What Is a Hydraulic Coupler?

Hydraulic coupling is a kind of hydraulic transmission device that uses the kinetic energy of liquid for energy transfer. It uses liquid oil as the working medium, and converts mechanical energy and liquid kinetic energy through pump wheels and turbines to connect the prime mover and the working machine Achieve power transmission. Hydraulic couplings can be divided into three basic types according to their application characteristics, namely ordinary type, moment-limiting type, speed regulating type and two derived types: hydraulic coupling transmission and hydraulic reducer.

Chinese name: Hydraulic coupling
Foreign name: fluid coupling

Also known as: Hydraulic coupling
working principle: Non-rigid coupling with liquid as working medium

Introduction to the concept of fluid coupling

Hydraulic coupling is a kind of hydraulic transmission device that uses the kinetic energy of liquid for energy transfer. It uses liquid oil as the working medium, and converts mechanical energy and liquid kinetic energy through pump wheels and turbines, thereby connecting the prime mover and the working machine. Achieve power transmission. Hydraulic coupling is a kind of non-rigid coupling using liquid as the working medium. The figure below is the physical picture of the hydraulic coupling.

Features of fluid coupling

Hydraulic coupler is a flexible transmission device. Compared with ordinary mechanical transmission device, it has many unique features: it can eliminate shock and vibration; the output speed is lower than the input speed, and the speed difference between the two shafts increases with the load. Increase; overload protection performance and starting performance are good, the input shaft can still rotate when the load is too large, will not cause damage to the power machine; when the load is reduced, the output shaft speed increases until it is close to the input shaft speed, so that the transmission torque Going to zero. The transmission efficiency of the fluid coupling is equal to the ratio of the output shaft speed to the input shaft speed. Generally, when the speed ratio of normal working condition of the hydraulic coupling is above 0.95, higher efficiency can be obtained. The characteristics of the fluid coupling vary depending on the shape of the working chamber, the pump wheel, and the turbine. It generally relies on the shell to dissipate heat naturally and does not require an externally cooled oil supply system. If the oil of the fluid coupling is emptied, the coupling is in a disengaged state and can function as a clutch. However, the hydraulic coupler also has the disadvantages of low efficiency and narrow efficiency range.

Classification of fluid couplings

Hydraulic couplings can be divided into three basic types according to their application characteristics, namely ordinary type, moment-limiting type, speed regulating type and two derived types: hydraulic coupling transmission and hydraulic reducer.

Structure and principle of fluid coupling

There are many structural forms of hydraulic couplers. Different hydraulic couplers have slightly different structures and principles, but the basic principles are the same. They all convert mechanical energy into liquid kinetic energy through the pump wheel, and then the flowing liquid Impact the turbine to realize the conversion of liquid kinetic energy to mechanical energy and output power, as shown in Figure 2. The typical structures and principles of ordinary, moment-limiting, and speed-regulating hydraulic couplers are introduced below.

Figure 1 Schematic diagram of hydraulic transmission

Hydraulic Coupler Ordinary Hydraulic Coupler

The ordinary type hydraulic coupler is the simplest type of hydraulic coupler, which is composed of the main components such as pump wheel 1, turbine 2, casing belt pulley 3, as shown in the figure below. Its working cavity has large volume and high efficiency (maximum efficiency reaches 0.96 to 0.98), and the transmission torque can reach 6 times to 7 times the rated torque. However, due to the large overload coefficient, the overload protection performance is poor, so it is generally used to isolate vibration, reduce starting shock or be used as a clutch.

Figure 3 Ordinary hydraulic coupler

Torque- limiting hydraulic coupler for hydraulic coupling

Common torque-limiting hydraulic couplers have three basic structures: static pressure relief type, dynamic pressure relief type, and composite relief type. The first two are widely used in construction machinery.

(1) Hydrostatic fluid coupling

The following figure is the structure diagram of the hydrostatic hydraulic coupling. In order to reduce the overload coefficient of the hydraulic coupler and improve the overload protection performance, it has a higher torque coefficient and efficiency at high transmission ratios. Therefore, it is different in structure from the ordinary hydraulic coupler. Its main features are the symmetrical arrangement of the pump wheel 2 and the turbine 3, and a baffle 5 and a side auxiliary cavity 4. The baffle is installed at the turbine outlet to guide and throttle. This fluid coupling works under partially filled conditions.

Figure 4 Hydrostatic fluid coupling

This type of hydraulic coupler has a small amount of oil stored in the side auxiliary cavity at high speed transmission ratio, so the transmission torque is large; while at a low transmission ratio, more oil is stored in the side auxiliary cavity, which makes the characteristic curve more flat and can be more Well meet the requirements of work machinery. However, it should be pointed out that, because the auxiliary chamber on the liquid inlet and outlet sides has a slow response speed following the load change, it is not suitable for working machinery with sudden load changes and frequent starting and braking. Because this type of hydraulic coupler is mostly used in the transmission of vehicles, it is also called a traction type hydraulic coupler.

(2) Dynamic pressure drain type hydraulic coupler

The dynamic pressure relief type hydraulic coupler can overcome the shortcoming of the static pressure relief type hydraulic coupler which is difficult to play an overload protection function when it is suddenly overloaded. The following figure is the structure diagram of the dynamic pressure hydraulic coupling.

Figure 5 Dynamic pressure-discharge type hydraulic coupling

In the figure above, the input shaft sleeve 1 is connected to the pump wheel 4 through the elastic coupling and the rear auxiliary cavity housing 9. The turbine shaft 7 is connected to the reducer or the working machine by the output shaft sleeve 8, and the fusible plug 6 is overheated. Protective effects. This hydraulic coupling has a front auxiliary cavity 2 and a rear auxiliary cavity 3. The front auxiliary cavity is a vaneless cavity at the center of the pump wheel and turbine; the rear auxiliary cavity is composed of the outer wall of the pump wheel and the rear auxiliary casing 9. The front and rear auxiliary chambers have small holes communicating with each other, the rear auxiliary chambers have small holes communicating with the pump wheel, and the front and rear auxiliary chambers rotate with the pump wheel.

The other function of the rear auxiliary cavity is "extended charging". The extended charging effect can improve the startability. When the engine starts (the turbine has not yet rotated), the working chamber fluid shows a large circulation, which makes the liquid fill the front auxiliary cavity and then passes through a small The hole f enters the rear auxiliary cavity. Because the working chamber is filled with a small amount of fluid and the torque is small, the engine can be started at light load. As the engine speed (that is, the speed of the pump wheel) increases, the liquid in the rear auxiliary chamber enters the working chamber along the small hole due to the increase in the pressure of the oil ring formed, and the filling volume of the working chamber increases. This is " Extended charge. " Due to the delay in filling the fluid, the torque of the turbine increases. After the torque reaches the starting torque, the turbine starts to rotate.

Hydraulic coupling speed regulating type hydraulic coupler

The speed-regulating hydraulic coupler is mainly composed of a pump wheel, a turbine, and a spoon tube chamber, as shown in the figure below. When the driving shaft drives the pump wheel to rotate, under the combined action of the blades and cavities in the pump wheel, the working oil will obtain energy and be sent to the outer peripheral side of the pump wheel under the action of inertia centrifugal force to form a high-speed oil flow. The high-speed oil flow on the outer circumferential side of the wheel combines the radial relative speed with the peripheral speed of the outlet of the pump wheel to form a combined velocity. It rushes into the inlet radial flow channel of the turbine and passes the oil flow moment along the radial flow channel of the turbine. The change causes the turbine to rotate, and the oil flows to the turbine outlet, and its radial relative speed and the peripheral speed at the turbine outlet form the combined velocity, flows into the radial flow path of the pump impeller, and regains energy in the pump impeller. Repeated like this from time to time, the circular flow of working oil in the pump wheel and turbine is formed. It can be seen that the pump wheel converts the input mechanical work into the kinetic energy of the oil, and the turbine converts the kinetic energy of the oil into the output mechanical work, thereby achieving power transmission.

Figure 6 Speed-regulating hydraulic coupler

The stepless speed change of the speed regulating hydraulic coupler is realized by changing the position of the spoon tube and changing the amount of working oil in the circulation circle. When the spoon tube is inserted into the deepest part of the fluid coupling chamber, the oil amount in the circulation circle is the smallest, and the rotation speed of the pump wheel and the turbine is large. The output speed is the lowest; when the spoon tube is inserted into the shallowest part of the fluid coupling chamber, the oil amount is the largest in the circulation circle. , The deviation between the speed of the pump wheel and the turbine is small, and the output speed is the maximum.

There is a certain difference between the speed of the pump wheel and the turbine of the speed regulating hydraulic coupler, which is called speed slip. It can be known from the properties of viscous fluid that the coupling slip loss and bearing friction loss will generate a large amount of heat and be absorbed by the coupler working oil. The larger the coupler slip, the greater the transfer power and the greater the heat generated. In order to keep the oil temperature of the coupler from exceeding the specified value, the oil circulation system must be used to take out the high-temperature oil, return to the coupler after cooling through the oil cooler, thereby ensuring the heat balance in the hydraulic coupler. Different hydraulic couplings have different oil cooling methods, which is also a more important issue in the application of hydraulic couplings. ^[1]