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Sổ tay tính toán cơ khí P24

Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS SECTION 24 MECHANICAL AND ELECTRICAL BRAKES Brake Selection for a Known Load 24.1 Mechanical Brake Surface Area and Cooling Time 24.3 Band Brake Heat Generation, Temperature Rise, and Required Area 24.6 Designing a Brake and Its Associated Mechanisms 24.8 Internal Shoe Brake Forces and Torque Capacity 24.15 Analyzing Failsafe Brakes for Machinery 24.17 BRAKE SELECTION FOR A KNOWN LOAD 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. | Source HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS __SECTION 24_ MECHANICAL AND ELECTRICAL BRAKES Brake Selection for a Known Load 24.1 Mechanical Brake Surface Area and Cooling Time 24.3 Band Brake Heat Generation Temperature Rise and Required Area 24.6 Designing a Brake and Its Associated Mechanisms 24.8 Internal Shoe Brake Forces and Torque Capacity 24.15 Analyzing Failsafe Brakes for Machinery 24.17 BRAKE SELECTION FOR A KNOWN LOAD 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 1 2 3 82.7 N m2 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. Calculation Procedure 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. 2. Compute the average brake torque required to stop the load Use the relation Ta Wk2n Z 308t where Ta average torque required to stop the load lb ft Wk2 load inertia including brake rotating member lb ft2 n shaft speed prior to braking r min t required or desired stopping time s. For this brake Ta 200 1800 Z 308 40 29.2 lb ft or 351 lb in 39.7 N m . 3. Apply a service factor to the average torque A service factor varying from 1.0 to 4.0 is usually applied to the average torque to ensure that the brake is of sufficient size for the load. Applying a service factor of 1.5 for this brake yields the required capacity 1.5 351 526 in lb 59.4 N m . 24.1 Downloaded from Digital Engineering Library @ McGraw-Hill www.digitalengineeringlibrary.com Copyright 2006 The McGraw-Hill Companies.