The present invention relates to drum brakes for vehicles. Drum brakes are well known. The main components of a drum brake for a vehicle are a rotating cylindrical drum and a set, usually an opposed pair, of brake shoes located in the drum. The shoes are pushed into contact with the drum, so that a friction force is generated between the shoes and drum when the drum is rotating which applies a drag to slow the drum. The drum is typically secured to a road wheel of the vehicle so that the drum rotates as the wheel rotates, and the shoes are fixed to a carrier or backing plate that does not rotate. An actuator applies a force to each shoe to press them onto the inner face of the drum. Commonly this actuator comprises a pair of opposed pistons in a common cylinder which can be pushed apart by forcing hydraulic fluid into the cylinder when braking is required. An electro-mechanical actuator may be used as an alternative

During the life of the brake, the shoes and to a lesser extent the inner cylindrical surface of the drum will wear. To ensure the brake shoes rest at a set distance from the drum when at rest an adjuster mechanism must be incorporated. These are commonly self-adjusting. A variety of different self-adjusting mechanisms are known, commonly involving the use of a two part threaded strut that is rotated by the action of a lever on a metal adjustment ratchet gear wheel when the wear has exceeded a set point. The thread causes the overall length of the strut to increase as it is rotated. The strut provides a stop against which the shoes are pulled by a retraction spring . This rotation by one tooth of the ratchet gearwheel repeats each time the wear has increased by that set amount, so the overall length of the adjuster increases in set increments to maintain the gap between the shoes and drum.

The adjuster mechanisms in prior art drum brakes are highly robust assemblies intended to bear high loads and in many cases resist long lasting compressive loads when the drum brake includes a parking brake function. They must also overcome the force of a return spring that commonly interconnects the two brake shoes to retract the shoes when the brake is released. According to a first aspect the invention provides a drum brake for a vehicle, comprising:

a drum;

a pair of brake shoes located inside the drum, each facing an inner lining of the drum;

an actuator mechanism which is connected at a first end to one of the brake shoes and at an opposing second end to the other of the brake shoes, operation of the brake causing the length of the actuator mechanism to increase, thereby pushing the two shoes away from each other to press the shoes onto the drum when braking is required, and

a self-adjuster assembly which interconnects the pair of brake shoes, the self- adjuster assembly comprising:

a strut having an adjustable length formed from two inter-engaging parts, and a retraction spring that interconnects the two shoes that in use forces the shoes back out of contact with the drum once the actuator mechanism is released,

characterised in that the strut comprises a two-part latching assembly that allows extension of the strut along a linear or curvilinear path, in which a first part of the latching assembly carries a set of teeth that are resiliently engageable with a corresponding set of teeth of the second part in a plurality of different positions to define a set of discrete lengths for the strut, the first part and the second part being configured such that the strut will resist a compressive force applied by the retraction spring but are free to move apart forcing the teeth to ride over one another when the actuator moves the brake shoes by a distance that exceeds the spacing between adjacent teeth on the first part.

The two parts of the latching assembly may provide a resistance to relative linear movement of the two parts that will cause the teeth of the latching assembly to skip from one incremental length to the next length that exceeds the compressive force that will, in use, be applied to the strut by the retraction spring. This ensures that as the strut increases incrementally in length to take up wear the teeth prevent the strut from being compressed back by the retraction spring.

However, the two parts should provide a low enough resistance to relative movement that it can be overcome by pressure applied by the actuator mechanism during braking when the stroke of the actuator exceeds the spacing between teeth. This enables the self-adjustment on braking when wear is present, typically increasing in length by the spacing between adjacent teeth. As more wear occurs, the strut will again increase in length when the stroke exceeds the spacing between teeth. One of the two parts may comprise a block having a bore which defines a plurality of inwardly facing ridges spaced along the bore, each defining a tooth of the part, and the other part may comprise a shaft having a set of circumferential ridges spaced along a length of the shaft, the shaft being located at least partially in the bore so that one or more of the ridges of the shaft engage the ridges of the bore.

The ridges on the bore and shaft may each define a set of triangular teeth.

As the brakes are applied the strut must be able to stretch slightly to permit the required movement of the shoes away from each other before it retracts as the brakes are released. This can be achieved by making the teeth suitably resilient or incorporating an element of backlash or clearance into the latching mechanism.

The teeth on one or both parts may be resilient. Alternatively the teeth may be rigid and may be supported by a resilient backing portion of each part to allow them to deflect and ride over one another as the length is adjusted.

The set of teeth may comprise one tooth, or two or more teeth. The number and spacing of teeth will define the range of discrete lengths that the self-adjusting strut can adopt.

Preferably one or both of the parts of the latching mechanism may comprise plastic components.

The latching mechanism may be bi-directional, whereby the length of the strut can be reduced when a force is applied to the two parts that exceeds the compressive force of the retraction spring. This feature will allow the strut to be retract in length in the event that the drum brake contracts and cools following a period of braking which has generated a high heat and caused the ratchet to increase in length as a result of expansion of the drum increasing the stroke of the actuator. Each end of the actuator mechanism may act on a corresponding end of each shoe, and the strut may interconnect the two shoes at those ends adjacent to the region where the actuator mechanism acts on the shoes. The actuator may comprise a hydraulic actuator. The actuator mechanism may comprise a pair of pistons located at opposed ends of a cylinder which receives an incompressible hydraulic fluid, pressurizing the fluid causing the pistons to move apart. Alternatively the actuator may comprise an electro-mechanical actuator which may comprise an electric motor in conjunction with a gear train to drive the shoes into contact with the drum.

There will now be described, by way of example only, one embodiment of the present invention with reference to the accompanying drawings of which:

Figure 1 is a view in plan of a drum brake assembly according to the present invention, and Figure 2 is a partial cross-sectional view of the strut of Figure 1.

As shown in Figure 1 , a drum brake 100 comprises a drum 102, which commonly rotates with a wheel of a vehicle (not shown), and a pair of brake shoes 104. Each one of the pair of brake shoes 104 includes a friction surface 106 that, in use, contacts an inner surface 108 of the drum. The friction generated by the action of the friction surface 106 rubbing against the inner surface 108 of the drum 102 converts kinetic energy of the drum 102 and wheel to heat energy, slowing the rotation of the drum 102 in relation to the brake shoes 104. Each brake shoe 104 is attached to a backing plate 1 10 of the drum brake 100 by a hold-down pin 1 12 and spring 1 14.

In order to contact the drum 102, the brake shoes 104 must be moved within the drum. 102 To this end, a cylinder 1 16 is provided, operating as an actuator mechanism. In the present case, the cylinder 1 16 is connected to a hydraulic braking circuit of a vehicle (not shown). Pressure build-up in the cylinder 1 16, caused by compression of a foot brake or otherwise, results in the expansion of the cylinder 1 16. Each brake shoe 104 is pivoted at one end about an anchor point 1 18 formed on an anchor block 120. The anchor block 120 is not fixed, allowing the brake shoes 104 to self-centre when in operation. The other end of each brake shoe 104 is attached to the cylinder 1 16. In use, the expansion of the cylinder 1 16 causes the end of each brake shoe 104 attached to the cylinder 1 16 to move further apart, causing pivoting of each brake shoe 104 about its anchor point 1 18 and thus forcing the friction surfaces 106 into contact with the drum 102. A retraction spring 122 acts between the brake shoes 104 in order to provide a return force that brings the brake shoes 104 back to a rest position once the pressure in the cylinder 1 16 decreases.

As the friction surfaces 106 wear over time, it is necessary to automatically adjust the rest position of the brake shoes 104 such that the cylinder 1 16 is not required to expand its range of motion. Any expansion of the range of motion of the cylinder 1 16 would translate into an expansion of the distance required for a user to depress a brake pedal in order to operate the cylinder 1 16. This is undesirable as it may result in sub- optimal response of the drum brake 100 and potential failure of the system. Therefore, a strut 124 is positioned between the two brake shoes 104, the strut 124 preventing the retraction spring 122 from pulling the brake shoes 104 further together than a distance set by the length of the strut 124.

In order to enable adjustment of the length of the strut 124 to allow for wear of the brake shoes 104, the strut 124 comprises two parts 126, 128 that inter-engage using a plurality of teeth 130. In the present embodiment, each part 126, 128 has a large number of teeth 130, for example ten, but it may be possible to provide one of the parts 126, 128 with a single tooth 130 and the other part 126, 128 with a plurality of teeth 130, as long as the strut 124 is capable of withstanding the forces applied by the retraction spring 122.

The movement of the two parts 126, 128 therefore results in the strut 124 adopting one of a set of discrete positions resulting in it having one of a set of discrete lengths, each length corresponding to a different positional relationship between the teeth 130 of the parts 126, 128. In the present embodiment as shown in Figure 1 , it can be seen that the first part 126 includes a plurality of teeth 130 on the outside of a shaft 132, the second part 128 having a plurality of teeth 130 within a bore formed in a block, thus forming a sleeve 134, the shaft 132 of the first part 126 being received within the sleeve 134 of the second part 128. The teeth 130 may also be considered to be ridges formed circumferentially around the shaft 132 and bore.

Resilient engagement of the teeth 130 of the two parts 126, 128 is provided in the present example by resilient deformation of the teeth 130. That is, under load, the respective teeth 130 deform to an extent that one set of teeth 130 is forced to ride over the other. However, it is also possible to provide the resilient engagement in other ways. For example, it may be desirable to have substantially rigid teeth, such as those formed by a metal or other material with limited elasticity. In this case, the resilience required to allow the teeth to be forced over one another, whilst still ensuring engagement when under lower loads, may be provided by additional components such as an elastic member that provides compression of the two sets of teeth towards each other. In this example, the compression of the elastic member must be selected carefully in order that the engagement or meshing of the teeth is overcome only under loads greater than those under which the strut is designed to remain rigid.

In another alternative, substantially rigid teeth on either or both of the first and second parts may be supported by a layer that gives a degree of resilience. For example, a rubberised or resilient polymer layer may allow the teeth of the first part and second part to move away from each other under sufficient load. Other such methods of providing resilient engagement of the teeth, through various adaptations or design characteristics, will be known to the person skilled in the art.

Referring to Figure 2, it can be seen that the teeth 130 do not engage fully but instead some level of relative movement of the teeth 130 is allowed by the provision of a backlash or clearance between the sets of teeth 130. The size of the clearance determines the amount of wear of the brake shoes 104 or the amount of expansion of the drum 102 that must occur before the strut 124 adjusts by the sets of teeth 130 riding over one another. Similarly, in the case of contraction of the drum 102, the teeth 130 may ride over one another in the opposite direction. The expansion and contraction of the drum 102 may be due to temperature fluctuations in the environment or temperatures produced during operation of the brakes, for example.

Although the strut 124 of the depicted embodiment is linear, it is also possible to provide a non-linear strut such as a curvilinear strut, whereby the strut follows a curved path. Use of such a design may be necessitated or desired due to the shape or placement of other parts of the drum brake 100, for instance.