ROAD AND WORKSHOP by Handyman
Page 36
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The Dropped Semi-trailer
WHEREVER there is intensified artic V, operation, there is almost certainly a somewhat involved repair situation—with everyone searching for simple answers. One particular annoyance for the maintenance engineer is the dropped semi-trailer—and the damage caused to the undercarriage and its operating mechanism.
This happens more often in the depot than on the road, fortunately, and despite the several styles of mechanism used to grip the standard fifth wheel pin, the mechanism is rarely at fault. If the trailer is empty things are not too bad, but dropping a fully loaded trailer is another matter and most certainly an expensive one. The causes of dropped trailers are several, but are actually divided into two groups: (1) false coupling and (2) landing gear failure.
False coupling arises from a failure of the jaws to close and lock around the pin, and recorded instances show a number of reasons for this:—
(a) A lack of lubrication to jaws and their hinge pins; (b) Road grit between sliding jaw and bedplate; (c) Rope end caught behind jaws and sheared end staying inside; (d) Rusted and weakened jaw return spring; (e) the human element—failing to check that locks are fully home and secure before driving off.
Regarding landing gear failure, reasons for this are:—
(a) Uneven ground may place a trailer with non-adjustable landing legs in such a position that it is dropped heavily on to its wheels—such a shock could collapse the undercarriage.
(b) Failure of the operator to wind radial-arm type undercarriage down to its limit; that is, vertical, and uncoup • ling with legs two or three degrees inside a right-angle, the legs could then buckle. All too often this fault is brought about by neglect and lack of lubrication to the threaded winding • mechanism. s17 (c) Failure to fully retract the legs before moving even a short distance. The wheels or feet could strike some fixed object (such as a rail crossing) hard enough to weaken them and they could fail when next used.
As the engineer is assured of a certain amount of unscheduled repair from the above causes, may he be permitted one criticism of a famous make and offer a suggestion. On the fifth wheel Dyson 10-ton L-type semi-trailer, the landing legs operate radially via worm and nut, the nut being coupled or connected to the leg stays. Other than the need for the occasional packings beneath the wheels to make up for uneven ground, there is not a single complaint that could be made until some mismanagement caused the trailer to be dropped when loaded. Then, instead of repairs to buckled stays with a possible bent worm shaft, the engineer is faced with a completely crushed servo cylinder, always found to be beyond any further repair. This cylinder is mounted alongside the wormshaft box at the nearside, and is directly in the path of the nearside landing leg and tie stay if these items are pushed further back than their normal retracted position.
It is suggested that this servo could be moved further to the rear of the wormshaft box—and clear of danger in the event of a trailer being dropped—thus making the unscheduled repair bill a little more bearable.