Tham khảo tài liệu 'giáo trình chế tạo ôtô - phần 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CONTENTS c H A p T E R 17 Power Screws 1. Introduction. 2. Types of Screw Threads used for Power Screws. 3. Multiple Threads. 4. Torque Required to Raise Load by Square Threaded Screws. 5. Torque Required to Lower Load by Square Threaded Screws. 6. Efficiency of Square Threaded Screws. 7. Maximum Efficiency of Square Threaded Screws. 8. Efficiency vs. Helix Angle. 9. Overhauling and Selflocking Screws. 10. Efficiency of Self Locking Screws. 11. Coefficient of Friction. 12. Acme or Trapezoidal Threads. 13. Stresses in Power Screws. 14. Design of Screw Jack. 15. Differential and Compound Screws. Introduction The power screws also known as translation screws are used to convert rotary motion into translatory motion. For example in the case of the lead screw of lathe the rotary motion is available but the tool has to be advanced in the direction of the cut against the cutting resistance of the material. In case of screw jack a small force applied in the horizontal plane is used to raise or lower a large load. Power screws are also used in vices testing machines presses etc. In most of the power screws the nut has axial motion against the resisting axial force while the screw rotates in its bearings. In some screws the screw rotates and moves axially against the resisting force while the nut is stationary and in others the nut rotates while the screw moves axially with no rotation. 624 CONTENTS Power Screws 625 Types of Screw Threads used for Power Screws Following are the three types of screw threads mostly used for power screws 1. Square thread. A square thread as shown in Fig. a is adapted for the transmission of power in either direction. This thread results in maximum efficiency and minimum radial or bursting h p a Square thread. h mtn b Acme thread. h p c Buttress thread. Fig. . Types of power screws. pressure on the nut. It is difficult to cut with taps and dies. It is usually cut on a lathe with a single point tool and it .
