Exploring Hydraulic Cylinder Types and Features

Power cylinder

A power cylinder is an actuator that converts hydraulic or pneumatic energy into mechanical energy to achieve linear reciprocating or oscillating motion. It is mainly divided into two categories: hydraulic cylinders and pneumatic cylinders. Its structure is simple and there is no transmission backlash, which eliminates the need for a speed reduction device. It is widely used in petrochemical, engineering machinery and other fields .

Hydraulic cylinders consist of a cylinder barrel, cylinder head, piston rod, piston, sealing device, buffer device, and venting device. They are classified into piston type (including single and double rod structures), telescopic type, and oscillating type. Pneumatic cylinders include piston type, plunger type, and oscillating type, among which oscillating cylinders can directly output torque. The sealing devices (such as V-type combination seals and Glyd rings) of both types of power cylinders directly affect operational stability. The cylinder barrel uses a rolling process to improve surface hardness and wear resistance. The buffer device converts kinetic energy into heat energy through throttling to avoid collisions, and the venting device ensures system stability. In the field of industrial automation, electric cylinders integrating electronic control technology are gradually replacing traditional hydraulic/pneumatic systems.

Types and characteristics of hydraulic cylinders

Hydraulic cylinders (oil cylinders) are mainly used to achieve linear reciprocating motion of mechanisms, and can also achieve oscillation. They have a simple structure, reliable operation, and wide application. The input of a hydraulic cylinder is the flow rate and pressure of the fluid, and the output is speed and force. Both hydraulic cylinders and hydraulic motors are hydraulic actuators, whose function is to convert hydraulic energy into mechanical energy. See the schematic diagram of a hydraulic cylinder.

Hydraulic cylinders can be classified as single-acting cylinders and double-acting cylinders according to the direction of oil supply; according to their structure, they can be classified as piston cylinders, plunger cylinders, telescopic sleeve cylinders, and swing hydraulic cylinders; and according to the piston rod type, they can be classified as single-piston rod cylinders and double-piston rod cylinders.

Piston Hydraulic Cylinders

Piston hydraulic cylinders (3 images) can be divided into single-rod and double-rod structures. They are fixed in two ways: by the cylinder body or by the piston rod. Based on the action of hydraulic pressure, they are classified as single-acting or double-acting. In a single-acting hydraulic cylinder, pressurized oil is supplied to only one chamber, and the cylinder moves in one direction due to hydraulic pressure. Reverse movement is achieved by external forces (such as spring force, weight, or external load). In a double-acting hydraulic cylinder, the piston moves in both directions by alternating oil supply to both chambers, relying on hydraulic pressure.

A double-piston rod hydraulic cylinder has piston rods at both ends of the piston, and it comes in two installation types: fixed cylinder body and fixed piston rod.

A single-piston rod hydraulic cylinder has a piston rod at only one end of the piston, allowing for bidirectional piston movement to achieve different speeds and output forces.

When pressurized oil is simultaneously supplied to both chambers of a single-piston rod cylinder, the effective working area of the rodless chamber is larger than that of the rod chamber, resulting in a greater force to the right than to the left. Therefore, the piston moves to the right, and the piston rod extends outward. Simultaneously, oil in the rod chamber is forced out and flows into the rodless chamber, thus accelerating the extension speed of the piston rod. This connection method in a single-piston rod hydraulic cylinder is called differential connection. Differential connection is an effective way to achieve rapid movement without increasing the capacity and power of the hydraulic pump.

plunger hydraulic cylinder

When the stroke of a piston-type hydraulic cylinder is long, the machining difficulty increases, leading to higher manufacturing costs. In some applications, hydraulic cylinders do not require bidirectional control; the plunger-type hydraulic cylinder is a cost-effective option that meets these requirements.

A plunger cylinder consists of a cylinder barrel, plunger, guide sleeve, sealing ring, and gland. The plunger does not contact the inner wall of the cylinder barrel, therefore the inner bore of the cylinder barrel does not require precision machining, resulting in good manufacturability and low cost. Plunger hydraulic cylinders are single-acting; their return stroke requires the aid of their own weight or other external forces such as springs. To achieve bidirectional motion, two plunger hydraulic cylinders can be used in pairs; to reduce the weight of the plunger, hollow plungers are sometimes manufactured.

Swing hydraulic cylinder

A swing hydraulic cylinder can achieve reciprocating swing motion with an angle of less than 360°. Because it can directly output torque, it is also called a swing hydraulic motor. It mainly comes in two structural forms: single-vane and double-vane. A single-vane swing hydraulic cylinder mainly consists of a stator block 1, a cylinder body 2, a swing shaft 3, vanes 4, left and right support plates, and left and right cover plates. The stator block is fixed to the cylinder body, and the vanes and swing shaft are rigidly connected. When pressurized oil is successively supplied to both oil ports, the vanes drive the swing shaft to reciprocate.

The swing angle of a single-vane oscillating hydraulic cylinder generally does not exceed 280º, while that of a double-vane oscillating hydraulic cylinder generally does not exceed 150º. When the input pressure and flow rate remain constant, the output torque of the double-vane oscillating hydraulic cylinder’s swing shaft is twice that of a single-vane oscillating cylinder with the same parameters, while its oscillation angular velocity is half that of a single-vane cylinder. Oscillating cylinders have a compact structure and high output torque, but sealing is difficult, and they are generally only used in medium and low-pressure systems for reciprocating oscillation, indexing, or intermittent motion.

Telescopic hydraulic cylinder

Telescopic hydraulic cylinders have two or more stages of pistons. The pistons extend in descending order of size, while retracting under no-load generally proceeds from smallest to largest. Telescopic cylinders can achieve long strokes and short retracted lengths, resulting in a compact structure. This type of hydraulic cylinder is commonly used in construction and agricultural machinery. Multiple pistons have single-stroke operation, and the output speed and force vary with each piston’s successive strokes. Telescopic double-acting cylinders have inlet and outlet ports A and B at both ends of the cylinder body. When oil enters through port A and exits through port B, the first-stage piston moves to the right. When the first-stage piston reaches its rightward end, the second-stage piston continues to move to the right under the pressure of the oil.

The characteristics of telescopic hydraulic cylinders are: long piston rod extension stroke and small structural size after retraction, making them suitable for dump trucks, crane telescopic booms, etc.

rack and pinion cylinder

The rack and pinion piston cylinder consists of a double-acting piston cylinder with a rack and pinion rod and a gear and rack mechanism. The reciprocating movement of the piston is converted into the reciprocating rotation of the gear shaft through the rack and gear mechanism.

Structure of hydraulic cylinder

The single-piston rod hydraulic cylinder mainly consists of a cylinder base 1, a cylinder barrel 6, a cylinder head 10, a piston 4, a piston rod 7, and a guide sleeve 8. One end of the cylinder barrel is welded to the cylinder base, and the other end is threaded to the cylinder head. The piston and piston rod are connected by a key. To ensure reliable sealing of the hydraulic cylinder, sealing rings 3, 5, 9, 11 and dustproof rings 12 are installed in corresponding locations.

Cylinder block assembly

The cylinder barrel is the main body of the hydraulic cylinder. Its inner bore is generally manufactured using precision machining processes such as boring, reaming, rolling, or honing, requiring a surface roughness of 0.1μm to 0.4μm. End caps are mounted at both ends of the cylinder barrel, forming a closed oil chamber with it, and also withstand significant hydraulic pressure. Therefore, the end caps and their connecting parts must have sufficient strength. Guide sleeves guide and support the piston rod or plunger. Some hydraulic cylinders do not have guide sleeves and directly use the end cap holes for guidance. For material selection and technical requirements of the cylinder barrel, end caps, and guide sleeves, please refer to the hydraulic design manual.

Piston assembly

Piston assemblies consist of pistons, seals, piston rods, and connecting parts.

Piston assemblies are primarily used to prevent hydraulic oil leakage. The basic requirements for sealing devices are good sealing performance and the ability to automatically improve sealing with increasing pressure. In addition, they should have low frictional resistance and be oil-resistant.

Hydraulic cylinders mainly use sealing rings. There are several types of sealing rings, including O-rings, V-rings, Y-rings, and combinations. The materials used are oil-resistant rubber, nylon, polyurethane, etc.

(1) O-rings

O-rings have a circular cross-section and are mainly used for static sealing. Compared to lip seals, they have greater dynamic resistance and are prone to torsion when used for dynamic sealing. Therefore, they are generally not used alone for dynamic sealing of hydraulic cylinders.

The principle of O-ring sealing: When installing any shape of sealing ring, an appropriate pre-compression must be ensured. Too little pre-compression will prevent sealing, while too much pre-compression will increase friction and easily damage the ring. Therefore, the groove dimensions and surface accuracy of the sealing ring must be strictly guaranteed according to the data given in the relevant manual. In dynamic seals, when the pressure exceeds 10 MPa, the O-ring will be squeezed into the gap and damaged. Therefore, a PTFE or nylon retainer ring needs to be installed on the low-pressure side of the O-ring. When subjected to high pressure in both directions, retainer rings must be added on both sides.

(2) V-ring

The V-ring has a V-shaped cross-section. A V-ring sealing device consists of a pressure ring, a V-ring, and a support ring. When the working pressure exceeds 10 MPa, the number of V-rings can be increased to improve the sealing effect. During installation, the opening of the V-ring should face the side with higher pressure.

(3) Y (Yx) shaped sealing ring

The Y-shaped sealing ring has a Y-shaped cross-section and belongs to the lip seal category. It is a sealing ring with low frictional resistance and a long service life, and is widely used. Y-rings are mainly used for sealing reciprocating motion. Depending on the length-to-width ratio of the cross-section, Y-rings can be divided into wide-section and narrow-section types.

Buffer Device

When a hydraulic cylinder drives a heavy component in a rapid reciprocating motion, the moving parts possess significant kinetic energy. Therefore, when the piston reaches the end of the hydraulic cylinder, it collides with the end cap, generating impact and noise. This mechanical impact not only damages parts of the hydraulic cylinder but also causes damage to other related machinery. To prevent this hazard and ensure safety, buffering measures should be implemented to control the speed of the hydraulic cylinder.

Air Venting Device

Hydraulic transmission systems often allow air to enter, causing instability, vibration, creeping, or forward lurching. In severe cases, this can prevent the system from functioning properly. Therefore, air removal must be considered when designing hydraulic cylinders.

For hydraulic cylinders requiring high speed stability and large hydraulic cylinders, a dedicated air venting device, such as an air plug or air valve, is often installed at the highest point of the cylinder. After loosening the locking screw of the air plug or valve and performing several low-pressure reciprocating movements, the air-filled oil will be expelled. Once the air is removed, tighten the screw, and the hydraulic cylinder will operate normally.

Types of pneumatic cylinders

Based on the common structural forms of pneumatic cylinders, they can be divided into four types:

Piston type

A single-piston rod pneumatic cylinder has a piston rod at only one end.

Plunger type pneumatic cylinder

(1)A plunger type pneumatic cylinder is a single-acting pneumatic cylinder that can only achieve movement in one direction by means of air pressure. The return stroke of the plunger depends on other external forces or the weight of the plunger itself;

(2) The plunger is supported only by the cylinder liner and does not contact the cylinder liner, making the cylinder liner very easy to machine, thus suitable for making long-stroke pneumatic cylinders;

(3) The plunger is always under pressure during operation, therefore it must have sufficient rigidity;

(4)The plunger is often quite heavy, and when placed horizontally, it is prone to sag due to its own weight, causing unilateral wear of the seals and guides, so vertical use is more advantageous.

Telescopic Type

Telescopic pneumatic cylinders have 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 and a shorter retracted length, resulting in a more compact structure. This type of pneumatic cylinder is commonly used in engineering and agricultural machinery.

Oscillating Type

Oscillating pneumatic cylinders are actuators that output torque and achieve reciprocating motion; they are also called oscillating pneumatic motors. They come in single-blade and double-blade forms. The stator block is fixed to the cylinder body, while the blades are connected to the rotor. Depending on the oil inlet direction, the blades drive the rotor to oscillate back and forth.

Machining of pneumatic cylinders

As a key component of products such as pneumatic cylinders, mining single-pillar supports, pneumatic brackets, and gun barrels, the quality of its machining directly affects the lifespan and reliability of the entire product. Cylinder machining demands high precision, with strict requirements for coaxiality and wear resistance. A fundamental characteristic of cylinders is deep-hole machining, which has always been a challenge for machining personnel.

Roller burnishing, by leaving residual compressive stress on the surface layer, helps to close micro-cracks and hinders the propagation of corrosion. This improves surface corrosion resistance and delays the initiation or propagation of fatigue cracks, thus increasing the cylinder fatigue strength. Through roller burnishing, a work-hardened layer is formed on the surface, reducing elastic and plastic deformation of the grinding surfaces, thereby improving the wear resistance of the cylinder inner wall and preventing burns caused by grinding. The reduced surface roughness after roller burnishing improves the fit properties.

Hydraulic cylinders are the most important components of engineering machinery. The traditional machining method is: broaching the cylinder body – precision boring the cylinder body – grinding the cylinder body. The roller burnishing method is: broaching the cylinder body – precision boring the cylinder body – roller burnishing the cylinder body. The process involves three parts, but in terms of time: grinding a 1-meter cylinder body takes approximately 1-2 days, while roller burnishing a 1-meter cylinder body takes approximately 10-30 minutes. The investment is also significant: grinding machines or honing machines cost tens of thousands to millions of dollars, while roller burnishing tools cost thousands to tens of thousands of dollars. The surface hardness of the hole is increased by approximately 30%, and the fatigue strength of the cylinder barrel’s inner surface is increased by 25%. If only the cylinder barrel’s influence is considered, the cylinder’s service life is increased by 2-3 times, and the boring and roller burnishing process is about 3 times more efficient than the grinding process. These data demonstrate that the roller burnishing process is highly efficient and can significantly improve the surface quality of the cylinder barrel.

After the cylinder is rolled, there are no sharp micro-cut edges on the surface. Long-term movement and friction will not damage the sealing ring or seals, which is especially important in the pneumatic industry.

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Contact: Nancy Zhu, Sales manager, JW GROUP.

Email: nancy@jwgroup.cc

Web: https://jwcylinder.com

Mobile/Whatsapp:+86 15902166721

HYDRAULIC CYLINDER and CNC parts specialist

JW GROUP is an integrated steel product group, products include: hydraulic cylinder, hydraulic cylinder spare parts, drilling forging parts, pneumatic actuator-scotch yoke, CNC machinery parts etc.

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Exploring Hydraulic Cylinder Types and Features

Power cylinder