Purpose of a rotary actuatorRotary actuators are standard engineered components used in automation applications to increase system flexibility by adding an axis of rotation. They are primarily used to orient parts in application segments such as machine tending and assembly. These units can be indispensible in an automation process. In general, 2-position pneumatically operated rotary actuators rotate their payloads at rotating angles of 90 or 180 degrees, while 3-position and infinitely adjustable units are also available.
Rotary actuators are designed to offer 3 essential functions: rotation, dampening, and guidance. Rotation, or the production of torque, gets the rotary actuator’s payload moving from rest and through the entire rotating angle. Dampening is the absorption of energy, created by the rotating load, at the end positions to allow for smooth actuation and long life. The dampening device is usually a hydraulic shock absorber, although, it could be an elastomer damper. Guidance is the rotary actuator’s ability to mechanically support the payload being rotated.
The main benefit of standard engineered rotary actuators is that they incorporate the above 3 characteristics in a compact, off-the-shelf component. In times past, engineers would have to design from scratch what now is available as a reliable standard product.
Choosing the rotary actuatorThe goal in selecting a rotary actuator is to make sure it can rotate, dampen, and support the rotating load. To start, the following application parameters should be obtained:
- Mass, size and location of each payload component with respect to the axis of rotation and pinion surface.
- Orientation of axis of rotation.
- Swivel angle.
- Swivel time.
- Cycles per hour.
- Operating air pressure.
Then, determine the rotational torque required. This is a product of the rotational mass moment of inertia (a measure of how easy it is to rotate an object about an axis of rotation) and the rotational acceleration. Note the moment of inertia is directly proportional to the square of the distance of the object perpendicular to the axis of rotation, so distance from the center of rotation is the major influencing factor.
Next, determine the dampening requirements. This is done by calculating the rotational mass moment of inertia, the kinetic energy generated, and the effective mass; the latter is a function of the impact velocity, the velocity at the point of contact with the damper, and the energy. These values are compared with the rotary actuator’s dampening device parameters.
It is possible to calculate the torque and dampening requirements by hand, however, due to the several factors involved and the many calculations to consider, most rotary actuator manufactures simplify the process by providing sizing software for relatively faster selection.
To determine the support capability required, the axial force and moment load created by the payload must be calculated. The moment load is calculated by multiplying the total payload mass by the acceleration (due to gravity and due to the robot/gantry) by the distance of the payload’s center of gravity to the rotating pinion.
Additional factors to consider are repeatability and environmental conditions. The required repeatability can simply be compared to the published value. For swivel applications in harsh environments, determine if the rotary actuator’s IP rating is acceptable.
A properly selected rotary actuator results in cycle time reduction, minimal downtime and long unit life, and ultimately a satisfied customer. Contact SCHUNK application engineers for assistance in sizing your next rotary actuator application.
Rotary actuator features SCHUNK incorporates several beneficial features in rotary actuator design:
- Multiple mounting and porting options.
- Ease of monitoring switch installation.
- Pneumatic or locked middle positions versions.
- Both large and small end position adjustability.
- Electrical and air feed-through and center bore through the rotating pinion.
Fred Wakim
Applications Engineer–Automation
SCHUNK Inc.
