Armature Actuated Brakes (AAB)

Armature brakes (also called power-off brakes or safety brakes) are spring-set, power-released DC brakes that develop braking and holding torque in the absence of electrical power.

Armature brakes can decelerate and hold a rotational load, or provide holding function only (when the motor is used as the dynamic brake).

AAB is a Stearns acronym for Armature Actuated Brakes. This best describes our spring-set electrically released (safety) DC direct acting brakes, which can also be rectified for use with AC voltage. Advantages include:

Available in IEC or NEMA frame sizes (smaller sizes also available for small gear motors)
Static torque range from 3 lb-in to 300 lb-ft
No additional wiring, controls or auxiliary electrical equipment required for most applications
Simple construction means less wear on parts & longer service life with high cycle rates
Quiet brake features available for sound-sensitive environments

Stearns armature brakes are UL recognized, RoHS3 compliant, ABS certified and manufactured in our ISO 9001:2015 facility in the USA.

Explore armature brake types below, find a distributor or contact us with questions.

Stearns AAB Products

Stearns offers armature-actuated motor brakes to meet a wide range of braking requirements. Features include Class180(H) magnet wire coils, corrosion resistance, and optional manual release lever. Explore our AAB product lineup below, or use our brake configurator to accelerate your project.

Stearns AAB 321 and 322 Series armature brakes shown with thru-shaft and manual release options

AAB 321 & 322 Series

Series 321 and 322 are totally-enclosed non-ventilated (TENV) versions of the Stearns 320 series brake and designed for high-cycling applications in dirty or contaminated environments. Series 322 is the same as series 321, with an IP54 enclosure rating offering protection against dust and water ingress.

The AAB 321 and 322 series brakes have a flexible design that can be customized to fit a variety of applications:

IEC Frame Size: 1.2 - 2.8
Torque (lb-in): 3 - 35
Torque (Nm): .34 - 3.95
IP Rating: 42, 54
Size 1.2 flange mount available for series 321 only
Location for external lead on backside of magnet body available for series 322 only
AAB 321/322 product sell sheet (PDF)

Explore AAB 321 Series Explore AAB 322 Series

AAB 321 and 322 series brakes are also available with NEMA C-Face mount:

AAB 321 and 322 Series brakes are part of Stearns Signature Series - available in as few as 5 days lead time.

Explore AAB 321 Signature Series Explore AAB 322 Signature Series

AAB 350 Series

AAB 350 series marine duty brakes are certified compliant with IEEE 45 Federal Standard 46 ABS and available with metric or US customary bore sizes. Wear adjustment is simple with an access hole for air gap inspection.

NEMA C-Frame: 182 - 184IC/284 - 286TC/324TC - 405TSC
IEC Frame Size: 196 - 278
Torque (lb-ft): 75 - 300
Torque (Nm): 102 - 400
Attributes: Ductile iron or aluminum housing
IP Rating: 56 (optional)

Explore AAB 350 Series Marine Duty Brakes

Stearns AAB 350 Series armature brake with enclosure and pressure plate mount

AAB 350 series brakes with pressure plate mount are ideal for portal crane applications with ABS, CSA and CE certification, featuring maintained manual release and a splined hub for quiet, dependable operation.

NEMA C-Frame: 182 - 184IC/284 - 286TC/324TC - 405TSC
IEC Frame Size: 196 - 278
Torque (lb-ft): 75 - 300
Torque (Nm): 102 - 400
Attributes: Ductile iron or aluminum housing
IP Rating: 56 (optional)

Explore AAB 350 Series Brakes with Pressure Plate Mount

AAB 360 Series

AAB 360 series marine duty brakes have a magnet body mount for both IEC or NEMA C-face mounting and are ideal for portal crane applications with ABS, CE and CSA certifications. Thanks to the modular brake assembly you can install and remove the brake without having to readjust the air gaps.

NEMA C-Frame: 182 - 184IC/284 - 286TC/324TC - 405TSC
IEC Frame Size: 170 - 278
Torque (lb-ft): 35 - 300
Torque (Nm): 47 - 400
Attributes: Optional external non-maintained (deadman) manual release
IP Rating: 56 (optional)

Explore AAB 360 Series Marine Duty Brakes

AAB 361 Series

AAB 361 series brakes feature the series 360 magnet body mount for both IEC or NEMA C-face mounting, as well as ABS and CE certification, an internal maintained manual release and captive brake gaskets so parts aren’t lost during maintenance.

NEMA C-Frame: 182 - 184IC/284 - 286TC/324TC - 405TSC
IEC Frame Size: 170 - 278
Torque (lb-ft): 35 - 300
Torque (Nm): 47 - 400
Attributes: Optional external non-maintained (deadman) manual release
IP Rating: 56 (optional)

Explore AAB 361 Series Brakes

AAB Accessories

Stearns brakes offers two different accessories for our AAB brake lines: proving switches and AC rectifiers.

Stearns electronic brake release indicators

Proving Switches

Electronic brake release indicators (aka proving switches) indicate when the brake is released by sensing the change in the brake coil current waveform when the brake armature is pulled to the magnet body during the brake release process.

Proving switches can be mounted in a remote location such as a control cabinet, and can operate in extreme temperatures from -40°C - 65°C (-40°F - 149°F). These electronic brake release indicators with silver-cadmium oxide relay contacts are not susceptible to issues common with mechanical switches, such as material fatigue, tolerances and vibration.

Proving switches are for use with series AAB 333, AAB 350 and AAB 360 brakes. Use with full-wave or Tor-AC full-wave rectifier only (cannot be used with half-wave rectifier).

View or Download Proving Switch Order & Installation Info

AC Rectifiers

Stearns offers combination Tor-AC Rectifiers/Line Filters for all sizes of Stearns AAB brake series.

By upgrading our Tor-AC rectifier products to incorporate line filters, the newly combined product can be used for AC drive applications and whenever electrical filtering is required to protect the rectifier from high-frequency pulses.

This combination product also costs less and is easier to use compared to installing and wiring separate components.

View or Download Tor-AC/Line Filter Install & Service Info

AAB Product Resources

Find information about torque selection, installation, maintenance, custom design options and more.

Sizes/Torque

Available sizes range from 3 lb-in up to 300 lb-ft static torque, in a variety of mounting options.

Torque Selection

Selecting the proper torque rating is crucial for armature-actuated brakes because it directly impacts braking effectiveness, safety, and temperature/wear control. Proper torque selection is also important for compliance with industry and application-specific safety regulations as well as compatibility with other machinery/vehicle system requirements.

Use these tables and formulae to select the proper AAB torque rating, and refer to the torque requirements (lb-in, lb-ft, Nm) for each AAB series.

Brakes used as holding-only duty require friction material burnishing at or before installation to ensure adequate torque (all Stearns armature brakes are factory burnished). When friction discs are replaced, the brake must be burnished again in order to produce its rated holding torque.

Mounting Options

No modifications are required to mount a Stearns armature actuated style brake in a vertical application. Stearns AAB style brakes are single disc brakes and can be universally orientated with no modifications required.

A short hub is available for face mounting or to provide for maximum space efficiency.

armature brake commercial and industrial applications

AAB Applications

The spring-set, electrically released disc brake is an important accessory to electric motors used in cycling and holding operations, including:

Cranes & Hoists
Small gear motors
Servo motors
Industrial automation
Marine applications:
- Shipboard cranes
- Anchor winches
- Windlasses
Agricultural equipment
Material handling equipment & conveyors
Packaging equipment

Generally the armature brake is mounted to an electric motor, but can also be mounted to gear reducers, hoists or machinery. A foot mount kit is also available for installation.

Installation & Service

Stearns provides detailed installation and service instructions for each AAB series. You can find them under the red “Documentation” tab on each product page.

General AAB installation notes:

Hub position should allow full engagement of friction disc without interfering with armature movement
Motor shaft end float should not exceed .020
Shaft runout should be within .002” TIR (total indicator reading/runout)
Motor mounting surface should be flat & perpendicular to within .004” of motor shaft
Prevent grease & oil from contacting friction surfaces
Hub should be a tight sliding fit (for shrink fit, consult technical support)

General AAB service notes:

To prevent electrical hazard, disconnect power source before working on bake (if power disconnect is out of reach, lock & tag disconnect in ‘off’ position)
To avoid damaging internal power supply, Hipot testing should not exceed 1500 volts/second & brake coils must be connected
Allow brake exterior surfaces to cool before servicing
Remove burnt/degraded friction material dust before servicing or adjusting brake - wear a respirator or mask, remove dust with vacuum or soft brush & place in sealed container (do not blow the dust - inhalation can be harmful)
Maximum ambient operating temperature should not exceed 40°C (140°F)

AAB are simple to maintain since the replaceable items (friction discs) can be easily changed.

Many spring-set motor brakes are equipped with features such as simple wear adjustment to provide optimum friction disc life, a visual wear indicator, torque adjustment and manual release.

Custom Designs & Modifications

Each AAB product page lists available modifications for the specific series. Common modifications include:

Maintained/non-maintained manual release
Encoder mount
Through-shaft
End cap plug
Brake release indicator switch
Internal/In-line AC rectifiers
Conduit box
Tach machining
Space heater
Band seal (boot)
Mounting plates
Double “D” disc
Carrier ring disc

Custom designs and modifications are possible—consult technical support for more information.

Armature Actuated Brake FAQ

Armature actuated brakes (AAB) are defined as spring-set, electrically released friction devices which develop holding and braking torque in the absence of electrical power. Armature brakes are also called power-off brakes or safety brakes because they can apply holding braking torque in the absence of electrical power.

What is an armature in a motor brake?

An armature is a metal plate positioned between the friction disc and coil (permanent magnet) inside spring-set friction brakes.

How do armature brakes work?

Armature brakes use friction to stop and hold a load. A hub attached to the shaft supports a rotatable friction disc. Brake torque is developed when springs apply a clamping force between the brake armature, friction disc and pressure plate.

When electrical power is applied (power on), the electromagnetic field from the magnet body assembly attracts the armature, drawing it away from the friction plate and compressing the springs. This allows the friction plate to rotate freely.

When electrical power is interrupted (power off), the electromagnetic force is removed and the pressure spring will mechanically force the armature plate to clamp the friction disc between itself and the pressure plate. This action generates torque which stops or holds the hub and shaft (dynamic torque is for stopping, and static torque is for holding).

What motors have armature brakes?

Armature brakes are used with electric motors, including small gear motors, servo motors, stepper motors, linear motors and others.

Are armature brakes better for stopping or holding?

Stearns armature brakes are rated for both dynamic/stopping and holding duty, and our product lineup includes models specifically for holding-only or dynamic/stopping-only.

For holding-only applications, our Armature Actuated Brake (AAB) products generally provide the longest life between servicing.

For applications which require dynamic stopping, our Solenoid Actuated Brake (SAB) products are best in class due to the self-adjusting feature, which means they require less maintenance and provide greater uptime on a torque-to-torque basis.

View or download the Stearns armature actuated brakes catalog section, find a distributor or contact us with questions.

AAB: Armature Actuated Brakes

Spring-Set Elecrically Released Brakes for Industry