Internal shoe drum brake
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an internal shoe drum brake, primarily for motor vehicles, and having a pair of brake shoes mounted on a backplate and arranged for expansion by a mechanism which includes a lever movable angularly in a plane generally parallel to the backplate. 
     2. Description of the Prior Art 
     It is common for the drum of such a brake to become grooved by one or both of the shoes after long service and it can become difficult to remove the drum in such circumstances, particularly when the brake includes an automatic adjuster for maintaining the shoe to drum clearance at a predetermined maximum value. In order to overcome this problem, such brakes are often provided with a releasable backstop carried by the backplate, and arranged so as normally to prevent the actuating lever from moving, in the brake release direction, beyond a predetermined retracted position, but, upon release, to permit the lever to move in the brake release direction to a second retracted position, thereby enabling the shoes to move inwardly to clear the drum groove. The drum may then readily be removed for servicing of the brake, for example. 
     Some examples of releasable backstops are described in our earlier British Patent No. 1247245, these being essentially in the nature of inserts placed in openings in the backplate and capable of manipulation to enable the lever to move from its normal retracted position to the second retracted position, when desired. These arrangements are satisfactory, provided that the backplate profile has sufficient depth to permit the backstop to be placed at a sufficiently steep angle to the plane of the backplate to provide a reliable stop. With some modern shallow backplate designs, these conditions cease to apply and the aforesaid type of backstop becomes unsatisfactory. 
     In another known arrangement, a resilient shoe hold-down device is provided with a projecting arm which extends to a position behind the plane of the shoe web to provide a backstop for the actuating lever. This can be satisfactory in some light-duty applications, but is not ideal for use in brakes intended for large saloon cars and light vans for example in which considerable shoe hold-down forces are required and hold down devices such as coil springs are then often preferred. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, an internal shoe drum brake comprises a pair of brake shoes mounted on a backplate and arranged for expansion by a mechanism which includes a lever movable angularly in a plane generally parallel to the backplate, and an abutment device mounted on one of the shoes and retained thereon by a separate shoe hold down device associated with that shoe, the abutment device extending to a position adjacent the lever to provide a backstop for the latter in order to set a predetermined retracted position for the lever. 
     In one convenient arrangement, the device is in the form of an elongate plate which lies against the web of the shoe and is provided with a projection out of the plane of the plate which engages an edge surface portion of the web to form the backstop. 
     Preferably, the projection extends through an opening in the shoe web to provide the backstop at the opposite side of the web to that on which the device is mounted, and engages an inner edge surface of the opening which constitutes the web edge surface portion. 
     The arrangement is preferably such that the backstop is disposed opposite to an opening in the backplate through which the abutment device may be moved so as to remove the backstop from the plane of movement of the lever, thereby permitting the latter to move to a further retracted position such that the shoes are able to move away from the drum. 
    
    
     The invention will now be described, by way of example, with reference to the accompanying drawings wherein: 
     FIG. 1 is an end elevation of one form of the internal shoe drum brake of the invention; 
     FIGS. 2A and 2B are detail cross-sectional views taken along line A--A of FIG. 1 showing a component in alternative operative positions; 
     FIGS. 3A and 3B are detail cross-sectional views taken along line B--B of FIG. 1; and 
     FIGS. 4A and 4B are views similar to FIG. 2 illustrating part of an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1 of the drawings, this shows an internal shoe drum brake having a pair of brake shoes 1, 2 mounted on a backplate 3, with one pair of adjacent shoe ends engaged by an hydraulic actuating cylinder 4 and the other pair engaging a fixed abutment 5. A strut 6 extends between the shoes and incorporates part of an automatic adjustment device indicated generally at 7, one end of the strut engaging a shoe-mounted lever 8 of the device 7 and the other end engaging a mechanical actuator lever 9 which has one of its ends pivotally mounted at 10 on the shoe 2, the other end being adapted for the connection thereto of an operating rod or cable. Shoe return springs 11, 12 urge the shoes inwardly to bring the shoe 1 and the lever 9 mounted on the shoe 2 into firm engagement with the strut, the length of which sets the normal retracted positions of the shoes. 
     The backstop is provided, in this embodiment by a thin flexible blade-like lever 13 which lies against the web 2A and is held in position by a coil spring 14 of a conventional shoe hold down assembly, of which a rod 15 passes through the lever 13 and web 2A to engage the backplate 3 (FIG. 2A). An end portion of the lever is bent out of the plane of the latter to form a tongue 16 which extends through a hole 17 in the web 2A across the plane of movement of the actuator lever 9 so that a lateral edge of the tongue is engaged by the lever 9 and provides a backstop for the latter. Since the lever 9 is pivotally mounted on the shoe 2 and abuts the strut, the return spring force creates a movement tending to rotate the lever outwardly, such outward movement being limited by the backstop. The various components are arranged so that the backstop sets a normal retracted position of the lever 9 which is suitable for normal brake operation and is somewhat further away from the drum than would be the case if the action of the shoe return springs on the lever were not restrained by the backstop. 
     When it is desired to remove the brake drum, it may be necessary, because of drum grooving, to retract the shoes further, and this may be achieved by passing a suitable implement through an opening 18 in the backplate located opposite to the tongue 16 and pushing on the tongue, so that it flexes back to the position shown in FIGS. 2B and 3B in which it is clear of the plane of movement of the lever 9, thereby enabling the latter to move further towards the drum under the action of the shoe return springs and consequently permitting said springs to effect further retraction of the shoes away from the drum. The opening 18 through which this manipulation is carried out is conveniently one which is normally provided for the purpose of inspecting the condition of the shoe linings without first removing the drum. 
     In the alternative embodiment illustrated in FIGS. 4A and 4B, the lever 13 is rigid, being in the form of an open channel having upstanding sides 13A. The physical arrangement of the lever 13 is identical to that described above, but since the lever is rigid, it requires resilient displacement of the hold down spring 14 in order to be able to reach the release position shown in FIG. 4B. 
     During initial assembly of the brake, it is necessary to adjust the length of the usual actuating cable (not shown), which connects the lever 9 to a driver-operated handbrake lever in the vehicle on which the brake is used, so that the lever rests firmly against the backstop with substantially no slack in the cable. This may be achieved in the brake of the invention by placing the lever 9 directly behind the backstop so that the latter is displaced outwardly, either against the inherent resilience of the blade 13 (FIG. 2B) and/or by deforming the hold-down spring 14 (FIG. 4B). The actuating cable is then adjusted to move the lever 9 inwardly until the backstop is able to snap into place behind the outermost edge of the lever. This action produces a readily audible &#34;click&#34; indicating that the lever is then in its correct position. 
     The invention will be seen to provide a simple backstop arrangement for an actuator lever in a shoe drum brake employing separate shoe hold-down springs, is cheap to produce and reliable in operation.