What is in this article?:
- Engineering Essentials: Rotary Actuators
- Bladder and vane actuators
Rotary actuators are used to pivot a joint when a conventional cylinder mounting proves impractical due to space, weight, or motion requirements.
Bladder and vane actuators
In a bladder actuator, Figure 2(a), the output shaft is driven by the expansion and contraction of a pair of bladders. When one bladder is pressurized, it pushes against a cup-shaped lever arm, rotating the shaft through an arc of up to 100°. A 4-way valve controls oil or air flow so that while one bladder is pressurized the other is exhausted.
Because this type of actuator has zero internal leakage, it is highly accurate, and insensitive to contamination. Almost any fluid medium can be used if the bladder is chemically compatible; special housing materials are not necessary. This actuator can transmit torque to 2,750 lb-in.
Another bladder design uses a rack-and-pinion mechanism driven back and forth by expanding and contracting air bags. The bags hold precharges indefinitely and provide rotation to 90° or 180°. Highest torque is at the beginning of the rotation cycle. Torques to 45,000 lb-in are possible.
A single-vane actuator, Fig. 2(b), has a cylindrical chamber in which a vane connected to a drive shaft rotates through an arc to 280°. Two ports are separated by a stationary barrier. Differential pressure applied across the vane rotates the drive shaft until the vane meets the barrier. Rotation is reversed by reversing pressure fluid at the inlet and outlet ports.
A double-vane actuator, Figure 2(c), has two diametrically opposed vanes and barriers. This construction provides twice the torque in the same space as a single-vane actuator, however, rotation is generally limited to 100°.
Vane actuators are easy to service because they have fewer parts and less-critical fits than many other types of rotary actuators. Their mechanical efficiencies range from 80% to 95%, depending on construction and application. The square corners of the vanes make sealing a challenge, and internal bypass leakage can be common in vane-type actuators. Position-holding may be limited without external controls. Vane-type actuators transmit torques to 700,000 lb-in.
To avoid excessive wear and premature failure, it is essential that very little or no thrust or overhung load be imposed on the actuator's output shaft unless it is equipped with bearings (such as tapered-roller bearings) to accommodate these loads. Use a flexible shaft coupling to eliminate side loading due to shaft misalignment. When side loading is unavoidable, support the output shaft with auxiliary bearings if the actuator is not equipped with adequate bearings to support such a load. Actuators coupled to gear trains belong in this category. Some helical and rack-and-pinion designs are available with integral bearings that can support significant overhung loads without additional external bearings.
To bleed trapped air, mount the actuator so the supply ports are on top. Or provide a suitable air bleeding device for the system. Larger rotary actuators often have built-in bleed valves.
In continuous-cycling applications, where hot hydraulic fluid may collect near the actuator, arrange for greater fluid circulation. Heat exchangers may be required. Do not install rotary actuators where contaminants are likely to collect — for example, at the system's low point.
Rotary actuators are used for mixing, dumping, intermittent feeding, screw clamping, continuous rotation, turning over, automated transfer, providing constant tension, and material handling. They are also suitable for turning, toggle clamping, indexing, positioning, oscillating, lifting, opening, closing, pushing, pulling, and lowering.
For example, in the steel industry, they up-end coils, turnstiles, and rollover devices, and tilt electric furnaces. In material handling, they switch conveyors, turn and position container clamps on lift trucks, tension, guide, operate valves, and brake. In marine operations they open and close hatches, swing cargo handling gear, operate booms and all types of large valves, position hydrofoils, and control steering.
They also perform such tasks as jib boom and work platform rotation on self-propelled aerial lifts, tool and implement positioning on construction equipment, percussion drill positioning on underground mining machinery, and hydraulic power steering on slip-form paving machines and piggyback forklifts.
For linear motion?
An actuator that rotates at constant speed can move heavy loads very efficiently in a linear direction by using a harmonic motion mechanical linkage. The harmonic motion produced offers a maximum mechanical advantage at the beginning of the stroke to accelerate the load quickly. Halfway through the stroke the load is at maximum velocity. The deceleration half of the stroke is a mirror image of the acceleration half. Heavy loads are slowed automatically and stopped with a force equal to that originally used to accelerate the load.
Automatic advance and return of a load at maximum speed can be obtained by using a 360° rotary actuator connected to a linkage rather than a 180° actuator. During deceleration, energy is not transmitted back into the hydraulic system but is used by the actuator to work the linked load. Less hydraulic fluid and horsepower is necessary. To size actuators for such applications, determine friction losses and the force needed to accelerate and decelerate the load.