Telescopic Hydraulic Cylinders
Long Stroke, Short Profile
When a long stroke is required in a tight space, a Telescoping Hydraulic Cylinder is the ideal solution.
A telescopic hydraulic cylinder is popular in many industries, especially in mobile applications. The main advantage of this cylinder design is its ability to provide an exceptionally long stroke while maintaining a relatively short retracted length. Therefore, when mounting space is tight and a long stroke is required, a telescopic cylinder is the answer.
A telescopic cylinder, also called a multi-stage cylinder, can have a collapsed length as small as 20% of the fully extended cylinder length depending on the number of stages.
Fig. 1 2-Stage Telescopic Cylinder Fully Extended
Fig. 2 Same 2-Stage Telescopic Cylinder Retracted
32% of the Total Extended Length!
A common application for a multi-stage cylinder is the dump body on a dump truck. In order to empty a load completely, the dump body must be raised sometimes to an angle of 60 degrees. A single rod cylinder is not suitable here as the retracted length would need to be longer than the required stroke. A dump truck chassis cannot accommodate this collapsed length and a telescopic cylinder is the ideal solution.
Telescopic Hydraulic Cylinder Design
Like a conventional single rod cylinder, the largest diameter tube is called the barrel and the smallest section is called the rod or plunger. A telescopic cylinder features two or more nested tubes known as stages, reducing in diameter from the outer barrel. The maximum number of stages seems to be six. Theoretically, a cylinder with more stages could be designed but stability becomes an issue. Three or four stage cylinders are quite common.
Typically, the stages of a telescopic cylinder extend from largest to smallest. The largest stage will complete its stroke before the next stage begins moving. This continues until all stages are fully extended and the cylinder has reached its overall extended length. And when being retracted, the smallest stage moves first until all stages have been retracted and the cylinder is fully collapsed.
Types of Telescopic Cylinders
Single Acting Telescopic Cylinders
A single acting telescopic cylinder is the simplest and most common design. Just like a single rod cylinder, it is extended using hydraulic or pneumatic pressure but relies on an external force, such as gravity, to retract. When fluid pressure is released to the reservoir, the weight of the load overcomes friction and mechanical losses retracting the cylinder. Single acting cylinders are used when some form of load is always present such as a dump body on a dump truck.
Double Acting Telescopic Cylinders
A double acting telescopic cylinder uses hydraulic or pneumatic pressure to extend and retract. The design is more complex than its single acting counterpart, but is the only option when an external retract force is not present or when control over the retract cycle is required. Classic applications are excavator shovels, garbage compactors, and roll on/off trucks.
Usually the hydraulic fluid ports are located on opposite ends of the cylinder. The extend port is located in the endcap or far end of the barrel, while the retract port is located at the end of the smallest stage, also known as the rod cap. Unfortunately with this design, the hydraulic hoses travel a great distance as the cylinder extends. A common approach here is to design both ports stationary on the barrel.
Single/Double Acting Combination
A Single/Double Acting Combination telescopic cylinder is typically used when retract force is only required for a portion of the stroke. A typical application is a mast raising cylinder on a drilling rig, where the load is pulled over center and then gravity takes over to complete the retract cycle. By enabling a number of stages to remain single acting, the design maintains simplicity and reduces the overall cost.