Introduction to Telescopic Cylinders

Telescopic cylinders provide a long stroke from a relatively small collapsed length. Depending on the number of stages, a telescoping cylinder may have a retracted length of 20 to 40% of its full extended length. This gives engineers a powerful yet compact actuator in machine design. They can thus install a telescopic cylinder in a space where a conventional rod cylinder with the required output stroke simply would not fit.

Above: This diagram shows the nested sections of a telescopic cylinder

An excellent example of this is a dump body trailer. In order to guarantee that the body will completely empty itself of its load, it must be capable of being tilted to an angle of about 60 degrees. A single stage rod cylinder capable of accomplishing this lift is very difficult to fit in the geometry available in the structure of a highway trailer. A multistage telescopic cylinder, however, accomplishes the same task very easily.

Telescopic cylinders are constructed by nesting a series of hydraulic tubes within each other in progressively smaller and smaller diameters. Each tube must be equipped with a series of hydraulic seals to contain the internal oil pressure. The tubes must also be equipped with bearings to support the unit weight and any side loads it may encounter as it extends. The largest diameter and outermost section of the cylinder is called the main bore or the barrel. The smaller sections nested within it are called the stages, with the smallest stage being called the plunger.

Telescopic cylinders are available with anywhere from 2 to 6 stages. Six stages seems to be the practical limit for design due to stability and cost factors.

Telescopic cylinders are available in both single acting and double acting designs.

Single acting are the most common, however, with gravity usually being used as the return force. This would be the case in the example of the dump body referenced above. The heavy weight of even the empty dump box would push down with sufficient force to retract the cylinder when the internal oil pressure was vented to the reservoir.

Double acting telescopic cylinders are much more complicated requiring special internal passageways to provide oil pressure to both extend and retract the stages hydraulically. Double acting telescopics are often required when the actuator is operating in a horizontal configuration such as in packer-ejector cylinders on waste trucks or push-off trailers. In these situations, gravity will not assist to help return the cylinder to its compressed position, so it must be done hydraulically.

Another design of telescopic cylinder is actually a combination of both single and double acting. In this design, only the final stage of the cylinder, the plunger, is double acting, while the other stages are all single acting. This unique design is used when an initial external return force is not available to retract the actuator. An example of a application using this design is the mast erection cylinder on some mobile oil drilling rigs. When the mast is returned from vertical to horizontal, the initial force to tilt back the mast must be hydraulic. Once the tilt has begun, gravity takes over and returns the cylinder to its fully retracted condition.

Above: A pair of very large bore, very long stroke telescopic cylinders erecting the mast of a mobile drilling rig.

Because of their additional complexity, double acting telescoping cylinders are much more expensive than their single acting cousins. Double acting telescoping cylinders are custom designed and manufactured for each application.

Because the nested cylinder sleeves slide along the pressurized bodies of the other stages, care must be taken to prevent any kind of over pressurizing condition. Over pressurizing a telescopic cylinder can result in the cylinder tubes ballooning outward slightly. This will cause the actuator to bind. In an extreme situation the entire unit could become jammed and refuse to move. Often telescopic hydraulic cylinders are limited to a maximum of 2000 psi service pressure to prevent such a disastrous occurrence.

Telescopic cylinders must be carefully designed into any application. This is to ensure that the unit will be able to function well in the application. A fully extended telescopic cylinder has a very long body length which is susceptable to side loads from its own weight as well as the mechanism it is pushing. A poorly designed application may cause the cylinder to buckle and collapse when fully extended. This would be very costly and dangerous.

Thus it is wise to work closely with the telescopic cylinder manufacturer to ensure a safe and efficient telescopic cylinder application.

Above: Another pair of large telescopic hydraulic cylinders on an oil well.

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