Patent GB-1177295-A (GB)
Title: Brake Actuators
Inventor: THOMAS GORDON FREDRICK FRANK
IPC: B60T17/16
Filed: 1967-07-07
Abstract: 1,177,295. Lock actuators for brakes. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. 2 July, 1968 [7 July, 1967], No. 31335/67. Headings F2E and F2F. The invention relates to lock actuators having a pressure-sensitive element to which a first fluid pressure is applied to effect a brake application through a brake-operating member, and having fluid-operated means for locking the brake against release, the locking means being normally rendered inoperative by application thereto of a second fluid pressure. In a lock actuator according to the invention, the brakeoperating member is non-rotatable and is screw-threaded with a " reversible " thread, and the locking means includes a nut engaged with this thread; the nut is mounted for rotation and for limited axial movement and carries a first clutch face engageable with a second normally-fixed clutch face to prevent rotation of the nut; the arrangement is such that movement of the brake-operating member to effect a brake application tends to move the nut so as to disengage the two faces, thereby freeing the nut for rotation, and movement of the brake-operating member to effect a brake release tends to move the nut so as to engage the two faces ; this latter movement of the nut is prevented only so long as the second fluid pressure is applied to the locking means. Operation.-For " normal " operation the second fluid pressure is applied continuously at a port 38. To apply the brake, the first fluid pressure is admitted to chamber 41, whereupon a non-rotatable piston 2 moves a brake-operating link 12 by means of an externally-threaded tubular part 7. A nut 21 threaded on the tubular part 7 and having a clutch face 23 normally in contact with a clutch face 25 on an annular member 26 is held away from clutch face 25 by the pressure at port 38, which is exerted through an annular ball race 36 in the form of a piston, the balls 29 and ball race 35. The nut 21 is thus free to be spun by the interengaging screw threads. To release the brake, the pressure in chamber 41 is relieved, whereupon the piston 2 and brake-operating link 12 are returned by the spring 40. Nut 21 is still maintained clear of clutch face 25 by the pressure at 38, so it is free to be rotated again in the opposite direction. For " parking " operation, the second fluid pressure is not applied at 38, but in this case, when the brake is applied, the nut 21 is forced downwards out of contact with the clutch face 25 by the tubular part 7 which overcomes the force of a spring 30 by which the nut is supported on an upper ball-bearing 28; the nut is therefore again free to turn. When the pressure in chamber 41 is relieved the force of the spring 40 acting through piston 2 and tubular part 7 brings clutch face 23 into contact with clutch face 25, so the nut 21 is no longer free to turn, and the brake is locked in the " on " position. To effect " brake release " from this situation, fluid pressure is restored at port 38, whereupon clutch faces 23 and 25 are separated, nut 21 is free to turn, and spring 40 is effective to release the brake as before. If for any reasonthe second fluid pressure fails, the brake may be released from the " locked on " position by rotating a worm 27 which engages gear teeth on the annular member 26; such rotation effects rotation of nut 21 because in this condition the clutch faces are held in contact by spring 40. Rotation of nut 21, assisted by spring 40, effects retraction of tubular portion 7 and link 12, to release the brake. As shown, the tubular part 7 forms the outer part of a two-part brake-operating member 6, the inner, axially-slidable push-rod 8 being surrounded by a stack of Belleville washers 9 which collapse under relative movement of part 7 and rod 8 when a certain limit of braking force, locked in the mechanism when the brake is applied, is exceeded. This construction of brake-operating member 6 is described and shown in Fig. 2, and claimed in Specification 1,165,248. In a modification, the brake-operating member is solid, and the stack of Belleville washers for the relief of braking force is provided between a shoulder on the shaft of worm 27 and a fixed shoulder.