FLUID POWER ACTUATORS Fluid PowerActuators Machinery's Handbook, 31st Edition
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Linear Actuators.— When a gas, typically air, is used in a linear actuator, it is called a gas cylinder or air cylinder. When a liquid is used, it is typically called a hydraulic ram or cyl inder. These are essentially the same in form and function. Linear actuators can be either single or double acting. Single Acting Cylinders: Single acting actuators with spring return use a spring to push the actuator to its free state when fluid pressure is removed. The free state of a single acting cylinder is usually fully retracted. A fluid pressure differential is used to extend the cylin der. Single acting cylinders use half the fluid of double acting cylinders. Single acting cylinders can be actuated with three way valves. Double Acting Cylinders: These cylinders require pressure differentials on both the ex- tend and retract strokes. When pressure equalizes across the cylinder, the state of the actuator becomes indeterminate. Double acting cylinders are most commonly actuated using four way valves. Double acting cylinders use twice the fluid of single acting cylin ders, which matters most in pneumatic applications where that air is exhausted and lost. Selection of Linear Actuators: Linear actuators are available with a variety of options. In addition to single and double acting versions, some cylinders are available with a magnet embedded in the piston to enable position sensing. For applications with side loading, a double rod cylinder or external bearing may be needed. When a cylinder is guiding a load that should not rotate, anti- rotation options such as a non-round rod or integral guide rods are commonly used. For heavy loads or high speeds, internal or external end cushions are usually offered to soften the impacts and allow the cylinder to withstand higher kinetic energy movements. Force Exerted by Linear Actuators: The force exerted by a linear actuator is calculated using the following equation: F η pA = where h hydraulic = 0.9 and h pneumatic = 1, and A is the effective area of the cylinder. Effec- tive area on the retract stroke is the piston area minus the rod area. It is good practice to oversize the load-carrying capacity of an actuator by 25 percent to ensure smooth operation and account for losses in the system. For high speed operation, oversizing by 50 percent is not uncommon. For a spring return cylinder, the force on the retraction stroke will be equal to the spring force rather than a function of air pressure. For applications where rapid retraction speed is critical, double acting cylinders may be the best choice. Speed of Linear Actuators: The speed achieved by an unloaded linear actuator depends on the effective area of the cylinder and the flow rate of the fluid. Actuation velocity can be calculated for an unloaded cylinder using the following equation: v 231 Q A = ------- where v is velocity in inches per minute, Q is flow rate in gallons per minute, and A is piston active area in square inches. Active area is reduced by the area of the rod when a rod is present on the acting side of the cylinder. Most manufacturers provide graphs of actuator speed versus loading for their prod- ucts. Fluid supply can be characterized by Coefficient of Velocity ( C v ) factor. Valves and circuits with a higher C v will be capable of moving an actuator faster because they can provide more air flow. If the C v of a valve and other system components is known, it can be used to determine the time required to extend or retract a cylinder. It is common to oversize the speed of an actuator by 25 percent in critical applications to account for losses.
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