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Multi-axle design spreads the load
Page 55
Page 56
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S THE tendency on the ontinent is to go for a 10-ton de, low-loaders will have to D for multi-axle designs as ayload requirements invase. This is how Craven asker (Andover) Ltd sees le low-loader market deveving over the next few ears, according to Len mith, sales director and oug Fisher, technical direct..
Although 13-ton axles will ive to be considered with a )ssible compromise at 11 or 2 tons, there is no doubt that N-loaders are going to develop ore wheels. This does not ean that the multi-axle achine will take over cornately as lots of small wheels ten means that the trailer can't taken over rough ground, so e conventional type of lowader will still have to be :ained for certain applications. The distribution of heavy ids over multiple axles is diffiIt with mechanical suspension full compensation for loading tween all the tyres can often ajudice the platform height — d with low bridges on most livisible load routes, this is eady a problem.
The Tasker solution to this iulted in a licensing agreeint with Delattre-Levivier in ince to manufacture multieeled transporters — it's difult to think of them as ailers.'" Constructed in modules with a self-elevating hydraulic suspension, they have an ultimate capability for loads of up to 1,200 tons. This particular suspension was chosen because, when operating with long loads — ie around 13m (42ft 8in) upwards — and with widths of around 5m (16ft 5in), a considerable amount of height compensation between the axles is necessary throughout the whole vehicle for only small variations in the road surface.
The principle of the Tasker's trailer involves sub-assemblies or modules consisting of three or four lines of axles which may be used alone as road bogies or coupled longitudinally or laterally to one or several other bogies.
These modules vary in individual payload capacity from 39 to 69 tons, depending on whether they are of the three or four-axle type and also on the required operating speed. This is how the 1,200 tons capability can be arrived at — by coupling some of these units together.
The wheel assembly incorporates a long-stroke hydraulic ram which gives a total travel of600mm (24in) to permit travelling over the rough stuff.
Craven Tasker thinks that this is how the low-loader market will be forced to develop, but as far as the engineering side is concerned their designers are worried about one possible weak link — the tyres.
Doug Fisher was more than happy about the capacity of the trailer, but because of the small wheels and tyres used on these multi-wheeled trailers and the inherent high pressures, he felt that the tyres were at their design limit.
This problem of tyre loading also occurs, but for different reasons, with mechanical suspensions linked by springs and levers. It is difficult to get enough compensation to protect the tyres when using more than three axles, but some Continental designs are now employing five axles with a mechanical suspension system.
In Tasker's opinion this is the wrong way to go, as even with a slight undulation a tremendous amount of suspension travel is needed and the lack of compensation on a multi-axle/ mechanical suspension lowloader will lead to tyre overload.
Mr Fisher was very aware of Tasker's responsibility in advising the operator. "It's incumbent on us to tell them that some conditions and some loads require higher tyre pressures. On this we have to work together with the tyre manufacturers. We now take the view that we should advise the operator on what pressures to run for that particular trailer and not allow him to rely on looking them up in a standard table."
It does not help of course when the recommended pressures for a particular loading capacity have.fo vary with make of tyre. As an illustration to the pressure increase which is often called for, the standard tyre pressure is around 81 OkN /sqm (117psi).
Tasker reckons this should be upped to 900/930kN/sqm in many cases.
Hydraulic suspension will be the way to go in the future, according to Tasker.
Air suspension is also very efficient, but it suffers from the major disadvantage that a lot of space is required on the trailer to accommodate the air bags. With an hydraulic suspension, smallbore piping can be used.
Where Tasker does see a use for an air system is for suspension distribution over a very long trailer. For a mechanical sus pension, semi-elliptic leaf springs are expected to reign supreme for a long time yet, as they combine function with cost. Torsion bars are an attractive alternative in some ways as they are progressively acting, but they are very expensive and often difficult to install in the trailer chassis.
Conventional road haulage trailers are designed with a very careful eye on the weight-saving aspect as a lower trailer weight means more payload.
However, when you start talking about indivisible loads and 100-ton payloads then an extra half-ton shaved off the trailer becomes irrelevant. In Tasker's experience the use of aluminium has been a disaster, as it is all too easy to get fatigue problems. High-tensile steel is used at Andover as then the stresses come within those accepted by the type approval people on the continent.
World markets
The good export markets vary from year to year for Tasker depending to a large extent on large-scale building developments.
At present Ghana is the top market while last year it was Nigeria. In general, the Middle East and West Africa have been happy hunting grounds for Tasker low-loaders. For these markets the more conventional type of trailer is used incorporating Tasker's own suspension and centre pivot bogies. This particular suspension was designed with a
maintenancerequirement high on the priority list.
The two bushes are resinbonded asbestos and are oilfilled and sealed although they can also function in the dry state. Straight bolts are used in the assembly in preference to U-bolts.
For loading and unloading, the removable axle method has been abandoned by Tasker in favour of the detachable neck. In fact the company won a Design Council Award for its Tasklift model a couple of years ago.
The latest version of this incorporates a new elevating neck which makes it possible to raise the trailer bed from a normal running height of 305 to 635mm (12 to 25in) above the road at the front end to get over an obstruction. Alternatively to get under low bridges it can be lowered to within 76mm (3in) of the ground.
As far as Len Smith and Doug Fisher can predict, the increasing tendency in low-loader operation to get the load per axle down will mean that if a trailer can be constructed to meet these requirements then the various Ministries and local authorities will insist on its use. So perhaps the multi-wheeled modular unit will take over for many applications.