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What is a hydraulic cylinder?

A hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and produces linear reciprocating motion (or oscillating motion). It has a simple structure and reliable operation. When used to achieve reciprocating motion, it can eliminate the need for a deceleration device and has no transmission clearance, ensuring smooth movement. Therefore, it is widely used in various hydraulic systems of machinery. The output force of a hydraulic cylinder is proportional to the effective area of the piston and the pressure difference on both sides of it.

Hydraulic Cylinder Structure Diagram

A hydraulic cylinder usually consists of main parts such as the rear cover, cylinder barrel, piston rod, piston assembly, and front cover. To prevent oil leakage from the hydraulic cylinder to the outside or from the high-pressure chamber to the low-pressure chamber, sealing devices are installed between the cylinder barrel and end cover, piston and piston rod, piston and cylinder barrel, and piston rod and front cover. In addition, a dustproof device is installed on the outer side of the front cover. To prevent the piston from hitting the cylinder head when it quickly returns to the end of the stroke, a cushioning device is also installed at the end of the hydraulic cylinder. Sometimes, an exhaust device also needs to be installed.

Classification of Hydraulic Cylinders

To meet the different needs of various machines, hydraulic cylinders come in many types. They can be classified according to the oil supply direction into single-acting cylinders and double-acting cylinders. Single-acting cylinders only input high-pressure oil to one side of the cylinder and rely on other external forces to make the piston return. Double-acting cylinders input pressure oil to both sides of the cylinder. The positive and reverse movements of the piston are both completed by hydraulic pressure.

According to the structural form, hydraulic cylinders can be divided into piston cylinders, plunger cylinders, swing cylinders, and telescopic sleeve cylinders. According to the form of the piston rod, they can be divided into single-piston rod cylinders and double-piston rod cylinders.

According to the special use of the cylinder, they can be classified into series cylinders, booster cylinders, speed-up cylinders, and step cylinders. These cylinders are not a single cylinder but are combined with other cylinders and components. Therefore, from a structural point of view, these cylinders are also called combination cylinders.

Working Principle Diagram of Hydraulic Cylinder

Hydraulic transmission principle - Using oil as the working medium to transmit motion by changing the sealed volume and transmitting power through the pressure inside the oil.

1. Power part - Converts the mechanical energy of the prime mover into the pressure energy (hydraulic energy) of the oil. For example, hydraulic pump.
2. Execution part - Converts the hydraulic energy input by the hydraulic pump into the mechanical energy that drives the working mechanism. For example, hydraulic cylinder and hydraulic motor.
3. Control part - Used to control and adjust the pressure, flow rate, and flow direction of the oil. For example, pressure control valve, flow control valve, and directional control valve.
4. Auxiliary part - Connects the above three parts to form a system, which plays a role in oil storage, filtration, measurement, and sealing. For example, pipelines and connectors, oil tanks, filters, accumulators, seals, and control instruments.

The pressure applied to any point on a certain volume of liquid can be transmitted equally to all directions. This means that when using multiple hydraulic cylinders, each hydraulic cylinder will pull or push at its own speed, depending on the pressure required to move the load.
In the case of hydraulic cylinders with the same bearing capacity, the hydraulic cylinder that bears the smallest load will move first, and the hydraulic cylinder that bears the largest load will move last.
To synchronize the movement of hydraulic cylinders to achieve the lifting of the load at the same speed at any point, control valves or synchronous lifting system components must be used in the system.

For more detailed information, please download the hydropneumatic cylinder white paper.