Power-off brakes are designed to hold or stop motion in the absence of power. Adding an electrical current releases the brake, freeing the load for motion. Given the safety ramifications of keeping a system locked in place until it is powered up, motion control system designers tend to specify power-off brakes more often than power-on brakes. There are, however, two different failsafe brake technologies: one uses compression springs to hold its load in place, and the other uses permanent magnets. Each has specific strengths and weaknesses, and knowing the difference can impact safety, durability, cost, and performance.
Spring Set Brakes
In spring set architectures (Figure 1), compression springs apply the braking force. When the power is off, the force from the springs pushes the non-rotating armature plate into a rotor. The rotor hub is designed to fit a splined or hex-shape shaft, which would be connected to the load. In this power-off mode, the force from the springs is enough to hold the rotor to keep its connected load clamped into place. The advanced friction material on the rotor maintains its torque.
Energizing the coil produces the electromagnetic magnetic force that pulls the armature away from the rotor, freeing it to turn. Once operating, keeping the springs compressed requires only one-quarter of the initial voltage. If that voltage is removed accidentally or through normal shutdown, the spring decompresses and pushes the armature plate back against the rotor, once again producing the friction that brings the load to a stop or holds it in place.
Spring set brake technology is simple, low-maintenance, and forgiving. Parts need little if any additional burnishing once implemented, and they can operate reliably amid fluctuating voltages — 5V DC is enough to release the armature’s hold on the rotor, and as little as 3V DC will keep it released. It is also possible to equip a spring set brake with a manual override, which has maintenance and safety advantages.
Permanent Magnet Brakes
Figure 2 illustrates a power-off architecture that uses a permanent magnet to hold its load in place. In normal power-off mode, the permanent magnet in the fixed field assembly creates an attractive force on the armature assembly, which is attached to the load shaft by means of set screws or pins, hence stopping or holding the load.
Energizing the coil negates the natural magnetic field, reducing its torque to zero, freeing the rotor and connected load to move (Figure 3). When the power is removed, the natural magnetic force takes over again, attracting the armature and holding the load in place. Because this magnetic force attracts the metal components, this is called “direct acting,” as compared with the pushing mechanism of spring set brakes, which is known as “reverse acting.”
Maximum performance of the direct acting designs requires metal-on-metal contact between the armature and the rotor. This eliminates play (backlash) while the brakes are locked on. Where the load stops is where the load stays. Direct action is efficient, delivering higher torque within a small footprint. It also enables higher-speed action with the brakes locking on or off at rapid intervals.
Achieving such advantages, however, requires a consistent supply of current. The brakes release over a narrow window, usually 10 percent of the applied voltage (Figure 4). Outside that window, they can reapply.
This narrow range causes permanent magnet brakes to be sensitive to variations in current, making voltage regulators essential. Because the effect is attained by reversing polarity, reversing the prongs of a contact plug, for example, will interfere with the action, which is why matching red and black leads on power cables is critical to performance.
Because temperature fluctuations impact current flow, permanent magnet devices require operation in controlled temperatures; however, the metal-to-metal contact still makes them highly sensitive to oxidation, which can reduce torque. If, for example, 10 inch-pounds of torque is required to hold a load, oxidation could reduce holding power to 8 inch-pounds over periods of inaction — even over the course of a weekend — requiring the need to burnish to regain the torque.
Technically, spring set and permanent magnet designs can perform the same basic function, but there are many good reasons to choose one technology over the other. Spring set brakes have advantages over permanent magnet brakes in applications where machine safety, maintainability, dynamic stopping, or cost are drivers. Permanent magnet brakes, on the other hand, are typically favored where zero backlash, smaller footprint, speed, or rapid cycling is needed.
Machine safety. New spring set and permanent magnet technologies provide comparable braking performance and are safer than power-on brakes. But with extended use and wear, air gaps form between the components, and these can contribute to failure of both types of brakes. The spring set brakes always fail safely closed; however, air-gap-related failure for them results in the inability to release the brake, which must be replaced or rebuilt before they can be used again. In air-gap-related failures of permanent magnet brakes, the growing gap makes it increasingly more difficult for the magnet to attract the armature, and the brakes will fail open, countering the initial advantage of the power-off design.
So the higher the potential for personal injury or equipment damage in the event of a brake failure, the more advantageous the spring set designs become. Possible applications include heavy robotic arms as might be found in an automotive production plant, large medical devices, actuators, motors, postal handling equipment, and packaging automation.
Dynamic stopping. Spring set brakes also are specified to stop a load as well as hold it in place once stopped because of the advanced friction materials. Permanent magnets are typically used only to hold the load in place. The metal-on-metal contact, which is already subject to increased wear and tear, would wear even faster — and noisier — if applied for stopping. Applications in which it is critical for the brakes to provide stopping include motors, actuators, and robotics.