Last time, I told you about my first day as Great Driver Mitch. It was a Friday, and we don't haul much on weekends this time of year. I drove just a couple of days in the second week. The third week, though, featured a long day every day-- with the partial exception of Tuesday. Work on Tuesday was artificially shortened.
The story isn't particularly technical, but I learned some time ago that many people know nothing at all about trucks. So, as usual, a little background is in order. You learn something new every day, after all-- unless you only read my site, in which case it's definitely not every day.
A tractor-trailer is held together by a large pin on the trailer (the "kingpin"), which is secured by locking jaws in a hinged plate on the tractor. That plate is called the "fifth wheel," presumably because it's vaguely round, and trucks only had four wheels back when it was invented. A fifth wheel looks like this.
A tractor-trailer is also known as a "semi," because the trailer is technically a semi-trailer; the weight is supported roughly half by the trailer axles, and half by the tractor. A full trailer would place relatively little weight (or none) on the vehicle pulling it-- like a trailer pulled by a car, or a child's little red wagon.
Now, since the front of a semi-trailer is so heavy, and it has to be about four feet off the ground in order to get the tractor under it, there has to be a rugged mechanism for holding it up. This consists of a pair of extendable legs, known (even officially) as "landing gear." A landing gear leg looks like this (external link). Both legs are conveniently adjusted by the single crank.
Most of time, a trailer is loaded and unloaded while still coupled to the tractor. Sometimes, though, this is a poor use of time or equipment, and a technique called "drop and hook" is employed. One form of drop and hook is 'spotting' empty trailers at several locations, where they can be loaded at the convenience of others, and then you come and haul the load once the trailer is full. This was the theory behind the third load of the day on Tuesday...
OK, so I wheeled into town 'bobtail' (without a trailer), to pick up an already loaded trailer, and then proceed merrily to Rochester. Since I don't have my license yet, I don't want to give the impression that I'm some sort of ham-handed idiot. As such, I dropped the air from the tractor rear suspension, and gently backed under just until the kingpin hit. Wouldn't want to bang into it like an unskilled fool. I then reversed direction and gave it a little tug. Wouldn't want to twist the driveshaft. Declaring (out loud, no less) the connection secure, I made the rest of the connections, and we were on our way.
My usual co-pilot is teaching me old-school techniques, and one of these is to use the trailer-only brake valve to test the trailer brakes-- and, in theory, the security of the coupling. So I did that, and the brakes worked. There was then a fairly loud and jarring bang. We figured that it must have been a particularly intense version of a common effect that follows using the trailer brakes-- because it couldn't possibly have been... no, that thought doesn't bear thinking on, because it couldn't be that!
The co-pilot then explains the importance of making sure you're hooked up properly, saying "Once you drop a trailer [accidentally], you never want it to happen again." I'm thinking that he's right-- that would be a very bad experience. A few seconds later, we slowly came to a small-- but steep-- rise, and there was a sound that I'd just as soon not hear again:
BOOM!
When the trailer fell, it sheared the pin on the landing gear foot that hit the pavement first. That single foot was designed to hold up 50 tons. Sheared it. We extracted the pin later, and it was shaped like a steel burrito.
As Shannon Foraker might say, "Oops."
So much for my looking like an idiot. Idiocy aside, though, we had to solve the problem of hooking back up. The very front of the trailer was nowhere near four feet from the ground, and the landing gear wasn't designed to lift a loaded trailer any great distance-- even with the crank in low gear.
The solution came in the form of a gigantic articulated wheel loader. Indeed, Caterpillar only makes three larger loaders. It picked up the front of the trailer-- perhaps fifteen tons-- and then the landing gear could be cranked down with ease. The trailer was backed back to the spot from whence it came, and the load was delayed due to the extremely uncool occurance. As it happens, I hauled it the next morning. Heavy bugger.
So, what had gone wrong? We'll never know, since if the problem had been detected before the disconnect, then nothing would have fallen off of anything. The working theory is that when I dropped the air on the tractor, the fifth wheel became too low, and the pin rode above the jaws that should have grasped it. There are a couple of other possibilities, but the bottom line is that it wasn't coupled properly in the first place. Car drivers will be happy to know that it is impossible for a Fontaine (our favored brand) fifth wheel to just come unhooked.
Needless to say, I am now paranoid about the possibility of a false coupling. It's just as my mentor said-- you never want it to happen again. Indeed, one of my college professors had a highly applicable saying: "You learn by doing-- and usually what you learn is 'don't do that.'"
Posted by Mitch at May 31, 2004 12:45 AM