Motion System Design

Actuator thrust requirements

An actuator's function is to provide thrust and positioning in machines used for production or testing. One type is the electromechanical actuator, which converts the torque of an electric rotary motor into linear mechanical thrust. To size such a linear actuator, the amount of thrust required by the load must be calculated. Without this information, it's easy to under or overestimate actuator size. Underestimating thrust can cause an actuator not to perform at all. Overestimating can lead to incorrect motor and drive selections, unnecessarily increasing system costs.

An additional actuator thrust requirement is sizing for payload, which necessitates thorough knowledge of the moment, normal, and side loads. It also requires a thorough knowledge of loading limitations based on carriage stiffness and the size, number, and spacing of linear bearing modules. Moment loads — rotational forces or torque defined by direction (pitch, yaw, roll) and magnitude (lb-in. or N-m) — are most obvious when the payload's mass is not centered on the carriage, especially on rodless actuators and precision positioning tables.

Q&A

Q: Are there any special considerations for applications with proportionately high accelerations?

A: When acceleration force (Wt/g)a is a large component of thrust, actuator mass must be considered before thrust is calculated. To find this, the actuator's weight (linear inertia) Wactuator is added to the load's weight WL and peak thrust for each section of the move profile is calculated.

Q: What is this move profile?

A: On a graph of the thrust profile, the acceleration, slew, and deceleration sections are depicted; their movements are translated into a second graph known as the move profile.

As an example, let's say a 447-lb. actuator is capable of accelerating up to 11.25 in/sec2. Then the acceleration section is calculated:

Ma = Wactuator (lb) / g x 11.25 (in/sec2)

Ftotal = 169 lb

The slew section is calculated:

Ma = 0 (lb), due to a = 0

The deceleration section is the same as the acceleration formula but is in the opposite direction:

Ma = Wactuator (lb) / g x 11.25 (in/sec2)

Ftotal = 193 lb

These equations demonstrate that additional “acceleration weight” increases the thrust needed during acceleration, but reduces the peak thrust for deceleration.

Q: How do different actuator types react to load thrust?

A: Three basic types of actuators exist: rod, rodless, and precision positioning tables. Rod actuators are appropriate for high axial force applications where moment and side loads (to be explained later) are properly supported. They are advantageous because they can extend into a work area during operation and retract to clear the area for subsequent operations. Moment and side loads are reduced on rod actuators by being designed into the system.

Screw-driven rodless actuators are best suited for areas where space is limited and where an actuator must support or carry the payload. Rodless actuators are effective when incorporated into Cartesian systems for some multi-axis applications. When high speed and low thrust are necessary, rodless actuators can also be driven with a timing belt.

Precision position tables are designed for applications requiring accuracy and repeatability instead of drive train axial thrust. They are also useful in less precise applications where adequate moment load support is necessary or as building blocks for multi-axis position systems.

Q: Are there any common problems?

A: A typical problem encountered with actuators is understanding the environmental challenges upfront. A number of standard options can address temperature and contamination issues, and actuators can be customized to face even more challenging requirements. The key is understanding what the actuator will be exposed to and performance expectations at the extremes.

This month's handy tips provided by David Kosewski, Product Manager, Precision Actuators & Tables at Danaher Motion. For more information, visit www.danahermotion.com.

Hide comments

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish