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When Failure Is Not an Option: The Evolution of Fail-Safe Actuators

Fail-safe actuators are those devices that return to a home position if the electrical power or pneumatic pressure supplied to the actuator is lost. Such actuators are often used on crucial damper and valve applications that require protection from excessive temperatures, such as preventing coils from freezing or rooms from overheating. They are required on most steam valve applications.

Back in the days when pneumatics ruled commercial HVAC controls, fail-safe protection was easy because pneumatic actuators are inherently fail-safe. Air pressure pushes on a diaphragm that compresses a spring and drives the actuator’s pushrod. When air pressure fails, the spring automatically returns the diaphragm and pushrod to the normal, home position.

When electric actuators began replacing pneumatics, however, they had no inherent fail-safe return. When power was lost, they were stuck in whatever position they happened to be in at the time. To remedy that, concepts from the old pneumatic spring technology were used to make spring-return fail-safe electric actuators. As an electric actuator drove a load, it also tensioned a spring. To maintain position, power had to be continuously supplied. When the electrical power failed, the spring would return the actuator to its home position. To make a fail-safe version of a particular standard electric actuator, however, the torque had to be essentially doubled to overcome the resistance of the spring in addition to the load.

Spring-return actuators have been around for a long time, and building specs sometimes still state that traditional spring-return actuators be used. But a newer, more convenient, more durable, more consistent, more energy efficient, and more economical solution for electric fail-safe is to use, instead of springs, capacitors that are continuously charged whenever power is applied. When electric power fails, the charge in the capacitor is used to drive the actuator back to its home position. KMC capacitor-driven actuators include the MEP-537x and MEP-7x5x models.

Capacitor-driven fail-safe models provide multiple advantages over spring-driven actuators:

  • Capacitor-driven actuators provide switch-selectable fail-safe direction, so that one model can easily be used for both CW and CCW fail-safe applications without flipping over the actuator and changing the shaft clamping mechanism. Not only can a capacitor-driven fail-safe easily change directions, it might also be turned off if desired (such as for testing purposes).
  • Capacitor-driven actuators usually have smaller, thinner cases and can attach to shorter shafts than bulkier spring-returns.
  • Capacitor-driven actuators usually have a quick-release button or lever that allows easy manual positioning of the shaft, but spring-returns usually require a wrench to manually “wind” the shaft into position (if manual positioning is available at all).
  • Capacitor-driven actuators provide higher energy efficiency over spring-return actuators. Spring return actuators require extra motor torque to overcome spring resistance on every cycle, and they consume much more power just to maintain a stationary position.
  • Because springs typically break after about 50–60K cycles, capacitor-driven actuators may last three or more times longer.
  • Spring returns typically drive much faster during fail-safe mode, potentially damaging equipment, but capacitor-driven actuators provide consistent torque during fail-safe as well as powered modes.
  • The longer equipment life and reduced energy costs mean that the life-cycle costs for typical capacitor-driven actuators are considerably lower than the equivalent spring-return models.

If shown these overwhelming advantages, few people will still insist on specifying old-school spring returns.

Additional Resources

Getting Physical with Actuators

ControlSet Actuators Brochure (SB-009)

Actuator Selection Tool

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