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Chapter 11

Chapter 11. Fasteners and Springs. Screw Threads. Helix or conical spiral formed on the external surface of a shaft or the internal surface of a hole Threads are used for: Fasteners Transmitting power Adjusting locations (leveling). Unified Thread Series.

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Chapter 11

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  1. Chapter 11 Fasteners and Springs

  2. Screw Threads • Helix or conical spiral formed on the external surface of a shaft or the internal surface of a hole • Threads are used for: • Fasteners • Transmitting power • Adjusting locations (leveling)

  3. Unified Thread Series • Developed to standardize threads so that there would be interchangeability • U.S. used American National System before 1949

  4. Screw Thread Terminology Axis-centerline of the cylindrical thread shape Body-portion of the screw left unthreaded Chamfer-angular relief at the last thread to help engagement Lead-the distance the thread travels in one revolution. Right hand thread-thread that engages when rotated clockwise (standard) Left hand thread-thread that engages when rotated counter-clockwise.

  5. Die-tool used for cutting external threads Tap-tool used for cutting internal threads. Tap drill-size of the drill used to make a hole before the tapping operation (each thread size requires a specific tap drill size) Fit-range of thread tightness or looseness. Included Angle-angle between the sides of the threads.

  6. Crest-top of external threads, bottom of internal threads Root-bottom of external threads, top of internal Depth of thread-distance from the crest to the root Major Diameter-size of the largest diameter on the thread Minor Diameter-size of the smallest diameter on the thread Pitch-the distance from a point on a thread form to the corresponding point on the next thread form Pitch diameter-diameter halfway between the major dia. and the minor dia.

  7. Thread form-profile of a thread (shape) Thread series-groups of major diameters and pitches. Threads per inch-number of threads included in one inch of length

  8. Thread FormsUnified Threads • Most common • Aka. Unified National • Several thread series: • UNC • UNF • UNEF

  9. American National • Old standard-pre 1949 • Flat root compared to Unified National • Harder to make

  10. Sharp V • Forms a tight seal • Difficult to make without damaging the crests and roots • Was original U.S. standard • Not common

  11. Metric Thread • ISO standard for metric sizes • Similar shape to Unified National

  12. Whitworth Threads • Original British standard • Developed in 1841 • Only used for replacement parts

  13. Square Threads • Used to transmit power (not fasten) • Difficult to manufacture

  14. Acme Threads • Transmit power • Replaced square threads • Used for screw jacks and vice threads

  15. Buttress Threads • Used to transmit power in one direction

  16. Dardelet Threads • Used where self locking threads are required • Resistant to loosening under vibration

  17. Rolled Threads • A formed thread (not cut) • Used for electrical parts (light bulbs, sockets)

  18. American National Standard Taper Pipe • Used for pipe fittings • Designed to be pressure tight

  19. Thread Representaion • Different methods of showing threads on engineering drawings. • 3 Methods

  20. DetailedThread Representation • Shows the true shape of the threads • Not commonly used because of the time needed to draw and computing time to model

  21. SchematicThread Representation • A simple way to graphically represent threads • Not commonly used any more

  22. SimplifiedThread Representation • Similar to schematic but shows more accurate information • Preferred method

  23. How to draw simplified

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