WHITWORTH THREAD

WHITWORTH THREAD

Whitworth threads were the world's first national screw thread standard, devised by Sir Joseph Whitworth in 1841. They are an imperial (inch-based) system characterized by a distinct 55° thread angle and rounded roots and crests. 

Key Characteristics

  • Thread Angle: All Whitworth threads, including BSW and BSF, have a symmetrical V-profile with a 55° included angle between the thread flanks. This is a key difference from the 60° angle used in modern metric and Unified threads.
  • Thread Profile: The threads feature unique rounded roots and crests, designed to distribute stress effectively and provide excellent fatigue resistance.
  • Designation: Dimensions are specified in imperial units, using inches and "Threads Per Inch" (TPI) rather than metric pitch measurements.
  • Standardization: The standard is codified in British Standard BS 84. 

BSW vs. BSF Threads

The Whitworth system encompasses two main series for general engineering:

  • BSW (British Standard Whitworth): This is the original, coarse-pitch thread series, primarily designed for general engineering applications like holding cast iron components or locomotives together. Its coarse pitch provides superior strength in softer materials and resistance to stripping.
  • BSF (British Standard Fine): This series was developed later as a fine-pitch alternative to BSW. BSF threads have the same 55° profile as BSW but a finer pitch, which provides increased core strength and better resistance to loosening from vibration, making it common in automotive and aviation applications before metric standardisation.

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