Brake it – don’t break it!
04 May 2014
What should you be looking out for when specifying spring applied electromagnetic release brakes for your application? Here are a few hints and tips from a prominent UK supplier.
Basic designs of spring applied electromagnetic release brakes have become standardised in a proven, highly reliable form that is also compact and economical. For example, they will have multiple coil springs, a well-developed non-asbestos friction material, a dc coil and torque typically in a range from 2 to 2,000Nm.
But while these suit most requirements, certain types of machines have stepped up their braking requirements based on the need for higher reliability.
There are two particular areas where a higher level of braking integrity might be needed: severe environments such as found with dockside cranes, cold stores and wind turbines, and those instances where industry standards require increased safety, such as with theatre hoists, passenger lifts and cranes in the nuclear industry.
In most cases brakes are required for emergency stop and holding duties, and customers will seek a very high level of reliability as well as failure modes that do not lead to disaster.
Where the environment is severe in terms of temperature extremes, humidity and the like, attention must be paid to the brake materials and enclosure. Electromagnetic spring applied brakes are designed to operate with friction surfaces that are clean and dry.
For dockside use, an enclosure rated toIP66 is appropriate. The outer casing of the brakes also needs to resist salt water corrosion. A problem area can be the provision of a manual release which allows the brake to be disengaged when power is off. The release mechanism requires two rods to pass through the casing but there are now brake models that achieve this while still retaining the IP66 rating.
Low temperatures also require special brake materials. Wind turbines are a particular environment where extreme temperatures are encountered. A special brake variant called CCV (Cold Climate Version) has been developed to suit a range from -40 to +40ºC.
The low temperatures bring the probability of condensation so the metal friction surfaces are chrome plated and the brake fixing bolts are manufactured from low-temperature resistant steel grades. These specifications are suitable for cold stores, particularly for braking electric vehicles that have to travel inside and outside of the cold area.
Where the requirement is for increased safety, there are a number of options supported by ‘B10’ safety data. The B10 rating denotes the number of operations before a 10 percent failure rate occurs according to DIN EN ISO 13849-1. Designers use this data to calculate the Safety Integrity Level (SIL) or Performance Level (PL) of a machine.
Spring applied brakes are available with B10 ratings typically in the range of three to ten million operations; exceptionally, 45 million is achievable. Failure modes are usually mechanical due to wear and the first step to increase the level of safety is to monitor the brake operation using microswitches.
These can supervise the on/off status, the amount of friction wear and the position of the manual release. Another simple option for increased safety is to piggy-back two brakes together so that if one fails there is still 50 percent stopping torque available. However, care is needed to ensure the lower level of torque is adequate, particularly with falling loads.
Brakes designed for passenger lifts meet regulations such as TRA200 and EN 81 by mechanical modifications to the basic design. There are models with dual electric circuits and split armature plates that ensure that at least 50 percent torque is available in the event of a failure.
Where this level of decreased torque is a problem, the solution lies in the use of spring applied calliper brakes. Here a number of independent brake callipers act on a rotating steel disc, and medium-to-high torque is possible. By way of an example, if five callipers were used and one failed, the drop in available torque is only 20 percent. Note that the B10 rating for a pair of spring applied calliper brakes is around six million operations so any such failure is unlikely.
Readers wishing to learn more about these various brake systems might care to consider the InTorq range of spring applied brakes, which is available in the UK from Techdrives. These include standard models, brakes with IP66 rated enclosures, CCV variants and lift brakes.
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