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'Hydraulic cylinders' (also called 'linear hydraulic motors') are mechanical actuators that are used to give a linear force through a linear stroke.

Contents

Operation

Parts of a hydraulic cylinder

Piston rod construction

Special hydraulic cylinders

Operation

Hydraulic cylinders are pressurized by hydraulic pressure and get their power from the hydraulic fluid under pressure.
They transform the oil's energy to linear work.
A Hydraulic cylinder works in a hydraulic system and is the motor side of this system. The generator side of the oil hydraulic system is the pump, that brings a fixed or regulated oilflow into the system. The hydraulic cylinder initiates the pressure of the oil, which can never be larger than the pressure that is asked by the load.
The cylinder consists of a cylinder barrel, in which a piston connected to a piston rod is moving. The barrel is closed by the cylinder bottom at the bottom side and by the cylinder head at the side where the piston rod comes out of the cylinder.
Mounting brackets or clevisses are mounted to the cylinder bottom as well as the piston rod.
The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber and the piston rod side chamber. By pumping hydraulic oil to the bottom side of the hydraulic cylinder, the piston rod starts moving out. The piston pushes the oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is zero, the pressure in the cylinder is now Push Force/Piston area.
If the oil is pumped into the piston rod side chamber and the oil from the piston area flows back to the reservoir without pressure, the pressure in the piston rod area chamber is Pull Force/(piston area - piston rod area). In such a way the hydraulic cylinder can push and pull.

Parts of a hydraulic cylinder

A hydraulic cylinder consists out of following parts:

★ Cylinder barrel

★ Cylinder bottom

★ Cylinder head

★ Piston

★ Cylinder bottom connection

★ Piston rod connection

★ (some times) Feet for mounting of the barrel
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. Therefore it is better to have a screwed or flanged connection. In that case also the barrelpipe can be maintained and/or 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 cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a big problem as well as the alignment during mounting.
A hydraulic cylinder should be used for pushing and pulling and no bending moments should be transmitted to the cylinder. For this reason the ideal connection of a hydraulic cylinder is a single clevis with a ball bearing.

Piston rod construction

The piston rod of a hydraulic cylinder operates both inside and outside the barrel, and consequently both in and out of the hydraulic fluid and surrounding atmosphere. Smooth and hard surfaces are desirable on the outer diameter of the piston rod and slide rings for proper sealing. Corrosion resistance is also advantageous. A chromium layer may often be applied on the outer surfaces of these parts. However, chromium layers may be porous, thereby attracting moisture and eventually causing oxidation. In harsh marine environments, the steel is often treated with both a nickel layer and a chromium layer. Often 40 to 150 micro-metre thick layers are applied. Sometimes solid stainless steel rods are used. High quality stainless steel such as AISI 316 may be used for low stress applications. Other stainless steels such as AISI 431 may also be used where there are higher stresses, but lower corrosion concerns. Due to shortcomings of metallic materials, ceramic coatings were developed. Initially ceramic protection schemes seemed ideal, but porosity was higher than projected.

Special hydraulic cylinders

'Telescopic cylinder'
The length of a hydraulic cylinder is the total of the stroke, the thickness of the piston, the thickness of bottom and head and the length of the connections. Often this length does not fit in the machine. In that case the pistonrod is also used as a piston barrel and a second pistonrod is used. This kind of cylinders are called "telescopic cylinders". If we call a normal cylinder "one stroke", telescopic cylinders can be two, three, four, five and even six stroke. In general telescopic cylinders are much more expensive that normal cylinders. Most telescopic cylinders are single acting (push). Double acting telescopic cylinders must be specially designed and manufactured.
'Plunger cylinder'

A hydraulic cylinder without a piston or with a piston without seals is called a plunger cylinder. A plunger cylinder can only be used as a pushing cylinder; the maximum force is pistonrod area
★ pressure. This means that a piston cylinder in general has a relatively thick pistonrod.
'Differential cylinder'
A differential cylinder acts like a normal cylinder when pulling. If the cylinder however has to push, the oil from the pistonrod side of the cylinder is not returned to the reservoir, but goes to the bottomside of the cylinder. In such a way, the cylinder goes much faster, but the maximum force the cylinder can give is like a plunger cylinder. A differential cylinder can be manufactured like a normal cylinder, and only a special control is added.