"S cam shaft" drum brakes are widely used in heavy commercial vehicles such as Trucks, Trailers and Buses. The brake shoes with the brake fining rest on the"S"shaped camshaft and are pressed against the brake drum during the brake application. The splincd portion of the"S catnshaft"protrucles out of the brake drum. The Automatic 13rakc Adjuster is fixed on the spline with the other end of the assembly connected to the output rod of the brake actuator suitable for air pressure operation. When air pressure is applied to the brake actuator, the mechanical output will be transmitted as braking force through the push rod to the"S camshaft" which expands the brake shoes and presses the lining against the brake drum to apply the brake.

During regular operation of the brakes, the clearance between brake lining and drum increases due to lining wear resulting in increasing the push rod stroke of the brake actuator to apply the brake. It is essential to maintain optimum clearance for effective braking and periodical adjustment to maintain the clearance. Therefore it has become necessary to provide the Automatic Brake Adjuster with an internal mechanism for sensing the excess wcar of the lining and automatically rotating, the worm shaft engaged with the worm wheel and in turn the"S camsllaft"for maintaining optimum clearnance. thereby maintaining the angular movement of the Automatic Drake Adjuster to the desired limit. It is because of this feature that it is called an Automatic Brake Adjuster.

The sensing of the slackness of the brake due to the lining wear by the Automatic Urake Adjuster can be either by stroke or clearance between the lining and the drum.

According to the stroke principle, the adjustment entirely depends on the push rod stroke of the brake actuator, or in other words the angular movement of Automatic Brake Adjuster is varied by the clearance between the brake lining and the brake drum.

According to the clearance principle which is claimed as an advanced system, the adjustment mechanism is able to sense various factors such as elasticity of brake drum and different structural parts in the brakes for varying the push rod stroke and differentiate the wear on the. brake lining to optimize the lining clearance ignoring the influence of excess push rod stroke due to other factors.

The automatic brake adjuster ts a satiety device working undo extremely bad environmental conditions andi atso under severe toad and vibration conditions, with limitation on space availability. This demands high reliability and prolonged life between servicing.

This invention will now be described in further detail with reference to the accompanying drawings, which illustrate by way ofexampte and not by way of timitation. a preferred embodiment of this invention.

'Fig ! is a Front view of the Automatic Brake Adjuster.

# Fig 2 is the End view sectioned along the line a\-A of Fig. 1.

# Fig 3 is the End view sectioned along the line A-A"of fig t to show the Pinion and Control arm assembly.

'Fig 4 is the End view sectioned along the line B-B" of Fig.3 to show the Ratchet and Pawl Mechanism.

In an Automatic Brake Adjuster the angular movement thereof is sensed by the centre) arm anchored to a fixed part on the axle for establishing the reference point. It is desirable to provide a flexible reference point for easy installation on various models of vehicles instead of having a fixed reference point mounting with reference to the Automatic Brake Adjuster body, which requires additional care for the initial installation and also for users during re-fitment after servicing.

Conventional Automatic Brake Adjusters have a fixed reference point for mounting on the vehicle chassis. During fitment on the new chassis accuracy of fitment is essential to obtain the proper function. This makes assembling the brake adjuster cumbersonne.

To overcome the above problems with the fixed reference point the angular position of the control arm of the Automatic Brake Adjuster housing is rendered inconsequential by ;) n arrangement having a floating reference point.

The adjustment mechanism in the Automatic Brake Adjuster with the clutch worm wheel and the worm shaft should be slow acting adjustment where by only a fraction of the whole excessive slack is to be compensated during every brake application. This necessitates a reduction gear mechanism. The gear ratios between a) Control gear and the l'inion and b) control worm screw and Ctutch worm wheel are fixed to achieve this.

Control gear wheel and pinion with control worm screw (as asscmbty) are arranged in parallel axis to each other whereas the clutch worm wheel and the worm shaft are positioned perpendicularly to the pinion assembly.

With the above features it is also important to have the adjustment of excess clearance made during the return stroke of the brake rather than during the brake application stroke to reduce the wear and thereby increase the life of the parts.

The proposed Automatic Brake Adjuster has the provision of a piîliol1 assembly comprising pinion and control worm screw incorporated with one way lock mechanism by means'of ratchet and pawl arrangement.

The controlled axial play of control worm screw in the spindle ensures desired slack or control distance for the Automatic Brake Adjuster for maintaining the clearance between the brake lining and the drum. The positioning of the control worm screw is determined by two compression springs during the to and fro movements.

The pinion assembly is retained by a screw to facilitate easy dismnntHng and re- assembling as shown in enclosed drawing.

Automatic Brake Adjuster is normally used as a lever to connect the output rod of the brake actuator and the splined camshaft of an "S Cam brake". The S cam brake consists of a brake drum, brake shoes and s-camshaft. The brake shoe will be pressed against the brake drum during the application of brakes by the S camshaft, the Automatic Brake Adjuster is used to transmit the braking force to the S cam brake by the hmke nctnator that is operated by air pressure.

Automatic Brake Adjuster body (1) is provided with a hole (2) in the tai) portimo lir connecting to the brake actuator. The other end of the body houses a rotatable worm whec) (3), which has internal splines for fixing the S camshaft. A rotatabte Worm shaft (4) meshing with the worm wheel (3) is located in the body (1) perpendicular to the axis of the worm wheel (3).

For the purpose of manual rotation by a spanner, one end of the worm shaft (4) has a hexagon portion (4') protruding out of the body (1). On the hexagona) portion side is mounted a clutch worm'wheel (6) along with a thrust bearing (7) and a bearing retainer (5) which is screwed in to the body (1) and guides the worm shaft (4). The bearing retainer (5) and the thrust bearing (7) are used to retain the clutch worm wheel (6) for free rotation against the heavy compression spring (9) load. Sealing ring (25) is provided in the groove of the worm shaft (4) to prevent the entry of foreign particles. On the other side of the worm shaft (4) is mounted the spring seat (10), heavy compression spring (9) and the spring retainer (11). The spring retainer (11) is screwed into the body (I) for setting the, desired spring load. The clutch worm wheel (6) forms a clutch by the serration (6') seating on the serration (4") of the worm shaft (4). The engagement of this clutch is ensured by a heavy compression spring (9) that exerts load on to the worm shaft (4).

The control arm assembly (12^15&17) is located in the body on the same bore of the worm wheel (3) and independently operational. The rotatable control gear wheel (12) of the control arm assembly having gear teeth circumferentially and integral with Control arm (13), which has fixing portion (13'). The cover plate (14) is located in between control gear wheel (12) and control arm (13) along with sealing rings (15) and (17) to avoiel cntry of foreign particles.. The control arm assembly (12-15&17) is fixed on thc body (1) with a Gasket. (8) by means of screws (18) in the periphery holes of the cover plate (14). Control arm (13) is a rotatable attachment of the control arm assembly (12- 15&17). The fixing portion of control arm (13') is to be rigidly mounted on the vehicle chassis and the purpose of this is to provide a reference point for the Automatic Brake Adjuster as explained below.

The Control gear wheel (12) meshes with the pinion (16) and is located in the worm wheel bote of the body (1). The location of the pinion assembly (16, 19, 20, 21, 23 &24) is shown in Fig. 2.

Pinion (16) and Control worm screw (19) connected by means of a ratchet (16') and pawl (26) mechanism and aligned in line by a spindle (23) with spring (24), Guide (20) and retained by the screw (21). The clearance provided in between the spindle step face and the control worm screw (19) bore face is to give free axial movement during the rotation of pinion (16) by control gear wheel (12) to achieve the desired slack between the brake shoe and the brake drum which is otherwise called as the clearance stroke of the Automatic Brake Adjuster. The two compression springs (22) and (24) are provided to ensure the proper axial movement to achieve the desired clearance stroke of the Automatic Brake Adjuster.

The ratchet (16') and pawl (26) mechanism between pinion (16) and control worm screw (19) is acting as a one-way lock, which allows free rotation of ratchet during the counter clockwise rotation of the pinion (16) and also permits axial movement of control worm screw (19).

In short the Automatic Brake Adjuster is a lever that transmits the braking force from the brake actuator to the S-cam shaft and a mechanical device to sense the excess lining clearance and adjusts the slack between the brake lining and the brake drum to an optimized value.

The operation of the Automatic Brake Adjuster according to the invention is described in further detail below l When the Brake is applied with excess lining clearance : - To begin with, durihg the initial brake application, the pinion (16) is rotated by the control arm assembly (l2-ls&ì7), which is rigidly mounted on the vehicle chassis, due to the movement of the lever. During this-operation the control worm screw (19) is<BR> <BR> <BR> <BR> <BR> allowed to move down axially to pre determined gap between step in the spindle (23) and<BR> <BR> <BR> <BR> <BR> control worm screw (19) bore face by the ratchet (16') and pawl (26) mechanism,<BR> <BR> <BR> <BR> <BR> <BR> compressing the spring (22) provided at the bottom on the control worm screw. This controlled axial play of the control worm screw (19) ensures predetermined clearance stroke of the Automatic Brake Adjuster. Simultaneously the worm wheel (3) is rotated in<BR> <BR> <BR> <BR> <BR> <BR> the counter clockwise direction along with Automatic Brake Adjuster body (1); in turn<BR> <BR> <BR> <BR> <BR> the S camshaft engaged with the worm wheel spline (3') is rotated to lift the brake lining<BR> <BR> <BR> <BR> <BR> towards the brake drum.

During the further rotation (after passing through clearance stroke cycle) of the ratchet mechanism in the pinion (t6), overrides and gets a new engagement with the pawl (26) fixed to the control worm screw (19).

This is because control worm screw (19) is prevented from rotation by dutch worm gear (6) due to excess friction on the clutch worm gear (6) with its serration (6') fully engaged on the worm shaft serration (4").

Once the brake lining engages the brake drum, the counter force increases and worm shaft (4) moves axially compressing the heavy compression spring (9) and dutch is disengaged as the serrated portion of worm shaft (4") is moved away from the clutch worm wheel serration (6').

As the clutch is how disengaged, the resistance oh the clutch worm wheel (6) is greatly reduced and permits the dutch worm screw (19) to rotate with the pinion (16) as a whole unit retaining the relative positions. Thereby the lever movement during this period (expansion/deflection zorie) is ignored.

When the brake is released, the pinion (16) is rotated in clockwise direction by the control gear wheel (12), opposite to the direction of brake application and the control worm screw (19) follows the rotation of pinion (16) as a whole unit along with clutch worm wheel (6) covering the deflection stroke of the Automatic Brake Adjuster.

However the worm shaft (4) remains static as the dutch is disengaged between serrations (4") and (6') Once the brake lining moves away from the brake drum, the force is reduced and the load of the heavy compression spring (9) moves the worm shaft (4) to engage the clutch preventing the free rotation of clutch worm wheel (6).

During the further release movement of brake actuator release, the body (1) keeps rotating in clock'wise direction, the control gear wheel (12) continues to rotate the pinion (16), but due to the friction on the control worm wheel (6) which is in clutch engaged condition, the control worm screw (19) is axially moved up closing the gap between the step in the spindle (23) and control worm screw (19) bore face. This movement covers the cleatance stroke of the Automatic Brake Adjuster, which was achieved during the initial period of brake application.

During the final releasing rotation of Automatic Brake Adjuster, the pinion (16) rotated by the control gear wheel (12) in turn rotates the control worm screw (19) by the one-way lock (ratchet (16') and pawl (26)) mechanism and the dutch worm wheel (6).'The worm shaft (4) that is now dhgaged with clutch worm wheel (6) is rotated and in turn rotates the worm wheel (3) and'S'camshaft to effect the adjustment of lining clearance. This is in propottion to the new engagement taken place during the brake application stroke explained earlier.

The function explained in last paragraph is applicable only for brake application with excess lining clearance, where the ratchet (16') is engaged in a new location and later disengaged as above. Whenever the brake is applied with optimum brake lining clearance, the totation of pinion (16) stops at the point completing the clearance stroke of the Automatic Brake Adjuster.