Telescopic cylinder
Encyclopedia
Telescopic cylinders are a special design of hydraulic cylinder
that 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. 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.
.svg.png)
Telescopic cylinders are usually powered by hydraulics
but some special light duty designs are 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. 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.
Telescopic cylinders are designed with a series of steel
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.
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 can be built with as many as 6 stages. Six stages seems to be the practical design limit as stability problems become more difficult with larger numbers of stages. 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.
Telescopic cylinders are often limited to a maximum hydraulic pressure
of 2000-3000 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 accumulator
s, to absorb pressure spikes.
design.
. 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.
faces to all of the cylinder stages and the difficulty in supplying pressurized fluid to the retraction pistons of the intermediate stages.
To accomplish the double acting feature, additional hydraulic seal
s are added to internally seal off the individual stages. In addition, internal oil passageways are machined so that as each stage completes retracting, an oil 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.
The extension and retraction fluid supply ports on double acting 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 hose
s 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.
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.
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.
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
.
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.
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.
. Yet small bore designs also exist that are used on lighter duty applications such as agricultural equipment. Often these cylinders are unnoticed by most people as they go unseen, buried deep inside common machinery such as a garbage truck.
Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
that 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. This feature is very attractive to machine design engineers when a conventional single stage rod style actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
will not fit in an application to produce the required output stroke.
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
but some special light duty designs are powered by compressed air
Compressed air
Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....
.
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
Dump truck
A dump truck is a truck used for transporting loose material for construction. A typical dump truck is equipped with a hydraulically operated open-box bed hinged at the rear, the front of which can be lifted up to allow the contents to be deposited on the ground behind the truck at the site of...
used in a construction
Construction
In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...
site. In order to empty the load of gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
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. It would be very challenging for the design engineer
Design engineer
Design Engineer is a general term that covers multiple engineering disciplines including electrical, mechanical, industrial design and civil engineering, architectural engineers in the U.S...
to fit the single stage cylinder into the chassis
Chassis
A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...
of the dump truck with the dump body in the horizontal
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...
rest position. This task is easily accomplished, however, using a telescopic style multi-stage cylinder.
Design and Technical Terminology
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
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.
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 can be built with as many as 6 stages. Six stages seems to be the practical design limit as stability problems become more difficult with larger numbers of stages. 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
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
to prevent failure in service due to side forces. Telescopic cylinders must only be used in machinery as a device for providing force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
and travel. Side forces and moment loads must be minimized. Telescopic cylinders should not be used to stabilize a structural component.
Telescopic cylinders are often limited to a maximum hydraulic pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
of 2000-3000 psi. This is because the outward forces produced by internal hydraulic pressure tends to expand the steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
sleeve sections. Too much pressure will cause the nested sleeves to balloon outward, bind the mechanism
Mechanism (engineering)
A mechanism is a device designed to transform input forces and movement into a desired set of output forces and movement. Mechanisms generally consist of moving components such as gears and gear trains, belt and chain drives, cam and follower mechanisms, and linkages as well as friction devices...
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 accumulator
Hydraulic accumulator
A 'hydraulic accumulator' is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. That external source can be a spring, a raised weight, or a compressed gas...
s, to absorb pressure spikes.
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 thrustThrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
design.
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 pressure but retracts using external forces when the hydraulic pressure is removed and relieved to the reservoirReservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
. 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
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of 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
Hydraulic fluid
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
out of the cylinder and cause it to retract to the fully collapsed position.
Double Acting
A double acting cylinder is extended and retracted using hydraulic 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 pistonPiston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
faces to all of the cylinder stages and the difficulty in supplying pressurized fluid to the retraction pistons of the intermediate stages.
To accomplish the double acting feature, additional hydraulic seal
Hydraulic seal
A hydraulic seal is a relatively soft, non-metallic ring captured in a groove or fixed in a combination of rings, forming a seal assembly, to block or separate fluid in reciprocating motion applications. Hydraulic seals are vital in machinery...
s are added to internally seal off the individual stages. In addition, internal oil passageways are machined so that as each stage completes retracting, an oil 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
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
rings or glass reinforced nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
seals.
The extension and retraction fluid supply ports on double acting 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 hose
Hose
A hose is a hollow tube designed to carry fluids from one location to another. Hoses are also sometimes called pipes , or more generally tubing...
s 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.
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
Excavator
Excavators are heavy construction equipment consisting of a boom, stick, bucket and cab on a rotating platform . The house sits atop an undercarriage with tracks or wheels. A cable-operated excavator uses winches and steel ropes to accomplish the movements. They are a natural progression from the...
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.
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
Hydraulic fluid
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
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.
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
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
spike occurs which can damage the actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
.
Single/Double Acting Combination
In some unique 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
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...
. 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
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
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.
Constant Thrust, Constant Speed
In some special applications, a telescopic cylinder is required to extend with a constant forceForce
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
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.
Conclusion
Telescopic cylinders are often seen used on heavy equipment such as construction equipmentConstruction Equipment
Construction Equipment is a trade publication and web site serving the information needs of construction contractors, materials producers, and other owners and operators of construction equipment....
. Yet small bore designs also exist that are used on lighter duty applications such as agricultural equipment. Often these cylinders are unnoticed by most people as they go unseen, buried deep inside common machinery such as a garbage truck.