Torque limitation device for a drum brake

Abstract

A drum brake, primarily for a trailer, which gives normal braking torque in forward direction and low torque in reverse. This is achieved by having a pivoted lever acting between the one ends of two shoes remote from the ends between which the brake actuator is located. The shoes act on the lever at different effective radii and the forces applied by the shoes to the lever hold it in a first position during forward rotation, thus allowing normal braking torque, and a second position during reverse rotation which allows the ends of the shoes acted on by the actuator to move apart thus allowing a limitation of the reverse braking torque by limiting the stroke of the actuator.

I claim: .[.1. A drum brake comprising a drum, two shoes mounted within the drum for movement between operative and inoperative positions, spring means biasing the shoes to their inoperative positions, a shoe actuator interposed between adjacent one ends of the two shoes, a shoe abutment and torque limitation means arranged to provide a clearance having a predetermined value when the shoes are in their operative positions and the drum is rotating in one direction and to eliminate or substantially to eliminate said clearance in response to the reactive forces generated by the shoes due to their engagement with the drum when the shoes are in their operative positions and the drum is rotating in the opposite direction thereby producing a lower torque between said drum and said shoes than when said drum is rotating in said one direction so that, when the drum is rotating in said opposite direction, a greater movement apart is required of said one ends of the shoes by said actuator from the positions said ends occupy when the shoes are in their inoperative positions to obtain a given braking torque than is required to obtain said braking torque when the drum is rotating in the one direction and the torque limitation means being arranged so that, with the drum rotating in either of said directions and with the shoes in their operative positions, said reactive force generated by one of the shoes acts to tend to reduce said clearance, the torque limitation means comprising a lever mounted to pivot with respect to a fixed part of the brake and being connected, with the other ends of the shoes, the engagement of the lever with said shoe ends being at different radii about the lever pivot for the two shoe ends, the lever being biased to a rest position which it takes up when the shoes are in their inoperative positions, whereas when the shoes are in their operative positions said reactive forces (1) hold the lever substantially in its rest position when the drum is rotating in said one direction and (2) pivot the lever to eliminate or substantially eliminate the clearance 
     
        when the drum is rotating in said opposite direction..]. 2. A brake according to claim .[.1.]. .Iadd.9 .Iaddend.wherein the shoes act respectively in a leading sense and in a trailing sense and wherein the lever engages the abutment, the zone of engagement between the lever and the abutment providing the .[.pivot.]. .Iadd.fulcrum .Iaddend.of the lever and the clearance being provided between .[.one of the shoes.]. .Iadd.said one shoe .Iaddend.and the abutment; and wherein .[.said radius about the lever pivot of the engagement of the other end of said one shoe is greater than the radius about the lever pivot of engagement of the other end of the other shoe.]. .Iadd.the moment arm between the lever fulcrum and the engagement of the lever with the one shoe is greater than the moment arm between the lever fulcrum and the engagement of the lever with the other 
     
     
        shoe. .Iaddend. 3. A brake according to claim 2 wherein the other end of said one shoe engages the lever through connecting means in the form of a 
     
     
        strut. .[.4.  A brake according to claim 1 wherein the shoes act respectively in a leading sense and in a trailing sense, and wherein the lever is pivoted about a fixed pivot axis on a fixed part of the brake, the lever being held against a first stop by resilient means and engagement with one of said shoes when the drum is rotating in said one direction, the said clearance being between the lever and a second stop; and wherein the radius about the lever pivot axis of the engagement of said other end of said one shoe is less than the radius about the lever pivot axis of the engagement of said other end of the other shoe..]. .[.5. A brake according to claim 4 wherein the stops are provided by opposite sides of a pin which is engaged in a slot in the lever..]. .[.6. A brake according to claim 1 arranged as a duo-servo brake and wherein the lever is pivoted about a fixed pivot axis on a fixed part of the brake; said clearance being provided between one of said shoes and the abutment and in that the radius about the lever pivot axis of engagement of said other end of the one shoe is less than the radius about said lever pivot axis of 
     
     
        engagement of the other end of the other shoe..]. 7. .[.A brake according to claim 1.]. .Iadd.A drum brake comprising a drum, two shoes mounted within the drum for movement between operative and inoperative positions, spring means biasing the shoes to their inoperative positions, a shoe actuator interposed between adjacent one ends of the two shoes, a shoe abutment and torque limitation means arranged to provide a clearance having a predetermined value when the shoes are in their operative positions and the drum is rotating in one direction and to eliminate or substantially to eliminate said clearance in response to the reactive forces generated by the shoes due to their engagement with the drum when the shoes are in their operative positions and the drum is rotating in the opposite direction thereby producing a lower torque between said drum and said shoes than when said drum is rotating in said one direction so that, when the drum is rotating in said opposite direction, a greater movement apart is required of said one ends of the shoes by said actuator from the positions said ends occupy when the shoes are in their inoperative positions to obtain a given braking torque than is required to obtain said braking torque when the drum is rotating in the one direction and the torque limitation means being arranged so that, with the drum rotating in either of said directions and with the shoes in their operative positions, said reactive force generated by one of the shoes acts to tend to reduce said clearance, to torque limitation means comprising a lever mounted to pivot with respect to a fixed part of the brake and being connected with the other ends of the shoes, the engagement of the lever with said shoe ends being at different moment arms about the lever pivot for the two shoe ends, the lever being biased to a rest position which it takes up when the shoes are in their inoperative positions, whereas when the shoes are in their operative positions, said reactive forces (1) hold the lever substantially in its rest position when the drum is rotating in said one direction and (2) pivot the lever to eliminate or substantially eliminate the clearance when the drum is rotating in said opposite direction, said brake .Iaddend.characterised by the provision of disabling means operable 
     
     
        to prevent pivotal movement of the lever to reduce the clearance. 8.  The combination of a brake according to claim 7 with first and second brake operating means operatively connected to the actuator, the first such means having sufficient travel to obtain a high braking torque when said clearance has its predetermined value but only sufficient travel to obtain a low braking torque when the clearance has been substantially eliminated, the second such means being associated with said disabling means to operate the same to prevent pivotal movement of the lever upon operation of the brake by the second such means. .Iadd. 9. A drum brake comprising a drum, two shoes mounted within the drum for movement between operative and inoperative positions, spring means biasing the shoes to their inoperative positions, a shoe actuator interposed between adjacent one ends of the two shoes, a shoe abutment between the other ends of the shoes and torque limitation means arranged to provide a clearance between the shoe abutment and the other end of one of the shoes, the clearance having a predetermined value when the shoes are in their operative positions and the drum is rotating in one direction, and the abutment then being engaged by the other end of the other of the shoes, and said torque limitation means being arranged to eliminate or substantially to eliminate said clearance in response to the reactive forces generated by the shoes due to their engagement with the drum when the shoes are in their operative positions and the drum is rotating in the opposite direction, thereby producing a lower torque between said drum and said shoes than when said drum is rotating in said one direction so that, when the drum is rotating in said opposite direction, a greater movement apart is required of said one ends of the shoes by said actuator from the positions said ends occupy when the shoes are in their inoperative positions to obtain a given braking torque than is required to obtain said braking torque when the drum is rotating in the one direction and the torque limitation means being arranged so that, with the drum rotating in either of said directions and with the shoes in their operative positions, said reactive force generated by one of the shoes acts to tend to reduce said clearance, the torque limitation means comprising a lever pivotally mounted on said other of the shoes adjacent to the other end thereof and being connected with the other end of said one of the shoes, the lever having a fulcrum on a fixed part of the brake, the respective moment arms of the shoes about the fulcrum of the lever differing from each other, the lever being biased to a rest position which it takes up when the shoes are in their inoperative positions, whereas when the shoes are in their operative positions, said reactive forces (1) hold the lever substantially in its rest position when the drum is rotating in said one direction and (2) pivot the lever to eliminate or substantially eliminate the clearance when the drum is rotating in said opposite direction, the brake being associated with parking braking means having a greater travel than said actuator and the clearance being such that when the brake is subjected to the greater travel of the parking braking means while the drum is stationary, the abutment is engaged by the other end of said one shoe so that full braking torque can be applied. .Iaddend.