Springing into action

Because brake-engaging springs immediately jump into action when power is cut, spring-set brakes are especially suitable for backup and protection applications. Widely used for static holding, general load stops, and occasional emergency stops, they’re also effective where VFDs, servos, and stepmotors slow loads first. While some use a diaphragm or bladder arrangement, most fluid power and electrically released types use springs for clamping when power is removed. The latter can be categorized into two groups.

Electrically released armature-activated brakes use springs to clamp the rotor (with attached friction pads) between a stationary end plate and a non-rotating armature. In these units a simple, fixedvoltage power supply keeps the brake released. With power applied to the brake coil, its magnetic force attracts the armature to compress the springs and release their hold.

Generally coils are rated for continuous duty and can be continually energized without overheating. When coils do heat up, they get hottest at coil engagement due to engaging, or pull-in amperage. For UL Class A insulation, coil temperatures shouldn’t exceed 105°C; Classes B and H limits are 130° and 180°C respectively. A high ambient temperature where the brake is actually mounted may limit the coil cycle rate. A coil temperature rating should include the wire, tape, insulation, and any coating materials.

Solenoid-actuated brakes are the second type of electrically activated springset brake. They come with either single or dual-voltage coils and can be wired to either single or three-phase motors. Compared to armature-actuated brakes, solenoid-actuated brakes have a crisp electrical response time. However, instantaneous rated voltage must be supplied to coils to ensure proper solenoid pull-in and maximum coil cycle rate.

It’s important to remember that these brakes must be wired separate from drives for proper control of each. Solenoid-actuated brakes are designed to a typical NEMA voltage range of 10% of the rated voltage and frequency. Equipment and system installers accustomed to typical NEMA design B motors may try to wire the coil across the motor windings; however, by design VFDs vary frequency, and that doesn’t work with a fixed-frequency product. An installation rule of thumb: Confirm wiring before powering. For example, suppose a brake is mounted to the accessory end of a drive intended for field-wire. An installer, familiar with standard motors, might assume the brake has been pre-wired, and begin to troubleshoot a faulty operation. In fact, in this situation the brake is not getting any electrical power. To prevent these misunderstandings, some drive manufacturers have a fixed voltage tap in their power box, which is a convenient arrangement for wiring a brake into the motor power box.

How big?

Emergency brakes cannot protect if they’re improperly selected; ISO and ANSI safety factors must be applied. So exactly how powerful should a specified brake be? It depends on whether the desired reaction is a very rapid emergency stop, or softer stop that sacrifices brakes to prevent disturbing loads or the overall system. Establishing maximum as well as typical conditions — from typical situations to worst-case scenarios — also influence requirements. The entire system (and not just one component) must have enough torque to meet the dominant prescribed condition: braking, clutching, tensioning, or limiting. The trick is knowing that condition ahead of time.

The easiest and most common method is to size a brake’s dynamic torque to the system’s motor torque. This technique does have limitations; motors can draw extra current for short bursts to provide more torque than their 100% rating. (This is especially important to remember for vertical applications, as brakes must be able to stop any falling of whatever items the motor lifts.) Even so, sizing to the system’s motor torque is a suitable method for the majority of cases.

Whether torque selection is robust or lean depends on design Driving a conveyor system intent. Because of the potential for fatigue or early wear of other system components (such as gearboxes or leadscrews) overdesigning is actually just as risky as under-designing. Specific industry standards — for automotive, packaging, medical, and general material hoisting applications — play a role in the type of planning required. In fact, each industry has different recommended sizing schedules. Motor manufacturer’s performance curves can be consulted for dynamic torque ratings at operating speeds that meet or exceed peak torques developed by the motor. Sometimes if the brake is small enough, the motor and load can drive through the brake.

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When an application’s’ maximum conditions involve extreme speeds, brake selection must be re-evaluated against recommended maximum rpms for balance and resonance. Different from stopping rpm, freewheeling rpm is the typical speed to which a drive can ramp. In some designs, the drive manufacturer may permit some overspeed rpm beyond the design rpm. In those situations it’s safer to size to stopping — not maximum — rpm. Stopping rpm can be a lower rpm or even zero for holding- only uses.

After an over-speed or fullload stop following the loss or removal of power, maintenance in the form of part replacement or inspection may be needed.

Friction interactions

Emergency stopping is generally a safety issue, but not always. When someone has caught clothing or body parts in a rotating piece of equipment, the hazard is obvious. However, less pressing situations can be dangerous too. Maybe misalignment is developing, a workpiece has dropped in upside down, or a load has shifted. Friction brakes are by far the most common solution for meeting these stopping requirements because their operation relies on offline mechanical mechanisms. There is a possible drawback to these brakes, though: The relative motion of slippage heats and changes friction materials, and can result in accelerated wear and torque loss. However, mating parts and friction materials themselves can be tweaked for improved performance if an application’s stops are predictable. New materials allow for technological advancements in these brakes: for example, compounded formula phenolic resin-based discs absorb more heat to improve part life and heat dissipation efficiencies. Spring-set brakes can also include an automatic wear-adjustment feature to keep brakes going strong even after they show some wear. In addition, specialty industries have spurred specialized design elements such as metal-to-metal disc contact, oil baths, and ceramic discs. These all aim to improve brake friction action.

Quick facts

• Torque is a measure of rotational force and in the case of a brake, is a retarding force.

• Emergency brakes functioning in failsafe use prevent shaft rotation when electrical power is cut off. When power is restored, brakes release.

• A user must know if an emergency brake is to be used once a year, once an hour, or at some interval in between. If a brake will not be used for extended periodsof time, the enclosure and related modifications can be adjusted. Brake selection should also consider the brake location and accessibility for maintenance and enclosure choice.

• Emergency brakes are everywhere — on escalators, airport baggage handlers, airport gate bridges, and even medical lifttables, or AGV. However, it’s important to prevent over-reliance on emergency stopping.

• Because most brakes require low current to keep brakes in the released position, the response time to set can be affected by emf voltages generated by motor windings. If this does occur, it may be necessary to isolate the brake coil from the motor winding and wire separately.

• Spring-set brakes are applied mechanically, so it’s easy to equip them with manual releases.

When it must stop

Variable frequency drives (VFDs) are growing in use for speed control. (A VFD determines rpm and can ramp loads down to zero rpm.) However, brakes remain crucial in applications when safety and compliance are concerns. A VFD can signal the motor to stop rotating, but an electromechanical brake can hold a load or rapidly stop and hold its gearbox and load when required. In fact, applications must often be fitted with rapid-stopping brakes to keep UL agency markings and to meet CE requirements; they are obligations already in place in many industries.

Besides quick stopping, emergency friction brakes guarantee secure loads at zero rpm so that nothing slips or causes workpiece waste, or worse, personal harm.

For more information, call the Stearns Division of Rexnord Corp. at (800)488- 9078 or visit www.rexnord.com/stearns.