Patent GB-1456663-A (GB)
Title: Brakes
IPC: B60T7/20
Filed: 1973-02-13
Abstract: 1456663 Trailer over-run brakes; internal shoe drum brakes; self-servo brakes L C CHOUINGS 4 Feb 1974 [13 Feb 1973] 6871/73 Heading F2E In an internal drum brake, a reduced braking effect is achieved for one direction of drum rotation by torque limiting means which provide a predetermined clearance between one shoe and its abutment when the brake is operated during the other direction of drum rotation and eliminate the clearance for operation in said one direction so that greater actuator travel is required to achieve a given braking effect, the torque limiting means including a lever which can be prevented from pivoting by overriding means such as a parking brake. In Figs. 3 and 5 the invention is applied to a leading-and-trailing internal shoe drum brake. The forward direction of drum 3, Fig. 3, is counter-clockwise. A lever 4 mounted on a pin 9 fixed in the web of the normally-trailing shoe 2 bears at one end against the table 5 of the shoe and at an intermediate point against a fixed abutment 6; the other end of the lever is connected by a spring 7 to the other shoe 1; engagement of the lever with pin 9 is by means of a pear-shaped hole 10. A pull-off spring (not shown) is fitted at the upper ends of the shoes. In operation, when the upper tips of the shoes are forced apart and the drum is rotating clockwise, shoe 2 is for the time being the leading shoe. During the first phase, spring 7 does not yield sufficiently to permit shoe 2 to move circumferentially, but subsequently it does so, whereupon the web of shoe 2 moves further into a slot 8 at the end of abutment 6, the edge of table 5 forcing lever 4 to pivot about its point of contact with abutment 6 to the position of Fig. 4 (not shown). The circumferential movement of shoe 2 causes the distance between the shoes at their upper ends to increase, and if the available travel of the operating means is limited to a value which will not permit the lower end of shoe 2 to reach the bottom of slot 8, the braking torque is limited, (the limitation being governed by the loading of spring 7, the dimensions of lever 4 and the brake geometry). The action of lever 4 may be over-ridden by a cranked lever 13, Fig. 5, pivotted on the back plate 14, and having attached to it a further lever 18 on the outside of the backplate. A second operating means is connected through a pre-loaded spring (not shown) to the lower end of lever 18, and when this second means is activated it firstly causes the cylindrical portion 15 of lever 13 to be interposed between lever 4 and shoe 2 and thereafter operates the brake normally, the torque the brake will resist (assuming the drum is at rest) being proportional to the applied operating effort. In Fig. 6 the invention is applied to a duoservo brake, where the forward direction of rotation is indicated by the arrow and a lever 22 is mounted on the primary shoe 21. The spring 26 is sufficient to support all normal forward loading, but on rotation in the reverse direction, after a certain limiting effort is reached, the shoes move circumferentially until gap 30 is closed, thus limiting the torque, when the first operating means is used. In Fig. 7, which is part of a leading-and-trailing brake oriented the opposite way round to Figs. 3 to 5, a lever 31 (equivalent to lever 4 of Fig. 3) is pivoted at 33 to the normally-trailing shoe 32, the forward direction being clockwise, and a spring 34, fitted between the lever and the shoe, pulls the lever on to the shoe table at 35. On reverse rotation, when the operating force 36 is great enough, shoe 32 moves circumferentially towards abutment 37, reducing gap 38 and increasing the travel at 36. As applied to a duo-servo brake, (Fig. 8, not shown), different from Fig. 6, the lever is acted on by a helical compression spring surrounding the strut between the lower ends of the shoes.