Technical Article October 2016
Understanding spring applied disc brakes
Electromagnetic brakes for dynamic, holding and emergency stop duties
Spring applied brakes with electromagnetic release are commonplace and often overlooked. More than two million are manufactured each year in Europe for industrial machinery. Torque ratings range from 0.1 to about 1500Nm although most fall in the range 2 to 150Nm. Typically they are found as part of brake motors, on hoists, lifts, wind turbines and electric vehicles. In this article we focus on the most common design type – single disc with DC coil actuation – and the function, operation plus the options that are available to customise brakes for particular duties.
A typical brake sectioned
The key part of the brake for generating torque is the rotor, also known as the brake disc. Generally this is an aluminium disc with friction material bonded to each side. The rotor is connected to the shaft that is to be stopped using a sliding spline connection onto a hub that is fixed on the shaft. The splines allow the relatively small amount of axial movement required during operation. On one side of the rotor is a fixed mounting surface, on the other is a guided and floating steel plate called the armature. The armature is pressed by multiple coil springs onto the rotor. This results in the brake torque being generated between the rotor and the two opposing surfaces: the armature and the mounting surface.
Brake release occurs by generating magnetic flux in a part called the stator which houses a DC coil. The magnetic flux pulls back the armature against the opposing spring pressure, through a small air gap, releasing the rotor to run freely. The process reverses for brake engagement: the DC current is interrupted, the springs push the armature plate back onto the rotor and the brake torque is generated again. The stator coil is continuously rated for a voltage of 24V DC suiting battery supply or low voltage control circuits, alternatively at about 190V DC for rectified AC supplies.
Spring applied brakes are often used in safety critical applications, for example holding loads in cranes and hoists, or achieving a safe stopping distance with moving machinery. Previously this type of brake has sometimes been called failsafe. However modern safety engineering recognises that a brake that is badly maintained, misused or incorrectly sized will not be safe. Thankfully such problems are rare. Today spring applied brakes can be properly engineered into machine safety systems and B10 safety data is available from some manufacturers. Higher safety requirements have led to a number of variants of spring applied brakes such as double brakes in theatres, brakes with twin operation systems for lifts and multi-
The design basics of spring brakes have changed little over the last 40 years. They are proven and reliable. However increasingly in recent years designs have been adopted with options to suit niche markets. Such options are readily available and can sometimes be used to advantage in other areas.
Long life brakes
Developed to minimise maintenance in cyclic and reversing applications such as found in intralogistics order pickers, long life brakes can extend the standard life from 1 million to 10 million operations. Standard brakes suffer from mechanical wear so this modification changes the guidance for the armature plate movement, also adds a plastic coating to the rotor splines that takes out backlash.
Brakes for extreme ambient conditions
The requirement for higher reliability in extreme temperatures and variable humidity comes in part from the wind turbine industry. Here brakes are used on the azimuth drives that point the blades into wind, and on blade pitch drives. Standard brakes risk jamming from corrosion and infrequent use which can results in unacceptable maintenance costs. Instead a modification called CCV (Cold Climate Version) is used with chrome plated rubbing surfaces plus other changes giving reliable operation over an extended range of ambient temperatures -
Brakes for electric vehicles
This growing market includes factory trucks, AGVs and commercial electric road vehicles. Requirements are varied leading to the development of special motor brakes, wheel brakes, back-
Brakes for passenger lifts
Here the highest levels of brake safety are required. Whilst the braking for normal stopping duties is generated electrically in the drive, spring applied brakes are used as a back-
Spring applied brakes have a range of further options in common use:
a manual brake release is sometimes required to allow maintenance although it can have safety implications. It can be fitted as an optional extra to most models of brakes.
brake friction material needs to be kept free of water and oil contamination so a high enclosure rating is required in certain environments. IP65 and IP66 are possible.
In recent years optional variants of brake friction material have become available. Examples are material that needs no running in, long-
Brake engage and release times are defined by the current rise and fall in the DC coil. Typically this gives brake engagement times from 30 to 200ms depending on brake size and release times up to 350ms. Using electrical control options it is quite easy to reduce the release times by applying a short term overvoltage. Engage times can be reduced by setting a lower holding voltage but manufacturer advice should be sought.
Spring applied electromagnetic brakes may seem at first glance to be simple devices, but in reality they are high developed and sophisticated. They can play an important role in the safety systems of modern high-
The company INTORQ is a world leader in spring applied brakes with manufacturing plants in Germany, USA and the Far East that produce over 800,000 units per year. Their main markets are in brake motors, industrial trucks, wind turbines, cranes, hoists and passenger lifts.
The INTORQ range is sold in the UK through Techdrives, http://www.techdrives.co.uk/spring_applied_brakes.html