What to look for when selecting linear brakes

Safety brakes acting in accordance with the fail-safe principle are the preferred choice for the quick and reliable braking of linear movements on gravity-loaded axes, because these brakes generate the braking force through thrust springs and are closed in a de-energised state. Reliable motor drives, on the other hand, may also be capable of bringing vertically moving loads to a standstill even in case of a power supply failure (SS1 operation), but are not then able to hold these loads without power.

Two tried and tested safety brake systems, which are frequently used for securing linear movements, are brakes which either act on separate round rods or on guide rails. These linear brakes are particularly suitable for use in gravity-loaded axes, as they are directly attached to the masses which are to be braked or held. Here there are no drive elements such as toothed belts, couplings or spindle nuts, which also transmit the braking torque in other braking systems and which could have an influence on safety. Furthermore, if linear brakes are used as a second braking unit in addition to a motor brake, for example, their mode of action - on a rod or a rail - ensures high-quality redundancy with the exclusion of the same fault.

More than just clamping units

Most linear brakes available on the market act as static clamping units and are designed to hold the axes at a standstill. However, persons may be standing under suspended loads during commissioning, maintenance or even during the production processes, without the load transmission previously being transferred onto a mechanical linear brake. Should a complete failure of the drive occur during these operating conditions, then only the linear brake is responsible for the reliable braking of the load. Such dynamic braking actions occur repeatedly in practical application. Therefore, during selection, the user should observe that the clamping units are designed so that they are also suitable for emergency braking.

Renowned manufacturers are capable of subjecting the units to dynamic testing on fall test stands especially designed for linear brakes. Such fall test stands enable different weights, the so-called load masses, to be accelerated to different fall speeds, after which these load masses are decelerated. As a result, influencing factors can be determined in practical application, because only products which have been tested under realistic conditions can subsequently fulfil all requirements without any problems. Ideally, test institutes independently confirm the suitability of products for dynamic braking.

Short switching times throughout the service life

For the safety of people and machines, short stopping distances are important, in particular for 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 linear brakes - and also 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 throughout the entire service life, it is also important that the linear brakes are equipped with a reliable switching condition monitoring system.

Force generation without self-reinforcing wedge effect

Furthermore, high-quality linear brakes operate without a self-reinforcing wedge effect, and therefore act in both directions of movement. Thanks to the force generation without self-reinforcing wedge effect and the extremely short reaction times, the axis does not drop when closing. Other linear braking systems which operate using a self-reinforcing wedge effect generate a high braking force, but the wedge effect only acts in one direction, and to create the wedge effect the brake must "settl" when closing. If the wedge effect of the brake is not limited, the braking force increases indefinitely. This impact has to be absorbed by the machine design. Furthermore, the clamp then often has to be made to move under massive force in order to release the brake again. This is not necessary for the linear brakes by renowned manufacturers.

High clamping positioning accuracy

High-quality linear brakes are also characterised by high clamping positioning accuracy of the brake rod or rail. This provides many advantages, especially in machine tools and machining centres: the additional rigidity of the NC axis increases the process accuracy, improves the cutting performance and can provide further technological advantages during heavy-duty cutting. The machining is low vibration and thus the surface quality of the workpiece can be improved.

The application fields for linear brakes are diverse, and therefore manufacturers offer a wide range of products. As a result, suppliers provide linear brakes in pneumatic, hydraulic and electromagnetic designs. Customers can select the appropriate function principle to match the respective application. Furthermore, such manufacturers also offer a wide selection of further tried and tested braking systems, for example, for axes with a toothed rack drive or for the attachment to servo motors. They are also capable of designing the brakes for special application cases, for example, with corrosion protection for the food sector.

Checklist for the procurement of linear brakes

• Please ensure that the linear 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 end check, 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 brakes based on different functional principles (electromagnetic, pneumatic or hydraulically released) provides flexibility for various applications. Furthermore, you will be able to obtain complete devices from a single source here.
• Rely on manufacturers with development, production and application experience, and a rapid global on-site service (especially if you market your machines globally) and competent consultancy. Request a consultation to determine 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!

For more information about linear brakes please visit www.mayr.com.