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The straight story on linear actuators

Linear actuators can be powered by pneumatics, hydraulics, or electric motors. Which is best for your job? Let's find out.

This hydraulic cylinder features tie-rod construction for extended life under high working pressures, and pressure-energized body seals to prevent leaks.

This hydraulic cylinder features tie-rod construction for extended life under high working pressures, and pressure-energized body seals to prevent leaks.

Pressure losses and the compressibility of air do sometimes make pneumatics less efficient than other linear technologies. Too, compressor and delivery system limitations dictate that pneumatic systems operate at lower pressures, providing lower forces than other linear systems. Component materials (mostly aluminum) and construction of fasteners and threaded end covers dictate lower operating pressures. As a result, pneumatic cylinders typically operate with compressed air at 100 psi or less, in contrast with hydraulic cylinders, which operate on pressurized hydraulic fluid at over 500 psi. So, speed ranges from a couple of inches per second to 60 in./sec. Force output is dependent on maximum pressure rating and related bore size: Aluminum actuators have a maximum pneumatic pressure rating of 150 psi with bore sizes ranging from to 8-in. for approximately 30 to 7,500 lb. Most steel actuators have a maximum pneumatic pressure rating of 250 psi with bore sizes ranging from approximately to 14 in. translating to about 50 to 38,465 lb of force.

Hydraulics: Rugged choice

They all put F = ma or T = Ia into action, but every linear actuator technology has specific strengths, and is only best for certain applications.

They all put F = ma or T = Ia into action, but every linear actuator technology has specific strengths, and is only best for certain applications.

Forms of hydraulic cylinders have been used for centuries to generate force and motion. Their operation is based on Pascal's principle: Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel. So, the amount of force applied by the cylinder is the product of the hydraulic input pressure times the area of the cylinder's piston.

Hydraulic systems are suitable for rugged applications that require high force output. However, hydraulic systems generate noise and, without proper maintenance, they can leak. If a pneumatic system leaks air, efficiency falls. But if a hydraulic system leaks, it's much more serious. Fluid is lost, causing cleanliness issues and possible damage if it gets on other system components or products.

More equipment is needed as well. A compressor, regulator, and directional valve are all that's needed for a pneumatic system. But hydraulic systems require a fluid reservoir, motors and pumps, release valves, and equipment to reduce noise and heat levels. They must be connected to a motor or gasoline engine-powered pump to draw fluid from a reservoir, a pressure relief valve to control maximum system pressure, and a directional valve to control the flow of hydraulic fluid into and out of the cylinder.

Controls: In a simple two-position unit, the directional-valve spool moves to an end and not an intermediate position and it cannot control cylinder velocity. Systems using this type of directional valve regulate piston velocity with a flow control valve that allows free flow of oil into one side of the piston and controlled flow (by adjusting an orifice) from the opposite side of the piston.

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© 2014 Penton Media Inc.

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