The term fail-safe brake refers to a type of brake that engages to prevent shaft rotation when electrical power is removed for any reason. When power is restored, the brake releases and stays in the off position.

Like all friction clutches and brakes, fail-safe brakes generate torque through friction surfaces that are clamped together. The source of the clamping force distinguishes the two basic types — permanent magnet and spring-set.

In general, permanent-magnet brakes are used in applications that require frequent on-off cycling and consistent performance, whereas spring-set brakes are better suited for static holding applications and low-cycle dynamic operation.

Permanent-magnet brakes

Fail-safe brakes of the permanent magnet type work essentially the same as electric brakes except that the magnets generate a flux that clamps the friction surfaces together, Figure 1. Shaped like a horseshoe, the magnet assembly directs magnetic flux through inner and outer (North and South) poles, which attracts the armature. When the armature contacts the magnet assembly, it completes a magnetic circuit to engage the brake and provide stopping torque through the shaft to the driven equipment. Friction material between the inner and outer poles contacts the armature and reduces wear.

When power is applied to the brake, a magnetic coil in the magnet assembly generates an equal, but opposite, magnetic force that counteracts the permanent- magnet flux and releases the brake.

Selecting the correct power supply for the coil is critical to the operation of permanent-magnet, fail-safe brakes. Though release voltages may be specified at 90 or 24 Vdc, these values vary slightly from one brake to another. Therefore, the power supply must have an adjustable voltage output so the voltage can be set at a value that causes the coil flux to cancel the permanent-magnet flux and cleanly release the brake.

To ensure a strong attractive force between armature and magnet, the friction surfaces must be clean and burnished. Burnishing is a process in which the manufacturer runs the unit to breakin the friction surfaces, thereby maximizing friction and torque. In dynamic applications, the friction surfaces slip upon each engagement (which continues the burnishing effect), keeping them free from corrosion and debris. This slippage maintains correct alignment and full contact of the friction surfaces.

Permanent-magnet brakes are usually equipped with some type of mechanism to automatically compensate for friction surface wear. This mechanism maintains a constant air gap between the armature and magnet assembly to ensure consistent stopping time throughout the brake life.

From a cycling standpoint, permanent magnet brakes provide more consistent performance — torque and stopping times remain the same throughout the life of the brake. And they are well suited to applications where cycling rates range from about 5 to 10 cpm or higher.

Spring-set brakes

The three basic categories of spring-set brakes are electrically released, hydraulically released, and pneumatically released. All types use springs to provide clamping pressure when power is removed. Though electrically released brakes are explained here in detail, hydraulically and pneumatically released brakes function in a similar way except that a hydraulic or pneumatic cylinder is used to release the brake rather than an electrical device. Also, hydraulic and pneumatic units have the same basic sizing and selection criteria as electrical types.

Electrically released spring-set brakes use a system of springs and a magnetic coil, Figure 2. Without electrical power, the springs clamp the rotor (with attached friction pads) between a stationary end plate and a non-rotating armature, generating brake torque. When power is applied to the coil, magnetic force pulls the armature toward the coil, compressing the springs and releasing the brake.

Power supplies for electrically released spring-set brakes are less critical to their operation than those for permanent- magnet brakes. A simple, fixed-voltage power supply is all that is required to release the brake.

Spring-set brakes are well suited to static holding applications where a servo or step motor brings the load down to speed. When the motor is turned off, the brake holds the load in position. This type of brake can also handle occasional emergency stops.

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