Telescopic cylinder - Wikipedia

Telescopic cylinders are a special design of a hydraulic cylinder or pneumatic cylinder as well as pulley system which provide an exceptionally long output travel from a very compact retracted length. Typically the collapsed length of a telescopic cylinder is 20 to 40% of the fully extended length depending on the number of stages.[1] Some pneumatic telescoping units are manufactured with retracted lengths of under 15% of overall extended unit length.[2] This feature is very attractive to machine design engineers when a conventional single stage rod style actuator will not fit in an application to produce the required output stroke.[1]

Link to EOOE

Heavy duty telescopic cylinders are usually powered by oil hydraulics, whereas some lighter duty units could also be powered by compressed air.

Telescopic cylinders are also referred to as telescoping cylinders and multi-stage telescopic cylinders.

An application for telescopic cylinders commonly seen is that of the dump body on a dump truck used in a construction site. In order to empty the load of gravel completely, the dump body must be raised to an angle of about 60 degrees. To accomplish this long travel with a conventional hydraulic cylinder is very difficult considering that the collapsed length of a single stage rod cylinder is approximately 110% of its output stroke.[1] It would be very challenging for the design engineer to fit the single stage cylinder into the chassis of the dump truck with the dump body in the horizontal rest position. This task is easily accomplished, however, using a telescopic style multi-stage cylinder.[3]

Design and technical terminology

Telescopic cylinders are designed with a series of steel or aluminum[2] tubes of progressively smaller diameters nested within each other. The largest diameter sleeve is called the main or barrel. The smaller inner sleeves are called the stages. The smallest stage is often called the plunger[3] or piston rod.

The cylinders are usually mounted in machinery by pivot mounts welded to the end or outer body of the barrel as well as on the end of the plunger.

Telescopic cylinders are commonly restricted to a maximum of 6 stages. 6 stages are commonly thought to be the practical design limit as stability problems become more difficult with larger numbers of stages. There are exceptions however, with one pneumatic cylinder manufacturer successfully incorporating up to 9 stages in their cylinder designs.[2]

Telescopic cylinders require careful design as they are subjected to large side forces especially at full extension. The weight of the steel bodies and the hydraulic oil contained within the actuator create moment loads on the bearing surfaces between stages. These forces, combined with the load being pushed, threaten to bind or even buckle the telescopic assembly. Sufficient bearing surfaces must therefore be incorporated in the design of the actuator to prevent failure in service due to side forces. Telescopic cylinders must only be used in machinery as a device for providing force and travel. Side forces and moment loads must be minimized. Telescopic cylinders should not be used to stabilize a structural component.[4]

Hydraulic telescopic cylinders are often limited to a maximum hydraulic pressure of - psi. This is because the outward forces produced by internal hydraulic pressure tends to expand the steel sleeve sections. Too much pressure will cause the nested sleeves to balloon outward, bind the mechanism and stop moving. The danger exists that a permanent deformation of the outer diameter of a sleeve could occur, thus ruining a telescopic actuator. For this reason, care must be taken to avoid shock pressures in a hydraulic system using telescopic cylinders. Often such hydraulic systems are equipped with shock suppressing components, such as hydraulic accumulators, to absorb pressure spikes.[5]

Basic design types of telescopic cylinders

Telescopic cylinders can usually be classified into two basic designs: single acting and double acting. A number of other special designs also exist including a hybrid single/double acting design, and a constant speed, constant thrust design.[3]

Single-acting

Single acting telescopic cylinders are the simplest and most common design. As with a single acting rod style cylinder, the single acting telescopic cylinder is extended using hydraulic or pneumatic pressure but retracts using external forces when the fluid medium is removed and relieved to the reservoir. This external retraction force is usually gravity acting on the weight of the load. This external weight must obviously be sufficient to overcome the friction and mechanical losses within the machine design even after the work portion of the machine cycle has been accomplished. In the example above of the dump truck, the weight of the dump body, now raised at an angle of 60 degrees but empty of the load, must be enough to force the unpressurized hydraulic fluid out of the cylinder and cause it to retract to the fully collapsed position.[6]

Double-acting

A double acting cylinder is extended and retracted using hydraulic or pneumatic pressure in both directions. Double acting telescopic cylinders are thus much more complex in design than the single acting type. This additional complexity is due to the requirement of adding retracting piston faces to all of the cylinder stages and the difficulty in supplying pressurized fluid to the retraction pistons of the intermediate stages.[3]

To accomplish the double acting feature, additional hydraulic seals are added to internally seal off the individual stages. In addition, internal air or oil passageways are machined so that as each stage completes retracting, a passage is open to supply the next stage with pressurized fluid to retract. Thus a double acting telescopic actuator usually retracts starting from the smallest diameter stage to finish with the largest stage retracting lastly. Because the seals used to accomplish this must pass over these internally machined fluid transfer holes, the seals are usually made from hard materials to resist wear and abrasion. They are often iron rings or glass reinforced nylon seals.[3]

Want more information on Multi Stage Hydraulic Cylinder? Feel free to contact us.

The extension and retraction fluid supply ports on double acting hydraulic telescopic cylinders are usually located at opposite ends of the cylinder assembly. The extension port is mounted at the base of the outer barrel and the retraction port is mounted in the end of the plunger section. This can, in some applications, prove to be very difficult to connect with hydraulic hoses due to the distance between these ports at full extension. In such a circumstance, both ports can be located in the barrel. An internal passageway must be fitted, however, so that the retracting fluid is supplied to the plunger section at full extension. This special passageway is in itself a telescopic assembly that extends with the cylinder and is outfitted with seals on the various stages.[3]

This additional complexity makes double acting telescopic cylinders very expensive. They are usually custom designed for each application.

Typical applications for double acting telescopic cylinders include the packer-ejector cylinders in garbage trucks and transfer trailers, horizontal compactors, telescopic excavator shovels, and roll-on/roll-off trucks. In all of these applications, the cylinder operates near horizontally and thus gravity is not available to retract the actuator. A double acting design is therefore required to both push and pull the telescoping mechanism.[6]

Care must be taken when controlling most double acting telescopic design cylinders. The effective retraction area is often much less than the extension area. Thus if the hydraulic fluid return line is blocked during extension a pressure intensifying effect can occur causing seal failure or even causing the metal sleeve to balloon outward. The cylinder could thus be rendered unable to retract because of failed seals or jam in position due to binding.[4]

Another problem can occur if a double acting telescopic cylinder encounters a load that pulls on the actuator during extension such as when a tilting load goes over center and opens the cylinder beyond the internal volume of the hydraulic oil. When the piston face catches up again and strikes the oil column a pressure spike occurs which can damage the actuator.[4]

Single- / double-acting combination

In some applications, a single acting telescopic cylinder is adequate to accomplish the work except for one stage that is required to be double acting.

An example of this is erecting the mast of a large mobile drilling rig. The mast is erected to the vertical position using a telescopic cylinder. However, to lower the mast, gravity is not available for the initial tilt back from the vertical position. Thus, the plunger stage only of the telescopic actuator is equipped as a double acting cylinder to provide the initial force to pull the mast back from vertical. Once the tilt back has been initiated, then gravity takes over and supplies the force to complete the full cylinder retraction. The remaining stages, therefore, are single acting. This special combination is much less complex and much less costly than using an entirely double acting design.[3]

Constant thrust, constant speed

In some special applications, a telescopic cylinder is required to extend with a constant force or constant speed. To accomplish this the cylinder is designed so that all the stages extend at the same time. This can also be accomplished in a double acting design by matching the extension and retraction areas of the pistons on all the stages.[3]

See also

• Scissors mechanism
• Linear actuator

At RHK Hydraulic Cylinder Services Inc., we are committed to engineering and innovating dependable and performance-driven hydraulic systems for industrial applications. One of the most fascinating and versatile components we manufacture is a telescopic hydraulic cylinder.

What Are Telescopic Hydraulic Cylinders?

Telescopic hydraulic cylinders are designed to extend and retract in a manner similar to the sections of a telescope. They consist of multiple nested cylinder sections, allowing them to achieve a significant stroke length while maintaining a compact retracted size. This unique design makes them ideal for applications where space is limited, and a long stroke is required.
The operation of these cylinders comes from hydraulic fluid being pumped into the cylinder, causing the inner sections to extend outward. This unique design allows for a dramatic increase in output force relative to the cylinder size, making them an effective choice for various heavy-duty industrial applications.

Why Use Telescopic Hydraulic Cylinders?

Choosing the specifications for your telescopic hydraulic cylinder can be customized to your requirements and use.

Space Efficiency
One of the most significant advantages of telescopic hydraulic cylinders is their space efficiency. The design allows for a long stroke without requiring a large footprint, which is invaluable in applications where space is at a premium. This efficiency means that equipment can be designed to be more compact, saving valuable space in industrial environments.
Increased Lifting Capacity
Telescopic cylinders can generate substantial force due to their multi-stage design. This means they can lift heavier loads compared to standard hydraulic cylinders of the same retracted length. This capability is essential in heavy-duty applications where the lifting of substantial weights is a daily requirement.
Improved Cycle Times
The rapid extension and retraction capabilities of telescopic cylinders enhance the overall efficiency of machinery. They allow for quicker operations, reduced cycle times and increased productivity. In industries where time is money, this efficiency can lead to significant cost savings.
Versatility
These cylinders can be tailored to meet specific needs across various applications. Customization options, including different stroke lengths, sizes, and mounting configurations, make them suitable for a wide range of industrial tasks.
Durability and Reliability
At RHK Hydraulic Cylinder Services Inc., we prioritize durability and reliability in our products. Telescopic cylinders are constructed to withstand harsh conditions, including extreme temperatures and exposure to contaminants. This resilience ensures that they can perform consistently over time, minimizing downtime and maintenance costs.

Industrial Uses and Applications

The versatility of telescopic hydraulic cylinders has led to their widespread use across multiple industries. Here are a few of the key applications RHK services:
Crane and Aerial Lifting
Telescopic cylinders are crucial in applications for cranes and aerial lifting, as they offer enhanced versatility and efficiency for these complex lifting operations. The multiple nested sections that can extend and retract allow cranes to reach greater heights with extended working radiuses without the need for additional equipment.
This ability to save space in situations where traditional fixed-length cylinders would be impractical, telescopic designs provide a compact solution. This is particularly useful in urban environments where space is limited and maneuverability is crucial.
Additionally, telescopic cylinders improve operational efficiency. By allowing cranes to extend to various heights seamlessly, operators can complete tasks more quickly and with precision. This adaptability not only enhances productivity, but also minimizes the risk of accidents, as cranes can operate within safer, more controlled parameters.
Maintenance is another key consideration. Modern telescopic cylinders are designed with durability in mind, RHK’s cylinders feature advanced sealing technologies that reduce wear and tear. This longevity translates to lower operational costs and increased uptime.
Oil and Gas
Telescopic cylinders are indispensable in applications involving equipment for oil rigs, fuel tankers, and many other hydraulic lifting systems used in the oil and gas industry including the transportation of materials. These cylinders enable equipment to reach significant heights or depths without requiring additional space or structural modifications.
For instance, in the oil and gas industry, loading and unloading heavy equipment, drilling tools, and fuel can be cumbersome without the right lifting and movement systems. Telescopic cylinders provide a reliable and efficient means of achieving these actions. Their ability to extend in stages allows for the required reach without needing overly long or cumbersome equipment. This is particularly valuable in environments with limited space or where the weight of materials demands high lifting capacity.
Another significant advantage of telescopic cylinders for truck and transportation is their role in maintaining balanced and efficient load distribution. Trucks transporting heavy equipment or hazardous materials, such as in the case of oil and gas logistics, demand precise control over the load. Telescopic cylinders are commonly used in dump trucks, tankers, and flatbeds to ensure that the load is lifted evenly and securely. This is particularly vital where transportation of equipment such as tanks, pipes, and chemicals must meet rigorous safety standards. RHK’s cylinders provide stable support, minimizing the risk of accidents and improving the efficiency of loading and unloading procedures.
Furthermore, telescopic cylinders are often employed in the operation of hydraulic trailers and cranes. In oil and gas operations, equipment such as rigs, pumps, and storage units are often too large and heavy to be lifted by standard hydraulic systems. The unique multi-stage extension of telescopic cylinders provides a high lifting capacity while still maintaining a manageable form factor. This ability to extend incrementally is ideal for handling the size and weight demands of this equipment. Additionally, the hydraulic action of these cylinders offers smooth and controlled lifting, reducing the potential for mechanical strain or failure during operation.
Construction Equipment
By utilizing hydraulic pressure across multiple stages of extension, telescopic cylinders help construction machinery perform precise, stable operations—critical for safety when lifting or moving heavy materials. Excavators, cranes, and dump trucks all rely on hydraulic systems that utilize telescopic cylinders to lift, extend, and maneuver large loads. RHK’s customized performance-driven cylinders are vital to these industrial applications due to their ability to provide powerful lifts and extensions to enhance both performance and safety in these construction environments.
In tight job sites, such as urban areas or in environments with limited access, equipment that uses telescopic cylinders can achieve significant reach and lifting capacity without requiring additional space. This is particularly valuable in applications including pile driving or material handling, where space and efficiency are at a premium.
Waste Management
Telescopic cylinders are an integral part of modern waste management equipment, offering key advantages in terms of space efficiency, lifting capacity, and operational versatility. In an industry where heavy lifting, compact design, and precise control are paramount, these cylinders help streamline waste collection, disposal, and recycling processes, ensuring both efficiency and safety.
Waste collection trucks, compactors, and recycling vehicles often require equipment that can handle large, heavy loads, such as dumpsters, trash bins, and recyclables, but must also navigate through tight urban streets and confined spaces. Telescopic cylinders, with staged extensions, allow these trucks to achieve significant reach without occupying excessive space. This ensures that operators can easily load and unload materials while maintaining maneuverability in congested areas.
Moreover, these cylinders contribute to smoother, more controlled lifting and dumping operations. Whether used in refuse trucks, roll-off containers, or material handling systems, these cylinders provide fine control over the movement of heavy loads, preventing abrupt or unsafe actions that could damage equipment or lead to accidents.
These cylinders are built to withstand the harsh environments and frequent heavy-duty use typical of waste collection, ensuring a longer service life matched with lower maintenance costs.
Agriculture
Telescopic cylinders are an essential component in modern agricultural machinery, providing significant benefits in terms of performance, efficiency, and space optimization. These cylinders, with their ability to extend and retract in stages, allows agricultural equipment to perform a variety of tasks with precision and ease. Agricultural machinery, such as loaders, harvesters, and tractors, often require equipment that can reach significant heights or depths to handle large bales of hay, transport harvested crops, or load supplies into vehicles. Telescopic cylinders allow these machines to perform tasks without the need for large, cumbersome lifting systems. By extending in multiple stages, telescopic cylinders provide the necessary reach and power required while keeping the machinery’s overall size manageable when in fields or tight spaces.
Additionally, telescopic cylinders are invaluable for tasks that involve precision lifting and handling. For example, when loading grain bins, telescopic cylinders ensure smooth and controlled movement of the boom or lifting arm, reducing the risk of damage to crops or equipment. The gradual extension of the cylinder also allows for fine adjustments, ensuring that loads are positioned accurately and safely.
Whether used in planting, irrigation systems, or material handling, these cylinders adapt to different applications, making them indispensable for farmers and agricultural contractors who need reliable, multi-purpose equipment.

If you want to learn more, please visit our website dual hydraulic cylinder.