汽车手动变速箱外文及其部分翻译

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 do not increase gearbox length.428 9. MANUAL GEARBOXES9.2 MECHANICAL EFFICIENCYThe mechanical eciency of an automotive gear wheel transmission is high com-pared to other mechanisms performing the same function; indeed, the value ofthis eciency should not be neglected when calculating dynamic performanceand fuel consumption. The continuous eort of to limit fuel consumption justi-es the care of transmission designers in reducing mechanical losses.Total transmission losses are conveyed up by terms that are both dependentand independent of the processed power; the primary terms are: Gearing losses; these are generated by friction between engaging teeth(power dependent) and by the friction of wheels rotating in air and oil(power independent). Bearing losses; these are generated by the extension of the contact area ofrolling bodies and by their deformation (partly dependent on and partlyindependent of power) and by their rotation in the air and oil (powerindependent). Sealing losses; they are generated by friction between seals and rotatingshafts and are power independent. Lubrication losses; these are generated by the lubrication pump, if present,and are power independent.All these losses depend on the rotational speed of parts in contact and,therefore, on engine speed and selected transmission ratio.Table 9.1 reports the values of mechanical eciency to be adopted in calcu-lations considering wide open throttle conditions; these values consider a pair ofgearing wheels or a complete transmission with splash lubrication; in the sametable we can see also the eciency of a complete powershift epicycloidal auto-matic transmission and a steel belt continuously variable transmission. For thetwo last transmissions, the torque converter must be considered as locked-up.TABLE 9.1. Mechanical eciency of dierent transmission mechanisms.Mechanism type Eciency (%)Complete manual gearboxwith splash lubrication 9297Complete automatic transmission(ep. gears) 9095Complete automatic gearbox(steel belt; without press. contr.) 7080Complete automatic gearbox(steel belt; with press. contr.) 8086Pair of cyl. gears 99.099.5Pair of bevel gears 90939.2 Mechanical eciency 429FIGURE 9.3. Contributions to total friction loss of a single stage gearbox designed for300 Nm as function of input speed.It is more correct to reference power loss measurement as a function ofrotational input speed rather than eciency. Figure 9.3 shows the example ofa double stage transmission, in fourth speed, at maximum power; the dierentcontributions to the total are shown.This kind of measurement is made by disassembling the gearbox step bystep, thus eliminating the related loss.In the rst step all synchronizer rings are removed, leaving the synchronizerhubs only; mechanical losses of non-engaged synchronizers are, therefore, mea-surable. The loss is due to the relative speed of non-engaged lubricated conicalsurfaces; the value of this loss depends, obviously, on speed and the selectedtransmission ratio.In the second step all rotating seals are removed.In the third step the lubrication oil is removed, and therefore, the bulk ofthe lubrication losses is eliminated; some oil must remain in order to leave thecontact between teeth unaected.By removing those gear wheels not involved in power transmission, theirmechanical losses are now measurable.The rest of the loss is due to bearings; the previous removal of parts canaect this value.A more exhaustive approach consists in measuring the complete eciencymap; the eciency can be represented as the third coordinate of a surface, wherethe other two coordinates are input speed and engine torque. Eciency calcu-lations can be made by comparing input and output torque of a working trans-mission.Such map can show how eciency reaches an almost constant value at amodest value of the input torque; it must not be forgotten that standard fuelconsumption evaluation cycles involve quite modest values of torque and there-fore imply values of transmission eciency that are changing with torque.Figure 9.4 shows a qualitative cross section of the aforesaid map, cut atconstant engine speed. It should be noted that eciency is also zero at input430 9. MANUAL GEARBOXESFIGURE 9.4. Mechanical eciency map, as a function of input torque at constantengine speed; the dotted line represents a reasonable approximation of this curve, to beused on mathematical models for the prediction of performance and fuel consumption.torque values slightly greater than zero; as a matter of fact, friction implies acertain minimum value of input torque, below which motion is impossible.A good approximation to represent mechanical eciency can be made usingthe dotted broken line as an interpolation of the real curve.9.3 MANUAL AUTOMOBILE GEARBOXES9.3.1 Adopted schemesIn manual gearboxes, changing speed and engaging and disengaging the clutchare performed by driver force only.This kind of gearbox is made with helical gears and each speed has a syn-chronizer; some gearboxes do not use show the synchronizer for reverse speed,particularly those in economy minicars.We previously discussed a rst classication; additional information is thespeed number, usually between four and six.Single stage gearboxes are used in trans-axles; they are applied, with someexceptions, to front wheel driven cars with front engine and rear driven cars withrear engine; this is true with longitudinal and transversal engines.In all these situations the nal drive is included in the gearbox, which istherefore also called transmission.Countershaft double stage gearboxes are used in conventionally driven cars,where the engine is mounted longitudinally in the front and the driving axle isthe rear axle. If the gearbox is mounted on the rear axle, in order to improve theweight distribution, the nal drive could be included in the gearbox.9.3 Manual automobile gearboxes 431By multi-stage transmissions, some gear wheels could be used for dierentspeeds. The number of gearing wheels could increase at some speeds; this nor-mally occurs at low speeds, because the less frequent use of these speeds reducesthe penalty of lower mechanical eciency on fuel consumption.Cost and weight increases are justied by transmission length reduction,sometimes necessary on transversal engines with large displacement and morethan four cylinders.In all these gearboxes synchronizers are coupled to adjacent speeds (e.g.:rst with second, third with fourth, etc.) in order to reduce overall length andto shift the two gears with the same selector rod.We dene as the selection plane of a shift stick (almost parallel to the xzcoordinate body reference system plane for shift lever on vehicle oor) the planeon which the lever knob must move in order to select two close speed pairs. Forinstance, for a manual gearbox following many existing schemes, rst, second,third, fourth and fth speed are organized on three dierent selection planes; thereverse speed can have a dedicated plane or share its plane with the fth speed.Figure 9.5 shows a typical example of a ve speed single stage gearbox. Therst speed wheels are close to a bearing, in order to limit shaft deection.In this gearbox the total number of tooth wheels pairs is the same as forthe double stage transmission shown in Fig. 9.6.While in the rst gearbox there are only two gearing wheels for each speed,in the second there are three gearing wheels for the rst four speeds and noneFIGURE 9.5. Scheme for a ve speed single stage transmission, suitable for front wheeldrive with transversal engine.432 9. MANUAL GEARBOXESFIGURE 9.6. Scheme of an on-line double stage gearbox for a conventional lay-out.for the fth. This property is produced by the presence of the so called constantgear wheels (the rst gear pair at the left) that move the input wheels of therst four speeds; the fth speed is a direct drive because the two parts of theupper shaft are joined together.The single stage gearbox in Fig. 9.5 shows the fth speed wheel pair posi-tioned beyond the bearing, witness to the upgrading of an existing four speedtransmission; in this case the fth speed has a dedicated selection plane.The double stage gearbox in Fig. 9.7 is organized in a completely dierentway but also shows the rst speed pair of wheels close to the bearing. The directdrive is dedicated to the highest speed; the fth speed shows a dedicated selectionplane.Six speed double stage gearboxes do not show conceptual changes in com-parison with the previous examples; synchronizers are organized to leave rstand second, third and fourth, fth and sixth speeds on the same selection plane.As already seen, the multistage conguration shown in Fig. 9.7 allows areasonable reduction of the length of the gearbox. In this scheme, only rst andsecond speeds benet from the second countershaft; power enters the counter-shaft through a constant gear pair of wheels and ows to the output shaft at areduced speed. Third, fourth and fth speed have a single stage arrangement.Reverse speed is obtained with a conventional idling wheel.9.3.2 Practical examplesFour speed gearboxes represented the most widely distributed solution in Europeuntil the 1970s, with some economy cars having only three speeds.9.3 Manual automobile gearboxes 433FIGURE 9.7. Scheme of a triple stage ve speed gearbox, suitable for front wheel drivencar with transversal engine.With the increase in installed power, the improvement in aerodynamic per-formance and increasing attention to fuel consumption, it became necessary toincrease the transmission ratio of the last speed, having the rst speed remain atthe same values; as a matter of fact car weight continued to increase and engineminimum speed did not change signicantly.To achieve satisfactory performance all manufacturers developed ve speedgearboxes; this solution is now standard, but many examples of six speed gear-boxes are available on the market, not limited to sports cars.Figure 9.8 shows an example of a six speed double stage transmission withthe fth in direct drive; here the rst and second pair of wheels are close to thebearing.This rule is not generally accepted; on one hand having the most stressedpairs of wheels close to the bearing allows a shaft weight containment. On theother hand, having the most frequently used pairs of wheels close to the bearingreduces the noise due to shaft deection.Synchronizers of fourth and third speed are mounted on the countershaft;this lay-out reduces the work of synchronization, improving shifting quality by anamount proportional to the dimension of the synchronizing rings. Synchronizersof rst and second gear on the output shaft are, because of their diameter, larger434 9. MANUAL GEARBOXESFIGURE 9.8. Double stage six speed gearbox (GETRAG).than those of the corresponding gear; the penalty of the synchronization work ispaid by the adoption of a double ring synchronizer.Synchronizers on the countershaft oer a further advantage: In idle positionthe gears are stopped and produce no rattle; this subject will be studied later on.9.3 Manual automobile gearboxes 435Figure 9.9 introduces the example of a single stage gearbox for a frontlongitudinal engine. The input upper shaft must jump over the dierential, whichis set between the engine and the wheels. The increased length of the shaftssuggested adopting a hollow section. Because of this length the box is dividedinto two sections; on the joint between the two sections of the box additionalbearings are provided to reduce the shaft deection.The input shaft features a ball bearing close to the engine and three otherneedle bearings that manage solely the radial loads. The output shaft has twotapered roller bearings on the dierential side and a roller bearing on the oppositeside. This choice is justied by the relevant axial thrust emerging from the bevelgears.The rst and second speed synchronizers are on the output shaft and featurea double ring.The reverse speed gears are placed immediately after the joint (the idlergear is not visible) and have a synchronized shift. Remaining synchronizers areset in the second section of the box on the input shaft. The output shaft endswith the bevel pinion, a part of the nal ratio.It should be noted that the gears of the rst, second and reverse speeds aredirectly cut on the input shaft, in order to reduce overall dimensions.Most contemporary cars use a front wheel drive with transversal engine; thenumber of gearboxes with integral helical nal ratio is, therefore, dominant.In these gearboxes geared pairs are mounted from the rst to the last speed,starting from the engine side. An example of this architecture is given in Fig. 9.10.Like many other transmissions created with only four speeds, it shows thefth speed segregated outside of the aluminium box and enclosed by a thin steelsheet cover; this placement is to limit the transverse dimension of the powertrain, in the area where there is potential interference with the left wheel in thecompletely steered position.This solution is questionable as far as the total length is concerned but showssome advantage i