When failure is NOT an option, fail-safe actuators are a must


Wouldn’t it be great if when things went wrong, you and I could immediately revert to a safe position and start again, no harm done. Dreaming aside, in the wonderful world of engineering this is a reality in the form of fail-safe devices, a tool that in the event of failure responds in a way that will cause no harm, or at least a minimum of harm, to other devices or people.

Fail-safe actuators are devices that return to a home position if the electrical power or pneumatic pressure supplied to the actuator is lost.This is vital when the risks are such that driving the device to its safety position must be possible at any time even in the absence of a power supply. A few examples include storage and distribution of gas and dangerous fluids, fire protection systems, chemical installation safety, climate control and ventilation of hazardous areas.

Historically, 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 this, 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.

Electric spring return actuators are one of the simplest ways to obtain fail-safe operation, because they depend only on electric supply for normal operation and provide fail-safe positioning regardless of the status of the external power source.

At Alltork, in addition to their normal function of on-off or modulating actuators, our FQ operators are designed to immediately close or open the driven device, thanks to the action of an integrated spring, and without any external power source, whenever they detect an emergency such as :

  • A loss of signal,
  • An abnormal temperature,
  • Or the loss of power itself.

When the power supply is restored no resetting of the spring is needed, the actuator is immediately available for normal operation. Business as usual.