Nov. 15, 2002
Actuators convert fluid energy into useful work.

Actuators convert fluid energy into useful work.

Fluid-power actuators are available in a number of forms to provide specific types of action. Cylinders work through linear extension; motors impart continuous rotary motion to objects; rotary actuators twist an object through only a partial arc.

Generally, all types of actuators are available for pneumatic or hydraulic operation. Often, the same cylinder can be used for either air or low-pressure oil operation. Air and hydraulic motors, though similar, are usually not interchangeable.

When fluid is pumped into a cylinder, piston and rod are forced to move in or out against a load. Movement can be all at once, or in increments; the piston may even rotate or carry fluid. It can be returned by gravity, springs, or reversed flow.

Single-acting cylinders, the simplest type, power a stroke in only one direction. When the fluid is allowed to drain from the cylinder, some external force must push the piston back to its starting position. This low-cost cylinder can be used for any job -- like a lift against gravity -- that provides a returning force.

Cushioned cylinders are single-acting cylinders with a built-in shock absorber. Where the function of the cushion is to reduce shock or hammering against the cylinder head, cushions are usually nonadjustable. But adjustable cushions are available for applications where precise cylinder speed is important.

Ram cylinders are usually single-acting types with a rod at or near full-piston diameter. The large-diameter ram is favored when column loads are extremely high or when the rod overhang in a horizontal cylinder could cause sagging. Ram cylinders are frequently used for large press applications and for jacking.

Spring-return cylinders are similar to the low-cost single-acting types, but with a spring added to return the piston to its starting point. This type is widely used in both pneumatic and hydraulic service, but is not always suitable for hydraulic service; if the spring is heavy enough for speedy piston return, it may require too much force to compress. The cylinder must be about twice as long as the required stroke to include space for the spring. Some cylinders are spring loaded in the opposite direction, so they extend with spring action and retract pneumatically or hydraulically.

Double-acting cylinders contain two fluid chambers so that pressure can be used to both extend and retract the rod. Sealing devices work in both directions. This type of cylinder is by far the most common, and can be used in nearly all types of applications. Effective working area of the rod side of the piston is less than that of the other side, so double-acting cylinders retract faster than they extend, and exert less force on the retraction stroke.

Double-end rod cylinders are double-acting types with a rod extending from each side of the piston. A chief advantage of this type is that working areas of both piston sides are equal. So the piston moves at the same rate and delivers equal forces in each direction. Double-end rod cylinders are available with a hollow rod, so that fluid or another machine element can be passed through the cylinder. In a design variation often used on planers, the hollow piston rod is restrained and the cylinder body is forced back and forth to shift the moving table.

Tie-rod cylinders, the oldest and most common, are typically used in industrial jobs. The cylinder body is held together by four or more tie rods that extend the full length of the body and pass through the end caps or mounting plate. In operation, they may perform any of the common cylinder functions except telescoping.

One-piece cylinders are most often used on mobile equipment and farm machinery. The body is either cast integrally, or head and body may be welded together. This is the least expensive type of cylinder; it is compact and simple. But it cannot be repaired when damaged or worn.

Threaded-head cylinders offer a compromise between tie-rod and one-piece units. Threaded units are relatively compact and streamlined, yet can be disassembled for repair by unthreading either or both ends from the cylinder body.

Telescoping cylinders provide a long stroke from a short body. Total stroke length may be as much as four times as long as the collapsed length of the cylinder. Available in either single or double-acting models, telescoping cylinders are widely used for vehicle applications; a typical job is lifting the bed of a dump truck. Force output varies with rod extension: highest at the beginning, when full piston area is used; lowest at the end of the stroke, when only the area of the final stage can be used to transmit force.

Diaphragm cylinders are used in either hydraulic or pneumatic service for applications that require low friction, no leakage across the piston, or extremely sensitive response to small pressure variations. They are frequently used as pneumatic actuators in food and drug industries because they require no lubrication and do not exhaust a contaminating oil mist. Spring-return models should not be pressurized in the reverse direction because reversals can pleat the diaphragm and shorten its life. Double-acting actuators with twin diaphragms are available for applications requiring pressure reversal.

Rotating cylinders impart linear motion to a rotating device. They are often used to actuate rotating chucks on turret lathes. Fluid is ported to the rotating cylinder through a stationary distributor. Rotating cylinders are available both with solid and hollow pistons.

Nonrotating cylinders are used in applications that demand both accurate linear position and precise angular orientation. Special guides can be added to standard cylinders to prevent rod rotation, but this is often expensive and unwieldy. More often, twin-rod or rectangular cylinders are used.

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