Power-off brakes, sometimes called safety, spring-set, or failsafe brakes, are widely used on the backs of motors to hold machines, pulleys, Z axes, and robotic arms in position in case of a power failure.

They come with various hub (motor coupling) designs, the hub being the part that attaches to the motor or driven shaft. Usually the rotor floats while the hub is connected rigidly to the shaft, so that the brake's rotor assembly disengages when power is applied and the motor runs at full rpm.

But brake backlash or free play — those degrees, plus and minus, that the shaft rotates while the brake is holding with no power fed to it — can undermine the actuation at the load end, or accuracy at the load end.

Q&A

If I am using a servomotor and power-off brake with a gear reducer already exhibiting backlash, should I be concerned about the brake's backlash?

Yes. Brake backlash is magnified by reducers. To illustrate: Say your gear ratio is 10:1. 1° of backlash at the input will yield 10° at the output. What's more, backlash increases under load. (This tendency is sometimes indicated in expressions for torsional stiffness or torsional rigidity.) Power-off brakes have internal clearances built into them to allow the units to function; when the brake is energized, the armature is pulled up against the case assembly that houses the coil. The armature plate is usually guided and held from rotating with the use of spacers or pins as guides. But at one-half static torque, the armature plate tends to slide to one side until it closes the clearance there. The outcome is more backlash.

Do different brake designs exhibit different backlash?

D drives use a round shaft with a flat ground into it for connection; double-D drives are simply two D drives back to back. Double Ds are similar to square drives: They have two points of contact that can overstress the rotor if inertial load is not considered during brake selection. In any case, with these square, or polygon or hex drives, there must be sufficient clearance between the hub and hole in the rotor to allow the friction-disc rotor to float mating parts. So, your larger design must allow for some misalignment in geometrical dimensioning and tolerances here. Worst-case scenario, the design might call for 0.002 in. clearance per side. Here, motor or drive inertial load must be considered so that when the brakes engage, they do not leave stress marks or indentations in mating parts.

Similar to a gear, the hub of a spline drive has a male set of teeth on the outer diameter that engage a female set on the rotor's inner diameter. This is a design with lower backlash than designs with double D or square connections.

Of straight and involute splines, involute varieties provide better sliding fits and are generally held to tighter tolerances. Backlash under no load is normally between 0.25° for small brakes (to 1.5 in. in diameter) and 1° for larger brakes. Many spline-connected brakes include a flat root-side fit, which minimizes the clearance at the teeth sides; the inner and outer diameters have significant clearance, and backlash is reduced.

In fact, there are thousands of spline sizes available; the smaller are most commonly used in smaller brakes and may have a clearance in the range of 0.0015 to 0.003 in.(total), while the larger can have double this clearance range. Clearance range also varies depending on the class of fit.

A zero-backlash drive takes it one step further: Comprised of a hub fixed to a flexible diaphragm or spring, its rotor assembly is one rigidly mounted diaphragm spring mounted to the hub. These drives have no play between the shaft, attachment hub, or friction-disc rotor. That said, under half load, they allow some free movement until the armature plate meets the spacers.

What design is least expensive?

Cost is related to the manufacturing process of the component parts, so in general, simple shapes are less expensive than spline or zero-backlash drives. But, depending on the shaft size as compared to brake size, hubs can sometimes be eliminated altogether, to leave the shaft to drive the rotor.

This month's handy tips courtesy Rocco Dragone, Product Specialist, Thomson Deltran of Danaher Motion, Wood Dale, Ill. Call (866) 993-2624 for more information.