Fluid Power Actuators & Motors
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An actuator is a device for converting fluid energy into mechanical energy. Fluid-power actuators are available in many forms to provide specific types of actions. Generally all types of actuators are available for either pneumatic or hydraulic operation.
Cylinders are by far the most common type of actuator and work through linear extension. When fluid is pumped into a cylinder, the piston and rod are forced to move in or out against a load.
- Single-Acting Cylinders: Simplest and lowest cost. Powers a stroke in only one direction. When fluid drains, an external force has to push the piston back to its starting position.
- Spring-Return Cylinders: A spring re-positions the piston to its starting point. Cylinders can be spring loaded to the extended or retracted position and use hydraulic and pneumatic power to move in the opposite direction.
- Double-Acting Cylinders: Contain two fluid chambers so that pressure both extends and retracts the rod. This type of cylinder is the most common. Double-acting cylinders retract faster than they extend.
- Rodless Cylinders: Has an internal piston that is connected physically or by magnetic force to an external carriage. It is more common in pneumatic applications than in hydraulics. They require a lot less mounting space than rod-type.
- Servo Actuators: Cylinders that transmit high forces while allowing computer control of rod velocity, acceleration, and positioning accuracy. The key to operation is precisely sensing cylinder rod position.
Rotary actuators turn an output shaft through a fixed arc. They produce high torque instantly in either direction, occupy little space, and are simple to mount. Two popular designs are rack-and-pinion and vane actuators.
- Rack-and-pinion: Uses fluid pressure to drive a piston connected to a gear rack, which rotates a pinion. Standard units rotate either 90, 180, and 360 degrees.
- Vane Actuators: Consist of a shaft mounted in a cylindrical housing with one or more vanes attached to the shaft. Single vane actuators are limited to about 280 degrees of rotation and double vane models to about 100 degrees.
Hydraulic and pneumatic motors are not as common as electric motors, but they are useful in specialized applications that have: high torque requirements, limited weight and space, the motor is subject to stalling and holding loads, and the use of electricity is prohibited because of safety concerns.
- Axial Piston Motors: Contain several pistons that are driven by high-pressure fluid. Typical maximum torque ratings are up to 20,000 pounds per inch at pressures to 5,000 psi, with maximum speeds to approximately 4,500 rpm.
- Radial Piston Motors: Reciprocating pistons cause shaft rotation. Pistons are arranged in a circle with their bases connected to a plate that is mounted off-center to the output shaft. A rotating valve ports fluid to the cylinders. They can develop over 1 million pounds per inch torque at pressures exceeding 5,000 psi. Speeds range from 0.1 to 2,000 rpm.
- Gear-on-Gear Motors: One of the most common for hydraulic units. Consist of a pair of matched spur or helical gears in a case. Typically develop maximum torques of about 6,000 pounds per inch and speeds to 3,000 rpm.
- Gear-within-Gear Motors (Gerotors): Very compact for their displacement. An inner gear seals against an outer one to prevent fluid leakage. Tooth velocities and wear are low and power density is high. Deliver torques exceeding 1,500 pounds per inch at speeds to over 5,000 rpm.
- Vane Motors: Used for both hydraulic and pneumatic operation. Consist of a slotted rotor mounted eccentrically within a circuit cam ring. As air or fluid enters, force is applied against the vane, turning the rotor and sweeping the fluid from inlet to outlet. A typical rating is 4,000 pounds per inch torque at 2,500 psi and 4,000 rpm top speed.
Content on this page was created using excerpts from the Power Transmission Handbook (5th Edition), which is written and sold by the Power Transmission Distributor’s Association (PTDA). The Power Transmission Handbook is just under 400 pages and is a valuable resource for anyone involved or interested in the power transmission industry.