> Main gear
Transmission
Purpose and types of main gears.
The main gear serves to increase the torque and change its direction at right angles to the longitudinal axis of the vehicle. For this purpose, the main gear is made of bevel gears. Depending on the number of gears, the main gears are divided into single bevel, consisting of one pair of gears, and double, consisting of a pair of bevel and a pair of cylindrical gears. Single bevel gears, in turn, are divided into simple and hypoid gears.
1 - driving bevel gear, 2 - driven bevel gear,
3 - driving spur gear, 4 - driven spur gear.
Single conical simple transfers(Fig. a) are used mainly on passenger cars mobiles and trucks mobiles small and medium load capacity. In these gears, the drive bevel gear 1 is connected to the cardan gear, and the driven gear 2 is connected to the differential box and through the differential mechanism to the axle shafts. For most vehicles, single bevel gears have hypoid gears (Figure 6). Hypoid gears have a number of advantages compared to simple ones: they have the axle of the drive wheel located below the axle of the driven one, which allows lowering cardan transmission, lower the floor of the car body. This lowers the center of gravity and increases vehicle stability. In addition, the hypoid gear has a thickened shape of the base of the gear teeth, which significantly increases their load capacity and wear resistance. But this circumstance determines the use of a special oil (hypoid) for lubricating the gears, designed to work under conditions of transmission of large forces that occur in contact between the gear teeth.
Double main gears (Fig. C) are installed on cars heavy duty to increase the overall gear ratio of the transmission and increase the transmitted torque. In this case gear ratio main gear calculated as the product of the gear ratios of the bevel (1, 2) and cylindrical (3, 4) pairs.
main transmission device.
The double main gear of the ZIL-130 car is part of the drive mechanisms rear axle, which are placed in its beam 8. The drive shaft of the final drive is made in one piece with the drive bevel gear 1. It is mounted on tapered roller bearings in a cup mounted on the crankcase 9 of the main drive. Here in the crankcase mounted on roller tapered bearings the intermediate shaft with the drive spur gear 12. The driven bevel gear 2 is rigidly fixed on the shaft flange, which is engaged with the gear 1. The driven spur gear 5 is connected to the left 3 and right 6 differential cups, forming its box. Differential parts are installed in the box: crosspiece 4 with satellites 11 and side gears 10.
Drive rear axle mechanisms
When the main gear is in operation, the torque is transmitted from the driveline to the flange of the drive shaft and its gear 1, then to the driven bevel gear 2, the intermediate shaft and its gear 12, the driven spur gear 5 and through the differential parts on the axle shaft 7 connected to the wheel hubs of the vehicle .
The transmission in the design of the car provides a change and transmission of rotation from power plant on the drive wheels. This component includes a number of nodes, among which is the main gear of the car.
Purpose, design features
The main task of this element is to change the torque before applying it to the wheel drive. The gearbox does the same, but it has the ability to change gear ratios by engaging certain gears. Despite the presence of a gearbox in the design of the car, the torque at the exit from it is small, and the speed of rotation of the output shaft is high. If you transfer the rotation directly to the drive wheels, then the resulting load will “crush” the engine. In general, the car simply will not be able to budge.
The main gear of the car provides an increase in torque and a decrease in rotational speed. But unlike the gearbox, the gear ratio is fixed.
The location of the main gear on the example of a conventional manual transmission
This transmission on a passenger car is a conventional gear single-stage constant-mesh gearbox, consisting of two gears of different diameters. The drive gear is small in size and is connected to the output shaft of the gearbox, that is, rotation is fed to it. The driven gear is much larger in size and it supplies the resulting rotation to drive shafts wheels.
The gear ratio is the ratio of the number of gear teeth in the gearbox. For passenger cars, this parameter is in the range of 3.5-4.5, and for trucks it reaches 5-7.
The larger the gear ratio ( more quantity driven gear teeth relative to the drive gear), the higher the torque applied to the wheels. In this case, the traction force will be greater, but the maximum speed will be lower.
The gear ratio of the main gear is selected based on performance indicators power plant, as well as other transmission units.
The final drive is directly dependent on design features the car itself. This gearbox can be either a separate unit installed in its crankcase (rear-wheel drive models) or included in the gearbox design (front-wheel drive cars).
Main gear in a rear-wheel drive car
As for some all-wheel drive cars, then they can use different layout. If in such a car the location of the power plant is transverse, then the main gear of the front axle is included in the design of the gearbox, and the rear is located in a separate crankcase. In a vehicle with a longitudinal layout, the main gears on both axles are separated from the gearbox and transfer case.
In models with a separated main gear, this gearbox performs another task - it changes the angle of rotation direction by 90 degrees. That is, the output shaft of the gearbox and the drive shafts of the wheels have a perpendicular arrangement.
The location of the final drive of the front axle Audi
IN front wheel drive models, where the main gear is included in the design of the gearbox, these shafts have a parallel arrangement, since it is not necessary to change the direction angle.
In a number of trucks, two-stage gearboxes are used. It is noteworthy that their design may be different, but the most widespread is the so-called spaced layout, which uses one central gearbox and two wheel (onboard) ones. This design allows you to significantly increase the torque, and, accordingly, the traction on the wheels.
The peculiarity of the gearbox is that it evenly divides the rotation into both drive shaft. In rectilinear motion, this condition is normal. But when cornering, the wheels of one axle travel a different distance, so it is necessary to change the rotation speed of each of them. This is part of the task of the differential used in the design of the transmission (it is installed on the driven gear). As a result, the main gear supplies rotation to the drive shafts not directly, but through the differential.
Types and their applicability
The main characteristic of the main gears is the type of gears and the type of engagement of the teeth between them. The following types of gearboxes are used on cars:
- Cylindrical
- Conical
- hypoid
- Worm
Types of main gears
Cylindrical gears are used in main gears front wheel drive cars. No need to change the direction of rotation and allows the use of such a gearbox. The teeth on the gears are oblique or chevron.
The gear ratio for such gearboxes is in the range of 3.5-4.2. A larger gear ratio is not used, since for this it is necessary to increase the size of the gears, which is accompanied by an increase in the noise of the transmission.
Bevel, hypoid and worm gears are used where it is necessary not only to change the gear ratio, but also to change the direction of rotation.
Bevel gearboxes are usually used on trucks. Their peculiarity boils down to the fact that the axes of the gears intersect, that is, they are on the same level. In such gears, oblique or curved teeth are used. On passenger cars, this type of gearbox is not used due to significant overall dimensions and increased noise.
On rear-wheel drive cars, a different type is most often used - hypoid. Its peculiarity boils down to the fact that the gear axles are displaced. Due to the location of the drive gear below the axis of the driven one, it is possible to reduce the dimensions of the gearbox. This type of transmission is characterized increased stability to loads, as well as smoothness and noiselessness of work.
Worm gears are the least common and are practically not used in cars. The main reason for this is the complexity and high cost of manufacturing components.
Primary requirements. Modern tendencies
The main gears are put forward a lot of requirements, the main of which are:
- Reliability;
- Minimum need for maintenance;
- High efficiency rates;
- Smoothness and noiselessness;
- Minimum possible overall dimensions.
Naturally, there is no ideal option, so designers have to look for compromises when choosing the type of final drive.
It has not yet been possible to refuse to use the main gear in the transmission design, therefore, all developments are aimed at improving performance.
It is noteworthy that changing the operating parameters of the gearbox is one of the main types of transmission tuning. By installing gears with a modified gear ratio, you can significantly affect the dynamics of the car, maximum speed, fuel consumption, load on the gearbox and power unit.
Finally, it is worth mentioning the design features robotic checkpoints With double clutch, which also affects the main gear device. In such gearboxes, paired and unpaired gears are separated, so there are two outputs secondary shaft. And each of them transmits rotation to its main drive gear. That is, in such gearboxes there are two drive gears, and only one driven gear.
Diagram of the DSG gearbox
This design feature allows you to make the gear ratio on the gearbox changeable. For this, only drive gears with a different number of teeth are used. For example, when using a number of unpaired gears to increase tractive effort a gear is used that provides a larger gear ratio, and a paired gear has a lower value of this parameter.
Material from the Encyclopedia of the magazine "Behind the wheel"
The main gear is a mechanism, part of the transmission of a car, which transmits torque from the gearbox to the driving wheels of the car. The main gear can be made in the form of a separate unit - the drive axle ( rear wheel drive vehicles classic layout), or combined with the engine, clutch and gearbox into a single power block(rear-engined and front-wheel drive vehicles).
According to the method of transmitting torque, the main gears are divided into jagged(gear) and chain. Chain final drives are currently only used on motorcycles and bicycles.
The chain final drive consists of two sprockets - a drive one mounted on the output shaft of the gearbox, and a driven one, combined with the hub of the driving (rear) wheel of the motorcycle. Somewhat more complicated in design is the main gear of a bicycle with a planetary gearbox. The driven sprocket, driven by the chain, drives the gears planetary box built into the wheel hub and through it - the driving rear wheel.
Sometimes, in motorcycles of a classic layout, a toothed reinforced belt is used in the main gear instead of a chain (for example, in the main gear of Harley-Davidson motorcycles). In this case, one usually speaks of a belt drive as a separate type of final drive.
Belted main the transmission is widely used in light motorcycles and in scooters (motor scooters) with a continuously variable variator. In this case, the variator serves as the final drive, since the driven pulley of the belt variator is integrated with the hub of the driving wheel of the motorcycle.
Classification of gear final drives
Double final drive
According to the number of pairs of engagement, the main gears are divided into single And double. Single main gears are installed on cars and trucks, contain one pair of bevel gears with constant meshing. Double main gears are installed on trucks, buses and heavy transport vehicles special purpose. In the double final drive, two pairs of gears are constantly engaged - bevel and cylindrical. double gear capable of transmitting more torque than a single one.
On three-axle trucks and multi-axle transport technology through main gears are used, in which the torque is transmitted not only to the middle drive axle, but also to the next, also drive. In the vast majority cars and two-axle trucks, buses, in other transport equipment with one driving axle, impassable main gears are used.
The most widely used single main gears by type of engagement are divided into:
- 1. Worm, in which the torque is transmitted by the worm to the worm wheel. Worm gears, in turn, are divided into gears with a lower and upper location of the worm. Worm final drives are sometimes used in multi-axle vehicles with through main gear (or with several through main gears) and in automobile auxiliary winches.
In worm gears, the driven gear wheel has the same type of device (always of large diameter, which depends on the gear ratio built into the design of the gearbox, and is always made with oblique teeth). And the worm can have a different design.
In shape, worms are divided into cylindrical and globoid. In the direction of the turn line - to the left and right. According to the number of grooves, the threads are divided into single-start and multi-start threads. According to the shape of the threaded groove - for worms with an Archimedean profile, with a convolute profile and an involute profile.
- 2. Cylindrical main gears, in which the torque is transmitted by a pair of cylindrical gears - helical, spur or chevron. Cylindrical main gears are installed in front-wheel drive vehicles with a transverse engine.
- 3. hypoid(or spiroid) final drives, in which the torque is transmitted by a pair of gears with oblique or curved teeth. Pair of gears hypoid gear either coaxial (less common), or the gear axes are offset relative to each other - with a lower or upper offset. Due to the complex shape of the teeth, the engagement area is increased, and the gear pair is capable of transmitting more torque than other types of final drive gears. Hypoid gears are installed in cars and trucks of classic (rear-wheel drive with front engine) and rear-engine layouts.
Double final drives according to the type of engagement are divided into:
- 1. Central one and two-stage. In two-stage final drives, gear pairs are switched to change the torque transmitted to the drive wheels. Such main gears are used on caterpillar and heavy transport vehicles for special purposes.
- 2. Spaced main gears with wheel or final drives. Such main gears are installed on Cars(jeeps) and trucks to increase ground clearance, on military wheeled transporters.
In addition, double main gears subdivided according to the type of engagement of pairs of gears into:
- 1. Conical-cylindrical.
- 2. Cylindrical-conical.
- 3. Conical planetary.
In automobiles, gear main gears are made in the form of a single unit with a differential - a mechanism for dividing torque between two wheels of the drive axle. In heavy motorcycles with cardan gear and rear wheel drive, the differential is not used. On motorcycles with a sidecar and all-wheel drive(on the rear wheel of the motorcycle and on the wheel of the sidecar) the differential is made as a separate mechanism. On such motorcycles, two independent main gears are installed, interconnected by a differential.
The principle of operation of the hypoid final drive
Torque is transmitted from the engine through the clutch, gearbox and driveshaft to the drive gear axle of the hypoid final drive. The axis of the drive gear is installed coaxially with the drive shaft of the engine and the driven shaft of the gearbox. During rotation, the drive gear, which has a smaller diameter than the driven gear, transmits torque to the teeth of the driven gear, causing it to rotate. Since the contact of the tooth surface is increased due to their special shape - oblique or curved - the transmitted torque can reach very high values. However, the complex shape of the teeth leads to the fact that their surface is affected not only by shock loads, but also by friction forces (due to slipping of the teeth relative to each other). Therefore, in hypoid main gears, special oil, which has high lubricating properties and provides long term gear pair service.
The principle of operation of the worm final drive
Due to design features, a large gear ratio (from 8 in steering gears, up to 1000 in especially powerful winches) and low efficiency, a worm gear pair is not used in automobile final drives (with rare exceptions). Most widespread she got in winches.
Torque is transmitted to the worm wheel through a power take-off connected to transfer case installed (as a rule, there are other kinematic schemes) behind the car's gearbox. The axes of the worm and the driven gear (driven wheel) are located at a right angle (but there is also a different arrangement of the axes of the worm pair). The worm wheel engages with a driven helical (to ensure tight contact and increase the engagement surface) gear wheel. Torque is transmitted from the helical groove of the worm to the teeth of the driven gear. The speed of the worm is much higher than the speed of the driven wheel. Due to this, the torque increases proportionally - the more gear ratio, the more effort the winch is able to develop.
The worm gear has a number of advantages over other types of final drives. It is highly wear resistant and does not require the use of high quality lubricants. It is capable of transmitting ultra-high torque. It features low noise and smooth running (due to the absence of shock loads on the worm groove and the surface of the teeth of the driven gear). Finally, the worm gear has the property of self-braking - when the transmission of torque to the worm stops, the rotation of the driven wheel automatically stops.
The disadvantages of a worm gear include a tendency to heat up due to friction forces, to seize the mechanism with little wear, and increased requirements for the accuracy of assembling a worm pair.
Worm final drive refers to irreversible gearboxes. If the force is transmitted from the driven gear wheel to the driving worm, that is, in reverse order, the worm will not rotate. Consequently, the worm main gear eliminates the movement of the car by inertia, coasting. Hence its use on low-speed transport equipment and special-purpose vehicles. On winches, to ensure free rotation of the drum, the worm pair is equipped with a free (reverse) clutch, which disconnects the drum and the driven gear when it rotates in the opposite direction - unwinding the winch cable.
Dual split final drive is used on heavy duty trucks when the gear ratio io ≥ 11 and on multi-purpose vehicles to obtain the required ground clearance.
Advantages transfers:
The transmission ratio can reach 20...30;
Smaller dimensions and weight of the cross-axle differential and the diameter of the axle shafts;
compact central part drive axle, which is important for obtaining low level floor and center of mass of the car, as well as to ensure the required ground clearance;
Possibility of adjusting the gear ratio of the main gear without changing the central part of the drive axle;
Onboard and wheel reduction gears carry only part of the load on the drive axle.
Flaws transfers:
High labor intensity of maintenance;
The complexity and cost of the design compared to the double central final drive due to a large number details;
Increase unsprung masses(especially when Not dependent suspension wheels).
Most often, the spaced main gear consists of a central gearbox (bevel or hypoid gear) and a wheel (or onboard) gearbox. Such a scheme is used both with a rigid axle housing (with dependent wheel suspension), and in the case of an articulated drive axle, when the central gearbox is placed on the body (or on the frame) of the car and is connected to the wheel gears by cardan gears (with independent wheel suspension).
Wheel gears used in vehicle designs can be with fixed shaft axles or planetary. Their main schemes are shown in fig. 6.12. The most widely used gearboxes are made according to the schemes of Fig. 6.12, a, c, d. Gearboxes, schematically shown in fig. 6.12, a, b, c, g, have fixed shaft axes, and the rest are planetary gears. In gearboxes (Fig. 6.12, aib), the drive gear can be located below the axis of the driven gear, which makes it possible to lower the level of the floor in the car body.
Lubrication of parts of wheel gears is carried out by spraying oil poured into their crankcases.
Determining the parameters of gear wheels of wheel reduction gears, selecting bearings and calculating shafts is carried out according to the same methods as for gearboxes. The materials used for the manufacture of gears and shafts are also similar.
When developing drive axles with wheel gears, the modular design principle is used. So, changing gear pairs in the planetary wheel reducer, you can change the total gear ratio of the main gear with a constant gear ratio in the central gearbox, i.e. receive a family of driving axles of various standard sizes.
Differentials
A differential is a transmission mechanism that performs the function of distributing the torque supplied to it between the wheels or axles and allowing the driven shafts to rotate with the same or different angular velocities, kinematically interconnected.
In addition to the general technical requirements for all transmission mechanisms, there is one requirement for differentials - they must distribute the torque between the wheels or axles in a proportion that provides the best operational properties vehicle (maximum traction, stability and controllability).
To increase the traction force of the car, it is necessary to distribute the torque along the wheels in proportion to the load on the wheel and the coefficient of adhesion. With different values of the friction coefficient under the wheels of the right and left sides, the traction forces along the sides will be different, as a result of which a moment of these forces will appear relative to the vertical axis passing through the center of mass of the vehicle, which worsens its stability and controllability. To ensure good stability of movement, the equality of traction forces on the wheels of the right and left sides is necessary. With different values of the friction coefficient under the wheels, this will lead to a limitation of the traction forces on all wheels by the traction force on the wheel with minimal grip, and, as a result, to a deterioration traction properties car. The noted contradiction is almost always resolved in favor of increasing the traction properties of the car.
It should be noted that the differential does not affect the overall gear ratio of the car's transmission. It ensures that the drive wheels roll without slipping when cornering and when driving over bumps in the road.
The main gear is designed to increase the torque supplied to it and transmit it through the differential and drive to the driving wheels of the vehicle at an angle of 90 degrees, as well as providing it top speed movement.
At front wheel drive vehicles main gear and differential are located in the gearbox housing. The engine of such cars is located not along, but across the axis of motion, which means that initially the torque from the engine is transmitted in the plane of rotation of the wheels. But, the function of increasing the torque and distributing it along the axes of the wheels remains unchanged in this case as well.
The principle of operation of the main gear
torque from crankshaft engine through the clutch, gearbox and driveline is transmitted to a pair of helical gears, which are in constant mesh.
torque transmission wheel
Rice. 2
Both wheels will rotate at the same angular velocity. But after all, in this case, turning the car is impossible, since the wheels must travel an unequal distance during this maneuver! If you take toy car, in which rear wheels connected to each other by a rigid axis, and
roll it a little on the floor, then the parquet in your house may noticeably suffer. With each turn of the car, one of its wheels will definitely slip, and leave a black trail behind it. Let's look at the tracks left on the turn by the wet wheels of any real car. Looking at these tracks with interest, you can see that the outer wheel from the center of the turn travels a much longer distance than the inner one. If the same number of revolutions were transmitted to each wheel, then turning the car without black marks on the “parquet” would be impossible. Hence, real car, unlike the toy one, it has a certain mechanism that allows it to make turns without “drawing” rubber wheels on asphalt. And this mechanism is called a differential.
