Available in a gamut of sizes, types, and design configurations, cylinder actuators form the main output device of a hydraulic motion system, and often the most visible part. These actuators convert hydraulic fluid pressure into rapid, controllable linear motion and force to move loads.
A typical actuator consists of the cylinder body, end caps, piston, piston rod, seals, and bearing surfaces for piston and rod. Designed for various pressures, ratings up to 3,000 psi (continuous) apply to industrial units; up to 5,000 psi for mill and press type units. Stroke lengths to 10 ft and cylinder diameters to 8 in. are available, with much larger units made for special applications.
From a basic relationship of hydraulics (Pascal's Law), linear force magnitude developed by a cylinder is the system fluid pressure times the piston's effective area over which pressure acts, or F = PA. Friction and other real-world losses will reduce results from this simple relation.
Many configurations
The simplest actuator configuration is a single-acting cylinder
, where fluid is ported to one side of the piston, producing output force and motion only in one direction. Gravity or external springs return the piston to its starting position as the fluid is returned to the reservoir. A double-acting cylinder ports fluid to either side of the piston to produce force/motion when extending and retracting the piston rod (see graphic). Sealing between piston OD and cylinder ID must handle both directions of motion. A variant here is a double-end rod cylinder that adds a piston rod extending through the cylinder's rear end cap.
A typical double-acting cylinder produces somewhat greater force when extending than when retracting—with equal pressure applied on each side of the piston—due to different effective areas exposed to the hydraulic fluid. The rod-side piston area is smaller than the head side, resulting in less force on the retract stroke, says Pascal's Law. A double-end rod cylinder has essentially the same area on each side of the piston, and doesn't exhibit this differential cylinder effect.
A variant of the above actuator types is a ram cylinder
, designed with a robust diameter piston rod approaching full-piston diameter that prevents rod bending in a long-stroke horizontal unit or buckling under high vertical loads as in a press or stamping application.
The piston connects to a high-strength steel piston rod whose opposite end comes with various rod-end connections (see graphic). Surface hardened or chrome-plated rods are often used with ultra-fine surface finish that's key to long seal life. Piston OD must fit precisely to the cylinder ID. Each must be precisely cylindrical and extremely finely finished for smooth output motion.
The latest industrial cylinders can incorporate sensor feedback and an electrohydraulic servo valve for sophisticated speed and position control of loads—not only full stroke (in/out) or rough positioning. Applications range from production machine tools and material handling to steel mill actuators, nuclear power plant controls, and passenger and freight elevators.