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.
