This invention relates to an automatic brake shoe adjuster for drum brakes of motor vehicles, more particularly, though not exclusively, of three wheeler and other similar vehicles to which this invention would be applicable. This invention specifically relates to automatic adjustment of brake shoe clearance.

Mostly drum brakes are used in medium to heavy duty vehicles to avoid wheel locking which is important for safe riding. For drum brakes to function correctly, brake shoes must remain close to the drum without touching It. If these shoes get too far away from the drum, as the shoes wear down, so will the piston require more fluid to travel the shoe distance, and the brake pedal will sink closer to the floor when brakes are applied, in the existing three wheeler brake system, brake shoes are adjusted manually for reducing the gap between shoes and drum by a rotating cam bolt. This leads to inaccurate adjustment and less life of liner with more pedal travel when there is no proper adjustment of cam. These kind of brakes will have a cam to adjust brake shoes whenever the shoes get worn out. However, in these brakes the shoe clearance is not maintained properly because the adjustment is manual and changes from person to person.

In order to avoid the above mentioned problems, an automatic shoe adjuster for drum brakes is proposed herein.

In drum brakes as the liner wears down, more space will form between the shoe and the brake drum. Whenever brakes are in applied condition, the shoe is pulled tight against the drum and when the gap between the shoe and the drum gets big enough (that is, when the shoe gets worn out) the adjusting lever rocks to advance the pinion by one tooth. The rack has teeth on it, so that it advances a little when it pulls, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can be advanced again, so that the shoes are always kept close to the drum.

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 its possible embodiments

Figure 1 showing a plan view of a drum brake in
accordance with the present invention;

Figure 2 shows a sectional view of the adjuster used in the drum brake of figure 1 in the direction of the arrow A in figure 1;

Figure 3 shows, on a larger scale, the adjuster with rack and pinion mechanism;

Figure 4 shows a view of adjuster and teeth on rack and pinion used in the brake of figure 1;

The drum brake illustrated in the accompanying drawings, comprises a back plate 1, a pair of brake shoes 2 and 3, a fixed abutment 10 and a double ended brake sleeve cylinder 4 for applying the service braking load to the adjacent tips of the respective brake shoes. Flg.1 shows back plate 1 having wheel cylinder 4 which Is clamped by means of bolts and nuts as shown. Brake shoes 2,3 are assembled by means of locating pin 8 and shoe holder flat 7. These shoes are tightly packed on back plate by means of top and bottom return spring 5, 6. An adjuster 9 is assembled at top spring location.

An adjuster consists of strut assembly 9a in which pinion 12 is riveted to strut. Strut assembly 9a is having a slot, which can engage into shoe 2 ribs by means of strut return spring 13. Rack 9b having similar slot which can assemble into shoe 3 rib by means of rack return spring 14.

Handbrake lever 11 is pivoted clockwise about the pin 15 as indicated by the arrow B in fig.2, the strut 9a forces the brake shoe 3 into contact with the associated brake drum via the contact surface of strut 9a and the reaction of this shoe contact is transmitted via the adjuster 9, handbrake lever 11, and pin 15 to 9ngage 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 adjuster 9 has two parts. First part is strut 9a, which is held in contact with the brake shoe 2 by a strut return spring 13, which is hooked through an aperture in the brake shoe 2 and into a hole in the strut 9a. In the second part rack 9b is guided through a hole relative to the strut 9a via rivet 20, which is riveted on strut 9a. Rack 9a and strut 9b are tightly held together by means of torsion spring 19 in order to engage rack and pinion as shown in fig.4.Rack 9b is provided with an array of teeth 17, which-are engaged by an array of teeth 18 on pinion 12. Rack 9a having similar slot which can be assembled into shoe 3 ribs by means of rack return spring 14. As best seen in figure 1, the rotation of the pinion, which is defined by drum and shoe clearance.

Thus under normal braking conditions, there is initial radial gap 'x' between drum and shoe. During normal hydraulic operation of the brakes, 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 in order to engage the drum does not exceed the initial radial gap 'x' between the drum and shoe, 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 initial radial gap x then, since strut part 9a is fixed to brake shoe 2 and the rack is fixed to shoe 3 by means return spring 14, there will be a tendency of the rack 9b to be moved relative to the strut 9a in the direction of arrow A of figure 2. If this tendency of movement exceeds the pitch of the inter engaging teeth on rack 9b and then rack 9b will be drawn away from strut 9a in direction A sufficiently to engage the next tooth formation and the effective length of an adjuster 9 is increased to take up the brake shoe clearance. Thus as the brake shoes wears down the effective length of an adjuster 9 progressively increases so that the retraction of the brake shoes under the action of spring 5 remain substantially constant.

In the tooth away configuration shown in figure 3 the teeth 17 extend in direction 'C and the teeth on pinion 12 in direction 'D' both of these directions being generally parallel to the direction A in which rack 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 teeth 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 teeth 18 at a slight angle 9 to the direction A in which the strut part 9b slides relative to the pinion as shown in figure 3. With the figure 3 arrangements there is an increased height 'h' of each tooth 17 available (see figure 3) to stop tooth slippage due to the angle of inclination 9. In a typical arrangement the Inclination 9 is 15 to 20 degrees with a tooth depth of say 1 mm and a tooth pitch of say 0.5 mm.

An adjuster can be used in a brake which does not have the brake lever 11 and which can only be ap plied by the wheel cylinder 4. In such an arrangement the adjuster strut 9a slot is held onto the shoe 2 by strut return spring 13, rack 9b having similar slot which can assemble into shoe 3 rib by means of rack return spring 14. An adjuster 9 thus still functions to limit the retraction of the brake 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.

We Claim:

1. An automatic brake shoe adjuster for the drum brakes of a motor vehicle comprising two parts,
the first part being a strut, which is held in contact with the brake shoe by a strut return spring, the second part being a rack, said strut and rack being tightly held together by means of a torsion spring in order to engage the rack and pinion; an array of teeth provided for the rack which are engaged by an array of teeth on pinion; the rotation of the pinion, being defined by drum and shoe clearance, whereby under normal braking conditions, there is an initial radial gap 'x' between drum and shoe; and as the brake shoes wear down the effective length of the adjuster progressively increases so that the retraction of the brake shoes under spring action remains substantially constant; the extensions of the teeth 17 and teeth 18 being inclined at a slight angle 9 to the direction in which the strut part slides relative to the pinion.

2. An automatic brake shoe adjuster as claimed in
Claim 1 wherein the strut return spring, is hooked through an aperture in the brake shoe and into a hole in the strut,

3. An automatic brake shoe adjuster as claimed in Claim 1 or Claim 2 wherein the rack is guided through a hole relative to the strut via rivet which is riveted on-the strut.

4. An automatic brake shoe adjuster as claimed In any one of the preceding Claims wherein the rack has similar slot which can be assembled into shoe ribs by means o1 rack return spring.

5. An automatic brake shoe adjuster for the drum brakes of a motor vehicle substantially as herein described with reference to, and as illustrated in, the accompanying drawings.