Hydraulic Rotary Actuators - Helical Spline

In its simplest form, a rotary actuator is a device that uses hydraulic fluid to generate rotary motion without a linkage. These devices come in handy when applications call for any form of transporting, securing, or positioning of parts. 

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How Do Rotary Actuators Work?

In terms of design, rotary actuators are most often configured as vane type, rack-and-pinon type, or helical spline actuators.

Our rotary actuators are the helical spline design. This means, a piston and a helix, or sliding spline, work together to convert the linear motion of the piston into rotational motion. Helical spline actuators function when the piston is axially displaced using hydraulic pressure causing simultaneous rotation of the piston.

Hydraulic rotary actuators are typically designed to operate at low speeds and produce high torque through arc lengths ranging between 90°, 180°, 270°, and 360°. They also can be configured up to 720° or more by making the spline gears longer while maintaining the same diameter.

Common Applications of Rotary Actuators

From manipulating the angle of a boom in construction applications to opening or closing flaps in the aircraft and aerospace industries, rotary actuators are dependable and durable devices used for a wide range of uses.

One common application is using a rotary actuator for tipping devices. A helical rotary actuator, for example, would be a suitable option for bin tippers in waste management applications due to its high torque in a compact package. 

Additionally, rotary actuators can function as simple tool changers. One example of this is using a rotary-linear actuator for a pallet changer that weighs 1,000 kg. This device is a combination of a rotary actuator and a dual-action linear cylinder, allowing for full flexibility in a minimal amount of space. Since the rotary actuator and the linear cylinder are hydraulically driven separately, any movement sequences can be selected: for example, rotating to the left and right, extending linearly, and retracting.

A rotary actuator could also be used for gate valve control too. While a hydraulic rotary actuator may be more expensive than pneumatic actuators, these devices have a higher level of precision, require less space, and can achieve high torque outputs. 

Other uses for rotary actuators include linear actuation, bending machines, work piece positioning, rotational devices, and transport applications.

Hydraulic Rotary Actuator Benefits

Hydraulic rotary actuators are known for their strong ability to produce power quickly. In comparison to pneumatic and electric rotary actuators, hydraulic actuators are the most powerful and have the capacity to achieve a torque output up to 2,200,000 Ibs-in (250,000 Nm) as a helical spine actuator and even higher as a rack and pinion actuator.

Rotary actuators also require a minimal amount of space while delivering high performance and flexibility due to its compact design. They also have high-efficient end cushioning and wear-resistant sliding surfaces.  

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The piston sleeve is hydraulically sealed between the housing and shaft. When hydraulic pressure is applied to the port to the left of the piston, three events occur simultaneously. The piston sleeve is displaced axially, moving to the right; it rotates clockwise (as viewed from the output shaft) as the gearing on its outside diameter and the housing's ring gear forces its rotation; and the gearing on the inside diameter of the piston sleeve causes the shaft to rotate clockwise. Applying pressure to the alternate port returns the piston sleeve to its original starting position and rotates the shaft counterclockwise.

The double helix, opposite hand design of the gear sets compound the rotation of the shaft, so its rotation is considerably more than that of the piston sleeve. For 30° helix designs, the rotation of the shaft is almost twice that of the piston sleeve, for 45° helix designs, it is even more. Features of this design include high torque from a compact configuration, constant torque through full angle of rotation, no internal leakage, and holding torque approximately two times the forward driving torque. With all gearing, moving parts, and bearings constantly bathed in hydraulic oil, helical actuators are virtually maintenance-free.

Because the effective hydraulic area is equal on both sides of the piston, equal torque is produced in the clockwise and counterclockwise directions. Average mechanical efficiency is about 70%. Since the angle of rotation is determined by actuator length, and because there are no internal barriers as in vane designs, any rotation is theoretically possible. Conversely, an appropriate internal stop tube can limit the rotation of an actuator to almost any intermediate angle. Most helical actuators, however, are available with 90°, 180°, and 360° rotations as standard. Due to the clearance needed between the internal spline teeth, helical actuators exhibit some backlash, typically ½° to 1° for smaller models and less for larger models.

Single-shaft designs are used where torque output is the primary factor in selecting the most economical actuator for the application. The double shaft extension with foot mounted housings is frequently used as a stable power hinge for heavy duty implements, tools, booms, cranes, and various machine components. Some helical actuators are also available with large diameter, drilled and tapped shaft flanges that permit loads to be bolt-mounted directly to the actuator. Hollow shaft designs are yet another option. Torque is directly proportional to pressure, and output torque can reach approximately 700,000 lb-in. with 3,000-psi fluid pressure.

An enclosed piston crank actuator, Figure 1(b), has an adjustable arc of up to about 100°. This actuator is compact and has few mechanical problems. Built-in bearing support overcomes side thrust forces. Fail-safe versions are equipped with a spring that returns the shaft to a safe position in case of power failure or loss of fluid. Torque generation follows a sinusoidal distribution. Maximum torque is produced at mid-stroke. Therefore, these actuators should be selected to drive a load based on their minimum torque.

A scotch-yoke actuator, Figure 1(c), has two pistons connected rigidly by a common rod. The central drive pin on the rod engages a double yoke keyed to the output shaft which turns through arcs to 90° maximum. Torque outputs at the beginning and end of the stroke (breaking torque) is twice that at mid-point (running torque). This characteristic is efficient because many applications require high breaking torques to move and accelerate the load. Fail-safe, single-acting, and double-acting models are available. Efficiencies range from 70% to 95%.

In a rack-and-pinion actuator, Figure 1(d), a long piston with one side machined into a rack engages a pinion to turn the output shaft. This gearset principle is adaptable for use in fail-safe, single-acting and double-acting models. Where balanced loading on the bearings is required, two bi-directional pistons with parallel racks are used; both racks engage the one centrally-located pinion. Rotation to 1,800° and torque to 50 million lb-in. are available. Torque is constant and equal in both directions.

Multiple-position rack-and-pinion actuators are available that rotate the output shaft to several positions by varying the pressure porting. Output positions can be in any sequence, allowing the actuator to stop at or pass any intermediate position.

Rack-and-pinion actuators are particularly useful for heavy-duty applications because they tolerate heavy side and end loads and can accommodate large bearings. Because of their constant torque output characteristics and resistance to drift, they are often used for precision control. Efficiencies range from 85% to 92% in single rack models and from 92% to 97% for double rack models.

In a piston-chain actuator, Figure 1(e), a circular drive chain is held taut over two sprockets. One sprocket converts linear motion into torque output; an idler sprocket maintains tension. Two piston-shaped links are located at equal distances on the chain; one piston is larger than the other. The housing containing the mechanism has two parallel piston chambers and a port on each of the two opposite ends. Pressurized fluid entering a port acts against both pistons; the chain moves in the same direction as the larger piston because of the differential forces being exerted. The smaller piston seals the return side of the chain to prevent fluid leakage. Rotation is reversed by reversing porting.

A piston-chain actuator provides rotation to five complete turns and torques to 23,500 lb-in. The design is limited by the strength of the chain and sprocket, and by its bulk for applications requiring extremely high torques. Torque is constant throughout the stroke.

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