This invention relates to an automatic brake adjuster system for a motor vehicle, in particular, to drum brakes where brake drum and shoe clearance is adjusted automatically.
For drum brakes to function correctly the brake shoes must remain close to the drum without touching it. If they get too far away from the drum (as the shoes wear down, for instance), an actuator will require more fluid to travel that distance, and brake pedal will sink closer to the floor when apply the brakes. This invention relates to drum brakes having two brake shoes, an actuator located between the shoes, which move the shoes apart into contact with brake drum to apply the brake, and a retraction spring which retract the shoes, away from the drum, when the actuator is released.
The object of the present invention is to maintain the distance through which the shoes are retracted when the actuator is released remain substantially constant throughout the life of the brake.
In the known systems, brakes are adjusted manually for reducing the gap between shoes and drum by rotating a cam bolt. This will lead to inaccurate adjustment, a tedious job for the operator, shorter life of the lining and more pedal travel when there is no proper adjustment of the cam.
For the drum brakes to function correctly, the brake shoes must remain close to the drum without touching it by maintaining minimum clearance. If they get too far away from the drum (as the shoes wear down, for instance), the piston will require more fluid to travel that distance, and brake pedal will sink closer to the floor when we apply the brakes.
The automatic brake adjuster system for a motor vehicle, according to this invention, comprises a drum brake having a pair of brake shoes; an actuator located between the shoes for moving the shoes apart into contact with the drum; a spring for retracting the shoes from the drum when the actuator is released; and a cross- strut which extends between the shoes, controlling the retraction, the cross-strut being in two parts with

teeth on each part engageable to hold the strut at a predetermined length, one part of the cross-strut moving with one shoe and the other part of the cross-strut having a nose portion engaging with a cut-out in the other shoe with a clearance allowing the other shoe to move relative to the nose for a predetermined distance, the shoes being movable in opposite directions by the actuator until the clearance is taken up, any further movement required to engage the shoes with the drum resulting in sliding movement between the first and second parts of the cross-strut, whereby whenever such movement exceeds the pitch of the engaging teeth on the strut parts, the effective length of the strut is permanently adjusted by re-engagement of the teeth in a new position with different elements of the teeth in contact in order to maintain the shoe retraction substantially constant, as the shoes wear.
This invention will now be described with reference to the accompanying drawings which illustrate by way of example, and not by way of limitation, one of possible embodiments of the brake adjuster system proposed herein, in which
Figure 1 shows a plan view of a drum brake in accordance with the present invention; Figure 2 shows a view of the cross-strut used in the drum brake of figure 1 in the direction of the arrow A of figure 1;
Figure 3 shows, on a larger scale, the adjusting teeth of the cross-strut; Figure 4 shows a view of part of the cross-strut used in the brake of figure 1;
Referring to fig.l, the drum brake has a pair of brake shoes 2 and 3 which react against a fixed abutment 10 mounted on a back plate 1 and are brought into contact with an associated drum by a hydraulic wheel cylinder 4 positioned between the upper ends of the brake shoes to provide a service brake function. The shoes are retracted from the drum by a coil spring 5, which acts between the shoes.
The brake shoes can also be applied mechanically to provide a parking/handbrake function by a handbrake lever 11, which is pivoted on cross strut 9 via a pin 12. The handbrake lever 11 is pivoted relative to the shoe 2 by a cable (not visible in figure 1). A

slot 14 in cross strut 9 engages brake shoe and the handbrake lever 11 and a surface of cross strut 9 engages the brake shoe 2.
Thus when the handbrake lever 11 is pivoted anti-clockwise about the pin 12 as indicated by the arrow B in fig.2, the cross-strut 9 forces the brake shoe 3 into contact with the associated brake drum via the contact surface of strut 9 and the reaction of this shoe contact is transmitted via the cross-strut 9, handbrake lever 11, and pin 12 to engage the other brake shoe 2 with the opposite side of the associated brake drum. Thus a handbrake function is provided in a conventional manner.
In accordance with the present invention, the cross-strut 9 is in two parts. A first part 9a, which is held in contact with the brake shoe 3 by a coil-spring 23, which is hooked through an aperture 22 in the brake shoe 2 and into a cut out 15 in the cross-strut part 9a. The second part 9b of cross-strut 9 is slideable relative to the first part 9a via a spring-loaded bracket 16 on part 9a. Cross-strut part 9a is provided with an array of teeth 17, which are engaged by an array of teeth 18 formed on the second cross-strut part 9b. A spring-loaded bracket with screw 16, which can be clamped on back plate 1, holds these engaging teeth in contact with each other.
The second part 9b of cross-strut 9 has a nose portion 20, which extends through a cutout 19 in the brake shoe 3. As best seen in figure 1, the effective right hand end of cut out 19 is defined by drum and shoe clearance. Thus at normal braking condition, there is clearance gap x between the edge of nose portion 18 and the edge of the cut out 19. During normal hydraulic operation of the brake the brake shoes 2 and 3 are applied against the associated drum by pressurizing the wheel cylinder 4 and providing the distance which the brake shoes require to move to engage the drum does not exceed the clearance x between the nose portion 20 of the cross-strut and the edge a of the slot 19, there is no tendency of the brake shoes to attempt to move the two parts 9a and 9b of the cross-strut relative to each other.

If the movements required to apply these shoes against the drum exceeds this clearance x then, since cross-strut part 9a is fixed to brake shoe 3 and the nose portion 20 of cross-strut part 9b is in slot 19, there will be a tendency of the cross-strut part 9b to be moved relative to the cross-strut part 9a in the direction of arrow A of figure 2. If this tendency of movement exceeds the pitch of the inter engaging teeth on cross-strut parts 9a and 9b then cross-strut part 9b will be drawn away from cross-strut part 9a in direction A sufficiently to engage the next tooth formation and the effective length of the cross-strut 9 is increased to take up the brake shoe clearance. Thus as the brake shoes wear down the effective length of the cross-strut 9 progressively increases so that the retraction of the brake shoes under the action of spring 5 remain substantially constant.
During mechanical application of the brake shoes using the handbrake lever 11 the spring 21 is sufficient to ensure that the inter engaging teeth on cross-strut portions 9a and 9b remain in contact so that there is no change in the length of the cross- strut 9.
In the tooth away configuration shown in figure 3 the teeth 17 extend in direction 'C and the teeth 18 in direction 'D' both of these directions being generally parallel to the direction A in which strut part 9b moves relative to part 9a. With such a configuration, when the brake has been lightly applied by the wheel cylinder 4 and the teeth 17 and 18 are only engaging each at their tips, if the handbrake lever 11 is then quickly applied the teeth may jump over each other which can lead to tooth damage and unintentional shortening of the effective length of strut 9.
The problem can be overcome by inclining the directions of extension 'C and 'D' of the teeth 17 and 18 at a slight angle 9 to the direction A in which the strut part 9b slides relative to the strut part 9a as shown in figure 2. With the figure 3 arrangement there is an increased height 'h' of each tooth 17 available (see figure 3) to stop tooth slippage due to the angle of inclination 0. In a typical arrangement the inclination 9 is 15 to 20 degrees with a tooth depth of say 0.4 mm and a tooth pitch of say 0.7 mm.

The cross strut can be used in a brake which does not have the brake lever 11 and which can only be applied by the wheel cylinder 4. In such an arrangement the cross strut part 9a is again held onto the shoe 2 by spring 21 and the nose portion 20 of cross strut part 9b again extends through cut out 19. The strut 9 thus still functions to limit the retraction of the brawhen ke shoes when the brake is released and, as the effective length of the strut adjusts as the shoes wear, the retraction of the brake shoes remains substantially constant throughout the life of the brake.
It will be appreciated that various other embodiments are possible without departing from the scope and ambit of this invention.

We Claim:
1. An automatic brake adjuster system for a motor vehicle comprising a drum brake
having a pair of brake shoes an actuator located between the shoes for moving the shoes
apart into contact with the drum; a spring for retracting the shoes from the drum when
the actuator is released; and a cross- strut which extends between the shoes, controlling
the retraction, the cross-strut being in two parts with teeth on each part engageable to
hold the strut at a predetermined length, one part of the cross-strut moving with one shoe
and the other part of the cross-strut having a nose portion engaging with a cut-out in the
other shoe with a clearance allowing the other shoe to move relative to the nose for a
predetermined distance, the shoes being movable in opposite directions by the actuator
until the clearance is taken up, any further movement required to engage the shoes with
the drum resulting in sliding movement between the first and second parts of the cross-
strut, whereby whenever such movement exceeds the pitch of the engaging teeth on the
strut parts, the effective length of the strut is permanently adjusted by re-engagement of
the teeth in a new position with different elements of the teeth in contact in order to
maintain the shoe retraction substantially constant, as the shoes wear.
2. An automatic brake adjuster system as claimed in Claim 1 incorporating a handbrake
lever pivotally mounted on one of the cross-struts and engaging one end of the brake
shoe to move the other brake shoe away from the former brake shoe via the cross strut,
the tooth means on the two parts of the cross strut remaining in contact to prevent
adjustment of the effective length of the strut when the brake is applied by pivoting the
handbrake lever.