Basic Knowledge of Hydraulic Cylinders

A hydraulic cylinder is a hydraulic device that converts hydraulic energy into mechanical energy, producing linear reciprocating or oscillating motion. It serves as a hydraulic actuator that transforms hydraulic energy into mechanical energy while delivering linear reciprocating (or oscillating) motion. With its simple structure and reliable operation, it can eliminate the need for a reducer when implementing reciprocating motion and features zero transmission backlash, making it widely used in various mechanical hydraulic systems. The output force of a hydraulic cylinder is directly proportional to the effective area of the piston and the pressure differential across its two sides. A typical hydraulic cylinder consists of a cylinder barrel and cylinder head, a piston and piston rod, sealing elements, cushioning devices, and venting mechanisms. While cushioning and venting devices depend on the specific application, other components may also be required. Due to its straightforward design and smooth reciprocating motion, hydraulic cylinders are extensively employed in construction machinery, marine equipment, machine tools, and even defense‑related applications.

Classification of Hydraulic Cylinder Mechanisms

Hydraulic cylinders can be classified structurally into piston cylinders, plunger cylinders, and oscillating cylinders. Pneumatic cylinders, on the other hand, are categorized into piston cylinders, diaphragm cylinders, and telescopic cylinders. The operating principle of a hydraulic cylinder is as follows: To understand how it works, let’s first examine its five basic components: 1) the cylinder body and cylinder head; 2) the piston and piston rod; 3) the sealing assembly; 4) the cushioning device; and 5) the venting mechanism. The working principle of each type of cylinder is similar. In a hydraulic cylinder, hydraulic fluid is manually pressurized via a lever‑operated pump (a manual hydraulic pump), with the fluid entering the cylinder through a single valve. At this point, the fluid cannot flow back due to the action of the check valve, forcing the piston rod to move upward. As long as the valve remains open, hydraulic fluid continues to enter the cylinder, driving continuous reciprocating motion. When the piston descends, the hydraulic valve is opened, allowing the fluid to return to the reservoir. This constitutes a straightforward operation; all other designs are essentially refinements built upon this fundamental principle.

Since the piston rods on both sides of the hydraulic cylinder have the same diameter, the effective working areas at both ends of the piston are identical. If hydraulic fluid at the same pressure and flow rate is supplied to both the left and right ports, the piston generates equal thrust and reciprocating speed. Such hydraulic cylinders are commonly used in applications where a constant reciprocating speed is required but relatively low thrust is sufficient, such as in the worktable of a grinding machine.

Precautions for Operating Hydraulic Cylinders

(1) Single‑rod hydraulic cylinders have a piston rod only on one side, resulting in unequal effective areas on either side of the piston. The larger the diameter of the piston rod, the greater the difference between the effective areas on the two sides. With equal supply pressure, the thrust generated on the rodless side is greater than that on the rod side. When the flow rate is constant, introducing pressurized fluid into the rodless chamber causes the piston rod to extend at a slower speed than when pressurized fluid is supplied to the rod side to retract the piston rod.

Note: Since the piston rod generates substantial thrust when extended, it can be moved rapidly during the return stroke, making this configuration ideal for applications where the piston rod is subjected to unidirectional loading and requires fast retraction under no‑load conditions. The thicker the piston rod, the greater the difference between thrust and pull forces, resulting in slower forward motion and faster return. A single‑rod hydraulic cylinder is used to achieve slow advance and rapid return for the hydraulic gantry worktable. Laizhou Hydraulic Co., Ltd.

(2) Double-rod hydraulic cylinder: This cylinder has piston rods on both sides of the piston. When the diameters of the two piston rods are identical and the supply pressure and flow rate remain constant, the reciprocating speed and force of the pistons are equal. Moreover, with two piston rods, the cylinder exhibits improved rigidity and stability.

Note: This operation requires substantial space, with a working stroke approximately three times the effective length. Hydraulic cylinders of this type are commonly used in grinding machine tables.

(3) Single-acting hydraulic cylinder: This type of hydraulic cylinder is a simple design. It can supply pressurized fluid to only one side of the piston, generating unidirectional force. The return stroke is accomplished by external loads, spring forces, or the self-weight of the piston rod or plunger. In other words, no hydraulic pressure is required for the return movement. Its advantages include reduced hydraulic power consumption and a simplified hydraulic circuit.

Note: The speed and force in the opposite direction cannot be controlled. The self-weight, load force, and spring force that drive the piston rod or plunger rod to move backward exceed the sum of the back pressure and frictional resistance in all parts of the hydraulic cylinder. For spring‑return hydraulic cylinders, the spring must have sufficient travel to allow its volume to increase.

Single-acting hydraulic cylinders are widely used in machine tool positioning and clamping, dump truck lifting, elevator lifting, ship crane booms, and more.

(4) Double-acting hydraulic cylinders are more widely used than single-acting hydraulic cylinders. Pressurized oil can be alternately supplied to both sides of the piston, driving it in a reciprocating motion, and allowing control over the speed of movement in both the pushing and pulling directions as well as the supply pressure. There are two types of double-acting hydraulic cylinders: single‑rod and double‑rod.

Note: Control systems are more complex than those of single‑acting hydraulic cylinders. Single‑rod double‑acting hydraulic cylinders are more widely used than double‑rod double‑acting hydraulic cylinders. Both reciprocating hydraulic cylinders for machine tools and various motion‑control hydraulic cylinders in construction machinery employ single‑rod double‑acting hydraulic cylinders.

(5) The piston cylinders of plunger hydraulic cylinders are typically single-acting. They feature a simple structure and are easy to manufacture and maintain. Plunger cylinders have thick, large, and heavy pistons, which offer superior rigidity compared to piston rods. Consequently, such cylinders should be used whenever possible in long-stroke hydraulic cylinders. Since the cylinder bore does not directly contact the plunger—guidance is provided solely through the guide sleeve—the inner surface of the cylinder can be either left unprocessed or subjected only to rough machining, resulting in excellent manufacturability and low processing costs.

Note: Plunger cylinders are relatively large in both volume and weight. When mounted horizontally, the plunger is pressed to one side under cold‑pressing conditions, which can lead to uneven wear on the seals and guide bushings. Therefore, plunger cylinders are best suited for vertical installation. During horizontal mounting and operation, a plunger support should be provided to prevent plunger sagging and to avoid the “loss of rigidity” phenomenon caused by initial flexural deformation and expansion of the flexible surface.

(6) Telescopic hydraulic cylinders, also known as multi‑stage cylinders, multi‑level cylinders, or composite cylinders, come in both single‑acting and double‑acting versions. These cylinders offer a long total stroke while maintaining a short retracted length, making them particularly well suited for applications where installation space is limited but a long stroke is required. Likewise, the more stages a cylinder has, the shorter its retracted length will be.