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Thread

A thread is a groove that in the case of fasteners is typically of a vee form. Enlarging a section of a thread it can be observed as a series of ridges and hollows. Specific names are given to parts of the thread.
The crest is the topmost section of the thread. The root is the bottom of the thread. The thread flanks lie between the roots and crest of the threads.

The major diameter of a thread is, for an external thread, the diameter over the crests of the thread. For an internal thread it is the diameter over the thread roots. The pitch diameter cannot be directly measured since there is no marking or indication on the thread to measure. To explain the pitch diameter, it is first necessary to understand what the pitch is. 

For inch threads, the pitch is defined as the number of threads per inch - or tpi. With metric threads the pitch is directly specified.

The pitch diameter is the diameter on the thread that the space between the threads is equal to the space across the thread. The pitch diameter is a critical dimension in determining the acceptability of threaded connections. It is sometimes referred to as the effective diameter. 

The minor diameter of an external thread is the diameter of the roots of the thread. For an internal thread, it is the diameter over the thread crests. Most thread forms have a radius at the root of an external thread. The radius improves the fatigue resistance of the thread.

Coarse and Fine Threads:

Threads are often referred to as coarse or fine. This has nothing to do with the thread quality. A coarse or fine thread refers to the pitch size, a coarse thread having a larger pitch than a fine thread for the same thread diameter. There are thread series for both coarse and fine threads. For example, the UNF thread refers to a Unified National Fine thread, UNC to a Unified National Coarse.
 

For a given thread diameter, a UNF thread has a smaller pitch - that is, a larger number of threads per inch - than a UNC thread. A standard metric thread is a coarse thread. There is a metric fine thread series but it is normally only specified for specialized applications.

There are advantages and disadvantages in selecting a coarse thread compared to a fine thread.

  • Size for size a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of the larger minor diameter).
  • Fine threads have also less tendency to loosen, since the thread incline is smaller and hence so is the off torque. The off torque is the torque that tends to undo the fastener and is present because of the preload (that is, the clamp force provided by the bolt).
  • Because of the smaller pitch, fine threads allow finer adjustments in applications that need such a feature.
  • Fine threads can be more easily tapped into hard materials and thin-walled tubes.
  • Fine threads require less torque to develop equivalent bolt preloads.

Fine threads also have a number of significant disadvantages:

  • Fine threads are more susceptible to galling than coarse threads. Galling is a type of thread seizure.
  • Fine threads need longer thread engagements and are more prone to damage and thread fouling.
  • Fine threads are also less suitable for high-speed assembly since they are more likely to seize when being tightened.
  • A key point is that normally a coarse thread is specified unless there is an overriding reason to specify a fine thread. Certainly, for metric fasteners, fine threads are more difficult to obtain.



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