FASTENER

Fasteners are used to secure or hold different parts together or to mount a component. Many types and sizes of fasteners are used in automobiles. Each fastener is designed for a specific purpose and condition. The most commonly used is the threaded fastener. Threaded fasteners include bolts, nuts, screws, and similar items that allow for easy removal and installation of parts (Figure).

The threads can be cut or rolled into the fastener. Rolled threads are 30% stronger than cut threads. They also offer better fatigue resistance because there are no sharp notches to create stress points. There are four classifications for the threads of Imperial fasteners: Unified National Coarse (UNC), Unified National Fine (UNF), Unified National Extrafine (UNEF), and Unified National Pipe Thread (UNPT or NPT). Metric fasteners are also available in fine and coarse threads.

NPT is the standard thread design for joining pipes and fittings. There are two basic designs: tapered and straight cut threads. Straight cut pipe thread is used to join pipes but it does not provide a good seal at the joining point. Tapered pipe threads provide a good seal because the internal and external threads compress against each other as the joint is tightened. Most often a sealant is used on pipe threads to provide a better seal. Pipe threads are commonly used at the ends of hoses and lines that carry a liquid or gas (Figure)

Coarse (UNC) threads are used for general- purpose work, especially where rapid assembly and disassembly are required. Fine threads (UNF) are used where greater holding force is necessary. They are also used where greater vibration resistance is desired.