Both spring-loaded and permanent-magnet brakes are authorized. But which is the right one for the respective application? Since very different aspects have to be considered in the selection, the user should rely on competent advice. Then of course it is advantageous if the consulting brake manufacturer has both principles of action in the program and advises without self-interest. The contribution of Kendrion shows that brake is not equal to brake.
Depending on the conditions of use, a distinction is first made between the holding brake and the working brake. The holding brake has the task of keeping loads at a standstill. The deceleration of the movement is taken over by the drive. Only in case of error, z. As in emergency stop, the holding brake performs braking work to bring the system to a halt and then keep at rest. In contrast, the work brake has the task of destroying the kinetic energy and then again to keep the system at a standstill in peace. Typical areas of application (Figure 1) for holding and working brakes can be found on elevators, monorails and wind turbines as well as in robotics and mechanical engineering.
The use of work brakes is becoming more and more obsolete, as modern drives and controls can cope safely with the braking task and also work wear-free. Applications in which work brakes are used, are operated by spring-applied brakes, as the organic friction systems used there - similar to the brake pads in the car - are well suited to provide a high overall work over the lifetime available. For the numerically significant portion of the holding brake applications both permanent magnet and spring brakes are used. The user here is spoiled for choice, as in so many other areas too. He has to decide which functional principle is best suited for his application. Both have characteristic properties that predestine them for the different fields of application.
Two principles of action - different propertiesFor both types of brakes, they are closed when de-energized. These are safety brakes: In the event of a power failure or failure of the power supply, such as a line break, the system is kept safe. In addition, however, there are fundamental differences. In the spring pressure brake, which is usually mounted on the B-bearing side of an electric motor, press springs in the de-energized state against the armature disk of the brake. The friction linings of the rotor, which is connected via a toothing with the motor shaft, are clamped between this armature disc and the mounting surface of the brake on the engine rear side. If the coil of the brake is energized, a magnetic field builds up, which attracts the armature disc and thus releases the rotor with the friction linings. The brake is released.
In the "permanent magnet" operating principle, on the other hand, in the de-energized state, the armature or the rotor is pulled by the permanent magnet field against the stator or the exciter system. In the energized state creates an electromagnetic field that cancels the attraction of the permanent magnets and thus solves the anchor by the tensile force of the springs between the armature and flange hub from the exciter system. The brake is released. Due to the non-positive connection between armature, hub and shaft, the permanent magnet brake is backlash-free. However, it is necessary to comply with defined installation conditions in order to ensure a defined air gap in the engine.
Permanent Magnet BrakesThese two principles of action with their different friction pairings, steel / steel for the permanent-magnet brake and organic friction linings / steel for the spring-applied brake, result in defined, essential characteristics that make typical applications possible for both brake types: Permanent magnet brakes (PE) are well suited for servomotors , for example in handling technology and robotics. Here they convince above all by their compact dimensions and their comparatively low weight. Thanks to the permanent magnets, the power density is twice as high as with spring-applied brakes (FD). But for other reasons as well, the lightweight and at the same time dynamic and virtually abrasion-free brakes in robotics will be given preference.
The abrasion resistance of the PE brake is ensured by the operating principle of the brake. The anchor is fully released by the spring. In the case of the FD brake, a start-up wear occurs, since an air cushion must first build up between the lining and the friction surfaces when the speed is increased. This wear can be caused by accelerations of the friction disc, z. B. increase the acceleration due to gravity in a vertical arrangement of the drive or by centrifugal forces in the rotation of the rotor blades of a wind turbine. Here, usually only one friction lining is affected.
When used as a pure holding brake with emergency stop function, the PE brake behaves differently than the FD brake. Due to its construction, the PE brake is free from residual torque. There is only abrasion at the emergency stops. In operation, the armature is fully released by the spring. In contrast, the FD brake has a starting torque, which leads to a certain amount of wear during each startup. To make matters worse, the above-mentioned wear by acceleration forces added. Often, this additional wear can not be determined exactly, as usually only one side of the friction is affected.
Another difference lies in the behavior over the temperature range. PE brakes are very temperature-stable and have a guaranteed high torque over the entire temperature range. The situation is different with the FD brakes. Here, the temperature stability is essentially influenced by the composition of the organic friction lining. This can be compared to a car tire, which is also developed for different conditions of use. Just as a Formula 1 tire can not be used in the winter, so does some organic brake friction pads.
With a high coefficient of friction, the lining has good adhesion, high torques are achieved, but the lining wears out very quickly. For linings in FD brakes, this means: linings with high coefficients of friction show a greater drop over the entire temperature range and in some cases have only half the torque at 120 ° or -40 ° C. In general, it can be said that FD brakes either achieve very good torques, but then are not as temperature stable or that the coefficient of friction is comparatively lower with a temperature-stable coating. It should be emphasized, however, that in a given temperature range, the torque of the FD brake can be set very precisely to the torque specified by the customer in the design process.
Allrounder spring pressure brakeLifting and travel drives with high braking energy and defined braking torque, ie controlled deceleration during emergency stop, can not be operated by PE. In addition, there are many applications that do not require high dynamics and power density. Cranes, overhead conveyors or roller shutters provide typical examples. In the event of a fall, the brake must brake, if necessary provide high deceleration values per brake during emergency stop and maintain the weight reliably. The switching time and power density play only a minor role. High braking work is no problem for the organic friction linings of the spring-applied brakes and they can also be used as a work brake if required.
In addition, the moment of inertia is lower than in permanent magnet brakes due to the comparatively low weight of the friction disc. In addition, IEC standard standard motors are usually used in these applications, where spring-applied brakes can be easily and quickly mounted. The brake, which is less complex than permanent magnet brakes, usually remains easily accessible. Therefore, anyone who can use a low-cost standard standard motor in an application will normally resort to a spring-applied brake. For the respective application, z. As for a certain operating temperature range, an optimized application on the friction lining can be selected.
The FD brake can be well adjusted to a desired torque with a relatively small tolerance, given proper selection of the organic friction lining and design of the springs. Moreover, if the temperature range is still comparatively small, the torque can be kept well over this range. In addition, the development has not stopped with spring-applied brakes: With the new Cobra brakes from Kendrion 80% more torque or three times the life compared to previously commercially available solutions can be achieved. By reducing the drive power, the power consumption is reduced by one third. There is less heat, which reduces the aging of the components.