th wheel locked in place on the trailer.
I then left it there and bobtailed back to Newark.
Shudder to think what could have happened if it didn’t happen there in the yard.
Winseer:
High tensile Steel alloy is a very tough metal,
Fixed that.
How long has that particular alloy been around for?
“Kingpins are older than the very latest alloy type” I’m thinking here.
I used to change the pins on trailers in one job I had. I often wondered the same thing. But the plate they are welded to is a good half inch thick. The pin its self is of some kind of steel alloy and has a 6 to 8 inch flange on it.
Took a good 4 or 5 hours to grind off the welds on the old flange, Then welding on the new ones required a lot of amperage and big sticks.
You need someone cleverer than us drivers to explain it, but there is big difference between the gross weight of the trailer and the lateral forces imposed on the king pin to make the trailer move. Think of the differences in the amount of throttle required to pull away on a steep hill compared to on the flat.
Winseer:
How long has that particular alloy been around for?“Kingpins are older than the very latest alloy type” I’m thinking here.
In general where strength is required it’s steel that’s chosen to do the job not iron and steel has been around more than long enough to make make truck trailer couplings.
Yeah irons not good, it fractures under torsional and tensile loads. Good for compressive loads. High tensile steel with the carbon is the job
The same can be said about the mechanism in the fifth wheel that secures the pin, I have only ever seen one break and another fail because the plate came off the trailer.
Their are several parts of a truck that defy logic, the propshaft transmits a massive amount of torque and power to the rear axle and is a hollow tube the steering mechanism that changes the direction of 44,000 kilos is relatively puny in construction.
These are one of the areas that should have been covered in the D.C.P.C a chance to save the industry millions every year related to abuse through ignorance.
It’s normally the welded in pins that come off ,we had tipper trailer once that was hit up the ■■■ by one of Ernest thorpes at low speed it pushed the cab back on Thorpes ,the back of the tipper trailer was not too bad the back doors still opened ,not long after the trailer came off turning in to a yard ,the pin was in the 5 th wheel still ,moose is the pin expert .
At Coaville a few weeks ago a skele with a container on lost it’s trailer apparently the whole plate was still attached to the pin and 5th.