Telescopic hydraulic cylinder is a linear actuator that comprises of series of tubular rods that are called sleeves. This is a multi-stage cylinder where the sleeves usually four or five in numbers are nested in each other. This type of design that is quite similar to the conventional telescope gave its name to be the telescopic hydraulic cylinder. The diameter of the nested sleeve decreases gradually that gives it a shape of a telescope. 

During the operation the pressurised hydraulic fluid enters the main barrel which is the biggest in diameter. With the increasing pressure of hydraulic fluid the sleeve of the barrel starts to extend and when it is extended to maximum extent, fluid enters the next barrel. Sequentially, sleeves of the telescopic hydraulic cylinder get extended as the pressure of the fluid is increased. Now this increase in the fluid pressure is done as the diameter of the sleeves get smaller from one to another. As the space is decreased the pressure of the fluid is increased however the same force is being applied. Gradually hydraulic fluid reaches the last sleeve that is called the plunger. So, with equal force the telescopic hydraulic cylinder extends the sleeves utilizing simple rule of fluid dynamics.

Types of Telescopic Hydraulic Cylinders

There are basically two types of telescopic hydraulic cylinders – single acting telescopic hydraulic cylinder and double acting telescopic hydraulic cylinder. Between these two types single acting telescopic hydraulic cylinders are most commonly used.

Sleeves of single-acting telescopic hydraulic cylinder extends due to hydraulic pressure but relies on gravitational force or some external force for retraction. Single-acting telescopic hydraulic cylinder are used in such systems where any form of load is always applied externally on cylinders. The most common use of single acting telescopic hydraulic cylinder are the dump trucks and trailers. The pressurized fluid within the telescopic hydraulic cylinder forces the cylinder to extend and raise one end of dumping trailer. After the full extension pressure is released and weight of dumper body releases fluid out of the cylinder automatically and cylinder retracts to its original position. 

In case of double-acting telescopic hydraulic cylinder both the ends of the cylinder are powered hydraulically. These cylinders do not require gravitational or external fore to be applied for retraction. Therefore, the double acting telescopic hydraulic cylinders are used in such applications where neither gravity nor external force is available to retract cylinder to its original position after the extension has reached its optimum limit. Example of such telescopic hydraulic cylinder is the aircraft. The wheels of the aircraft are extended and retracted back with double acting telescopic hydraulic cylinders.

Telescopic hydraulic cylinders are preferred when the plunger needs to reach is longer but the space of installation of cylinder is restricted. This type of cylinder is especially helpful where the conventional rod and piston type cylinders cannot be installed. The telescopic hydraulic cylinders perform well when you need to lift heavy loads but you have really small space to install the cylinders. Therefore, heavy duty equipment such as dumping trucks, excavators use telescopic hydraulic cylinders.

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