I suppose that any part of a coupler can fail, but where does it occur most often? In the locking pin (not sure if this is what it is called)? I have seen extra knuckles hanging on the side of locomotives, are there extra pins aboard? I would guess that the pins would shear and/or the “pin hole” in the knuckle would tear away as the most likely spot for failure.
Northtowne
The pin and the thrower hook seem to be the most common.
The pin wears eventually, and the shock of coupling will snap it in two.
As for pins being a normal part carried on the locomotive like spare knuckles, not often…you can rob one from another car, use it to hold the knuckle open and in place while coupling, then either leave it there or put it back in the car you robbed it from.
The pin only provides a pivot point, once the knuckle is closed, it is locked in place and the pin serves no purpose until the knuckle is opened again.
In most yards, there is a stock pile of pins, often stacked up by the yard men and car men so they have handy access to them, as it is in yards that most pins fail.
Knuckles themselves do fail, they break across the face most of the time, or fail because of a fracture caused by being switched or humped against with the knuckle closed, or a casting defect.
The knuckle is designed to fail before the rest of the draft system (coupler, yoke, key and draft gear). It’s much easier to replace than the rest. It’s also much easier to see when cracks start to develop.
That’s the theory.
Ed’s post has the what happens in practice.
I’ve heard some hog heads say that they always seem to break 15 cars back…
Knuckles can be broken anywhere in a train…They have broken between the lead two engines of the engine consist and between the rear two cars in a train - and everywhere in between.
Personal observation from trains getting knuckles on the territory I supervise. I would appear that more knuckles are shattered by buff forces (slack run in) than by draft forces (pulling). Draft forces are somewhat limited by power, tonnage and grade that the train is operating on. Buff forces can be relatively uncontrolled by bad train handling decisions by the engineer and his manipulations of throttle and brake.
Former (now retired) BNSF locomotive engineer Al Krug wote “It always breaks 10 Cars Back” - see the bottom half of his webpage on “Drawbar Limits” (“How Much Force can a Coupler Withstand?”) at: http://www.alkrug.vcn.com/rrfacts/drawbar.htm
Murphy’s Law dictates that the coupler on the most inaccessible car will fail. Failure rates also increase directly with the distance to the replacement part(s) needed, the difference between the current temperature and 75 degrees, and the amount of precipitation.