10 Drum Brakes - Automotive Engineering .pptx

Automotive Engineering

1. Automotive Chassis Systems Eighth Edition Chapter 11 Drum Brakes

2. Objectives Identify drum brake component parts. Discuss the advantages and disadvantages of drum brakes. Explain the function of the backing plate, wheel cylinders, and drum brake shoes. Describe the operation of non-servo brakes. Explain the operation of dual-servo brakes. Discuss automatic brake adjusters.

3. Drum Brakes Background Drum brakes were the first type of brakes used on motor vehicles. Even today, over 100 years after the first “horseless carriages,” drum brakes are still used on the rear of most vehicles, The drum brake has been more widely used than any other automotive brake design. Although the disc brake has proven its superiority in extreme braking conditions, and has replaced the drum brake on the front axle of vehicles, the drum brake continues to have a number of advantages that contribute to its widespread use on the rear axle of most automobiles. FIGURE 11–1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.

4. Drum Brake Advantages Self-Energizing and Servo Action The primary advantage of drum brakes is that they can apply more stopping power for a given amount of force applied to the brake pedal than can disc brakes. This is possible because the drum brake design offers a self-energizing action that helps force the brake linings tightly against the drum. Parking Brake Service One advantage of drum brakes is that they make excellent parking brakes. A simple linkage fitted to the brake assembly allows relatively low effort from the driver to hold a heavy vehicle in place when parked.

5. Drum Brake Disadvantages Brake Fade Drum brakes are not very efficient at dissipating heat. The brake drum covers the linings, and most of the heat produced during braking must pass through the drum, from the inside out into the air. The greatest drawback of drum brakes is that they are susceptible to fade. Brake fade is the loss of stopping power that occurs when excessive heat reduces the friction between the brake shoe linings and the drum. The four types of brake fade include the following: Mechanical fade. Mechanical fade occurs when the brake drum gets so hot that it expands away from the brake linings Lining fade . Lining fade occurs when the friction coefficient of the brake lining material drops off sharply because intense heat makes it “slippery.” Gas fade . Gas fade occurs under extended hard braking from high speeds. Water fade . Water fade occurs when moisture is trapped between the shoes and drum, where it acts as a lubricant.

6. Drum Brake Disadvantages Brake Adjustment Another disadvantage of the drum brake design is its need for an adjusting mechanism. As the brake shoe lining material wears, the clearance between the linings and drum increases, resulting in longer brake pedal travel. To maintain a high brake pedal, a mechanism must be included in the brake assembly for adjustment of the clearance between the brake shoe and drum. Brake Pull Brake pull occurs when the friction assemblies on opposite sides of the vehicle have different amounts of stopping power. These differences can be caused by brake fade or misadjustment of the clearance between the brake linings and drum.

7. Backing Plate and Wheel Cylinders Backing Plate The foundation of every drum brake is the backing plate that mounts to the steering knuckle on the front brakes, or to the suspension or axle housing on the rear brakes. The backing plate serves as the mounting surface for all the other friction assembly parts. The backing plate also functions as a dust and water shield to keep contaminants out of the brake assembly. In addition to mounting holes for the various brake parts, the backing plate may also have openings that are used to inspect the wear of the brake linings, or adjust the lining-to-drum clearance.

8. Backing Plate and Wheel Cylinders Shoe Anchors Shoe anchors prevent the brake shoes from rotating with the drum when the brakes are applied. The majority of drum brakes have a single anchor, but some drum brake designs use two or more. Many anchors are a simple round post that is permanently mounted on the backing plate. The brake shoes have semicircular cutouts where they contact the anchor, and the anchor positively locates the shoe on the backing plate. Another type of anchor is the self-centering or keystone anchor.

9. Backing Plate and Wheel Cylinders Piston Stops Some backing plates incorporate piston stops that prevent the wheel cylinder pistons from coming out of their bores when the friction assembly is disassembled for servicing. The stops may be part of a reinforcing plate positioned under the anchor or they can be stamped directly into the shape of the backing plate itself. When piston stops are used, the wheel cylinder must be removed from the backing plate before it can be taken apart for servicing. Shoe Support Pads The shoe support pads are stamped into the backing plate and contact the edges of the brake shoes to keep the linings properly aligned with the center of the friction surface inside the brake drum. These pads are also called ledges or shoe contact areas.

10. Backing Plate and Wheel Cylinders Wheel Cylinders Hydraulic pressure is transferred from the master cylinder to each wheel cylinder through brake fluid. The force exerted on the brake fluid by the driver forces the piston inside the wheel cylinder to move outward. Through pushrods or links, this movement acts on the brake shoes, forcing them outward against the brake drum.

11. Drum Brake Shoes Terminology The linings of drum brakes are attached to curved metal assemblies called brake shoes. Most shoes are made of two pieces of sheet steel welded together in a T-shaped cross section. The outer edge is lined with a friction material that contacts the brake drum to generate the actual stopping power. The ends of the linings on most brake shoes are tapered to prevent vibration and brake noise The curved metal piece on the outer portion of the shoe is called the lining table, the shoe rim, or platform. The lining table supports the block of friction material that makes up the brake lining. The metal piece of the shoe positioned under the lining table and welded to it is called the shoe web.

12. Drum Brake Shoes Terminology One end of the web usually has a notch where the wheel cylinder touches the shoe, while the other end commonly has a flat or curved surface where the shoe meets an anchor or adjusting link. The upper ends of the webs on dual-servo brake shoes have semicircular cutouts called anchor eyes. Brake shoes are parts that can be relined and reused many times if the web and lining table are not damaged. Brake shoes are parts that can be relined and reused many times if the web and lining table are not damaged. Brake shoes for any given application are usually available in both “new” and “relined” versions from suppliers. At the time of purchase, a core charge is added to the cost of the relined parts.

13. Drum Brake Shoes Primary and Secondary Brake Shoes In a dual-servo drum brake system, the shoes in a dual-servo brake perform different jobs. The primary shoe ( forwardfacing shoe) is self-energized by drum rotation to create a servo action that forces the secondary shoe more firmly against the drum. To help deal with the added friction, heat, and wear it undergoes, the lining of the secondary shoe extends nearly the full length of the shoe lining table. On most dual-servo brake primary shoes, the lining is positioned near the center of the lining table.

14. Drum Brake Shoes Lining Assembly Methods There are two main methods used to mount brake linings to the brake shoe. RIVETING. The brake block is attached to the lining table or backing plate with copper or aluminum rivets. Riveting has the following advantages: The major advantage of riveting is that it allows a small amount of flex between the brake block and lining table or backing plate. Rivets are also very reliable and will not loosen at high temperatures. Riveting has the following disadvantages: Rivet holes, however, do create stress points in the lining where cracks are likely to occur. Riveted lining has a reduced service life because the linings must be replaced before the rivet head contacts the brake drum. The rivet can cause deep grooves to be cut into the drum, often requiring replacement if the worn brakes are not detected early enough to prevent damage.

15. Drum Brake Shoes Lining Assembly Methods BONDING. Bonded linings use high-temperature adhesive to glue the brake block directly to the shoe lining table or pad backing plate. Heat and pressure are then applied to cure the assembly. Bonding is a common form of shoe and pad assembly and is most often used to mount organic friction materials.

16. Drum Brake Shoes Lining Assembly Methods Bonding has several advantages including the following: Without rivets, bonded linings can wear closer to the lining table or backing plate and provide a longer service life. Bonded linings also have fewer problems with cracking because they have no rivet holes to weaken the brake block. Bonded linings have the following disadvantages: If a bonded lining gets too hot, the bonding adhesive will fail and allow the brake block to separate from the lining table or backing plate. Bonded linings are also more prone to be noisy because they do not allow any vibration absorbing flex between the brake block and lining table or backing plate.

17. Drum Brake Shoes Return Springs The brake shoe return springs retract the shoes to their unapplied positions when the brake pedal is released. This helps prevent 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. The coils on these springs are very tightly wound and contact one another when the spring is relaxed. Some vehicles have a single, large, horseshoe-shaped return spring. The type, location, and number of return springs vary from one brake design to the next. All springs are installed in one of two ways. Some connect directly from shoe to shoe, while others connect from one shoe to the anchor post.

18. Drum Brake Shoes Brake Shoe Hold-Downs While the return springs retract the brake shoes to their unapplied positions, the brake shoe hold-downs keep the shoes securely against the support pads on the backing plate. The hold-downs prevent noise, vibration, and wear, but still allow the shoes to move out and back as the brakes are applied and released. The hold-downs also provide enough freedom of movement to allow adjustments of the shoes outward as the linings wear. The most common design is a steel pin installed through a hole in the backing plate and a corresponding hole in the brake shoe web. Another type of hold-down is a taper-wound coil spring with a hook formed on its end.

19. Drum Brake Shoes Parking Brake Linage Most rear drum brake friction assemblies include a parking brake linkage. The linkage commonly consists of a cable, lever, and strut system that spreads the brake shoes apart to apply the brake mechanically. The parking brake strut plays a large part in many of the automatic brake adjusters. The brake drum is not connected to the backing plate, but turns with the wheel. The drum mounts on the hub or axle, and covers the rest of the brake assembly.

20. Non-Servo Brake Design Purpose and Function A non-servo brake feature is that each brake shoe is applied individually. The action of one shoe has no effect on the action of the other. Many non-servo drum brakes use self-energizing action to improve their braking performance. Parts and Operation Self-energizing action occurs when the forward or leading shoe contacts the drum and the drum attempts to rotate the shoe along with it. The drum also attempts to rotate the reverse or trailing shoe as it contacts the drum. When this type of brake is applied with the vehicle backing up, the roles of the forward and reverse shoes are switched. The reverse shoe becomes the leading shoe, which is self-energized by drum rotation, while the forward shoe becomes the trailing shoe, which is de-energized. A leading shoe is always energized by drum rotation. A trailing shoe is always de-energized by drum rotation.

21. Non-Servo Brake Design Parts and Operation The first shoe from the wheel cylinder in the direction of drum rotation is the leading shoe. Leading shoes generally wear at a faster rate than trailing shoes because they are applied with greater force. Double-Trailing Brake The least powerful non-servo drum brake is the double-trailing brake. This design has two trailing shoes and does not use any self-energization. Both shoes have the same size and shaped linings that are applied with equal force by a pair of single-piston wheel cylinders. Each shoe is anchored at the end opposite the wheel cylinder that applies it. In many double-trailing brakes, the backside of one-wheel cylinder serves as the anchor for the brake shoe actuated by the other wheel cylinder.

22. Non-Servo Brake Design Leading-Trailing Brake Design The non-servo leading-trailing brake has one leading shoe and one trailing shoe. Typically, a single, two-piston wheel cylinder is mounted at the top of the backing plate and the two brake shoes are anchored at the bottom of the backing plate. The brake design has one energized and one de-energized shoe regardless of whether it is applied while the vehicle is traveling forward or in reverse. Leading-trailing brakes are popular on the rear wheels of many small and front-wheel-drive vehicles because, although they are not as powerful as a servo brake, they are less prone to lockup.

23. Dual-Servo Brake Design Purpose and Function The dual-servo brake is the most common drum brake design. The name “servo” comes from the fact that one shoe “serves” the other to increase application force. All servo brakes used on automobiles are of the dual-servo design that works with equal force in both directions. The primary advantage of the dual-servo brake is that it is more powerful than any of the non-servo designs. Another advantage of the dual-servo brake is that it makes a good parking brake. Dual-servo brakes are more susceptible to pull than other brake designs, and their greater application force can lead to faster fade under extreme braking conditions.

24. Dual-Servo Brake Design Dual-Servo Brake Construction The basic dual-servo brake uses one anchor and a single two-piston wheel cylinder ADJUSTING LINK. The adjusting link consists of a star-wheel that is part of an adjusting screw, a pivot nut that one end of the adjusting screw threads into, and a socket that rotates freely on the opposite end of the adjusting screw. PRIMARY AND SECONDARY BRAKE SHOES. Although dual-servo brakes make use of self-energizing action to help provide servo action, the two brake shoes are not called leading and trailing parts as in non-servo brakes. Instead, they are identified as the primary shoe and the secondary shoe. The secondary brake shoe provides approximately 70% of the total braking power in a dual-servo brake.

25. Dual-Servo Brake Design Dual-Servo Brake Operation When a dual-servo brake is applied, the wheel cylinder attempts to force the tops of both brake shoes outward against the drum. Although the wheel cylinder attempts to push the top of the secondary shoe outward, the rotational force developed by friction between the brake shoes and drum is much greater than the force developed by hydraulic pressure in the wheel cylinder. Servo Action Once all clearance is taken up between the brake shoes, adjusting link, and anchor, both brake shoes become self-energized like the leading shoes in a non-servo brake. When a dual-servo brake is applied with the vehicle moving in reverse, the primary and secondary shoes switch roles. 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.

26. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Servo brakes use three styles of starwheel adjusters: Cable. Lever. Link. All three adjusters mount on the secondary brake shoe and adjust only 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. Some servo brakes with cable-actuated starwheel automatic adjusters have an over-travel spring assembly on the end of the cable. In this design, the adjuster pawl is mounted under the starwheel , and adjustment is made as the brakes are applied rather than released.

27. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters The lever starwheel adjuster makes the adjustment as the brakes are applied rather than released. The operation includes the following actions: As the secondary shoe moves away from the anchor, the solid link between the anchor and the top of the adjuster lever forces the lever to rotate around the pivot point where it attaches to the brake shoe. This moves the bottom half of the lever downward, which causes the pawl to rotate the starwheel and make the adjustment. The separate pawl piece is free to pivot on the lever to prevent damage if the starwheel will not rotate. When the brakes are released, the return springs lift the lever. If the brakes have worn enough, the end of the lever engages the next tooth on the starwheel and additional adjustment will be made the next time the brakes are applied.

28. Automatic Brake Adjusters Non-Servo Automatic Adjusters The starwheel automatic adjusters used on non-servo brakes may be mounted on either the leading or trailing shoe. These types of adjusters work whenever the brakes are applied—in either the forward or reverse direction. When the brakes are not applied, the adjuster pawl is held in position by the parking brake strut. When the brakes are applied and the primary shoe moves out toward the brake drum (away from the parking brake strut), the pawl spring pivots the pawl downward where it mounts on the brake shoe and rotates the starwheel to adjust the brake.

29. Automatic Brake Adjusters Non-Servo Automatic Adjusters The trailing-shoe non-servo starwheel adjuster works somewhat like the leading-shoe design, but it makes the adjustment as the brakes are released rather than applied. The upper shoe return spring in this design returns the brake shoes and operates the automatic adjuster, as follows: When the brakes are not applied, spring tension holds the trailing shoe and the adjuster pawl tightly against the parking brake strut. When the brakes are applied, the trailing shoe moves out toward the drum and away from the parking brake strut. If the brakes have worn far enough, the arm will engage the next tooth of the starwheel . When the brakes are released, the return spring pulls the brake shoes back together and the parking brake strut levers the adjuster pawl downward to rotate the starwheel and adjust the brakes.

30. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Most ratchet automatic adjusters use movement of the brake shoes to adjust the lining-to-drum clearance. The adjustment of a ratchet adjuster is carried out by two parts that have small interlocking teeth. As the adjustment is made, the two toothed elements ratchet across one another. Once adjustment is complete, the teeth lock together to hold the brake shoes in their new positions. LEVER-LATCH RATCHET AUTOMATIC ADJUSTER. The lever-latch automatic adjuster installs on the leading shoe of a non-servo brake and operates whenever the brakes are applied.

31. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters STRUT-QUADRANT RATCHET AUTOMATIC ADJUSTER. The strut-quadrant automatic adjuster is used on some non-servo brakes. The strut-quadrant adjuster consists of three basic parts: The parking brake strut Adjusting quadrant A quadrant spring The strut has a toothed post solidly mounted on its underside. The adjuster quadrant pivots on a pin that slips into a notch in the end of the strut, and the backside of the quadrant has a toothed, cam-shaped surface that interlocks with the toothed post on the strut.

32. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters STEP 1 When the brakes are applied, the leading shoe moves out toward the brake drum. STEP 2 If there is sufficient wear of the brake lining, the edge of the slot in the shoe web contacts the inner side of the adjuster quadrant arm and pulls it outward. STEP 3 When this happens, the toothed section of the quadrant is lifted away from the post on the parking brake strut. STEP 4 The quadrant spring then rotates the quadrant until its pivot pin is bottomed in the slot in the parking brake strut. STEP 5 When the brakes are released, the quadrant returns inward with the leading shoe. STEP 6 The toothed section of the quadrant then engages the teeth on the strut post, causing the quadrant arm to remain in its new extended position that holds the shoes farther apart and reduces the lining-to-drum clearance.

33. Summary Drum brakes were the first type of brakes used on motor vehicles and are still used on the rear of most vehicles. The primary advantage of drum brakes is that they can apply more stopping power for a given amount of force applied to the brake pedal than can disc brakes. The greatest drawback of drum brakes is that they are susceptible to fade. The foundation of every drum brake is the backing plate that mounts to the steering knuckle on the front brakes, or to the suspension or axle housing on the rear brakes. Hydraulic pressure is transferred from the master cylinder to each wheel cylinder through brake fluid. The linings of drum brakes are attached to curved metal assemblies called brake shoes. Most shoes are made of two pieces of sheet steel welded together in a T-shaped cross section. A non-servo brake feature is that each brake shoe is applied individually. The action of one shoe has no effect on the action of the other. Many non-servo drum brakes use self-energizing action to improve their braking performance.

34. Summary Cont …. The dual-servo brake is the most common drum brake design. The name “servo” comes from the fact that one shoe “serves” the other to increase application force. Servo brakes use three styles of starwheel adjusters: Cable Lever Link All three adjusters mount on the secondary brake shoe and adjust only when the brakes are applied while the vehicle is moving in reverse.