Care & Maintenance
Page 52
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Beware cheap offers
THERE IS always a temptation to use a slightly lighter and cheaper vehicle for the job than is really needed, especially when business is tight.
There are times when only one vehicle is available for a particular job, and it is a case of either using that vehicle or losing the job, but these are exceptional circumstances. Even then, there are times when it is wiser not to take the job on. But at the outset, the vehicles should be purchased on the basis of the anticipated duty and traffic, since paring costs here is a false economy — and could be dangerous, Specialist heavy-duty machines are designed for continuous heavy use by the haulage industry, with design targets for intervals between major overhauls of 400,000800,000km {250,000 500,000 miles). Or the other hand, the lighter vehicles are designed more for own-account operators, or for lighter duty generally, rather than two-shift working.
Although, as a general rule, it is fair to say that the lower the power:weight ratio, the harder the engine has to work, this depends to some extent on the torque characteristics of the engine.
An engine that can lug well at low speeds can cope better with heavy-duty operation than one that is always revving flat out. Nor is power output necessarily a clue to fuel consumption, which is now such an important part of the haulier's costs. There is quite a lot of evidence to suggest that a vehicle with a good power: weight ratio and good torque can use less fuel than one with a smaller engine. In addition, because less gearshifts are needed, the driver has an easier time, which means that safety is improved.
In any case, the overworked lightweight is going to end up ri the workshops more often than its heavier brother, which cart upset those expected cost benefits.
What is equally important is that the faults that crop up can make the vehicle less safe, while the lighter structure can lead to dangerous situations resulting from any overloading — in gross terms or locally. For example, the use of too light a vehicle can result in an accident at a roundabout.
Inspection of vehicles that have overturned at roundabouts quite often shows that the search for weight-saving in platform and sub-frame design has affected the pattern of the accident.
Sometimes, outer members or outriggers sag under loadsway, and then simply failed to return to their normal shape, while light channel section cross-bearers bend, and open up and also take on a -permanentset''.
What happens here is simply this: when any piece of metal is deflected under load, it will normally spring back to its original shape. However, if the material is overstressed, and it is deflected too far, or the load is too great, it will reach its -yield pointand will not spring back but will remain in its bent shape. If the load is increased still further, the material may break.
You can check this easily enough by flexing a piece of mild steel strip. If you don't apply much load, it will spring back and forth, but if you really bend it, the strip will remain bent, and because you have overstressed it, arty attempt to straighten the strip will leave a tell-tale ripple.
When cornering, the result of the overstressing of the members is that the load shifts, thus increasing the weight on the outer wheels, and this can be enough to cause the vehicle to overturn, If the members are not overstressed, they spring back, and the vehicle continues on its way -in any case, they will have deflected less than the overstressed members.
Even if these light members do not actually bend perman
ently, each time the vehicle corners they are highly stressed, so eventually they are likely to crack. Obviously if a crack appears just before a roundabout, then the member can fail very quickly and perhaps with disastrous results.
Although the decking of platforms and trailers is usually well up to the job, some care is needed in the selection of the decking, arid in ensuring that it is properly positioned.
For general freight, timber flooring is still very popular, and there is a lot to be said for the soft-wood floor. It gives great flexibility, arid sharp edges at the corners of loads can dig in, which helps hold them in place, It can shrink and swell, but this extra security it gives is well worth while; in any case, the face is less slippery than hardwood, which is a good point.
However, there is an important point to watch when purchasing trailers, and that is the relationship between the height of the decking and the longitudinal frame members. On many trailers, the frame members are designed to come up to floor level, the steel face in fact taking the place of a floor-board.
To keep the load secure, it is important that the frame members should be slightly below the level of the floor. If the frame members are slightly proud of the floorboards, they will carry all the load, on a very small area, so the load can too easily slide over the side.
Trailer manufacturers are, of course, aware of this problem, but the thickness of the timber can vary slightly; there is a tolerance on the height of the cross-bearers, while the timber can dry out slightly thinner than when fitted, and, of course, it can warp. Then, there may be an attempt to cut costs by using thinner timber — not surprising with the current price of timber — and these boards can then be depressed below the level of the frame members following the continuous movement of loads, Then, there is the normal wear to be considered — and the clue' here is that the top faces of the frame members start to get polished as soon as they become flush with the floorboards.
It may seem to be an expensive business to replace floorboards just because they have worn below the level of the frame members, but it is cheaper than losing a heavy load.
But the time to take care is when the trailer is being purchased -make sure that the timber platform is at least 3mrn (0.12in) above the level of the frame members.