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Thread Standards and Specifications in Mechanical Engineering

Learn about Unified and International thread standards, terminology, bolt and screw specifications, nuts, and locking devices. Practice questions included.

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Thread Standards and Specifications in Mechanical Engineering

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  1. MECH 1200 Topics • Introduction • Terminology • Thread Specifications • Bolts and Screws • Nuts and Locking Devices • Exercise Questions • Note: Refer to Chapter 6 in the text.

  2. MECH 1200 Introduction • There are two thread standards: • The Unified Screw Thread Standard: Used in the U.S. It is based on a 60o thread angle. • Unified National Coarse (UNC) series: this is the most common series. Bolts, screws, and nuts of this series are used for general applications in which a quick assembly is required. • Unified National Fine (UNF) series: threads of this series are used for handling high tensile stresses, or when the length of engagement is limited such as screws in thin walls. • Unified National Special (UNS) series • The International Screw Thread Standard: most common standard, based also on the 60o thread angle.

  3. MECH 1200 Terminology • Thread angle: the angle included between the sides of the thread, The ISO thread angle is 60 degrees. • Pitch: the distance between one point on a thread to the corresponding point on the next thread. • Number of threads per inch • Minor (root) diameter • Major (Outside) diameter • Lead: the distance a screw thread advances in one revolution. • Thread Fit: How tight is the thread engagement. Three classes of fit are used in the unified system, loose, medium, and tight fits. A for external and B for internal fits. (see p. 110) Source: Siemens TechnikAkademie Berlin

  4. MECH 1200 Thread Specifications • Unified system threads: • Example: ¼ - 20 UNC-BX1 indicates a thread with an outer diameter is ¼ inch, with 20 threads per inch, the type of thread is UNC, and a B (internal) fit and a thread length of 1 inch. • Metric system threads: • Example: M10 1.5-6g indicates a Metric thread with an outside diameter of 10 mm and a pitch of 1.5 mm, the fit of the thread is 6g. Bolts and Screws: The two words are used interchangeably, but generally, bolt is used when a nut is required to be used. The shank of a bolt has a diameter that is equal to the major diameter. Source: Siemens TechnikAkademie Berlin

  5. MECH 1200 Common Bolts and Screws

  6. MECH 1200 Source of images in this slide: Siemens TechnikAkademie Berlin

  7. MECH 1200 Nuts Hex nuts: Have a hexagonal shape, and has different grades Castle Nut: Named so because it looks like a castle, used in applications where heavy vibrations occur, such as on wheels. Can handle light torques.only. Wing nuts Cap (Acorn) Nut

  8. MECH 1200 Fastener Locking Devices • Screw locking devices are used to keep the nut from getting loose. • They can be friction based or non-friction based: Friction based Locking devices Internal tooth lock washer External tooth lock washer Spring lock washer Lock washer Safety wires Non-Friction based Locking devices Split pins Nut lock plate Tab washer

  9. MECH 1200 Exercise Questions • What are the two thread standards? The Unified Screw Thread System and the ISO metric thread standard. • Thoroughly describe the bolt given by: ¼ - 28 UNF 2A The bolt has an outer diameter of ¼ inch and a fine thread of 28 threads per inch (Unified National Fine thread series) and has a medium fit with external thread. • Thoroughly describe the thread bolt given by: M10 x 1.25 The bolt is metric and its outer diameter is 10 mm and has a pitch of 1.25 mm

  10. MECH 1200 • If a thread has 40 threads per inch what is its pitch? Pitch = 1 inch / 40 = 0.025 • Is each of the screw locking devices below friction based or not? Yes Yes No No No

  11. MECH 1200 • What is the component shown in the figure below? Castle Nut: used in applications that require low amount of torque, and where vibrations are common.

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