With the development of modern industry, hydraulic transmission technology has been greatly applied and developed in many industries around the world, such as loader, bulldozer and roller of construction machinery; Forklifts, belt transporters and car cranes for lifting and transportation machinery; Piling machines, hydraulic jacks and ground machines for construction machinery; Agricultural machinery, automotive industry, mining machinery, metallurgical machinery ...
Hydraulic transmission equipment usually consists of power, execution, control and auxiliary four components. And the hydraulic cylinder acts as a linear reciprocating motion or less than 360. The hydraulic mechanism of reciprocating swing movement is simple in structure and reliable in operation. It is also a widely used main execution element in hydraulic systems. It is the liquid pressure energy generated by driving the hydraulic pump in the form of mechanical energy. It becomes a direct drive load for reciprocating motion or less than 360. An energy conversion device for the mechanical energy of reciprocating motion. To understand hydraulic cylinders, we must know the following basic knowledge.
Classification and composition of hydraulic cylinders
According to the structural form, hydraulic cylinders can be divided into four categories: piston cylinders, plunger cylinders, swing cylinders and special cylinders; According to the rated pressure is divided into high pressure and ultra-high pressure hydraulic cylinders, medium high pressure hydraulic cylinders and medium low pressure hydraulic cylinders.
Cylinder: Cylinder is the main part of the cylinder. It forms a closed cavity with the cylinder head, piston and other parts to promote the movement of the piston. There are 8 commonly used cylinder structures, which are usually selected according to the connection form between the cylinder and the end cover. Materials generally require sufficient strength and impact toughness, and good welding performance for welding, so commonly used materials are: 25, 45, ZG200 ~ 400, 1Cr18Ni9 and so on. Cylinder rough mostly uses cold drawn or hot seamless steel pipes, so the process is usually to adjust the quality(ensure the strength of the cylinder, Make it able to withstand oil pressure without deformation and damage) → honing or boring and rolling pressure(ensure the roughness, roundness, cylindrical and straightness of the inner diameter of the cylinder, etc.. Make the piston seal remain unchanged after long-term reciprocating motion → car(ensure the design size requirements such as the full length of the cylinder) → drill(process the oil hole, ensure access to the oil road) → pliers
Cylinder cover: Cylinder cover is installed at both ends of the cylinder, forming a tight oil cavity with the cylinder. There are usually many types of connections such as welding, threads, bolts, gridlock, and pull rods. Generally, they are selected based on factors such as working pressure, connection mode of the cylinder, and use environment.
3, piston rod: Piston rod is the main component of hydraulic cylinder transmission force. The material is generally selected as carbon steel(such as steel No. 45). When the cylinder is working, the piston rod is subjected to thrust, pull or bending torque, etc., and it is necessary to guarantee its strength. And the piston rod often slides in the guide sleeve. The fit should be suitable, too tight, the friction force is large, too loose, and it is easy to cause stuck phenomenon and unilateral wear. This requires that the surface roughness, straightness, and roundness be suitable. Therefore, the piston rod process is usually a rough car → quality adjustment → semi-precision car → quenching → plating front mill → Chrome plating → plating rear mill → precision car.
Pistons: Pistons are the main components that convert hydraulic energy into mechanical energy. Its effective working area directly affects the force and speed of hydraulic cylinders. Pistons and piston rods are connected in many forms. Commonly used are clamps, bushings, and nuts. When there is no guide ring, the piston is made of high-strength cast iron HT200 ~ 300 or ductile cast iron; When there is a guide ring, the piston uses high-quality carbon steel No. 20, No. 35 and No. 45.
5, guide sleeve: guide sleeve to guide and support the piston rod, it requires high precision, small friction resistance, good wear resistance, can withstand the piston rod pressure, bending force and impact vibration. It is equipped with a sealing device to ensure that the cylinder has a rod cavity seal, and a dust-proof ring is installed on the outside to prevent impurities, dust and water from being brought to the sealing device and damage the seal. Metal guide sleeves generally use bronze, gray cast iron, ductile cast iron and oxidized cast iron with small friction coefficient and good wear resistance; Non-metallic guide sleeve can use PTFE and PTFCM.
Buffer: Pistons and piston rods have great momentum when they move under liquid pressure. When they enter the end cover and bottom part of the cylinder, they will cause mechanical collisions, resulting in great impact pressure and noise. Buffers are used to avoid such collisions. The working principle is to allow the oil liquid(all or part of the cylinder) in the low-pressure cavity of the cylinder to convert kinetic energy into heat energy through throttle, and the heat energy is brought from the circulating oil to the outside of the hydraulic cylinder. The structure of the buffer device is divided into two types: constant current area buffer device and variable flow buffer device.
Main parameters of hydraulic cylinder
The main parameters of the hydraulic cylinder include pressure, flow, size specifications, piston stroke, motion speed, push force, efficiency, and hydraulic cylinder power.
1, pressure: pressure is the pressure of the oil on the unit area. Calculate formula P = F/A, that is, the load acting on the piston is divided by the effective working area of the piston. From the above formula, it can be seen that the establishment of the pressure value is due to the existence of the load. The greater the load, the greater the pressure required to overcome the load on the working area of the same piston. In other words, if the effective working area of the piston is certain, the greater the oil pressure, the greater the force generated by the piston. Usually the rated pressure we refer to is the pressure that the hydraulic cylinder can work with for a long time. According to the rated pressure, the pressure classification of the hydraulic cylinder is shown in the following table: Unit MPa
The maximum allowable pressure refers to the ultimate pressure that the hydraulic cylinder can withstand in an instant; The pressure-resistant test pressure refers to the test pressure that must be borne when checking the quality of the hydraulic cylinder. Most of these two types of pressure countries specify a rated pressure of less than or equal to 1.5 times.
2, flow: flow is the amount of oil per unit time through the cylinder effectively cut area. Calculate formula Q = V/t = vA, where V represents the volume of oil and liquid consumed during a single stroke of the hydraulic cylinder piston, T represents the time required for a single stroke of the hydraulic cylinder piston, V represents the piston rod movement speed, and A represents the piston's effective working area.
3, piston stroke: Piston stroke refers to the piston reciprocating movement in the distance between the two poles. In general, after satisfying the stability requirements of the cylinder, according to the actual work schedule to select a standard trip similar to its.
4, the speed of motion of the piston: The speed of movement is the distance per unit of time when the pressure oil pushes the piston to move, which can be expressed as V = Q/A.
Size Specifications: Size Specifications mainly include the inner and outer diameter of the cylinder, piston diameter, piston rod diameter and cylinder head size, etc.. These dimensions are based on the use environment of the hydraulic cylinder, the installation form, the push force required and the stroke required. Calculate, design and check.
III. Common Problems and Maintenance of Hydraulic Cylinders
The hydraulic cylinder, as a component, a working device, like all mechanical equipment, inevitably produces different degrees of wear, fatigue, corrosion, loosening, aging and deterioration, and even damage in its structural components during the long-term operation. phenomenon, The working performance and technical condition of the hydraulic cylinder are deteriorated, which directly causes the failure of the entire hydraulic equipment and even fails. Therefore, it is very important to eliminate and repair the common problems in the daily work of hydraulic cylinders.
The Development of Hydraulic Cylinders
With the deepening popularization of hydraulic technology and the expansion of fields and occasions, new requirements have been put forward for the working performance, construction, scope of use, manufacturing accuracy, appearance, materials, and test methods of hydraulic cylinders. Therefore, the development and progress of hydraulic cylinders are constantly promoted.
Its general trends are:
1, high-pressure, miniaturization. Hypertension is an effective way to reduce the radial size and weight of the hydraulic cylinder and reduce the volume of the entire hydraulic device.
2, new material, lightweight. With the high-pressure, miniaturization, hydraulic cylinder use of the environment test, new materials, lightweight has become one of the solutions.
3, novel structure compounding. In order to adapt to the expansion of the scope of application of hydraulic cylinders, various novel structures of hydraulic cylinders continue to appear, such as automatic hydraulic cylinders, self-locking hydraulic cylinders, steel cable hydraulic cylinders, peristaltic hydraulic cylinders and composite hydraulic cylinders.
4, high performance, multiple varieties.
5, energy-saving and corrosion resistance.