Interrupted screw
Encyclopedia
An interrupted screw or interrupted thread is a mechanical device typically used in the breech
Breech-loading weapon
A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....
of artillery
Artillery
Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...
guns. It was invented circa 1845.
It is a screw
Screw
A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...
that has a section of thread along its axis removed. The screw is mated with a hole at the rear of the weapon's chamber; i.e., where the screw has threads the hole does not, and vice versa. The screw can thus be smoothly inserted all the way into the gun, after which as little as of a turn can engage the two sets of threads securely, sealing the breech. Interrupted screws are occasionally seen in loose gunpowder rifles, as this mechanism was historically one of the few practical ways to achieve a gas-proof seal with a breech-loading firearm which does not employ cartridges
Cartridge (firearms)
A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...
. An earlier method was the use of a wedge to block the rear of the gun.
Sealing was further improved with the de Bange obturator in 1872.
The major weakness of the original designs was that only half of the circumference of the breechblock could be threaded, hence a fairly long breechblock was still required to achieve a secure lock. Axel Welin
Axel Welin
Axel Welin , was a Swedish inventor and industrialist.Ernst Axel Martin Welin studied at the Royal Institute of Technology in Stockholm from 1879 to 1884. Between 1886 and 1888, Welin worked as a weapons designer for Thorsten Nordenfelt in London. In 1889 he started his own engineering firm, the...
solved this problem with his stepped interrupted screw design : the Welin breech block
Welin breech block
The Welin breech block was a revolutionary stepped, interrupted thread design for locking artillery breeches, invented by Axel Welin in 1889 or 1890...
of 1890. This design has threads of the block and breech cut in steps of successively larger radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
. This allows e.g. a breechblock with 4 steps to allow 4/5 of the block circumference to be threaded, allowing for a much shorter breechblock while still only requiring 1/5 turn to open or close. This is the basic design still in use with bagged charge artillery.