Put on the Brakes: Know Your Disks, Hubs, and Driveshafts

Most electric robots today use servomotors for direct-drive operation. While servomotors eliminate the need for clutches, brakes are still required to stop a robotic arm when necessary. One of the major risks of robotics — and what OEMs certainly never want to see — is an uncontrolled robot!

A Tech Briefs feature article (Choosing the Right Brake for Robotic Applications) explores the key elements of a spring-engaged friction brake:

  • The field coil, which is fixed to the machine frame or a backstop. The field coil generates an electromagnetic field.
  • The armature. Attracted by the electromagnetic field, the armature initiates a compression of the brake’s springs.
  • The friction disk. In addition to the compression, an air gap allows the friction disk to rotate freely with the hubs, driveshafts, and loads.
  • The pressure plate.

The article was written by Rocco V. Dragone, an application engineer at SEPAC, a manufacturer of clutches and brakes in Elmira, NY.

Spring-engaged friction brakes are typically coupled to the driveshafts with a hub, which is fixed to the shaft with a set screw.

When power is off, the force of attraction between the field coil and armature falls to zero. The spring between the armature and the magnet body pushes the armature into contact with the friction disk. The friction disk is then squeezed between the pressure plate and armature, transmitting torque and stopping (or holding) the disk, hub, and driveshaft.

The Tech Briefs article also explores a third type of brake commonly used in robotics: a permanent-magnet friction brake.

How does a permanent-magnet friction brake work? With no power, the permanent magnet pulls the armature into contact with the friction faces, holding the driveshaft, hub, and load in place. When power returns, the field coil generates a magnetic field of opposite polarity to that of the permanent magnet, which brings the force at the face of the armature to zero. The armature disengages from the electromagnet housing, assisted by a diaphragm spring.

“Choosing the Right Brake” also explores a condition called backlash, which occurs when the driveshafts rotate while the brake is holding. This lost motion depends upon the hub used to connect the driveshaft to the brake.

Learn the hub options; each hub must provide sufficient clearance to enable the armature to move axially along the driveshaft. A spring-engaged brake, for example, will always have some degree of backlash.

OEMs building robots have a variety of brake options for their systems. Spring-engaged friction brakes do the job at an economical price point. For space-constrained equipment that requires high holding torque, spring-engaged tooth brakes are quite effective, but also more expensive. Permanent-magnet friction brakes offer wide bores and infinite resolution, as well as zero-backlash operation.

Review your options, and partner with their supplier to find the best solution.

Read the Tech Briefs feature article: Choosing the Right Brake for Robotics Applications.