How much do you know about hydraulic cylinders?

With the development of modern industry, hydraulic transmission technology has been widely used and developed in many industries around the world, such as loaders, bulldozers and rollers of engineering machinery; forklifts, belt conveyors and truck cranes of lifting and transportation machinery; pile drivers, hydraulic jacks and graders of construction machinery; agricultural machinery, automobile industry, mining machinery, metallurgical machinery...

Hydraulic transmission systems are usually composed of four components: power, execution, control and auxiliary. As a hydraulic mechanism that realizes linear reciprocating motion or reciprocating swing motion less than 360 degrees, the hydraulic cylinder has a simple structure and reliable operation. It is also one of the main actuators that are widely used in hydraulic systems.

Since the hydraulic cylinder has such an important position in the hydraulic transmission system, we must understand the hydraulic cylinder. The following editor will first introduce you to the basic knowledge about the hydraulic cylinder. Classification of hydraulic cylinders

According to the structural form: it can be divided into piston type, plunger type, sleeve type and gear rack type, etc.; according to the movement mode: it can be divided into linear reciprocating type and rotary swing type;

According to the action form: it can be divided into single-acting type and double-acting type; according to the installation form: it can be divided into tie rod type, earring type, foot type, hinge shaft type, etc.;

According to the pressure level: it can be divided into low pressure, medium pressure, medium and high pressure, high pressure, and ultra-high pressure.

Structural composition of hydraulic cylinders

Single-rod double-acting piston hydraulic cylinder, this type of hydraulic cylinder is the simplest and most widely used one. The following is an example of the structural composition of the hydraulic cylinder using a single-rod double-acting piston hydraulic cylinder.

The hydraulic cylinder is usually composed of the main parts such as the rear end cover, cylinder barrel, piston rod, piston assembly, and front end cover. In order to prevent the oil from leaking out of the hydraulic cylinder or from the high-pressure chamber to the low-pressure chamber, sealing devices are provided between the cylinder barrel and the end cover, the piston and the piston rod, the piston and the cylinder barrel, and the piston rod and the front end cover. A dustproof device is also provided on the outside of the front end cover. In order to prevent the piston from hitting the cylinder head when it quickly returns to the end of the stroke, a buffer device is also set at the end of the hydraulic cylinder, and sometimes an exhaust device is also required.

Main parameters of hydraulic cylinders

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 acting 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. From the above formula, it can be seen that the establishment of the pressure value is generated by the existence of the load. On the same effective working area of the 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 that the hydraulic cylinder can work for a long time.

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

2. Flow rate: Flow rate is the volume of oil passing through the effective cross-sectional area of the cylinder per unit time. The calculation formula is 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 working area of the piston.

3. Piston stroke: The piston stroke refers to the distance traveled between the two poles when the piston reciprocates. Generally, after meeting the stability requirements of the cylinder, select a standard stroke similar to it from the table below according to the actual working stroke.

4. Piston movement speed: The movement speed is the distance that the pressure oil pushes the piston to move per unit time, which can be expressed as v=Q/A. The speed of the hydraulic cylinder should be appropriate. When the speed is too high, it often causes overheating and wear of the seal, and also aggravates the wear of the piston rod, guide sleeve and cylinder. When the speed is too low, it is easy to cause unstable conditions such as creeping. When using rubber seals, the maximum speed of the hydraulic cylinder should generally not exceed (24-30) m/min, that is, (0.4-0.5) m/s, and should not be lower than 6 m/min (0.1 m/s). It is a safe method to refer to the speed value of similar hydraulic cylinders.

5. Dimensions: Dimensions mainly include the inner and outer diameters of the cylinder, piston diameter, piston rod diameter and cylinder head size. These dimensions are calculated according to the use environment, installation form, required push and pull force and stroke of the hydraulic cylinder. After design and verification, they are rounded from the table below.

Development Trends of Custom Hydraulic Cylinders

With the in-depth popularization of hydraulic technology and the increasing expansion of application fields and occasions, new requirements are constantly put forward for the working performance, structure, scope of use, manufacturing accuracy, appearance, materials and test methods of hydraulic cylinders, which continuously promotes the development and progress of hydraulic cylinders. 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 materials and lightweight. With the high pressure, miniaturization, and the test of the use environment of hydraulic cylinders, new materials and lightweight have also become one of the solutions.

3. Novel structural composite. In order to adapt to the expansion of the application range of hydraulic cylinders, various novel structures of hydraulic cylinders continue to appear, including hydraulic cylinders, self-locking hydraulic cylinders, steel cable hydraulic cylinders, peristaltic hydraulic cylinders and composite hydraulic cylinders.

4. High performance and variety. The design of hydraulic cylinders will tend to be integrated and modular, simplifying the installation and maintenance of hydraulic systems, while improving the flexibility and scalability of the system.

5. Energy saving and corrosion resistance. Hydraulic cylinders will adopt more efficient sealing technology to reduce internal leakage, while optimizing the design to reduce energy consumption and meet increasingly stringent environmental protection standards.

6. Intelligence and digitalization. With the advancement of Industry 4.0, hydraulic cylinders will integrate more intelligent sensors and control units to achieve self-diagnosis, predictive maintenance and intelligent monitoring, and improve the reliability and efficiency of the system.