# Welin breech block

Welin breech-block of a 16-inch Mk 6 gun on USS Alabama (BB-60), 1943. Note the four separate thread "steps" on the block which engage with matching steps in the breech when the block is swung up and inwards and rotated slightly clockwise

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.

## Design

Leftward-opening breech of British 12-inch gun on HMAS Australia, 1918. This breech was locked by rotating counter-clockwise

The breech block incorporated a screw design with multiple "steps" of threads of progressively increasing radius, each step occupying the same circular sector.

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 is:

${\displaystyle {\frac {\text{number of steps}}{1+{\text{number of steps}}}}\times {\text{length of the screw}}}$

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the Welin block allowed it to be shorter, allowing faster opening as it could be swung down or to the side after being withdrawn a much shorter distance than previous designs.[1] It was also simpler and more secure.

A female worker cleans the breech threads of a 15-inch gun from inside the barrel in the Coventry Ordnance Works during WWI

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 (114 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.[citation needed]