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Safely stopping loads
11 January 2016
Permanent magnet brakes offer a safe, lower cost option for servo motors
It is already the standard solution in Asia, and now it is on the rise in Europe too: the use of spring-applied brakes in servo motors as an alternative to permanent magnet brakes. Servo motors are often fitted with brakes for holding and emergency stop under conditions of power failure. A typical case is automated storage and retrieval units in intralogistics. The new BFK417 brakes from specialist INTORQ give advantages of lower costs, more reliable braking and less maintenance.
Working on the failsafe principle, servo motor brakes are used to secure static positions while in operation or during system shutdown. Because zero backlash is desirable for this function, permanent magnet (PM) brakes that use diaphragm springs instead of sliding splines have so far mainly been used. However the price increase of rare earth magnets together with a shift in application requirements are creating a noticeable trend towards the use of spring applied brakes. With suitably adapted friction linings, spring applied brakes can not only fulfil the purely static holding function, they can also be used a working brake for highly dynamic emergency stops. This feature means that the spring applied brake is attractive in many servo applications, such as in material handling or in wind energy.
In a power cut, for example, it is extremely important that a servo brake can be applied from a dynamic motion mode. At high energy levels, PM brakes are pushed to their physical limits because of the metal to metal friction at the magnetic poles that is the norm. In comparison to spring applied brakes, they exhibit poorer and less predictable emergency stop performance. This is due to the formation of metal particles on the friction surface which lead to a sudden build-up of braking torque or, through a reduced dynamic friction coefficient, which can result in brake slippage and load crashes. Examples of the new trend can be found in the intralogistics industry. As a result of the increasing automation in material-handling technology, more and more automatic storage and retrieval units are being installed. In the horizontal and vertical axes, safety requirements decree that the possibility of a load crash from the lifting unit or an over-run of the travelling unit has to be excluded. In an emergency, the working brake has to protect the high cost components from damage, even with maximum driving dynamics, and it also has to help minimise unplanned downtime caused by faults.
Adjustable braking torque
Spring applied brakes in servo motors are not only reliable with low wear, they can also be customised to meet the needs of a specific application. In storage and retrieval units, for example, a hand-release is often required in the event of a fault. The hand-release is a standard option with spring applied brakes. Some motor manufacturers are going away from mounting the brake at the drive end of the motor and mounting at the back B-side to aid access and maintenance. The sliding splines of the brake rotor makes it easy to mount spring-applied brakes onto the motor shaft with standard bearings.
By contrast, PM brakes have to be fitted with fixed bearings. The most important advantage of the spring-applied brake, however, is the easy way that static and the dynamic braking torques can be adjusted. A large range of organic friction material makes it possible to meet the challenging customer requirements. When dimensioning brakes for storage and retrieval units, defined tolerances in torque have to be met in order to safely brake the dynamic loads on the one hand and to exclude an over-braking and skidding of the drive wheels on the other hand. The experience of INTORQ is that the entire range of requirements in storage and retrieval units can only be met by spring applied brakes.
- The use of spring-applied brakes in servo motors as an alternative to permanent magnet brakes is on the rise
- Spring applied brakes in servo motors are reliable with low wear, and can also be customised for the specific application
- Brake rotor sliding splines make it easy to mount spring-applied brakes onto the motor shaft with standard bearings.