Hydro-mechanical gearbox: design features and advantages

What is the hydro-mechanical transmission and how it works

Clutch and gearbox are traditional nodes of any domestic or foreign car. The transmission is the element that provides the flow of torque from the power unit to the wheels. If previously most vehicles were equipped with a manual transmission, today more and more motorists prefer a hydromechanical automatic transmission. This is partly due to the fact that driving the car is simplified, since there is no clutch pedal, and the gear shift is automatic.


Purpose of the combined transmission of a passenger car

The lifestyle of modern drivers is changing significantly and today there are more and more demands for optimal comfort while driving. Standard units of automobiles undergo significant changes, the combination of mechanical and hydraulic transmission can be highlighted among the brightest examples. If we talk about the hydromechanical transmission and what it is, the first thing to understand is what its purpose is. The main difference is the smooth change of rotational motion. The lightweight control allowed to abandon the use of the clutch, because the combined transmission is responsible for all processes. With the automatic transmission, you can talk about the following situations concerning the driving of the car:

  • The transmission is disengaged from the powertrain during gear shifting.
  • If road conditions change, the value of torque will also change its value.

Using an automatic transmission on a car allows you to get several undeniable advantages. In addition to automating gear shifting, it is also worth noting the increase in the performance characteristics of the power unit and the box and improve the passability of the vehicle in off-road conditions.

Hydraulic transmission

Varieties of hydromechanics

Automatic transmissions for a long time were installed only on cars of the middle class and premium category. To date, the unit has received mass use and is increasingly popular among motorists. Automatic transmission can significantly improve comfort while driving, but it is worth considering that such units have different varieties, each of which has its own advantages and disadvantages. After understanding the principle of operation of hydro-mechanical gearboxes, it will be possible to determine which type of automatic transmission is suitable for a particular driver. It is worth mentioning the following types of hydromechanical gearboxes:

  • Hydromechanical automatic. This is one of the first transmissions of this kind, which appeared as an alternative to “mechanics”. The design is a combination of a torque converter and a planetary gearbox. The presence of electronic components allows to considerably increase the functional features of the unit.
  • Variator transmission. It is less popular because it does not have the usual fixed gears. The advantages include the maximum smoothness of motion, and it is explained just by the absence of the change of gears. The design of the continuously variable transmission is as follows: the usual hydraulic converter is used for the transmission of torque, and the change in torque occurs by changing the diameter of the drive and idler pulley. These components are connected by means of a belt and chain, and the change in diameter will depend on speed and load.
  • Robotic box. Massively began to be used about 20 years ago. There are few differences from the mechanics, there is a clutch, but the difference is that the clutch is controlled automatically. The advantages of the “robot” can include a low cost, dynamic acceleration and fuel economy. As for the disadvantages, the main one is the reduction of the comfort level.
  • Pre-selective double clutch boxes. These include DSG or Powershift units. The unit can be attributed to the robotized transmission, but with higher technical characteristics. The design resembles the usual mechanics, but this time the engineers used two units at once, placed in one box.
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Robotic units and automatic transmission are devices, the purpose of which is to simplify the interaction of the driver with the transmission.

The function of the torque converter

A hydraulic transformer is essentially an advanced hydraulic clutch. A conventional clutch performs the task of simply rotating, then in the case of an automatic transmission, an increase in torque position is added. The unit performs several basic functions, one of which is the damping action during rotational movement. If there is a constant difference in rotational speed, losses occur, so lockup occurs, causing torque to be transmitted through the damping springs. The lock-up clutch performs another useful function, the prevention of increased fuel consumption. Speaking of the functions of the car hydromechanical transmission, it is worth noting some negative factors.

Important! When locking, there is often an increased pressure on important components of the motor and transmission. Friction components can wear out more quickly and friction particles can enter the oil. As a result, driving performance can be impaired and gear shifts can no longer be smooth. It is imperative that car owners take care of the transmission during acceleration and braking.

Torque Converter Design

What is the structure of the hydro-mechanical transmission, you can understand by studying its design. The main components are the torque converter, the mechanical transmission and the controls. The torque converter is the main component, and it performs almost the same function as the clutch. Having studied the design of this part, you can notice that it consists of three wheels with a special shape. The first wheel is a pump wheel, its purpose is to serve as a link between the hydraulic unit and the power unit. The second ring is the turbine ring, it forms the connection with the primary shaft of the box. The third wheel is the reactor wheel, its function is to amplify the torque. All three components are enclosed by means of a special housing, the inner volume of which is three-quarters filled with lubricant. From the engine, the torque flows to the pump part, then through rotary movements directs the lubricant to the turbine wheel, resulting in the force being transmitted to the primary shaft. As the load is applied, the torque converter will automatically change the torque of force, which in turn, being transmitted to the mechanical components, will be switched through the friction components. The pressure of the fluid flowing from the pressure disc to the turbine is also regulated in automatic mode.

Torque converter

Planetary gearbox

An automatic is usually equipped with a planetary gearbox. Despite its simple design, the torque is properly adjusted and routed to the sun gear. The planetary gear is meshed with the freewheeling satellites gears, which have a special driver to communicate with the shaft. The torque will be transmitted through the driver if the pinion is in the braked mode, and if the pinion is unbraked, the satellites will start to send torque to it. You can understand how a hydro-mechanical gearbox works by examining its design. The planetary gearbox is one of the varieties of the combined system. The name of the node is due to the fact that the satellites rotate around the central gear like the planets of the solar system. The use of these components in the automatic machine is due to the simplicity of modifying the transmission ratio. For this purpose, it is enough to brake one of the parts of the node or to connect several elements by means of a friction clutch.

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The electronic part of the hydro-mechanical automatic transmission

Modern automatic boxes are equipped with electronic control, which allows you to maintain the specified moments with greater accuracy. Whereas the older units were 6 to 8 percent, the electronically controlled controls are accurate to 1 percent. There are new possibilities, based on speed and engine load the computer can determine the weight of the vehicle and make the necessary corrections. The main components of the electronic control system are the control lever and the electronic unit. This system also includes certain subsystems, such as:

  • Manual control subsystem.
  • A system that generates controlled signals.
  • Operating elements.
  • Automatic protection systems.
  • Measuring nodes.
  • Actuating system.

Pros and cons of hydromechanics

Cars equipped with an automatic transmission provide safer and more comfortable driving, since they provide an opportunity to concentrate on the road without being distracted by unnecessary actions. Beginner drivers, who find it difficult to use the mechanics, get a particular advantage.

Important! If in a driving school a student is trained on a car with an automatic transmission, he will not be able to drive a vehicle with a manual gearbox, as in the driving license there will be a corresponding note.

The following can be attributed to the advantages of the automatic transmission:

  • The gears do not need to be shifted manually;
  • There is an even flow of power. Cars equipped with an automatic transmission are characterized by a smooth stroke during gear shifting.
  • In the case of a manual transmission, there may be difficulties with starting, with a sharp lowering of the clutch, the engine can stall. In vehicles with “automatic” this process is controlled by electronic components.

The automatic transmission also has its disadvantages, the main of which – it is expensive to service. It is worth noting and high requirements for operating conditions. Another disadvantage is the absence of the possibility to start the car with a “pusher”, with a dead battery.

Hydromechanics is the choice of those car owners, who are not straitened with money and are not ready to sacrifice their comfort. With competent management and maintenance, a car with an “automatic” is more reliable and safer to drive.

What is a hydro-mechanical gearbox

One of the elements of the car control system is a hydromechanical transmission. Thanks to it, the driver can shift gears smoothly and without jerks. Hydromechanical transmission – what is it? Let’s find out.

Hydromechanical gearbox

Hydromechanical transmission

The role of the automatic transmission with hydromechanical control

For a car and similar vehicle, the transmission is the assembly that transmits torque from the engines to the wheels. This is what it looks like in cars with a clutch, but they are gradually being displaced from the market by automatic transmissions. “Automatics” are being put in more and more today. They don’t have a clutch, and the gears shift automatically. Hydromechanics help make it easier to change gears while driving. In classic transmissions, the following processes are performed when driving the car:

  • disconnecting the transmission from the engine at the time of gear change;
  • changing torque levels as road conditions change.
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The body of the torque converter rotates together with the pump wheel. The turbine is not connected to the housing (except for GT lock-up period) - it is connected to the box shaft. The reactor in this case is fixed through an overrunning clutch - it doesn't let it rotate under the flow pressure, when the difference in speed of the pump and turbine wheels is great, but allows to rotate together with them in one direction, when the car moves with a constant speed and the GT slip is minimal. This way it is possible to raise the efficiency of the box.

The torque converter housing rotates with the pump wheel. The turbine is not connected to the housing (except for GT lockup period) – it is connected to the box shaft. The reactor in this case is fixed through an overrunning clutch – it doesn’t let it rotate under the flow pressure, when the difference in speed of the pump and turbine wheels is great, but allows to rotate together with them in one direction, when the car moves with a constant speed and the GT slip is minimal. In this way it is possible to raise the efficiency of the box.

The hydro-mechanical automatic transmission is needed to perform these actions. It acts as a clutch and transmission at the same time. This box was specially invented for use in urban conditions, where it can be problematic to constantly depress the clutch because of frequent stops in traffic jams. The car with hydromechanics is operated with the brake and gas pedals.

Varieties of hydromechanics

This transmission necessarily includes a torque converter, components of the control system and a mechanical box. It can be one of several systems:

  • multi-shaft;
  • twin-shaft;
  • three-shaft;
  • planetary.

The last type of box is the most common. It is often installed on passenger cars, because it is not highly metal-intensive. It is characterized by less noise during operation, high durability and compactness.

Shaft gears can be found on trucks and buses. They have multi-disc clutches for shifting gears, which are encased in oil. First gear and reverse are engaged via a toothed clutch. Due to the special design of the shaft boxes, gear shifting is done by the crankshaft. The speed is not taken off, the torque and power are not broken.

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Main purpose of the automatic transmission

Main function of the automatic transmission

The function of the torque converter

The torque converter acts as the clutch in a modern automatic transmission. It allows the transmission to move quickly and smoothly from a standstill. This reduces dynamic forces, thus providing a gentle treatment of the engine and increasing durability. Transmission parts last much longer with a torque converter. The driver is less fatigued because of the reduced number of gears. Torque converters are recommended for use on off-road vehicles, as with their help it is possible to increase the cross-country ability of the vehicle in difficult conditions – on snow or sand.

Important: In Russia, it is also worth choosing transmissions with this node, as in the winter time, special equipment often does not have time to clean the roads. Thanks to the torque converter, a stable traction force is created with a low speed of rotation of the drive wheels, which increases their traction with the road surface.

Torque converter

Torque Converter Design

The converter is located between the engine and the mechanical end of the transmission. It consists of interconnected discs with blades. The first is the pump wheel, which is the driving wheel. It connects the engine and the transformer. The turbine wheel is the slave, it contacts the primary shaft. The reactor wheel is responsible for torque amplification. Turbines are practically buried in oil (immersed in it by three quarters). They are covered by a casing which protects them from ingress of foreign particles into the oil. During turbine operation, the engine torque force is directed to the pump disc. At the same time a pressurized flow of oil is directed to the turbine disc. It is spun by the reactor wheel, which is located in the central part. The resulting force is transmitted to the gearbox shaft.

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The torque converter works by a special circulation of oil, which enters it from the outer part of the pump disc, then moves to the turbine wheel and returns through the central part of this assembly. The oil circulation cycle is completed on the pump disc.The torque change in the torque converter occurs automatically as the engine load increases. This unit sends the torque force to the transmission, where friction clutches are used to engage the gears. The ratio is automatically determined by the transformer and the pressure of the circulating oil varies depending on the value.

Sectional view of automatic transmission converter

Sectional view of automatic transmission

Planetary mechanism

In most modern automatic transmissions, the torque converter acts in tandem with the planetary system. It transmits torque to the friction clutches. In the simplest version, the force is directed to the central pinion (sun gear). Two additional satellites (auxiliary gears) are in constant coupling with the central gear thanks to the teeth applied to these elements. The satellites are not fixed, but rotate freely around their axes. The gear mechanism is located inside the crown wheel, which, depending on the gear engaged, is locked or comes into motion. At the moment the crown gear locks, the driven shaft starts to move (the force is transmitted to it). Otherwise, the satellites transmit torque to the crown gear, leaving the driven shaft stationary. Friction clutches are installed in planetary automatic transmissions to shift gears. Each of them looks like several discs, which are thin plates of smooth metal. Each plate is coated with a special friction compound that prevents it from wearing out. Splines can be found on part of them. There are spacers between the clutches. They are pressed together by a hydraulic piston which functions when the working fluid is supplied. When the pressure in it increases, the friction clutches close tightly, becoming almost one unit. When the fluid pressure in the hydraulic piston drops, the friction discs are brought back into place by a spring. The operation of the friction clutches is closely related to the function of the brake and planetary mechanisms. The transmission commands of the transmission control system and the engine torque are transmitted to these clutches. Without their participation, engine braking and starting in tow is not possible. The mechanical unit works smoothly and accurately.

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planetary system

Important: When in neutral, the friction and brake mechanisms are deactivated. When accelerating and shifting gears, the friction clutches come into action and the planetary systems rotate in sync.

The electronic part of the hydro-mechanical automatic transmission

Electronic controls are essential to the precise shifting of gears in modern automatic transmissions. Nowadays, it is almost impossible to find transmissions whose operation would not be supported by electronic components. They are responsible for:

  • The function of the automatic transmission. In hydromechanics, this system consists of pressure regulators and pumps.
  • Collection of information about the current control program.
  • Generation of control impulses.
  • Execution of commands during gear shifting.
  • Engine and transmission protection in case of a dangerous situation.
  • For manual control, the unit is responsible for all operations, and the control is done by the lever.

electronic part of automatic transmission

The electronic part of the hydro-mechanical automatic transmission

Strengths and weaknesses of the hydromechanics

The hydro-mechanical box is a serial connection of the transformer, the planetary unit with the frictions of the hydraulic control system. Its main advantage is that there is no need for the driver to shift gears manually. The electronics do it precisely, so there is no discomfort and the engine is not overloaded. The absence of overload helps to keep it in one piece for a long time. When you start driving, the power transfer also occurs without interruption and jerking, which makes the hydromechanics more advanced, surpassing the performance of manual transmissions. Not for nothing are they used not only in the automotive industry, but also installed on tanks (in America and Germany).

Important: If you choose a car in which you will mainly move around the city, it is worth choosing a hydromechanical automatic transmission. With its help, you will not have discomfort when stopping in traffic jams or at traffic lights.

Another weak point of such automatic transmissions is the torque converter.

The weakest part of such automatic transmission is the torque converter.

The disadvantage of this mechanism is its high cost and technical complexity. When shifting gears, you can notice a loss of performance due to the slippage of the friction and brake belts. The weak part of such an automatic transmission is also the torque converter, due to which the torque is lost. In spite of the obvious advantages, the efficiency of the hydro mechanicals according to the measuring results makes 86%, whereas with the usual box it reaches 98%. Another disadvantage is the necessity of installing make-up systems for the cooling of the hydraulic unit. They occupy space under the hood, and because of that the engine-transmission compartment has big dimensions. Also cars with installed hydro-mechanics can not be started by pushing or moving it with a cable. For this kind of box, as well as in all automatics, the lack of possibility to regulate fuel consumption is characteristic. The described variant of the hydro-mechanical automatic transmission is one of the most primitive. Today, more advanced transmissions are being developed, which are installed on passenger cars produced in recent years. Hydromechanics is recommended for those who have recently sat behind the wheel. For the beginner, it is indispensable because there is no need to switch gears on your own.

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