ROAD AND WORKSHOP by HANDYMAN
Page 39
If you've noticed an error in this article please click here to report it so we can fix it.
Unusual Cause of Dropped Semi-trailer
S0ME months ago operational problems of articulation were discussed at length, and it could well be said at the time that just about every feature and hazard was covered. Yet there is always some new surprise in transport, and although the affair I will outline may turn up only once in perhaps several million miles, turn up it did, and the answer took a little finding.
A maximum-capacity tractor and trailer was proceeding empty through a new trading estate to make a collection. As its driver was approaching a partly erected factory, he saw ahead of him a recently filled trench across the road that had sunk after filling and had not had the early topping up and levelling attention it required.
Knowing that_ he was due for a jolt, the driver did all he could to reduce both his speed, the impact on wheels, springs and axles. He applied the footbrake hard, hut as he came up to the ditch he eased off the foot pedal as the tractor was about to drop into the 6 in. or Sin. trench and instead applied the trailer emergency brake to the trailer axles. So when the trailer first axle dropped in, its brakes were still on. Then, as the rearmost wheels came out, the brake was taken off and the driver began to accelerate from, in his estimation, around 6 to 10 m.p.h.
His annoyance over the badly filled ditch was nothing to his surprise a few seconds later, when his tractor shot ahead leaving the trailer crashing down on to
the road behind him. Although the van was empty, the landing legs were knocked back, leg tubes bent and air and lamp connections torn out, together with damage to the lower front part of the alloy-van. Unable to do anything himself, he called for assistance, leaving the tractor just where it stopped.
Examination of the tractor coupling showed it to be fully unlocked, but with nothing misplaced or broken and no undue wear. The trailer van was jacked up and recoupled, and as the bent legs were useless the outfit was taken back to base. The trailer coupling was tested time and again but was found faultless, and it appeared that we had found ourselves a real mystery, until the driver reminded us again of the badly filled trench and his actions on arriving there.
This caused further thought, so the coupling—and particularly the release levers—were again closely examined, when some light began to dawn. The tractor in question was equipped with a first release lever that had to be lifted from a slot and moved laterally, the jaw release lever then being free to be moved towards the cab. Originally, the jaw release lever was on the short side, and a later modification lengthened the lever, giving easier reach and better purchase. This lever was now quite long, and included a fairly sizeable round steel grip. Both levers moved rather freely when the load was taken off them.
It now remained to find out what could happen in identical conditions to the road incident. A short chain was secured between tractor and trailer, just to make sure they did not move too far apart, then the outfit was driven over 4 in. planks. Nothing happened other than the jaw release lever did try to move
forward. The Its release was then lifted' and moved to its free position, and on the trailer wheels hitting a 6 im plank, the long lever shot forwards, releasing the jaws from around the trailer pin. This was considered fair enough and it was decided that the tractor wheels dropping heavily into the ditch could jump the first lever, and with the trailer already under heavy braking, dropping the trailer wheels into the ditch could shock the long lever forward, releasing the king pin.
One point remains unexplained, however, as on being lifted from its slot the first release lever required lateral movement. Nevertheless, this was considered near enough, and on this particular model the first release was fitted with a slip-on retaining spring.