Hydraulic cylinders – structure, principle of operation, types and use

Hydraulic cylinders, very often called ‘linear hydraulic motors or engines’, constitute main parts of hydraulic systems. They are rather simple and small devices but can do large amounts of work. For these reasons, hydraulic engines (cylinders) are widely used both in light and heavy machines in almost all fields of economy.

In this article, structure, classification, principles of operation and use of hydraulic cylinders are briefly described.

How is a hydraulic cylinder built and how does it work?

Generally, a typical hydraulic cylinder is built of a cylindrical body, a cap, a head, a piston, a piston rod and seals. Plungers or membranes can be used instead of a piston.

As hydraulic cylinders have to stand both fluid high pressure and a large working load, they are made of toughest materials, mainly of seamless chrome-molybdenum or carbon steel pipes or forgings as well as - though rarely - of cast-iron castings (mainly of grey iron) or aluminium. Forgings are more durable, so are used for production of short cylinders intended for high pressures.

A principle of operation of hydraulic cylinders is as follows: a pressurised fluid is pumped into a cylinder on one or both sides of a piston and causes working motion of a piston. The piston pumps a fluid under high pressure to outflow channels and changes hydrostatic energy into mechanical energy. In general, the larger diameter both of a cylinder and a piston, the more work a cylinder can do. A piston goes back to an initial position due to its weight, springs, external force or pressurised fluids.

Classification of hydraulic cylinders

Because of action, hydraulic cylinders are divided in 2 types:

• single-acting cylinders

• double-acting cylinders.

In simpler single-acting cylinders working fluids act on one side of a piston, so a working stroke of a piston can be done only in one direction.

In case of double-acting cylinders, fluids act on both sides of a piston so working strokes are possible in both directions alternately. For this reason, double-acting cylinders are more efficient.

Usage of hydraulic cylinders

Although hydraulic cylinders are rather simple and relatively small (in comparison to whole machineries or electromechanical servomotors), they can do incredible large amounts of work. They are commonly used in hydraulic systems of industrial (mainly in hydraulic presses), construction, mining, road construction and agricultural machines, lorries, planes and other vehicles. Single-acting cylinders are simpler and more commonly used but rather as auxiliary elements. Double-acting ones are especially useful when very high force is required in both directions (e.g. in cranes) or when external force is not available (e.g. due to lack of space) to retract a piston.

Summary

Use of hydraulic cylinders brings numerous advantages. First off all, cylinders can do really incredible amounts of work with relatively low electricity consumption so are very efficient. Their compact dimensions enable using large machines in tight places and - due to simplicity and strength of materials - they are not very damageable.