A proposed design for an electromagnetic brake would increase the reliability while reducing the number of parts and the weight, relative to a prior commercially available electromagnetic brake. The reductions of weight and the number of parts could also lead to a reduction of cost. A description of the commercial brake is prerequisite to a description of the proposed electromagnetic brake. The commercial brake (see upper part of figure) includes (1) a permanent magnet and an electromagnet coil on a stator and (2) a rotor that includes a steel contact plate mounted, with tension spring loading, on an aluminum hub. The stator is mounted securely on a stationary object, which would ordinarily be the housing of a gear drive or a motor. The rotor is mounted on the shaft of the gear drive or motor. The commercial brake nominally operates in a fail-safe (in the sense of normally braking) mode: In the absence of current in the electromagnet coil, the permanent magnet pulls the contact plate, against the spring tension, into contact with the stator. To release the brake, one excites the electromagnet with a current of the magnitude and polarity chosen to cancel the magnetic flux of the permanent magnet, thereby enabling the spring tension to pull the contact plate out of contact with the stator.
The fail-safe operation of the commercial brake depends on careful mounting of the rotor in relation to the stator. The rotor/stator gap must be set with a tolerance between 10 and 15 mils (between about 0.25 and about 0.38 mm). If the gap or the contact pad is thicker than the maximum allowable value, then the permanent magnetic field will not be strong enough to pull the steel plate across the gap. (For this reason, any contact pad between the contact plate and the stator must also be correspondingly thin.) If the gap exceeds the maximum allowable value because of shaft end play, it becomes impossible to set the brake by turning off the electromagnet current. Although it may still be possible to set the brake by applying an electromagnet current to aid the permanent magnetic field instead of canceling it, this action can mask an out-of-tolerance condition in the brake and it does not restore the fail-safe function of setting the brake when current is lost.
In the proposed brake (see lower part of figure), the contact pad would be mounted on the stator via compression springs instead of on the rotor via tension springs. Optionally, a steel or ablative brake pad would be mounted on the rotor. There would be no permanent magnet. Instead of using a permanent magnet to pull the contact plate across the rotor/stator gap, one would use the compression springs to push the contact plate into the rotor. An electromagnet would be used to pull the contact plate against the compression springs to release the brake. If the critical gap between the contact plate and the electromagnet were to grow beyond the reach of the electromagnetic field, the brake could not be released: the contact plate would remain pushed against the rotor — that is, in the braked or fail-safe configuration.
In the proposed design, longitudinal movement of the shaft could be accommodated by increasing the throw of the compression springs. The tolerance on the rotor/stator gap could be increased to as much as tenths of an inch (several millimeters), and the failure mode would change from not being able to set the brake to not being able to release the brake. Also, inasmuch as the frictional braking contact would no longer be between the steel contact plate and the actuating electromagnet, a contact pad of any thickness or material could be mounted on the rotor.
This work was done by Toby B. Martin of Johnson Space Center. For further information, contact the Johnson Commercial Technology office at (281) 483-0837. MSC-23226