What is a hydraulic cylinder?
Hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and does linear reciprocating movement (or swinging movement). It is simple in structure and reliable in operation. When it is used to achieve reciprocating movement, the deceleration device can be eliminated, and there is no transmission gap, and the movement is smooth, so it is widely used in various mechanical hydraulic systems. The output force of the hydraulic cylinder is proportional to the effective area of the piston and the pressure difference between the two sides;
Hydraulic cylinder structure
The hydraulic cylinder is usually composed of a rear end cover, cylinder, piston rod, piston assembly, front end cover and other main parts; To prevent oil leakage outside the hydraulic cylinder or from the high pressure chamber to the low pressure chamber, a sealing device is arranged between the cylinder and the end cover, the piston and the piston rod, the piston and the cylinder, the piston rod and the front end cover, and a dustproof device is also arranged outside the front end cover; To prevent the piston from hitting the cylinder head when it quickly returns to the end of the stroke, the hydraulic cylinder end is also provided with a buffer device; Sometimes an exhaust device is required.
Cylinder block assembly
The sealing cavity formed by the cylinder assembly and the piston assembly is subjected to oil pressure, so the cylinder assembly should have sufficient strength, high surface accuracy, and reliable sealing.
(1) Flange type connection, simple structure, easy processing, reliable connection, but the cylinder end is required to have enough wall thickness to install bolts or screw, it is a commonly used connection form.
(2) Half ring connection, divided into two connection forms of outer half ring connection and inner half ring connection. The ring connection process is good, reliable, and compact in structure, but it weakens the strength of the cylinder. The application of a ring connection is very common, and it is often used in the connection of a seamless steel pipe cylinder and end cap.
(3) threaded connection, there are two kinds of external thread connection and internal thread connection, which are characterized by small size, light weight, and compact structure, but the cylinder end structure is complex; this connection form is generally used for occasions requiring small size and light weight.
(4) The tie rod connection has a simple structure, good process, and strong versatility, but the volume and weight of the end cover are larger, and the tie rod will be stretched longer after being stressed, affecting the effect. Only suitable for medium and low pressure hydraulic cylinders with small length.
(5) Welded connection, high strength, simple manufacturing, but easy to cause cylinder deformation during welding.
The basic form of action of the hydraulic cylinder:
Standard dual action: power travel in both directions, and used in most applications:
Single-acting cylinder: When thrust is required in only one direction, a single-acting cylinder can be used;
Twin cylinder: When equal displacement is required on both sides of the piston, or when a load is attached to each end, when mechanically advantageous, the additional end can be used to install cams for operating stroke switches, etc.
Spring return single-acting cylinder: Usually limited to very small, short stroke cylinders used to hold and clamp. The length required to accommodate the return spring makes them a nuisance when long journeys are required;
Plunger type single acting cylinder: only one flow chamber, this type of cylinder is usually installed vertically, load resets the cylinder retraction, they are also called "displacement cylinder", and are practical for long travel;
Multistage telescopic cylinder: up to 4 sleeves, shorter than the standard cylinder. There are single action or double action, they are more expensive than the standard cylinder, usually used for installation space is small, but requires a larger stroke occasion.
Tandem cylinder: A tandem cylinder foot consists of two coaxially mounted cylinders, the pistons of the two cylinders are linked by a common piston rod, and the rod seal is arranged in front of the two cylinders so that each cylinder can double function when the mounting width or height is limited. The tandem cylinder can increase the output;
Double cylinder: A double cylinder is composed of two cylinders mounted a coaxially way. The two pistons are not connected. A rod seal is arranged between the two cylinders so that each cylinder can double function, and the two cylinders can be mounted on the piston rod (as shown) or back-to-back. Usually used to provide three position work.
Working principle of hydraulic cylinder
Working principle and structure analysis of a complete set of hydraulic cylinders (Animation demonstration)
Hydraulic transmission principle - with oil as the working medium, the movement is transmitted through the change of the sealing volume, and the power is transmitted through the pressure inside the oil.
1. The power part converts the mechanical energy of the prime mover into the pressure energy of the oil (hydraulic energy). For example: hydraulic pump.
2..The executive part - the hydraulic pump input oil pressure energy into mechanical energy to drive the working mechanism. For example: hydraulic cylinder, hydraulic motor.
3. The control part - used to control and adjust the oil pressure, flow, and flow direction. For example: pressure control valve, flow control valve, and direction control valve.
4. The auxiliary part - the first three parts are connected to form a system, which plays the role of oil storage, filtration, measurement, and sealing. Examples include pipes and joints, fuel tanks, filters, accumulators, seals, and control instruments.
Pressure applied at any point on a given volume of liquid can be transmitted equally in all directions. This means that when multiple hydraulic cylinders are used, each cylinder will pull or push at its own speed, and these speeds depend on the pressure required to move the load.
In the case of the same bearing capacity of the hydraulic cylinder, the hydraulic cylinder carrying the smallest load will move first, and the hydraulic cylinder carrying the largest load will move last.
In order to synchronize the movement of the hydraulic cylinder so that the load is jacked at the same speed at any point, it is necessary to use a control valve or synchronous jacking system element in the system.
Classification of hydraulic cylinders
In order to meet the different uses of various main engines, there are many types of hydraulic cylinders.
According to the direction of oil supply, it can be divided into single acting cylinder and double acting cylinder. The single-acting cylinder only inputs high-pressure oil to one side of the cylinder and relies on other external forces to reverse the piston. The double acting cylinder feeds pressure oil to both sides of the cylinder. The forward and reverse movement of the piston is accomplished by hydraulic pressure.
According to the structure, it can be divided into piston cylinder, plunger cylinder, swing cylinder, and telescopic sleeve cylinder. According to the form of the piston rod, it can be divided into single piston rod cylinder and double piston rod cylinder.
According to the special purpose of the cylinder, it can be divided into series cylinder, booster cylinder, speed cylinder, stepping cylinder, and so on.
This type of cylinder is not a simple cylinder, but a combination of other cylinders and components, so from the structural point of view, this type of cylinder is also called a combination cylinder.
1. Differential hydraulic cylinder
The differential principle of the hydraulic cylinder is that both ends are connected to the oil supply pipeline at the same time; one end is connected because the piston rod area is smaller than the other end. The differential principle is used to achieve movement.
When two cavities of a single rod piston cylinder are fed into the pressure oil at the same time, because the effective area of the rodless cavity is greater than the effective area of the rodless cavity, the force of the piston to the right is greater than the force of the left. Therefore, the piston moves to the right and the piston rod extends out. At the same time, the oil with the rod cavity is extruded so that it flows into the rodless cavity, thus speeding up the extension speed of the piston rod. This connection method of the single piston rod hydraulic cylinder is called the differential connection. In the differential connection, the effective area of the hydraulic cylinder is the cross-sectional area of the piston rod, and the speed of the table is larger than that of the rodless cavity, while the output force is reduced.
Working principle and structure analysis of a complete set of hydraulic cylinders (Animation demonstration)
Differential connection is an effective way to achieve fast movement without increasing the capacity and power of the hydraulic pump.
2. Single lever hydraulic cylinder
A single piston rod hydraulic cylinder has a piston rod at only one end. It is a single piston hydraulic cylinder. The inlet and outlet oil ports A and B at both ends can pass pressure oil or return oil to achieve two-way movement, so it is also called double acting cylinder. When it is used to achieve reciprocating movement, the deceleration device can be eliminated, and there is no transmission gap, and the movement is smooth, so it is widely used in various mechanical hydraulic systems.
Features:
(1) Oil inlet without rod cavity, oil return with rod cavity.
(2) Oil inlet with rod cavity, oil return without rod cavity.
(3) Differential connection - the left and right cavities are connected, and the pressure oil is passed.
Single rod cylinder three comparison, as shown in the figure below:
Working principle and structure analysis of a complete set of hydraulic cylinders (Animation demonstration)
3. Single rod piston cylinder
The piston of the single rod piston cylinder has only one end with the piston rod, because the effective area of the left and right cavities of the single rod piston cylinder is different, so it is characterized by: when the fluid pressure and flow Q are unchanged alternating into the two cavities of the cylinder, the output thrust F of the piston cylinder in the left and right directions is not equal, the reciprocating speed port is not the same, and the larger the diameter of the piston rod, the greater the difference. However, when the cylinder block is fixed and the piston rod is fixed, the motion range of their corresponding workbench is the same.
Working principle and structure analysis of a complete set of hydraulic cylinders (Animation demonstration)
4. Double rod piston cylinder
The rod diameters at both ends of the double rod piston cylinder are usually equal, so the effective area of the piston at both ends is also equal. When the two chambers of the cylinder alternately input the same flow and pressure of fluid, the maximum thrust and motion speed generated on the piston are also equal. However, when the cylinder block is fixed and the piston rod is fixed, the motion range of their corresponding workbench is different
The structure of the double rod piston cylinder is similar to that of the double rod hydraulic cylinder, and the graphic symbol is the same.
The double rod hydraulic cylinder is a hydraulic cylinder with a piston rod on both sides of the piston, which is generally driven by two-way hydraulic pressure and can achieve constant speed reciprocating movement.
Features:
(1) Oil inlet without rod cavity, oil return with rod cavity.
(2) Oil inlet with rod cavity, oil return without rod cavity.
(3) Differential connection - the left and right cavities are connected, and the pressure oil is passed.
Working principle and structure analysis of a complete set of hydraulic cylinders (Animation demonstration)
5. Gas-liquid booster cylinder
Gas-liquid booster cylinder is also known as gas-liquid booster cylinder, generally referred to as booster cylinder. Hydraulic oil and compressed air are strictly isolated, the piston rod in the cylinder contacts the working parts and starts automatically, the action speed is fast, and it is more stable than the pneumatic transmission, the cylinder block device is simple, the output adjustment is easy, under the same conditions can achieve the high force of the hydraulic press, low energy consumption, soft landing does not damage the mold. Easy to install and special booster cylinder can be installed at any Angle of 360 degrees, occupies a small space, causes less trouble without temperature rise, long life, is low-noise, and has other core characteristics. The booster cylinder can achieve the high force of the hydraulic cylinder by using the general air pressure, and no hydraulic unit is required. Booster cylinders can be generally divided into: pre-pressure booster cylinders, direct pressure booster cylinders, adjustable stroke booster cylinders, increased return pull booster cylinders, compact parallel booster cylinders, mini booster cylinders, fast booster cylinders, and oil and gas isolation booster cylinders.
The working frequency of the booster cylinder, according to different strokes and cylinder diameters, is generally 10~70 times/min.
Actuation mode: double-acting operating speed: 50~1000mm/s Output range: 1~100 tons Application range: Stamping marks, bending profiles, die punching, punching steel, profile touch welding, extrusion molding, flattening and straightening, riveting and forging, sheet metal finishing, tight assembly, riveting and joining, metal stamping.
6. Telescopic hydraulic cylinder
The telescopic hydraulic cylinder is a hydraulic cylinder with a multistage telescopic piston rod that can get a longer working stroke, and the telescopic hydraulic cylinder is also known as the multistage hydraulic cylinder. The telescopic hydraulic cylinder is composed of two or more piston hydraulic cylinders, and the piston rod of the first piston cylinder is the cylinder of the second piston cylinder.
When the pressure oil enters from the rodless cavity, the cylinder with the largest effective area of the piston begins to extend, and when the line reaches the end, the cylinder with the second largest effective area of the piston begins to extend. The extension order of the telescopic hydraulic is extended from large to small, which can obtain a long working stroke. The smaller the effective area of the overstretched cylinder, the faster the extension speed. Therefore, the extension speed is slow and fast, and the corresponding hydraulic thrust decreases from large to small; This change law of thrust and speed is suitable for the requirements of various automatic loading and unloading machinery for thrust and speed. The order of retraction is generally from small to large, and the axial length of the retraction is short, the space is small, and the structure is compact. It is often used in the hydraulic system of construction machinery and other walking machinery, such as cranes, dump cars, etc.
7. Plunger cylinder
Plunger cylinder is a structural form of hydraulic cylinder.
A single plunger cylinder can only achieve one direction of movement, and the reverse depends on external force, as shown in Figure A below. With two plunger cylinders combined, as shown in Figure b, reciprocating motion can also be achieved with pressurized oil. When the plunger cylinder moves, it is guided by the guide sleeve on the cylinder head, so the inner wall of the cylinder does not need to be finished. It is especially suitable for longer trips. In addition, the plunger cylinder is divided into a radial plunger cylinder and an axial plunger cylinder.
