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Always Learning

Always Learning. CHAPTER 6 Transmission Theory. OBJECTIVES. After studying Chapter 6, the reader should be able to: Identify the parts of a standard transmission. Explain the purpose of each standard transmission part. Trace the power flow for the various speeds.

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Always Learning

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  1. Always Learning

  2. CHAPTER 6 Transmission Theory

  3. OBJECTIVES After studying Chapter 6, the reader should be able to: • Identify the parts of a standard transmission. • Explain the purpose of each standard transmission part. • Trace the power flow for the various speeds. • Calculate transmission gear ratios. • Explain synchronizer operation. • Understand the requirements for good transmission operation.

  4. Automated manual Transmissions (AMT) Ball bearing Blocker rings Bushing Cam Closed case Cluster gear Clutch shaft Composite ring Constant mesh Detent Dog clutch teeth Dogs Double cone Hub Idler gear (for reverse) Input shaft Insert plates Interlock Keys Main drive gear Mainshaft Manual transmission fluid (MTF) Mineral oil Needle bearing Oiling funnel Open case Output shaft Overdrive Pitch Rail Reverse idler shaft Shift fork Sleeve Speed gears Splines Split case Struts Synchronizer assembly Synchronizer rings Synthetic oil Tapered roller bearing Trough Tunnel case Viscosity KEY TERMS

  5. INTRODUCTION • The purpose of the transmission is to provide neutral, forward speeds or ranges, and reverse gears. • It must be able to provide a gear ratio that is low enough, when multiplied by the final drive ratio, to increase the engine’s torque sufficiently to accelerate the vehicle at the desired rate. • The highest gear ratio should allow the vehicle to cruise at an engine speed that is low enough to conserve fuel and decrease noise.

  6. FIGURE 6–1 A five-speed transmission gear train. Power enters through the input shaft and leaves through the transmission output shaft. INTRODUCTION

  7. CONSTRUCTION • A RWD manual transmission has four shafts, one of which is nearly all gears and is commonly called a cluster gear, countershaft gear, or layshaft. • Besides the cluster gear, there is the input shaft (also called a main drive gear or clutch shaft), the output shaft (also called a mainshaft), and the reverse idler shaft. • Manual transmissions are also called standard transmissions and manual-step layshaft transmissions.

  8. FIGURE 6–2 Bearings support the input shaft, countershaft, mainshaft, and speed gears (shaded). (Courtesy of Chrysler Corporation) CONSTRUCTION

  9. FIGURE 6–3 (a) A very simple transmission in neutral (no output gears in mesh); (b) first gear—the first–reverse gear is slid into mesh with the cluster gear; (c) second gear—the 2–3 sliding gear is in mesh with the cluster gear; (d) third gear—the 2–3 sliding gear is in mesh with the input shaft; and (e) reverse—the first–reverse gear is in mesh with the reverse idler gear. CONSTRUCTION

  10. FIGURE 6–4 A transmission’s gear ratios are computed by determining the ratios for each gear set and then multiplying the input gear set ratio by the ratio of the first, second, and reverse sets. GEAR RATIOS

  11. FIGURE 6–5 Dividing the difference between two ratios by the lower of the ratios gives the percentage difference between them, and this can be used to determine rpm drop or the torque loss after a shift. CLOSE RATIO/WIDE RATIO

  12. FIGURE 6–6 The close-ratio Muncie 4-speed has ratio changes of 25%, 22%, and 22%. The wide-ratio Muncie has the same first and second gear changes with a larger 32% 3-4 change. CLOSE RATIO/WIDE RATIO

  13. FIGURE 6–7 (a) The input shaft rotates in a clockwise direction; the countershaft rotates in a counterclockwise direction; and the first–reverse gear drives the output shaft in a clockwise direction. (b) When meshed with the idler gear, the first–reverse gear will be driven in a counterclockwise direction. A simple (single) idler is shown. REVERSE

  14. SYNCHRONIZERS • Synchronizer assemblies are used for all the shifts except reverse. A few transmissions, however, do use a synchronizer for the reverse shift. • A synchronizer assembly includes: • A hub • A sleeve with internal splines • A pair of blocker rings, also called synchronizer rings, • A set of three spring-loaded keys, also called struts, insert plates, or dogs. • A speed gear with a set of dog clutch teeth and a polished, external cone clutch surface on the speed gear

  15. FIGURE 6–8 A synchronizer assembly that uses brass blocker rings. Note the shape of the speed gear clutch teeth and the sleeve’s inner splines. (Courtesy of Toyota Motor Sales USA, Inc.) SYNCHRONIZERS

  16. FIGURE 6–9 A synchronized shift begins as the shift fork moves the sleeve toward the speed gear (1). The keys push the blocker ring against the cone (2). The cone’s rotation will rotate the ring out of alignment with the sleeve so the sleeve will force the ring against the cone (3). As the speed of the sleeve and cone become equal, the blocker ring rotates to align with the sleeve, and the sleeve will slide into mesh with the clutching teeth (4). SYNCHRONIZERS

  17. FIGURE 6–10 Exploded (a) and cutaway (b) views of a triple-cone synchronizer. The inner and outer rings rotate with the synchronizer sleeve while the middle ring rotates with the speed gear. (Courtesy of Toyota Motor Sales USA, Inc.) SYNCHRONIZERS

  18. FIGURE 6–11 The splines in the sleeve and the dog clutch teeth have a slight back taper or back cut to hold them in mesh as the vehicle is driven. (Courtesy of Toyota Motor Sales USA, Inc.) SYNCHRONIZERS

  19. FIGURE 6–12 Power flows through a four-speed transmission. Note how the 1–2 and 3–4 synchronizer sleeves and the reverse idler gear are moved for the shifts. POWER FLOW • Shifting into a gear involves moving a synchronizer sleeve or a gear into mesh to make a connection between the mainshaft and that speed gear. • A four-speed transmission has five possible power paths: first (low), second, third (high), fourth, and reverse.

  20. FIGURE 6–13 A comparison of three-, four-, five-, and six-speed transmissions shows the major differences to be the length of the unit and the number of synchronizer assemblies. POWER FLOW

  21. FOUR-SPEED TRANSMISSIONS • A four-speed transmission requires five gears on the cluster gear, a matching gear on the mainshaft, and a synchronizer to connect this gear to the mainshaft. • See Figure 6-14 in the text. • See Figure 6-15 in the text.

  22. FIGURE 6–16 The T4 transmission is available in two different ratios, as shown here. The close-ratio version has a 3.50:1 first-gear ratio, and the wide-ratio version has a 4.03:1 first-gear ratio. (Courtesy of BWD Automotive Corporation) FOUR-SPEED TRANSMISSIONS

  23. FIVE-SPEED TRANSMISSIONS • A five-speed transmission requires six gears on the cluster gear and on the main-shaft plus another synchronizer.

  24. FIGURE 6–17 The power flows for a five-speed, T5 transmission. Note that they are similar to those for a four-speed unit, with one additional gear on the mainshaft and an additional gear with a synchronizer assembly on the cluster gear. FIVE-SPEED TRANSMISSIONS

  25. FIGURE 6–18 Ratios for three versions of a T5 transmission. (Courtesy of BWD Automotive Group) FIVE-SPEED TRANSMISSIONS

  26. FIGURE 6–19 The M5R4 five-speed transmission has a long input shaft with four fixed gears, one speed gear, and the 3–4 synchronizer assembly. The countershaft has two fixed gears plus two synchronizer assemblies and four speed gears. The mainshaft has only one fixed gear and the dog clutch teeth for fourth gear (direct drive). FIVE-SPEED TRANSMISSIONSFIVE-SPEED VARIATION

  27. FIGURE 6–20 A T56, six-speed transmission. First through fourth gears are in the main case; fifth, sixth, and reverse gears are located in the extension housing. (Courtesy of Transmission Technologies Corporation, TTC) SIX-SPEED TRANSMISSIONS • A six-speed transmission requires one more gear on the cluster, an additional speed gear, and one-half of a synchronizer assembly.

  28. TRANSMISSION TORQUE CAPACITY • A transmission is designed to be strong enough to handle the engine’s torque output. • High torque requires large input shafts, even larger output shafts, wide gears, and large bearings. • A method of comparing transmission torque capacity is the centerline distance, which is the distance between the centers of the input and cluster shafts.

  29. SHIFT MECHANISMS • Many transmissions use internal linkage. • See Figure 6-21 in the text. • Some transmissions use external linkage, with shift motion transmitted from the shift lever to the transmission by a group of two or three metal rods mounted on the side of the transmission.

  30. FIGURE 6–22 External linkage (shift rods) is used to transmit shift motions from the shift levers to the transmission levers. SHIFT MECHANISMS

  31. SHIFT MECHANISMS • A shift mechanism must include two features, an interlock system and a series of detents. • See Figure 6-23 in the text. • Some transmissions also include a reverse lockout. • The interlock prevents engagement of more than one gear at a time.

  32. FIGURE 6–24 The detent notches in the shift shafts are designed to hold the shift shaft in one of the three positions. (Courtesy of Chrysler Corporation) SHIFT MECHANISMS

  33. SHIFT MECHANISMSEXTERNAL LINKAGE • External shift linkage is normally attached to the transmission extension housing for floor shifts or is part of the steering column for column shifts. • The gear selector arm is mounted in such a way that it can swing and pivot to select the desired gear.

  34. SHIFT MECHANISMSINTERNAL LINKAGE • In light-duty vehicle transmissions, the gearshift lever is normally mounted in the extension housing with one or more shift rails that transmit the shifting motions to the main case. • In trucks and heavy-duty vehicles, the shift assembly is built into the top cover of the transmission case. • See Figure 6-25 in the text. • See Figure 6-26 in the text.

  35. AUTOMATED MANUAL TRANSMISSIONS • Manual transmissions are considered more fuel-efficient than automatic transmissions, but many drivers prefer the driving ease of an automatic transmission. • Automated manual transmissions (AMT), also called dual-clutch transmissions (DCT) or direct shift gearbox (DSG), provide easy shift control, very fast shifting, and good fuel economy.

  36. FIGURE 6–27 An automated manual transmission (AMT) uses two clutches (a). One of them drives the odd-numbered gears, and the other one drives the even-numbered gears (b). The actuators that apply the clutches and shift the synchronizers are operated by a control module (c). AUTOMATED MANUAL TRANSMISSIONS

  37. FIGURE 6–28 The electronic controls for an AMT include a control module that receives signals from the transmission rpm sensor(s), engine control module, and driver-operated switches. AUTOMATED MANUAL TRANSMISSIONS

  38. GEAR LUBRICATION • Manual transmissions, transfer cases, and drive axles must be lubricated to reduce heat and friction. • Lubricants can be either refined petroleum or synthetic products. • Refined natural products, also called mineral oils, are usually less expensive. • Synthetic oils are man-made from petroleum or vegetable oil. • The lubricant’s job is to: • Reduce friction. • Transfer heat away from the gears and bearings. • Reduce corrosion and rust. • Flush dirt and wear particles away from the moving parts.

  39. FIGURE 6–29 The fluid level of most transmissions is at the bottom of the fill plug opening; the fluid is moved around inside the case when the gears rotate. (Courtesy of Chrysler Corporation) TRANSMISSION LUBRICATION • The transmission cluster gears run in a bath of lubricant, and as they spin, their motion will throw the lubricant throughout the case. • The lubricant can be gear oil, ATF, or manual transmission fluid, as determined by the manufacturer.

  40. FIGURE 6–30 This transmission was operated without oil. Note the heat-darkened color and the lack of teeth on the main drive gear. The gear got so hot that the metal reached its plastic state, and the teeth were removed or reshaped. TRANSMISSION LUBRICATION

  41. FIGURE 6–31 This extension housing (a) includes a trough to deliver oil to the cluster gear bearing and (b) delivers oil to the center of this shaft, where it will flow on to lubricate the speed gears. (Courtesy of Chrysler Corporation) TRANSMISSION LUBRICATION

  42. DESIGN FEATURES • Knowledge of certain design features will increase your understanding of transmission operation, make problem diagnosis easier and more accurate, and improve service and overhaul techniques. • GEARS • MAINSHAFT • COUNTERSHAFT AND CLUSTER GEAR • CASE • BEARINGS

  43. FIGURE 6–32 Speed gears float on their mainshaft journals and have thrust surfaces on each side to keep them properly positioned. DESIGN FEATURES

  44. FIGURE 6–33 Notice the gear teeth in this transmission; those on the fifth gears have a finer pitch and a greater helix angle, to produce quieter operation while cruising. DESIGN FEATURES

  45. FIGURE 6–34 The left ends (circled) of the reverse gear teeth around this 1–2 synchronizer sleeve are beveled to allow easier engagement with the reverse idler during shifts into reverse. Constant mesh gears normally have teeth that are squared off at the ends. (Courtesy of Chrysler Corporation) DESIGN FEATURES

  46. FIGURE 6–38 Two gears, the input (3) and third (4), are press fit onto the cluster gear in a TR-3550 transmission. DESIGN FEATURES • See Figure 6-35 in the text. • See Figure 6-36 in the text. • See Figure 6-37 in the text.

  47. FIGURE 6–39 The back of the countershaft (cluster gear) of a T56 transmission has an internal spline (a); the countershaft extension (b) has mating splines that fit into it so it can drive the fifth, sixth, or reverse gears. (Courtesy of Transmission Technologies Corporation, TTC) DESIGN FEATURES

  48. FIGURE 6–41 The types of frictionless bearings are cylindrical or straight roller bearings (a), tapered roller bearings (b), ball bearings (c), and needle bearings (d). (Courtesy of CR Services) DESIGN FEATURES

  49. What Is an Overdrive? • From the mid-1930s to the early 1960s, each domestic vehicle manufacturer offered a manual transmission with an overdrive. In most cases, the unit was manufactured by Borg Warner. In England and Europe, several vehicle manufacturers used an overdrive unit known as the Laycock de Normanville. In the mid-1980s, an overdrive unit manufactured by Doug Nash Industries was used with a version of the Borg Warner T10 transmission and installed in Corvettes. Each of these units provides two ratios: a 1:1 direct drive, sometimes called underdrive or passing gear; and an overdrive with a ratio somewhere between 0.8:1 and 0.6:1.

  50. What Is an Overdrive? • The Borg Warner and de Normanville units use true planetary gear sets with a sun gear that can rotate or be held stationary. The planet carrier is driven by the transmission mainshaft, a ring gear that is connected to the output shaft, and a one-way clutch assembly that allows the carrier to drive the output shaft. Overdrive occurs when the sun gear is held stationary. When the carrier is driven, the planet pinion gears will be forced to “walk” (rotate on their axis) around the sun gear. This causes the outer teeth of the planet gears, the ones in mesh with the ring, to move faster than the carrier; as a result, they will drive the ring gear at an overdrive ratio.

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