40-ton payload trailers at Tilbury
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• Skeletal models, plated for a gross weight of no less than 50 tons, are included in the latest batch of semi-trailers supplied by Primrose Third Axle Co Ltd, Blackburn, Lanes, to the Port of London Authority.
Loading one of these 40ft-long trailers even to half its design gross weight would bring the articulated vehicle of which it was part very close to the present limit of 32 tons gross; but at Tilbury all journeys are restricted to private roads entirely within the precincts of the docks. There the trailers are hauled by Douglas KS8 Conbase Tugmaster tractors, each equipped with an elevating fifth-wheel coupling and powered by a GM diesel engine driving through Allison automatic transmission.
The purpose of the trailers is to move 20, 30 and 40ft ISO containers between the freight container terminal and container ships in any berth.
Designing for a gross trailer weight of 50 tons has been necessary in order to cater for containers carrying badly distributed loads. This rating provides for a 20 tons imposed load at the upper fifth-wheel plate and a ground load of 15 tons by each axle—the Primrose trailers being plated accordingly.
Although rated at 15 tons each, the two axles are spaced the normal distance apart as Construction and Use legislation does not apply in this instance. For the same reason, only a two-line air-pressure system is employed to operate the 15f4n. x 8+in. cam-expanded brakes. Suspension is by a multi-leaf four-spring system of non-reactive layout, while the tyres are Michelin XB 10.00-20in. twin formation. Typical unladen weight of one of these trailers is 5 tons 8+ cwt.
Where only single 20ft containers have to be conveyed at a time, the PLA uses smaller versions of these trailers, each rated at 32 tons gross trailer weight. Fifteen of these were supplied along with the 15 40ft 50-tonners.
A further order currently in hand for the PLA is for five more Primrose 40ft skeletals, of similar construction and complying fully with C and U legislation. These will have mechanically wound two-speed landing gears, instead of fixed legs, and three-line braking. Designed for operation at 50 tons gtw, They will be used for operations within Tilbury Docks; but they can, if required, be run at a restricted loading on the public highway, hauled by conventional tractive units.