Video detailing operating specifications. Video Credit: AgriSupplyHowTo / CC BY-SA 4.0

• 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:

F_τ = P(∏r²)

Where:

F_τ 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:

F_τ = P∏(r₁² - r₂² )

Where:

F_τ 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

Cylinder Configuration

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

Cylinder Action

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.

Mounting Options

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.

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.

Applications

Hydraulic cylinders are used in many industries including agriculture, military and government, oil and gas, and fire and rescue. They are easily sized and are ruggedly designed so they can be used in various applications and environments.

Standards

A-A-52410 -- Boot, dust and moisture seal: hydraulic brake cylinder, synthetic rubber

AWS D14.9/D14.9M -- Specification for welding of hydraulic cylinders

BS ISO 10100 -- Hydraulic fluid power - cylinders - acceptance tests

Resources

How to Select Air Cylinders

How Hydraulic Machines Work- The Basic Idea

Image Credit: Bailey | Grainger