Introduction to Hydraulic Cylinders
Hydraulic cylinders are often also referred to as hydraulic rams, hydraulic jacks, hydraulic pistons or hydraulic actuators. These different terms are generally synonymous, although a ram is usually a cylinder with a very large piston rod diameter and a jack normally refers to a short stroke single acting hydraulic cylinder.
Above: A cutaway of a typical hydraulic cylinder with major components labelled with the correct technical terms.
Types of Hydraulic Cylinders
Hydraulic cylinders may be classified into two groups, double acting and single acting cylinders.
Single Acting Hydraulic Cylinders
A single acting hydraulic cylinder employs hydraulic force in only one direction, usually to extend the cylinder. The single acting cylinder is returned to its start position by an external force such as gravity or a spring. Of course, this can only be done after the hydraulic fluid within the cylinder has been depressurized and is allowed to return to the oil reservoir.
Double Acting Hydraulic Cylinders
A double acting hydraulic cylinder employs hydraulic force in two directions, both extension and retraction. This requires valving between the pump and the cylinder to direct the flow of oil alternately between the two sides of the piston. A double acting cylinder is more complex in design than a single acting cylinder as it has two oil supply ports and additional seals to retain the pressurized fluid within the cylinder and to prevent it from leaking past the piston rod.
Above: A simple double acting hydraulic cylinder system.
Hydraulic cylinders may be further classified into two other basic design types, Rod Cylinders and Telescopic (or Telescoping) Cylinders.
Rod Style Cylinders
Rod cylinders have a single stage barrel with a piston moving within it. A rod cylinder can only produce a motion or stroke much less than its overall length. It can be said that it works within the length of the barrel. The output motion is limited to the length of the barrel minus the length of the internal piston and the cylinder end caps.
Above: A small bore rod style cylinder.
Rod cylinders are the most common type of hydraulic cylinder. Most are single rod end with the piston rod extending out through a rod gland on one end of the actuator only. Double rod end cylinders have a piston rod attached to both sides of the internal piston. When one rod is extended, the other rod is retracted. (for more information, see the section "Double Rod End Cylinders" below, under the Heading "Cylinder Optional Features".)
Telescopic Hydraulic Cylinders
Telescopic cylinders are also sometimes called multi-stage telescopic cylinders. They may have 2, 3, 4, 5 or even 6 stages. These consist of hydraulic tubes nested within each other. They enable the telescopic cylinder to extend to a length much longer than the cylinder's fully retracted length. This system gives engineers great flexibility when designing a machine. A telescoping cylinder is, however, much more complex and expensive than a rod cylinder.
Above: A cross sectional diagram of a telescopic cylinder.
Telescopic cylinders are also available in both single and double acting configurations. Double acting telescopic cylinders are very complex and require special procedures in their use and application to prevent damage. See our separate Tutorial on Telescopic Cylinders.
Finally, hydraulic cylinders are categorized by method of construction. The two most common methods of constructing hydraulic cylinders are the Tie Rod Style and the Welded Body Style.
Tie Rod Style Cylinders
Tie Rod Cylinders use high strength steel rods to hold the end caps onto the cylinder barrel. Miniature cylinders (1/2 or 3/4" bore) may have 2 tie rods, small to intermediate bore size cylinders (1" to 8" bore) may have 4 tie rods, and larger bore size cylinders may have as many as 20 tie rods. The tie rod design is easy to assemble and disassemble but suffers from some design limitations. (See the Tutorial on Welded Body Cylinders - Advantages)
Above: A typical tie rod style cylinder.
Welded Body Cylinders
With Welded Body Cylinders, the end caps are welded to the cylinder barrel. This requires more careful construction but also produces a more robust cylinder design. Thus welded cylinders are the design of choice for mobile hydraulic applications and heavy industry. (See the separate tutorial on Welded Body Cylinders.)
Above: A typical welded style cylinder with cutaway showing internal components.
How Hydraulic Cylinders Work
Hydraulic cylinders produce linear force and motion by employing the flow of pressurized fluid. This fluid is usually supplied by a mechanical pump. In the most simple and basic application, the pump may be hand or foot operated. In a mechanized application, the pump is usually powered by an electric motor or an internal combustion engine. The distance the piston rod of a hydraulic cylinder is able to push a load is called the "stroke".
Once it reaches the hydraulic cylinder, the pressurized oil exerts pressure upon the area of the piston inside the cylinder barrel. This pressure produces a large force that moves the piston. In order to prevent the hydraulic pressure from being lost by passing over the piston to the opposite side, hydraulic seals are installed in the piston. These seals are often made from a rubber or urethane compound. They may be O-rings, U-cups, Stacked V-cups or another style of seal design. These seals usually fit into grooves machined into the outside diameter of the piston where it meets the inside diameter of the cylinder barrel. In addition to the piston seals, the piston may also have a bearing surface to enable it to endure side load forces without damaging the smooth inside diameter of the cylinder barrel. Often the piston bearing is a replaceable flexible band fitted into a groove on the outside diameter of the piston.
The internal piston is coupled to a shaft called the piston rod. This rod is in turn attached to the work piece or load that the cylinder is required to push or move. It may be coupled to the load using a machined thread on the end of the shaft or one of a number of typical mounting attachments including rod eyes and rod clevises. These mounting attachments may be welded or threaded onto the end of the piston rod. In some cases a pivot hole is simply machined into the end of the piston rod.
Above: A simple double acting cylinder ciircuit.
The piston rod exits the cylinder through a sealed gland called the rod gland. The rod gland is equipped with elastomeric seals that prevent the oil from leaking out of the cylinder when that end of the actuator is pressurized. It is also often outfitted with a rod wiper which prevents external contaminants from entering the cylinder when the rod motion is reversed and the piston rod is retracted back into the actuator. Finally, the rod gland has a bearing that supports and guides the rod as it moves back and forth. This bearing must be of sufficient size to support the weight of the piston rod and any external forces, especially side forces, exerted on the rod. This is particularly critical in long stroke applications. (See the Tutorial on Cylinder Design Considerations)
The opposite end of the cylinder to the rod end is called the cap end, rear end or blind end. The cap end is sealed off with a plate that is either welded or bolted to the cylinder barrel. The cap end often also serves as a mounting surface for the actuator as many hydraulic cylinders produce motion that requires they pivot through an arc. Thus the cap ends of many hydraulic cylinders are attached to clevis, trunnion or eye mounts.
The amount of force produced by the cylinder is directly proportional to both the oil pressure and the effective area of the piston. This force can be calculated using the equation F=PA , where F= Force, P = oil pressure, and A = the effective area of the piston.
Above: A diagram comparing force outputs for small and large pistons at the same pressure. Exert a force on the small piston on the left and the large piston on the right will exert a much larger force.
The effective area of a piston is different on both sides of a rod cylinder. (except in the case of a double rod end cylinder) The piston rod on the one side of the piston occupies a section of the piston face preventing the pressurized oil from acting on that area. Thus the net effective area of the rod end of a hydraulic cylinder is less than that of the cap end. This must be included in any design calculations when selecting a hydraulic actuator for an application particularly when it is required to produce force in the retraction stroke.
This retraction force of a rod cylinder can be calculated using the equation F=P(Ap-Ar), where F= Force, P = oil pressure, and Ap = the total area of the piston and Ar = the area of the piston rod.
The volume of the rod end of a rod cylinder is also much less than that of the cap end due to the volume taken up by the piston rod. This difference in volume causes the cylinder to retract much more quickly than it extends as the pump is able to fill the smaller rod end volume much faster. As well, the rate of oil flow returning to the oil reservoir will be much higher on the return stroke. In fact, this flow will be higher than the pump flow. The cylinder port size, fitting, hose and tube sizes, and the flow capacity of the return line oil filter must be selected based on the return flow on retraction. This is very important in systems with cylinders having very large piston rod diameters.
Cylinder Optional Features
Double Rod End Cylinders
Double rod end cylinders have a piston rod extending out both sides of the piston and have a rod gland at both ends of the cylinder barrel. In this case, if the rod diameters are the same on both sides, the extension and retraction speed would be exactly the same. The force output would also be exactly the same on extension and retraction.
Above: A small bore double rod end cylinder.
Double rod end cylinders are sometimes mounted by the two rod ends. The load is then attached to the body of the cylinder which moves back and forth while the piston rods are held stationary.
Double rod cylinders are sometimes employed as a means to adjust the output stroke of the actuator. The piston rod extending from the rear is equipped with a mechanism so that it strikes external stops. This arrangement can limit the extension, retraction or both.
In a similar fashion, the rear rod may also be outfitted with a mechanism to activate position sensors or some other form of feedback device indicating the cylinders stroke position.
In either of the latter two uses, the second piston rod may not have to be the same diameter as the primary rod coupled to the workload. Using a smaller secondary piston rod for stroke limiting or position sensing may thus save on unit weight and cost.
Hollow Piston Rods
Double rod end cylinders may also be built with a hollow piston rod so that a continuous passageway extends through the cylinder from one end to the other. This may be used to allow a cylinder to extend closing a die mold and then inject a material through its hollow rod into die mold.
Single rod end cylinders may also be equipped with a hollow piston rod. Often a hollow rod is used to reduce the weight of a large diameter piston rod. A hollow rod is much lighter and yet retains the column strength of a solid rod. Additionally, a hollow rod can accommodate a Linear Velocity Displacement Transducer (LVDT) which are used to provide very accurate electronic measurements of cylinder stroke.
Cylinders are often equipped with end of stroke cushions on one or both ends. Cushions are an internal feature that slows down the approach speed of the piston and rod assembly as it nears the end caps. This reduces the high impact forces that might otherwise cause damage to the internal components of the cylinder or to the machine that the cylinder is driving.
Cushions are in essence small secondary pistons that are mounted on either side of the main cylinder piston. As this small piston approaches the cylinder end cap it enters the passage way the leads to the cylinder port. This shuts off the flow of oil leaving the cylinder. The oil exiting the cylinder is now forced to leave the cylinder through a second passageway that is usually fitted with an adjustable needle which controls the volume of oil flow. This reduced oil flow slows the cylinder down during the last part of its stroke. To enable the cylinder to leave the end cap with full oil flow when it is reversed, a second passageway equipped with a check valve allows full pump oil flow to bypass the restricted cushion flow and reach the piston face.
Above: A cut away showing the details of a cushioned cylinder head.
Cushion pistons (sometimes also called cushion bosses) are often tapered so that they produce a progressive reduction in oil flow at the end of stroke. This results in a more gradual speed reduction and less of a jerk in the motion.
Caution must be exercised when using cushions with very heavy loads as the momentum of a large load may produce a very large pressure spike in the cushion chamber. This spike may be severe enough to exceed the pressure rating of the cushion seals. With damaged cushion seals the impact forces would not be reduced at end of stroke and significant machine damage could occur.
Stop Tubes and Dual Pistons
On cylinder applications that may encounter large side load forces, a number of methods may be employed to increase the cylinders ability to withstand these forces. This is particularly true in cylinders with very long strokes. If a very long stroke cylinder is fully extended, side load forces may cause the actuator to buckle and collapse. A general rule of thumb is to consider additional internal bearing support if the stroke exceeds ten times that of the cylinder bore size.
One such method of providing additional internal bearing support is the dual piston. A dual piston may be simply a longer piston that has extra piston bearing area or it may be two pistons separated some distance on the piston rod. The effect of this design is also to reduce the moment forces acting on the rod gland when the cylinder is at full extension by keeping the rod bearing and the piston bearing separated by a certain distance. Dual pistons retain the ability of a cylinder to have rod end cushions.
A second similar method of achieving this is the stop tube. The stop tube methods is simpler and less costly. It involves installing a large section of tube around the piston rod so as to prevent the cylinder from fully extending. Thus the piston bearing is kept a distance away from the rod bearing and side load capacity to resist buckling is maintained. The use of a stop tube does, however, preclude the installation of a rod end cushion.
It is obvious that the installation of either a dual pistons or a stop tube increases the overall length of a cylinder.
Hydraulic Cylinders in Mechanisms
Some hydraulic cylinders are assembled to a rack and pinion mechanism attached to a shaft in order to produce a rotary motion. This type of unit is called a hydraulic rotary actuator. The units can produce extremely high torque outputs in the order of tens of thousands of foot pounds.
Other mechanisms serve to guide or support the load that the cylinder is moving. It must be remembered that, while hydraulic cylinders are very powerful, the load that they are moving must be supported by rails or shafts in order to prevent damage to the cylinder. An improperly supported load may cause the piston rod to bend, or it may apply excessive side load forces to the piston and rod bearing which will score the inside diameter of the barrel and the rod bearing. The result is a reduced service life from the actuator and machine downtime while it is repaired.
Hydraulic Cylinder Mountings and Attachments
Cylinders are mounted in machines using a wide variety of methods. Mounting styles may be separated into two distinct classifications: rigid and flexible.
Rigid mounting styles hold the cylinder firmly in place and do not allow the body of the cylinder to move when it extends and retracts. These fixed mountings include foot mounts, flange mounts, side tapped holes, and threaded face mounts.
Flexible mounts allow the body of the cylinder to move as it extends and retracts. A cylinder pushing on a lever requires a flexible mount to allow the cylinder to follow the lever as it moves through an arc. Sometimes a flexible mount is used to allow for a slight misalignment between a cylinder and a load that is firmly guided. Flexible mounting styles include rear pivot mounts, clevis mounts, trunnion mounts, and spherical eye mounts.
Above: A hydraulic cylinder with spherical mounts on both ends.
Mounting attachments are sometimes bolted to a cylinder with high strength fasteners. Often they are welded to the body or the piston rod for maximum strength and cycle life expectancy.
Hydraulic Cylinder Materials of Construction
Hydraulic cylinders must be manufactured from high strength materials such as steel. Yet many applications are in areas with high temperature, humidity, corrosive elements, and abrasive elements. To accommodate these difficult environments, steel components are often surface treated to resist corrosion and abrasion. These treatments can include nitriding, chrome plating, and epoxy painting. In some cases, piston rods or entire cylinders may be made from stainless steel for maximum corrosion resistance. Care must be taken in the selection of cylinder materials as some corrosion resistant materials may lack tensile strength or surface hardness and thus prove unsatisfactory.
The Future of Hydraulic Cylinders
Hydraulic cylinders will continue to be the primary source of industrial heavy muscle for some time to come. Although great strides have been made in the area of electric motor driven linear actuators (sometimes called electric cylinders), these still do not have the power density or ruggedness of high pressure hydraulic cylinders. This means that designers will continue to turn to hydraulic cylinders as the main solution for high force output actuators.