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Hydraulic » Hydraulic Article Resources » Advantage Telescopic Cylinder


Advantage Telescopic Cylinder

SEARCH The Industrial Sourcebook [November 2003]

Mobile applications often call for cylinders with long strokes but small profiles. Here is where telescopic cylinders come in. Dave Combs and Steve Ruth from Parker Hannifin describe the pros of using such cylinders...

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

Consider this. A dump body has to be tilted 60° to become completely empty. 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 cannot 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.

A telescopic hydraulic cylinder is a relatively simple device, but its successful application requires an understanding of its idiosyncrasies. Knowledge of the working of a telescopic a cylinder, and of the special application criteria that have to be considered, will help you integrate it into the equipment safely and economically.

Main and Stages

As the name implies, telescopic cylinders or are constructed like a is telescope. Sections of steel tubing to with successively smaller diameters nest inside each other. The section with the largest diameter of is called main or barrel; moving sections an with smaller diameter are called stages and the smallest stage is called the plunger. In of practice, the maximum number of moving stages is six. Theoretically, it is possible to design cylinders with more stages, but solving their stability problem is a daunting task.

Telescopic cylinders normally extend from the largest stage to the smallest. This means the largest stage, with all the smaller stages inside it, will move first, a 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 moving. This continues till all stages are nested back into the main.



Basic Cylinder Types

As with conventional cylinders, the two basic types of telescopic 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. They are used in applications where some form of load is always on the cylinders. Classic single-acting telescopic applications are dump trucks and dump trailers. Pressurised oil extends the telescopic cylinder to raise one end of the dump body. When pressure is released, the weight of the dump body forces oil out of the cylinder causing it to retract.

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 cytinder. Such cylinders are well suited to non-critical Positioning applications that need 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 retracts with the platen so more material can be added. Since gravity is of no help, a double- acting cylinder is used.

Bearings and Seals

At least two bearings support each stage within every successively large stage. One is at the piston end of the stage and the other at the packing section of the next larger stage. The distance between these two bearings determines the degree by which one stage overlaps the next. 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 a telescopic cylinders. One of the most common designs is one that uses either several hinged chevron V-seal or one-piece, multi-lip seal with hinged lips moulded in place, or both. These seals are held in place by a stop or snap rings and packing nut. 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 vary with cylinder manufacturers. The style of seal also depends on its particular function. Zero-leakage, multiple-lip soft seals are usually found in the internal a 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 at the oil in the cylinder. The upper end of the cylinder, where soft seals are normally found, now contains a bearing for guidance. In case a seal is used in the upper end of this design, it usually re consists of a wiper/seal combination to prevent contaminants from entering the cylinders. With any type, the many sealing surfaces must compensate for normal deflection of stages as the cylinder extends.

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

Double-Acting Telescopic Cylinders

Normally, extension of a double- acting telescopic cylinder occurs in the same manner as that of a single-acting type. Retraction of double-acting telescopic cylinders is made possible by sealing each moving stage's piston area outside diameter with the next larger stage's 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 is normally located at the top of the smallest stage. Oil flows through this port and into the smallest stage. The oil-transfer hole allows the oil to enter and pressurise the volume between the next stage's internal diameter and the smaller stage's 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 retraction. This process continues automatically till 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 are the ones that move as the cylinder extends.

In some cases, a double-acting telescopic cylinder where both ports are located in the stationary main barrel can be designed. Cylinder size (diameter and stroke) and the number of moving stages determine if 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 telescopic cylinders are normally manufactured from hard substances like 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

Some unusual types of telescopic cylinders are designed for specific applications. For example, a manufacturer of oil well equipment will use 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 rig's work is done, the telescopic cylinder pulls the rig to begin the transition from vertical back to horizontal. However, once the rig starts tilting, no more pull force is needed because of the rig's weight. Gravity will continue retracting 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 a double-acting one, while the others are single-acting. The small stage can provide push force to raise the rig, and pull force to start it back down. It is not unusual to design this type of cylinder as a skip-a-sleeve. Skip-a-sleeve (as the name implies) is where a sleeve or stage is skipped during design. Normally a telescopic stage diameter increases by approximately an inch. For example, a sleeve with diameter of 3.75 inches may fit into a 4.25 inches bore; 4.75 inches may fir into a 5.25 inches bore and so on. 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. For example, a plunger's diameter is 2.75 inches and fits into the 4.25 inches bore of the 4.75 inches sleeve, thus increasing effective area and retract force.

Contstant-thrust, constant-speed

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

(Courtesy: Parker Hannifin)