A cylinder is a device that converts fluid power into linear mechanical force and motion. A hydraulic cylinder is a linear actuator used for converting fluid energy to an output force in a linear direction for performing various jobs. These include: pulling or pushing in engineering applications such as in machine tools, earth moving equipments, construction equipments and space applications.

The hydraulic cylinders are used for all fields of engineering that comprise of: earth moving, mining industry, construction machinery, plant engineering, defence technology, automotive engineering, mechanical engineering, textile industries, railways, power plants, agricultural machinery etc.

Types of Cylinders
As per their functions, cylinders are classified as:
1. Single acting cylinders
In these, the oil pressure is fed only on one side of the cylinder either during extension or retraction. When the oil pressure is cut-off, these cylinders return to the normal position either by a spring or by an external load.

2. Double acting cylinders
These are operated by applying oil pressure to the cylinder in both directions. Due to inherent mechanical problems associated with the spring, single acting cylinders with spring return are not used in applications using larger stroke lengths. They may be either single rod ended or double rod ended type.

Apart from the above commonly used functional designs, there are special cylinders such as:
1. Plunger or ram cylinders
These are used as a single acting cylinder in a vertical position so that the load on the cylinder can retract when the oil supply is stopped. Eg. Cylinders used as lifts in automobile service stations.

2. Telescoping cylinders
These cylinders provide long working strokes in a short retracted envelope and are used in mobile applications such as tilting of truck dump bodies and fork lift trucks, hydraulic cranes etc.

3. Cable cylinders
These are double acting cylinders that can be powered either pneumatically or hydraulically and find usage in applications requiring relatively long strokes and moderate forces and can be operated in limited spaces.

4. Diaphragm cylinders
These are often used in pneumatic applications and are either of the rolling diaphragm or flat diaphragm-type. They have very low break-out friction with absolute zero leak across the piston.

5. Bellows cylinders
These are used for very low force applications in sensitive pneumatic control systems. The pressure and the spring rate of the bellows determine the amount of tension and contraction and may be used for basic servo-control systems since metal bellows have a linear spring rate.

6. Tandem cylinders
These are commonly used in hydraulic and pneumatic systems, two cylinders are mounted in line with the pistons connected to a common piston rod in order to multiply the force in a limited lateral space.

Mounting Styles
The mounting style of hydraulic cylinders is an important parameter both from design and maintenance consideration. The various forms of mounting arrangements used are: Plain, Threaded, Clevis, Flange, Tongue or eye etc. Mounting styles for the cylinder body can be: Plain, Foot, Square bracket (single-ended), Square bracket (double-ended), Trunnion-Cap or head-ended, Trunnion-Center, Flange-rectangular or square, Clevis, Tongue and bracket or Tie rod extended.

Checklist
Designing a hydraulic cylinder for a particular purpose is a complex task and designing of the hydraulic cylinder should be done after taking into consideration all the relevant factors that determine its reliability. These factors are: metallurgical and physical parameters, manufacturing parameters, frictional parameters, pressure parameters, environmental conditions, structural considerations and operating conditions.

The component design of any hydraulic system should provide and function that will contribute to the optimum maintainability with minimum downtime ensuring ease of repairing and rapidity of recommissioning to serve at best possible accuracy at nominal economic liability.