In Asia, it is already the standard solution, and now it is on the rise here, too: the use of spring-applied brakes for automated storage and retrieval units and high-load applications in intralogistics. Working on the failsafe principle, servo motors are usually fitted with holding brakes in order to secure static positions while in operation or during system standstill. Because it is vitally important to have zero backlash for this function, permanent magnet (PM) brakes have so far mainly been used.
However, both the price development of rare earths and a shift in the requirement profile 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 springapplied brake is predestined for use in many servo applications, such as in material handling or in wind power. In a power cut, for example, it is extremely important that a servo brake can be applied from a dynamic motion mode in an emergency stop situation.
At high energy levels, PM brakes are pushed to their physical limits because of the metal friction that is common with this product. Also, in comparison to springapplied brakes, they exhibit worse, or undefined, emergency stop properties. This is due to the formation of striations or metal pellets in the surfaces of the friction partners, which can be observed in load-bearing metal pairings. In a braking process, this can either lead to a very abrupt or sudden buildup of braking torque or, through a reduced dynamic friction coefficient, it can result in brake slippage and, in an emergency, load crashes. A very clear example of the trend towards working brakes in servo motors 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 travelling axes of these storage systems, safety requirements decree that the possibility of a load crash from the lifting unit and a collision of the travelling unit at the end of the operating aisle has to be excluded. In an emergency, the working brake has to protect the cost-intensive components of the automated machine from damage, even with maximum driving dynamics, and it also has to help minimise unplanned downtimes caused by faults.
Spring-applied brakes in servo motors are not only reliable with loads and in terms of wear and tear, they can also be adjusted 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. Servo motors with a B-side brake mounting are also currently being planned, with the aim of improving the maintainability. The axial relocatability of the brake rotor makes it easy to mount spring-applied brakes on the loose bearing of the servo motor. By contrast, PM brakes have to be fitted onto the fixed bearing, for functional reasons. The most important advantage of the spring-applied brake, however, becomes apparent when both the static and the dynamic braking torque have to be adjusted. A large range of organic friction linings also makes it possible to meet the challenging customer requirements. When dimensioning brakes for storage and retrieval units, we have to adhere to defined tolerances in the braking torque in order to safely brake the dynamic loads on the one hand and to exclude an over-braking of the drive wheels on the other hand, which would result in what are called brake marks and in the uneven running of the drive wheels. Our experience shows that the entire package of requirements in storage and retrieval units can only be met by spring-applied brakes.
The INTORQ company, with headquarters in Aerzen, started developing spring-applied brakes for servo motors at an early stage with its BFK418 range. Based on the PM brake, the BFK418 was designed for mounting on the A-bearing side of the motor. Since then, the servo motor technology has been continuously refined. There has been progress especially in the friction linings and in the increased power of the magnetic circles. So a spring-applied brake that has been designed as a pure holding brake can achieve the braking torque of a PM brake that is the same size. In Asia, the spring-applied brake in a servo motor has long been established as the standard solution. In Europe, the change-over has already started, and not only for reasons of cost. The new price- and performanceoptimised servo brake from INTORQ is called BFK417. Like its predecessor, it is designed for mounting on the A-bearing side of the motor, and it can be provided as a holding brake or as a working brake, depending on the application.