How to specify brakes for safety-critical applications

Suspended loads in machines and systems, in particular in machine tools, handling systems, assembly facilities, lifting devices, cranes and stage winches, represent a substantial risk potential - especially when people have to stand underneath them. Therefore, it is important to already permanently exclude the risk of an inadvertent descent of loads as well as unpermittedly long stopping distances in the design phase. Decisive for this purpose is the correct selection of safety brakes, as well as their proper integration into the overall system. For safety-critical applications, safety brakes designed in accordance with the fail-safe principle are the best choice, as these brakes generate the braking force through thrust springs and are closed in de-energised condition. Reliable motor drives, whilst also being in a position to decelerate vertically moved loads to a standstill in case of power supply failure (SS1 operation), cannot subsequently hold loads suspended without a supply of power.

Reliable spring force

A further important criterion for safety brakes is that the basic safety principles are applied, such as for example the reliable spring force. Renowned manufacturers rely on tried and trusted springs, which comply with EN ISO 13849-2. In addition, they consistently take into account the multiplication of parts as a proven safety principle in compliance with EN ISO 13849-2. In addition to sufficient spring guidance, it is important that the coil distance on the thrust springs is smaller than their wire diameter. Prospective users should also query the safety factor under continuous load and the dimensioning in the dynamic fatigue strength range.

For the safety of people and machines, short stopping distances are important, in particular for safety-critical and vertical axes. The switching times of the brake are decisive for the braking distance, because during the free-fall time until the brake closes and deceleration takes effect, the mass also accelerates - possibly to such extremes that the permissible values of the brake are exceeded. Users should therefore observe verified switching times which are as short as possible when selecting safety brakes - and also assure themselves that these switching times can be maintained throughout the entire service life of the brake. To achieve the greatest possible level of safety in the respective application, and to ensure the planned switching times over the entire service lifetime, it is also important that the brakes are equipped with a reliable switching condition monitoring system.

Known static and dynamic braking behaviour

To guarantee that the brakes always ensure the highest level of reliability in the respective application, it is important that the static and dynamic braking torques correspond to the respective customer specifications. For this purpose, it is necessary to check these two application conditions in trials. Therefore, it is highly recommended that users obtain information about the manufacturer's test field, and question whether the brakes are subjected to tests that are as realistic as possible: Does the manufacturer also subject the safety brakes to climatic tests with realistic temperature and air humidity ranges? Are parameters such as braking torque, speed, temperature, coefficient of friction and friction work determined and checked under various conditions? Are the friction value progression and the wear characteristics of the friction system evaluated? Only in this way is it possible to verify that the safety brake has been correctly selected and dimensioned for the application concerned.

At renowned manufacturers, the data gained from the trials flows into a calculation tool for safety brakes. Here, for example, the permitted friction work dependent on the speed and compression, or the friction values dependent on the temperature can be determined. Furthermore, amongst other things, these manufacturers can make statements regarding the friction value progression dependent on the speed, compression and friction work, or regarding the wear depending on the compression, sliding speed and friction work. In this way, they are able to derive design guidelines for construction. This means that renowned manufacturers can make reliable statements, for example on how the braking system will behave under certain conditions, and what is possible in a special application or not. This creates a basis for a high level of service quality - and safety.

100 per cent quality inspections

In addition to quality assurance measures conducted during the construction process, a renowned manufacturer executes a comprehensive final inspection. This means that every individual safety which that leaves the plant is subjected to a 100 per cent inspection following complete assembly and adjustment. During the course of the inspection, function-relevant parameters such as pull-in and drop-out voltages, coil resistance, dielectric strength of the coil, insulation of the coil to the housing, axial force, armature disk stroke and the operating principle of the switching condition monitoring are determined. And also series inspection of the braking torque or the braking force is possible or can be made a standard procedure. At renowned manufacturers, all the determined measurement values are archived together with the corresponding serial numbers of the brakes in an electronic database. Only in this way 100 per cent traceability can be ensured.

The application fields for safety brakes are diverse, and therefore renowned manufacturers offer a wide range of products. As a result, leading suppliers provide a wide selection of rotatory and linear brakes: These include, amongst other things, various brake designs for rotating shafts, for example for attachment to servomotors, redundant double brakes, disk brakes, brake systems for axes with rack-and-pinion drive, linear brakes for profiled rail guides, but also linear brakes for piston rods in pneumatic, hydraulic and electromagnetic design. Customers can select the appropriate function principle to match the respective application. Renowned manufacturers are also able to design the brakes for special application cases, for example with corrosion protection or in particularly robust designs for the food sector or the maritime sector.

Checklist for the procurement of safety brakes for safety-critical applications:

• Please ensure that the safety brakes are also suitable for dynamic braking actions and have been tested under realistic conditions. Ask about the testing possibilities utilised by the manufacturer.
• Ensure that the supplier implements a 100 per cent final inspection, including the automated storage of all test data. For gapless traceability, the brakes must also be labelled with a unique serial number.
• Check the response times of the brake (attraction/drop-out). You can only achieve short, reliable braking distances using a quick brake and constant switching times throughout the service life.
• Rely on a supplier with a wide standard product range. A large selection of rotatory and linear brakes based on different functional principles (electromagnetic, pneumatic or hydraulically released) provides flexibility for various applications. Furthermore, you will be able to obtain complete solutions from a single source here.
• Rely on manufacturers with development, production and application experience, a rapid global on-site service (especially if you market your machines globally) and competent consultancy. Request a consultation to see the differences.
• Take the time to view the production facilities of the supplier, especially in case of important purchases. You can convince yourself what is really going on behind the scenes during a visit to the plant.

Learn more about brakes for safety-critical applications at www.mayr.com.