Hydraulic transmission refers to a transmission method that uses liquid as a working medium for energy transmission and control. In liquid transmission, according to its different forms of energy transmission, it is divided into hydraulic transmission and hydraulic transmission. Hydraulic transmission is mainly a transmission method that uses liquid kinetic energy for energy conversion, such as a fluid coupling and a hydraulic torque converter. Hydraulic transmission is a transmission method that uses liquid pressure energy for energy conversion. Hydraulic drive technology on the machine can simplify the structure of the machine, reduce machine quality, reduce material consumption, reduce manufacturing costs, reduce labor intensity, improve work efficiency and work reliability.
Hydraulic transmission characteristics:
Compared to mechanical drives, hydraulic drives have the following main advantages:
(1) Since oil is generally used as the transmission medium, the hydraulic components have good lubrication conditions; the working fluid can be piped to any position, allowing the hydraulic actuator and the hydraulic pump to maintain a certain distance; the hydraulic transmission can conveniently transfer the original The rotational motion of the motive becomes a linear motion. These features are very suitable for the needs of various construction machinery and mining equipment. Typical examples of their applications are the single hydraulic props and hydraulic supports used in coal mines.
(2) A wide range of stepless speed regulation can be realized during operation, and the transmission ratio can be as high as 1:1 000, and the speed regulation performance is not limited by the power level.
(3) It is easy to implement load control, speed control and direction control, and can perform centralized control, remote control and automatic control.
(4) The hydraulic transmission can realize the gapless transmission, so the transmission is stable, the operation is labor-saving, the reaction is fast, and the high-speed start and frequent commutation can be achieved.
(5) Hydraulic components are standardized, serialized and generalized products, which are easy to design, manufacture and promote.
The main advantages of hydraulic transmission compared to electric drives are the following:
(1) The quality is small and the volume is small. This is because the motor is limited by magnetic saturation, and the tangential force per unit area is several tens of times different from the hydraulic pressure that the hydraulic machine can withstand.
(2) The motion inertia is small and the response speed is fast. The torque inertia ratio of the hydraulic motor (ie, the ratio of the driving torque to the moment of inertia) is much larger than that of the motor, so its acceleration performance is good. For example, accelerating a medium-powered motor typically takes a second to a few seconds, while a hydraulic motor that accelerates the same power takes only about 0.1 s. This kind of good dynamic characteristics is more important for hydraulic control systems.
(3) The low speed stability of low speed hydraulic motors is much better than that of electric motors.
(4) The application of hydraulic transmission can simplify the electrical system of the machine. This is good for underground mine operations where there is a risk of explosion. 
(1) In the transmission process, since the energy needs to be converted twice, there are pressure loss, volume loss and mechanical friction loss, so the total efficiency is usually only 0.75 to 0.8.
(2) The working performance and efficiency of the transmission system are greatly affected by the temperature. The general hydraulic transmission works in high temperature or low temperature environment, which has certain difficulties.
(3) The liquid has a certain compressibility, and there is inevitably a leak in the mating surface, so the hydraulic transmission cannot guarantee a strict transmission ratio.
(4) The working fluid is very sensitive to pollution. The working fluid after the pollution is very harmful to the hydraulic components. Therefore, the fault of the hydraulic system is more difficult to find, and the technical level of the operation and maintenance personnel has higher requirements.
(5) The manufacturing precision, surface roughness, material material and heat treatment requirements of hydraulic components are relatively high, so the cost is high.
In general, the advantages of hydraulic transmission are primary. Some of its shortcomings are gradually being overcome with the development of production technology. If you take advantage of other transmission methods, you can use the combined drive of electro-hydraulic, gas, and liquid to make full use of its characteristics.
Hydraulic mechanical composition
The hydraulic mechanical system mainly consists of five parts: power component (oil pump), actuator (cylinder or hydraulic motor), control components (various valves), auxiliary components and working medium.
1. Folding power components
Gear pump, vane pump, plunger pump, screw pump.
Its function is to use liquid to convert the mechanical energy of the prime mover into hydraulic pressure energy; it is the power part of the hydraulic transmission.
2. Folding actuator
Cylinder, hydraulic motor
Hydraulic cylinder: piston hydraulic cylinder, plunger hydraulic cylinder, swing hydraulic cylinder, combined hydraulic cylinder
Hydraulic motor - gear hydraulic motor, blade hydraulic motor, plunger hydraulic motor
It converts the hydraulic energy of a liquid into mechanical energy. Among them, the cylinder makes a linear motion and the motor performs a rotary motion.
3. Folding control components
Including pressure valves, flow valves and directional valves. Their role is to adjust the speed of the liquid motor steplessly as needed, and to regulate and control the pressure, flow and flow direction of the working fluid in the hydraulic system.
Pressure control valve - relief valve, pressure reducing valve, sequence valve, pressure relay, etc.
Flow control valve - throttle valve, speed control valve, diverter valve
4. Folding auxiliary components
Other than the above three parts, including pressure gauges, oil filters, energy storage devices, coolers, pipe fittings (flare, welded, ferrule), high pressure ball valves, quick-change joints, soft Pipe assemblies, pressure taps, pipe clamps, heaters, tubing, pressure gauges, flow meters, seals, and fuel tanks are equally important.
5. Folding working medium
Working medium refers to hydraulic oil or emulsion in various hydraulic transmissions. It converts energy through oil pump and liquid motor.