Topics

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 (UTS): 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: special threads having 8 threads per inch (TPI) irrespective of dia of the threads. It is also known as 8UN or 8 pitch series. • The International Screw Thread Standard (ISO metric): most common standard internationally, 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 – pitch is determined by dividing 1 inch/# of threads per inch. • Example: If a thread has 50 threads per inch what is its pitch? Pitch = 1 inch / 50 = 0.020 • Number of threads per inch • Minor (root) diameter • Major (Outside) diameter • Lead: the distance a screw thread advances in one revolution. Source: Siemens TechnikAkademie Berlin

4. MECH 1200 Terminology • Thread Fit: How tight is the thread engagement. Three classes of fit are used in the unified system: • Loose • Medium • Tight We use A for external and B for internal fits. • Internal threads (B) refer to those on nuts and tapped holes. • External threads (A) are those on bolts, studs, or screws.

5. MECH 1200 Terminology • Unified Thread Fits

6. MECH 1200 Terminology • ISO Thread Fits

7. 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.

8. MECH 1200 Fastener Identification 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

9. MECH 1200 Common Bolts and Screws

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

11. MECH 1200 Nuts Hex nuts: Have a hexagonal shape, and has different grades. Used in a large majority of nut and screw application. Inexpensive and usually universal. 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. These nuts are used together with a bolt and a cotter pin to create a secure nut in place so that it cannot be tightened or loosened Wing nuts: have protruding 'wings' to make hand tightening and loosening easier. ... The ease of use also make wing nuts popular for applications in which convenience is vital, such as on boats and bicycles. Cap (Acorn) Nut: hide the unattractive and potentially damaging end of bolts and cap screws. They provide a seamless, smooth look for finished products and help to improve conditions in applications involving insulation (buildings, boats, automobiles, etc.)

12. 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 - use the coefficient of friction between the two contacting surfaces Internal tooth lock washer External tooth lock washer Spring lock washer Lock washer Safety wires

13. MECH 1200 Fastener Locking Devices Non-Friction based Locking devices – locking based on tension, bending, or pinning. Split pins Nut lock plate Tab washer Keys: small piece of metal imbedded partially in the shaft and partially in the hub to prevent rotating of a gear or pulley on a shaft. Safety wire/locking-wire: prevents fasteners from loosening or falling out due to vibration and other forces. The presence of safety wiring may also serve to indicate that the fasteners have been properly tightened.

14. Threading Process Thread Rolling Thread Cutting Thread Tapping Thread Size/Pitch

15. 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

16. 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

17. 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.