chap68 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 68, the reader should be able to: • • • • • Prepare for ASE Brakes (A5) certification test Explain kinetic energy and why it is so important to brake design Discuss mechanical advantage and how it relates to the braking system Explain the coefficient of friction Describe how brakes can fade due to excessive heat 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: brake fade • coefficient of friction • energy friction • fulcrum gas fade • inertia kinetic energy • kinetic friction • leverage • lining fade mechanical advantage • mechanical fade pedal ratio • static friction weight bias • weight transfer • work 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 ENERGY PRINCIPLES Figure 68–1 Energy, which is the ability to perform work, exists in many forms Energy is the ability to do work.  Chemical, mechanical, and  electrical energy are the most   familiar kinds involved in the  operation of an automobile When the ignition key is turned  to the “Start” position, chemical  energy in the battery is converted  into electrical energy to operate  the starter motor.  The starter motor converts the  electrical energy into mechanical  energy to crank the engine 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 Work is the transfer of energy from one physical system to   another—especially the transfer of energy to an object through  the application of force This occurs when a vehicle’s brakes are applied: The force of the  actuating system transfers the energy of the vehicle’s motion to  the brake drums or rotors where friction converts it into heat  energy and stops the vehicle 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 KINETIC ENERGY Kinetic energy is a fundamental form of mechanical energy. It is  the energy of mass in motion Every moving object possesses kinetic energy, and amount of that  energy is determined by the object’s mass and speed. The greater  the mass of an object and the faster it moves, the more kinetic  energy it possesses Even at low speeds, a moving vehicle has enough kinetic energy  to cause serious injury and damage. The job of the brake system is to dispose of that energy in a safe and controlled manner 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 Engineers calculate kinetic energy using the following formula: See these formulas on Pages 818 and 819 of your textbook.  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 68–2 Kinetic energy increases in direct proportion to the weight of the vehicle 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 If a 3,000­lb vehicle traveling at 30 mph is compared with a 6,000­lb  vehicle also traveling at 30 mph as shown in Figure 68–2, the equations  for computing respective kinetic energies look like this: The results show that when the weight of a vehicle is doubled from  3,000 to 6,000 lb, its kinetic energy is also doubled from 90,301 ft­ lb to 180,602 ft­lb. In mathematical terms, kinetic  energy increases  proportionally as weight increases. In other words, if the weight of a  moving object doubles, its kinetic energy also doubles. If the weight  quadruples, the kinetic energy becomes four times as great 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 68–3 Kinetic energy increases as the square of any increase in vehicle speed 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 FRICTION AND HEAT The function of the brake system is to convert kinetic energy into  heat energy through friction. But just how much heat is created by  this conversion process?  Although there are too many variables to obtain exact temperature  increase of any specific component, the average temperature rise of the brakes during a single stop can be computed as follows: See this formula on Page 822  of your textbook.  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 Consider a 3,000­lb vehicle with a combined brake drum and rotor  weight of 20 lb that is brought to a complete stop from 30 mph.  We calculated this vehicle has 90,301 ft­lb of kinetic energy, and  since the vehicle is coming to a full stop, the change in kinetic  energy during the stop will equal the entire 90,301 ft­lb.  Based on this information, the equation for computing the rise in  brake temperature reads: Figure 68–11 Brake temperature increase is determined mostly by vehicle weight, drum and rotor weight, and the change in kinetic energy 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 total brake temperature increase in this case is 58°F (32°C).  This increase is relatively small, but the weight and speed of the  vehicle are also rather low It is the change in kinetic energy that determines the amount of  temperature increase, and kinetic energy increases proportionately  with increases in weight, and as the square of any increase in  speed. If the vehicle weight is doubled to 6,000 lb, the change in  kinetic energy required to bring it to full stop will be 180,602 ft­lb 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 temperature increase computed with this equation is the  average of all the friction generating components. Some of the  heat is absorbed by the brake drums and rotors, some goes into the  shoes and pads, and some is conducted into the wheel cylinders,  calipers, and brake fluid.  NOTE: The possibility of brake fade caused by heat can be reduced if the  transmission gear selector is placed in a lower gear when descending a steep or long hill to reduce amount of braking needed to maintain a safe  road speed. Engine braking will help keep vehicle speed under control 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 HEAT-CAUSED BRAKE FADE The temperature of a brake drum or rotor may rise more than  100°F (55°C) in only seconds during a hard stop, but could take  30 seconds or more to cool to the temperature before the stop If repeated hard stops are performed, brake system components  can overheat and lose effectiveness, or possibly fail altogether.  This loss of braking power is called brake fade The point at which brakes overheat and fade is determined by a  number of factors including the brake design, its cooling ability,  and the type of friction material being used.  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 Three primary types of brake fade caused by heat: Mechanical fade Lining fade Gas fade Mechanical fade occurs when a brake drum overheats and  expands away from the brake  lining.  To maintain braking power, the  brake shoes must move farther  outward, which requires  additional brake pedal travel.  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Figure 68–12 Mechanical fade occurs when the brake drums become so hot they expand away from the brake lining ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 When the drum expands to a point where there is not enough pedal  travel to keep the lining in contact with the drum, brake fade  occurs. Sometimes, partial braking power can be restored by  rapidly pumping the brake pedal to move the brake shoes farther  outward and back into contact with the drums Mechanical fade in drum brakes is  reduced by using larger or  heavier drums that absorb more heat before they expand too far.  Cooling fins are also added to the drums or make them partially of  aluminum to help speed heat transfer to the passing air Mechanical fade is not a problem with disc brakes because as a  brake rotor heats up it expands toward the brake pads rather than  away from them 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 Lining fade affects both drum and disc brakes, and occurs when  the friction material overheats to the point where its coefficient of  friction drops off.  Figure 68–13 Some heat increases the coefficient of friction, but too much heat can cause it to drop off sharply Partial braking power can  sometimes be restored by  increasing pressure on the  brake pedal Extra pressure increases the  amount of heat and fade.  Disc brakes lining fade is  possible, but less a problem  because of disc brakes’ ability  to dissipate heat.  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 Gas fade is a relatively rare type of brake fade that occurs under  very hard braking when a thin layer of hot gases and dust particles  builds up between the brake drum or rotor and linings. The gas layer  acts as a lubricant and reduces friction.  Figure 68–14 One cause of brake fade occurs when the phenolic resin, a part of the friction material, gets so hot that it vaporizes The vaporized gas from the disc brake pads gets between the rotor (disc) and the friction pad Because the friction pad is no longer in contact with the rotor, no additional braking force is possible In most cases brake fade is a temporary  condition. Brakes will return to normal  once they have all been allowed to cool  in all except extreme situations where  the heat has been so great it has damaged  the friction material or melted rubber  seals within the hydraulic system 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 WATER-CAUSED BRAKE FADE If a vehicle is driven through deep water or during a severe  rainstorm, water can get between the brake drum and the linings When this occurs, no stopping power is possible until water is  pushed out and normal friction is restored. While water fade is  most likely with drum brakes, it can also occur on disc brakes After driving through deep water, the wise driver should lightly  apply the brakes to check the operation and to help remove any  water trapped between the friction material and the rotor or drum 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 DECELERATION RATES Deceleration rates are measured in units of “feet per second per second”  (No, this is not a misprint) What it means is that the vehicle will change in velocity during a  certain time interval divided by the time interval Deceleration is abbreviated “ft/sec2” (pronounced “feet per second per  second” or “feet per second squared”) or meters per sec 2 (m/sec2) in the  metric system 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 Typical deceleration rates include the following: Comfortable deceleration is about 8.5 ft/sec2 (3 m/sec2) Loose items in the vehicle will “fly” above 11 ft/sec2 (3.5  m/sec2) Maximum deceleration rates for most vehicles and light  trucks range from 16 to 32 ft/sec2 (5 to 10 m/sec2) An average deceleration rate of 15 ft/sec2 (3 m/sec2) can stop a  vehicle traveling at 55 mph (88 km/h) in about 200 ft (61 m) in  less than 4 seconds.  During a standard brake system test, a vehicle is braked at this  rate 15 times. Temperatures at the front brake pads can reach  1300°F (700°C) or higher, sometimes reaching as high as 1800°F  (980°C). Brake fluid and rubber components may reach 300°F  (150°C) or higher 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 Energy is the ability to do work. A vehicle in motion represents  kinetic energy, which must be absorbed by the braking system  during a stop The front brakes must provide a higher percentage of the  braking force due to weight bias and weight transfer The brake pedal uses mechanical advantage to increase the force  applied by the driver to the master cylinder Coefficient of friction represents the amount of friction between  two surfaces 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.) Friction creates heat during a stop and the braking system must  be able to absorb this heat Brake fade results when the heat generated by the brakes causes  changes in the friction materials that reduce the braking force or  by water that can get between the brake drum and the linings Deceleration rates are expressed in feet per second per second or  ft/sec2 or m/sec2 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