Tips for Selecting Motor Brakes

July 24, 1998
Some helpful guidelines can put an end to brake selection dilemmas.

Pat Watson
Application Manager
Stearns Div., Rexnord Corp.
Milwaukee, Wis.

Wherever electric motors are used, electromechanical brakes are usually not far away. Applications range from electric wheelchairs and positioning drives to automated storage systems and conveyors.

Electromechanical brakes fall into two main categories based on their method of actuation — electrically set and spring set. In electrically set brakes a magnetic coil provides actuation or “set” force, so power must be continuously supplied to ensure brake torque. As the name implies, springs provide the actuation force in spring-set brakes, so the brake applies torque anytime power is disconnected.

Spring-set brakes are well suited for most requirements and are traditionally used for rapid stop and hold applications. They are increasingly used as fail-safe holding brakes on inverter or vector drives where the drive controls speed, and the spring-set brake holds a load stationary. Another growing application for spring-set brakes is on exported machinery governed by European safety standards.

Electrically engaged brakes are generally selected for high-cycling applications and are often combined with a clutch. These brakes also may be suitable for soft-release or soft-engagement applications.

Most engineers select a brake based on the application and then turn to sizing calculations to finalize the design. Here’s a look at some factors that must be considered.

Cycle rate describes how frequently a brake is used. Because brake friction generates heat, cycle rates affect thermal sizing and selection. The physical structure and construction materials affect the efficiency with which a brake stores and transfers heat to the environment. Demands such as jogging, repetitive proof-load testing, emergency stops, or temperature extremes can also affect brake size. For rapid cycling, a clutch or clutch-brake is a good design alternative and may be used occasionally with an overexcitation control.

Environmental conditions such as heat, cold, and moisture can mean a quick death to the wrong brake. Fortunately, an IEC “Index of Protection” (IP) or NEMA rating can take some of the guesswork out of the specification when the brake must work in less-than-ideal environments. Many variations of standard brakes are available to meet specific needs, such as those with built-in space heaters, coil insulation, coil encapsulation, alternative materials such as stainless steel or brass, epoxy paints, and special gasket materials.

Power-supply considerations also affect brake selection, beginning with the power available where the brake will be installed. Length and sizing of power line runs may result in enough line drop and voltage variation to affect a brake’s operation. In turn, the brake may affect sensitive equipment sharing a common power supply.

Brakes with a coil that can be changed on-site by maintenance staff provide greater design flexibility and reduce part stocks. Some variable-speed motor manufacturers supply motors with taps for a dedicated voltage brake.

Although brakes generally are supplied with motors, wiring a brake through the motor increases reaction time. Wiring to a dedicated relay separates the brake electrically and speeds operation. On the other hand, the appropriate wiring and a timer circuit prevent a too-rapid stop. Consider this option in overhauling-load or high-slip motor applications.

Mounting brakes to the high-speed shaft, ahead of a speed reducer, is generally recommended by brake manufacturers. Mounting on the low-speed shaft increases the braking torque required which usually means a larger, more costly brake. Also keep in mind that a brake is but one component of a system. Excessive brake torque, applied dynamically, can strip or overheat a gearbox, cause torsion failure of a shaft, and introduce shock loads. Systems that require redundancy can use caliper, air, shoe, or oil brakes near the load, partnered with the smaller friction brake mounted on the motor.

Space and accessibility are both selection factors. Many brakes are designed to fit the C-face register of an electric motor. An ideal package is a close-coupled C-face mount brake, mounted by the motor manufacturer and sold as a package. Alternatively, brakes can be free standing, shaft mounted, or coupled to the motor drive end or a parallel high-speed gearbox shaft.

Noise is another concern, given that machinery operating in close proximity to workers must meet OSHA and other regulations. Several methods can reduce brake noise. For instance, cast-iron brake covers dampen sound, while hard surfaces and vibrating machinery add to noise levels. Sizing a brake for a less-abrupt stop reduces the overall system noise during deceleration. Substituting softer diskpack material, such as a brass or bronze alloy, often produces less noise than steel. Maintaining the overall system eliminates belt slip, overheated bearings, or lubrication problems that generate noise when the brake changes the speed of a system.

Industry standards can also play a major role in brake selection. Some are written for specific designs, such as shoe brakes, or specific industries, such as the Crane Manufacturers Association of America (CMAA) standards. Other standards consider the brake as a component of a specific system, such as IEEE 45: Recommended Practice for Electric Installations on Shipboards. Some sources of standards include: • American National Standards (ANSI) • American Society of Mechanical Engineers (ASME) • Association of Iron & Steel Engineers (AISE) • Crane Manufacturers Association of America (CMAA) • Institute of Electrical and Electronic Engineers (IEEE) • National Electrical Manufacturers Association (NEMA).

After determining the general type of brake for a system, sizing for optimal perfoperformance and long life are next on the agenda. Manufacturers generally size brakes based on the power-transmission equation,

T = 5,252PFs ⁄ Nb

This method provides a quick means of sizing brakes for standard applications. Many catalogs also provide quick reference sizing charts that consider the service factor Fs that can vary by manufacturer and brake design.

For instance, applications where the drive controls speed and requires brakes only for holding typically use a smaller service factor. Brakes that must hold even if the motor is accidentally powered should use the nameplate peak torque or NEMA design rating found in NEMA MG1 Part 12: Test and Performance of AC/DC Motors.

For oversize motors or unique performance requirements, size the brake by factors such as inertia or stop time. The following calculations are appropriate where there is high inertia, in critical applications, as well as for many overhauling load conditions that require both stopping and holding torque.

Reflected inertia in linear applications is determined with

Wkl2 = W(V ⁄ 2πNb)2.

For rotary applications, reflected inertia is

Wkr2 = Wk2(N ⁄ Nb)2.

Dynamic torque is calculated using

Td = Wk2N ⁄ 308t.

The accompanying box takes a look at how the equations are used.

This first example considers an application in which a brake must stop a load on a horizontal belt conveyor in a specified time. Conveyor pulley speed Np = 32 rpm; the load weight W = 30 lb; pulley and belt inertia Wkp 2 = 4 lb-ft2; pulley diameter dp = 1 ft; and required stopping time t = 0.25 sec.

First convert rotational pulley speed to linear belt speed,

Vb = πdpNp = 100 fpm.

Next determine load inertia,

Wkw2 = W(Vb ⁄ 2πNp)2 = 7.4 lb-ft2.

Then calculate the total inertial load,

Wkt2 = Wkw2 + Wkp2 = 11.4 lb-ft2.

It’s now possible to find the dynamic torque required to stop the conveyor load in 0.25 sec.

Td = Wkt2Np ⁄ 308t = 4.7 lb-ft.

Convert dynamic torque to static torque with

Ts = Td ⁄ 0.8 = 5.9 lb-ft. Select a brake with a standard static torque rating of 6 lb-ft.

A second example deals with rotary conditions. Consider a brake that must stop an overhauling load driven through a gear reducer. Motor speed Nm = 1,150 rpm; motor inertia Wkm 2 = 0.65 lb-ft2; gear reduction R= 300:1; drum diameter D= 1.58 ft; weight of load W= 4,940 lb; drum inertia WkD 2 = 600 lbft 2; and the required stopping time t = 0.5 sec.

First calculate all inertial loads reflected to the brakemotor shaft. Drum speed Nd = Nm ⁄ R = 3.83 rpm. From this, determine cable speed, V = NdDπ = 19.0 fpm.

The equivalent inertia of the load reflected to the brakemotor shaft is

Wki2 = W(V ⁄ 2πNm)2 = 0.034 lb-ft2.

The equivalent inertia of the drum at the brakemotor shaft speed is

Wkd2 = WkD2(Nd ⁄ Nm)2 = 0.0067 lb-ft2.

Finally, the total inertia the brake will retard is

Wkt2 = Wkm2 + Wki2 + Wkd2 = 0.691 lb-ft2.

Dynamic torque required to decelerate the total inertia is

Td = Wkt2Nm ⁄ 308t = 5.16 lb-ft.

Now calculate the dynamic torque to overcome the overhauling load. To = WD ⁄ 2 = 3,903 lb-ft. Reflected to the brakemotor shaft, this becomes Tm = To ⁄ R = 13.0 lb-ft. Then the total dynamic torque to stop and hold the overhauling load is the sum of the dynamic torques,

Tt = Td + Tm = 18.16 lb-ft.

Converting to static torque,

Ts = Tt ⁄ 0.8 = 22.7 lb-ft.

Select a brake with a standard torque rating of 25 lb-ft.

© 2010 Penton Media, Inc.

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