The great advantage telescopic cylinders have over any other type of cylinders is their ability to provide an exceptionally long stroke from a compact initial package. The collapsed length of typical telescopic cylinders varies between 20% to 40% of their extended length. Thus, when mounting space is limited, and the application needs a long stroke, a telescopic cylinder is a logical solution.

For example, assume a dump body needs to be tilted 60° in order to empty completely. If the body or trailer is fitted with a conventional rod-type cylinder — with a one-piece barrel and stroke long enough to attain that angle — the dump body could not return to a horizontal orientation for highway travel because of the cylinder's length, even when fully retracted. A telescopic cylinder easily solves this problem.

Telescopic hydraulic cylinders are relatively simple devices, but their successful application requires an understanding of this components' idiosyncrasies. Knowledge of how telescopic cylinders work and which special application criteria to consider will enable you to design them safely and economically into equipment.

Main and stages
As the name infers, telescopic cylinders are constructed like a telescope. Sections of steel tubing with successively smaller diameters nest inside each other. The largest diameter section is called the main or barrel; the smaller-diameter sections that move are called stages; the smallest stage is also called the plunger. The maximum practical number of moving stages seems to be six. Theoretically, cylinders with more stages could be designed, but their stability problem would be daunting.

Telescopic cylinders normally extend from the largest stage to the smallest. This means the largest stage — with all the smaller stages nested inside it — will move first, and complete its stroke before the next stage begins to move. This procedure will continue for each stage until the smallest-diameter stage is fully extended. Conversely, when retracting, the smallest-diameter stage will retract fully before the next stage starts to move. This continues until all stages are nested back into the main.

Basic types

As with conventional cylinders the two basic types of telescropic hydraulic cylinders are single- and double-acting. Single-acting telescopic cylinders extend under hydraulic pressure and rely on gravity or some external mechanical force for retraction. Single-acting cylinders are used in applications where some form of load is always on the cylinders. The classic single-acting telescopic applications are dump trucks and dump trailers. Pressurized oil extends the telescopic cylinder to raise one end of the dump body. When pressure is released, the weight the dump body forces oil out of the cylinder, it retracts.

Double-acting telescopic cylinders are powered hydraulically in both directions. They can be used in applications where neither gravity nor external force can retract the cylinder. They are well suited to non-critical positioning applications requiring extension and retraction movement of a substantial load.

A classic application is the packer-ejector cylinder in refuse vehicles and transfer trailers. The horizontally mounted cylinder pushes a platen to compress the load, then must retract with the platen so more material can be added. Gravity cannot help, so a double-acting cylinder is used.

Bearings and seals

Each stage is supported within each successively larger stage by at least two bearings. One is at the the next larger stage. The distance between these two bearings determines the degree by which one stage overlaps the next. Generally, this distance must increase as overall stroke increases in order to resist deflection caused by the weight of extended stages and the load.

There are several designs for sealing telescopic cylinders. One of the most common designs is the use of several hinged chevron V seals, one-piece, multi-lip seals with hinged lips molded in place, or both. These seals are held in place by a stop ring or snap ring and packing nut and they use guide bearings on the sleeve piston. The internal diameter of each stage is sealed against the outer diameter of the next smaller stage nested inside it.

The style and placement of these seals varies among cylinder manufactures. The style of seal also depends on its particular function. Zero-leakage, multiple-lip soft seals are usually found in the internal diameter at the packing section of the main and moving stages. Low-leakage hard seals are found on the piston end of double-acting telescopic cylinders. These piston seals allow the cylinder to retract under pressure.

Another design used on some single-acting telescopic cylinders is soft, zero leakage seals on the piston, which in turn use the full bore of the next larger stage as the effective area for extend force. These same seals contain the oil in the cylinder. The upper end of the cylinder, where the soft seals normally would be found, now contains a bearing for guidance. If any type of seal is used in the upper end of this telescopic cylinder design,-it is usually a wiper/seal combination to exclude contaminants from entering the cylinders. With either type, the many sealing surfaces must compensate for normal deflection of stages as the cylinder extends.

The cylinder design with the bearing on the piston and the seal on the other end is called a displacement-type cylinder. The single-acting design with a seal on the piston and a bearing at what normally would be the packing end approaches the classification of ram-type cylinder. Performance is similar to a double-acting rod-type cylinder with pressurized oil being supplied only to the piston side. All the telescopic stages would stroke in this way.