GB1355339A - Vehicle wheel brakes - Google Patents

Abstract

1355339 Locking brakes "on"; transmitting- mechanism; slave cylinders GIRLING Ltd 30 June 1971 [21 July 1970] 35246/70 Headings F2E and F2F A lock actuator, e.g. for a vehicle internal shoe-drum brake includes an adjustable-length strut comprising a pair of relatively-rotatable members 8, 12, Fig. 1, and which is normally free, in use, to extend and contract to follow up movements of a brake applying member, and friction clutch means (comprising, as shown, the left-hand face 13 of member 12 and an adjacent inner face of rotationally-fixed tappet 6) which are selectively operable to lock the strut in an extended condition, there being resilient means 17 acting on the clutch means to disengage it, and a fluid-pressure-actuated motor 14, 16 for clutch engagement. As shown, nut member 8 is prevented from rotating by pin 9. To apply the lock, main working chamber 4 is pressurized to separate further two pistons 2, 3, whereupon the strut parts 8, 12, are moved outwards with their respective tappets 7, 6. Chamber 16 is then pressurized, causing piston 14 to move spindle 12 leftwards to engage the friction clutch 6, 13 (overcoming the pressure of spring 18) and thereby lock the strut parts against relative rotation. Next, chamber 4 is relieved of pressure so that shoe retraction forces, through the tappets, hold the clutch engaged, and in turn the strut holds the brake shoes applied. Finally, chamber 16 is relieved, and the lock remains applied until the clutch is next disengaged by pressurization of chamber 4. The resilient means may alternatively take the form of a block of elastomeric material behind a disc, a coil spring behind a ball, or a conical spring disc; in Fig. 5, a spring 18 works onanhydraulic plunger 21 in an cylinder 22 which is pressurized to compress the spring and thereby relieve the spindle 12 of the spring force when the clutch is to be engaged. To guard against accidental release of the parking brake lock by operation of the foot brake, there is inserted in the line from the service brake pressure source to the wheel cylinder, a shuttle valve which serves to isolate the working chamber 4 from the pressure source when the locking function has been selected, and until the lock release function is next selected.