Patent GB-999538-A (GB)
Title: Improvements relating to hydraulic braking systems
IPC: B60T17/22
Filed: 1961-11-21
Abstract: 999,538. Hydraulic brakes; brake indicators. R. T. HAMILTON. April 27, 1962 [Oct. 30, 1961; Nov. 21, 1961], Nos. 38707/61 and 41547/61. Headings F2E and F2F. [Also in Division H2] A differential pressure responsive device for hydraulic braking systems for road vehicles consists of a plurality of separately movable hydraulic pressure responsive members (such as pistons 34, 36 in Fig. 3) adapted to be associated respectively with separate hydraulic circuits of the system such that when substantially equal pressures obtain in the circuits the members are moved in a relatively opposed balanced manner and when unequal hydraulic pressures obtain the members are moved in a relatively unbalanced manner, indicator means being associated with the movable pressure responsive members to give an indication of the normal condition and failure of an hydraulic circuit or circuits. In the vehicle hydraulic braking system shown in Fig. 2, a master cylinder 10 supplies fluid under pressure to the front wheel brakes 17 via pipe 15 and another master cylinder 11 actuated by a common pedal 14 supplies fluid under pressure to the rear wheel brakes 18 via pipe 16. Alternatively, a tandem master cylinder may be used. Between pipes 15 and 16 is a differential pressure-responsive device 23 and as shown in Fig. 3 this device takes the form of a hollow body 24 provided with connections 26, 27 leading to pipes 15 and 16 respectively. Opposed piston members 34, 36 provided in the hollow body 24 form the movable pressure responsive members and normally abut stop extensions 50, 51 by reason of the spring 49. In this position, switches 54, 55 are actuated by electrically insulated flange parts 42, 48 of pistons 34, 36 so that lamps 60, 61 are not illuminated. Providing that the two circuits associated with the front wheels 17 and the rear wheels 18 are in efficient working order when the brake pedal 14 is actuated hydraulic pressure enters the cylinders 33 and 35 and operates on the pistons 34 and 36 to cause the pistons to move towards each other against the action of the spring 49 in a relatively opposed balanced manner until the shanks of the pistons abut, such that the contacts 56 and 58 make electrical contact with the electrically conductive sleeves 40 and 46 respectively, whereby the two lights 60 and 61 are illuminated. When the pressure on the brake pedal is released the two pistons move apart to their fullest extent so that both lamps are extinguished to give an indication to the driver of the vehicle that both hydraulic circuits are working efficiently. Should a fault develop in one hydraulic circuit, full hydraulic pressure will not be developed in the appropriate cylinder when the brake pedal 14 is actuated and the respective piston will remain in the position shown in Fig. 3 with its associated light extinguished to give an indication of failure to the driver of the vehicle. An alternative embodiment is disclosed (Fig. 5, not shown) in which spring biasing means are associated with the pistons which are connected together by lever means to effect the balancing of the pistons. A further embodiment is disclosed in Fig. 6, wherein the separately movable pressure responsive members are in the form of Bourdon tubes 92, 93 which are biased in a relatively opposed manner and against hydraulic pressures obtaining in the circuits. The Bourdon tubes are arranged to be associated with a common movable member 98 associated with switch contacts 104, 105 wired to indicator lamps 108, 109 respectively. When the two hydraulic circuits are in efficient working order, both free ends 96 and 97 of the Bourdon tubes will move inwardly towards each other to bear against extensions 112 and 113 on the member 98 so that the electrical circuits are closed to illuminate lamps 108 and 109. In one hydraulic circuit has failed only one free end 96 or 97 will bear against its respective contact extension so that only one lamp will be illuminated and the arm 98 may be locked in its offset position so that one indicator light remains permanently illuminated until the hydraulic circuit failure is rectified.