Slack adjusters for vehicle brakes
This invention relates to slack adjusters for vehicle brakes and particularly to automatic slack adjusters of the kind commonly used in association with the actuating cam of internal expanding shoe type brakes and comprising a worm wheel adapted to be keyed or splined on to the brake cam shaft, a body member attached to or formed integrally with a brake actuating arm movable around the axis of the cam shaft, a worm rotatably mounted in the body member and meshing with the worm wheel and means for automatically rotating the worm to adjust the angular position of the worm wheel relative to the brake actuating arm when a pre-determined angular movement of said arm is exceeded, said means including a member oscillatably mounted on the body member and co-acting with a fixed member to impart motion through a uni-directional clutch to the worm. The object of the present invention is to provide an improved slack adjuster of this kind and in particular to safe-guard the adjuster mechanism against damage should it perform an adjusting stroke before brake loading on the worm has been removed. 
     According to the invention the uni-directional clutch is a sprag clutch and there is interposed in the drive to the worm and in addition to said sprag clutch a friction clutch which automatically dis-engages if an attempt is made to rotate the worm whilst it is still under brake loading. 
    
    
     The invention will now be described in more detail and with reference to the accompanying drawings, wherein: 
     FIG. 1 is a part sectional side view of one embodiment of the slack adjuster; 
     FIG. 2 is a part sectional end view thereof, and 
     FIG. 3 is a fragmentary sectional view on the line III--III of FIG. 1. 
    
    
     The adjuster shown is of conventional form in that it comprises a banjo-shaped body member 1, the upper end of which constitutes a brake-actuating arm 1a and is formed at 2 for attachment to a brake rod or like brake actuating gear (not shown), the body member being apertured at its lower end to receive a worm wheel 3 which in use is splined upon a brake cam shaft. The worm wheel is engaged by a worm 4 which is located in a recess 5 in the body member and is secured fast on a worm shaft 6 journaled in the body member. The recess 5 extends to the end face 1b of the body member and located in the outer end portion of the recess is a friction clutch comprising a rotatable member 7 recessed at one end to form the female part of the clutch and a disc or plate 8 received within the recessed member and constituting the male part of the clutch. A screw 9 extends with clearance through the member 7 and into a stem portion 8a of the clutch disc 8, a spring or disc spring assembly 10 beneath the head of the screw serving to urge the two clutch parts together. Located between the friction clutch and the worm 4 is a sleeve 11 formed integral with the worm shaft 6, pawls, balls or equivalent spragging elements being interposed between the sleeve 11 and the stem 8a to form a one-way sprag clutch providing uni-directional drive from the stem to said sleeve. 
     Detachably secured to the end face 1b is a cover plate 12 in which is journaled a hub member 13, said member being secured to an arm 14 constituting the oscillatable member and which is forked to receive with clearance a stud 14a or other member fast with the chassis or the fixed brake structure whereby when the arm is rocked in the direction of the arrow A to apply the brake the arm 14 and its hub member 13 rotate relative to the body 1. Fast on the outer end of the clutch member 7 and accommodated in a recess in the cover plate 12 is a lever 15, the free end of this lever being slotted to receive a stud 16 carried by the hub member 13. 
     The worm shaft 6 is formed at one end as shown at 6a for application of a tool to rotate the worm manually when setting up the linkage. 
     The slack adjuster operates in the following manner: Under normal operating conditions movements imparted to the arm 1a of the body member by the brake operating gear are transmitted rigidly through the worm and worm wheel to the brake cam shaft. During these movements the arm 14 will move relative to the stud 14a but, assuming that the degree of slack in the system is not excessive, the brake will be applied before the clearance between the arm and the stud is taken up. If, however, due to excessive slack developing in the brake actuating linkage or to excessive shoe lining wear the arm 1a on brake application moves beyond a predetermined degree, the arm 14 will contact the stud 14a and thereafter will be rocked relative to the body 1 and, through the lever 15 and friction clutch 7, 8 will impart rotation to the inner member 8a of the sprag clutch. In this direction of rotation no movement is transmitted through the sprag clutch but on the return movement of the arm 1a rotation is transmitted from arm 14, which rocks oppositely upon again contacting stud 14a, through member 13, lever 7 and the friction clutch and through the outer member 11 of the sprag clutch and shaft 6 to the worm 4 thereby adjusting the worm wheel 3 relative to the body member 1 to take up the excessive slack in the brake linkage. If, however, the adjusting mechanism should attempt to transmit movement to the worm shaft while a braking effort is still being exerted, that is, while there is still a braking load on the worm, the friction clutch 7, 8 will slip thus safeguarding the adjusting mechanism. The friction clutch is normally set to slip at about 100 lbs/ins torque, but this figure is adjustable as previously described.