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1、<p>  MANUAL GEARBOXES</p><p>  9.1 MANUAL GEARBOX CLASSIFICATION</p><p>  Gearboxes are normally classi?ed according to the number of toothed wheel</p><p>  couples (stages) i

2、nvolved in the transmission of motion at a given speed; in the</p><p>  case of manual vehicle transmissions, the number to be taken into account is</p><p>  that of the forward speeds only, wit

3、hout consideration of the ?nal gear, even if</p><p>  included in the gearbox.</p><p>  Therefore there are:</p><p>  ? Single stage gearboxes</p><p>  ? Dual stage or

4、countershaft gearboxes</p><p>  ? Multi stage gearboxes</p><p>  Figure 9.1 shows the three con?gurations for a four speed gearbox.</p><p>  It is useful to comment on the generally

5、 adopted rules of these schemes.</p><p>  Each wheel is represented by a segment whose length is proportional to the pitch</p><p>  diameter of the gear; the segment is ended by horizontal strok

6、es, representing</p><p>  the tooth width. If the segment is interrupted where crossing the shaft, the</p><p>  gear wheel is idle; the opposite occurs if the segment crosses the line of the<

7、/p><p>  shaft without interruption. Then the wheel rotates with the shaft. Hubs are</p><p>  represented according to the same rules, while sleeves are represented with a</p><p>  pai

8、r of horizontal strokes. Arrows show the input and output shafts.</p><p>  Single stage gearboxes are primarily applied to front wheel driven vehicles,</p><p>  because in these it is useful tha

9、t the input and the output shaft are o?set; in</p><p>  G. Genta and L. Morello, The Automotive Chassis, Volume 1: Components Design, 425</p><p>  Mechanical Engineering Series,</p><p

10、>  c Springer Science+Business Media B.V. 2009426 9. MANUAL GEARBOXES</p><p>  FIGURE 9.1. Schemes for a four speed gearbox shown in three di?erent con?gurations:</p><p>  a: single stage, b

11、: double stage and c: triple stage.</p><p>  conventional vehicles, on the other hand, it is better that input and output shafts</p><p>  are aligned.</p><p>  This is why rear whee

12、l driven vehicles usually adopt a double stage gearbox.</p><p>  The multi-stage con?guration is sometime adopted on front wheel driven</p><p>  vehicles with transversal engine, because the tra

13、nsversal length of the gearbox</p><p>  can be shortened; it is used when the number of speeds or the width of the gears</p><p>  do not allow a single stage transmission to be used.</p>

14、<p>  It should be noted that on a front wheel driven vehicle with transversal</p><p>  engine, having decided on the value of the front track and the size of the tire,</p><p>  the length

15、of the gearbox has a direct impact on the maximum steering angle of</p><p>  the wheel and therefore on the minimum turning radius.</p><p>  The positive result on the transversal dimension of m

16、ulti-stage gearboxes is</p><p>  o?set by higher mechanical losses, due to the increased number of engaged gear</p><p><b>  wheels.</b></p><p>  It should be noted that

17、in triple stage gearboxes, shown in the picture, the</p><p>  axes of the three shafts do not lie in the same plane, as the scheme seems to</p><p>  show. In a lateral view, the outline of the t

18、hree shafts should be represented as</p><p>  the vertices of a triangle; this lay-out reduces the transversal dimension of the</p><p>  gearbox. In this case and others, as we will show later,

19、the drawing is represented</p><p>  by turning the plane of the input shaft and of the counter shaft on the plane of</p><p>  the counter shaft and of the output shaft.</p><p>  Gea

20、r trains used in reverse speed are classi?ed separately. The inversion of</p><p>  speed is achieved by using an additional gear. As a matter of fact, in a train of</p><p>  three gears, the out

21、put speed has the same direction as the input speed, while</p><p>  the other trains of two gears only have an output speed in the opposite direction;</p><p>  the added gear is usually called i

22、dler.</p><p>  The main con?gurations are reported in Fig. 9.2.</p><p>  In scheme a, an added countershaft shows a sliding idler, which can match</p><p>  two close gears that are

23、not in contact, as, for example, the input gear of the</p><p>  ?rst speed and the output gear of the second speed. It should be noted that, in</p><p>  this scheme, the drawing does not preserv

24、e the actual dimension of the parts.9.1 Manual gearbox classi?cation 427</p><p>  FIGURE 9.2. Schemes used for reverse speed; such schemes ?t every type of gearbox</p><p><b>  lay-out.<

25、/b></p><p>  Scheme b shows instead two sliding idlers, rotating together; this arrange-</p><p>  ment o?ers additional freedom in obtaining a given transmission ratio. The coun-</p>&

26、lt;p>  tershaft is o?set from the drawing plane; arrows show the gear wheels that match</p><p>  when the reverse speed is engaged.</p><p>  Scheme c is similar to a in relation to the idler;

27、 it pairs an added speci?c</p><p>  wheel on the output shaft with a gear wheel cut on the shifting sleeve of the ?rst</p><p>  and second speed, when it is in idle position.</p><p>

28、;  Con?guration d shows a dedicated pair of gears, with a ?xed idler and a</p><p>  shifting sleeve.</p><p>  The following are the advantages and disadvantages of the con?gurations</p>&

29、lt;p>  shown in the ?gure.</p><p>  ? Schemes a, b and c are simpler, but preclude the application of synchro-</p><p>  nizers (because couples are not always engaged), nor do they allow the

30、use</p><p>  of helical gears (because wheels must be shifted by sliding).</p><p>  ? Scheme d is more complex but can include a synchronizer and can adopt</p><p>  helical gears.&l

31、t;/p><p>  ? Schemes a, b and c do not increase gearbox length.428 9. MANUAL GEARBOXES</p><p>  9.2 MECHANICAL EFFICIENCY</p><p>  The mechanical e?ciency of an automotive gear wheel t

32、ransmission is high com-</p><p>  pared to other mechanisms performing the same function; indeed, the value of</p><p>  this e?ciency should not be neglected when calculating dynamic performance

33、</p><p>  and fuel consumption. The continuous e?ort of to limit fuel consumption justi-</p><p>  ?es the care of transmission designers in reducing mechanical losses.</p><p>  Tota

34、l transmission losses are conveyed up by terms that are both dependent</p><p>  and independent of the processed power; the primary terms are:</p><p>  ? Gearing losses; these are generated by f

35、riction between engaging teeth</p><p>  (power dependent) and by the friction of wheels rotating in air and oil</p><p>  (power independent).</p><p>  ? Bearing losses; these are ge

36、nerated by the extension of the contact area of</p><p>  rolling bodies and by their deformation (partly dependent on and partly</p><p>  independent of power) and by their rotation in the air a

37、nd oil (power</p><p>  independent).</p><p>  ? Sealing losses; they are generated by friction between seals and rotating</p><p>  shafts and are power independent.</p><p

38、>  ? Lubrication losses; these are generated by the lubrication pump, if present,</p><p>  and are power independent.</p><p>  All these losses depend on the rotational speed of parts in cont

39、act and,</p><p>  therefore, on engine speed and selected transmission ratio.</p><p>  Table 9.1 reports the values of mechanical e?ciency to be adopted in calcu-</p><p>  lations c

40、onsidering wide open throttle conditions; these values consider a pair of</p><p>  gearing wheels or a complete transmission with splash lubrication; in the same</p><p>  table we can see also t

41、he e?ciency of a complete powershift epicycloidal auto-</p><p>  matic transmission and a steel belt continuously variable transmission. For the</p><p>  two last transmissions, the torque conve

42、rter must be considered as locked-up.</p><p>  TABLE 9.1. Mechanical e?ciency of di?erent transmission mechanisms.</p><p>  Mechanism type E?ciency (%)</p><p>  Complete manual gear

43、box</p><p>  with splash lubrication 92–97</p><p>  Complete automatic transmission</p><p>  (ep. gears) 90–95</p><p>  Complete automatic gearbox</p><p> 

44、 (steel belt; without press. contr.) 70–80</p><p>  Complete automatic gearbox</p><p>  (steel belt; with press. contr.) 80–86</p><p>  Pair of cyl. gears 99.0–99.5</p><p

45、>  Pair of bevel gears 90–939.2 Mechanical e?ciency 429</p><p>  FIGURE 9.3. Contributions to total friction loss of a single stage gearbox designed for</p><p>  300 Nm as function of input s

46、peed.</p><p>  It is more correct to reference power loss measurement as a function of</p><p>  rotational input speed rather than e?ciency. Figure 9.3 shows the example of</p><p> 

47、 a double stage transmission, in fourth speed, at maximum power; the di?erent</p><p>  contributions to the total are shown.</p><p>  This kind of measurement is made by disassembling the gearbo

48、x step by</p><p>  step, thus eliminating the related loss.</p><p>  In the ?rst step all synchronizer rings are removed, leaving the synchronizer</p><p>  hubs only; mechanical los

49、ses of non-engaged synchronizers are, therefore, mea-</p><p>  surable. The loss is due to the relative speed of non-engaged lubricated conical</p><p>  surfaces; the value of this loss depends,

50、 obviously, on speed and the selected</p><p>  transmission ratio.</p><p>  In the second step all rotating seals are removed.</p><p>  In the third step the lubrication oil is remo

51、ved, and therefore, the bulk of</p><p>  the lubrication losses is eliminated; some oil must remain in order to leave the</p><p>  contact between teeth una?ected.</p><p>  By remov

52、ing those gear wheels not involved in power transmission, their</p><p>  mechanical losses are now measurable.</p><p>  The rest of the loss is due to bearings; the previous removal of parts can

53、</p><p>  a?ect this value.</p><p>  A more exhaustive approach consists in measuring the complete e?ciency</p><p>  map; the e?ciency can be represented as the third coordinate of

54、a surface, where</p><p>  the other two coordinates are input speed and engine torque. E?ciency calcu-</p><p>  lations can be made by comparing input and output torque of a working trans-</p

55、><p><b>  mission.</b></p><p>  Such map can show how e?ciency reaches an almost constant value at a</p><p>  modest value of the input torque; it must not be forgotten tha

56、t standard fuel</p><p>  consumption evaluation cycles involve quite modest values of torque and there-</p><p>  fore imply values of transmission e?ciency that are changing with torque.</p&g

57、t;<p>  Figure 9.4 shows a qualitative cross section of the aforesaid map, cut at</p><p>  constant engine speed. It should be noted that e?ciency is also zero at input430 9. MANUAL GEARBOXES</p>

58、;<p>  FIGURE 9.4. Mechanical e?ciency map, as a function of input torque at constant</p><p>  engine speed; the dotted line represents a reasonable approximation of this curve, to be</p><p

59、>  used on mathematical models for the prediction of performance and fuel consumption.</p><p>  torque values slightly greater than zero; as a matter of fact, friction implies a</p><p>  cert

60、ain minimum value of input torque, below which motion is impossible.</p><p>  A good approximation to represent mechanical e?ciency can be made using</p><p>  the dotted broken line as an interp

61、olation of the real curve.</p><p>  9.3 MANUAL AUTOMOBILE GEARBOXES</p><p>  9.3.1 Adopted schemes</p><p>  In manual gearboxes, changing speed and engaging and disengaging the clut

62、ch</p><p>  are performed by driver force only.</p><p>  This kind of gearbox is made with helical gears and each speed has a syn-</p><p>  chronizer; some gearboxes do not use show

63、 the synchronizer for reverse speed,</p><p>  particularly those in economy minicars.</p><p>  We previously discussed a ?rst classi?cation; additional information is the</p><p>  s

64、peed number, usually between four and six.</p><p>  Single stage gearboxes are used in trans-axles; they are applied, with some</p><p>  exceptions, to front wheel driven cars with front engine

65、and rear driven cars with</p><p>  rear engine; this is true with longitudinal and transversal engines.</p><p>  In all these situations the ?nal drive is included in the gearbox, which is</p

66、><p>  therefore also called transmission.</p><p>  Countershaft double stage gearboxes are used in conventionally driven cars,</p><p>  where the engine is mounted longitudinally in t

67、he front and the driving axle is</p><p>  the rear axle. If the gearbox is mounted on the rear axle, in order to improve the</p><p>  weight distribution, the ?nal drive could be included in the

68、 gearbox.9.3 Manual automobile gearboxes 431</p><p>  By multi-stage transmissions, some gear wheels could be used for di?erent</p><p>  speeds. The number of gearing wheels could increase at so

69、me speeds; this nor-</p><p>  mally occurs at low speeds, because the less frequent use of these speeds reduces</p><p>  the penalty of lower mechanical e?ciency on fuel consumption.</p>

70、<p>  Cost and weight increases are justi?ed by transmission length reduction,</p><p>  sometimes necessary on transversal engines with large displacement and more</p><p>  than four cylind

71、ers.</p><p>  In all these gearboxes synchronizers are coupled to adjacent speeds (e.g.:</p><p>  ?rst with second, third with fourth, etc.) in order to reduce overall length and</p><

72、p>  to shift the two gears with the same selector rod.</p><p>  We de?ne as the selection plane of a shift stick (almost parallel to the xz</p><p>  coordinate body reference system plane for

73、 shift lever on vehicle ?oor) the plane</p><p>  on which the lever knob must move in order to select two close speed pairs. For</p><p>  instance, for a manual gearbox following many existing s

74、chemes, ?rst, second,</p><p>  third, fourth and ?fth speed are organized on three di?erent selection planes; the</p><p>  reverse speed can have a dedicated plane or share its plane with the ?f

75、th speed.</p><p>  Figure 9.5 shows a typical example of a ?ve speed single stage gearbox. The</p><p>  ?rst speed wheels are close to a bearing, in order to limit shaft de?ection.</p>&l

76、t;p>  In this gearbox the total number of tooth wheels pairs is the same as for</p><p>  the double stage transmission shown in Fig. 9.6.</p><p>  While in the ?rst gearbox there are only two

77、 gearing wheels for each speed,</p><p>  in the second there are three gearing wheels for the ?rst four speeds and none</p><p>  FIGURE 9.5. Scheme for a ?ve speed single stage transmission, sui

78、table for front wheel</p><p>  drive with transversal engine.432 9. MANUAL GEARBOXES</p><p>  FIGURE 9.6. Scheme of an on-line double stage gearbox for a conventional lay-out.</p><p&g

79、t;  for the ?fth. This property is produced by the presence of the so called constant</p><p>  gear wheels (the ?rst gear pair at the left) that move the input wheels of the</p><p>  ?rst four s

80、peeds; the ?fth speed is a direct drive because the two parts of the</p><p>  upper shaft are joined together.</p><p>  The single stage gearbox in Fig. 9.5 shows the ?fth speed wheel pair posi-

81、</p><p>  tioned beyond the bearing, witness to the upgrading of an existing four speed</p><p>  transmission; in this case the ?fth speed has a dedicated selection plane.</p><p>  

82、The double stage gearbox in Fig. 9.7 is organized in a completely di?erent</p><p>  way but also shows the ?rst speed pair of wheels close to the bearing. The direct</p><p>  drive is dedicated

83、to the highest speed; the ?fth speed shows a dedicated selection</p><p><b>  plane.</b></p><p>  Six speed double stage gearboxes do not show conceptual changes in com-</p>&l

84、t;p>  parison with the previous examples; synchronizers are organized to leave ?rst</p><p>  and second, third and fourth, ?fth and sixth speeds on the same selection plane.</p><p>  As alrea

85、dy seen, the multistage con?guration shown in Fig. 9.7 allows a</p><p>  reasonable reduction of the length of the gearbox. In this scheme, only ?rst and</p><p>  second speeds bene?t from the s

86、econd countershaft; power enters the counter-</p><p>  shaft through a constant gear pair of wheels and ?ows to the output shaft at a</p><p>  reduced speed. Third, fourth and ?fth speed have a

87、single stage arrangement.</p><p>  Reverse speed is obtained with a conventional idling wheel.</p><p>  9.3.2 Practical examples</p><p>  Four speed gearboxes represented the most w

88、idely distributed solution in Europe</p><p>  until the 1970s, with some economy cars having only three speeds.9.3 Manual automobile gearboxes 433</p><p>  FIGURE 9.7. Scheme of a triple stage ?

89、ve speed gearbox, suitable for front wheel driven</p><p>  car with transversal engine.</p><p>  With the increase in installed power, the improvement in aerodynamic per-</p><p>  f

90、ormance and increasing attention to fuel consumption, it became necessary to</p><p>  increase the transmission ratio of the last speed, having the ?rst speed remain at</p><p>  the same values;

91、 as a matter of fact car weight continued to increase and engine</p><p>  minimum speed did not change signi?cantly.</p><p>  To achieve satisfactory performance all manufacturers developed ?ve

92、speed</p><p>  gearboxes; this solution is now standard, but many examples of six speed gear-</p><p>  boxes are available on the market, not limited to sports cars.</p><p>  Figure

93、 9.8 shows an example of a six speed double stage transmission with</p><p>  the ?fth in direct drive; here the ?rst and second pair of wheels are close to the</p><p><b>  bearing.</b&g

94、t;</p><p>  This rule is not generally accepted; on one hand having the most stressed</p><p>  pairs of wheels close to the bearing allows a shaft weight containment. On the</p><p>

95、  other hand, having the most frequently used pairs of wheels close to the bearing</p><p>  reduces the noise due to shaft de?ection.</p><p>  Synchronizers of fourth and third speed are mounted

96、 on the countershaft;</p><p>  this lay-out reduces the work of synchronization, improving shifting quality by an</p><p>  amount proportional to the dimension of the synchronizing rings. Synchr

97、onizers</p><p>  of ?rst and second gear on the output shaft are, because of their diameter, larger434 9. MANUAL GEARBOXES</p><p>  FIGURE 9.8. Double stage six speed gearbox (GETRAG).</p>

98、<p>  than those of the corresponding gear; the penalty of the synchronization work is</p><p>  paid by the adoption of a double ring synchronizer.</p><p>  Synchronizers on the countersh

99、aft o?er a further advantage: In idle position</p><p>  the gears are stopped and produce no rattle; this subject will be studied later on.9.3 Manual automobile gearboxes 435</p><p>  Figure 9.9

100、 introduces the example of a single stage gearbox for a front</p><p>  longitudinal engine. The input upper shaft must jump over the di?erential, which</p><p>  is set between the engine and the

101、 wheels. The increased length of the shafts</p><p>  suggested adopting a hollow section. Because of this length the box is divided</p><p>  into two sections; on the joint between the two secti

102、ons of the box additional</p><p>  bearings are provided to reduce the shaft de?ection.</p><p>  The input shaft features a ball bearing close to the engine and three other</p><p> 

103、 needle bearings that manage solely the radial loads. The output shaft has two</p><p>  tapered roller bearings on the di?erential side and a roller bearing on the opposite</p><p>  side. This c

104、hoice is justi?ed by the relevant axial thrust emerging from the bevel</p><p><b>  gears.</b></p><p>  The ?rst and second speed synchronizers are on the output shaft and feature<

105、/p><p>  a double ring.</p><p>  The reverse speed gears are placed immediately after the joint (the idler</p><p>  gear is not visible) and have a synchronized shift. Remaining synchr

106、onizers are</p><p>  set in the second section of the box on the input shaft. The output shaft ends</p><p>  with the bevel pinion, a part of the ?nal ratio.</p><p>  It should be n

107、oted that the gears of the ?rst, second and reverse speeds are</p><p>  directly cut on the input shaft, in order to reduce overall dimensions.</p><p>  Most contemporary cars use a front wheel

108、drive with transversal engine; the</p><p>  number of gearboxes with integral helical ?nal ratio is, therefore, dominant.</p><p>  In these gearboxes geared pairs are mounted from the ?rst to th

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