chap74 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 74, the reader should be able to: • • • • • Prepare for ASE Brakes (A5) certification test content area “B” (Drum Brake Diagnosis and Repair) Identify drum brake component parts Describe the operation of non-servo brakes Explain the operation of dual-servo brakes Discuss drum brake adjusters 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 eyes • automatic adjusters beehive holddown • bonded linings • brake fade • brake shoe holddown • brake shoe return springs • brake shoes core charge double-trailing brake • dual-servo brake • duo-servo gas fade labyrinth seal • leading shoe • leading-trailing brakes • ledges • lining edge codes • lining fade • lining table 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 • nibs • non-servo brakes • over-travel spring pawl • piston stops • platform • primary shoe riveted linings secondary shoe • self-energizing action • servo brake • shoe anchors • shoe contact areas • shoe rim • shoe support pads • shoe web trailing shoe water fade 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 DRUM BRAKE ADVANTAGES Drum brakes were the first type of brakes used on motor vehicles.  Today, over 100 years after the first “horseless carriages,” drum  brakes are still used on the rear of most vehicles Figure 74–1 Typical brake system components showing disc brakes on the front and drum brakes on the rear 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 74–2 An exploded view of a typical drum brake assembly The drum brake has been more  widely used than any other  automotive brake design.  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 drum brake has advantages that contribute to its widespread  use on the rear axle of most automobiles Self­Energizing and Servo Action  Drum brakes can apply more  stopping power for a given amount of force applied to the brake  pedal 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 Some drum brake designs use an effect called servo action that  enables one brake shoe to help apply the other for increased  stopping power.  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 Parking Brake Service  Drum brakes 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 Disc brakes, which do not benefit from self­energizing or servo  action, require a complex set of extra parts to provide enough  application force to work well as parking 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 DRUM BRAKE DISADVANTAGES Drum brake disadvantages fall into three areas: Brake fade Brake adjustment Brake pull Brake Fade  The greatest drawback of drum brakes is that they  are susceptible to fade. Brake fade is loss of stopping power  when excessive heat reduces the friction between shoe linings and the brake drum 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 Mechanical Fade  When the brake drum gets so hot it expands  away from the brake linings mechanical fade occurs. The shoes  move outward to maintain contact with the drum, causing the  brake pedal to drop toward the floor as additional brake fluid  moves into the hydraulic system Lining Fade  When the friction coefficient of the brake lining  material drops off sharply, lining fade occurs, because intense  heat makes it “slippery.” The pedal becomes hard and there is a  noticeable loss of braking power Gas Fade  Rare type gas fade occurs under extended hard braking  from high speeds. A thin layer of hot gasses and dust particles can  build up between the brake shoe linings and drum, acts as a  lubricant and reduces friction. The brake pedal is hard 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 74–35 A non-servo brake with a lever-activated starwheel automatic adjuster on the leading shoe This type of adjuster makes the adjustment as the brakes are applied 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 74–36 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 The trailing­shoe non­servo  starwheel adjuster shown here  works like the leading­shoe  design, but makes 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.  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 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. This allows the  adjuster pawl, which is restrained by the return spring, to pivot  where it attaches to the brake shoe, causing the adjuster arm to  move upward 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 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 Cool the Brakes Before Backing Self-adjusters can over adjust the rear drum brakes if the brake drums are hot and have increased in diameter due to the heat A typical example of how this can occur involves a pickup truck towing a boat down a long, steep grade to reach the lake and the boat ramp If the brakes are used to slow the truck and trailer, the rear brake drums can become larger in diameter due to the heat created during braking Then when the truck is backing the boat trailer down the ramp, the brakes can over adjust if the driver repeatedly depresses and releases the brake pedal while backing 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 movement of the truck up the ramp NOTE: Some drum brakes are equipped with a bimetallic heat sensor  that prevents the self­adjusters from working if the brakes are 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 Ratchet Automatic Adjusters  Most ratchet automatic adjusters  use movement of the brake shoes to adjust the lining­to­drum  clearance The adjustment is carried out by two parts that have small  interlocking teeth. As adjustment is made, the two toothed  elements ratchet across one another Once complete, the teeth lock together to hold the brake shoes in  their new positions 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 Lever­Latch Automatic Adjuster The lever­latch automatic  adjuster installs on the leading shoe of a non­servo brake and  operates whenever the brakes are applied.  This design consists of a large  lever and a smaller latch with  interlocking teeth.  A spring on the latch piece  keeps contact with the lever to maintain the adjustment.  One end of the parking brake  strut hooks into an opening in  the lever and the other end is  held against the trailing brake  shoe by a strong spring Figure 74–37 A lever-latch ratchet automatic adjuster Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 As brakes are applied and shoes move outward toward the drum,  the parking brake strut pulls on the adjuster lever and forces it to  pivot inward from where it attaches to the top of the leading shoe If brakes are sufficiently worn, the bottom of the lever will ratchet  one or more teeth on the latch. When the brakes are released, the  parking brake strut, which bottoms against the lever, will hold the  shoes farther apart to reduce the lining­to­drum clearance 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 Strut­Quadrant Automatic Adjuster  The strut­quadrant  automatic adjuster is used on some non­servo brakes.  The strut­quadrant adjuster  consists of three basic parts: The parking brake strut Adjusting quadrant A quadrant spring The strut has a toothed post  mounted on its underside.  The adjuster quadrant pivots on a pin that slips into a notch in the end of the strut Figure 74–38 A strut-quadrant ratchet automatic adjuster Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The backside of the quadrant has a toothed, cam­shaped surface  that interlocks with the toothed post on the strut. The quadrant  also has an arm that extends through an opening in the web of the  leading brake shoe The outer side of this arm serves as the brake shoe stop when the  brakes are released. The quadrant spring holds the quadrant in  contact with the post to maintain the adjustment When the brakes are applied, the leading shoe moves out toward  the brake drum. 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 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 When this happens, the toothed section of the quadrant is lifted  away from the post on the parking brake strut. The quadrant  spring then rotates the quadrant until its pivot pin is bottomed in  the slot in the parking brake strut When the brakes are released, the quadrant returns inward with  the leading shoe. 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 lining­to­drum clearance 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 Drum brake shoes include the lining table, 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, the second 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 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 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 Dual­servo 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 self­energizing Leading­trailing brakes use two brake shoes that are not  connected 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.) Leading­trailing brakes operate on a more linear basis and are  therefore more suited than dual­servo for ABS Most self­adjusting mechanisms usually operate from the  secondary or rearward facing brake shoe and adjust the brakes  as the brakes are released 10 Some self­adjusters operate on the primary shoe and adjust the  brakes as they are being applied 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 AUTOTECH BOOK COVER 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