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Understanding Automotive Transmissions: Components, Gear Ratios, and Systems

This chapter explores the requirements for an automotive drivetrain, the major components of an automatic transmission and transaxle, how gear ratios are obtained in a planetary gearset, and the different systems within an automatic transmission.

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Understanding Automotive Transmissions: Components, Gear Ratios, and Systems

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  1. OBJECTIVES After studying Chapter 3, the reader should be able to: • Explain the requirements for an automotive drivetrain. • Identify the major components of an automatic transmission and transaxle. • Explain how different gear ratios can be obtained in a planetary gearset. • Describe the systems within an automatic transmission and how they relate to each other.

  2. TRANSMISSIONS: THEIR PURPOSE • A vehicle would be difficult to operate without a transmission. • Every driver is familiar with the gear shift lever that is moved to control a vehicle’s motion. • This lever determines gear selection in the transmission, which determines the driving mode of the vehicle.

  3. TRANSMISSIONS: THEIR PURPOSE • These selections are: • P: Park, allows the engine to run without moving the vehicle and mechanically locks the drivetrain to hold the vehicle stationary. • R: Reverse, allows the vehicle to be driven backward. • N: Neutral, allows the engine to run without moving the vehicle or locking the drivetrain. • O or OD: Overdrive, allows the engine to run slower while cruising for improved fuel mileage and lower emissions, shifts automatically from 1st through OD. • D: Allows automatic shifts from 1st through D, drives high gear with a 1:1 ratio. • I: Intermediate, prevents high-gear operation and provides compression braking. • L: Low gear, multiplies the engine’s torque so there will be enough power to move the vehicle in difficult conditions, prevents upshifts, and provides compression braking.

  4. FIGURE 3-1 A RWD drivetrain uses a transmission to provide the necessary gear ratio and a single driveshaft to transfer power to the rear axle (a). A FWD drivetrain uses a transaxle that combines the transmission, final drive, and differential (b). A driveshaft is used for each of the two front drive wheels. TRANSMISSIONS: THEIR PURPOSE

  5. FIGURE 3-2 Transverse (a) and longitudinal (b) mounted FWD drivetrains. Note that B can easily be redesigned to drive a shaft to the rear wheels of a 4WD or AWD vehicle. TRANSMISSIONS: THEIR PURPOSE

  6. FIGURE 3-3 Torque is a twisting force like that produced when you pull on a wrench. TORQUE AND HORSEPOWER

  7. FIGURE 3-4 A typical automotive engine horsepower and torque chart. Note that the maximum horsepower (155) occurs at about 3,400 rpm and that maximum torque occurs between 2,000 and 2,500 rpm. The rpm range and maximum power output will vary depending on engine size and design. TORQUE AND HORSEPOWER

  8. FIGURE 3-5 The common methods of transferring torque are belt, gear, and chain. TORQUE AND HORSEPOWER

  9. FIGURE 3-6 Transmissions and transaxles are designed with a torque capacity to match the engine with which it is used. (Reprinted with permission of General Motors) TORQUE AND HORSEPOWER

  10. GEAR RATIOS • The term gear ratio refers to the relative size of two gears (a gearset). • The ratio can be determined using either the diameter or the number of teeth on the two gears. • A pair of gears of different size will have different numbers of teeth, and the number of teeth is directly related to the diameter of the gears.

  11. FIGURE 3-7 In a matched set of gears, the number of teeth on each gear is related to the diameter. Gear A is half the size of C and has half the number of teeth. Gear B is one and a half times the diameter of A and has one and a half times as many teeth. GEAR RATIOS

  12. FIGURE 3-8 The gear ratio is determined by dividing the number of teeth on the driven (output) gear by the number of teeth on the driving (input) gear. GEAR RATIOS

  13. FIGURE 3-9 The forward gear ratios of a five-speed transmission are shown along with mph speed of the vehicle per 1,000 rpms of engine speed. The overall gear ratio is determined by multiplying the transmission gear ratio by the final drive ratio. A final drive ratio of 3.55:1 and a first-gear ratio of 3.97:1 will produce an overall ratio of 14.09:1. GEAR RATIOS

  14. FIGURE 3-10 The teeth of a spur gear (a) are cut parallel to the shaft, which produces a straight pressure between the driving and driven gears. The teeth of a helical gear (b) are cut on a slant, which causes an axial or side thrust. GEAR RATIOS

  15. FIGURE 3-11 The three major styles of bevel gears are spur or plain bevel gears (a), spiral bevel gears (b), and hypoid gears (c). Note the differences in the shape of their teeth.A worm gearset (d) also transmits power between angled shafts. (Reprinted with permission of General Motors) GEAR RATIOS

  16. FIGURE 3-12 External gears (a) rotate in opposite directions. An idler gear in an external gearset (b) changes the direction of rotation so the input and output gears turn in the same direction without changing the ratio. An internal and external gearset (c) rotate in the same direction. GEAR RATIOS

  17. GEAR RATIOS • Some important rules to learn about gearsets are: • Two mated external gears will always rotate in opposite directions. • Mated internal and external gears will rotate in the same direction. • An idler gear allows the drive and driven gears to rotate in the same direction. • To find the ratio, divide the driven gear by the drive gear. • When power transfers through an even number (two or four) of gears, the input and output gears will rotate in opposite directions.

  18. GEAR RATIOS • When power transfers through an uneven number (one, three, or five) of gears, the input and output gears will rotate in the same direction. • To find the overall ratio of multiple gearsets, multiply the ratios of the gearsets. • Two gears transferring power push away from each other in an action called gear separation. The gear separation force is proportional to the torque being transferred. • All gearsets have backlash to prevent binding. • The smaller gear(s) in a gearset is often called a pinion.

  19. FIGURE 3-13 The pitch diameter is the effective diameter of a gear. Note how the contact points slide across the gear tooth. Backlash is the clearance on the nonloaded side of the gear tooth. GEAR RATIOS

  20. TRACTIVE FORCE • An engineer uses the term tractive force to describe the power in a vehicle’s drivetrain. • It is a product of the engine’s torque multiplied by the gear ratio and can be plotted in a graph.

  21. FIGURE 3-14 Tractive force (a) is determined by multiplying engine torque times the gear ratio. Note how it is greatest in first gear. The chart in (b) shows the same curves related to engine and vehicle speed. This curve is based on a typical 5-liter V8 engine. TRACTIVE FORCE

  22. FIGURE 3-14 (CONTINUED) Tractive force (a) is determined by multiplying engine torque times the gear ratio. Note how it is greatest in first gear. The chart in (b) shows the same curves related to engine and vehicle speed. This curve is based on a typical 5-liter V8 engine. TRACTIVE FORCE

  23. FIGURE 3-15 The speed of a vehicle is determined by the diameter of the drive tires and how fast the tires are turned. TRACTIVE FORCE

  24. TRACTIVE FORCE

  25. FIGURE 3-16 A standard transmission provides several gear ratios and a method to shift them. MANUAL TRANSMISSION AND CLUTCH • A manual transmission, also called a standard transmission, has several sets of gears to produce the necessary gear ratios.

  26. FIGURE 3-17 Power flows through each of the gear ranges of a five-speed, RWD, standard transmission. Note that the synchronizer sleeves are used to shift the power flow. (Courtesy of Chrysler Corporation) MANUAL TRANSMISSION AND CLUTCH

  27. FIGURE 3-17 (CONTINUED) Power flows through each of the gear ranges of a five-speed, RWD, standard transmission. Note that the synchronizer sleeves are used to shift the power flow. (Courtesy of Chrysler Corporation) MANUAL TRANSMISSION AND CLUTCH

  28. FIGURE 3-17 (CONTINUED) Power flows through each of the gear ranges of a five-speed, RWD, standard transmission. Note that the synchronizer sleeves are used to shift the power flow. (Courtesy of Chrysler Corporation) MANUAL TRANSMISSION AND CLUTCH

  29. FIGURE 3-18 The shift fork slides the synchronizer sleeve into mesh with the dog teeth of the desired gear (a). This allows torque to be transferred between the speed gear and the synchronizer hub. An exploded view of a synchronizer assembly (b) shows all of the parts. (a is courtesy of Chrysler Corporation) MANUAL TRANSMISSION AND CLUTCH

  30. FIGURE 3-19 A clutch cover (pressure plate assembly) is bolted onto the flywheel with the clutch disc between them (a). The release bearing and fork provide a method to release the clutch. When the clutch is engaged, the disc is squeezed against the flywheel by the pressure plate (b). Releasing the clutch separates the disc from the flywheel and pressure plate. MANUAL TRANSMISSION AND CLUTCH

  31. FIGURE 3-20 A FWD transaxle. Note that it combines a five-speed transmission with the final drive gears and differential. (Courtesy of Chrysler Corporation) MANUAL TRANSMISSION AND CLUTCH

  32. SIMPLE PLANETARY GEARSETS • A planetary gearset is a combination of a sun gear, two or more planet gears, a planet carrier, and a ring gear. • The ring gear, also called an annulus gear, is an internal gear. • All the other gears are external gears. • The carrier holds the planet gears (also called pinions) in position and allows each of these gears to rotate in the carrier

  33. FIGURE 3-21 A simple planetary gearset is a combination of a sun gear, a planet carrier with a group of planet pinion gears, and an annulus/ring gear (a). A transmission planetary gear train contains a combination of complex gearsets (b). (Courtesy of Chrysler Corporation) SIMPLE PLANETARY GEARSETS

  34. FIGURE 3-22 If the planet carrier is held with the sun gear rotating, the planet gears simply rotate in the carrier and act as idler gears between the sun and ring gears (a). If the sun or ring is held, the planet gears will walk around that stationary gear; they rotate on their shafts as the carrier rotates (b). If two parts are driven and no parts are held, the planet gears are stationary on their shafts, and the whole assembly rotates as a unit (c). SIMPLE PLANETARY GEARSETS

  35. FIGURE 3-23 Nine possible modes of operation for a simple planetary gearset. In all cases, solid black indicates a reaction member, light shading an input member, and medium shading the output member. SIMPLE PLANETARY GEARSETS

  36. COMPOUND PLANETARY GEARSETS • Most automatic transmissions use a more complicated compound planetary gearset that combines a simple planetary gearset with portions of one or more planetary gearsets in such a way that three or more gear ratios are possible. • The most common compound planetary gearset used in three-speed transmissions is known as the Simpson gear train, named after its designer, Howard Simpson.

  37. FIGURE 3-24 A cutaway and an exploded view of a Simpson gear train. Note that the gearset uses a double sun gear with two carriers and ring gears. The two clutches allow either the forward ring gear or the sun gear to be driven. The two bands allow either the sun gear or the rear carrier to be held in reaction. The one-way clutch will keep the rear carrier from turning counterclockwise. (Courtesy of Chrysler Corporation) COMPOUND PLANETARY GEARSETS

  38. FIGURE 3-24 (CONTINUED) A cutaway and an exploded view of a Simpson gear train. Note that the gearset uses a double sun gear with two carriers and ring gears. The two clutches allow either the forward ring gear or the sun gear to be driven. The two bands allow either the sun gear or the rear carrier to be held in reaction. The one-way clutch will keep the rear carrier from turning counterclockwise. (Courtesy of Chrysler Corporation) COMPOUND PLANETARY GEARSETS

  39. CONTROL DEVICES • Power flow through a gearset is accomplished by the control devices: the clutches, bands, and one-way clutches. • They are also called apply devices or friction members. • A gearset has several paths for power flow, and each path provides a different gear ratio

  40. FIGURE 3-25 A sectioned view of a modern, four-speed, RWD transmission. You should be able to identify all of these parts, know what they do, and be able to service them at the completion of this book. CONTROL DEVICES

  41. FIGURE 3-26 A multiple-disc clutch combines lined friction discs and unlined steel plates. (Courtesy of Toyota Motor Sales USA, Inc.) CONTROL DEVICES

  42. FIGURE 3-27 Fluid pressure forces the piston against the clutch pack to apply a clutch (left). When the fluid pressure is released, the return springs push the piston away from the clutch pack (right). (Courtesy of Toyota Motor Sales USA, Inc.) CONTROL DEVICES

  43. FIGURE 3-28 A band is applied when fluid pressure forces the piston and rod inward (right). When the pressure is released, a spring pushes the piston back (left). (Courtesy of Toyota Motor Sales USA, Inc.) CONTROL DEVICES

  44. FIGURE 3-29 Two types of one-way clutches. The roller clutch (a) will release if the inner race turns clockwise and lock up if it turns counterclockwise. The sprag clutch (b) will operate in the same manner. CONTROL DEVICES

  45. TRANSMISSION HYDRAULICS • As soon as the engine starts, a pump driven by the torque converter sends fluid into the transmission’s hydraulic passages. • Fluid is pumped from the sump in the transmission pan into the hydraulic circuit where the pressure and flow is controlled by the valve body. • This fluid pressure is used to: • Apply the clutches and bands, • Maintain a full torque converter so it can transmit power, and • Lubricate and cool the internal parts of the transmission.

  46. FIGURE 3-30 A simplified hydraulic control circuit for a nonelectronic transmission. The main control unit (valve body) sends oil pressure to the clutches and bands to control the shifts. It uses signals from the manual shift lever, governor, and throttle valve. TRANSMISSION HYDRAULICS

  47. FIGURE 3-31 The end of this transmission case shows most of the fluid passages. These connect the valve body to passages in the case. TRANSMISSION HYDRAULICS

  48. TRANSMISSION HYDRAULICS • The valve body contains most of the transmission’s control valves. • Sometimes, there are one or more valves elsewhere. • The major valves used in nonelectronic transmissions are as follows: • Manual valve • Pressure regulator • Throttle or modulator valve • Governor • Shift valves • Shift modifier valves • Torque converter clutch valve

  49. FIGURE 3-32 The electronic controls for an electronic transmission include input sensor signals from the engine and the transmission to the PCM and output devices in the transmission. The test connector is the diagnostic link used to diagnose transmission problems. TRANSMISSION HYDRAULICS

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