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Tham khảo tài liệu 'machinery's handbook 2th episode 4 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Guide to Machinery s Handbook 27th Edition 132 PROBLEMS IN MECHANICS 0.75 6.2832 X 0.150 X 2 2 F 6000 X -7- - - - - -- . T- X - - 6.2832 X 2-0.150 X 0.75 10 254 pounds Example 9 What force F will be required to lower a load of 6000 pounds using the screw referred to in Example 8 In this case the load assists in turning the screw hence 6.2832 X 0.150 X 2-0.75 2 F 6000 X - - ---- - -- _ ----- - X - - 107 pounds 6.2832 X 2 0.150 X 0.75 10 F Coefficients of Friction for Screws and Their Efficiency. According to experiments Professor Kingsbury made with square-threaded screws a friction coefficient p of 0.10 is about right for pressures less than 3000 pounds per square inch and velocities above 50 feet per minute assuming that fair lubrication is maintained. If the pressures vary from 3000 to 10 000 pounds per square inch a coefficient of 0.15 is recommended for low velocities. The coefficient of friction varies with lubrication and the materials used for the screw and nut. For pressures of 3000 pounds per square inch and by using heavy machinery oil as a lubricant the coefficients were as follows Mild steel screw and cast-iron nut 0.132 mild-steel nut 0.147 cast-brass nut 0.127. For pressures of 10 000 pounds per square inch using a mild-steel screw the coefficients were for a cast-iron nut 0.136 for a mild-steel nut 0.141 for a cast-brass nut 0.136. For dry screws the coefficient may be 0.3 to 0.4 or higher. Frictional resistance is proportional to the normal pressure and for a thread of angular form the increase in the coefficient of friction is equivalent practically to p secp in which p equals one-half the included thread angle hence for a sixty-degree thread a coefficient of 1.155p may be used. The square form of thread has a somewhat higher efficiency than threads with sloping sides although when the angle of the thread form is comparatively small as in an Acme thread there is little increase in frictional losses. Multiple-thread screws are much more efficient .
