Electric Brakes Selection Guide: Types, Features, Applications | GlobalSpec

Electric brakes are devices that use an electrical current or magnetic actuating force to slow or stop the motion of a rotating component. They are used in industrial and vehicular braking applications that require fast response times and precise tension control.

 

There are two main types of electric brakes: magnetic and friction. Each has various subtypes. As described below, the way an electric brake works depends upon these characteristics.

 

In addition to type, the GlobalSpec SpecSearch database allows industrial buyers to search for electric brakes by operating specifications, engagement mechanism, measurements and shaft configuration, brake materials, and features.

 

 

Types of Magnetic Brakes

 

Magnetic brakes are non-contact brakes that use magnetic fields to actuate the braking components. There are four types.

 

Permanent Magnet Brakes

 

Permanent magnet brakes stop or hold a load when electrical power is either accidentally lost or intentionally disconnected. They are sometimes called “fail safe” brakes and use a permanent magnet to attract a single face armature. As the brake is engaged, the magnets create magnetic lines of flux, which can turn to attract the armature to the brake housing.  To disengage the brake, power is applied to the coil, which sets up an alternate magnetic field that cancels out the magnetic flux of the permanent magnets. Permanent brakes are engaged when no power is applied to them and can hold or stop when power is lost or unavailable.

 

Design Tip: Multiple disks can also be used to increase brake torque, without increasing brake diameter.

 

Electromagnetic Brakes

 

Electromagnetic brakes have a coil in a shell, a hub, and an armature. An electrical circuit engages the brake as it energizes the coil. The current runs through the coil and generates a magnetic field. The magnetic flux acts directly between the armature and field. The armature is pulled into contact with the rotor when the magnetic flux overcomes the air gap between the armature and field. All of the torque comes from the magnetic attraction and coefficient of friction between the steel of the armature and the steel of the brake field. Deceleration occurs when the armature contacts the field, and the torque transfers into the field housing and machine frame. Turing off power causes the flux to fall rapidly, the armature to separate, and disengagement to occur. Springs are used to help push the armature away from the surface and maintain an air gap.

 

Eddy Current Brakes

 

Eddy current brakes develop torque by the direct magnetic linking of the rotor to the stator.  A magnetic field induces a voltage in moving objects and the induced voltage causes an eddy current to flow in any conducting objects. The electrical current is sent to coils, which alternate polarities, creating an electromagnetic field. This change in magnetic flux induces a small circulating current in the conductor called an ‘eddy current’.

 

Eddy currents are generated in two rotors as they spin through the field and slow the rotation of the driveshaft.  The first current created generates an opposing current.  The counter-opposing flux and Lorentz force reduces the velocity of the object. Ohmic losses and significant heating are also produced by the current. Eddy current brakes must have a slip between the rotor and the stator to generate torque.

There are two types of Eddy current brakes.

 

Rotational or circular brakes are connected to a rotating coil and magnetic field between the rotor and the coil, creating a resistance that’s used to generate electricity. A braking force is possible when electric current is passed through the electromagnets.

 

Linear eddy current brakes consist of a magnetic yoke with electrical coil positioned along the rail. The coils are magnetized, alternating as south and north magnetic poles.

 

Hysteresis Powered Brakes

 

Hysteresis powered brakes have a wide torque range. They have a reticulated pole structure and a specialty steel rotor/shaft assembly that are fastened together, but not in physical contact. The drag cup can spin freely until the field coil is energized by a current/voltage, creating an internal magnetic flux. The air gap between the pole structure and the rotor becomes a flux field and magnetically restrains the rotor. This provides the braking action between the pole structure and the rotor.

 

When electricity is removed from the brake, the rotor is free to turn, and no relative force is transmitted between either part. Torque is only produced through a magnetic air gap that does not use friction or shear forces. Control over torque is done through the DC current to the field coil. The amount of braking torque transmitted by the brake is proportional to the amount of current flowing through the field coil.

 

Magnetic Brake Comparison

 

This table compares the different types of magnetic brakes.

 

Type

Application

Advantages

Disadvantages

Permanent magnetic brake

For electric motors, robotics, holding brakes for Z axis ball screws and servo motor brakes

High and accurate torque, long life, unaffected by power supply, safe and easy to use

Require a constant current control to offset the permanent magnetic field

Electromagnetic brake

Copy machines, conveyor drives, packaging machinery, printing machinery, food processing machinery and factory automation

Fast response time, smooth, reliable, and backlash free operation, produce high torque, automatic air gap available

Braking force diminishes as speed diminishes, load cannot be held at a standstill causing safety concern. 

Eddy current brakes

Train and roller coaster brakes

Noncontact, Frictionless, resettable, light weight, few moving parts

Unusable at low speeds, generates heat

Hysteresis powered brakes

Food and drug packaging operations, clean rooms, environmental test chambers, load simulation for life testing on rotating devices, capping, bolting and other screwing applications

Long, maintenance-free life, cost effective, operational, smoothness, torque repeatability, broad speed range, environmental stability, high-dissipation capability. The torque remains constant and smooth and responds with increases and decreases in current.

Experience a salient-pole phenomenon called “cogging”, an undesirable, pulsating output torque which prevents smooth and efficient operation of these systems

 

Types of Electrically Actuated Friction Brakes

 

Although many electric brakes use mechanical methods for actuation, others rely upon friction. There are several types of frictional brake devices. Each is described below.

 

Band Brakes

 

Band brakes are the simplest electric brake configuration. They are often used as a back-stop mechanism to prevent reverse rotation. These brakes have a flexible band of leather, rope, or steel with a friction lining that is wound around a rotating drum. One end of the band passes through the fulcrum of the actuating lever and frictional torque is then generated when tension is applied to the band. The band will lock up the brake for rotation in one direction and when friction is placed on the band, it loosens for rotation in the opposite direction.

 

Band brakes are often used in lifting applications to prevent the object being hoisted from falling when the user stops pulling. The ratio of band tensions is given by

Where, T1  = tension in the taut side

             T2 = tension in the slack side

             µ  = coefficient of kinetic friction

             β  = angle of wrap

If the band is wound around a drum by a radius R, then the braking torque is:

 

 

Drum Brakes

 

Drum brakes are commonly used on automobile rear wheels. They operate by forcing the friction-lined brake shoes against the inner surfaces of the rotating drums. A drum brake has two brake shoes, a piston, an adjuster mechanism, and an emergency brake mechanism and springs. The shoes expand against the inside surface of the brake drum, and slow the wheel down. The harder the linings are forced against the brake drum, the higher the braking force that is applied.

 

Many drum brakes are self-actuating, which means that shoe mounting can be designed to assist in their own operation. The self-actuating mechanism uses a wedging action to assist the lining to grip the rotating drum when the brakes are applied.  This extra braking force allows drum brakes to use a smaller piston than disc brakes. The springs are used to pull the shoes away from the drum when the brake is released, as well as to help hold the brake shoes in place and return the adjuster arm after it actuates.

 

Three drum brake designs are generally used.

 

Single leading shoe designs use a single wheel cylinder with two pistons. As the brakes are applied, both shoes (leading and trailing) press against the brake drum. The leading shoe is self-actuating, while the trialing shoe is forced off the drum. This arrangement works well going forward and reverse. 

 

Twin leading shoe designs have an actuator for each brake shoe

 

Duo-servo designs use a single wheel cylinder with two pistons with a high self-actuating force. Since the lower ends of the shoes are linked but not firmly anchored to the backing plate, the shoe floats within limits. As brakes are applied, both shoes are carried around the drum, until the secondary shoe contacts the anchor pin and the self-actuating force of the primary shoe is transferred to the secondary shoe through their lower linkage. Force is applied to the secondary shoe from both ends, causing the wheel to slow. This design is common on rear wheels and it works in both the forward and reverse direction.

For the drum brakes to function correctly, the brake shoes must remain close to the drum without touching it. As the shoes wear down, they can get too far away from the drum and the piston will require more fluid to travel that distance. To correct this, an automatic adjuster is used to fill the gap created as the brakes wear.

 

 

Design Tip: With hard use, brake “fade” can occur eventually. Brake fade is the gradual loss of brake stopping power during prolonged or strenuous use. Very high temperatures occur at the brake drum, and that causes deterioration in the frictional value of the lining or pad material. This is common in drum brakes.

 

Disc Brakes

 

Disc brakes consist of a caliper that squeezes brake pads against a rotating disc. They can be used on all four wheels of a vehicle or on the front brakes in conjunction with rear drum brakes. The most common type of disc brake is the single-piston floating caliper. The main components of this type are the brake pads and the caliper, which contains a piston; and the rotor, which is mounted to the hub. On driving wheels, the disc is mounted onto the driving axle and may be held in place by the wheel.

 

The brake caliper system is attached to the vehicle axle housing or suspension and the brake is usually attached as close to the wheel as possible. When no current is flowing in the coil, the motor is braked by two compression springs squeezing the brake pads and the brake rotor together. Friction between the pads and rotor slows the disc down. When current flows into the core, it counteracts the piston force, pulling the brake pads towards it, and the brake is released. A lot of friction is generated during braking, and this creates heat in the system. Since the brake parts must be able to withstand high temperatures and are often exposed to air and vented, cooling is much faster than for drum brakes.

 

The single-piston floating-caliper disc brake is self-centering and self-adjusting. Self-centering means that the caliper can slide from side to side so it will move to the center each time the brakes are applied. The pads always stay in light contact with the rotor so less force is needed to engage the brakes.

 

Cone Brakes

 

Cone brakes include a cone that is lined with heat- and wear-resistant material that presses against a mating cup surface. They have a cup and a cone, which is lined with a heat- and wear-resistant friction material. During actuation, the cone is pressed against the mating cup surface. Cone brakes are not commonly used.

 

Friction Brake Comparison

 

This table compares different types of friction brakes.

 

Mechanism

Application

Advantages

Disadvantage

Band

Automatic transmissions, backstops, bucket conveyors, hoists

Simple, inexpensive, easy to make, reliable, low maintenance

Poor heat dissipation capacity, wear friction lining is uneven from one end to the other.

Drum

Rear automobile brakes

Shoe mounting can be designed to assist their own operation (self-actuating), don’t need a break booster

The friction area is covered by lining, so most of the heat must be conducted through the drum to reach the outside air to cool, difficult to get water out of the drum if driven through water, not as efficient in reverse

Disc

Automobile brakes, often front wheel

Brake fade is rare, can operate after being exposed to water

Generate high heat from friction, no self-servo effect

 

Friction brakes are used in industrial applications such as in agriculture, ATVs, aerospace and construction equipment, or mining, milling and manufacturing projects.

 

Operating Specifications

 

Specifications for electric brakes include:

 

Torque rating is the turning force of an object. The maximum torque rating for the brake should be greater than or equal to the application’s requirement. To determine this requirement, the following equation can be used.

 

Where,

T = full-load motor torque (in lb-ft)

5252 = constant (33,000 divided by 3.14 x 2)

HP = motor horsepower

Rpm = speed of motor shaft (rotations per minute)

 

Power is defined as the rate of doing work. For rotational power, such as a brake, the calculation can be done by:

 

Where,

Prot = rotation mechanical power

M = torque

ω = angular velocity

 

It is important to consider the units involved when making the power calculation. Such a reference is used to convert the torque-speed product to units of power (Watts). Conversion factors for commonly used torque and speed units are given in the following table.

 

Torque Units

Units Speed

Conversion Factor

oz-in

RPM

0.00074

oz-in

rad/sec

0.0071

in-lb

RPM

0.0118

in-lb

rad/sec

0.1130

ft-lb

RPM

0.1420

ft-lb

rad/sec

1.3558

N-m

RPM

0.1047

 

 

Speed rating applies only to rotary brakes

 

Operating voltage is the input voltage that must be supplied to the electric brakes. Electric brakes can be designed to run on either AC or DC power. A wide range of input voltage specifications is available to suit various applications.

 

Brake capacity depends on five factors:

• Unit pressure between brakes

• Contacting area of braking surface

• The radius of the brake drum

• The coefficient of friction

• The ability of the brake to dissipate heat that is equivalent to the energy being absorbed.

Engagement Mechanisms

 

There are a variety of engagement methods for electric brakes. 

• Non-contact brakes use methods such as magnetic fields and eddy currents.
• Magnetic particle brakes have a space between the coil and the shaft, which is filled with magnetic powder. The brakes engage with the application of an electric current to stainless steel particles in the gap between the output disk/shaft assembly and the housing. As the current is increased, the magnetic flux becomes stronger and increases the torque produced.

• Friction brakes generate friction between contact surfaces.

• Wrap spring brakes transmit torque from the input to the output through a wrapped spring that uncoils to disengage the brake.

• Electric brakes with teeth engage only during stops or at slow speeds.

• Oil shear brakes achieve engagement through the viscous shear of transmission fluid between the brake plates.

• Spring-return brakes require power to engage. The spring engages during operation and requires power to disengage. Spring-actuated brakes are also called power-off brakes, fail-safe brakes, and safety brakes.

• Spring-actuated brakes require power to disengage. A spring is used to disengage the brake. Spring-return brakes are also called power-on brakes and non-fail-safe brakes.

Brake Measurements

 

Selecting electric brakes requires an analysis of measurements and mounting configurations. Important measurements include:

• Diameter — The bigger the rotor, the less force required to apply to the brake. The smaller the rotor, the more pressure or force needed for the brake (compared to the larger rotor) to stop.

• Length — The dimension along the axis of rotation.

• Weight — The weight of the brake.

Brake Material Properties

 

Desirable properties for friction materials/linings include:

• A high coefficient of friction.

• Resistant to wear effects, such as scoring, galling, and ablation.

• Constant friction value over a range of temperatures and pressures

• Resistant to the environment (moisture, dust, pressure)

• Good thermal properties, high heat capacity, good thermal conductivity, ability to withstand high temperatures and contact pressures

• Good shear strength to transferred friction forces to structure

• Safe to use and safe for the environment

This table describes important properties of materials used for brakes and clutches.

 

Material Combination

Coefficient of Friction

Temp.(max)

Pressure (Max)

Wet

Dry

Deg.C

MPa

Cast Iron/Cast Iron

0,05

0,15-0,20

300

0,8

Cast Iron/Steel

0,06

0,15-0,20

300

0,8-1,3

Hard Steel/Hard Steel

0,05

0,15-0,20

300

0,7

 

Shaft Configurations

 

Brakes are mounted on a shaft which, can come in three configurations:

 

• In-line shafts are along the axis of the load

• Parallel shafts are parallel to the load but not in-line with it.

• Perpendicular shafts are at a right angle to the axis of the load

Special Features

 

Electric brakes are available with a variety of special features. Some devices use:

• Electrical or electronic signals to monitor parameters such as position, speed, torque, lockup, or slip status.

• Prevention of play or backlash during load engagement and prevent direction reversal during load disengagement.

• Washdown-capable electric brakes use housing materials that are rated for washdown cleaning.

References

 

Advantages of Hysteresis Devices

 

Bhandari, V. B. Design of Machine Elements. New Delhi: Tata McGraw-Hill, 2007. Print.

 

Brakes

 

Hysteresis Brakes and Clutches (pdf)

 

Industrial Clutch/Brakes: How they work

 

Module 12: Design of Brakes (pdf)

 

Orthwein, William C. Clutches and Brakes: Design and Selection. New York: M. Dekker, 2004. Print.

 

What’s Inside: Brakes

 

 

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Ogura | PrecisionTork | Scioly | How Stuff Works

 

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