DUMPER'S HYDROSTATIC TRANSMISSION
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Oo a recent visit to Dowty Hydraulic Units Ltd., Ashchurch, Glos, by a member of The Commercial Motor staff details were obtained of the Dowty Dowmatic hydrostatic drive fitted to the new DD105 12-cu.-yd. 4 x 4 articulated dumper produced by Whitlock Bros. Ltd., Great Yeldham, Essex.
The machine represents an outstanding application of hydrostatic drive in a number of important ways, apart from the avalability of an infinitely variable gear ratio in forward and reverse, notably that no other means exists to drive the rear wheels of a 4 x 4 articulated vehicle except the use of a separate power unit for the trailer; it greatly facilitates mounting of the Perkins 6.354 95-b.h.p. engine in front of the tractive-unit axle; it enables the front axle to be driven without the rear axle for relatively high speeds on the road, and with four-wheel drive in operation the driver can progressively vary torque distribution between front and rear axles with the Van-Speed singlelever control. Also, with the aid of individually operated wheel brakes, it can check spin of any of the wheels.
The variable-displacement pump of the Dowmatic system is of the axial-pistontilting-head type and is driven direct from the crankshaft of the Perkins 'engine through a flexible coupling. The drive is transmitted to the wheels of both axles by a fixed-head axial piston motor, mounted centrally on the chassis; in each case
through a conventional type of differential. When the four wheels are being driven the pump output is divided between the two motors, whilst the entire output is directed to the one motor when the front wheels only provide the traction for road work and the speed capacity of the machine is doubled. The maximum road speed is about 20 m.p.h.
Details of the Dowmatic system include a torque multiplication of 4 to 1, a maximum pump input speed of 2,250 r.p.m. and a maximum motor torque of 710 lb. ft. Overall efficiency does not fall below 80 per cent, the optimum being about 90
per cent at half output speed. When the rear drive is " free-wheeling ", the fluid is circulated round the casing of the motor to provide cooling.
Air-pressure braking is employed and a conventional type of pedal is used for normal brake applications, four footactuated buttons being provided for operation of the brakes individually. Steering is assisted by a hydraulic servo mechanism and the machine has a turning radius of only 18 ft. 6in.
A " commuting link" is a special feature of a new type of Dowty hydrostatic transmission announced a few weeks ago, the pump and motor of which are both of the tilting-head type. The units are arranged in line or on parallel axes and the link gives balanced hydraulic loading. For vehicle applications, the pump is coupled to the engine crankshaft and the motor is directly geared to the road wheels. The unit is designed for a maximum continuous output of 30-50 b.h.p. at 2 ,500 r.p.m. and a maximum output torque of 600 lb. ft.