What is a hydraulic cylinder?
A hydraulic cylinder is a key hydraulic component. It serves as a hydraulic consumer that converts the energy of the hydraulic fluid into useful work. Its input value is the hydraulic fluid under pressure acting on the surface of the hydraulic cylinder piston. This causes a linear movement of the piston and thus the piston rod, which is connected to the load. Therefore, the energy of the hydraulic fluid is transformed into a controllable power, which acts in a straight line. The hydraulic medium is usually mineral oil, and in hydraulics, synthetic oils and emulsions, as well as water (water hydraulics), are also used.
Hydraulic cylinder components
Hydraulic cylinders are composed of two main elements, namely the barrel and the piston with an attached piston rod. The cylinder bottom and the cylinder head close both sides of the barrel respectively. The piston rod exits through the cylinder head. The piston, equipped with seals and sliding rings, divides the inside of the cylinder into two chambers, the lower pressure chamber and the upper piston rod chamber. The hydraulic pressure is generated by the piston that moves the piston rod in a linear direction. This type of cylinder is also called a double–acting hydraulic cylinder.
A more detailed description of the components:
The barrel is typically a seamless forged tube, which is ground and/or honed internally.
The base is welded or screwed to the cylinder barrel, although the base of some cylinders can be with or without a flange.
The head is typically attached with a screw connection, but can also be welded or attached to the barrel with a spring ring. The grooved inside of the cylinder head contains piston rod seals, a rod wiper and glands.
The piston divides the cylinders into two chambers. The piston is sealed with a piston seal guided with guide rings, which prevents the passing of the oil between the chambers. The pressure difference between both chambers triggers cylinder movement.
Piston rods are typically made from high–tolerance, hard chrome–plated cold–rolled steel. Piston rods are mounted onto the piston on one side and enter from the cylinder through the cylinder head on the other. An eye or flange is typically attached to the exterior side of the piston rod. Piston rods can be tempered, double chromed, hollow or stainless steel.
These are used to supply the hydraulic fluid to and from the cylinder. Welded ports are available in inch and metric diameter values.
There are several types of cylinder seals. They vary according to their shape, material and installation method, as well as the operating temperature range and pressure of the hydraulic cylinder.
The rod end is intended for connecting the cylinder to the load through the sliding bearing or bushes.
The self–locking nut is used for attaching the piston rod to the piston in certain types of applications.
It is used to limit the stroke of the plunger cylinder and to mount the cylinder head (with a spring ring).
For a pivotal installation of the cylinder.
It is used in flange connection.
Screw plugs protect the welded connections, and thus the interior of the cylinder, from impurities.
Bushes are usually built into the welded rod ends of the cylinders. There are steel and self–lubricating bushes.
Sliding bearings are built into the cylinder heads that serve as a connection and attachment of the cylinder to the load.
How does a hydraulic cylinder function?
A hydraulic cylinder usually has only one piston rod. This type of cylinder is referred to as differential hydraulic cylinder. A feature of this cylinder is that the surface area of the piston under working pressure differs from the surface area of the piston rod. This cylinder has a greater pushing force in comparison to the pulling force in the amount of the surface area ratio. The opposite applies to the speed of the working and return stroke of the cylinder. The return stroke is faster than the working stroke in the amount of the surface area ratio. Cylinders with double–acting piston rods typically have identical piston surface areas and identical working and return strokes.
Fields of application
Hydraulic cylinders are used wherever great strength is required. This applies to all branches of industry, particularly to:
In mechanical engineering hydraulic cylinders are used for transportation devices, hoists, hydraulic presses, forming machines, feeding devices, automated production lines, plastic forming machines, production of ecological equipment, etc.
In agriculture hydraulic cylinders are used for working machinery (tractors, combine harvesters, loaders, etc.), tractor attachments, stable equipment, hydraulic wood splitters, etc.
In civil engineering hydraulic cylinders are used for working machinery (excavators, bulldozers, trenchers) and attachments, hoists, material preparation, etc.
In shipbuilding hydraulic cylinders are used for transportation equipment, steering, hoists, etc.
In the energy industry hydraulic cylinders are most commonly used in hydroelectric stations to control the gates and
regulate the water flow, etc.
In the food industry hydraulic cylinders are used for conveyor belts, production lines, etc.
MOTOR VEHICLE INDUSTRY
Classification of hydraulic cylinders
According to the method of functioning hydraulic cylinders are classified as double–acting, single–acting, plunger cylinders and telescopic cylinders.
Double–acting cylinders are the most common type of cylinders. Double–acting cylinders apply strokes in both directions, whereby the pushing force exceeds the pulling force in the amount of the surface area ratio.
These type of cylinders are equipped with a piston rod on both sides of the piston. The volume in both pressure chambers always remain the same, and so does the entry and exit velocity of the cylinder.
The characteristic of the single–acting piston hydraulic cylinder is that the operating stroke is only generated in a single direction; whereas, the return stroke is enabled by the load, spring or any other exterior force. Typically the working stroke can be generated as cylinder extraction, therefore this cylinder is a push cylinder; or the working stroke can be generated as cylinder contraction. This type of cylinder is called a pull cylinder.
A plunger cylinder is a single–acting cylinder without a piston, whose function is assumed by the piston rod itself. Therefore, the operating surface of the cylinder is the cross–section of the piston rod itself. Due to the absence of a piston, the cylinders should be equipped with additional axial guidance.
Telescopic cylinders or multi–stage cylinders are composed of a number of cylinders stacked on top of each other. Where the cylinder barrel also acts as a piston rod. Cylinders can have two, three, four, five or even six stages. They are most often used where the cylinder installation length is less than the required stroke. Most telescopic cylinders are single–acting, but there are also double–acting telescopic cylinders available.
Tandem cylinders are two interconnected cylinders. The piston rod of the first cylinder enters through the base of the second cylinder and pushes its base. In this manner the greater effective surface area of both pistons generates greater force, despite a small cylinder diameter and unaltered operating pressure.
Furthermore, hydraulic cylinders can be classified according to the method of installation:
- MM - basic construction without attachment,
- MA - eyelet attachment with a spherical bearing,
- MB - eyelet attachment with bushes,
- MD - trunnion attachment,
- MF - front flange attachment,
- MF - rear flange attachment,
- special designs.
We have only provided you with a brief overview of the well–known hydraulic cylinders and there are still more which are also produced by us. Special and custom products are very welcome in our production plant, even in small quantities. Thanks to our size we are very flexible and adaptable for all our customers’ requests.
There are many different sub–types of hydraulic cylinders, such as:
- Hydraulic cylinders with sensors,
- Hydraulic cylinders with yardsticks,
- Hydraulic cylinders for high temperatures,
- Hydraulic guiding cylinder,
- Hydraulic cylinders with hydraulic or mechanical locking.
All our hydraulic cylinders are economical, efficient and reliable. We can also produce any kind of special or custom hydraulic cylinders. We try to make all products manufactured by us as space-efficient as possible, but not at the cost of performance or efficiency. The possible applications are endless in the field of power transmission.