Main gear and differential. Purpose, device and types

5.4.1 The purpose of the main gear

The main purpose of the drive gear in the transmission is to transfer engine thrust to, so to speak, the "end consumer" - the wheels. Drive gear distributes traction at an angle of 90 ° between the right and left wheels. In case if the car is rear-wheel drive, then the thrust from the gearbox through the cardan drive is transmitted to the drive gear, and that, in turn, redirects the power flow to the wheels via the axle shafts (if the rear suspension is dependent and has a bridge) or drive shafts with hinges of equal angular speeds (this will be described later). In case if the car is front-wheel drive, then the drive gear is connected directly to the gearbox via a gear, and the thrust from the engine is transmitted via drive shafts with hinges of equal angular speeds.

5.4.2 Continuous bridge

There is such a concept as a solid axle. This means that the drive gear together with the differential are located in the housing, to which initially two axle shafts are connected or cast together with it. Half shafts are the shafts that connect the differential of the final drive to the wheels.

This structure is part of the dependent suspension of the car, because it rigidly connects the right and left drive wheels. Half shaft rigidly connects the wheel and the drive gear. That is, while overcoming any obstacle, the entire axle moves along with the wheels and all contents. If we remove the casing of half shafts, install the drive gear housing on the body or a sub frame, connect the wheels with the drive gear with the help of drive shafts through the hinges of equal angular speeds - and we get a split axle and independent wheel suspension. All this is described in more detail below in the "Arrangement of Drive Gear" section.

5.4.3 Final drive function

Note
The drive gear is used for reducing the number of revolutions that are transmitted from the engine to the wheels and increasing the tractive effort. It provides the transmission of rotation from the cardan shaft to the semi-axle at an angle of 90 ° for the classic vehicle layout (which is described in detail in Chapter 3). Drive gear implements gear-wheel drive, single or double gears. Dual gears, in turn, can be center or spaced.

5.4.4 Final drive device

Примітка
Below is a description of the most common design on passenger cars - a single main bevel gear.

The drive gear consists of two gears, or rather, a drive gear and a driven wheel (shown in Figure 5.33).

Figure 5.33 Drive gear of rear solid axle.

5.4.4.a Gear

The gear is the drive element (traction from the gearbox and the engine is supplied to it), and the wheel is driven (takes thrust from the gear and redirects it at an angle of 90 °).

The gears are made in the form of a cone with spiral teeth, which increases their strength. This also increases the number of teeth simultaneously meshing, and the gears run more smoothly and quietly.

5.4.4.b Hypoid transmission

In addition to the bevel main gear transmission, in which the axes intersect mutually, a hypoid transmission is used in passenger cars (shown in Figure 5.34). In this transmission, the teeth have a special profile, and the axis of the drive gear is offset downward relative to the center of the driven wheel by a certain distance (S). This makes it possible to position the cardan shaft lower and reduce the height of the convex upper part of the tunnel in order to accommodate the shaft in the floor of the body, as a result of which a more convenient placement of passengers in the body is achieved. In addition, it is possible to slightly lower the center of gravity of the vehicle and increase its driving stability. Hypoid gear has smoother operation, higher tooth strength and wear resistance.

Note
However, the hypoid gear has one unpleasant feature: the jamming threshold during the back stroke. The calculations of this gear, of course, exclude such a possibility, but it is always worth remembering that this drive gear can jam when the design speed is exceeded (when rotating in the opposite direction). Thus, be careful when shifting to a reverse gear.

For hypoid transmission, it is necessary to use special grades of lubricant due to the high pressure between the teeth during operation and high speeds of relative sliding between the teeth. In addition, higher gear mounting accuracy is required.

Figure 5.34 Drive gear elements. Tapered and hypoid gears.

5.4.5 Differential assignment

The differential allows the right and left drive wheels to rotate at different speeds when turning the car and when driving on uneven roads.

When the car moves in a turn (as shown in Figure 5.35), the inner drive wheel travels a shorter distance than the outer one, and in order to ensure rolling without slipping, it must rotate slower than the outer wheel. In order for the wheels to rotate at different speeds, they are connected through drive shafts to a differential, which is rigidly connected to the driven wheel of the drive gear.

5.4.6 Differential working principle

The differential consists of (see Figure 5.35) half-axle gears [1], satellites [2], the spider pin [3] (which can be cruciform, if there are four of them) and the differential casing [4]. Semi-axial bevel gears are fixed on the inner ends of the half shafts [5], on the outer ends of which the drive wheels are attached. Satellites that are small bevel gears, fit loosely on the axis.

Figure 5.35 Differential flow chart.

When the car moves around a turn, the inner wheel travels a shorter distance and, due to traction, starts rotating more slowly. In this case, the satellites that are rotating, start rolling along the axle shaft gear of the inner wheel that has slowed down its rotation. As a result, the satellites start rotating about their axes, increasing the number of revolutions of the second axle shaft gear and the outer wheel, respectively.

Note
In case of availability of a differential between the number of wheel revolutions, there is a certain relationship in which the sum of the wheel revolutions is always equal to twice the number of revolutions of the differential box. That is, when the number of revolutions of one wheel decreases, the number of revolutions of the other wheel increases by the same amount. In case of a stationary differential box, if one of the wheels rotates, the other wheel will rotate in the opposite direction.

5.4.7 Differential negative impact

However, the operation of the differential and its result is positive only in case of movement on a dry road. Under certain conditions, the differential can adversely affect the movement of the car.

Thus, in case if one of the wheels hits a slippery surface (ice, mud) due to insufficient adhesion to the road, it starts slipping. In case of a significant deterioration in the adhesion of the slipping wheel to the road, the tractive effort on it becomes very low. In this case, the second wheel, which has sufficient adhesion to the road, stops, since due to the property of the differential to distribute the force (torque) between the wheels equally, the tractive effort on the second wheel also becomes very small and insufficient for the movement of the car. The skidding wheel rotates at twice the speed and the car stops completely.

5.4.8 Varieties of differentials

Differentials can be symmetrical and asymmetrical, as well as free or lockable.

Note
Depending on the installation location, differentials may be:
- inter-wheel;
- interaxle;
- inter-board.

Note
A differential, which distributes the thrust from the engine equally between the wheels or between the axles, is called symmetrical. In case if, however, the interaxle differential (divides the thrust from the engine in an all-wheel drive vehicle between the front and rear axles), it can be asymmetrical, that is, transfer less traction to one of the axles than to the other.

If the symmetrical distribution does not always provide benefit for the controllability or cross-country ability of the car, then this problem must be solved.

This is how locking differentials were invented. The blocking process can be left at the mercy of the driver with the removal of the control lever in the car, or transferred to the electronics and can be fully automatic or controlled by controllers in the car.

Also, as an option, a limited slip differential is often used. This kind of differential in case of more complicated road situations simply will not allow all traction to "go" to a wheel that has lost traction.