Basic knowledge of hydraulic cylinders

With the development of modern industry, hydraulic transmission technology has been greatly applied and developed in many industries around the world, such as loaders, bulldozers and road rollers for construction machinery; forklifts, belt conveyors and truck cranes for lifting and transporting machinery; construction machinery Pile driver, hydraulic jack and grader, etc.; agricultural machinery, automobile industry, mining machinery, metallurgical machinery...

Hydraulic transmission equipment is usually composed of four elements: power, execution, control and auxiliary. And the hydraulic cylinder can realize linear reciprocating motion or less than 360. The hydraulic mechanism of reciprocating oscillating motion is simple in structure and reliable in work. It is also the main executive element widely used in hydraulic systems. It is the hydraulic pressure energy generated by the prime mover driving the hydraulic pump in the form of mechanical energy, and then it can be directly driven by the load. Reciprocating motion or less than 360. An energy conversion device for the mechanical energy of reciprocating swing motion. To understand hydraulic cylinders, we must know the following basic knowledge.

1. Classification and composition of hydraulic cylinders

According to the structure, hydraulic cylinders can be divided into four types: piston cylinders, plunger cylinders, swing cylinders and special cylinders; according to the rated pressure, they are divided into high pressure and ultra high pressure hydraulic cylinders, medium and high pressure hydraulic cylinders and medium and low pressure hydraulic cylinders.

1. Cylinder: The cylinder is the main part of the hydraulic cylinder. It forms a closed cavity with the cylinder head, piston and other parts to push the piston to move. There are 8 types of commonly used cylinder structures, which are usually selected according to the connection form of the cylinder and the end cover. Materials generally require sufficient strength and impact toughness, and good welding performance for welding. Therefore, commonly used materials are: 25, 45, ZG200~400, 1Cr18Ni9 and so on. Cylinder blanks are mostly cold drawn or hot-rolled seamless steel pipes, so the process is usually quenching and tempering (ensure the strength of the cylinder so that it can withstand oil pressure without deformation and damage) → honing or boring and rolling (ensure the inner diameter of the cylinder The roughness, roundness, cylindricity and straightness, etc. of the piston will not change after long-term reciprocating motion) → Car (ensure the design size requirements such as the full length of the cylinder) → Drill (process the oil outlet hole to ensure entry and exit Oil circuit) → clamp

2. Cylinder head: The cylinder head is installed at both ends of the hydraulic cylinder and forms a tight oil cavity with the cylinder barrel. There are usually multiple connection methods such as welding, thread, bolt, snap key and tie rod, which are generally selected according to the working pressure, the connection method of the cylinder, the use environment and other factors.

3. Piston rod: The piston rod is the main component of the hydraulic cylinder to transmit force. The material is generally medium carbon steel (such as No. 45 steel). When the cylinder is working, the piston rod is subjected to thrust, tension or bending moment, so it is necessary to ensure its strength; and the piston rod often slides in the guide sleeve, the fit should be appropriate, too tight, the friction is too large, too loose, easy It causes jamming and unilateral wear, which requires proper surface roughness, straightness and roundness. Therefore, the process of the piston rod is usually rough turning → quenching and tempering → semi-finishing turning → quenching → pre-plating grinding → chrome plating → post-plating grinding → finishing turning.

4. Piston: Piston is the main component that converts hydraulic energy into mechanical energy. Its effective working area directly affects the force and movement speed of the hydraulic cylinder. There are many forms of connection between the piston and the piston rod. Commonly used are the snap ring type, the sleeve type and the nut type. When there is no guide ring, the piston is made of high-strength cast iron HT200~300 or ductile iron; when there is a guide ring, the piston is made of high-quality carbon steel No. 20, No. 35 and No. 45.

5. Guide sleeve: The guide sleeve plays a guiding and supporting role for the piston rod. It requires high matching accuracy, low friction resistance, good wear resistance, and can withstand the pressure, bending force and impact vibration of the piston rod. A sealing device is installed inside to ensure the sealing of the rod cavity of the cylinder, and a dust ring is installed on the outside to prevent impurities, dust and moisture from being brought to the sealing device and damaging the seal. Metal guide sleeves generally use bronze, gray cast iron, ductile iron and oxide cast iron with low friction coefficient and good wear resistance; non-metallic guide sleeves can use polytetrafluoroethylene and polychlorotrifluoroethylene.

6. Cushioning device: The piston and piston rod have a great momentum when they are driven by hydraulic pressure. When they enter the end cover and bottom of the cylinder, they will cause mechanical collisions, resulting in great impact pressure and noise. The buffer device is used to avoid such collisions. Its working principle is to make the oil (all or part) in the low-pressure chamber of the cylinder barrel convert kinetic energy into heat energy through throttling, and the heat energy is carried out of the hydraulic cylinder by the circulating oil. The structure of the buffer device is divided into two types: a constant throttle area buffer device and a variable throttle buffer device.

Second, the main parameters of the hydraulic cylinder

The main parameters of hydraulic cylinders include pressure, flow, size specifications, piston stroke, movement speed, push-pull force, efficiency and hydraulic cylinder power.

1. Pressure: Pressure is the pressure of oil on a unit area. The calculation formula p=F/A, that is, the load acting on the piston divided by the effective working area of the piston. It can be seen from the above formula that the establishment of the pressure value is produced by the existence of the load. On the effective working area of the same piston, the greater the load, the greater the pressure required to overcome the load. In other words, if the effective working area of the piston is constant, the greater the oil pressure, the greater the force generated by the piston. The rated pressure we usually talk about is the pressure at which the hydraulic cylinder can work for a long time. According to the rated pressure, the hydraulic cylinder pressure classification is shown in the following table: unit MPa

The maximum allowable pressure refers to the ultimate pressure that the hydraulic cylinder can withstand at an instant; and the pressure test pressure refers to the test pressure that the hydraulic cylinder needs to withstand when checking the quality of the hydraulic cylinder. Most of these two pressures are less than or equal to 1.5 times the rated pressure in various countries.

2. Flow rate: Flow rate is the volume of effective cross-sectional area of oil passing through the cylinder in a unit time. The calculation formula Q=V/t=vA, where V represents the volume of oil consumed in one stroke of the hydraulic cylinder piston, t represents the time required for one stroke of the hydraulic cylinder piston, v represents the movement speed of the piston rod, and A represents the effective work of the piston area. 3. Piston stroke: Piston stroke refers to the distance traveled between the two poles when the piston reciprocates. Generally, after meeting the stability requirements of the oil cylinder, a standard stroke similar to it is selected according to the actual working stroke.

4. The movement speed of the piston: the movement speed is the distance that the pressure oil pushes the piston to move in a unit time, which can be expressed as v=Q/A.

5. Size specifications: The size specifications mainly include the inner and outer diameters of the cylinder, the piston diameter, the piston rod diameter and the cylinder head size, etc. These dimensions are calculated according to the use environment of the hydraulic cylinder, the installation form, the required push-pull force and the stroke. , Design and verification.

3. Common problems and maintenance of hydraulic cylinders

As a component and a working device, hydraulic cylinder, like all mechanical equipment, will inevitably produce different degrees of wear, fatigue, corrosion, loosening, aging, deterioration or even damage in its structural parts during long-term operation. This phenomenon makes the working performance and technical condition of the hydraulic cylinder deteriorate, which directly causes the failure or even failure of the entire hydraulic equipment. Therefore, it is very important to eliminate and repair common problems in the daily work of hydraulic cylinders.

Fourth, the development trend of hydraulic cylinders

With the in-depth popularization of hydraulic technology and the increasing expansion of application fields and occasions, new requirements have been continuously raised for the working performance, structure, scope of use, manufacturing accuracy, appearance, materials, and test methods of hydraulic cylinders. Therefore, hydraulic cylinders are continuously promoted. Development and progress.

The general trend is:

1. High pressure and miniaturization. High pressure is an effective way to reduce the radial size and weight of hydraulic cylinders, and reduce the volume of the entire hydraulic device.

2. New material and light weight. With the high pressure and miniaturization, and the test of the use environment of hydraulic cylinders, new materials and light weight have become one of the solutions.

3. The novel structure is compounded. In order to adapt to the expansion of the application range of hydraulic cylinders, various hydraulic cylinders with novel structures have appeared constantly, such as self-control hydraulic cylinders, self-locking hydraulic cylinders, steel cable hydraulic cylinders, peristaltic hydraulic cylinders and composite hydraulic cylinders.

4. High performance and multiple varieties.

5. Energy saving and corrosion resistance.