Patent GB-1182579-A (GB)
Title: Vehicle Brake Systems
IPC: B60T8/74
Filed: 1968-07-18
Abstract: 1,182,579. Fluid-pressure brake control. GENERAL MOTORS CORP. 18 July, 1968 [4 Aug., 1967], No. 34230/68. Heading F2F. An automatic control system for preventing skidding due to excessive braking, of the type in which a sensor unit (e.g. a flywheel, cam and valve arrangement) is used to control a modulator (e.g. a valve mechanism) for reducing pressure in the wheel brake cylinder, is characterized mainly in that at the end of the pressurereduction phase, and before the pressurerestoration phase, a pressure-holding phase is interposed, at which the reduced pressure is maintained constant: this phase begins after the wheel has begun to accelerate again, and ends when the wheel has finished accelerating, or before this. As shown in Fig. 1, applied to the rear brakes of a motor vehicle, a shaft 74, driven by the propeller shaft 75 of the vehicle, has a lost motion connection (crosspin 80, and slot) with a cam 82, and the cam has a lost motion connection (slot 86 and pin 84) with a flywheel 76. Three balls 88 (see also Figs. 2 to 4) are located in depressions 90 in the flywheel and co-operate with cam slots 92. A spring 134 acting through a valve rod 110 and a ball 96 urges the cam to the left. (Figs. 2 to 4 are a diagrammatic simplification of the relevant parts of Fig. 1). The valve section 34 of the sensor unit 30 has an inlet 106 connected to atmosphere, an inlet 120 connected to vacuum at 38, and an outlet 36 connected to a chamber 48 above a diaphragm 44 and power piston 42 in the brake-pressure modulator 28; the lower chamber 46 therein is also connected to vacuum at 38. In operation, excessive deceleration causes the flywheel 76 to over-run the cam 82-the position of Fig. 3, where the flywheel has moved downwards in relation to its position in Fig. 2, the normal position. Valve 128 has now closed while valve 122 has now opened, whereby atmospheric pressure is applied above diaphragm 44. Piston 42 thereupon descending, first permits valve 60 to seat by extension 68 on piston 66 secured to it, to cut off communication between master cylinder 22 and brakes 14, 16, and thereafter reduces pressure in the brakes by increasing the volume of passage 64 communicating with conduit 26. When the pressure above 44 is balanced by spring 70, downward movement ceases. Upon brake-pressure reduction, the wheels stop decelerating and start accelerating again: this brings about the position of Fig. 4, where flywheel 76 has underrun cam 82 (and is upwards in relation to its position in Fig. 2), but because of the shape of slot 92 valve rod 110 is not so far to the right as in Fig. 3. Both valve 128 and 122 are now closed, so the position of the diaphragm-piston assembly 44, 42 is fixed for the time being ; this is the pressure-holding phase. When the wheel acceleration has decreased substantially to zero, relative displacement between flywheel and cam is also zero, and the position of Fig. 2 is reached, where valve 122 is still closed, but valve 128 is open. The diaphragm and piston in modulator 28 now return to the Fig. 1 position, where the master cylinder is placed in communication with the brakes once more. Preferably the vacuum supply 38 is the engine intake manifold, so that when the engine is not running the chambers 46 and 48 are both at atmospheric pressure, whereby spring 70 ensures that valve 60 is open and the brakes may be applied.