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 .
The support pads are slightly coated with special high-temperature silicone brake grease to minimize wear, prevent rust, and eliminate squeaking that can occur when the shoes move slightly on the pads during a stop.
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.
FIGURE 10–7 Cross-section of a wheel cylinder that shows all of its internal parts. The brake line attaches to the fluid inlet. The cup extender prevents the cup seal lip from collapsing when the brakes are released.
DRUM BRAKE PARTS WHEEL CYLINDERS FIGURE 10–8 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.
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.
FIGURE 10–9 Steelbrake shoes are made from two stampings welded together—the web and the lining table.
DRUM BRAKE SHOES FIGURE 10–11 Typical drum brake shoe and the names of the parts. (Courtesy of Allied Signal Automotive Aftermarket) FIGURE 10–10 Tapered ends on the linings help to reduce brake noise.
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 (forward facing shoe) is self-energized by drum rotation to create a servo action that forces the secondary shoe more firmly against the drum.
Because of this, the two shoes have physical differences and cannot be interchanged.
DRUM BRAKE SHOES PRIMARY AND SECONDARY BRAKE SHOES FIGURE 10–13 Primary shoe lining may vary depending on the application. FIGURE 10–12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing).
Purchase Quality Brake Linings for Best Performance
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.
Servo action enables a drum brake to provide increased stopping power, but it can also cause the brakes to grab and lock if they get too far out of adjustment. As clearance between the shoes and drum increases, the primary brake shoe is allowed a greater range of movement. The farther the shoe moves, the more speed it picks up from the rotating brake drum. At the moment the slack is taken up between the brake shoes, adjusting link, and anchor, the speed of the primary shoe is converted into application force by servo action. If the primary shoe is moving too quickly, it will apply the secondary shoe very hard and fast, causing the brakes to grab and possibly lock the wheels.
Servo brakes use three styles of starwheel adjusters:
AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–29 A cable-actuated starwheel adjuster. This type of adjuster makes the adjustment as the brakes are released.
AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–30 A lever-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment when the brakes are applied.
AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–31 A link-actuated starwheel adjuster. This type of adjuster makes the adjustment when the brakes are released.
AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–32 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel.
AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–33 A cable-actuated starwheel adjuster with an over-travel spring.
The starwheel automatic adjusters used on nonservo 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.
FIGURE 10–34 A non-servo brake with a lever-actuated starwheel automatic adjuster on a leading shoe. This type of adjuster makes an adjustment as the brakes are applied.
AUTOMATIC BRAKE ADJUSTERS NON-SERVO STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–35 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.
Self-adjusters can over-adjust the rear drum brakes if the brake drums are hot and have increased in diameter due to the heat. For example, if a pickup truck towing a boat had to brake while backing down a long, steep grade to the boat ramp, the rear brake drums could become larger in diameter due to the heat created during braking. The brakes could overadjust if the driver repeatedly depresses and releases the brake pedal while backing the trailer down the boat ramp. Then, after the boat has been removed from the trailer and the rear brakes have cooled, the drums will shrink and keep the rear brakes from releasing, preventing the truck from moving up the ramp.
Drum brake shoes include the lining table and shoe web plus holes for the springs to attach and semicircular anchor eyes.
Brake linings can be attached using rivets or bonding.
Lining edge codes identify the manufacturer and include two letters at the end, which identify the coefficient of friction of the material. The first letter indicates the coefficient when the lining is cold and the second indicates the coefficient when the lining is hot.
Brake shoes are forced outward against a brake drum by hydraulic action working on the brake shoes by the piston of a wheel cylinder.
The curved arch of the brake shoe causes a wedging action between the brake shoe and the rotating drum. This wedging action increases the amount of force applied to the drum.
Dual-servo brakes use primary and secondary brake shoes that are connected at one end. The wedge action on the front (primary) shoe forces the secondary shoe into the drum with even greater force. This action is called servo self-energizing.
Leading-trailing brakes use two brake shoes that are not connected. Leading-trailing brakes operate on a more linear basis and are therefore more suited than dual-servo for ABS.
Most self-adjusting mechanisms usually operate from the secondary or rearward facing brake shoe and adjust the brakes as the brakes are released.
Some self-adjusters operate on the primary shoe and adjust the brakes as they are being applied.
1. Two technicians are discussing drum brake shoes. Technician A says that forward and rearward facing shoes are the same and can be installed in either position on any drum brake system. Technician B says that the darker color lining should always be placed toward the front of the vehicle. Which technician is correct?
2. Two technicians are discussing brake lining edge codes. Technician A says that the code can identify the manufacturer. Technician B says that all friction material from the same manufacturer will have the same edge codes. Which technician is correct?
3. Technician A says that starwheel adjusters use different threads (left- and right-handed) for the left and right sides of the vehicle. Technician B says that a pawl controls the teeth of the starwheel adjuster. Which technician is correct?
4. Technician A says that drum brakes can fail to slow the vehicle if driven through deep water. Technician B says that when drum brakes get hot, the brake pedal will drop because the drum expands away from the shoes. Which technician is correct?
5. Technician A says that self-adjusters used on most drum brakes work when the brakes are applied, then release while traveling in reverse. Technician B says that some adjusters can overadjust if the brake drums are hot. Which technician is correct?
9. Two technicians are discussing drum brake self-adjusters. Technician A says that a frozen starwheel adjuster can cause the brakes to lock up due to the adjusting lever being unable to move the adjuster causing the linkage to bind. Technician B says that some brakes self-adjust when the brakes are applied rather than when released. Which technician is correct?