05 June 2017
If you need to hold a load, you need a brake. The question is which type of brake and why?
The first thing you need to decide is whether you want the brake to be applied when there is no power, or when power is applied. In most cases you want to hold the load when power is off, but there are applications like gates and turnstiles, where the brake must disengage in the case of a power failure. Power-to-energise brakes are also used in process control often in conjunction with a clutch to give a stop-start action. However these applications are increasingly being solved with Servos or steppers, removing the need for brakes altogether.
For power to release applications, there are two main technologies available, spring applied or Permanent Magnet (PM) brakes. The majority of brakes are spring applied fail-safe brakes, this is because a spring applied brake is a rugged, low cost, solution that will work over a wide temperature and voltage range. Extras such as a hand release, or wear indicator microswitch can be easily added. Traditionally theses brakes have been designed for dynamic operation, but now brakes can be specified for holding. In the case of only emergency stop applications, the torque density can be increased by 50 percent, providing better value and space saving when the brake is used in conjunction with a motor under Inverter or Servo Control.
With PM brakes, the holding force is provided by high power rare-earth magnets. The brake incorporates an electromagnetic coil, this is used to cancel out the flux from the permanent magnets, releasing the load when powered. The advantage of PM brakes is a superior torque density, faster response, and backlash free operation. This makes them a popular choice for incorporation into servo motors and for precision functions such as robotics, medical, military and laboratory applications. The disadvantage over spring applied brakes is higher cost, and difficulties in operating over an extended temperature and voltage range. The design of a system with a PM brake needs to consider the armature mounting arrangement carefully because if the airgap between the armature and the magnet is exceeded, then the brake may not operate as “Fail-Safe”.
Brakes are sized on torque so it is usual to place the brake at the high speed end of the power train, often on the back of the motor, or within the casing of a servo motor. If this cannot be achieved or for safety reasons the brake needs to be at the low speed high torque end then Caliper, Drum or Multidisc types are used on larger systems. This article does not cover these brake technologies. For applications up to approximately 1000Nm, standard spring applied and PM brakes can be used successfully across all points in the drive system.
Calculating the optimum brake torque size, needs to take into account the torque needed to stop or hold the load, the safety factor and the operating cycle. Care needs to be taken to ensure the brake is not over sized, particularly for dynamic applications, as the inertia of the brake itself can become significant. Bigger is not always better. Another consideration is duty cycle; Friction brakes produce heat every time they stop the load, so for stop/start applications it needs to be taken into account, particularly when large inertias are involved.
Short-cut brakes are often sized by comparing their torque to the motor torque, but this is not ideal as it can lead to “Safety Factor Inflation” where additional margin is added at every stage of the design, as well as adding unnecessary cost, it can produce a design that is inherently less dynamic and in extreme cases failures can be caused by the shock load of an oversized brake being applied in an emergency stop situation.
With correct sizing an electromagnetic brake can produce a safe trouble-free machine, as long as the appropriate technology is chosen, and the design is correct.
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