Normally, extension of a doubleacting telescopic cylinder occurs in the same manner as with the single-acting type. Retraction of double-acting telescopic cylinders is made possible by sealing each moving stages piston area outside diameter with the next larger stages inside diameter and building internal oil-transfer holes into each moving stage. The oil-transfer holes are located just above the pistons in the body of the stage. The retraction port normally is located in the top of the smallest stage. Oil flows through this port and into the smallest stage. The oiltransfer hole allows oil to enter and pressurize the volume between the next stages internal diameter and the smaller stages outer diameter. Pressure in this volume generates the force to move or retract the smaller stage into the larger stage.

Once this stage is fully retracted, the oil-transfer hole in the next larger stage is exposed to allow oil flow for it to retract. This retraction process continues automatically until all stages have retracted into the main. The seal on each stage selects the areas against which pressure will work.

Locating the retract port on the top of the smallest stage is the simplest way to design a double-acting telescopic cylinder, but this port location typically requires an arrangement of hoses, hose protection, and hose reels to deliver oil to the moving stage. To avoid having fluid power ports spaced far apart when the cylinder is fully extended, most double-acting telescopic cylinder designs locate both fluid ports in the smallest stage or plunger. The cylinder is then mounted so that the smallest stage or plunger is stationary and the larger and heavier stages would be the ones that move as the cylinder extends.

In some instances, a double-acting telescoping cylinder can be designed where both ports are located in the stationary main barrel. Cylinder size (diameter and stroke) and the number of moving stages determine whether this is possible. If it is, the more-complicated internal passages for oil flow require a double wall and or a special trombone type telescopic design.

Piston seals on double-acting telescoping cylinders are normally manufactured from a hard substance-such as cast iron, ductile iron, or glass-reinforced nylon. The hard seals are needed to limit abrasion between the oil transfer holes and ports over which they must pass.

Single-and double-acting combinations

There are a few unusual types of telescoping cylinders designed for specific applications. For example, a manufacturer of oil well equipment uses a type composed of both single-and double-acting stages to position a work-over rig. The work-over rig is a derrick or tower that is transported horizontally to the well site on a trailer. There, telescopic cylinders extend to swing the rig into a vertical position. When the rigs work is done, the telescopic cylinder pulls the rig to begin the transition from vertical back to horizontal. However, once the rig has started to tilt, no more pull force is need because of the rigs weight, and gravity will continue to retract the cylinder. In other words, the cylinder needs hydraulic power for the first part of its retraction stroke, but then operates as a single-acting unit.

In this type of design, the smallest moving stage is designed to be double-acting; the others are single-acting. The small stage can then provide push force to raise the rig, and pull force to start it back down. It is not unusual to design this type cylinder as a skip-asleeve design. Skip-a-sleeve (as its name implies) is where a sleeve or stage is skipped during design. Normally a telescopic stage diameter increases approximately every inch, example; sleeve diameter may be 3.75-in., fits into a 4.25-in. bore; 4.75-in. fitting into 5.25-in. bore, etc. In a skip-a-sleeve design, a sleeve is removed to increase the effective area and the retract force of the smallest sleeve or plunger, example; plunger diameter is 2.75-in. and fits into the 4.25-in. bore of the 4.75-in. sleeve, thus increasing effective area and retract force.

Constant thrust, constant speed

A special telescopic cylinder — known as a constant-thrust/constant-speed cylinder — is configured so that all moving stages will extend at the same time, providing an overall constant speed as well as a constant push force throughout its stroke when extending or retracting. This type of cylinder has been used to drive a drill head in underground mining, where such performance parameters are necessary, and space is at a premium. The more complicated design accomplishes the required action by trapping oil internally, matching extend and retract areas, and limiting the number of moving stages.