Patent GB-1470570-A (GB)
Title: Hydraulic braking system with skid control arrangement
IPC: B60T8/40
Filed: 1974-05-07
Abstract: 1470570 Anti-skid braking system NISSAN MOTOR CO Ltd 7 May 1974 [13 June 1973] 20087/74 Heading F2F An anti-skid hydraulic brake control system incorporates a skid control arrangement between a master cylinder MC and a wheel cylinder WC which skid control arrangement comprises a main passage control section 12 and a by-pass control section 14; a source S of operating fluid normally communicates with a chamber 20 of the section 12 via a solenoid operated directional control valve 60, and the pressure in chamber 20 acts on a piston 36 connected to a plunger 34 connected to a valve 26; should a skid develop a control module 66 energizes the valve 60 and chamber 20 is connected to a reservoir 62 to reduce pressure in chamber 20 whereby piston 36, plunger 34 and valve 26 are moved rightwardly to close valve 26 on a passageway 34 to disconnect chambers 16, 18 of section 12 and relieve braking pressure in wheel cylinder WC. The source S comprises an engine-driven pump 46 an accumulator 48 communicating with the pump 46 through an unloading valve 52 with a cut-in pressure Pi and a cut-out pressure Po. The source Si is in constant and direct communication with a chamber 72 in the section 14 by a line 125 which chamber 72 has axially slidable therein a control piston 88 connected to a control plunger 86 slidable in a bore 68, the plunger 86 being connected to a ball valve 102 by a stem 104. The section 14 also includes a bore 70 and a chamber 74, a counter plunger 92 being axially movable in bore 70 and an attached counter piston 94 being axially movable in chamber 74. The counter plunger 92 is formed with a diametrical hole 106 in constant communication with a circumferential groove 82 in the bore 70, an axial passageway 98 opening into chamber 74 and an axial bore 100. A preload spring 116 biases ball valve 102 away from counter plunger 92 and the counter plunger 92 is biased away from the ball valve 102 by a pre-load spring 108. The pistons 88, 94 preferably have substantially equal pressure working areas, as do the plungers 86, 92. A switch unit 124 projects into chamber 74 and is closed when engaged by the counter piston 94 and is responsive to an operating fluid pressure which is determined in terms of a given braking fluid pressure as indicated by line m in Fig. 2. A pressure responsive contact unit 126 detects an absolute minimum level of operating fluid pressure Pmin and opens in response to an operating fluid pressure lower than this. The units 124, 126 are serially connected to an input terminal of the module 66 which is operative when both switches are concurrently closed. Relative movements of plungers 86, 92 are brought about by a difference between the operating fluid pressure P in chamber 72 and the braking fluid pressure Q in chamber 74. The braking pressure Q is supplied to the chamber 74 from chamber 16 via a passage 84, groove 82 and passage 98 although as an alternative to passage 98 a passageway may be formed in housing between passage 84 and chamber 74 or directly between chambers 16, 74. The pressure P in chamber 20 moves the piston 36 leftwardly and the master cylinder MC communicates with the wheel cylinder WC via chamber 16, passageway 24, grooves or recesses 38 in plunger 34, chamber 18, passageway 80, bore 68, passageway 78 and bore 70. If under these conditions an impending skid is sensed the valve 60 is energized by module 66 and the resulting loss of pressure in chamber 20 causes braking pressure in the brake cylinder WC to be relieved, until the skid condition is relieved. Even if the operating fluid pressure is reduced below the cut-in pressure Pi, skid controlled braking may still be effected as indicated on Fig. 2. If the operating pressure P during an anti-skid operation is reduced too far the plungers 96, 98 are moved rightwardly to open switch unit 124 and disable the module 66. The valve 60 therefore supplies the pressure remaining in accumulator 48 to chamber 20 to move back the plunger 34 to open valve 26. If the vehicle is running on a slippery road surface, skid-controlled braking can continue until pressure P falls below Pmin when contact unit 126 will open.