08 November 2001
Choosing a Holding Brake
by Greg Cober
In recent years, design engineers have had access to an expanding variety of options for holding (also called fail-safe) brakes. Although all holding brakes perform the same basic function, engineers can choose from a wide range of mounting configurations, engagement or release methods, and torque ranges. Each of these differences liberates design engineers to find the best choice for their applications, thus increasing efficiency, enhancing machine performance, reducing costs, and improving safety.
What Is a Holding Brake?
Quite simply, a holding brake engages when it is cut off from its power source. Electric holding brakes engage when deprived of electricity; pneumatic units engage when air pressure is lost.
Holding brakes' principal use is to safely keep a load in place in situations where the power either is turned off or fails. Typical applications include automated storage and retrieval systems, or machine tools where a motor drives the load vertically but can't hold the load without the constant application of power. In these applications, the brake will engage upon power loss, holding the load safely in position. Similarly, a holding brake on a decline conveyor or overhead crane will engage to keep the load safely under control.
Holding brakes are available in two common configurations: dynamic stopping and static holding. Dynamic stopping brakes are commonly used as cycling brakes, which take the wear and tear of constant on/off engagements while the shaft is rotating and still provide long life. Static holding brakes are for simple load-holding applications. For example, a typical servo application uses a static holding brake because the motor drives the load into the proper position. The brake's only function is to hold the load stationary until the motor moves it again.
A static holding brake that's incorrectly applied in a frequent cycling application will wear out and fail quickly because it isn't designed for wearing applications. Similarly, a unit designed as a dynamic brake needs engagements at speed to maintain its full torque rating. A dynamic brake that's used solely as a holding brake may experience torque degradation, reducing its performance over time due to a loss of burnish.
The most common method for engaging a holding brake is to use a spring-set design (Figure 1). In this arrangement, springs compress a friction plate against the housing when input power is removed, thereby engaging the brake (Figure 2). Applying input power compresses the springs, releasing the friction plate that's connected to the shaft through a splined hub. The shaft is then free to rotate.
Spring-set units are commonly available in electric, pneumatic, and hydraulic designs. Given the design's simplicity, spring-set brakes can range from quite small (1 ft∑lb) to very large (18,000+ ft∑lb).
Permanent Magnet Brakes
A less common but equally flexible design is the permanent-magnet, electrically released brake (Figure 3). In this configuration, permanent magnets are built into an electromagnetic friction brake's shell. Without applied power, the attractive force from the permanent magnets clamps the unit's halves together, stopping and holding the load. Applying power to the coil within the shell creates a magnetic field of opposite polarity. This puts the two fields in balance, eliminates the magnetic attraction at the friction faces, and allows the armature to release and spin freely.
The challenge in this design is to keep the two opposing magnetic fields in balance. The best solution is an adjustable power supply, which can accurately adjust the electrically created magnetic field to fully negate that created by the permanent magnets.
Choosing Your Brakes
Spring-set brakes have a simple on/off function, giving them a consistent performance without concerns about power supply. Permanent-magnet units, however, require an adjustable power supply. If a permanent-magnet brake has a fluctuating power supply (i.e., input AC power varies more than 10%), the two opposing fields won't be in balance. Consequently, the brake will "hang up" or drag.
While the permanent-magnet designs involve a somewhat more complicated setup, they provide the advantage of a wider variety of mounting styles than spring-set brakes. Further, while some spring sets can be used as cycling brakes, all permanent-magnet units are capable of long life in high cycle rate applications.
Holding brakes are available in several different mounting styles. Both permanent-magnet (Figure 4) and spring-set designs are available as flange-mounted and bearing mounted/shaft-mounted types. Some manufacturers also have designs that are appropriate for NEMA C-face and typical servo frame mountings.
To select the proper holding brake, first define whether the application requires a static holding or dynamic stopping brake. For a holding brake, select a unit based on the required holding torque and the desired mounting style. For a dynamic stopping brake, calculate torque with the following formula:
Next, select a brake that provides the needed torque at the operating revolutions per minute (rpm).
Note that at a higher rpm, some brakes have significantly less torque (compared with their static torque). If inertia is unknown, calculate torque with the following formula:
This formula may yield a brake that's larger than necessary, but it accommodates those situations where inertia is unknown or can't be reliably calculated.
The Future of Holding Brakes
To predict the future of holding brakes, you can easily look at the increased options available in standard clutches and brakes. As holding brakes become an increasingly desirable choice for machine designers, the number of variations will multiply. Only a few years ago, flange- and shaft-mounted versions of these brakes were the only options. Recent additions include versions for C-face mounting and servo/stepper mounting. Future versions will be determined by the needs of the marketplace, as manufacturers seek to meet requirements with creative new solutions. MC
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