Chap74

[object Object],[object Object],[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 74, the reader should be able to:

[object Object],[object Object],KEY TERMS: Continued

DRUM BRAKE ADVANTAGES ,[object Object],Continued Figure 74–1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.

Figure 74–2 An exploded view of a typical drum brake assembly. ,[object Object],Continued

DRUM BRAKE DISADVANTAGES ,[object Object],Continued ,[object Object],[object Object],[object Object],Brake Fade The greatest drawback of drum brakes is that they are susceptible to fade. Brake fade is loss of stopping power when excessive heat reduces the friction between shoe linings and the brake drum.

[object Object],Continued Quick-and-Easy Drum Brake Adjustment Check Tap the brake drum lightly with a hammer or wrench. If the brake shoes are not contacting the drum, the drum will ring like a bell. If the shoes are contacting the drum, the sound will be muffled.

DRUM BRAKE PARTS ,[object Object],Continued Figure 74–3 The backing plate is the foundation of every drum brake. The foundation of every drum brake is the backing plate. It mounts to the steering knuckle on the front brakes; to the suspension or axle housing on the rear brakes.

Figure 74–4 The flange on the backing plate is designed to come close to a notch or groove on the brake drum, forming a type of seal that helps prevent debris and water from getting onto the drum brake. Continued

Figure 74–5 A keystone anchor allows the brake shoes to self-center in the drum. Continued

Figure 74–6 Piston stops prevent the wheel cylinder from coming apart Continued

Figure 74–7 Cross-section of a wheel cylinder that shows all of its internal parts. The brake line attaches to the fluid inlet. The cup expander prevents the cup seal lip from collapsing when the brakes are released. ,[object Object],Force exerted on the brake fluid by the driver forces the piston inside the wheel cylinder to move outward. Continued

Figure 74–8 The pushrods are held in place by the rubber dust boots. As the wheel cylinder pistons move outward, the pushrods transfer the movement to the brake shoes. Through pushrods or links, this movement acts on the brake shoes, forcing them outward against the brake drum.

DRUM BRAKE SHOES ,[object Object],Continued Most shoes are made of two pieces of sheet steel welded together in a T-shaped cross-section. Figure 74–9 Steel brake shoes are made from two stampings welded together.

Figure 74–10 Tapered ends on the brake linings reduce noise. ,[object Object],On some, the lining table edge has small V- or U- shaped notches called nibs . Continued

Figure 74–11 Typical drum brake shoe and the names of the parts. (Courtesy of Allied Signal Automotive Aftermarket) Continued

[object Object],Figure 74–12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing). Continued

Figure 74–13 Primary shoe lining may vary depending on the application. Continued

[object Object],Continued ,[object Object],[object Object],[object Object],Riveting The oldest method of lining attachment is used on riveted linings . The brake block is attached to the lirining table or backing plate with copper or aluminum rivets. See Figure 74–14.

Figure 74–14 Riveted brake linings are quiet and reliable at high temperatures. ,[object Object],This play enables the assembly to absorb vibration, and operate more quietly than bonded linings. Rivets are very reliable and will not loosen at high temperatures. Continued

[object Object],Figure 74–15 The brake line is bolted to the shoe on most heavy trucks and busses. Continued

[object Object],Figure 74–16 Many brake linings are bonded. Continued Heat and pressure are applied to cure the assembly. Bonding is a common form of shoe and pad assembly.

LINING EDGE CODES ,[object Object],Continued ,[object Object],[object Object],[object Object],See Figure 74–17.

Figure 74–17 Typical drum brake lining edge codes. Continued

[object Object],Continued These codes were established by the SAE (Society of Automotive Engineers): Code C 0.00 to 0.15 Code D 0.15 to 0.25 Code E 0.25 to 0.35 Code F 0.35 to 0.45 Code G 0.45 to 0.55 Code H 0.55 and above Code Z ungraded

[object Object],Continued These letters should not be interpreted to mean relative quality of the lining material.

NOTE: While many brands of replacement brake lining provide acceptable stopping power and long life, purchasing factory brake lining from a dealer is usually the best opportunity to get lining material that meets all vehicle requirements. Aftermarket linings are not required by federal law to meet performance or wear standards that are required of original factory brake linings. Lining wear, fade resistance, tensile strength, heat recovery rate, wet friction, noise, and coefficient of friction must be considered when purchasing high-quality linings. There are no standards that a purchaser can check regarding all of these other considerations. For best brake performance, always purchase the best-quality name-brand linings that you can afford.

Figure 74–18 Typical drum brake showing support plate (backing plate), anchor pin, and shoe guide plate. ,[object Object],This prevents brake drag, and aids the return of brake fluid to the master cylinder reservoir. Most brakes use closed-coil return springs to retract the brake shoes. Continued Type, location & number of springs varies from one brake design to the next.

Figure 74–19 A single spring-steel return spring is used on some drum brakes. ,[object Object],Some vehicles have a single, large, horseshoe-shaped return spring. Some connect directly shoe to shoe; others from one shoe to the anchor post. Continued

Figure 74–20 Various types and styles of hold-down springs. Continued

[object Object],Figure 74–21 A mechanical parking brake linkage is part of most rear drum brakes. Continued

[object Object],Figure 74–22 An aluminum brake drum with a cast-iron friction surface. The cooling fins around the outside help dissipate the heat from the friction surface to the outside air. Brake drums are made of cast iron or cast aluminum with a cast-iron liner. Many of these drum types may have ribs or fins on their outer edge to help dissipate heat.

DRUM BRAKE DESIGN ,[object Object],All drum brakes fall in two basic categories: Continued ,[object Object],[object Object],Early automotive drum brake friction assemblies were non-servo designs, and are still in many rear-wheel applications. The more powerful servo drum brakes were developed later and are still used on some vehicles.

NON-SERVO BRAKES ,[object Object],Continued

[object Object],Figure 74–23 Self-energizing action can increase or decrease the stopping power of a brake shoe. Continued As the leading shoe contacts the drum, the drum attempts to rotate the shoe. The shoe cannot rotate; its far end (relative to drum rotation) is fixed in place. Drum rotation energizes the shoe by forcing it outward and wedging it tightly against the drum,

[object Object],Continued A leading shoe is always energized by drum rotation. A trailing shoe is always de-energized by drum rotation .

Figure 74–24 A double-trailing non-servo drum brake. Continued

Figure 74–25 A leading-trailing non-servo drum brake. Continued

[object Object],Continued NOTE: Dual-servo brakes are also called Duo - Servo , which is a brand name of the Bendix Corporation.

[object Object],Figure 74–26 A typical dual-servo drum brake. Continued The anchor is usually mounted at the top of the backing plate with the wheel cylinder directly beneath it. The tops of the brake shoes are held against the anchor by individual return springs. The bottoms of the shoes are spaced by an adjusting link held by a third return spring that connects the two shoes.

[object Object],Continued NOTE: Adjusting links generally have specific left- or right-hand threads, and must be installed on the correct side of the vehicle. The outer ends of the pivot nut and socket are notched to fit over the brake shoe webs. Some adjusting links have a steel thrust washer and/or spring washer installed between the socket and the starwheel. These washers allow easier rotation of the starwheel and help reduce brake squeal. See Figure 74–27.

Figure 74–27 A dual-servo brake adjusting link. NOTE: Adjusting links generally have specific left- or right-hand threads, and must be installed on the correct side of the vehicle. Continued

[object Object],Figure 74–28 Dual-servo brake operation. The primary shoe on the left exerts a force on the secondary shoe on the right. As the primary shoe makes contact it rotates with the drum because the far end is not anchored to the backing plate. As the primary shoe rotates, it forces the adjusting link and secondary shoe to rotate until the secondary shoe seats firmly against the anchor. Continued

Figure 74–29 Servo action greatly increases the application force on the secondary shoe. ,[object Object],The primary shoe is forced against the anchor while the secondary shoe moves outward and rotates with the drum to apply the primary shoe with a greater force. Continued

[object Object],Rear Wheel Lockup? Check the Adjustment

AUTOMATIC BRAKE ADJUSTERS ,[object Object],Continued ,[object Object],[object Object],[object Object],All three adjusters mount on the secondary brake shoe and adjust when the brakes are applied while the vehicle is moving in reverse. As the brakes are applied on a vehicle with a cable or link automatic adjuster, the wheel cylinder and drum rotation combine to move the secondary shoe away from the anchor. Movement of the shoe causes the cable or linkage to pull up on the adjuster pawl . See Figure 74–33.

Figure 74–30 A cable-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment as the brakes are released. Continued

Figure 74–31 A lever-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment as the brakes are applied. Continued

Figure 74–32 A link-activated starwheel automatic adjuster. This type of adjuster makes the adjustment as the brakes are released. Continued

Figure 74–33 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel. ,[object Object],Continued When the brakes are released, the pawl return spring pulls the pawl down, rotating the starwheel and moving the brake shoes apart to reduce the lining-to-drum clearance.

Figure 74–34 A cable-actuated starwheel automatic adjuster with an over-travel spring. ,[object Object],This moves the bottom half of the lever down, causing the pawl to rotate the starwheel and make the adjustment. Continued

Figure 74–35 A non-servo brake with a lever-activated starwheel automatic adjuster on the leading shoe. This type of adjuster makes the adjustment as the brakes are applied. Continued

Figure 74–36 A non-servo brake with a lever-actuated starwheel automatic adjuster on the trailing shoe. This type of adjuster makes the adjustment as the brakes are released. ,[object Object],The upper shoe return spring in this design returns the brake shoes and operates the automatic adjuster. Continued

[object Object],Cool the Brakes Before Backing NOTE: Some drum brakes are equipped with a bimetallic heat sensor that prevents the self-adjusters from working if the brakes are hot.

Figure 74–37 A lever-latch ratchet automatic adjuster. ,[object Object],Continued This design consists of a large lever and a smaller latch with interlocking teeth. A spring on the latch piece keeps contact with the lever to maintain the adjustment. One end of the parking brake strut hooks into an opening in the lever and the other end is held against the trailing brake shoe by a strong spring.

Figure 74–38 A strut-quadrant ratchet automatic adjuster. ,[object Object],[object Object],[object Object],[object Object],The strut-quadrant adjuster consists of three basic parts: The strut has a toothed post mounted on its underside. The adjuster quadrant pivots on a pin that slips into a notch in the end of the strut Continued

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],[object Object],Continued ( cont. )

SUMMARY ,[object Object],[object Object],[object Object],( cont. )

Chap74

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Chap74