Normally Open

A “Normally Open” brake is a unit that is disengaged in its default, de-energized state. In this state, the shaft is free to rotate without any braking force being applied. To activate the brake and stop or hold the load, power must be supplied.

This is often referred to as a “power-applied” or “energize-to-engage” brake.

The principle of operation is the inverse of a spring-applied (fail-safe) brake:

• Default State (Power Off): There is no force—or only a very light release spring force—acting on the friction disc. The brake is “open,” and the shaft is free.
• Braking State (Power On): Power (typically hydraulic pressure, pneumatic pressure, or an electric current creating a magnetic field) is actively applied to generate a clamping force on the friction disc, engaging the brake and creating braking torque.

Crucial Safety Implication: A “Normally Open” brake is not fail-safe. If there is a loss of control power or system pressure, the braking force is lost, and the brake will disengage.

Primary Applications: Because they are not fail-safe, “Normally Open” brakes are used for specific service or process control functions, not for critical safety holding:

• Tension Control: In unwind stands for materials like paper, steel, or wire, a “Normally Open” pneumatic or electric brake can apply a continuous and precisely modulated amount of drag to maintain constant tension in the material as it is unspooled.
• Dynamic Service Braking: They are used as the primary service brake in applications where a separate, fail-safe emergency or parking brake is also installed. For example, the service brakes on a vehicle or a moving trolley.
• Dynamometers: Used to apply a variable, controlled load to test the performance of engines and motors.

In summary, “Normally Open” describes a brake that requires power to stop a load. Its function is primarily for controlled, in-process braking, and it is fundamentally unsuitable for applications where a load must be safely held in the event of a power failure.