According to the movement form, hydraulic cylinders can be divided into linear reciprocating hydraulic cylinders and swing hydraulic cylinders. According to the oil supply direction, it can be divided into single-acting cylinders and double-acting cylinders. According to the structure, it can be divided into piston cylinder, plunger cylinder, telescopic cylinder, swing cylinder, rack piston cylinder and booster cylinder. According to the form of the piston rod, it can be divided into a single piston rod cylinder and a double piston rod cylinder.
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Piston-type hydraulic cylinders can be divided into two structural forms: double-rod type and single-rod type, and there are two ways to install them: cylinder barrel fixing and piston rod fixing.
Both ends of the piston of the double-piston-rod hydraulic cylinder have piston rods, which can be divided into two installation forms: fixed cylinder and fixed piston rod, as shown in the figure.
Because the diameters of the two piston rods of the double-piston rod hydraulic cylinder are equal, when the input flow rate and oil pressure are constant, its reciprocating motion speed and thrust are equal. Then the moving speed V and thrust F of the cylinder are respectively:
This hydraulic cylinder is often used on occasions that require the same speed of reciprocating motion.
The piston of the single-piston rod hydraulic cylinder only has a piston rod at one end, and the two-way movement of the piston can obtain different speeds and output forces. The schematic diagram and oil circuit connection method are shown in the figure below.
Calculation of speed and thrust when the rodless cavity enters oil:
Calculation of speed and thrust when oil enters the rod cavity:
Comparison of the above two oil inlet methods:
Differential connection: When the two cavities of the single-rod piston cylinder are fed with pressure oil at the same time (below). Since the effective active area of the rodless chamber is greater than that of the rod chamber, the rightward acting force of the piston is greater than the leftward acting force. Therefore, the piston moves to the right, and the piston rod extends outward. At the same time, the oil in the rod cavity is squeezed out to make it flow into the rodless cavity, thereby speeding up the extension speed of the piston rod. This connection method of the single piston rod hydraulic cylinder is called a differential connection.
Calculation of speed and thrust in differential connection:
When differentially connected, the effective area of the hydraulic cylinder is the cross-sectional area of the piston rod, and the movement speed during operation is larger than that of the rodless chamber when oil is fed, while the output force is smaller. Differential connection is an effective way to achieve fast movement without increasing the capacity and power of the hydraulic pump.
When the stroke of the piston hydraulic cylinder is long, the processing is difficult, which increases the manufacturing cost. The hydraulic cylinder used in some occasions does not require two-way control, and the plunger hydraulic cylinder is a low-cost hydraulic cylinder that meets this requirement.
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The plunger cylinder is composed of a cylinder barrel, plunger, guide sleeve, sealing ring, and gland. The plunger does not touch the inner wall of the cylinder barrel, so the inner hole of the cylinder barrel does not need to be finished, and the manufacturability is good and the cost is low. The plunger hydraulic cylinder is single-acting, and its return stroke needs to be completed by other external forces such as self-weight or springs. If two-way movement is to be obtained, two plunger hydraulic cylinders can be used in pairs. In order to reduce the weight of the plunger, it is sometimes made into a hollow plunger. The following figure.
The swing hydraulic cylinder can achieve a reciprocating swing motion with an angle of less than 360°. Because it can directly output torque, it is also called a swing hydraulic motor. There are mainly two structural forms: single-blade and double-blade. The single-blade swing hydraulic cylinder is mainly composed of stator block 1, cylinder body 2, swing shaft 3, blade 4, left and right support plates, left and right cover plates, and other main parts. The stator block is fixed on the cylinder body, and the vane and the swing shaft are fixedly connected together. When the two oil ports are successively supplied with pressure oil, the vane drives the swing shaft to swing back and forth.
When considering the mechanical efficiency, the swing shaft output torque of the single vane cylinder is as follows:
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The swing angle of the single-blade swing hydraulic cylinder generally does not exceed 280 º, and the swing angle of the double-blade swing hydraulic cylinder generally does not exceed 150 º. When the input pressure and flow are constant, the output torque of the swing shaft of the double-vane swing hydraulic cylinder is twice that of the single-vane swing cylinder with the same parameters, and the swing angular velocity is half of that of the single-vane. The swing cylinder has a compact structure and a large output torque, but it is difficult to seal. It is generally only used for reciprocating swing, indexing, or intermittent movement in medium and low-pressure systems.
The characteristics of the telescopic hydraulic cylinder are: the stroke of the piston rod is long, and the structure size after contraction is small, which is suitable for dump trucks, telescopic arms of cranes, etc.
There are inlet and outlet ports A and B at both ends of the cylinder. When oil enters port A and returns oil at port B, first push the primary piston to move to the right. When the primary piston travels right to the end, the secondary piston continues to move to the right under the action of pressure oil.
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The rack piston cylinder is composed of a double-acting piston cylinder with a rack rod and a rack and pinion mechanism. The reciprocating movement of the piston becomes the reciprocating rotation of the pinion shaft through the rack and gear mechanism.
A hydraulic cylinder consists of the following parts:
The cylinder barrel is mostly a seamless thick-walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally.
In most hydraulic cylinders, the barrel and the bottom are welded together; this can damage the inside of the barrel if done poorly. Therefore, some hydraulic cylinder designs have a screwed or flanged connection from the cylinder end cap to the barrel. In this type the cylinder barrel can be disassembled and repaired in future.
The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general however, the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger hydraulic cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.
The piston is a short, cylinder-shaped metal component that separates the two sides of the cylinder barrel internally. The piston is usually machined with grooves to fit elastomeric or metal seals. These seals are often O-rings, U-cups or cast iron rings. They prevent the pressurised hydraulic oil from passing by the piston to the chamber on the opposite side. This difference in pressure between the two sides of the piston causes the cylinder to extend and retract. Piston seals vary in design and material according to the pressure and temperature requirements that the hydraulic cylinder will see in service. Generally speaking, elastomeric seals, made from nitrile rubber or other materials, are best in lower temperature environments while seals made of viton are better for higher temperatures. The best seals for high temperature are cast iron piston rings.
The piston rod is typically a hard, chrome-plated piece of cold-rolled steel which attaches to the piston and extends from the cylinder through the rod-end head. In double rod-end hydraulic cylinders, the actuator has a rod extending from both sides of the piston and out both ends of the barrel. The piston rod connects the hydraulic actuator to the machine component doing the work. This connection can be in the form of a machine thread or a mounting attachment such as a rod-clevis or rod-eye. These mounting attachments can be threaded or welded to the piston rod or, sometimes, they are a machined part of the rod-end.
The hydraulic cylinder head is fitted with seals to prevent the pressurised hydraulic oil from leaking past the interface between the rod and the head. This area is called the rod gland. It often has another seal called a rod wiper which prevents contaminants from entering the hydraulic cylinder when the extended rod retracts back into the cylinder. The rod gland also has a rod bearing. This bearing supports the weight of the piston rod and guides it as it passes back and forth through the rod gland. In some cases, especially in small hydraulic cylinders, the rod gland and the rod bearing are made from a single integral machined part.
A hydraulic cylinder should be used for pushing and pulling only. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, the ideal connection of a hydraulic cylinder is a single clevis with a spherical ball bearing. This allows the hydraulic actuator to move and allow for any misalignment between the actuator and the load it is pushing.
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