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Chapter 8

Chapter 8. Screws, Fasteners, and the Design of Nonpermanent Joints. Chapter Outline. 8-1 Thread Standards and Definitions 8-2 The Mechanics of Power Screws 8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness

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Chapter 8

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  1. Chapter 8 Screws, Fasteners, and the Design of Nonpermanent Joints

  2. Chapter Outline 8-1 Thread Standards and Definitions8-2 The Mechanics of Power Screws8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt Strength 8-7 Tension Joints-The External Load8-8 Relating Bolt Torque to Bolt Tension 8-9 Statically Loaded Tension Joint with Preload 8-10 Gasketed Joints 8-11 Fatigue Loading of Tension Joints 8-12 Shear Joints 8-13 Setscrews 8-14 Keys and Pins 8-15 Stochastic Considerations

  3. Announcements • HW #5 Ch 18, on WebCT • Due Date for HW #5 is Mon. DEC. 31, 2007 • Quiz on Ch. 18, Mon. DEC. 31, 2007

  4. LECTURE 33 8-1 Thread Standards and Definitions

  5. Introduction The fundamental operation in manufacture is the creation of shape - this includes assembly, where a number of components are fastened or joined together either permanently by welding (Ch9) for example or detachably(nonpermanent) by screws,nuts and bolts and so on. Since there is such a variety of shapes in engineering to be assembled, it is hardly surprising that there is more variety in demountable fasteners than in any other machine element. Fasteners based upon screw threads are the most common, so it is important that their performance is understood, and the limitations of the fastened assemblies appreciated.

  6. Introduction • There are two distinct uses for screw threads and they usually demand different behavior from the threads : • a   power screw such as a lathe leadscrew or the screw in a car lifting jack which transforms rotary motion into substantial linear motion (or vice versa in certain applications), and

  7. Fasteners • a   Threaded Fastener similar to a nut and bolt which joins a number of components together again by transforming rotary motion into linear motion, though in this case the translation is small.

  8. Thread Profiles (c) Thread profiles. a) Square (b) ACME; (c) UN, ISO

  9. Thread Profile Parameters Figure 8-1 Terminology of screw threads. Sharp Vee Threads shown for clarity; the crests and roots are actually flattened or rounded during the forming operation Lead=L=n p

  10. Threads (a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws. Text Reference: Figure 15.2, page 667

  11. Thread Systems A thread 'system' is a set of basic thread proportions which is scaled to different screw sizes to define the thread geometry. Whitworth, Sellers, British Standard Pipe (BSP) are just three of the many systems which proliferated before the adoption of the ISO Metric thread system. The American National (Unified) Thread standards is used mainly in the US. Square and ACME

  12. Thread geometry The basic profile of ISO Metric threadsis built up from contiguous equiangular triangles of height  H disposed symmetrically about a pitch line which becomes the   pitch cylinder of diameter   d2 when the profile is rotated about the axis to form the thread. The distance between adjacent triangles - the pitch - is   p = 2 H /√3. The tips of the triangles are truncated by h/8 to form the major diameter ( size ) d of the thread, and the bases are truncated by h/4 to form the minor diameter  d1. It follows that d1 =  d - 5 h/4 = d - 1.08 p. This leads to the rule of thumb for suitable tapping size

  13. Thread Profile For Metric System (M, MJ) major H= 0.5(3)1/2 p pitch US N= # threads/in minor a ISO 68= a American National (Unified) thread standard= 60°

  14. Thread Standards • ISO Metric thread system: Table 8-1 Major diameter (mm), • 2a= 60°Standard thread is RH • Specifications: e.g.: M12x1.75 or • M = Basic Metric, J = round root; 12 = nominal major diameter (mm); 1.75 = pitch (mm) • Tensile stress area At =p/4(dm+dr)/2 (see footnote T.8.1) MJ12x1.75

  15. Thread Standards • The American National (Unified) Thread standards is used mainly in the US: Table-8-2 (Size designation) use d • UN=regular thread, UNR=round root (use root radius) • Specifications: 5/8”-18 UN, UNC, UNF • UNR, UNRC, UNRF • 5/8”=d 18 = N (thread size) • UN = Unified, F=fine, C=Coarse, R =Round Root

  16. Thread Standards • Square (a) and The ACME Threads (b)-used mainly in power screws Table 8.3 gives preferred pitches for ACME threads

  17. Power screw thread forms. [Note: All threads shown are external (i.e., on the screw, not on the nut); dm is the mean diameter of the thread contact and is approximately equal to (d + dr)/2.]

  18. ACME Thread Profile Figure 15.5 Details of ACME thread profile. (All dimensions are in inches.) Text Reference: Figure 15.5, page 670

  19. ACME Thread Properties N Crest diameters, threads per inch, and stresses for Acme thread.

  20. Mechanics of Power Screws • A power screw is a device that is common to tools or machinery that are used to change angular motion into translation. It is also capable of developing a large amount of mechanical advantage. Familiar applications include clamps or vises, presses, lathes lead screws, and jacks. The joyce warm-gear screw jack

  21. Mechanics of Power Screws Weight supported by three screw jacks. In each screw jack, only the shaded member rotates.

  22. Mechanics of Power Screws Example of catalogue: http://www.roton.com/web/quick.power_screws.jsp

  23. Vehicle Screw Jack

  24. Power Screw with collar Different types of collars

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