Hydraulic cylinder What are the categories?

　　Hydraulic cylinders come in a wide variety of designs, and they can be classified in numerous ways. By motion type, they are divided into linear reciprocating and rotary oscillating types. Based on the mode of hydraulic pressure application, they are categorized as single‑acting and double‑acting. According to their structural configuration, they include multi‑stage telescopic cylinders, rack-and-pinion types, and others. Depending on the mounting method, they are available with lever, eye‑bolt, bracket, hinge, or other configurations. By pressure rating, they are specified at 16 MPa, 25 MPa, 31.5 MPa, and so forth.

　　Piston type:

　　A single‑rod hydraulic cylinder has a piston rod on only one side. As shown in the figure, this is a single‑rod hydraulic cylinder. The inlet and outlet ports at both ends, designated A and B, can be supplied with pressurized oil or return oil to produce bidirectional motion; hence, it is referred to as a double‑acting cylinder.

　　It can only perform unidirectional motion; the reverse direction requires an external force. However, its stroke is generally longer than that of a piston-type hydraulic cylinder.

　　What is the working principle of a piston-type hydraulic cylinder?

　　Piston hydraulic cylinders can be classified as single‑rod or double‑rod types. They are secured using either a cylinder block or a rod‑end mounting. Depending on the direction of fluid pressure, they exhibit either single‑acting or double‑acting operation. In a single‑acting hydraulic cylinder, pressurized fluid is supplied to only one port; hydraulic pressure drives the piston in one direction, while the opposite direction is achieved by an external force—such as spring force, gravity, or an applied load. In a double‑acting hydraulic cylinder, both directions of motion are accomplished by alternately pressurizing the two chambers, with the movement driven by hydraulic pressure.

　　As shown in the figure, this is a schematic diagram of a single‑rod double‑acting hydraulic cylinder. With the rod mounted on only one side, the effective cross‑sectional areas of the two chambers differ. When the flow rate is the same, the piston speed varies between the two chambers. Moreover, for a given external load, the required supply pressure differs depending on whether the oil is drained through the same port; alternatively, after adjusting the system pressure, the maximum load that can be overcome in each direction of the hydraulic cylinder will also differ.

　　Plunger type:

　　(1) A plunger hydraulic cylinder is a single-acting hydraulic cylinder that can move in only one direction; its return stroke is accomplished by an external force or by gravity.

　　(2) The piston is supported solely by the cylinder liner and does not come into direct contact with it; consequently, the cylinder liner is easy to machine and well suited for long-stroke hydraulic cylinders.

　　(3) During operation, it is constantly subjected to pressure, thus requiring sufficient stiffness.

　　(4) Plungers are often quite heavy; when mounted horizontally, they may sag under their own weight. Unilateral wear on the seals and guide rails makes vertical mounting more advantageous.

　　Retractable:

　　Telescopic hydraulic cylinders may have two or more stages; the extension sequence proceeds from largest to smallest, while the retraction sequence typically follows the reverse order, from smallest to largest. Telescopic cylinders can achieve long strokes, yet remain compact in their retracted state. These cylinders are commonly used in construction and agricultural machinery. When subjected to single‑stroke or multi‑stroke operation, both the output speed and output force vary with each individual stroke.

　　Swing type:

　　A swing hydraulic cylinder is an actuating component that delivers torque and produces reciprocating motion, available in various configurations such as single‑blade, double‑blade, and helical‑swing types. In the vane type, vanes are mounted on the cylinder body and connected to the rotor; depending on the direction of fluid flow, the vanes cause the rotor to oscillate back and forth. There are helical‑swing, single‑helix swing, and double‑helix designs; currently, the double‑helix configuration is more widely used. It employs two helical pairs to convert the linear motion of the piston inside the hydraulic cylinder into both linear and rotational motion, thereby generating oscillatory movement.