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01.What is a hydraulic cylinder
A hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy, producing linear reciprocating (or oscillating) motion. It features a simple structure and reliable operation. Reciprocating motion can be achieved without a reduction gear, and with zero transmission backlash and smooth movement, it is widely used in hydraulic systems of various machines. The output force of a hydraulic cylinder is proportional to the effective area of the piston and the pressure differential across it.
02.Structure of hydraulic cylinder
A hydraulic cylinder is usually composed of main parts such as the rear end cover, cylinder barrel, piston rod, piston assembly, and front end cover. In order to prevent oil from leaking out of the hydraulic cylinder or from the high-pressure chamber to the low-pressure chamber, sealing devices are provided between the cylinder barrel and the end cover, the piston and the piston rod, the piston and the cylinder barrel, and the piston rod and the front end cover. A dustproof device is also installed on the outside of the front end cover. In order to prevent the piston from hitting the cylinder cover when it quickly returns to the end of the stroke, a buffer device is also provided at the end of the hydraulic cylinder. Sometimes an exhaust device is also required.
03.Cylinder barrel tube assembly
The sealed cavity formed by the cylinder barrel tube assembly and the piston assembly is subjected to the action of oil pressure. Therefore, the cylinder barrel tube assembly must have sufficient strength, high surface accuracy and reliable sealing.
(1) Flange connection: simple structure, easy processing, reliable connection, but requires sufficient wall thickness at the end of the cylinder barrel tube to install bolts or screw in screws. It is a commonly used connection form.
(2) Semi-ring connection: divided into two connection forms: outer semi-ring connection and inner semi-ring connection. Semi-ring connection has good processability, reliable connection and compact structure, but it weakens the strength of the cylinder barrel tube. Semi-ring connection is widely used and is often used in the connection between seamless steel pipe cylinder barrel tube and end cover.
(3) Threaded connection: there are two types: outer thread connection and inner thread connection. Its characteristics are small size, light weight and compact structure, but the structure of the cylinder barrel tube end is complex. This connection form is generally used in occasions requiring small size and light weight.
(4) Tie rod connection: simple structure, good processability and strong versatility, but the end cover is large in size and weight. The tie rod will stretch and lengthen after being stressed, affecting the effect. Only suitable for medium and low pressure hydraulic cylinders with short length.
(5) Welding connection has high strength and simple manufacturing, but it is easy to cause deformation of the cylinder barrel tube during welding.
04. Basic working form of hydraulic cylinder:
Standard double-acting: Power travels in both directions and is used for most applications:
Single-acting cylinder: When thrust is required in only one direction, a single-acting cylinder can be used.
Double-rod cylinder: Used when equal displacement is required on both sides of the piston, or when connecting a load to each end is mechanically advantageous. The additional end can be used to mount a cam for operating a travel switch, etc.
Spring-return single-acting cylinder: Usually limited to very small, short-stroke cylinders for holding and clamping applications. The length required to accommodate the return spring makes them unsuitable for long strokes.
Plunger-type single-acting cylinder: With only one fluid chamber, this type of cylinder is usually mounted vertically. Retracting the load causes the cylinder to retract. They are also called “displacement cylinders” and are practical for long strokes.
Multi-stage telescopic cylinder: Can accommodate up to four sleeves and has a shorter retracted length than a standard cylinder. Available in single-acting or double-acting, these cylinders are more expensive than standard cylinders and are typically used where space is limited but a longer stroke is required.
Tandem cylinder: A tandem cylinder consists of two coaxially mounted cylinders, with their pistons connected by a common piston rod. A rod seal is placed between the two cylinders, allowing each cylinder to function as a double-acting cylinder. This can increase output when installation width or height is limited.
Double cylinder: A double cylinder consists of two coaxially mounted cylinders. The pistons are not connected. A rod seal is placed between the two cylinders, allowing each cylinder to function as a double-acting cylinder. The two cylinders can be mounted rod-to-piston (as shown) or back-to-back. It is typically used to provide three-position operation.
05.Working principle of hydraulic cylinder
Hydraulic transmission principle: Using oil as the working medium, motion is transmitted through changes in sealed volume, and power is transmitted through the pressure within the oil.
1. Power section: Converts the mechanical energy of the prime mover into the pressure energy (hydraulic energy) of the oil. An example is a hydraulic pump.
2. Actuator: Converts the pressure energy of the oil input by the hydraulic pump into mechanical energy that drives the working mechanism. Examples are hydraulic cylinders and hydraulic motors.
3. Control section: Controls and regulates the pressure, flow, and direction of the oil. Examples are pressure control valves, flow control valves, and directional control valves.
4. Auxiliary section: Connects the previous three sections to form a system, providing functions such as oil storage, filtration, measurement, and sealing. Examples include piping and fittings, oil tanks, filters, accumulators, seals, and control instruments.
Pressure applied at any point on a given volume of fluid is transmitted equally in all directions. This means that when using multiple hydraulic cylinders, each will pull or push at its own speed, determined by the pressure required to move the load.
If the cylinders have the same load capacity, the cylinder carrying the smallest load will move first, and the cylinder carrying the largest load will move last.
To synchronize the movement of the hydraulic cylinders so that the load is lifted at the same speed at any point, control valves or synchronized lifting system components must be used in the system.
06.Classification of hydraulic cylinders
To meet the diverse needs of various machine types, hydraulic cylinders come in a variety of types.
Based on the direction of oil supply, they can be categorized as single-acting and double-acting. Single-acting cylinders only supply high-pressure oil to one side of the cylinder, with external force driving the piston in the reverse direction. Double-acting cylinders supply pressurized oil to both sides of the cylinder. Both forward and reverse piston movement are achieved by hydraulic pressure.
Based on their structural form, they can be categorized as piston cylinders, plunger cylinders, swing cylinders, and telescopic sleeve cylinders. Based on the type of piston rod, they can be categorized as single-rod cylinders and double-rod cylinders.
Based on their specific application, they can be categorized as tandem cylinders, booster cylinders, speed-increasing cylinders, and stepper cylinders. These cylinders are not simply a single cylinder barrel, but are combined with other cylinder barrels and components. Therefore, from a structural perspective, they are also called combination cylinders.
1. Differential hydraulic cylinder
The differential principle of a hydraulic cylinder involves connecting oil supply lines to both ends. Because the piston rod’s active area at one end is smaller than the other, movement is achieved using the differential principle.
When pressurized oil is simultaneously introduced into both chambers of a single-rod piston cylinder, the effective active area of the rodless chamber is larger than that of the rod chamber, causing the piston’s force to the right to be greater than its force to the left. Consequently, the piston moves rightward, extending the piston rod. Simultaneously, oil in the rod chamber is squeezed out, flowing into the rodless chamber, accelerating the extension of the piston rod. This connection method for a single-rod hydraulic cylinder is called a differential connection. With a differential connection, the cylinder’s effective active area is equal to the cross-sectional area of the piston rod. The worktable’s movement speed is greater than when oil is flowing into the rodless chamber, while the output force is reduced.
Differential connection is an effective way to achieve rapid movement without increasing the capacity and power of the hydraulic pump.
2. Single-rod hydraulic cylinder
A single-piston rod hydraulic cylinder has a piston rod at only one end. It is a single-piston hydraulic cylinder. Its inlet and outlet ports A and B at both ends can be connected to pressure oil or return oil to achieve bidirectional motion, so it is also called a double-acting cylinder. When using it to achieve reciprocating motion, the reduction device can be eliminated, and there is no transmission gap, and the movement is smooth. Therefore, it is widely used in the hydraulic systems of various machines.
Features:
(1) Oil enters the rodless chamber and oil returns to the rod chamber.
(2) Oil enters the rod chamber and oil returns to the rodless chamber.
(3) Differential connection – the left and right chambers are connected and both are connected to pressure oil.
3. Single-rod piston cylinder
A single-rod piston cylinder has a piston rod at only one end. Because the effective areas of the left and right chambers of a single-rod piston cylinder are unequal, its characteristics are: when the pressure and flow Q of the fluid alternately entering the two chambers remain constant, the thrust F output by the piston cylinder in the left and right directions is unequal, and the reciprocating speeds are also different. The larger the piston rod diameter, the greater this difference. However, when the cylinder body and the piston rod are used for fixing, respectively, the corresponding worktable motion range is the same.
4. Double-rod piston cylinder
The rod diameters at both ends of a double-rod piston cylinder are usually equal, so the effective working area at both ends of the piston is also equal. When the two chambers of the cylinder are alternately fed with fluids of the same flow rate and pressure, the maximum thrust and movement speed generated on the piston are also equal. However, when the cylinder body and the piston rod are respectively fixed, the movement range of their corresponding workbenches is different.
The structure of a double-rod piston cylinder is similar to that of a double-piston rod hydraulic cylinder, and their graphic symbols are also the same.
A double-rod hydraulic cylinder is a hydraulic cylinder with piston rods on both sides of the piston. It is generally bidirectionally hydraulically driven and can achieve constant speed reciprocating motion.
Features:
(1) Oil enters the rodless chamber and oil returns to the rod chamber.
(2) Oil enters the rod chamber and oil returns to the rodless chamber.
(3) Differential connection – the left and right chambers are connected, and both are connected with pressure oil.
5. Gas-liquid booster cylinder
A gas-hydraulic booster cylinder, also known as a gas-liquid booster cylinder or simply a booster cylinder, is an improved design that combines the advantages of both air and oil cylinders. The hydraulic oil is strictly isolated from the compressed air. The piston rod in the cylinder automatically activates upon contact with the workpiece, resulting in faster actuation and greater stability than pneumatic transmissions. The cylinder assembly is simple, and output adjustment is easy. Under the same conditions, it can achieve the highest output of a hydraulic press. It also offers low energy consumption, a soft landing without damaging the mold, and easy installation. Special booster cylinders can be installed at any 360-degree angle. These core features include a small footprint, minimal malfunctions, no temperature rise issues, a long lifespan, and low noise. Booster cylinders utilize standard air pressure to achieve the high output of a hydraulic cylinder, eliminating the need for a hydraulic unit. Booster cylinders are generally categorized as preloaded, direct-pressure, adjustable-stroke, increased-return-tension, compact parallel, mini, fast-acting, and air-oil-isolated.
The working frequency of the booster cylinder is generally 10~70 times/minute according to different strokes and cylinder diameters. Actuation mode: double action. Operating speed: 50~1000mm/s. Output range: 1~100 tons. Application range: stamping marks, bending profiles, die punching, punching steel, profile welding, extrusion molding, flattening and straightening, riveting forging, shaping sheet metal, tight assembly, riveting connection, metal stamping.
6. Telescopic hydraulic cylinder
Telescopic hydraulic cylinders are hydraulic cylinders with multi-stage, sleeve-shaped piston rods that achieve a long working stroke. They are also called multi-stage hydraulic cylinders. Telescopic hydraulic cylinders are constructed by assembling two or more piston-type hydraulic cylinders, with the piston rod of the first stage connecting to the cylinder barrel of the next stage.
When pressurized oil enters the rodless chamber, the cylinder barrel with the largest piston effective area begins to extend. When it reaches its end point, the cylinder barrel with the second smallest piston effective area begins to extend. Telescopic hydraulic cylinders extend sequentially from largest to smallest, achieving a very long working stroke. The smaller the effective area of the extended cylinder barrel, the faster the extension speed. Consequently, the extension speed gradually increases, and the corresponding hydraulic thrust decreases. This thrust and speed variation is ideally suited to the thrust and speed requirements of various automatic loading and unloading machines. Retraction, on the other hand, generally occurs sequentially from smallest to largest, resulting in a shorter axial length during retraction, smaller footprint, and a more compact structure. These cylinders are commonly used in the hydraulic systems of construction machinery and other mobile machinery, such as cranes and dump trucks.
7. Plunger cylinder
A plunger cylinder is a type of hydraulic cylinder.
A single plunger cylinder can only move in one direction; reversing requires external force, as shown in Figure a below. Combining two plunger cylinders, as shown in Figure b, can also achieve reciprocating motion using pressurized oil. The plunger cylinder’s movement is guided by a guide sleeve on the cylinder head, eliminating the need for precision machining of the inner cylinder wall. It is particularly suitable for applications with long strokes. Plunger cylinders are also categorized as radial and axial.