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.