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.
High tolerance of voltage variability. When consistent voltage is needed, especially in outdoor or other highly variable environments, spring set brakes again are preferred. The small window required for the electromagnetic force to counter the natural magnet makes permanent brakes highly unreliable amid fluctuating temperatures, electromagnetic interference, or other forces that might alter the magnetic field or impact voltage levels. Using permanent magnet brakes in highly variable temperatures, for example, could cause them to break or reapply at undesirable times.
Spring set brakes are made more desirable in challenging environments by the fact that they can be sealed properly, coated with protective materials, or incorporate friction-enhancing materials that don’t rust. It also is possible to build shock and vibration resistance into spring set brakes, which is not possible with permanent magnet designs because of the sensitivity of the magnetic field and the need for high-precision, metal-on-metal contact.
For example, one would never see a permanent magnet on an external component of a plane, such as the horizontal trim flap, which needs to provide consistent torque at 40,000 feet of altitude and temperatures as low as -30 °C (-22 °F), and be subjected to extreme shock, vibration, and humidity. Other applications challenging torque consistency — and thus are more appropriate for spring set brakes — include aerospace and defense, agricultural, and other outdoor applications.
Manual control. Spring set brakes also make it possible to implement a manual release, utilizing a lever or other mechanism to decompress the springs that are pushing the armature against the rotor. Releasing the hold of a permanent magnet, however, requires electrical energy. Manual release can be valuable for maintenance, or in applications such as medical or factory automation equipment.
Maintainability and durability. Spring set brakes also require less maintenance than permanent magnet brakes. The simpler design and advanced friction material means that they will wear less, run quieter, and last longer — all of which contribute to a lower cost of operation and ownership. The metal-on-metal contact required for permanent brakes contributes more wear and tear, more particulates in the process, and noisier operation, resulting in higher maintenance time and expense, and shorter life. Maintenance and durability are important factors in any application. Likewise, quieter operation has widespread appeal, but could be of special interest in healthcare applications such as imaging machining equipment (CAT/PET scans or X-rays).
Price. All other factors being equal or comparable, spring set devices will typically win out on price. These cost less than permanent magnet brakes because they don’t have the baseline cost of a magnet composed of rare earth metals, nor do they require as much complex, precision machine tooling. Price is especially critical in high-volume applications such as semiconductor pick-and-place, and automotive applications. Permanent magnets are seldom found in high-volume applications because there is virtually no economy of scale in the cost of the magnets.
Advantages of Permanent Magnets
Zero backlash. One significant advantage of permanent magnet brakes over spring set designs is the fact that when they are holding, there is no play (near zero backlash) because of the armature/diaphragm/hub configuration. The spring set brake rotor-to-hub fit and armature-to-spacer clearance contribute to backlash when the rotor is being held. For example, if there is one degree of play on a five-foot torque arm, it will be multiplied by the length of the arm, and could contribute to undesirable wobble in immediate operation as well as wear over time.
Consider a medical application in which a doctor is performing eye surgery with a five-foot-long robotic arm. If the power goes off, the surgeon needs the arm to be in the exact same spot when the power returns. However, one degree of backlash multiplied by the length of the arm would introduce an unsafe level of precision. Other applications in which zero backlash is critical include pick-and-place robots and many medical uses.
Of course, if the permanent magnet brake fails completely while in operation, it can introduce safety concerns of a different sort. This is why braking systems intended for such applications are designed with extra care and precision, and it is critical to monitor them closely and frequently for early signs of wear.
It also is worth noting that spring set breaks can be adapted for zero backlash operation through the addition of a customized hub/diaphragm/rotor configuration. This does, however, add costs, and can reduce available torque by as much as half.
Small footprint.Because it is electromagnetic force that does most of the work in applying the magnetic attractions, permanent magnets operate more efficiently with smaller components. As such, they need less space to operate and can be deployed in tighter quarters. This is increasingly important in medical device applications such as X-ray and mammography scanners, as well as in static holding applications.
Speed and rapid cycling. Permanent magnets have an air gap between the output hub assembly and the magnet. With nothing contacting it, the output hub assembly can rotate at the speed the motor can achieve, which in a servo-driven application, for example, could be as high as 20,000 RPM. Spring set brakes, on the other hand, have more moving parts between the hub and splined shaft, limiting RPM. Balancing spring set systems can increase the speed potential, but at a cost. Also, the direct-acting electromagnetic force of permanent magnet brakes means that they can pull in and drop off faster, which is an advantage where quick response time is needed.
Managing the Tradeoffs
The table in Figure 4 compares spring set and permanent magnet brakes using the dimensions we have been discussing. From a performance standpoint, about a third of all applications would be better served by spring set, a third by permanent magnet, and a third by either. And, in most cases, it is pretty clear which is which.
One would almost always specify spring set brakes for the airline trim application mentioned previously because it is a safety-critical application, and the permanent magnet would not operate effectively in the extreme physical conditions a plane might encounter in flight. By contrast, you would always specify a permanent magnet brake for medical devices, in which zero backlash and smaller footprints are desirable, and voltage is more controllable.
In those areas where either a spring set or permanent magnet would provide comparable performance — such as a printer, where safety, backlash, and current supply do not come into play — the choice often comes down to price. Other factors may include delivery time, availability of spare parts, and what often trumps everything else — personal preference. For many engineers, the safest and most effective alternative is the one they know.
This article was written by John L. Pieri, Senior Product Line Manager at Thomson Industries, Deltran Clutches and Brakes, Radford, VA. For more information, Click Here .