Hydraulic cylinders come in various structural forms and can be classified in multiple ways: by motion (linear reciprocating and rotary swing); by hydraulic pressure (single-acting and double-acting); by structure (piston, plunger, multi-stage telescopic sleeve, rack and pinion, etc.); by mounting method (connecting rod, clevis, foot, hinge pin, etc.); and by pressure rating (16 MPa, 25 MPa, 31.5 MPa, etc.).
Piston Type
A single-piston hydraulic cylinder has a piston rod at only one end. Figure 1 shows a single-piston hydraulic cylinder. Its inlet and outlet ports A and B can both supply pressurized oil or return oil, achieving bidirectional motion; hence, it is called a double-acting cylinder.
Plunger Type
(1) A plunger-type hydraulic cylinder is a single-acting hydraulic cylinder. It relies on hydraulic pressure to achieve movement in only one direction. The plunger's return stroke depends on external forces or its own weight.
(2) The plunger is supported only by the cylinder liner and does not contact it. This makes the cylinder liner very easy to machine, thus suitable for long-stroke hydraulic cylinders.
(3) The plunger is always under pressure during operation, therefore it must have sufficient rigidity.
(4) The plunger is often quite heavy. When placed horizontally, it is prone to sagging due to its own weight, causing unilateral wear on the seals and guides. Therefore, vertical use is more advantageous.
Telescopic Type
A telescopic hydraulic cylinder has two or more stages of pistons. The pistons extend in a sequence from largest to smallest, while the retraction sequence under no-load is generally from smallest to largest. Telescopic cylinders can achieve a longer stroke, while the retracted length is shorter, resulting in a more compact structure. This type of hydraulic cylinder is commonly used in engineering machinery and agricultural machinery. It has multiple pistons that move in one stroke. The output speed and output force of each piston change with each stroke.
Oscillating Type
Oscillating hydraulic cylinders are actuators that output torque and achieve reciprocating motion. They come in several forms, including single-vane, double-vane, and helical oscillating types. Vane type: The stator block is fixed to the cylinder body, while the vanes and rotor are connected together. Depending on the oil inlet direction, the vanes drive the rotor to oscillate reciprocally. Helical oscillating types are further divided into single-helical and double-helical types. Double-helical types are more commonly used now. They rely on two helical pairs to transform the linear motion of the piston inside the hydraulic cylinder into a composite motion of linear motion and rotational motion, thereby achieving the oscillating motion.
