The Latest Mardi Servo System
Page 46
If you've noticed an error in this article please click here to report it so we can fix it.
AN improved Marelli vacuum-servo braking system has recently been developed by Feeny and Johnson, Ltd., 134, Ealing Road, London, W. It is a remarkably clever device and not its least interesting feature is its simplicity.
The apparatus consists of a valve, connected in the brake rod, and a servo cylinder-, hung at one end by an arm from the rod and at the other end by a link, whichis anchored on a fulcrum to any convenient part of the chassis, and which is connected by a pin to the rod.
The rods and links hitherto used in the Marelli system have been eliminated and the only point of anchorage of the device to the frame is the fulcrum of the lever.
Situated within the valve is a spring—slightly weaker in effect than the brake-return spring; this transmits the straight pull from the brake pedal to the brake rod ; thus, when the pedal is depressed, the body of the valve will move relatively to the inner part, the amount of the motion being proportional to the pressure of the foot. This relative movement causes air to be drawn from the bellows through the valve to the suction tube, the quantity being regulated by the distance of the cone-faced valve head from its seating. The consequent contraction of the bellows exerts a forward pull upon the lower end of the lever. The reactionary force at the other end of the bellows is, of course, transmitted as a backward pressure, to the arm, at the point where the brake rod is coupled, with the result that a powerful turning moment is set up in the arm about its point of anchorage. Pressure, however, cannot be transmitted along the pedal rod, for unless this is in tension, the valve will close and air will pass to the bellows from the atmosphere; thus the brake-rod tension is controlled entirely by the pressure of the foot.
The travel of the brake rod is limited only by the dimensions of the bellows and the arm, and the apparatus can be accommodated anywhere on the chassis, for, being anchored to the frame at only one point, it may, if desired, be fixed to a cross member as readily as to a longitudinal.
The action is stated to he extremely flexible and progres
sive, and it is alleged that the force exerted cannot "build up." It requires little or no upkeep, and it provides a perfectly straight pull. Should the engine stall, the vacuum would be maintained by the face on the back of the valve head closing the passage to the atmosphere.
The device, in its smallest size, exerts a pull on the brake' rod of 750 lb. (less pedal effort) and weighs 10 lb. It is also made in a number of larger sizes. The prices are provisionally fixed at from £4 4s. upwards.
In the works of Feeny and Johnson, Ltd., we examined and tested a demonstration model fitted to a chassis, and so arranged as to lift, by means of a system of bell cranks, a loaded platform. Working on a vacuum of between 5 ins. and 10 ins., a weight of 1 cwt. could be lifted with the utmost ease by the pressure of one hand on the brake pedal.