A hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy, producing linear reciprocating motion (or oscillating motion). It features a simple structure and reliable operation. When used for reciprocating motion, it can eliminate the need for speed-reducing devices, has no transmission gaps, and delivers smooth movement, making it widely employed in the hydraulic systems of various machines.

The output force of a hydraulic cylinder is directly proportional to the effective area of the piston and the pressure difference across its two sides. A hydraulic cylinder typically consists of a cylinder barrel and cylinder head, a piston and piston rod, sealing elements, a cushioning device, and an exhaust port. The cushioning and exhaust devices depend on the specific application, while the other components are essential.

First, classification of hydraulic cylinders: Hydraulic cylinders come in a wide variety of designs, and accordingly, there are numerous ways to classify them. Based on their mode of motion, they can be divided into linear reciprocating motion and rotary oscillating motion. According to the type of hydraulic pressure applied, they can be categorized as single-acting or double-acting. Depending on their structural design, they include piston-type, plunger-type, multi-stage telescopic‑type, and gear‑rack types. By mounting configuration, they may feature lever‑type, eye‑type, base‑type, or hinge‑type arrangements. Lastly, based on pressure ratings, they are available in 16 MPa, 25 MPa, 31.5 MPa, and other classes. 1. Piston‑type single‑rod hydraulic cylinders have a piston rod extending from only one end. As shown in the figure, this is a single‑rod hydraulic cylinder. Both ends—ports A and B—can accommodate bidirectional movement via pressurized oil or return oil; thus, it is classified as a double‑acting cylinder. The piston itself moves in only one direction, with the opposite motion requiring an external force. However, its stroke length is typically greater than that of a standard piston‑type cylinder. Single‑rod and double‑rod configurations exist, with the rod secured either to the cylinder body or directly to the piston; depending on whether the fluid pressure acts on one side or both sides, these cylinders exhibit single‑rod or double‑rod functionality. In a single‑acting hydraulic cylinder, pressurized oil enters only one port, driving the cylinder in one direction, while the reverse motion relies on an external force—such as spring force, gravity, or an external load. For a double‑acting cylinder, both chambers are alternately supplied with fluid under hydraulic pressure, enabling motion in both directions. A schematic diagram of a single‑rod, double‑acting piston hydraulic cylinder is shown below. Since the piston rod is attached to only one side of the piston, the effective working areas of the two chambers differ. With equal supply volumes, the piston’s speed varies between chambers. When the resisting load is constant, differing required supply pressures—or adjustments in system pressure—result in variations in the load‑carrying capacity achievable during bidirectional operation. 2. Plunger‑type hydraulic cylinders: (1) These are single‑acting hydraulic cylinders where pressurized fluid drives motion in only one direction, and the plunger must return by means of an external force or its own weight. (2) Plunger cylinders rely solely on the cylinder liner for support, without direct contact with the liner, making the liner easier to machine and suitable for long‑stroke applications. (3) During operation, the plunger is constantly subjected to pressure, necessitating sufficient rigidity. (4) Due to their substantial weight, plunger cylinders tend to sag under horizontal placement; therefore, vertical installation is preferable to minimize unilateral wear on seals and guide rails. 3. Telescopic hydraulic cylinders: These cylinders feature two or more stages of pistons, with the pistons extending outward in descending order of size, while retraction occurs in ascending order. Telescopic cylinders can achieve very long strokes, yet remain compact when retracted. They are commonly used in construction and agricultural machinery. When multiple pistons move sequentially, both output speed and force vary with each piston’s displacement.