Brake Systems. Introduction

Transcription

1 Brake Systems Figure 1. A Typical Brake System Introduction The brake system (Figure 1) is designed to slow and halt the motion of a vehicle. To do that, various components within a hydraulic brake system must convert the momentum of the vehicle into heat. They do so by using friction. Friction is the resistance to movement exerted by two objects in contact with each other. Friction always converts moving, or kinetic, energy into heat. The greater the friction between two moving surfaces, the greater the amount of heat produced. As the brakes on a moving automobile are applied, rough-textured pads or shoes are pressed against rotating parts of the vehicle either rotors (discs) or drums. The kinetic energy, or momentum, of the vehicle is then converted into heat energy by the friction of rubbing surfaces and the car or truck slows down. NAPA FastTrack Counter Sales Training Brake Systems Page 1

2 Braking Power The four factors that determine a vehicle s braking power are: 1) pressure, which is provided by the hydraulic system; 2) coefficient of friction, which represents the frictional relationship between pads and rotors or shoes and drums, and is engineered to ensure optimum performance; 3) frictional contact surface, meaning that bigger brakes stop a car more quickly than smaller brakes; and 4) heat dissipation, which is necessary to prevent brake fade. Principles of Hydaulic Brake Systems The hydraulic system (Figure 2) uses brake fluid to transfer pressure from the brake pedal to the pads or shoes. At the wheels, sets of friction pads are forced against rotors or drums to slow their turning and bring the car to a stop. Mechanical force (the driver stepping on the brake pedal) is changed into hydraulic pressure, which is changed back into mechanical force (brake shoes and disc pads contacting the drums and rotors). Figure 2. A Typical Hydraulic Brake System Brake fluid is the lifeblood of any hydraulic brake system. It is what makes the system operate properly. Every can of brake fluid carries the identification letters of SAE and DOT. These letters (and corresponding numbers) indicate the nature, blend, and NAPA FastTrack Counter Sales Training Brake Systems Page 2

3 performance characteristics of each particular brand of brake fluid. The Department of Transportation (DOT) has set requirements for the three types of brake fluid used in this country. DOT 3 and DOT 4 are glycol-based brake fluids, while DOT 5 is silicone-based. DOT 3 is most often recommended for newer vehicles. DOT 3 and DOT 4 brake fluids are amber colored, whereas DOT 5 is purple in color. DOT 5 fluid is not recommended for vehicles with anti lock brake systems (ABS). The brake pedal is where the brake s hydraulic system gets its start. When the brake pedal is depressed, force is applied to the master cylinder. The brake master cylinder (Figure 3) changes the mechanical force of the driver s foot on the brake pedal to hydraulic pressure. The master cylinder has a bore that contains a piston assembly and return spring. Seals on the piston prevent fluid leakage. A cup seal on the forward end keeps the brake fluid ahead of the piston when it is put under pressure. Although master cylinders differ in number of pistons, reservoir design, and integrated hydraulic components, the operation of all master cylinders is basically the same. Figure 3. An Exploded View of a Master Cylinder NAPA FastTrack Counter Sales Training Brake Systems Page 3

4 When a master cylinder is faulty, poor pedal feel or total brake failure can result. Master cylinders are replaced or rebuilt. To save time and frustration, most technicians will install a rebuilt or new master cylinder. All modern vehicles have dual brake systems. These systems are designed to allow two of the four brakes to operate when there is a hydraulic failure in one of the circuits. These systems typically use a dual master cylinder, which has a piston for each pair of brakes. The brake circuits normally are divided into front and rear or are diagonally split, that is one front brake coupled with one rear brake on the opposite side of the vehicle. Hydraulic Tubes and Hoses Steel tubing and flexible synthetic rubber hosing are used to connect the various parts of the hydraulic system. Most brake line tubing consists of double-wall steel tubing in diameters ranging from 1/8 to 3/8 inch. Because of the high pressure in the lines, special tubing, fittings, and hoses must be used. Assorted fittings are used to connect steel tubing to junction blocks or other tubing sections. The most common fitting is the double or inverted flare style. Double flaring is important to maintain the strength and safety of the system. Fittings are constructed of steel or brass. Brake hoses offer flexible connections to wheel units so that steering and suspension members can operate without damaging the brake system. Typical brake hoses range from 10 to 30 inches in length and are constructed of multiple layers of fabric impregnated with a synthetic rubber. Brake hose material must offer high heat and highpressure resistance and withstand harsh operating conditions. Replacement brake hoses must be of the correct length. If they are not, they may rub against suspension parts and quickly develop a leak. Hydraulic System Safety Switches and Valves Switches and valves are installed in the brake system hydraulic lines to act as warning devices or pressure-control devices. A pressure differential valve is used in all dual brake systems to operate a warning light switch. Its main purpose is to tell the driver if pressure is lost in either of the two hydraulic systems. The stop light switch and mounting bracket assembly is attached to the brake pedal bracket and is activated by pressing the brake pedal. There are two types of stop light switches: a mechanically operated and a pressure-activated. The mechanical stop light switch is operated by contact with the brake pedal or with a bracket attached to the pedal. The hydraulic switch is operated by hydraulic pressure developed in the master cylinder. The pressure on the switch causes the brake lights to come on. Metering and proportioning valves are used to balance the braking characteristics of disc and drum brakes. A metering valve in the front brake line holds off pressure going from the master cylinder to the front disc calipers. This delay allows pressure to build up in the rear drum brakes first. The valve provides for better balance of the front and rear brakes. NAPA FastTrack Counter Sales Training Brake Systems Page 4

5 It also aids directional stability by keeping pressure from the front brakes until the rear brakes have started to operate. The proportioning valve is used to control rear brake pressures, particularly during hard stops. It regulates rear brake system pressure and adjusts for the difference in pressure between front and rear brake systems. This keeps front and rear braking forces in balance. Most newer cars have a combination valve in their hydraulic system. This valve is simply a single unit that combines the metering and proportioning valves with the pressure differential valve and switch. Drum Brake Assemblies A drum brake assembly (Figure 4) consists of a cast-iron drum, which is bolted to and rotates with the vehicle s wheel, and a fixed backing plate to which are attached the shoes and other components-wheel cylinders, automatic adjusters, and linkages. There might also be some extra hardware for parking brakes. The shoes are surfaced with frictional linings, which contact the inside of the drum when the brakes are applied. The shoes are forced outward by pistons located inside the wheel cylinder. They are actuated by hydraulic pressure. As the drum rubs against the shoes, the energy of the moving drum is transformed into heat. This heat energy is passed into the atmosphere. When the brake pedal is released, hydraulic pressure drops, and the brake shoes are pulled back to their unapplied position by return springs. Front of vehicle Figure 4. An Exploded View of a Rear Drum Brake NAPA FastTrack Counter Sales Training Brake Systems Page 5

6 To activate or apply drum brakes, hydraulic pressure from the master cylinder is delivered to the brake assembly. Here the pressure is applied to a wheel cylinder which increases the hydraulic pressure and exerts a pressure onto the brake shoes. A wheel cylinder is typically composed of two pistons, one at each side. When hydraulic pressure is delivered to the middle of the unit, the pressure pushes the pistons out. This movement of the pistons is what applies the pressure on the brake shoes. Because the shoes quickly come in contact with the drum, they move very little. The pistons in the wheel cylinder also move very little. This allows pressure to build up in the wheel cylinder. Increased pressure pushes the shoes against the drum with a greater force. If the pressure on the pistons is able to leak off, low braking pressure will result. When the brake pedal is released, pressure from the master cylinder is relieved. Springs on the shoes pull the shoes away from the drum. At the same time, the inward movement of the shoes pushes the pistons back into the wheel cylinder. Although wheel cylinders can be rebuilt, it is highly recommended that customers always replace the wheel cylinders. If the original wheel cylinders have leaked, the brake shoes should be replaced no matter how much lining material remains on them. The leaking brake fluid soaks into the lining and reduces the effectiveness of the brake shoes. Brake shoes are held against the anchor pin by the return springs (Figure 5). The shoes are held to the backing plate by hold-down springs (Figure 6) or spring clips. There are many different sizes and designs of brake springs. It is important that the correct springs are installed in their proper location. Sometimes they are color-coded to help identify their location. Opposite the anchor pin, a star wheel adjuster (Figure 7) links the shoe webs and provides a threaded adjustment that permits the shoes to be expanded or contracted. The shoes are held against the star wheel by a spring. Figure 5. An Assortment of Return Springs NAPA FastTrack Counter Sales Training Brake Systems Page 6

7 Figure 6. A Typical Hold-down Spring Kit Figure 7. An Assortment of Star Wheel Adjusters NAPA FastTrack Counter Sales Training Brake Systems Page 7

8 Since the early 1960s, automatic drum brake adjusters have been used on nearly all vehicles. There are several types of automatic adjusters used. Adjusters, whether cable, crank, or lever, are installed on one shoe and operated whenever the shoe moves away from its anchor. Automotive brake drums are made of heavy cast iron (some are aluminum with an iron or steel sleeve or liner). They have a machined surface inside, against which the linings on the brake shoes generate friction when the brakes are applied. This results in the creation of a great deal of heat. Remind your customers that the drums should be inspected and measured whenever new brake shoes are installed. Brake drums can be refinished either by turning or grinding on a brake lathe. When the drum on one side of the car is machined, the drum on the other side must also be machined to the same diameter so that braking is equal. Drums should be measured with a drum micrometer to make sure they are within the safe oversize limits. Brake drums are stamped with a discard dimension. This is the allowable wear dimension and is not the minimum diameter allowed. There must be inch left for wear after turning the drums. If the drum is within safe limits, even though the surface appears smooth, turning should be considered to assure a true drum surface and remove any possible contamination in the surface from previous brake linings, road dust, etc. If deep scores or grooves cannot be removed or if the diameter exceeds the limits, the drum must be replaced. The brake shoe lining provides friction against the drum to stop the car. It contains heatresistant fibers. The lining is molded with a high-temperature synthetic bonding agent. Different lining materials (each with a different friction value) are used, depending on the normal operation of the vehicle. The use of a lining with a friction value that is too high can result in a severe grabbing condition. A friction value that is too low can make stopping difficult because of a hard pedal. The two common ways of attaching the lining to the shoe are riveting and bonding. When selecting brake shoes for a customer, make sure it is the correct one for the vehicle. Brake shoes come in different sizes for different applications. However, there are also differences in web thickness, shape of web cutouts, and positions of any reinforcements. Disc Brake Assemblies Disc brakes (Figure 8) resemble the brakes on a bicycle: the friction elements are in the form of pads, which are squeezed or clamped on both sides of a rotating disc. With automotive disc brakes, this wheel is a separate unit, called a rotor. The rotor is made of cast iron. Since the pads clamp against both sides of it, both sides are machined smooth. Usually the two surfaces are separated by a finned center section for better cooling (such rotors are called ventilated rotors). The pads are attached to metal shoes, which are actuated by one or more pistons, the same as with drum brakes. NAPA FastTrack Counter Sales Training Brake Systems Page 8

9 Figure 8. A Disc Brake Assembly The piston is contained within a caliper assembly, a housing that wraps around the edge of the rotor. The caliper (Figure 9) is a housing containing the piston and related seals, and boot as well as the cylinder and fluid passages necessary to force the friction linings or pads against the rotor. Figure 9. An Exploded View of a Brake Caliper with Brake Pads NAPA FastTrack Counter Sales Training Brake Systems Page 9

10 The caliper housing is usually a one-piece construction of cast iron or aluminum. A cylinder bore is located in the caliper. In the cylinder bore is a groove that seats a squarecut seal. This groove is tapered toward the bottom of the bore to force the lower edge tighter against the piston. The top of the cylinder bore is also grooved as a seat for the dust boot. A fluid inlet hole is machined into the bottom of the cylinder bore and a bleeder valve outlet hole is located near the top of the casting. The top of the pistons is grooved to accept the dust boot. The dust boot seats in a groove at the top of the cylinder bore and also in a groove in the piston. The dust boot prevents moisture and road contamination from entering the bore. The piston hydraulic seal prevents fluid leakage between the cylinder bore wall and the piston. This rubber ring also acts as a retracting mechanism for the piston when hydraulic pressure is released, causing the piston to return in its bore. The seal distorts to allow the piston to be pushed out to bring the pad into contact with the rotor. When the brake pedal is released, the seal works like a return spring to pull the piston back into its bore. Disc brake pads are metal plates with the linings either riveted or bonded to them. Pads are placed one in each side of the caliper and together straddle the rotor. The inner brake pad, which is positioned against the piston, is sometimes not interchangeable with the outer brake pad. The linings are made of semimetallic or other nonasbestos material, as are brake shoes (Figure 10). Figure 10. Brake Pads You will sell a lot of brake pads. Each time you do, make sure to ask the customer about the condition of the calipers mounting hardware and other parts of the disc brake assembly. This includes the condition of the rotor. The surfaces on both sides of the rotor should be smooth but not glazed. It should also be checked for cracks and other damage. The rotor s thickness should be measured and compared to specifications for minimum thickness. Rotors should also be measured for uneven wear, runout, flatness, and depth of scoring. Often problems can be corrected by resurfacing the rotor, however if the rotor becomes too thin after machining, it should be replaced. Brake calipers are either rebuilt by the customer or replaced with a new or remanufactured caliper assembly. Whenever brake pads are replaced, the caliper and related hardware should be inspected (Figure 11). NAPA FastTrack Counter Sales Training Brake Systems Page 10

11 Figure 11. Examples of Brake Hardware for Disc Brakes Power Brakes Power brakes are nothing more than a standard hydraulic brake system with a booster unit located between the brake pedal and the master cylinder to help activate the brakes. Two basic types of power-assist mechanisms are used. The first, is vacuum assist (Figure 12). These systems normally use engine vacuum to help apply the brakes. The second type of power assist is hydraulic assist (Figure 13). It is normally found on larger vehicles. This system uses hydraulic pressure developed by the power-steering pump or other external pump to help apply the brakes. NAPA FastTrack Counter Sales Training Brake Systems Page 11

12 Figure 12. Vacuum-assist Power Brake Unit Figure 13. Hydraulic-assist Power Brake System NAPA FastTrack Counter Sales Training Brake Systems Page 12

13 Both vacuum and hydraulic assist act to multiply the force exerted on the master cylinder pistons by the driver. This increases the hydraulic pressure delivered to the wheel cylinders or calipers while decreasing driver foot pressure. Parking Brakes Parking brakes are designed to hold the vehicle when the vehicle is parked. These brakes are applied by a brake lever or a separate pedal assembly. Applying the parking brakes mechanically moves the rear brake shoes or pads against the drum or rotor. By moving the parking brake lever or pedal, a parking brake cable pulls on the parking brake assembly to provide the braking action. Parking brakes are an integral part of the regular brake system. The most common part of the parking brake system that is replaced or serviced is the brake cable. Sometimes, however, it is necessary to replace some of the hardware for the parking brake that is located in the brake assembly. On vehicles with 4- wheel disc brakes, the parking brake mechanically operates the rear calipers. Anti-Lock Brakes Many late-model cars and trucks have anti-lock brake systems (ABS) (Figure 14). These systems are designed to bring the vehicle to a controlled stop during hard braking. ABS does this by electronically watching the speed of the wheels. When one wheel is decelerating faster than the others, the brakes to that wheel are quickly held, released, then applied again very rapidly. This action creates a pulsing feel at the brake pedal. Since slower speeds mean that the wheel is skidding or locking; holding, then releasing the brakes at that wheel, prevents further locking. NAPA FastTrack Counter Sales Training Brake Systems Page 13

14 Figure 14. Typical (ABS) Anti-lock Brake System There are many different anti-lock brake systems. Most of them have a hydraulic pump and wheel speed sensors. The pump is what quickly releases and reapplies the brakes. The wheel sensors measure the speed of each wheel. In a controllable stop, all wheels will be at the same speed. Most anti-lock brake systems have warning lights to alert the driver that the system has a fault. Because these systems are quite complex and operate under high pressures, only experienced individuals should be servicing them. If you sense that the customer is not qualified to work on ABS, gently caution him/her about the dangers of the system and refer him/her to a technician. Normal brake work (pad replacement, etc) on ABS systems are the same as non-abs equipped vehicles. Key Terms ABS Anti-lock Brake System. Caliper a housing containing one or more pistons and related seals, and boots as well as the cylinders and fluid passages necessary to force the friction linings or pads against the rotor. Combination valve a single unit that combines the metering and proportioning valves with the pressure differential valve. Disc brakes the type of brake system that works to clamp brake pads against the sides of a rotating disc to stop the vehicle. DOT Department of Transportation; the certifying agency for many products including brake fluid. NAPA FastTrack Counter Sales Training Brake Systems Page 14

15 Drum brakes an assembly which relies on brakes shoes being forced into a rotating drum to stop the vehicle. Friction the resistance to movement exerted by two objects in contact with each other. Hold-down springs the springs that hold the brake shoes in place on the backing plate. Hydraulic assist power brakes normally found on larger vehicles, this system uses hydraulic pressure developed by the power steering pump or other external pump to help apply the brakes. Master cylinder the device that reacts to the movement of the brake pedal to send pressure to the brakes at the wheels. Metering valve located in the front brake line to hold pressure going from the master cylinder to the front disc calipers. This delay allows pressure to build up in the rear drums first, to provide for better balance of the front and rear brakes. Parking brakes a mechanical brake system designed to hold the vehicle when it is parked. Proportioning valve used to control rear brake pressures, particularly during hard stops. It regulates rear brake system pressure and adjusts for the difference in pressure between front and rear brake systems. Return springs the set of springs that holds the brake shoes tightly against the anchor pin. Rotor made of cast iron, this wheel is the clamping surface for the brake pads. SAE Society of Automotive Engineers. Star wheel adjuster the device that is used to manually or automatically adjust the clearance between the brake shoes and the brake drum. Vacuum assist power brakes use engine vacuum to help apply the brakes. Ventilated rotors a brake rotor with its two surfaces separated by a finned center section for better cooling. Wheel cylinder the device in a drum brake which applies force to move the brake shoes against the brake drum. The wheel cylinder is controlled by hydraulic pressure. NAPA FastTrack Counter Sales Training Brake Systems Page 15

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