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Mechanical Engineering Drawing MECH 211/M

Lecture # 10 Chapter 13 Dr. John Cheung. Mechanical Engineering Drawing MECH 211/M. Screws, Nuts and Bolts. Used to assemble machine parts Used to transmit power / motion The threads are cut or rolled Bolts and nuts must be matched. Screw and Thread Terminology.

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Mechanical Engineering Drawing MECH 211/M

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  1. Lecture #10 Chapter 13 Dr. John Cheung Mechanical Engineering DrawingMECH 211/M

  2. Screws, Nuts and Bolts Used to assemble machine parts Used to transmit power / motion The threads are cut or rolled Bolts and nuts must be matched

  3. Screw and Thread Terminology • Screw Thread - A ridge of uniform section in the form of a helix on the external or internal surface of a cylinder. • Major Diameter -The largest diameter of a screw thread. • Minor Diameter - The smallest diameter of a screw thread.

  4. Axis – the longitudinal center line of the original work (blank) or hole Chamfer – the angular relief at the beginning or end of the thread to allow easier engagement with the mating part Crest – the peak of the top of a thread Depth – the distance between the crest and the root Screw and Thread Terminology

  5. External thread – the screw thread on the outside of a cylindrical surface Internal thread – the screw thread on the inside of a cylindrical surface Lead – the distance that a screw will travel along the axis when turned by 360° Screw and Thread Terminology

  6. Pitch – the distance between corresponding points on adjacent thread forms, measured parallel to the axis expressed in 1 divided by the number of pitch in one inch Pitch diameter – the diameter of an imaginary cylinder that is located equidistant between the major and the minor diameter Screw and Thread Terminology

  7. Root – the bottom of the screw thread cut in a cylinder Thread angle – the angle between the surfaces of two adjacent threads Thread series – the number of threads per inch for a given diameter Screw and Thread Terminology

  8. Tap and Die

  9. ANSI Y14.6 - 1998 Thread form Thread series Major diameter Class of fit Threads per inch Thread Specifications – Imperial System

  10. Screw Thread Forms • Imperial & Metric have same proportion • Sharp V was original American national thread • American National now has flattened root and crest to increase strength • Unified thread is agreed as standard in US, Canada and Britain, the crest may be flat or rounded but the root is rounded. Otherwise similar to American national

  11. Screw Tread Forms • ISO Metric is the most common of all. The depth is smaller than that of Unified National thread • Knuckle thread is rolled or cast (used in light bulbs and sockets) • Square and Acme threads are used for transmitting power • Buttress thread takes pressure on one side ( to the axis). Unidirectional motion.

  12. Power Transmission & Measurement Screws Controls and positioning applications

  13. Measuring Thread Pitch • Pitch is the distance along the axis between corresponding points in adjacent threads (i.e crest to crest or root to root) • Metric: Measured in millimeters (i.e. M10 X 1.5) • Imperial: Measured in threads per inch • ThreadPitch is measured with scale or a thread pitch gage

  14. Thread Types • Series: Varies with the pitch and the major diameter of the thread • Coarse Series (UNC) • General purpose usage • Fine Series (UNF) • Used when a great deal of force is necessary • Extra Fine Series (UNEF) • Used when the length of engagement is short and there is a high degree of stress • 8-Pitch (8UN), 12-Pitch (12UN), 16-Pitch (16UN) • Does not depend on the diameter

  15. Metric Thread Series Simplified compared to Unified Series Only two series Coarse (General Purpose) Fine

  16. Left & Right Handed Threads • Threads are assumed to be right handed (RH) • A nut advances when turned clockwise • Left handed (LH) threads turn counter clockwise to tighten

  17. Single and Multiple Thread Forms

  18. Thread Symbols • Three Types: Simple, Schematic or Detailed • For major diameters >1” Detailed is preferred

  19. Simplified & Schematic Internal SymbolsNormal & Section Drawings

  20. Thread Clearance • Class 1 • Loose fit where quick assembly is required and play between parts is acceptable • Class 2 • High quality general purpose commercial class of fit for bolts, nuts and screws used in mass production • Class 3 • Very high quality threaded fasteners with a close fit used for precision assembly subjected to vibrations • A is for external threads and B is for internal threads

  21. Metric Thread Class of Fit Tolerance grade 3 to 9 for external threads 4 to 8 for internal Tolerance position Letter between A and H Capitals for internal threads and lowercase letters external threads Tolerance Class Tolerance grade plus a tolerance position, First for the pitch diameter Second major diameter Example, “4g6g”

  22. Imperial Thread Notes

  23. Thread Identification • Thread form symbol – M • Nominal size • Pitch size • Tolerance Class

  24. Basic Metric Thread Note

  25. Complete Metric Threading Note • For example, grade 4 < grade 6 < grade 8 tolerances • In addition to the tolerance grade, a positional tolerance is required • For external threads: • Tolerance position e (large allowance) • Tolerance position g (small allowance) • Tolerance position h (no allowance) • For internal threads: • Tolerance position G (small allowance) • Tolerance position H (no allowance)

  26. How to Represent a Threaded Hole

  27. Screws, Nuts and Bolts

  28. Large variety of bolts (dimensional, head shape, etc.) • Material, quality, finishing • Grade

  29. Unfinished bolts are not machined anywhere except for the thread portion • Finished bolts have machined face for washer holding or flush location on parts

  30. Fastener Locking

  31. Fastener Locking Durlock nut Castellated nut Jam nut

  32. Screw Types • Head can have different shapes • Pass through a clearance hole to screw into member • Socket screws are used while in crowded condition

  33. Screw Types - Continued • Similar to cap screws but smaller in size. • General diameters between .06 to .75 inches • Hex head (not shown here) may be slotted if desired • Other heads are available as either slotted or recessed • Generally used for screwing into thin materials

  34. Bolt Representation

  35. Cap Screw Representation

  36. Screw Assembly Representation

  37. Bolt Assembly Representation

  38. Non-Permanent FastenersSnap Rings

  39. Non-Permanent FastenersPins

  40. Once assembled, the parts of the assembly (including the fastener) would be destroyed to disassemble the assembly. • Rivets, soldering, brazing, welding

  41. Used to permanently fasten mechanical components

  42. Riveting process

  43. Fastening With Rivets

  44. Common Riveted Joints

  45. Self-Piercing Rivets Blind rivets

  46. Blind Rivets

  47. Joining of two part using a third component – filler that joins the parts when in liquid state • Soldering and brazing – low temperature binding materials – Sn-Pb, Cu-Ag alloys • Limited capability to face thermo-mechanical loading

  48. Very well regulated activity • It requires license to practice • Welders bear significant responsibility • The activity is based on rigorous rules and regulations • Designer prescribe welding based on mechanics of materials calculations • Symbols indicate the type of welding

  49. Basic Welding Note

  50. Basic Welding Symbols

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