Innovative solution with proven technology Electromagnetic, spring applied, fail safe brake for servo motors.

Characteristics and advantages

• Extreme low inertia

• Easy and safe assembly without complicated, time consuming adjustment

• Insensitive to voltage fluctuations

• Insensitive to temperature variations can also be used at the B-bearing side.

• High permissible friction work

• Customer specific applications

Brakes for Servo motors

Today, servo motors are being used in manifold applications with various requirements. Initially, however, when servo motors came to market, no absolute positioning encoders were used and zero-backlash was an important requirement. Because permanent magnet brakes offered significantly less backlash than conventional spring-loaded brakes, those permanent magnet brakes have often been used for servo motors. Whereas conventional spring loaded brakes have often unacceptable backlash and unsuitable material for higher temperatures, permanent magnet brakes have also a few weak points in their design.

Disadvantages of permanent-magnet brakes

Permanent-magnet brakes have a few weak points in their construction. When braking from speed there is a mixture of friction work between iron/iron and iron/friction lining contact. This causes partially strong friction value fluctuations and results in low permissible friction work and friction capacity, which is typical for this type of friction pairing. Another serious disadvantage is sensitivity to overvoltage conditions, which can cause demagnetisation and damage of the permanent-magnets.

The technically superior concept

The new ROBA®-servostop®does not have these disadvantages of brakes with permanent-magnets. Torque is transmitted from the motor shaft via the spline to the rotor, which is designed with special organic friction linings. These temperature resistant linings have an impressively high and uniform friction coefficient, and can be used with high ambient temperatures - up to 120 °C. The ROBA®-servostop®is electromagnetically released and actuated via spring pressure. In the case of a closed brake (power off) the rotor, with friction linings, is kept between armature disc and brake plate holding the shaft. If the electromagnetic coil is energised the magnetic field attracts the armature disc to the coil carrier against the force of the springs. The brake is released and the rotor is able to rotate.

Backlash is no issue

As most modern servo motors are designed with an absolute positional encoder, the subject of "backlash-free" designs has moved into the background. An insignificant amount of play is allowable and is compensated via motor matching. In spite of the possibility of play compensation, it is a requirement of the new brake to minimise backlash in order to guarantee high wear resistance.

Assembly errors excluded

Some relatively expensive work can be avoided by using this simple and robust design. The operating-air gap does not have to be adjusted. Also no exact axial positioning on the motor shaft is necessary. The brake always works accurately and reliably with a constant air gap and is not influenced by bearing play and temperature expansion. The impact on assembly is therefore substantially smaller than with permanent magnetic brakes. This saves costs and additionally, assembly errors are almost excluded.

Mounting on either A or B side

Usually servo brakes are preferably installed in the A-bearing housing of the motor, as the support bearing is located here and temperature expansions cannot seriously influence the brake. However, one drawback with this location is that the connection cable of the brake must be guided through the internal body of the motor to the rear and into the terminal box. The ROBA®-servostop®can also be integrated in the B-bearing side of the motor without restriction, as temperature expansions and bearing play do not have any negative effect on function and reliability of the brake.

Servo drives – controlled drives

•Controlled operation

In a controlled operation the servo motor undertakes acceleration/deceleration as well as holding of an axis in a neutral position.

Task of the brake

In a controlled operation

Only if the drive is switched off or the servo motor is out of control, does the brake hold the axis in position => holding operation for the brake e. g. holding of a load of a vertical axis with a switched off drive

Emergency – OFF situation (not controlled)

e. g. power failure until standstill In this case the brake undertakes the deceleration of the axis => friction work for the brake

Emergency – OFF situation (controlled)

e. g. Actuation of the emergency – switch OFF The servo motor controls the deceleration of the axis