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 construction 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 most widely used main actuators in hydraulic systems.

1. Classification of hydraulic cylinders

Structural form: it can be divided into piston type, plunger type, sleeve type and gear rack type, etc.;

Movement mode: it can be divided into linear reciprocating type and rotary swing type;

Action form: it can be divided into single-acting type and double-acting type;

Installation form: it can be divided into pull rod type, earring type, foot type, hinge shaft type, etc.;

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

2. The structure of hydraulic cylinder

Single-rod double-acting piston hydraulic cylinder, this type of hydraulic cylinder is the simplest and most widely used. The following will take the single-rod double-acting piston hydraulic cylinder as an example to explain the structural composition of the hydraulic cylinder.

The hydraulic cylinder is usually composed of the rear end cover, cylinder barrel, piston rod, piston assembly, front end cover and other main parts. 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 dust proof device is also installed on the outside of the front end cover. In order to prevent the piston from hitting the cylinder cover when it quickly returns to the end of the stroke, a buffer device is also provided at the end of the hydraulic cylinder, and sometimes an exhaust device is also required.

(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 common cylinder structures, which are usually selected according to the connection form between the cylinder and the end cover.

(2) Cylinder head: The cylinder head is installed at both ends of the hydraulic cylinder and forms a tight oil chamber with the cylinder. There are usually many connection methods such as welding, threading, bolts, keys and tie rods. Generally, the selection is based on factors such as working pressure, cylinder connection method, and use environment.

(3) Piston rod: The piston rod is the main component for transmitting force in the hydraulic cylinder. The material is generally medium carbon steel (such as 45 steel). When the cylinder is working, the piston rod is subjected to thrust, tension or bending torque, so it is necessary to ensure its strength; and the piston rod often slides in the guide sleeve, and the fit should be appropriate. If it is too tight, the friction is large, and if it is too loose, it is easy to cause jamming and unilateral wear, which requires its surface roughness, straightness and roundness to be appropriate.

(4) Piston: The 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, and the commonly used ones are the clamp 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 guides and supports 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 cylinder rod cavity, 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 are generally made of bronze, gray cast iron, ductile iron and oxidized cast iron with low friction coefficient and good wear resistance; non-metallic guide sleeves can be made of polytetrafluoroethylene and polytrifluorochloroethylene.

(6) Buffer device: When the piston and piston rod move under the drive of hydraulic pressure, they have great momentum. When they enter the end cover and the bottom of the cylinder, they will cause mechanical collision, generate great impact pressure and noise. The buffer device is used to avoid such collision. Its working principle (as shown in the figure below) is to convert the kinetic energy of the oil (all or part) in the low-pressure chamber of the cylinder into heat energy through throttling, and the heat energy is carried out of the hydraulic cylinder by the circulating oil. The most commonly used are the adjustable throttle type and the variable throttle type.

3. Common problems and repairs of hydraulic cylinders

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