New Seddon Tractor Unit With Two-speed Axle
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Seddon Tractor with Scammell Coupling and Two-speed Axle for Operation as a lo-ton Articulated Unit THIS week, "The Commercial Motor" is able to announce a new Seddon forward-control oilengined tractor chassis fitted with an Eaton two-speed rear axle and equipped with the Scammell gear for semi-trailer operation. A prototype model has been subjected to rigorous tests at Oldham and Watford, and Seddon Motors, Ltd., in conjunction with Scammell Lorries, Ltd., has now started assembling production models.
Fuel-consumption tests made during the trials at Watford with. a 10-ton payload produced a return of 13.8 m.p.g.
The Seddon chassis frame, which has been specially shaped to accept the Scammell standard ramps and coupling gear, is fitted with the Perkins P6 oil engine, which develops 79 b.h.p. at 2,400 r.p.m. Standardization of units with other models of the Seddon range has been observed in the new tractor, which has a Borg and Beck 12-in. diameter clutch and David Brown fivespeed gearbox. A one-piece propeller shaft with Hardy Spicer needle-roller zouplings is used between the gearbox and two-speed axle. The combination of the five-speed gearbox and twospeed axle affords a variation of ten forward and two reverse ratios.
To provide sufficient clearance between the additional tubular crossmember and the diaphragm controlling the speed change of the axle, the diaphragm has been removed from the casing and fitted to the frame side member with a short cable connection to the operating lever in the axle. A second modification at the rear axle has been to attach the brake compensating arrangement to the front of the case, again to provide clearance under the coupling gear when the semi-trailer is loaded to the maximum. The frame downsweep at the rear requires reverse spring hanger-brackets and the springs are of a stiffer pattern than normal to accommodate the load. All wheels are fitted with 34 by 7-in. tyres, with twin tyres at the rear. The tractor has a 9-ft. wheelbase, and the turning circle is 34 ft. The Marks doubleroller cam-and-lever steering is conventionally linked to the front axle.
To obtain a good balance of braking between the tractor and semi-trailer,
the prime mover is fitted with Girling wedge units of 15-degree angle in place of the normal 13-degree wedge supplied on other Seddon vehicles. The Clayton Dewandre and Lockheed hydraulic braking system is specified, and the servo for operating the trailer brakes is piped from the end of the master cylinder.
These brakes only are vacuumservo operated, and the brake linkage is arranged so that the trailer brakes are always applied before those on the tractor. The trailer brakes are automatically connected in the coupling operation. A light guard surrounds the master servo and hydraulic unit to protect it against damage when loading the spare wheel on its carrier which is built horizontally across the frame behind the cab.
The guide ramps for the semi-trailer rollers are secured externally to the main chassis frame and the tubular cross-member carrying the coupling gear and stays is fitted inside the frame
where it picks up the same bolts which hold the ramps. This is the Scammell standard equipment which is fitted to other makes of chassis and ensures that the Seddon tractor can be used in conjunction with any existing semi-trailer unit having the Scammell 6-ton dolly and coupling arrangement.
There are two additional levers in the cab, one for operating the parking brakes on the trailer unit, whilst the second locks the coupling gear. With the Scammell system, the semi-trailer can be coupled or disconnected from the cab without the driver leaving his seat. The unladen weight of the tractor, after conversion, in taxation condition is 2 tons 13 cwt. 2 qrs. which gives a maximum gross train weight of 14 tons 14 cwt. 3 qrs.
Two-speed Control Apart from the additional levers in the cab the driving compartment is arranged in the same manner as the Seddon standard tractor. The control valve unit, actuating the two-speed arrangement of the rear axle, is in the form of a trigger attached to the gear lever. To engage the high ratio, the trigger is lifted, and to obtain the low ratio it is depressed.
Interconnected with the control valve, a vacuum gearbox unit incorporated in the speedometer drive provides the necessary adjustment to give a true reading to the instrument for both ratios.
The Seddon tractor, which was among the first vehicles to be equipped with a two-speed axle in this country, formed the subject elf a road test reported in "The Commercial Motor" for February 6, 1948. With a total running weight of 14 tons 8 cwt. a fuel return of 10.4 m.p.g. was obtained, and accelerating from rest to 30 m.p.h. using the high axle-ratio and all four gears occupied 44 seconds. The axle ratios during this test were 6.33 and• 8.81 to 1. This 'tractor was ,equipped with a four-speed gearbox and 34 by 7-in. tyres.