chap76 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 76, the reader should be able to: • • • • Prepare for ASE Brakes (A5) certification test content area “C” (Disc Brake Diagnosis and Repair) Describe how disc brakes function Name the parts of a typical disc brake system Describe the construction of disc brake pads Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 76, the reader should be able to: • • Describe the difference between fixed caliper and floating or sliding caliper Explain the difference between a standard caliper and a lowdrag caliper Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: anchor plate • antirattle clips • aramid fiber bonded linings • brake block • brake pad carbon fiber-reinforced carbon (CFRC) fixed brake caliper • floating caliper • gas fade integrally molded Kevlar lining fade • low-drag caliper Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: mechanical fade • moldbonded lining NAO • NAS • natural frequency • nonasbestos pad wear indicators • pin-slider caliper riveted linings semimets • sintered metal • sintering • sliding caliper • swept area water fade • ways Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 DISC BRAKES 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 term and commonly used spelling 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 fade Disc brakes are well suited for use as front brakes, which must provide 60% to 80% of the vehicle’s total stopping power Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 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 disc brake friction assembly has several significant strong points, and only a few relatively minor weak points See Figure 76-1 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 76–1 A typical disc brake assembly Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The main advantages of the disc brake include the following: Fade resistance Self-adjustment Freedom from pull Fade Resistance When a disc brake is compared with a drum brake of similar diameter, its biggest advantage is a much greater ability to resist fade Disc brakes are more resistant to all kinds of fade, including the following: Mechanical fade Lining fade Gas fade Water fade Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Disc brakes avoid heat-induced fade because all of the major parts of a disc brake are exposed to the air flowing over the friction assembly They also have greater swept area, the amount of brake drum or rotor friction surface that moves past the brake linings every time the drum or rotor completes a rotation A larger swept area allows the heat generated in braking to be transferred more rapidly into the rotor for better cooling A disc brake has swept area on both sides of the rotor A drum brake has swept area only on the inside of the drum Large diameter rotors, however, require larger diameter wheels to provide the necessary clearance This is why high-performance vehicles use 17-inch diameter and larger wheels Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Bushings that support floating calipers are made from a number of materials including rubber, Teflon, and nylon Figure 76–27 Floating calipers are supported by rubber O-rings or plastic bushings O-rings are generally made of 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 See Figure 76–28 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 76–28 Metal guide pins and sleeves are used to retain and locate floating calipers Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Floating calipers depend on proper lubrication of their pins, sleeves, bushings, and O-rings for smooth operation If these parts become rusted or corroded, the caliper will bind and stick, causing loss of braking power that is usually accompanied by rapid and unusual wear of brake pads Special high-temperature brake grease must be used to lubricate these parts any time the caliper is disassembled Many manufacturers recommend that floating caliper pins, sleeves, bushings, and O-rings be replaced whenever the caliper is serviced These parts come in a “small parts kit” available from brake part suppliers Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 What is a Low-Drag Caliper? - Part A low-drag caliper differs from a standard caliper in the area of the square-cut Oring A V-shaped cutout allows the O-ring to deflect more and, as a result, is able to pull the caliper piston back into the bore when the brakes are released Because of this further movement, the brake pads are pulled further from the rotor and are less likely to drag The negative aspect of this design is greater volume of brake fluid is needed to achieve a brake application To compensate for this need for greater brake fluid volume, a quick-takeup master cylinder was designed and is used whenever low-drag calipers are used Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 … Low-Drag Caliper? - Part Figure 76–29 In a standard disc brake caliper, the square-cut O-ring deforms when the brakes are applied and returns the piston to its original (released) position due to the elastic properties of the rubber seal In a low-drag caliper design, the groove for the square-cut O-ring is V-shaped, allowing for more retraction When the brake pedal is released, the piston is moved away from the rotor, resulting in less friction between the disc brake pads and the rotor when the brakes are released Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Sliding Calipers The body of a sliding caliper mounts in direct metal-tometal contact with the anchor plate Figure 76–30 Exploded view of a typical sliding brake caliper Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Instead of pins and bushings, sliding calipers move on ways cast and machined into the caliper body and anchor plate Figure 76–31 Sliding calipers move on machined ways Retaining clips and design of the caliper prevent the body from coming out of the ways once the caliper is assembled On some, the ways may have to be filed for proper clearance between caliper body and anchor plate if the caliper is replaced Sliding calipers depend on good lubrication for proper operation If not properly coated with high-temperature brake grease, the ways can corrode, causing the caliper to drag or seize Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 REAR DISC BRAKES In recent years, four-wheel disc brake systems have become more common In most 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 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 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 Figure 76–32 Exploded view of a typical rear disc brake with an integral parking brake The parking brake lever mechanically pushes the calliper piston against the rotor See Figures 76–33 and 76-34 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 76–33 This single-piston brake caliper is mechanically actuated to serve as a parking brake Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Use mechanically actuated drum brakes inside the rear rotors Figure 76–34 Drum parking brakes are fitted inside the rotors on this vehicle equipped with rear disc brakes Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Disc brakes are superior to drum brakes because they are fade resistant, self-adjusting, and are less likely to pull during braking Disc brakes, however, lack self-energization requiring greater force be applied to the brake pedal compared with the drum brakes Disc brakes are more prone to noise than drum brakes A typical disc brake assembly includes the caliper assembly, splash shield, brake pads, and brake rotor Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) The three basic types of disc brake calipers include fixed, floating, and sliding designs A low-drag caliper requires the use of a quick-take-up master cylinder Some disc brakes are equipped with integral parking brakes Brake pads can be attached using rivets, bonding, or integrally molded Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) Typical semimetallic brake linings contain phenolic resin, graphite or carbon particles, steel fibers, ceramic and metal powders plus other modifiers such as rubber scraps 10 Other types of friction material include nonasbestos organic (NAO), nonasbestos synthetic (NAS), and carbon fiberreinforced carbon (CFRC) 11 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 Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458