Double-acting hydraulic cylinder. Single acting hydraulic cylinder.
Hydraulic cylinders are actuation devices that utilize pressurized hydraulic fluid to produce linear motion and force. Hydraulic cylinders are used in a variety of power transfer applications. The basic principle behind hydraulic cylinders is that the force applied at one point is transferred to another point using an incompressible fluid. The hydraulic fluid is typically some type of oil. A simple hydraulic system consists of two pistons connected by an oil filled pipe. As a force is applied to one piston, it is transmitted to the second piston through the oil in the pipe. Oil is incompressible and therefore is a very efficient medium for the force to travel through; almost all the force applied to the first piston appears in the second.
Hydraulic cylinder cutaway. Video Credit: Whirlbird57
Besides efficiency, hydraulic cylinders are advantageous in many systems because the pipe connecting the pistons can be any length, size, or shape. The pipe can fork to create a master cylinder which operates several slave cylinders throughout the system. They are also able to provide large amounts of power to machinery in areas away from the source of power generation. Possible power sources include electric motors, diesel engines, turbines, or hand or foot pumps. Hydraulic cylinders have a very efficient power to weight ratio and power to size ratio. These cylinders include variable speed control, positioning, and automatic overload protection. They are often ruggedly designed to be used in extreme environments.
Hydraulic cylinders consist of a piston that moves through a smooth round cylinder or tube. This cylinder is sealed at both ends with end plates (also referred to as end caps or cylinder heads) and firmly connected to a shaft (piston rod) that exits the cylinder through a hole in one end cap. The opposite end is called the cap end because it does not have an eye for the rod to stick out.
Hydraulic cylinder parts. Image Credit: Hyco Canada
The cylinder is typically made from high strength seamless steel tubing. The tubing has been honed or skive roller burnished to a fine finish inside so the hydraulic piston can slide through smoothly. The tube must also be thick enough to withstand the pressure within the system.
Choices for cylinder type include tie-rod, welded, and ram.
A tie-rod cylinder is a hydraulic cylinder that uses one or more tie-rods to provide additional stability. High strength rods can be small bore ranging from 1 inch to 8 inch bore, usually there are four tie rods holding the end caps together. Large bore cylinders may require as many as 24 tie rods in order to retain the huge internal force being generated. Tie-rods are typically installed on the outside diameter of the cylinder housing. At high pressures, the tie rods stretch slightly, and may stretch to such an extent that the tube detaches from the end caps. This causes a loss of pressure and leakage occurs. Additional support is required for long stroke tie rod cylinders to prevent sagging. In many applications, the cylinder tie-rod bears the majority of the applied load. Image Credit: AutomationDirect
A welded cylinder is a smooth hydraulic cylinder that uses a heavy-duty welded cylinder housing to provide stability. The cylinder is built by welding the steel end caps to a heavy gauge steel tube. The rod gland is then bolted or threaded to a flange welded to the rod end of the cylinder. The barrel of a welded cylinder must be strong enough to withstand the mechanical forces, and provide the necessary rigidity to support the loads of the actuator body. They are narrower and shorter than tie-rod cylinders. Image Credit: Energy Manufacturing
A ram cylinder is a type of hydraulic cylinder that acts as a ram. A hydraulic ram is a device in which the cross-sectional area of the piston rod is more than one-half the cross-sectional area of the moving component. Hydraulic rams are primarily used to push rather than pull, and are most commonly used in high pressure applications. Image Credit: Hydraulic Ram Repair
For more information please read GlobalSpec's guide How to Select Air Cylinders.
Video detailing operating specifications. Video Credit: AgriSupplyHowTo
Stroke- The distance that the piston travels through the cylinder.
Stroke lengths- The length of the piston. Length varies from fractions of an inch to many feet.
Operating pressure- The operating pressure range specifies the full-required range of operating pressure.
Bore size- Bore size is the inner diameter measure of a barrel.
Body material- Common body material choices include aluminum, steel, stainless steel, and plastic.
Rod diameter- The rod diameter determines how much load the piston is able to handle before it buckles. The table below shows the minimum rod diameter under various load conditions. The exposed length of the piston, listed at the top of the table, is typically longer than the stroke length of the cylinder. The vertical scale is in English tons (1 ton = 2000lbs.)
Figures in body of chart are suggested minimum rod diameters. Image Credit: Mead
If the piston rod is too small then the rod may buckle under the force of the load. If the piston rod is too big, it will have a larger initial cost, require a more expensive mounting attachment, and produce high end stroke impact forces in a high velocity application. The large rods reduce the effective area of the piston on the rod end resulting in smaller retraction forces.
Speed - The speed is controlled by valves, which control the flow of the hydraulic oil entering or leaving the cylinder. Cylinders are typically allowed to run at their maximum natural speed. This maximum speed is determined by cylinder size, port size, air pressure, bore and length of the hoses, and the load against which the cylinder is working. From this natural speed, the user can increase speed, or more often, reduce it. Smaller valves have slower cylinder movement; however the maximum natural speed of these cylinders can often be achieved with a valve that is one or two sizes smaller than the cylinder port size.
Force- Force is related to the diameter of the piston, but they are not directly proportional. In general, the larger the piston area, the more force is produced. Force ratings can differ somewhat in opposite directions. The instroke (pull) force is less than the outstroke (thrust) force when powered hydraulically by the same supply of compressed fluid, due to the effective cross sectional area reduced by the area of the piston rod. The relationship between force on outstroke, pressure, and radius is as follows:
Fτ = P(∏r2)
Fτ is the resultant force
P is the pressure distributed load on the surface
∏ is pi, approximately equal to 3.14159
r is the radius of the piston
∏r2 represents the effective area of the piston surface where the pressure is acting on.
Instroke uses a similar equation as outstroke force; however, the cross section area is less than the piston area so the relationship with the radius is different. The relationship between force exerted for instroke, pressure, radius of the piston, and radius of the piston rod is as follows:
Fτ = P∏(r12 - r22 )
Fτ is the resultant force
P is the pressure distributed load on the surface
∏ is pi, approximately equal to 3.14159
r1 is the radius of the piston
r2 is the radius of the piston rod
In hydraulic cylinders, the force can easily be multiplied or divided throughout the system. This is commonly known as trading force for distance and can be seen in many other mechanical systems. To achieve this in hydraulic systems, the size of the piston and cylinder must be changed relative to one another. As you can see from the images below, the piston on the right has an area nine times greater than the piston on the left. This means that for every nine units of force applied to the piston on the left, the piston on the right will move one unit.
Hydraulic multiplication. Image Credit: How Stuff Works. Adjustments by Author
Choices for cylinder configuration are simple configuration or telescopic figuration.
A simple configuration hydraulic cylinder consists of a single cylindrical housing and internal components.
A telescopic configuration hydraulic cylinder uses "telescoping" cylindrical housings to extend the length of the cylinder. A series of hydraulic tubes are nested like sleeves that telescope within each other. They are used to provide a long total output travel with as many as 6 stages or sleeves. Telescopic configuration cylinders are used in a variety of applications that require the use of a long cylinder in a space-constrained environment. Telescopic cylinders are single acting cylinders and are retracted via gravity.
Telescopic cylinder. Image Credit: Hyco Canada
Hydraulic cylinders can be single action or double action.
A single action hydraulic cylinder is pressurized for motion in only one direction. It is a simple, inexpensive design. Once the work is completed, the oil is depressurized and returned to the fluid reservoir. The piston returns to the starting position by an external force such as gravity or a compressed spring.
A double action hydraulic cylinder can move along the horizontal (x-axis) plane, the vertical (y-axis) plane or along any other plane of motion. This design uses pressurized hydraulic fluid to extend and retract the piston rod. This requires fluid ports at both ends of the actuator in order for the oil to be directed onto both sides to the piston.
Choices for mounting method include flange, trunnion, threaded, clevis or eye, and foot. The mount location can be cap, head, or intermediate. It's important to select the right the mounting style since certain mounting styles can cause the end caps of a tie-rod cylinder to misalign causing loss of fluid and pressure. Image Credit: Bailey
Fixed mounting styles, such as front and rear flanges, threaded side tapped mounts, and foot mounts, allow the actuator to be fastened to a flat surface. When using a fixed mount it is important to ensure that the load doesn't exert side forces on the piston, which will cause wear inside the cylinder. Side loading causes a shorter service life, and erratic cylinder movement.
Pivoting mounts, including rear pivot, spherical eye mounts, and trunnion mounts, allow for misalignment since the actuator is able to swing or pivot through an arc.
Hydraulic cylinders are used in many industries including agriculture, military and government, oil and gas, and fire and rescue, just to name a few. They are easily sized and are ruggedly designed so they can be used in various applications and environments.
For common features, please refer to GlobalSpec's guide How to Select Linear Actuators.
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Air cylinders are pneumatic linear actuators that are driven by a pressure differential in the cylinder's chambers. They may be single-acting (with a spring return) or double-acting.
Electric linear actuators have an output rod that provides linear motion via a motor driven ball screw, lead screw, or ACME screw assembly. The actuator's load is attached to the end of a screw or rod and is often unsupported.
Linear thrusters use double-acting pneumatic cylinders or hydraulic cylinders mated to shafts and plates to provide reoccurring linear motion They are used in conveying, inspection systems, and lifting applications, or to apply thrust load
Rodless cylinders are linear devices that use pressurized fluid to move a load within many power transfer operations.