How-to-Guide / Automated Assembly / Lean Manufacturing Assembly
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How To Select a Rotary actuator

March 5, 2014
Trans

Purpose of a rotary actuator

Rotary actuators are standard, engineered components used in automation applications to increase system flexibility by adding an axis of rotation.  These units can be indispensible in a wide variety of automation processes.  Conventional rotary actuators are pneumatically operated with optional 2 or 3 positions.  However, servo-electric rotary actuators are available providing a high degree of flexibility in terms of rotational position, speed, and acceleration.

A well designed rotary actuator should offer 3 essential functions: rotation, dampening, and guidance.  Rotation, or the production of torque, moves the rotary actuator’s payload from rest through the entire rotating angle.  Dampening is the process of bringing the rotating load to a smooth, impact-free stop.  The dampening element is usually a self-compensating hydraulic shock absorber or an elastomer damper.  The shock transfers the inertial energy into heat energy. Guidance is the rotary actuator’s ability to mechanically support the payload while being carried by the robot, slide or gantry.

The advantage of using standard, engineered rotary actuators is that they incorporate the above 3 characteristics in a compact, off-the-shelf component.  They also include many other features and benefits for ease of use and integration.  For example, accommodations for position monitoring sensors, pneumatic and electric feed-through, and easy to use end position adjustment to name a few.

Choosing a pneumatic rotary actuator

The goal in selecting a rotary actuator is to make sure it can rotate, dampen, and support the rotating load while meeting all other specific customer requirements.  The first step is to list the application parameters:

  • Mass, size, and location of each payload component with respect to axis of rotation and pinion surface.
  • Orientation of axis of rotation.
  • Required swivel angle.
  • Required swivel time.
  • Required cycles per hour.
  • Available operating pressure.
  • Repeatability.
  • External acceleration and process forces
  • Environmental conditions.

Next, determine the rotational mass moment of inertia given the payload data.  This can be calculated using solid modeling software or by using rotary actuator sizing software provided by many rotary actuator manufacturers, or simply by doing hand calculations. 

The inertia value is then entered in the rotary sizing software.  The software effortlessly calculates the required torque (a product of the rotational mass moment of inertia and the rotational acceleration), the kinetic energy generated, and the effective mass (a function of the shock absorber plunger’s impact velocity).  These values are compared with the rotary actuator’s available torque and dampening parameters to output a recommended actuator.

For an alternative selection process, some manufacturers like SCHUNK, have produced inertia versus actuation time graphs for the offered rotary actuators.  SCHUNK’s cataloged graphs can be used for applications where the swiveling angle is 90° or 180°, using 87 psi operation pressure, and with a vertical axis of rotation (or horizontal axis of rotation is with a balanced/symmetrical payload with respect to the axis of rotation).

Then confirm that the rotary actuator’s guidance can support the forces and moment loads generated by the payload.  This is simply done using Newton’s second law of motion, F=ma, and calculating the moment load, given the force and distance to the center of gravity of the payload.

Finally additional factors to consider are repeatability and environmental conditions.  For applications in harsh environments, check the rotary actuator’s IP rating and operating temperature range.

A properly selected rotary actuator results in cycle time reduction, minimal downtime, and long unit life, and ultimately a well cared for customer.  Contact SCHUNK application engineers to provide experienced support in sizing your next rotary actuator application.

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 

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