Intelligent monitoring of brakes for vertical axes

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Bernd Keys of Mayr Transmissions offers a view on how machine builders can put a stop to things when they really need to.

In many safety-critical applications, safety brakes are often the only protection against crashes or the unintentional descent of suspended loads. Our business is providing a range of reliable safety brakes for these vertical axes and also solution concepts for predictive maintenance of them. Here, the focus is placed on our ROBA-brake-checker module, which is able to monitor and supply brakes without the use of sensors. In an advanced version, it also provides data, thus enabling scheduled and cost-effective maintenance.

Suspended loads in machines and systems, in particular in machine tools, handling systems, assembly facilities, hoisting devices, cranes and stage winches, represent a substantial risk potential, especially when people have to stand underneath them. Therefore, it is important to permanently exclude the risk of an inadvertent descent of loads as well as unacceptably long stopping distances in the design phase. A key factor here is the correct selection of the safety brakes as well as their proper integration into the overall system. Safety brakes (according to the fail-safe principle) are the first choice for applications with vertical axes, as they generate the braking force through thrust springs and are closed in a de-energised condition. Reliable motor drives, whilst also being able to decelerate vertically-moved loads to a standstill even in case of power supply failure (SS1 operation), cannot subsequently hold loads suspended without a supply of power.

Safety through reliable linear brakes

Two proven safety braking systems that are frequently used to safeguard linear movements are brakes that act either on separate round rods or on guide rails. These include, for example, our own ROBA-linearstop brakes (round rod) and the ROBA-guidestop brakes (guide rail). These linear brakes are particularly suitable for use in gravity-loaded axes, as they are mounted directly on the masses which are to be braked or held. Drive elements such as toothed belts, couplings or spindle nuts, which in other braking systems also transmit the braking torque and could have an influence on safety, do not exist here. If linear brakes are also used as a second brake unit, for example in addition to a motor brake, their operating principle – either on a rod or rail – also ensures high-quality redundancy with the exclusion of the same errors. Linear brakes can be used in many different application fields.

More than just clamping units

Most linear brakes available on the market today function as static clamping units, and have been designed to hold the axes safely at a standstill. However, there might be people under suspended loads during initial operation, maintenance or even during the production process without the load transfer to the mechanical linear brake having taken place first. If the drive fails completely during these operating conditions, the linear brake alone is responsible for the deceleration of the load. Such dynamic braking actions repeatedly occur in practice. Therefore, when making their selection, users should make sure that the clamping units are designed in such a manner that they are also suitable for emergency braking actions. Here at mayr power transmission we subject the units to dynamic tests on a drop test stand specially designed for linear brakes. This makes it possible to accelerate different weights, the so-called load masses, to different drop speeds and then decelerate the load masses. This way, the influencing factors can be determined in a realistic setting, because only products that have been tested under realistic conditions can subsequently meet all requirements without any problems.

Never compromise on safety

Depending on the design, there are further possibilities for integrating or retrofitting safety brakes. One possible position is directly between the servomotor and the spindle. With the ROBA-topstop brake system, users have an independent module which holds the vertical axis safely in any position, even if the motor is disassembled for maintenance or transport. The axis does not have to be additionally supported, which considerably accelerates the replacement of the drive motor, for example. This reduces costs, and also, downtime for repairs is considerably reduced. Thanks to its adapted flange dimensions, the brake can also be easily integrated into existing constructions.

The safety brake meets high IP standards and is currently available up to IP65, which means it is already well protected against the penetration of grease or oil mist. A voluntary type examination by the German Social Accident Insurance (DGUV) confirms the braking device as a “tried and tested component” in terms of Category 1 acc. DIN EN ISO 13849-1. For axes with a rack and pinion drive, the ROBA-pinionstop can be used as an additional braking system. This electromagnetically opening safety brake with integrated, bearing-supported pinion directly locks into the toothed rack at any required position and therefore operates independently of the drive motor. It can also be retrofitted in existing systems.

Short switching times over the entire service lifetime and reliable switching condition monitoring

In particular on vertical axes, short stopping distances are important for the safety of people and machines. Here the brake switching times are decisive for the braking distance, because during the free-fall time until the brake closes and the retardation takes effect, the mass additionally accelerates – possibly to such extremes that the permitted brake values are exceeded. Users should therefore pay attention to 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 lifetime of the brake. In order to achieve the highest possible level of safety in each application and to guarantee the planned switching times throughout the entire brake lifetime, it is also important that the brakes are equipped with a reliable switching condition monitoring system.

Intelligent brake monitoring

At mayr power transmission we offer solution concepts for the predictive maintenance of electromagnetic safety brakes based on the principle of permanent inspection. The monitoring of the safety brakes takes place without the use of sensors with the retrofittable ROBA-brake-checker module. Users benefit from the permanent inspection of the safety brakes by the ROBA-brake-checker module. If, for example, the limit temperature is reached, this is an indication of damage to the brake, of brake failure or even incorrect dimensioning. The ROBA-brake-checker also shows when critical wear values have been reached. This permits predictive maintenance, which in turn ensures higher system availability. Temperature progressions and changes in parameter across a product’s lifetime are also made visible. In the case of previous solutions, such as contactless release monitoring, users are only able to see the failure or destruction pattern, but do not know how the error occurred. Using the ROBA-brake-checker, on the other hand, progressions are made visible and error analyses can be used and even transferred onto other user systems. All this data from malfunctions and normal operation thus supply valuable input for future improvements and optimisations, for example for more system safety or an extended performance limit.

Checklist: Safety brakes for vertical axes

  • Ensure that the safety brakes are also suitable for dynamic braking actions and have been tested under realistic conditions. Also enquire about the testing facilities of the manufacturer
  • Make sure 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 stopping distances using a quick brake and constant switching times throughout the service lifetime
  • 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 design flexibility for different applications. Further, you are more likely to obtain complete solutions from a single source
  • Rely on manufacturers with development, production and application experience, a rapid global on-site service (especially if you market your machines worldwide) and competent consultancy. Ask for a consultation so you can evaluate the differences between suppliers
  • If you can, take the time to view the production facilities of a potential supplier, especially if a critical application is in hand. You can convince yourself what is really going on behind the scenes during a visit to the plant

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