MANUAL GEARBOXES9.1 MANUAL GEARBOX CLASSIFICATIONGearboxes are normally classied according to the number of toothed wheelcouples (stages) involved in the transmission of motion at a given speed; in thecase of manual vehicle transmissions, the number to be taken into account isthat of the forward speeds only, without consideration of the nal gear, even ifincluded in the gearbox.Therefore there are: Single stage gearboxes Dual stage or countershaft gearboxes Multi stage gearboxesFigure 9.1 shows the three congurations for a four speed gearbox.It is useful to comment on the generally adopted rules of these schemes.Each wheel is represented by a segment whose length is proportional to the pitchdiameter of the gear; the segment is ended by horizontal strokes, representingthe tooth width. If the segment is interrupted where crossing the shaft, thegear wheel is idle; the opposite occurs if the segment crosses the line of theshaft without interruption. Then the wheel rotates with the shaft. Hubs arerepresented according to the same rules, while sleeves are represented with apair of horizontal strokes. Arrows show the input and output shafts.Single stage gearboxes are primarily applied to front wheel driven vehicles,because in these it is useful that the input and the output shaft are oset; inG. Genta and L. Morello, The Automotive Chassis, Volume 1: Components Design, 425Mechanical Engineering Series,c Springer Science+Business Media B.V. 2009426 9. MANUAL GEARBOXESFIGURE 9.1. Schemes for a four speed gearbox shown in three dierent congurations:a: single stage, b: double stage and c: triple stage.conventional vehicles, on the other hand, it is better that input and output shaftsare aligned.This is why rear wheel driven vehicles usually adopt a double stage gearbox.The multi-stage conguration is sometime adopted on front wheel drivenvehicles with transversal engine, because the transversal length of the gearboxcan be shortened; it is used when the number of speeds or the width of the gearsdo not allow a single stage transmission to be used.It should be noted that on a front wheel driven vehicle with transversalengine, having decided on the value of the front track and the size of the tire,the length of the gearbox has a direct impact on the maximum steering angle ofthe wheel and therefore on the minimum turning radius.The positive result on the transversal dimension of multi-stage gearboxes isoset by higher mechanical losses, due to the increased number of engaged gearwheels.It should be noted that in triple stage gearboxes, shown in the picture, theaxes of the three shafts do not lie in the same plane, as the scheme seems toshow. In a lateral view, the outline of the three shafts should be represented asthe vertices of a triangle; this lay-out reduces the transversal dimension of thegearbox. In this case and others, as we will show later, the drawing is representedby turning the plane of the input shaft and of the counter shaft on the plane ofthe counter shaft and of the output shaft.Gear trains used in reverse speed are classied separately. The inversion ofspeed is achieved by using an additional gear. As a matter of fact, in a train ofthree gears, the output speed has the same direction as the input speed, whilethe other trains of two gears only have an output speed in the opposite direction;the added gear is usually called idler.The main congurations are reported in Fig. 9.2.In scheme a, an added countershaft shows a sliding idler, which can matchtwo close gears that are not in contact, as, for example, the input gear of therst speed and the output gear of the second speed. It should be noted that, inthis scheme, the drawing does not preserve the actual dimension of the parts.9.1 Manual gearbox classication 427FIGURE 9.2. Schemes used for reverse speed; such schemes t every type of gearboxlay-out.Scheme b shows instead two sliding idlers, rotating together; this arrange-ment oers additional freedom in obtaining a given transmission ratio. The coun-tershaft is oset from the drawing plane; arrows show the gear wheels that matchwhen the reverse speed is engaged.Scheme c is similar to a in relation to the idler; it pairs an added specicwheel on the output shaft with a gear wheel cut on the shifting sleeve of the rstand second speed, when it is in idle position.Conguration d shows a dedicated pair of gears, with a xed idler and ashifting sleeve.The following are the advantages and disadvantages of the congurationsshown in the gure. Schemes a, b and c are simpler, but preclude the application of synchro-nizers (because couples are not always engaged), nor do they allow the useof helical gears (because wheels must be shifted by sliding). Scheme d is more complex but can include a synchronizer and can adopthelical gears. Schemes a, b and c