How does a hydraulic cylinder work?

Introduction and history of Hydraulic Cylinders

Controlling the motion of fluid is nothing new, for decades this phenomena has been used to generate energy. Fluid power is a term originally derived from the idea that anciently people used water to push levers and turn wheels. This principle is still applied today to produce powerful forces. French physicist Blaise Pascal noticed that a certain amount of liquid applies the same force in all directions and that these forces can be controlled. Then, later in 1795 the first hydraulic press was patented by Joseph Bramah. Later it was noticed that oil is better hydraulic fluid than water. Due to its certain properties like more density, Non corrosive, handle higher loads and resist evaporation. Hydraulic power is growing further each year. Many applications involves i.e. building skyscrapers, on cranes, aircraft landing gears, moving heavy objects, mining, drilling and manufacturing.

Hydraulic cylinders are  linear drivers which use fluid pressure to counter the movement under load. This device also helps in pushing and pulling of the load. They provide motion of a fluid in straight line. They are also known as actuators. Hydraulic cylinder is actually the device which converts pressure energy to mechanical energy. They are used to transmit power. The output power depends on the pressure drop around actuator, rate of flow and overall efficiency. There are different types of hydraulic cylinders such as

  • Single acting cylinders
  • Tandem cylinders
  • Double acting cylinders
  • Telescopic cylinders
  • Through rod cylinders
  • Displacement cylinders

Working and parts of hydraulic cylinders

There are some specifications of hydraulic cylinders which should be considered before operating. Like cylinder type, bore diameter, stroke, maximum operating pressure and rod diameter. Bore diameter is diameter of cylinder and rod diameter is the diameter of piston in the cylinder. Stroke is the distance a piston travels across the cylinder. Stoke length can vary accordingly and it can be some fractions of an inch or few feet. The maximum operating pressure is the pressure that a cylinder can suffer or support. All the power of hydraulic cylinder depends on the type of hydraulic fluid used in it. The most common used hydraulic fluid is oil.

Parts of hydraulic cylinders include

Cylinder barrel: It is a main body of cylinder, used to hold the pressure. The cylinder barrels have smooth surface from inside, durable for use, high strength, high corrosion resistance and high precision tolerance.

Cylinder base: This is also known as cylinder cap. It is used to seal the pressure chamber at one end. Bending stress determines the size of the cap. These caps can be welded to the body or can be joined by using bolts, threading or tie rods.

Piston: Piston separates the two pressure zones present inside the barrel. The expanding and retracting of cylinder is due to the pressure difference between two sides of piston.

Piston rod: A piston rod serves as a connection between hydraulic actuator and machine component which is doing the work. It is highly precise and polished to prevent leakage and provide suitable seal.

Seal gland and seal

The area where seal and cylinder head is fitted is called seal gland. This fitting prevents the leakage of oil due to pressure. The leakage usually occurs between rod and the cylinder head. Seals are of many types and suitable choice of seal is depending on many factors like operating temperature, cylinder type, cylinder speed, working pressure, working application and medium. To contain the pressurized fluids in a hydraulic cylinder system and keep it in motion, a complex configuration of high-performance seals in two basic categories are required: (1) static seals and (2) dynamic seals

How does a hydraulic cylinder work?

The power of hydraulic cylinder is depending on the hydraulic fluid being used because it gets most of its power from this fluid. The most commonly used hydraulic fluid is oil. A piston is connected to piston rod and moves back and forth. This arrangement is present inside cylinder barrel. One end of the barrel is closed with the cap and other end is closed with gland. Through gland the piston rod escape from the cylinder. There are two parts between the cylinder. These are divided by piston rod. One is the upper camber or head part and other is a bottom camber or cap part. The mounting attachments enable the connection between cylinder and machine which is pulling and pushing. The cylinder is a motor side of hydraulic system and hydraulic pump is the generator side of hydraulic system. The pump provides a fixed flow of oil to move the piston in the cylinder. The piston pushes the oil in the other chamber back to the reservoir. When the oil enters from bottom end during the extension stroke and the pressure at the other end is nearly zero then the force applied on the piston rod is:

F = P . A
Where, A= Area of the piston   P= Pressure in the cylinder.

Hydraulic cylinders provide push and pull by extending and retracting the piston rod. By this mechanism it drives the externally exerting load in a straight-line path.  Hydraulic motors are used for continuous angular movement. For semi angular movements we use semi rotatory actuators. In double acting cylinders the piston is covered with rod attached to it and due to this arrangement, there is a force difference between the two sides of piston. This force difference occurs when the cylinder reverse the input and output pressure. The force that is applied for retraction stroke can be reduced when the surface area of rod is reduced. When the oil pumped into the rod end and it flows back into the reservoir though the cap end without any pressure then at the rod end the fluid pressure is equal to pull force/(area of piston – area of piston rod).

Hydraulic cylinders provide push and pull by extending and retracting the piston rod. By this mechanism it drives the externally exerting load in a straight-line path.  Hydraulic motors are used for continuous angular movement. For semi angular movements we use semi rotatory actuators. In double acting cylinders the piston is covered with rod attached to it and due to this arrangement, there is a force difference between the two sides of piston. This force difference occurs when the cylinder reverse the input and output pressure. The force that is applied for retraction stroke can be reduced when the surface area of rod is reduced. When the oil pumped into the rod end and it flows back into the reservoir though the cap end without any pressure then at the rod end the fluid pressure is equal to pull force/(area of piston – area of piston rod).

Hydraulic cylinders provide push and pull by extending and retracting the piston rod. By this mechanism it drives the externally exerting load in a straight-line path.  Hydraulic motors are used for continuous angular movement. For semi angular movements we use semi rotatory actuators. In double acting cylinders the piston is covered with rod attached to it and due to this arrangement, there is a force difference between the two sides of piston. This force difference occurs when the cylinder reverse the input and output pressure. The force that is applied for retraction stroke can be reduced when the surface area of rod is reduced. When the oil pumped into the rod end and it flows back into the reservoir though the cap end without any pressure then at the rod end the fluid pressure is equal to pull force/(area of piston – area of piston rod).

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