The Welin breech block was a revolutionary stepped, interrupted thread design for locking artillery breeches, invented by Axel Welin in 1889 or 1890. Shortly after, Vickers acquired the British patents.
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The screw design incorporated multiple "steps" of threads of progressively increasing radius, each step occupying the same circular sector, such as the four steps seen in the accompanying photograph of a 16-inch gun breech and block.
Each step engaged with its matching thread cut in the gun breech when inserted and rotated. A gap in the thread steps was still necessary for the insertion of the largest step before rotation, so the area of the breech secured by threads in the block was number of steps / (1 + number of steps) x length of the screw.
This was a major improvement on previous non-stepped designs such as the de Bange system, which had only a single thread step and hence only half of the block's circumference had a thread which engaged with the breech, necessitating a fairly long screw to achieve a secure lock. The much greater threaded area of the Welin block allowed it to be shorter, allowing faster opening as it could be swung down or to the side after being withdrawn a much shorter distance than previous designs.[1] It was also simpler and more secure.
The Welin breech was a single motion screw, allowing it to be operated much faster than previous interrupted-thread breeches, and it became very common on British and American large calibre naval artillery and also field artillery above about 4.5 inches (110 mm).
Though the US Navy was offered the design a year or two after its invention, they declined and the American Bethlehem Steel spent the next five years in trying to circumvent Welin's patent, before having to buy it through Vickers.