A New Make of Vacuum Servo Brake
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ANEW entrant into the field of vacuum-brake makers is Hamilton Motors (London), Ltd., 466-490, Edgware Road, London, W.2. The concern's latest product is the Theed servo apparatus, and among its desirable features are simplicity and cheapness. The price of the servocylinder, vacuum valve and fittings as constructed for Fords and Bedfords is £4, a figure that does not includ the cost of installation. This, however, is a simple matter well within the capabilities of any semi-skilled mechanic.
The vacuum cylinder is a drawn steel tube with, of course, closed ends, and provided with a leather-cupwasher-type piston and rigid piston rod. It is mounted at its front end by a hinge having a horizontal transverse axis, the rod extending rearwards and terminating in a clevis, pinned to an arm secured by a clip and high-tensile steel grub-screw to the brake cross-shaft. No alteration to the chassis, excepting the addition of a few parts, is involved in the attachment of either the cylinder or valve.
The body of the latter is mounted to any convenient part—in the Bedford to the top of the gearbox—whilst the operating arm is fixed to the pedal rod in such a manner that the application of the brake moves the valve plunger forwards against a spring. A lost-motion device is provided at the point of attachment of the rod to the crossshaft.
The plunger is of silver steel and is a, ground fit in the B24 mild-steel baud. For part of its length it is of reduced diameter, the section thus machined tapering from front to rear. The annular space so formed is connected to the plain front end by an axial passage. To the same end of the barrel the pipe from the cylinder is connected, whilst that from the induction manifold communicates with a port in the side of the valve barrel. There are further ports at the front open to the atmosphere.
Normally the plunger is held by the spring in such a position that it closes the suction port but leaves the atmosphere ports open. When the brake is applied the latter are closed and the former opened, the admission of air being gradual, by reason of the taper. In this position air is exhausted from the cylinder, via the axial passage, annular space and piping, and the pedal partly or wholly relieved of braking effort. On release of the pedal the suction is cut off and air permitted to enter the cylinder by way of the atmosphere ports. The pedal position at which the servo motor becomes effective can be adjusted by moving the valve axially in its clips. The apparatus should be reliable and lasting, the piston rod and valve plunger being protected by leather bellows.
The results of a series of tests which we carried out with the apparatus installed on a Bedford lorry that had covered 37,943 miles, and of which the brakes were badly worn, having an efficiency of barely 30 per cent., are shown in an accompanying graph. They, reveal the fact that the addition of the gear gives a new lease of life to an old chassis, bringing the braking efficiency up to nearly 50 per cent., and afford an indication of its effect upon the braking of a new machin-.