Today’s AGV Brake Technology … When You Need to Stop a 50-Ton Automatic Guided Vehicle – Safely

By Gary Zwick, Product Manager, Stearns Division – Rexnord Industries, LLC.

When it’s a 50 ton AGV, selection of the right braking system is critical. For Mentor AGV – recently acquired by Jervis B. Webb Company, Farmington Hills, Michigan - the choice was an electric brake design made by the Stearns Division of Rexnord. Jervis B. Webb Company is a major manufacturer of large Automatic Guided Vehicles (AGVs). As a pioneer in the development of AGVs and AGCs, Webb offers one of the most advanced selections of standard and customized Automatic Guided Vehicles, as well as all types of guidance options including laser, inertial, wire and magnetic tape. Stearns is a leader in brake technology, and the #1 supplier of spring-set electrically-released brakes to the North American motor manufacturers.

AGV Description

This AGV is used to manufacture the F-35 Joint Strike Fighter, and has been in operation for about three years at the Lockheed Martin plant in Fort Worth, TX. It is about 12’ tall x 12’ wide x 36’ long and carries a load of 30,000 pounds. Because of its sheer size, the design includes many holes in order to reduce the weight.
The load is either the “forward fuselage” mounted in a fixture, or the “wing carry-through component” in a holding fixture. There are two rams on the AGV which engage the load. It can lift the load 3’ and side shift the load 4’.

This side loading AGV was chosen as the best technology to transport these components. It is very maneuverable and can move forward, backward, sideways, or rotate about a point. The AGV transfers the components to 16 different stations within a new (5)-axis milling machine (which is 35’ wide, 30’ high - and 358’ long). It docks the load precisely at each station using the bidirectional wheels and laser triangulation. The AGV is also equipped with obstacle-detection sensors to prevent collisions.
The AGV uses a total of (5) Stearns electric brakes – (1) on each of the two drive motors, (1) on the side shift motor, (1) on the lift motor, and (1) on one of the lift actuators. All (5) are Stearns model 331 (see image above) “Armature Actuated Brakes” (AABs), which is a spring-set / electrically-released design. They all operate on 48 VDC, are rated for 25 lb-ft static torque, sealed for IP 54 rating, and incorporate encoders to provide feedback for position control.

This laser-guided AGV is driven by a steer-drive package that contains a planetary gearbox driven by a 48-volt DC motor, with the brake mounted on the back of the motor. The package fits within the driven wheel hub of the AGV. The AGV is designed for easy maintenance. It includes a door that opens to provide access to the steer-drive package, and a hand crank that allows the motorized wheel to be oriented to face the door. Mounting the brake on the high-speed end of the motor not only aids accessibility but also allows the use of a smaller brake with a lower torque rating.
Braking System

The drive system brakes provide two functions. During normal operation, the motor controls slow the vehicle to a stop and the brakes then hold the unit in its parked position. When the emergency stop switch is activated – or when the vehicle processor determines that there is a problem - all power to the drive system is interrupted, and the brakes are automatically applied to stop the vehicle. A spring-applied brake was required to meet safety requirements.

Stearns also provides a “proving switch” for each of the drive system brakes. This switch is important for proper AGV operation, as it signals when the brake is released. Frequently, there is no operator to notice if the vehicle tries to move before the brake is disengaged. A signal from the brake is needed to tell the processor that the brake is released and the AGV is ready to move. This switch also functions as a wear indicator, since it provides a signal when the brake does not fully engage due to a worn down friction disc.

Brake Design

The Stearns model 331 AAB design is a spring-set and electrically-released brake. It decelerates or holds inertial loads when voltage is disconnected from the brake coil. The brake design is direct-acting with only two moving parts. When electrical power is applied, an electromagnetic force in the brake’s magnetic body pulls the armature, which overcomes the spring action. This allows the friction disc to rotate freely. When electrical power is interrupted, the electromagnetic force is removed, and the spring pressure mechanically forces the armature plate to clamp the friction disc between itself and the pressure plate. This develops the torque needed to stop or hold the load.

Brake Design Features

• A key feature of the Stearns design is the reliability of the braking torque – without the need for a “run-in” period. The Stearns design includes greater spring pressure than comparably rated competitive units. In addition, Stearns tests and burnishes every brake on an automated test stand – to insure that the brake provides rated torque “out-of-the-box”, and throughout its life. This brake feature is critical for the safe operation of any industrial vehicle. 
• Coils are class H insulated and potted/encapsulated, to provide the necessary heat dissipation and environmental protection.
• Standard brake sizes accommodate larger shaft diameters than competitive designs, to cover a wider range of NEMA & IEC motor shaft sizes.
• 100% RoHS Compliant

Brake Design Options

• Armature 'shim', located between the magnet body and armature, gives the Stearns design a quicker engagement – about 25% faster than competitive brakes.
  AC rectifiers (either a separate model, or - for the smaller brake sizes - a rectifier that is in-line with the brake).
• Band seal / boot – for IP54 rating.
• Tachometer / encoder mounting (Stearns design can accommodate most of the popular encoder brands and designs).

Mentor AGV had previously used a competitor’s brake made in Germany, but found it difficult to get the necessary technical support. The Engineering Manager at Mentor advised that “the Stearns brakes are U.S.–made, bolt- on systems, and Stearns service has been excellent.”

Stearns Division is the largest supplier of spring-set electrically-released brakes in North America. With 90+ years of brake design and manufacturing experience, Stearns has built a reputation of being responsive, innovative and reliable – along with achieving an unmatched record for on time delivery. For more information on Stearns and Stearns products, contact Stearns Division, 5150 S. International Drive, P. O. Box 100640, Cudahy, WI 53110, Phone: (414) 272-1100, Fax: (414) 277-4364. or visit stearns.rexnord.com

About the Author:

Gary Zwick is a Product Manager for Stearns Division – Rexnord Industries LLC. Gary is responsible for the development and commercialization of brake, clutch, and control products for the industrial vehicle market and other industrial markets. He has 35 years experience in engineering, sales and marketing for power transmission products. Gary has a bachelor’s degree in mechanical engineering (Marquette University), and a master’s degree in business management (Durham University). His hobbies include basketball, boating, community and church projects. Gary is married and has one son.