Contributed by Josh Cosford
Telescopic cylinders are a unique type of actuator able to extend far longer than a traditional rod-style cylinder. Telescopic cylinders are manufactured using two or more nested tubes inside one another, and these tubes act as both the rod and barrel. Telescopic cylinders have as many as seven stages or more, providing a compact footprint with a surprisingly long stroke length. A cylinder with 120 in. of stroke only needs 50 in. of retracted space, while a traditional cylinder will always be longer than whatever its stroke length is.
The most common use for telescoping cylinders is for dump bodies, dump trailers or other dump applications such as garbage or vacuum trucks. Because the dump bed pivots as it lifts, so too must the cylinder freely travel through an arc as it extends, necessitating a pivoting mount such as cross tube, trunnion or rod eye. As well, some telescopic cylinders are configured “upside-down” where the rod is fixed to the chassis as the cap end lifts into the air.
Telescopic cylinders are manufactured as either single- or double-acting, although single-acting cylinders are much more common. For applications where a dump box or machine goes over center, a double acting cylinder can pull it back to align with gravitational retraction. All telescopic cylinders have ports at only the bottom end mounted close to the chassis, since running hoses up through 10 to 20 ft of stroke and an arc of rotation would be exceedingly difficult. In fact, double-acting telescopic cylinders run both ports through the head — one into the many bodies, and the other through the hollow core.
The stroke length of a telescopic cylinder ranges from less than 80 to more than 300 in. From a technical standpoint, a telescopic cylinder can be viewed as multiple cylinders in the same envelope rather than a homogenous product. Each cylinder stage extends sequentially based on its effective area. These cylinders aren’t immune from the physics of fluid power, so the stage with the largest area will also provide the path of least resistance as it extends.
Because the effective area lessens as bore diameter shrinks when each stage comes online, pressure jumps upward to compensate for the reduced surface area. It’s important to consider how this affects the cylinder’s ability to move a load … the cylinder must be sized accurately to continue to supply enough force to lift the load when it reaches its final stage. The first stage may see less than 1,000 psi to get the dump bed moving but could rise to over 3,000 psi at the final stage, depending on the design and settings of its hydraulic system.