Crossley Introduces a
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Four-wheeled Trolleybus IN addition to the six-wheeled trolleybus chassis, which was exclusively announced by The COM Fnercial Motor in October last, Crossley Motors, Ltd., Gorton, Manchester, is now producing a four-wheeled chassis suitable for 50-56 seater double-deck bodies. In many ways the electrical equipment follows the design, of the six-wheeler, but there is a number of new features
in the mechanical .specification. The frame, for example, is new and, naturally, the layout of the major components is also different from that of the larger chassis, which has been on the market for nearly a year.
As in all Crossley products sturdy construction, is apparent, whilst the detail work is well finished. It is clear that the chassis has been planned by a practical engineer who realizes the value of such attributes as good accessibility, simplicity of construction and the need for ample bearing surfaces for all working parts.
Low Frame Height, Before describing the items in the mechanical and electrical specification, it might be opportune to give here a few of the leading dimensions. The chassis has a wheelbase of 16 ft. and front and rear tracks of 6 ft. 7 ins. and 5 ft. 9 ins, respectively, whilst the overall length and width of the chassis are, respectively, 25 ft. 31 ins. and 7 It. 5i ins. When laden, the top of the frame stands 2 ft. above the ground, the floor height being 2 ft. 7 ins, at the forward end and 2 ft. 5 ins. at the rear. These dimensions are distinctly moderate considering the fact that the motor is placed amidships, beneath the floor.
To minimize the space required for floor bearers, etc., a i-in. sheet-sfeel plate is used above the motor, thereby
allowing the ground clearance under the power unit to be 10i ins, when using 9.75-20tyres. Naturally enough, the side members of the frame have had to be specially designed, with a long sunken portion between the axles.
Above the front wheels, the tops of -the side members run parallel with the ground and the webs taper in depth froni 5 ins, at the front to 101ins, in the most heavily loaded portion amidships. A pronounced kick-up over the rear axle gives adequate clearance for wheel Movement, and as the webs of the channels are maintained at a reasonable depth throughout, the overhung portion of the frame at the rear is adequately supported. ' The frame height at the platform is 1 ft. 1 in.
Adequate cross-bracing is afforded by the provision of a large number of ties of channel and tubular section. Right at the front is a. channel-sectioned pressing connecting the side members immediately above the spring pins, whilst a tubular member is installed. approximately in line with the
rear shackles of the front springs. Amidships are two channels, each wellgusseted to the frame sides and set 3 ft. 7 ins, apart to form the support for the cradle carrying the motor.
Two further large-diameter tabes, coincident with the fore and aft supports of the rear springs, stiffen up the structure at this heavily loaded part of the frame, whilst a. wide cross-bracing at the platform end helps to cut out any lozenging tendency.
Motor Suspension.
As in, the Crossley six-wheeled chassis, the motor is mounted on a separate sub-frame, the longitUdinals far this component being attached to the main frame at four points, which are insulated by means of sileatbloc rubber bushes carried by the fixing bolts. As has already been indicated the motor itself is made by Metropolitan-Vickers Electrical Co., Ltd., and is compound wound for regenerative and rheostatic control. The system employed enables the speed of the vehicle to be controlled by one pedal, both when motoring and braking.
The motor characteristics permit an output of 80 h.p. for one hour and a continuous development of 60 h.p,, the current consumed under these conditions being 132 amps. and 100 amps. respectively, at 500 volts. Metropolitan-Vickers control equipment is also used, the connections providing eleven notches, of which the first five are resistance notches, the next a full-field notch with no resistance, and the remaining five, notches giving weakened shunt field. This gives a total of six economical running positions.
Acceleration and Braking.
Acceleration is, of course, obtained in the normal manner by depressing a pedal which operates the master controller. On the return movement of the pedal the shunt field is strengthened, causing regeneration, the degree of retardation increasing, relative to the return movement of the pedal, up to the point 'where the full field strength is attained.
Effective retardation due to regeneration ceases at about 12 m.p.h., below which speed rheostatic braking connections are automatically established and effective control is main
tamed down to about 2 m.p.h. In the interests of economy a regenerative relay is provided which prevents the rheostatic braking circuit from being completed until full advantage has been taken of the regeneration to the line. An incidental feature of this type of zontral is that the maximum torque so obtained is controlled by the motor characteristics and riot by the driver.
The layout of the cab, which, of course, accommodates the contactor panels, the master controller, the reverser, etc., is on exactly the same lines as that of the six-wheeled vehicle. On the near side is the main contactor panel, whilst the scissor-type master controller and reverser are situated under the driver's seat. Immediately behind the cab and slung from the frame, is a cradle formed to carry the manceuvring batteries. These are of 48-amp.-hr. capacity, and operate at 60 volts—by coupling the' two 30-volt lighting batteries in series.
A full charge is maintained by a special single-shaft motor generator which operates on the line voltage, and as the motor is a series machine with no shunt winding, the use of necessarily extremely thin wire for a 500-volt shunt coil is avoided. The generator itself is of the constant-voltage type, and the arrangement of the low-tension circuit is such that advantage can be taken of the whole output during normal daylight running for charging purposes.
By using a relatively large-capacity low-tension equipment, the battery manmu,vring speed has been brought up to 3 m.p.h.—a feature which should be particularly attractive to operators in clistri.cts where the change-over from one route to another cannot be made conveniently" 'on the overhead
For the rest, the chassis follows wellestablished principles, and the detail work is carried out on the lines of the Crossley six-wheeled chassis. A HardySpicer propeller shaft couples the motor and the worm of the rear axle, a sliding splined sleeve at the forward end of the shaft taking care of the axial motion caused by axle movements. The resultant end thrust is withstood by a large-diameter ball thrust bearing mounted in the end case of the motor ; in this way the armature is relieved of end loading.
As already. stated, Westinghouse brakes are employed; the pedal is connected with the control valve through a linkage of simple design, the connections to the operating cylinders on each hub being effected by seamless steel tubing.
Air Compressing Plant.
The motor-driven compressor unit is mounted on an insulated base on the off side of the frame, rubber bushes with a large creepage surface being used for mechanical and electrical insulation. Air is drawn by the compressor through a filter, the pipe from which passes through the foremost tubular cross-member of the frame— an arrangement which, it is claimed, helps considerably to maintain a high degree of silence. An air reservoir of the usual type is also installed on the off side of the chassis.
A pull-on type of hand brake is provided and is linked up to the rear axle by means of rods and levers to operate on the same sets of shoes as those controlled by the Westinghouse cylinders.