TABLE OF CONTENTS Choose a suitable brake to stop a 50-hp (37.3-kW) motor automatically when power is cut off. The motor must be brought to rest within 40 s after power is shut off. The load inertia, including the brake rotating member, will be about 200 lb ft 2 (82.7 N m 2); the shaft being braked turns at 1800 r/min. How many revolutions will the shaft turn before stopping? How much heat must the brake dissipate? The brake operates once per minute.
1. Choose the type of brake to use
Table 1 shows that a shoe-type electric brake is probably the best choice for stopping a load when the braking force must be applied automatically. The only other possible choice the eddy-current brake is generally used for larger loads than this brake will handle.
| Type of brake | Typical characteristics |
|---|---|
| Block | Wooden or cast-iron shoe bearing on iron or steel wheel; double blocks prevent bending of shaft; used where economy is prime consideration; leverage 5:1 |
| Band | Asbestos fabric bearing on metal wheels; fabric may be reinforced with copper wire and impregnated with asphalt; bands are faeed with wooden blocks; used where economy is a major consideration; leverage 10:1 |
| Cone | Friction surface attached to metal cone; popular for cranes; coefficient of friction = 0.08 to 0.10; useful for intermittent braking applications |
| Disk | Have one or more flat braking surfaces; effective for large loads; continuous application |
| Internal-shoe |
