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Fasteners, Powers Screws, Connections

Fasteners, Powers Screws, Connections. Helical thread screw was an important invention. Power Screw, transmit angular motion to liner motion Transmit large or produce large axial force It is always desired to reduce number of screws. Definition of important Terminologies.

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Fasteners, Powers Screws, Connections

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  1. Fasteners, Powers Screws, Connections Helical thread screw was an important invention. Power Screw, transmit angular motion to liner motion Transmit large or produce large axial force It is always desired to reduce number of screws

  2. Definition of important Terminologies Major diameter d, Minor diameter dr Mean dia or pitch diameter dp Lead l, distance the nut moves for one turn rotation

  3. Single and Double threaded screws Double threaded screws are stronger and moves faster

  4. Screw Designations • United National Standard UNS • International Standard Organization Roots and crest can be either flat or round Pitch diameter produce same width in the thread and space,

  5. Coarse thread Designated by UNC • Fine Thread UNF, is more resistance to loosening, because of its small helix angle. • They are used when Vibration is present • Class of screw, defines its fit, Class 1 fits have widest tolerances, Class 2 is the most commonly used • Class three for very precision application • Example:1in-12 UNRF-2A-LH, A for Ext. Thread and B for Internal, R root radius • Metric M10x1.5 10 diameter mm major diameter,1.5 pitch

  6. Some important Data for UNC, UNF and M threads • Lets Look at the Table 8-1 on Page 398

  7. Square and Acme Threads are used for the power screw Preferred pitch for Acme Thread

  8. Mechanics of Power Screws

  9. Used in design to change the angular motion to linear motion, Could you recall recent failure of power screw leading to significant causalities

  10. What is the relationship between the applied torque on power screw and lifting force F

  11. Torque for single flat thread If the thread as an angle α, the torque will be Wedging action, it increases friction

  12. Stresses in the power Screw Shear stress in the base of the screw Bearing stress Bending stress at the root of the screw Shear stress in the thread nt number of engaged thread

  13. Loading to the fasteners and their Failure considerations

  14. Bolts are used to clamp two or more partsIt causes pre tension in the bolt Grip length is the total thickness of parts and washers d t

  15. ld h t2 lt=L’- ld L’ effective grip= h+t2 if t2<d =h=d/2 for t2 d

  16. Failure of bolted or riveted joints

  17. Type of Joints • Lap Joint (single Joint) But Joint

  18. Example 1

  19. Example 2

  20. Example 2

  21. Example 3

  22. Weld

  23. Weld under Bending

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