Control device for a motor vehicle pneumatic braking system

Abstract

A control device for a pneumatic braking system which is particularly suitable for industrial trucks and lorries the axle loading of which varies widely depending on whether the truck is empty or fully laden. The device has an inlet port which in use is fed with the braking control pressure, and an outlet port which feeds a servo-valve to determine the pressure fed to the brakes of, for example, the rear wheels. The control device comprises a generally cylindrical housing separated into two chambers by a piston, the two chambers communicate with each other through a controlled restrictor passage the size of the opening of which is controlled by the position of the piston. The exposed area of the piston in one chamber is greater than in the other, so that when the pressure in both chambers is the same, the piston experiences a resultant force towards the other chamber but movement is resisted by a force proportional to the axle loading derived by a mechanical or fluid pressure arrangement. If the braking control pressure in the two chambers reaches a value such that the resultant force on the piston is greater than the applied force the piston moves partly closing the restrictor thereby reducing the pressure in the one chamber until the forces on the piston are balanced. The outlet port from the device is fed from the one chamber so that the outlet braking pressure is reduced with respect to the inlet pressure in these circumstances.