Mechanical transmission

5.2.1 Elements of manual transmission

Let’s consider in more detail the elements of the mechanical transmission of a car with a classic layout (if you have forgotten what a layout is, then you should return to Chapter 3 "Components of the Car and Layout Diagrams"). Thus, the transmission with this arrangement of the units has the following elements (Figure 5.1):

• clutch;
• gearbox;
• drive shaft with hinged joints (in this case – cardan shaft);
• drive gear with differential;
• half shafts.

Note
All the elements in the following image are shown schematically. Taking into account the fact that the differential and the axle shafts are located inside the drive gear housing and in the axle cases, respectively, only the drive gear is shown in Figure 5.1.

Figure 5.1 Engine and transmission components.

Note
In case if the car is front-wheel drive, then the thrust from the engine is transmitted to the front wheels through two separate - right and left - drive shafts, that are joined hinged with the main gear and wheel hobs (the hobs are described in the chapter "Chassis"). This will be discussed in the section "Hinges of Equal Angular Velocities". The same shafts and hinged joints are installed on cars with independent suspension of the rear drive wheels, but more on that later. It should be mentioned that for all-wheel drive vehicles - those in which the thrust from the engine is transmitted to all wheels - a transfer case is also installed in the transmission. We will talk about this below.

5.2.2 Clutch assignment

The clutch is used to temporarily disconnect the transmission from the engine, as well as to smoothly connect them. Disconnection is necessary when stopping and braking the vehicle and when shifting gears. Smooth connection is needed when starting the car from a place and after shifting gears.

5.2.3 Clutch device

Clutch is the initial and one of the most important elements of the entire manual transmission. This is a coupling, which is designed for connecting and disconnecting two separate shafts. The appearance of a regular clutch is shown in Figure 5.2.

Figure 5.2 Clutch.

The clutch consists of two discs (shown in Figures 5.3 and 5.4): one of them is pressure disk (driving), the second is driven disk (friction). The pressure disk is installed in the casing on the flywheel, which, in turn is attached to the engine crankshaft. The driven disc is installed on the splines of the input shaft of the gearbox and has the ability to move along the splines of this shaft.

5.2.4 Clutch operation

The simplified diagram (Figure 5.3) shows that one clutch disc is connected through the casing and the flywheel to the crankshaft, and the second one is installed on the input shaft of the gearbox. This figure (5.3) also shows the pressure disk, which has the ability to move, as it is attached to the diaphragm spring. This spring, if viewed from the side, has the form of a truncated cone with slots along the entire circumference (the spring is clearly visible in Figure 5.2). These slots form blades, upon pressing which the entire "truncated cone" will bend in the opposite direction. So it turns out that the release bearing presses on the ends of the blades, the spring bends. And since the clutch pressure plate is connected to its outer circumference, it moves and releases the driven disc. At this moment, both discs rotate separately from each other, the crankshaft rotates independently of the gearbox shaft. This means that traction is not transmitted to the wheels - the clutch is disengaged. We release the clutch pedal smoothly - the release bearing moves away from the diaphragm spring The spring takes its original position, the pressure plate tightly presses the driven disk to the flywheel, thereby connecting the crankshaft with the gearbox shaft - the clutch is engaged.

Figure 5.3 Simplified layout of clutch design.

Interesting 
In case if the clutch discs do their job with dry rubbing surfaces, such a clutch is called dry. Conversely, if the clutch workflow takes place in a fluid bath, then the clutch is called wet. The clutch can be single-disc, double-disc and multi-disc. In this case, the definition depends on the number of driven discs in the clutch. The clutch release drive can be cable or hydraulic (in cars) and hydro-pneumatic (in trucks). Also, the clutch can be with mechanical, semi-automatic (automated) and automatic shutdown drive.

Figure 5.4 Arrangement of one-disk dry clutch.

The clutch is engaged smoothly in case of a smooth release of the clutch pedal at the beginning of the movement. That is, the pressure disk gradually presses the driven disc against the flywheel (the driven disc slips a little). The number of revolutions of the crankshaft differs significantly from the number of revolutions of the input shaft of the gearbox, but traction is already being transmitted. The car starts moving, and you continue to release the clutch pedal. The driven disk is pressed more and more tightly against the flywheel, the revolutions of the crankshaft and the gearbox shaft are equalized. When the clutch pedal is fully released, the clutch kind of unites the two shafts, and the thrust from the engine is fully transmitted to the gearbox, and from it to the wheels.

Good to know
In case if you do not fully release the clutch pedal for a long time, the driven disc will begin to burn out, as it were, because its working surface is special non-metallic friction linings that tend to wear out. When the clutch disc burns out, the passenger compartment is filled with a characteristic, rather unpleasant, smell.

5.2.5 Mechanical clutch release drive

In case if the drive is mechanical, then everything is extremely simple (in confirmation of this figure 5.5): a cable is attached to the clutch pedal, the other end of which is connected to the lever. The lever, in turn, acts on the release bearing, and then the process proceeds as described above.

Figure 5.5 Clutch release shift cable.

5.2.6 Hydraulic clutch release

In case if the drive is hydraulic, then everything is a little more complicated (but not as much as it might seem when looking at figure 5.6). There is a small cylinder with a piston (called the "clutch release hydraulic transmission master cylinder"). This cylinder is connected through a rod to the clutch pedal on one side, and almost the same cylinder with a piston (it is called "clutch release hydraulic transmission slave cylinder"), which is connected through a rod to a release fork, which has the ability to act on the release bearing. Nowadays, the more common design is when the release bearing and the clutch release cylinder are combined into one module. Everything is good from the point of view of reliability. But regarding the maintenance of the system, everything is much sadder if compared to the classic layout of hydraulic transmission, which is described above.

Figure 5.6 Clutch release hydraulic drive.

5.2.7 Adjustment of the clutch release drive

Whether it is a hydraulic or mechanical drive for disengaging the clutch, the designers foresee the possibility of its adjustment, because either after disassembling the transmission elements, or due to excessive wear of the clutch disc, adjustment may be necessary.

Good to know
The later the clutch starts "grabbing", the more worn out its driven disc would be (unless, of course, any disassembly and assembly work of the gearbox was carried out). The point is that the higher the clutch pedal is (when it is slowly released) when starting off the car, the more wear the clutch driven disc has. In case if you squeezed the clutch pedal, shifted into gear without any problems, started smoothly releasing the pedal and almost immediately the car started moving, then everything is in order. In case if the gears are switched on normally, but the car cannot really drive onto a small hill, the engine speed sometimes goes off scale, and the car drives at an inappropriate speed. In this case passenger compartment is filled with a caustic characteristic smell of burnt friction linings, which means that the clutch is "slipping". It is time to replace it (or it is necessary to adjust the clutch release drive). In case if the gears, on the contrary, are shifted very tightly, sometimes with a bump (if it is not a GAZ-53, of course), or do not turn on at all, the reason, again, may lie in the clutch. It simply does not turn off completely, that is, the discs do not separate and the crankshaft is practically not completely disconnected from the gearbox shaft. In this case one says that the clutch is “slipping”. Most likely, the clutch release drive is to blame, which must be adjusted as soon as possible, otherwise the gearbox may fail.

5.2.8 Gearbox

The gearbox (GB) is a very simple mechanism, which consists of a body (and it, in turn, of several parts) and shafts with gears inside. The thrust from the engine is supplied on one side of the gearbox, and on the other side, the already increased thrust is transmitted further to the main gear and wheels.

5.2.9 Gearbox functions

The gearbox (GB) is a very simple mechanism, which consists of a body (and it, in turn, of several parts) and shafts with gears inside. The thrust from the engine is supplied on one side of the gearbox, and on the other side, the already increased thrust is transmitted further to the main gear and wheels.

The gearbox also serves to change the tractive effort on the wheels of the car and to obtain reverse gear, as well as for a long-term separation of the engine thrust from the transmission to the drive wheels. This disconnection is necessary in order not to keep the clutch release pedal constantly depressed.

Flexibility is one of the functions of the gearbox. Its essence is as follows: the tractive effort on the wheels, necessary to overcome all the resistances that arise when the car is moving, must change depending on the operating conditions of the car.

In case if the car is moving on a flat road at a low speed, then the traction, which is required to overcome air resistance and losses due to rotation of wheels and all mechanisms, will be small. In order to obtain this tractive effort, only a small fraction of the power that the engine can deliver is needed. All excess engine power is accumulated until the driver needs to accelerate the car in order to obtain a higher speed of its movement. But when the car is driving on a bad road or uphill, the resistance to movement increases significantly. In order to overcome these resistances, the tractive effort on the driving wheels must be increased accordingly. Also, when the car starts moving, the tractive effort on its wheels should be especially large, since this requires, in addition to the resistance of all mechanisms, to overcome the inertia of the car, which is associated with its mass.

Note
At a certain engine power, its torque and the number of revolutions of the crankshaft, various tractive forces on the driving wheels of the car can be obtained by changing the ratio of the number of revolutions of the engine crankshaft and driving wheels. When the number of wheel revolutions decreases if compared to the number of revolutions of the crankshaft, the tractive effort on the wheels increases, but the speed of the vehicle decreases (recall the example with a bicycle). With an increase in the number of revolutions of the wheels, the tractive effort on them decreases, and the vehicle speed can be increased.

5.2.10 Gearbox device

The change in the ratio between the number of revolutions of the crankshaft of the engine and the driving wheels and the change as a result of this tractive effort on the wheels is carried out using toothed gear wheels (gears), of which the gearbox consists. An example of a gear pair, which consists of a drive and driven gear, is shown in Figure 5.7.

Figure 5.7 Gear set (gear pair)

When the small drive gear rotates (Figure 5.7), the large driven gear, which is meshed with it, will rotate more slowly as many times as the number of its teeth. In this case, the torque on the axis of the driven gear will increase by the same amount.

The ratio of the number of teeth (or diameters) of the driven gear to the number of teeth of the driving gear is called the ratio of number of teeth (or gear ratio). The greater the gear ratio of the gear pair,  the more significantly the number of revolutions of the gear shafts and the torque on them change. The action of gearboxes is based on changing gear ratios by engaging gears with different numbers of teeth.

Enabling the car to go in reverse is another of the important functions of the gearbox. This function is based on the inclusion of an intermediate gear between the driving and driven gears - the so-called "idle gear".

Thus, when transferring force in the box without turning on the parasitic (intermediate) gear, the drive shaft (Figure 5.8) and the driven shaft rotate in the same direction. When the idle gear is turned on, the driven shaft starts rotating in the opposite direction and the car moves back.

Interesting
Gear ratios are never integers. Rather, when calculating them, one tries to choose so that they are not integers. For clarity, it is worth providing an example: there are two gears, we take a marker and put two marks - one opposite the other - on both gears. In case if the gear ratio between these gears is, for example, three, then after exactly three revolutions the marks applied to the gears will meet again. This will reset the teeth to their original position every three turns. This will lead to the fact that one pair of teeth will be three times more loaded than the others and, as a result, will wear out faster or break altogether. That is why, when selecting gear ratios, manufacturers try to make them not integer.

Figure 5.8 Simplified layout of single-stage gearbox.

5.2.11 Types of gearboxes

Nowadays, five- and six-speed manual transmissions are used in passenger cars. This indicates that the gearbox has five or six forward gears and one reverse gear.

Structurally, two- and three-shaft gearboxes are distinguished.

Gears with helical teeth are used in the gearbox (shown in Figure 5.9) of passenger cars that are intended for movement on public roads.

This gear coupling is used for reducing the size of the gear, improving the smoothness and quiet operation, and also for reducing wear in gearboxes.

Straight tooth gears (Figure 5.10) are used in sports cars (to be more precise - rally cars), where noiselessness is the last indicator in the list of requirements.

Special mechanisms, the so-called speed synchronizers, are used for facilitating gear shifting in all two-shaft and three-shaft manual gearboxes.

Note
Figure 5.11 shows one of the options for the gear shifting speed synchronizer.

5.2.12 Twin shaft gearboxes

Note
Gearboxes of this design are often used on vehicles with front wheel drive or front wheel drive with a connected rear axle.

This design has a drive shaft and a driven shaft. The drive shaft is connected to the crankshaft via a clutch. The gears on the drive shaft are rigidly mounted and mesh with the gears on the driven shaft, the rotation from which, in turn, is transmitted to the drive gear and differential, and from there to the wheels.

Note
Figure 5.12 shows an example of a two-shaft gearbox with an indication of the thrust path from the engine to the drive gear and wheels, depending on the gear currently engaged.

Figure 5.12 Example of two-shaft gearbox. Power flow when engaging different gears.

The advantage of this construction arrangement is its compactness (to confirm the words, Figure 5.13), as well as ease of maintenance and repair. The gearshift drive in this case is carried out by rods (outdated drive method) or cables, which can be easily adjusted.

Figure 5.13 Sectional view of manual two-shaft gearbox

5.2.14 Three-shaft gearbox with two output shafts

Manufacturers do various design tricks in order to make the gearbox as compact as possible. One of these is the use of two output shafts in the design. Both driven shafts are driven from one input shaft and further transmit the thrust through the gears to the drive gear (an example is shown in Figure 5.15). All gears are synchronized (that is, speed synchronizers are used for engaging gears). Durability and compactness are the main advantages of this gearbox layout. The thing is that the load, which in a two-shaft gearbox falls on one output shaft, in a three-shaft gearbox is distributed over two shafts.

Figure 5.15 Example of three-gear gearbox with two output shafts.

5.2.15 Sequential boxes

Sequential gearboxes (an example is shown in Figure 5.16) are also worth mentioning. In these gearboxes gears may be switched exclusively sequentially and alternately. Such a gearbox differs from a classic type gearbox in the principle of operation of the gearshift mechanism. It is extremely useful when you need to reach the shift lever with your foot (for example, on a motorcycle) or when you need to change gears as quickly as possible (for example, in a sports car).

The gear shifting mechanism can be either of direct action, for example, on motorcycles, or with a servo drive (automatic or non-automatic).

Figure 5.16 Sequential gearbox.

5.2.16 The purpose of the transfer case

In case if in a passenger car the tractive effort from the engine must be transferred not to one, but to both axles, that is, to make it all-wheel drive, then the question arises – how to do this? The solution to this task is the installation of an intermediate mechanism - a transfer case. This mechanism is engaged in distributing tractive effort from the engine to the front and rear axles. The transfer case is installed right after the gearbox. Figure 5.17 shows an example of its device.

Transfer cases can be single-stage or with a reduction row.

Figure 5.17 Example of transfer case.

Note
The output shaft of the gearbox is connected to the input shaft of the transfer case. The latter has two output shafts: one to the rear axle, the other to the front. In case if it is possible to make the number of revolutions of the output shafts of the transfer case less than the number of revolutions of the output shaft of the gearbox, then one says that there is a reduction row in the transfer case.

There are transfer cases with the ability to completely disable the front or rear axle.

The transfer case can be controlled directly by the drive lever or by means of servos that are controlled by small switches on the center console or dashboard.