For Heavy Haulaw it Top Level
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Powered by a Cummins 250 b.h.p. supercharged engine the Wynn-Pacific 6 x 6 tractor is equipped with a S.C.G. eight-speed gearbox and a three-range transfer box. By P. A. C. Brockington, A.114.1.Mech.E. WHILST the Pacific 6 x 6 forwardVV control tractor, built by Robert Wynn and Sons Ltd., Newport, is based on the frame and rear drive units of an army type articulated tractor produced by Pacific Truck and Trailer Ltd., of Vancouver, the latest type combines a variety of advanced features that reflect the company's extensive prictical experience of heavy haulage and preparedness to cater for indivisible loads of progressively increasing weight. In place of the original Hall Scott petrol engine, the Pacific is equipped with a Cummins NHRS-6-B supercharged six-cylindered diesel engine developing 250 b.h.p. at 2,100 r.p.m., the maximum torque being 750 lb. ft. at 1,550
The supercharger is of the Roots type and the crankshaft is fitted with a viscoustype torsional-vibration damper. Drive is transmitted through a Lipe twin-plate dry clutch to a Self-Changing Gears R.V. 30 eight-speed gearbox and thence to a three-range transfer box. Propeller shafts relay the torque from the transfer box to a Knuckey double-reduction front axle and to a single double-reduction rear axle. The latter is located centrally in the bogie to provide chain drive to the wheels, the sprockets being outrigged from the assembly on each side. This form of drive enables the final torque to be stepped up without overloading the half-shafts, the chain ratio being 2-5 to 1.
Size Variations Of importance in the construction of the Pacifies, variations in the size of tyres fitted to the front and rear axles can easily be accommodated by fitting suitable sprockets.
The layout of the rear transmission is such that a half-shaft can be changed in one hour, and a light auxiliary hoist on the main platform provides for replacement of the differential unit in three hours, spares being carried'on the vehicle. Reducing the time required for such replacements may be all important in a particular operation.
Braking Braking is applied to the rear wheels by four piston-type air-pressure actuators (of the type fitted to the original Pacific chassis) and independently by four airdiaphragm units installed in the Wynn workshops, which approximately double the braking power available in addition to providing a higher safety factor. Three reservoir tanks are supplied by a Westinghouse engine-driven compressor, one of which is employed for gearbox actuation and two for operation of the brakes, alt the tanks being equipped with non-return check valves. The drum brake mechanism is of the conventional lever and S-cam type, whilst a mechanically operated transmission brake is used for parking.
The brake operating pressure of the combined tractor-trailer system is MO p.s.i., and if the pressure drops to 60 p.s.i. a tractor-protection valve automatically cuts off the air supply to the trailer and isolates the two sections of the system, so that braking power is still available for the tractor. in the event of the complete failure of the tractor system, manual operation of a bleed valve in the emergency line of the trailer system automatically applies the trailer brakes in the normal way. This is in addition to the safeguard provided by the emergency breakaway system.
Operation The foot pedal operates all the brake cylinders of the tractor and trailer. Controlled by a hand lever on the steering column, the original piston-type system is employed for braking on gradients and for parking. Manual control can be used for these operations, because the lever can be positioned appropriately for long periods without sustained effort on the part of the driver. Another hand lever controlling the trailer brakes is also arranged to operate the emergency bleed valve by a check valve system which links the emergency and service
Engine installation details include a roof-mounted header tank at the rear of the .cab, which, in conjunction with a 4-p.s.i_ pressure cap, effectively eliminates air, entrainment in the cooling system. Although engine-noise level is average, the cab is lined with 2 in. of foamed rubber acoustic insulating material in the interests of sound damping so that messages over the Tannoy transistorized inter-communication system can be more readily heard. The cab is externally lined with aluminium panels and incorporates plastics corners. Bolted to the main structure at waist level the cab can be removed from the vehicle in less than one hour, which simplifies engine changes. Supplied with the chassis, power-steering gear of the hydraulic-piston type is incorporated in the column and is energized by a Vickers' engine-driven pump.
Experiments
The Wynn company has also experimented in the case of the latest Pacific with a S.C.G. 17D torque converter and RV 48 gearbox, which could, with advantage, be equipped with one or, more extra ratios, but particular tribute is paid to its smooth operating characteristics which are of special value in many operations. The torque converter is fitted with an overrun clutch to give positive enga2ement of the engine and to provide full engine braking. A heat exchanger for the converter fluid is fitted on the engine and an oil-to-air cooler is located in front of the radiator. The original radiator has an additional unit fitted below it, cooled by a separate shaft-driven fan. Overall length of the vehicle is 26 ft. 9 in., whilst the width of the standard Pacific is 11 ft The standard tyre size is 14-00-24, but, in the case of the latest tractor, 14-00-24 front tyres are employed and 12-00-24 rear tyres. This necessitated changing the existing sprockets to an appropriate size-and re-designing the rear hubs.
A total of 20 Wynn vehicles are equipped with Cummins 7-39-litre diesels, which include Diamond Ts, two Scamme11 Mountaineers and a Lorain I7-ton mobile crane, in addition to the Pacifies. The NHRS-6-B units fitted to the Diamond T tractors are further derated to 220 b.h.p. at 1,700 r.p.m. and give a maximum torque of 6501b. ft. at 1,300 r.p.m. A naturally aspirated unit, the NHE-180, is installed in the Seammell tractors and Lorain crane and develops its normal rated power of 173 b.h.p. at 1,950 r.p.m., maximum torque being 520 lb. ft. at 1,100 r.p.m.