Test of
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YORK AIR-LOAD TRANSFER
By A. J. P. Wilding
A.M.I. MECH E.
FOR off-the-road work few operators would specify a sixor eight-wheeler with a single-drive rear bogie. Yet it is not very satisfactory having to incur the permanent loss of payload often associated with double drive when the vekicle spends little of its time on rough ground. There is also the extra cost of maintenance which must be considered.
As a solution to this problem the York Trailer Co. has for some time offered a system called Air-Load transfer for vehicles it converts to six-wheelers. This has the object of transferring load from the rear axle of the bogie on to the driving axle when difficult conditions are encountered. More recently Scammell Lorries Ltd. and Primrose Group Sales have introduced devices to fulfil the same purpose.
The opportunity to observe the effect of the York design was taken recently when the concern was able to make available a Thames Trader with a York third axle and air-load transfer temporarily in its possession. The vehicle had previously been in service for 12 months with the Salop Sand and Gravel Supply Co. Ltd.
With the York conversion to sixwheeler, the added wheels are mounted independently on stub axles which pivot on brackets mounted to the chassis frame, the front ends being linked to the rear of the existing springs on the driving axle. The air-load transfer consists of two air diaphragms—one attached to each stub axle—which apply pressure to the rocker beams in front of the pivot and in so doing take weight from the rearmost wheels, applying the same amount to the driving axle wheels.
With maximum air pressure in the system, the total load exerted by the two air cylinders is about 8,000 lb, The actual load which can be transferred to the driven axle will depend on the chassis maker's weight limit for his axle and will vary from make to make. in the case of the Trader, the theoretical load transfer is just under 3,400 lb.
A quarry close to the York works at Corby, Northamptonshire, was used for a series of tests to check the performance of the load transfer system. The weighbridge at the quarry was used to discover the actual load transferred and in the unladen state—kerb weight was 5 ton 17-cwt.--it was found that byapplying the air load transfer, I ton 5 cwt. was taken from the rearmost wheels to the driving axle. A number of runs around the quarry, up and down gradients and through thick mud and so on were made and it was found that the operation of the system made definite improvements in traction. The tipper was driven up inclines, for example, until traction was lost by one wheel slipping and where the vehicle could not be restarted by normal methods. With the load transferred, although there was not a startling return of traction, the vehicle could be driven away.
After these runs the body was loaded with about 9 tons of limestone and reweighed before carrying out laden runs. It was discovered that trouble must have developed in the air system for the load transfer, as only about 4 cwt. was being transferred. It was subsequently found that, in fact, the control valve, which is located in the cab, had developed a fault.
When the valve had been replaced and the system checked round, laden tests were carried out. The load for these was almost I 0 tons-15 ton 14 cwt. gross. The load on the driving axle was 7 ton 4 cwt. and on the third axle 5 ton 2 cwt. without air-load transfer in operation and when the system was applied just 1 ton was transferred onto the driving axle. This was much less than the theoretical figure and the difference could have been due to a number of factors, the main one being inaccurate weighing as it is difficult to obtain accurate figures for the individual axles in a bogie on conventional weighbridges.
It was obvious that the addition of only a ton or so onto the driving axle would not result in any startling improvement in traction and in fact this was the case. Similar runs to those when unladen were carried out, the main ones being with the vehicle driven onto an incline until all traction was lost and then the air-load transfer brought into operation. The improvement in traction was found to be only marginal, sometimes the vehicle could be driven on, at other times it could not. But what was important was that there was some improvement. It is not possible to say how much improvement is obtained with a load transfer system unless extensive testing is carried out, including runs with other types of vehicles, such as double drive sixwheelers, four-wheelers and so on. And even then it would be hard to be dogmatic, as the actual state of the ground on which the wheels were resting could vary from place to place.
What the system does do is to give the six-wheeler at least equal traction to a four-wheeler and, at around £100, fitted, it is certainly worthy of consideration.