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With the development of modern industry, hydraulic transmission technology has been widely applied and developed in many industries around the world. For example, in construction machinery such as loaders, bulldozers, and rollers; in lifting and transport machinery such as forklifts, belt conveyors, and mobile cranes; in construction machinery such as pile drivers, hydraulic jacks, and graders; and in agricultural machinery, the automotive industry, mining machinery, and metallurgical machinery.
Hydraulic transmission systems typically consist of four components: power, actuator, control, and auxiliary. Hydraulic cylinders, as hydraulic mechanisms that achieve linear reciprocating motion or reciprocating and oscillating motion of less than 360 degrees, feature a simple structure and reliable operation, making them one of the most widely used and primary actuators in hydraulic systems.
Since the hydraulic cylinder plays such an important role in the hydraulic transmission system, we must understand the hydraulic cylinder. Now I will introduce to you the basic knowledge about the hydraulic cylinder.
1. Classification of Hydraulic Cylinders
- By structure: they can be divided into piston, plunger, sleeve, and rack-and-pinion types;
- By motion: they can be divided into linear reciprocating and rotary swing types;
- By action: they can be divided into single-acting and double-acting types;
- By mounting: they can be divided into tie rod, earring, foot, and hinged shaft types;
- By pressure level: they can be divided into low pressure, medium pressure, medium-high pressure, high pressure, and ultra-high pressure.
2. Hydraulic Cylinder Structure
A single-rod, double-acting piston hydraulic cylinder is the simplest and most widely used type of hydraulic cylinder. Below, we’ll use a single-rod, double-acting piston hydraulic cylinder as an example to explain its structure.
A hydraulic cylinder typically consists of a rear end cap, cylinder barrel, piston rod, piston assembly, and front end cap. To prevent oil leakage from the cylinder or from the high-pressure chamber to the low-pressure chamber, seals are installed between the cylinder barrel and end cap, the piston and piston rod, the piston and cylinder barrel, and the piston rod and front end cap. A dust guard is also installed on the outside of the front end cap. To prevent the piston from striking the cylinder head when rapidly retracting to the end of its stroke, a buffer device is installed at the end of the hydraulic cylinder, and sometimes an exhaust device is also required.
1. Barrel tube: The barrel tube is the main component of the hydraulic cylinder. It forms a sealed chamber with the cylinder head, piston and other parts to drive the piston. There are eight common barrel tube structures, which are usually selected based on the connection between the barrel tube and the end cover.
2. Cylinder Head: The cylinder head is installed at both ends of the hydraulic cylinder, forming a tight oil chamber with the cylinder barrel. Various connection methods are commonly used, including welding, threads, bolts, keying, and tie rods. The choice is generally based on factors such as the operating pressure, the cylinder connection method, and the operating environment.
3. Piston Rod: The piston rod is the primary force transmission component in the hydraulic cylinder. It is generally made of medium-carbon steel (such as 45-grade steel). During cylinder operation, the piston rod is subjected to thrust, tension, and bending torque, so ensuring its strength is essential. Furthermore, the piston rod frequently slides within the guide sleeve, so the fit should be appropriate. Too tight results in excessive friction, while too loose can easily cause binding and unilateral wear. Therefore, the surface roughness, straightness, and roundness must be adequate.
4. Piston: The piston is the primary component that converts hydraulic energy into mechanical energy. Its effective working area directly affects the force and speed of the hydraulic cylinder. There are various types of connections between the piston and the piston rod, with the most common being the retaining ring, sleeve, and nut types. When a guide ring is not used, the piston is made of high-strength cast iron HT200-300 or ductile iron. When a guide ring is used, 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 precision, low friction, and excellent wear resistance to withstand the pressure, bending force, and impact vibration of the piston rod. It contains an internal seal to ensure the seal between the cylinder barrel and the rod cavity, and a dust seal on the outside to prevent impurities, dust, and moisture from entering the seal and damaging it. Metal guide sleeves are generally made of bronze, gray cast iron, ductile iron, and oxidized cast iron, which have low friction coefficients and good wear resistance. Non-metallic guide sleeves can be made of polytetrafluoroethylene and polychlorotrifluoroethylene.
6. Buffer: When the piston and piston rod move under hydraulic pressure, they possess significant momentum. When these forces enter the end cap and bottom of the cylinder, they can cause mechanical collisions, generating significant impact pressure and noise. Buffers are used to prevent these collisions. Their operating principle (as shown in the figure below) is to convert the kinetic energy of the oil (in whole or in part) in the low-pressure chamber of the cylinder into heat energy through throttling. This heat energy is then carried out of the hydraulic cylinder by the circulating oil. The most commonly used types are adjustable orifice and variable orifice.
3. Main Parameters of Hydraulic Cylinders
The main parameters of hydraulic cylinders include pressure, flow rate, dimensions, piston stroke, speed, push-pull force, efficiency, and cylinder power.
1. Pressure: Pressure is the force exerted by the oil per unit area. The calculation formula is p = F/A, which is the load acting on the piston divided by the piston’s effective working area. As can be seen from the above formula, the pressure value is determined by the presence of load. For the same piston’s effective working area, the greater the load, the greater the pressure required to overcome it. In other words, for a given piston’s effective working area, the greater the oil pressure, the greater the force generated by the piston. The rated pressure, as we commonly refer to it, is the pressure at which the hydraulic cylinder can operate for a long period of time.
The pressure ratings for hydraulic cylinders are shown in the table below: Unit: MPa
The maximum allowable pressure refers to the ultimate pressure a hydraulic cylinder can withstand momentarily; the pressure test pressure refers to the test pressure required to inspect the quality of a hydraulic cylinder. Most countries stipulate that both 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 during one stroke of the hydraulic cylinder piston, t represents the time required for one stroke of the hydraulic cylinder piston, v represents the piston rod speed, and A represents the effective working area of the piston.
3. Piston Stroke: Piston stroke refers to the distance traveled between the two ends of the piston during reciprocating motion. Generally, after meeting the stability requirements of the cylinder, a standard stroke approximating the actual working stroke is selected from the table below.
4. Piston Speed: Speed is the distance the pressurized oil pushes the piston per unit time and can be expressed as v = Q/A. The speed of the hydraulic cylinder should be appropriate. Excessive speeds often cause overheating and wear of seals, as well as increased wear of the piston rod, guide sleeve, and cylinder barrel. Excessive speeds can easily lead to instability such as creep. When using rubber seals, the maximum speed of the hydraulic cylinder should generally not exceed 24-30 m/min (0.4-0.5 m/s), nor should it be less than 6 m/min (0.1 m/s). A safe approach is to reference the speed 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 based on the use environment, installation form, required push and pull force and stroke of the hydraulic cylinder. After design and verification, they are rounded off from the table below.
4. Common Hydraulic Cylinder Problems and Repair
As a component and working device, hydraulic cylinders, like all mechanical equipment, are subject to varying degrees of wear, fatigue, corrosion, loosening, aging, deterioration, and even damage to their structural components over long-term operation. These problems can deteriorate the cylinder’s performance and technical condition, leading directly to malfunction or even failure of the entire hydraulic equipment. Therefore, troubleshooting and repairing common hydraulic cylinder problems during daily operation are crucial
5. Development Trends of Hydraulic Cylinders
With the increasing popularity of hydraulic technology and the ever-expanding scope of its applications, new requirements are constantly being placed on the performance, structure, application range, manufacturing precision, appearance, materials, and testing methods of hydraulic cylinders. This is driving the development and advancement of hydraulic cylinders. The general trends are:
1. Higher pressure and smaller size. Higher pressure is an effective way to reduce the radial dimensions and weight of hydraulic cylinders, thereby reducing the overall volume of hydraulic systems.
2. New materials and lighter weight. With the increasing pressure and smaller size, and the challenges faced by hydraulic cylinder operating environments, new materials and lighter weight have become one solution.
3. Novel and complex structures. To adapt to the expanding application range of hydraulic cylinders, various novel hydraulic cylinder structures are constantly emerging, including hydraulic cylinders, self-locking hydraulic cylinders, cable-type hydraulic cylinders, peristaltic hydraulic cylinders, and composite hydraulic cylinder
4. High performance and a wide variety of products. Hydraulic cylinder designs will tend towards integration and modularization, simplifying the installation and maintenance of hydraulic systems while improving system flexibility and scalability.
5. Energy efficiency and corrosion resistance. Hydraulic cylinders will adopt more efficient sealing technologies to reduce internal leakage, while also optimizing their designs to lower energy consumption and meet increasingly stringent environmental standards.
6. Intelligence and Digitalization. With the advancement of Industry 4.0, hydraulic cylinders will integrate more intelligent sensors and control units to enable self-diagnosis, predictive maintenance, and intelligent monitoring, improving system reliability and efficiency.