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Breech from Russian 122 mm M1910 howitzer, modified and combined with 105 mm H37 howitzer barrel An interrupted screw or interrupted thread is a mechanical device typically used in the
breech Breech may refer to: * Breech (firearms), the opening at the rear of a gun barrel where the cartridge is inserted in a breech-loading weapon * breech, the lower part of a pulley block * breech, the penetration of a boiler where exhaust gases leave ...
of
artillery Artillery is a class of heavy military ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during si ...
guns. It is believed to have been invented in 1845. The system has also been used to close other applications, including the joint between helmet (bonnet) and breastplate (corselet) of
standard diving suit Standard diving dress, also known as hard-hat or copper hat equipment, deep sea diving suit or heavy gear, is a type of diving suit that was formerly used for all relatively deep underwater work that required more than breath-hold duration, which ...
helmets, and the locks of diving chambers.


Design

An interrupted screw has a section of thread along its axis removed. The screw is mated with a partially threaded hole in the receptacle: threadless channels in the breechblock screw line up with the threaded parts of the screw, and vice versa. The screw can thus be smoothly inserted all the way into the receptacle, after which as little as one-eighth of a turn can engage the two sets of threads securely, sealing the joint. The amount of rotation required to achieve full closure depends on the number of unthreaded sectors. The minimum balanced arrangement has two sectors of 90°, and requires 1/4 turn to lock. three sectors of 60° requires 1/6 turn to lock, and the typical arrangement on a diving helmet, four sectors of 45°, requires 1/8 turn to lock.


Applications

M109 howitzer breech plug Illustration of M109 breech plug In the artillery application, the screw is set into the breechblock and mated with a partially threaded hole at the rear of the weapon's chamber. American engraver Benjamin Chambers, Sr. received in 1849 for a
wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" ...
built-up gun A built-up gun is artillery with a specially reinforced barrel. An inner tube of metal stretches within its elastic limit under the pressure of confined powder gases to transmit stress to outer cylinders that are under tension.Fairfield (1921) p. ...
"dissected screw breech" with "sectional screws" "cut therein for the purpose of speedily opening the breech for swabbing, depositing the load, and readily closing it again when the gun is to be discharged". His gun was built and tested with metallic cartridges, center-primed with a
percussion cap The percussion cap or percussion primer, introduced in the early 1820s, is a type of single-use percussion ignition device for muzzle loader firearm locks enabling them to fire reliably in any weather condition. This crucial invention gave rise ...
, because
US Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...
was initially interested, and chief of the
Bureau of Ordnance The Bureau of Ordnance (BuOrd) was a United States Navy organization, which was responsible for the procurement, storage, and deployment of all naval weapons, between the years 1862 and 1959. History Congress established the Bureau in the Departme ...
Charles Morris recommended more developments, but the system was too expensive for the time and cheaper
Dahlgren gun Dahlgren guns were muzzle-loading naval artillery designed by Rear Admiral John A. Dahlgren USN (November 13, 1809 – July 12, 1870), mostly used in the period of the American Civil War. Dahlgren's design philosophy evolved from an accidental e ...
s were adopted instead. Breech sealing was improved with the de Bange obturator in 1872, and all interrupted screw breeches with this system were generally known as the De Bange type. 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 Ernst Martin Axel Welin (10 November 1862 – 27 July 1951), was a Swedish inventor and industrialist. He was married to Agnes Welin from 1889. Axel Welin studied at the Royal Institute of Technology in Stockholm from 1879 to 1884. Between ...
solved this problem with his stepped interrupted screw design: the Welin breech block of 1890. This design has threads of the block and breech cut in steps of successively larger
radius In classical geometry, a radius (plural, : radii) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', ...
. For example, this allows a breechblock with four steps to allow four-fifths of the block circumference to be threaded, allowing for a much shorter breechblock while still requiring only one-fifth of a turn to open or close. This is the basic design still in use with bagged charge artillery. 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 that does not employ metallic cartridges. An earlier method was the use of a wedge to block the rear of the gun. The system has also been used to close other applications, including the joint between helmet (bonnet) and breastplate (corselet) of
standard diving suit Standard diving dress, also known as hard-hat or copper hat equipment, deep sea diving suit or heavy gear, is a type of diving suit that was formerly used for all relatively deep underwater work that required more than breath-hold duration, which ...
helmets, and the locks of diving chambers.


See also

*
Rifled breech loader A rifled breech loader (RBL) is an artillery piece which, unlike the smoothbore cannon and rifled muzzle loader (RML) which preceded it, has rifling in the barrel and is loaded from the breech at the rear of the gun. The spin imparted by the ...


References


External links


Interrupted screw breech mechanisms


Screws Firearm components {{artillery-stub