Cranes Offer Air-mechanical Disc Brakes
Page 47
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ADOPTION of disc brakes for heavy vehicles has been hampered by the need for extremely high pressures at the brake pads. It is difficult to provide adequate power with an air-operated mechanical system. Now Cranes (Dereham), Ltd., Dereham, Norfolk. independently of established brake manufacturers, have developed :a mechanically operated disc •brake for heavy trailers which, although. it has yet to be tested in this application, promises to be completely practicable.
It is fitted to a 45-60-ton trailer for Pickfords, which will be shown on the Cranes stand at the Commercial Motor Show. Cranes are also to exhibit a new air-sprung side-by-side-axle bogie fitted to a straight-frame semi-trailer, and another air-sprung bogie with independently suspended stub axles.
The latter unit is being fitted to frameless road tankers by the Yorkshire Engineering and Welding Co., Ltd. A feature shared by the two air-sprung designs is the use of Ferrobestos selflubricating fabric bushes at the main pivot points.
The disc-braked trailer is a conventional crankedframe type but uses a novel layout; originally developed for South African operators, for the twin detachable rear axles. Each wheel is carried on a stub axle attached at right angles to a solid longitudinal beam.
Thus each axle unit, which on normal trailers of this type would be straight, is Z-shaped in plan. This places the inner pair of wheels behind the outer pair without the complication of separate axles and, by distributing the bogie load over a greater area, simplifies the routeing of the laden unit.
Each brake disc is keyed to the wheel hub by 12 sliding dowels which locate in bores in a rim at the periphery of the 18-in.-diameter 11-in.-thick cast-steel disc. The wide rim aids heat dissipation, whilst the avoidance of a rigid connection between the disc and the hub limits heat transference to the latter unit.
Bearings Protected
This protects the wheel bearings and simplifies lubrication. As the disc can move freely on its mountings alignment problems are overcome.
The brake shoes and i-in.-thick brake pad segments, which are bonded and riveted to them, are forced into contact with the disc by a shaft which passes through a slot in it.
The brake pad on the hub side of the disc is pressed on to it by retraction of the shaft. This is performed by a screw mechanism turned by a lever actuated by
an air-pressure cylinder. Initial movement of the brake pad tends to force the disc off its locating dowels against the other fixed brake pad.
To provide for wear of the pads, a ratchet is interposed between the operating arm and the screw-actuating mechanism, and this gives automatic slack adjustment. When air pressure is released after a brake application, the return spring in the servo draws the actuating arm back, whilst the opposing pads are pushed off the disc by coil springs incorporated in the shoe assembly.
Power exerted by the 30-sq.-in., air cylinder is multiplied 10 times by the lever-and-screw mechanism. Braking effect obtained from the discs is expected to be about double that provided by the 15-in.-diameter drums with 7-in.-wide shoes used on previous trailers of this type. This should also be achieved without the fade which has been a problem on these large trailers.
Although the disc-brake system fitted to the 45-60-tort trailer has not been tested, similar but lighter equipment has completed a year's experimental service on a tandem-axled semi-trailer, coupled in this case with air suspension.
The in-line-axle semi-trailer for the Show is one ofsix ordered by Fisher and Stacey. Ltd. Each axle is carried on a large box-section trailing beam fabricated from -in.-thick material and pivoted on large Ferrobestos bushes at II-in. centres.
The beam is 3 ft, 6 in. long and the tubular axle is mounted above it, 2 ft. behind its leading end, on trunnions which incorporate Ferrobestos bushes and thrust washers.
A small pedestal at the rear of the beam supports the lower mounting plate for the Firestone two-convolution air spring, the upper face of which abuts on a similar plate attached to a member dropped from the frame.
Rebound is restricted by a %rubbercushioned rod and the suspension is controlled by a Clayton Dewandre two-stage levelling valve. This is mounted on the upper spring abutment and is linked to the lower spring-mounting plate.
Transverse axle movement is checked by stops on the trunnion, which permit one wheel to be 5i in. above its twin at maximum deflection. Each hub carries a 151-in.-diameter brake drum with 31-in.-wide linings, the brake unit being operated by an air cylinder mounted on the axle.
• An interestingpoint is the absence of conventional dampers, although most authorities consider them essential for air suspension, which has no inherent damping. Cranes have found that the drag exerted by the Ferrobestos bushes in the trailing beam pivots (they tend to swell slightly after installation) provides adequate restraint.
The trailer to be shown with in-lineaxle air, suspension arrangements will be a 12-15-ton model with a rolled-steel frame. It will be exhibited without bodywork and costs about £150 more than comparable leaf-sprung types.
Cranes' other new air-suspension design gives independent suspension and is primarily intended for road tankers, The complete single-axled bogie is carried on .a compact simple sub-frame.
Short Side Members The frame consists of two short side members joined by an 8-in.-diameter cross-tube which contains the surge tanks for the air bellows. .Three brackets welded to the bottom of the tube carry a cross-shaft on which the separate 1-ft 441-in.-long trailing beams for the stub-axle pivot. Ferrobestos bushes are employed for bearings.
The stub axles are welded to the beams, and platforms behind the axles form the lower mounting plates for the two-convolution air springs.
Clayton levelling valves and rebound rods are fitted. Telescopic dampers are provided on the bogie supplied to Yewco but, because of the action of the Ferrobcstos bushes, Cranes do not consider them essential.