One special type of hydraulic cylinder that we find most often in our everyday lives is a telescopic cylinder, also known as a multistage hydraulic cylinder. This kind of cylinder produces a longer cylinder from an initial short length. Each section of the cylinder slides into a larger section. Typically the collapsed length of a telescopic cylinder is 20 to 40 per cent of its fully extended length depending on the number of stages. The stages can be as few as two to as many as five or six.

Basic Types
A telescopic cylinder can be one of two types—single action or double action.

The simplest and most common design uses single action. In this type of cylinder, the pressure of the hydraulic fluid at one end causes the cylinder to extend. When the pressure of the fluid is cut off, a spring or other external load is used to return the top of the cylinder to its original position.

More complex in design are double acting cylinders in which the cylinder is extended and retracted using pneumatic or hydraulic pressure in both directions. The double acting feature is accomplished by adding hydraulic seals to internally seal off the individual stages. Machining is also done to the internal air or oil passageways so that as each stage completes its retraction, pressurized fluid can be supplied to an open passage so that the stage can retract. Therefore, in a double acting telescopic unit, the retraction starts from the retraction of the smallest diameter stage and ends with the retraction of the largest stage.

Real World Applications
These pneumatic telescoping units are best suited for lifts, cranes, forklifts and other commercial vehicle applications such as the dump body on a dump truck. When used on a construction site, the dump body needs to be raised to an angle of 60 degrees to empty its load.

The use of a regular hydraulic cylinder for this would be very difficult as the collapsed length of a single stage rod cylinder is approximately 110 per cent of its output stroke. In addition, it would be an engineering challenge to fit a single stage cylinder into the chassis of the dump truck when the dump body is in its horizontal resting position. The use of a telescopic style multi-stage cylinder easily accomplishes this task.

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