13 Disc Brakes - Automotive Engineering .pptx

Automotive Engineering

1. Automotive Chassis Systems Eighth Edition Chapter 13 Disc Brakes

2. Objectives Describe the parts and operation of disc brakes. Describe the construction of disc brake pads. Discuss the brake pad assembly methods and brake lining composition. Describe the difference between fixed caliper and floating or sliding caliper. Discuss brake rotors, disc brake designs, and rear disc brakes.

3. Disc Brakes Parts and Operation Disc brakes use a piston(s) to squeeze friction material (pads) on both sides of a rotating disc (rotor). Disc may be spelled disk by some manufacturers, but disc is the SAE (Society of Automotive Engineers) term and the most commonly used spelling in the industry. The rotor is attached to and stops the wheel. Disc brakes are used on the front wheels of late-model vehicles, and on the rear wheels of an increasing number of automobiles. Disc brakes were adopted primarily because they can supply greater stopping power than drum brakes with less likelihood of fade. This makes disc brakes especially wellsuited for use as front brakes, which must provide 60% to 80% of the vehicle’s total stopping power.

4. Disc Brakes Disc Brake Advantages Although increased federal brake performance standards hastened the switch to disc brakes, the front drum brakes would eventually have been eliminated anyway because disc brakes are superior in almost every respect. The main advantages of the disc brake include the following: FADE RESISTANCE. The main design feature that helps disc brakes avoid heat-induced fade is their cooling ability because all of the major parts of a disc brake are exposed to the air flowing over the friction assembly. FIGURE 13–1 A typical single-piston, floating-caliper-type disc brake assembly.

5. Disc Brakes Disc Brake Advantages Disc brakes are resistant to all kinds of fade, including the following: Mechanical fade is not a problem with disc brakes because, unlike a brake drum, the disc brake rotor expands toward the brake linings as it heats up rather than away from them. The primary symptom of lining fade is a hard brake pedal that requires the driver to apply greater force to maintain stopping power.

6. Disc Brakes Disc Brake Advantages Increased pressure will, however, create even more heat, and if brake lining temperatures continue to increase, gas fade and vapor lock of the hydraulic system can occur. Gas fade is a problem only under severe braking conditions when hot gases and dust particles from the linings are trapped between the brake linings and rotor, where they act as lubricants. The pedal becomes hard and increased force is required to maintain stopping power. Even though disc brakes operate at higher temperatures than drum brakes, they have fewer problems with gas fade for a number of reasons. Disc brakes do not have a drum to contain gases and particles in the area around the brake linings. The constant flow of air over the brake carries away contaminants that might otherwise build up. The surface area of the brake lining material in a disc brake is smaller than that of a comparable drum brake and this allows gases and particles to escape more easily. To help prevent gas fade, many brake pads have slots cut in the lining material.

7. Disc Brakes Disc Brake Advantages Water fade is not a big problem with disc brakes because centrifugal force created by the spinning rotor throws off most moisture, and the brake pads positioned only a few thousandths of an inch away from the rotor continuously wipe it clean. Although far more resistant to water fade than drum brakes, disc brakes are not entirely free from its effects. SELF-ADJUSTING ABILITY. Disc brakes are self-adjusting because any wear of the linings is automatically compensated for by the action of the brake caliper. When the brakes are applied, the caliper pistons move out as far as needed to force the brake pads into contact with the rotor. When the brakes are released, the piston retracts only the small distance dictated by rotor runout and piston seal flex. FREEDOM FROM PULL. A disc brake will stop straighter under a wider range of conditions than will a drum brake. A disc brake is self-cleaning, will throw off most water, and is less likely to pull. Disc brakes do not have self-energizing or servo action. Because disc brakes do not use friction between the linings and rotor to increase their braking power, the effects of a loss of friction on one side of the vehicle are far less pronounced than with drum brakes.

8. Disc Brakes Disc Brake Disadvantages The most notable fact about the disadvantages of disc brakes is that there are so few. The weaknesses of disc brakes include the following: No self-energizing or servo action—The disc brake’s lack of self-energizing or servo action is a disadvantage for two reasons. It contributes to poor parking brake performance and requires the driver to push harder on the brake pedal for a given stop It is easier to modulate the brakes for the exact amount of stopping power desired. Brake noise—Probably the biggest complaint about disc brakes is that they sometimes make various squeaks and squeals during a brake application.

9. Disc Brakes Disc Brake Disadvantages Some calipers use special shims between the brake pad backing plate and the caliper piston to damp vibrations. BRAKE DUST. Because the lining is exposed on a disc brake, rather than being enclosed as on a drum brake, some brake dust can accumulate on the wheels. This brake dust is often dark brown or black and can stain wheels if not cleaned often or protected from the dust. POOR PARKING BRAKE PERFORMANCE. The lack of self-energizing or servo action plays a large part in poor disc brake parking brake performance. The lining-to-rotor contact area of a disc brake is somewhat smaller than the lining-to-drum contact area of a drum brake.

10. Disc Brake Construction A disc brake is relatively simple compared with a drum brake. The major disc brake friction assembly components include the following: Caliper With the exception of the rotor, the caliper is the largest part of a disc brake friction assembly. The brake caliper uses hydraulic pressure to create the mechanical force required to move the brake pads into contact with the brake rotor. At the front axle, the caliper mounts to the spindle or steering knuckle

11. Disc Brake Construction Caliper Rear disc brake calipers mount to a support bracket on the axle flange or suspension. Splash Shield The splash shield bolts to the front spindle or steering knuckle, or in rear disc brake applications, to the axle flange or a suspension adapter plate. The job of the splash shield is to protect the inner side of the brake rotor from water and other contaminants, whereas the outer side of the rotor is protected by the wheel. Most splash shields are made of stamped steel or plastic.

12. Disc Brake Pads The lining of a disc brake is part of an assembly called the brake pad. Compared to a brake shoe, a brake pad is a relatively simple part that consists of a block of friction material attached to a stamped steel backing plate. Some pad backing plates have tabs that bend over the caliper to hold the pad tightly in place and help prevent brake noise. Other pad backing plates have tabs with holes in them. A pin slips through the holes and fastens to the caliper body to hold the pads in position. Still other pad backing plates have a retainer spring attached that locates the pad in the caliper by locking it to the caliper piston. As with brake shoes, the lining material of a disc pad can be any one of a number of products that can be fastened to the backing plate in several ways. The edges of the lining material on a brake pad are usually perpendicular to the rotor surface, although a few larger pads do have tapered edges to help combat vibration and noise

13. Disc Brake Pads Pad Wear Indicators Although not required by law, a growing number of vehicle manufacturers are fitting pad wear indicators to their brakes for safety reasons. Pad wear indicators are either mechanical or electrical and signal the driver when the lining material has worn to the point where pad replacement is necessary. A mechanical wear indicator is a small spring-steel tab riveted to the pad backing plate. FIGURE 13–15 Typical pad wear sensor operation. It is very important that the disc brake pads are installed on the correct side of the vehicle to be assured that the wear sensor will make a noise when the pads are worn. If the pads with a sensor are installed on the opposite side of the vehicle, the sensor tab is turned so that the rotor touches it going the opposite direction. Usually the correct direction is where the rotor contacts the sensor before contacting the pads when the wheels are being rotated in the forward direction.

14. Disc Brake Pads Pad Wear Indicators Electrical wear indicators use a coated electrode embedded in the lining material to generate the warning signal. The electrode is wired to a warning light in the instrument panel and when the lining wears sufficiently, the electrode grounds against the rotor to complete the circuit and turn on the warning light. On newer vehicles, the pad wear sensor is a two wire input to the electronic brake control module (EBCM). The module can detect faults in the sensor wiring.

15. Disc Brake Pads Pad Assembly Methods As mentioned previously, there are several methods that are used to mount brake linings, including the following: Riveted linings take advantage of the oldest method of lining attachment still in use. In this system, the brake block is attached to the backing plate with copper or aluminum rivets. The major advantage of riveting is that it allows a small amount of flex between the brake block backing plate. However, the rivet holes create stress points in the lining where cracks are likely to develop. Bonded linings use high-temperature adhesive to glue the brake block directly to the shoe pad backing plate. Bonding offers several advantages. Without rivets, bonded linings can wear closer to the backing plate and provide a longer service life. Mold-bonded linings are found on some disc brake pads. Mold bonding is a manufacturing process that combines the advantages of bonding with some of the mechanical strength of riveting.

16. Disc Brake Pads Pad Assembly Methods To make a mold-bonded pad, one or more holes are punched in the pad backing plate, and a high-temperature adhesive is applied to it. The backing plate is then installed in a molding machine where uncured friction material is formed onto the plate and forced into the holes. Once the pad is cured under heat and pressure, the bonding adhesive combines with the portions of the lining that extend into the backing plate holes to solidly lock the brake block in place.

17. Brake Rotors The brake rotor provides the friction surfaces for the brake pads to rub against. The rotor, the largest and heaviest part of a disc brake, is usually made of cast iron because that metal has excellent friction and wear properties. There are two basic types of rotors: Solid —Solid rotors are most often used on the rear of vehicles equipped with four-wheel disc brakes. Vented —Vented rotors have radial cooling passages cast between the friction surfaces.

18. Disc Brake Designs There are three types of calipers: Fixed Floating Sliding Fixed Caliper Design The fixed brake caliper is the earliest design. The fixed caliper has a body manufactured in two halves, and uses two, four, or six pistons to apply the brake pads. FIGURE 13–19 (a) Many fixed caliper disc brakes use a simple retaining pin to hold the disc brake pads. (b) Removing the retainer pin allows the brake pads to be removed. (c) Notice the crossover hydraulic passage that connects both sides of the caliper. Most fixed calipers use four (two on each side) or six pistons (three on each side) of the rotor.

19. Disc Brake Designs Fixed Caliper Design FIXED CALIPER ADVANTAGES. Fixed calipers are relatively large and heavy, which enables them to absorb and dissipate great amounts of heat. This allows the brake rotor and pads to run cooler, and reduce the amount of heat transferred to the brake fluid. The size and rigid mounting of a fixed caliper also means it does not flex as much as other designs. A caliper that is flexing is usually felt by the driver as a spongy brake pedal. The strength and heat-dissipating abilities of fixed calipers make them best suited for heavy-duty use such as in most race vehicles. FIXED CALIPER DISADVANTAGES. The size and weight of fixed calipers are advantages in heavy-duty use, but they add weight to the vehicle. Another disadvantage of fixed calipers is that with multiple pistons and split bodies, service is more difficult and allows greater opportunity for leaks.

20. Disc Brake Designs Floating and Sliding Caliper Design The front brakes of most vehicles are fitted with either floating or sliding calipers, which are not rigidly mounted. The caliper is free to move within a limited range on an anchor plate that is solidly mounted to the vehicle suspension. The anchor plate may be cast into a suspension member (often the front spindle) or it can be a separate piece that bolts to the suspension. When the brakes are applied, the caliper piston moves out of its bore and applies the inner brake pad. At the same time, the caliper body moves in the opposite direction on the anchor plate and applies the outer brake pad. With a floating or sliding caliper, the caliper body moves every time the brakes are applied.

21. Disc Break Designs Floating and Sliding Caliper Design NORMAL CALIPER OPERATION. The piston moves just enough to distort the caliper seal and returns to the original position when the brake pedal is released. WEAR COMPENSATION. The piston moves more than the caliper seal can distort. It moves through the seal until the pad contacts the rotor. The caliper piston returns to the released position by the seal distortion, the same as during normal operation, except now in a different, more applied position. As the wear occurs and the piston moves, additional brake fluid is needed behind the piston. Although the inboard piston location of floating and sliding calipers provides good cooling, these designs can never absorb as much heat (and therefore have the fade resistance) as a fixed caliper with similar stopping power.

22. Disc Break Designs Floating Caliper Operation The body of a floating caliper does not make direct metal-to-metal contact with the anchor plate. Instead, the caliper body is supported by bushings and/or O-rings that allow it to “float” or slide on metal guide pins or locating sleeves attached to the anchor plate.

23. Disc Brake Designs Floating Caliper Design The bushings that support floating calipers are made from a number of materials including rubber, Teflon, and nylon. The O-rings are generally made from high-temperature synthetic rubber. The guide pins and sleeves are made of steel and come in a variety of shapes and sizes for different caliper designs. Floating calipers depend on proper lubrication of their pins, sleeves, bushings, and O-rings for smooth operation.

24. Disc Brake Designs Sliding Calipers Unlike a floating caliper, the body of a sliding caliper mounts in direct metal-to-metal contact with the anchor plate. Instead of pins and bushings, sliding calipers move on ways cast and machined into the caliper body and anchor plate. Retaining clips and the design of the caliper prevent the body from coming out of the ways once the caliper is assembled. Like floating calipers, sliding calipers depend on good lubrication of their ways for proper operation. If not properly coated with high-temperature brake grease, the ways can rust or corrode, causing the caliper to drag or seize.

25. Rear Disc Brakes In recent years, four-wheel disc brake systems have become more common. In most rear-wheel applications, drum brakes are adequate to provide the relatively small portion of a vehicle’s total braking power required of them. Because rear drum brakes are lightly loaded, fade is a problem only in extreme conditions when the front brakes fade and force the rear brakes to take on a larger part of the braking load. The automatic adjusting ability of disc brakes is also less of an advantage in slow-wearing rear brakes.

26. Rear Disc Parking Brakes There are two methods of providing parking brakes when rear discs are installed on a vehicle. Adapt the disc brake to also function as the parking brake. This is done by installing a series of cables, levers, and internal parts to mechanically actuate the brake caliper.

27. Summary Disc brakes use a piston(s) to squeeze friction material (pads) on both sides of a rotating disc (rotor). Disc may be spelled disk by some manufacturers, but disc is the SAE (Society of Automotive Engineers) term and the most commonly used spelling in the industry. The rotor is attached to and stops the wheel. The brake caliper uses hydraulic pressure to create the mechanical force required to move the brake pads into contact with the brake rotor. At the front axle, the caliper mounts to the spindle or steering knuckle. A brake pad is a relatively simple part that consists of a block of friction material attached to a stamped steel backing plate. Some pad backing plates have tabs that bend over the caliper to hold the pad tightly in place and help prevent brake noise. The brake rotor provides the friction surfaces for the brake pads to rub against. The rotor, the largest and heaviest part of a disc brake, is usually made of cast iron because that metal has excellent friction and wear properties. There are three types of calipers: Fixed Floating Sliding

28. Summary Cont …. There are two methods of providing parking brakes when rear discs are installed on a vehicle. Adapt the disc brake to also function as the parking brake. This is done by installing a series of cables, levers, and internal parts to mechanically actuate the brake caliper. Use mechanically actuated drum brakes inside the rear rotors .