Brake application with fully worn linings

(Fig. 11.11(d)) When the brakes are operated with fully worn linings the shoes move outwards before they contact the drum. During this outward movement the rear end of the slot contacts the inner edge of the quadrant lever, disengaging it from the pinion. At the same time the quadrant lever rotates until the fingered end of the lever touches the side of the shoe web. Releasing the brakes permits the shoes to retract until the quadrant lever contacts the pinion at its least return position near the quadrant's edge, furthest away from the new lining retraction position.

If any more lining wear occurs, the quadrant is not able to compensate by moving into a more raised position and therefore the master cylinder pedal movement will become excessive, providing a warning that the linings need replacing.

11.3.3 Strut and cam brake shoe expander (Fig. 11.12(a and b))

This type of shoe expander is used in conjunction with leading and trailing shoe brakes normally operated by air pressure-controlled brake actuators connected to a lever spline mounted on the camshaft, which is itself supported on a pair of plain bronze bushes.

The camshaft mounted in the expander housing is splined at its exposed end to support and secure the actuating lever (Fig. 11.12(a)). The other end, which is enclosed, supports an expander cam which has two spherical recesses to accommodate a pair of ball-ended struts. The opposite strut ends are located inside a hollow tappet plunger (follower). Mounted on the end of each tappet plunger is a tappet head abutment which guides and supports the twin web shoes. This construction enables the linings to follow the drum shape more accurately. The tappet head abutments are inclined to provide a means for self-centralizing the brake shoes after each brake application. The cam strut lift relative to the camshaft angular movement tends to give an approximately constant lift rate for the normal angular operating range of the cam between new and worn linings conditions.

When the brakes are applied (Fig. 11.12(b)) the camshaft is rotated, causing the struts to move outwards against the hollow tapper plungers. The tappet head abutments force the shoes into contact with the drum, thereby applying the brakes.

11.3.4 Wedge shoe adjuster unit (Fig. 11.13) The adjuster housing is made from malleable iron and is spigoted and bolted firmly to the back plate (Fig. 11.13). A hardened steel wedge is employed

-----Expander housing

I - Plunger seal

-----Expander housing

I - Plunger seal

(a) Brake released [b| Brake applied

Fig. 11.12 (a and b) Strut and cam brake shoe expander

(a) Brake released [b| Brake applied

Fig. 11.12 (a and b) Strut and cam brake shoe expander

Tappet Plunger Assembly

with a screw adjuster stem rotating within the wedge which does not rotate, but moves at right angles to the inclined faced tappet plungers. So that accurate adjustment for each brake assembly is possible, a clicker spring is located between the screwed stem and the wedge. This spring fits onto two flats provided on the screw stem (not shown). The clicker spring has two embossed dimples which align and clip into shallow holes formed in the back of the wedge when the shoes are being adjusted; they therefore enable a fine adjustment to be made while also preventing the adjuster screw stem unwinding on its own while in service.

Cam expander requirements The object of a cam brake shoe expander (Fig. 11.14(a and b)) is to convert an input camshaft leverage torque into a shoe tip force. The shape of the cam profile plays a large part in the effective expansion thrust imposed on the shoe tips as the shoe linings wear and the clearance between the drum and linings increase.

Early S-shaped cams were derived from an Archimedean spiral form of locus which gives a constant rate of lift per degree of cam rotation, but varying cam radius. The present tendency is for the S cam to be generated from an involute spiral (Fig. 11.14(a)) which gives a slight reduction in lift per degree of cam rotation, but maintains a constant cam effective radius so that the shoe tip force always acts in the same direction relative to the cam shoe roller, no matter which part of the cam profile is in contact with the roller (Fig. 11.14(b)). By these means the shoe tip force will remain approximately constant for a given input torque for the whole angular movement of the cam between new and worn lining conditions. Note this does not mean that the effective input torque will be constant. This depends upon the push or pull rod and the slack adjuster lever remaining perpendicular to each other which is unlikely.

Cam profile geometry The involute to a base circle is generated when a straight line is rolled round a circle without slipping; points on the line will trace out an involute. The involute profile may be produced by drawing a base circle and a straight line equal to its circumference and dividing both into the same number of equal parts (Fig. 11.14(a)). From the marked points on the circle draw tangents to represent successive positions of the generated line. Step off the unwrapped portion of the circumference along each tangent and then plot a smooth curve passing through the extending tangential lengths. The locus generated is the involute to base circle, this shape being the basic shape of the so-called S cam.

Cam and shoe working conditions With new shoe linings the leading shoe works harder than the trailing shoe so that initially the leading shoe wear will be higher than that of the trailing shoe. If there is adequate camshaft to bush bearing clearance, shoe wear will eventually be sufficient to permit the camshaft to float between the shoe tips, allowing the trailing shoe to produce the same friction drag as the leading shoe, thus producing the equal work condition.

If the shoe tip force applied by the cam is equal, then the camshaft floats on its bearing. In practice, because the shoe tip force is not always equal, a resultant reaction force input will be provided by the camshaft to maintain equilibrium. Therefore the frictional force between the shaft and bearing can be significant in the mechanical losses between the input camshaft torque and the shoe tip force. The input camshaft torque may be derived from both the shaft frictional torque and the cam to roller contact torque (Fig. 11.14(c)).

Let mC = coefficient of camshaft to bearing friction R = resultant camshaft radial load (N) rc = camshaft radius (m) rb = base circle radius (m) F1 and F2 = roller contact forces (N) Then

Camshaft frictional torque = mc Rrc (Nm)

Cam design considerations To give the highest shoe factor, that is the maximum shoe frictional drag to input torque, a low rate of cam lift is desirable. This conflicts with the large total lift needed to utilize the full lining thickness which tends to be limited to 19 mm.

Typical rates of cam lift vary from 0.2 to 0.4 mm/ deg which correspond to brake factors of about 12 to 16 with the involute cam profile.

Involute Rolling Angle
Fig. 11.14(a-c) Air operated foundation brake assembly

As the cam lifts (Fig. 11.14(c)) the pressure angle ^ which is made between the cam and roller centre lines and the base circle tangential line decreases. For the cam to be self-returning the pressure angle should not be permitted to be reduced below 10°.

One approach to maximize cam lift without the rollers falling off the end of the cam in the extreme wear condition is to use the involute cam up to the point where the lining rivets would contact the drum and relining would be required. Beyond this point the cam is continued in a straight line, tangential to the cam profile (Fig. 11.14(c)). By this method, total cam lift is achieved for the normal thickness of lining within the designed angular movement of the cam, which is not possible with the conventional involute cam. Shoe tip force efficiency does drop off in the final tangential lift cam range but this is not a serious problem as it is very near the end of the linings' useful life. One important outcome of altering the final involute profile is that the blunting of the cam tips considerably strengthens the cam.

11.3.6 Wedge type brake shoe expander with automatic adjustment (Fig. 11.15) The automatic brake shoe adjustment provides a self-adjusting mechanism actuated by the expander movement during the on/off brake application cycle, enabling a predetermined lining to drum clearance to be maintained. When a brake application takes place, an adjusting pawl mechanism senses the movement of an adjusting sleeve located in one of the wedge expander plungers. If the sleeve travel exceeds 1.52 mm, the spring-loaded pawl acting on the sleeve teeth drops into the next tooth and automatically makes the adjusting screw wind out a predetermined amount. An approximate 1.14 mm lining to drum clearance will be maintained when the brakes are released, but if the adjusting sleeve and plunger outward travel is less than 1.52 mm, then the whole plunger assembly will move back to its original position without any adjustment being made.

Description (Fig. 11.15) The automatic adjustment is built into one of the expander plungers. With this construction the adjusting screw is threaded into an adjusting sleeve, the sleeve being a free fit inside the hollow plunger. A hollow cap screw, spring, and an adjusting pawl are pre-assembled and act as a plunger guide. The end of the adjusting pawl has sawtooth type teeth which engage corresponding helical teeth on the outside of the adjusting sleeve.

Operation (Fig. 11.16) As the brakes are applied, the plunger sleeve and screw move outwards and the sloping face of the teeth on the adjusting sleeve lifts the adjusting pawl against the spring. When the brake is released, the rollers move down both the central wedge and the two outer plungers' inclined planes to their fully released position. As the linings wear, both the plunger strokes and resulting pawl lift gradually increase until the pawl climbs over and drops into the next tooth space. The next time, when the brake is released and the plunger is pushed back into its bore, the upright face of the pawl teeth prevents the sleeve moving directly back. It permits the sleeve to twist as its outside helical teeth slide through the corresponding guide pawl teeth in its endeavour for the whole plunger assembly to contract inwards to the off position, caused by the inward pull of the shoe retraction spring. The partial rotation of the adjusting sleeve unscrews and advances the adjusting screw to a new position. This reduces the lining clearance. This cycle of events is repeated as the lining wears. The self-adjustment action only operates in the forward vehicle direction. Once the brake shoes have been installed and manually adjusted no further attention is necessary until the worn linings are replaced.

11.3.7 Manual slack adjuster (Fig. 11.17)

Purpose A slack adjuster is the operating lever between the brake actuator chamber push rod and the camshaft. It is used with 'S' type cam shoe expanders and features a built-in worm and worm wheel adjustment mechanism enabling adjustment to be made without involving the removal and alteration of the push rod length.

Operation (Fig. 11.17) The slack adjuster lever incorporates in its body a worm and worm wheel type adjuster (Fig. 11.17). The slack adjuster lever is attached indirectly to the splined camshaft via the internal splines of the worm wheel located inside the slack adjuster body. For optimum input leverage the slack adjuster lever and the push rod should be set to maintain an inside angle just greater than 90° with the brakes fully applied. Once the push rod length has been set, further angular adjustment of the'S' cam is made by rotating the worm shaft so that the large gear reduction between the meshing worm and worm wheel will slowly turn the worm wheel and camshaft until the cam flanks take up the excess shoe lining to drum clearance. Owing to the low reverse efficiency of the worm and worm wheel gearing, the worm and shaft will not normally rotate on its own. To prevent the possibility of the worm and shaft unwinding, caused perhaps by transmitted oscillatory movement of the slack adjuster during periods of applying and releasing the brakes, a lock sleeve is utilized.

Slack Adjuster Sleeve
Fig. 11.15 Twin wedge foundation brake expander and automatic adjuster

Adjustment (Fig. 11.17) Cam adjustment is provided by the hexagon head of the worm shaft situated on the side of the slack adjuster body (Fig. 11.17). To adjust the cam relative to the slack adjuster lever, the lock sleeve is depressed against the worm lock spring by a suitable spanner until the worm shaft is free to turn. The worm shaft is then rotated with the spanner until all the excess

Rotating Locking Mechanism

[3) Brakes fully applied

Fig. 11.16 Wedge expander and automatic clearance adjuster

[3) Brakes fully applied

Fig. 11.16 Wedge expander and automatic clearance adjuster shoe lining to drum clearance is eliminated. The worm shaft is then prevented from unwinding by the worm lock spring forcing the lock sleeve against the hexagonal head of the worm shaft. The removal of the spanner permits the worm lock spring to push the internally serrated lock sleeve up to and over the hexagonal bolt head. To prevent the lock sleeve rotating, a guide pin fixed in the slack adjuster body aligns with a slot machined on the sleeve.

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