Double Acting Telescopic Cylinders and its applications ?

A double acting cylinder is extended and retracted using hydraulic or pneumatic pressure in both directions. Double acting telescopic cylinders are thus much more complex in design than the single acting type. This additional complexity is due to the requirement of adding retracting piston faces to all of the cylinder stages and the difficulty in supplying pressurized fluid to the retraction pistons of the intermediate stages.
The extension and retraction fluid supply ports on double acting hydraulic telescopic cylinders are usually located at opposite ends of the cylinder assembly. The extension port is mounted at the base of the outer barrel and the retraction port is mounted in the end of the plunger section. This can, in some applications, prove to be very difficult to connect with hydraulic hoses due to the distance between these ports at full extension. In such a circumstance, both ports can be located in the barrel. An internal passageway must be fitted, however, so that the retracting fluid is supplied to the plunger section at full extension. This special passageway is in itself a telescopic assembly that extends with the cylinder and is outfitted with seals on the various stages.

This additional complexity makes double acting telescopic cylinders very expensive. They are usually custom designed for each application.

Typical applications for double acting telescopic cylinders include the packer-ejector cylinders in garbage trucks and transfer trailers, horizontal compactors, telescopic excavator shovels, and roll-on/roll-off trucks. In all of these applications, the cylinder operates near horizontally and thus gravity is not available to retract the actuator. A double acting design is therefore required to both push and pull the telescoping mechanism.

Care must be taken when controlling most double acting telescopic design cylinders. The effective retraction area is often much less than the extension area. Thus if the hydraulic fluid return line is blocked during extension a pressure intensifying effect can occur causing seal failure or even causing the metal sleeve to balloon outward. The cylinder could thus be rendered unable to retract because of failed seals or jam in position due to binding.

Another problem can occur if a double acting telescopic cylinder encounters a load that pulls on the actuator during extension such as when a tilting load goes over center and opens the cylinder beyond the internal volume of the hydraulic oil. When the piston face catches up again and strikes the oil column a pressure spike occurs which can damage the actuator