Interrupted screw: Difference between revisions
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An interrupted screw is not a screw at all and breeches are machined with true spiraling threads which continue down to and resume spiraling from the interruptions not in the screw but in the smooth tapered plug that becomes a screw when threaded and has interrupted threads to permit it to block the breech bore instead of plug it. |
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[[Image:Interrupted breech screw.jpg|thumb|250px|Illustration of an [[M109 howitzer|M109]] breech plug]] |
[[Image:Interrupted breech screw.jpg|thumb|250px|Illustration of an [[M109 howitzer|M109]] breech plug]] |
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An '''interrupted thread''' is a mechanical device typically used in the [[Breech-loading weapon|breech]] of [[artillery]] guns. It is believed to have been invented in 1845.<ref name="Bunch">[https://books.google.com/books?id=MlQ7NK9dw7IC&pg=PA355 ''The history of science and technology'' by Bryan H. Bunch p.355]</ref> |
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The gun's breech is threaded internally and the breech block is threaded externally which effectively turns the block into a male pipe plug screwed into a female pipe. However, because the threads would tend to foul as well as "cross" given the extreme taper necessary to prevent the block from seizing in the breech, the breech bore and block typically have two flutes cut across the threads and axial to the parts themselves with the breech flutes cut[["offset" to the block's flutes so that the breech resembles a [[thread die]] and the block resembles a [[thread tap]] but having only a pair of flutes rather than typical four flutes those threading tools have to create space for the actual cutting of the threads and ejection of the chips. The screwing action of the block into and out of the breech is retained and the commonly used term "interrupted screw" is incorrect. The threads themselves are interrupted by the flutes and those flutes allow the block to slide fully into the breach where its impossible for the threads to "cross", the plug has to be fully seated in the breech for any thread engagement to occur and the block only has to rotate a fraction of one revolution to be fully threaded into the breech instead of one revolution per thread. |
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It is a [[screw]] set into the breechblock that has a section of thread along its axis removed. The screw is mated with a partially threaded hole at the rear of the weapon's chamber: threadless channels in the breechblock screw line up with the threaded parts of the breech, and vice versa. The screw can thus be smoothly inserted all the way into the gun, after which as little as one-sixth 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 that does not employ [[Cartridge (firearms)|cartridges]]. An earlier method was the use of a wedge to block the rear of the gun.<ref name="Bunch" /> |
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The taper and interrupted thread design are crucial to safe, efficient and reliable closing and opening of the breech and the taper and allow the breech block to be hung on a swinging hinge mechanism that allows the block to swing entirely out of the way for open access to the breech for the crew and any type of ammunition and loading systems from bagged powder charges and projectiles with no cartridge case to standard cartridge-type ammunition which is identical in form and function to small arms cartridge ammunition of the rimless variety the overwhelming majority of the time due to the necessity of headspacing the cartridge cases on their shoulders and "preloading" them with the closed and locked breech block to safely handle the very high pressures and velocities smokeless, rimless and bottlenecked cartridges generate during firing. The breech block upon initial closing when the ammunition and gun are properly designed, constructed and assembled will never physically contact the chamber/breech of the gun anywhere but on the threads themselves and upon closing on a loaded cartridge case will be perfectly aligned with the threads while resting flat and lightly on the rear face of the fully chambered and seated case. |
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Sealing was further improved with the [[de Bange#De Bange breech obturator system|de Bange obturator]] in 1872, and all interrupted screw breeches with this system were generally known as the De Bange type. |
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The result is zero [[headspace]] with the case fully into the chamber, the block is fully into the breech and the threads precisely aligned ready to engage with block rotation providing the final compression and clamping of the cartridge block into the chamber and breech. The breech, which is screwed into the breech and the process of screwing it into the breech are not only not "interrupted", they are greatly accelerated and abbreviated by reducing what would take one full revolution of the breech plug per thread simply to screw it firmly against the case head while leaving it "loose" in the breech to a fraction of one revolution once the block "drops" to the bottom of the threaded breech bore just as the bolt of a rifle or shotgun sliding directly and completely into its receiver in a straight line into a "threaded" receiver upon bottoming out against a cartridge case depending on the design and at most requires approximately 100 degrees of rotation to as few as 60 degrees of rotation if equipped with 3 or 6 locking lugs. |
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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]] 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]]. 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 [[Shell (projectile)#Separate loading bagged charge|bagged charge]] artillery. |
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==See also== |
==See also== |
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==Notes== |
==Notes== |
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{{reflist |
{{reflist} |
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==External links== |
==External links== |
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Revision as of 06:55, 19 December 2018
An interrupted thread is a mechanical device typically used in the breech of artillery guns. It is believed to have been invented in 1845.[1]
The gun's breech is threaded internally and the breech block is threaded externally which effectively turns the block into a male pipe plug screwed into a female pipe. However, because the threads would tend to foul as well as "cross" given the extreme taper necessary to prevent the block from seizing in the breech, the breech bore and block typically have two flutes cut across the threads and axial to the parts themselves with the breech flutes cut[["offset" to the block's flutes so that the breech resembles a thread die and the block resembles a thread tap but having only a pair of flutes rather than typical four flutes those threading tools have to create space for the actual cutting of the threads and ejection of the chips. The screwing action of the block into and out of the breech is retained and the commonly used term "interrupted screw" is incorrect. The threads themselves are interrupted by the flutes and those flutes allow the block to slide fully into the breach where its impossible for the threads to "cross", the plug has to be fully seated in the breech for any thread engagement to occur and the block only has to rotate a fraction of one revolution to be fully threaded into the breech instead of one revolution per thread.
The taper and interrupted thread design are crucial to safe, efficient and reliable closing and opening of the breech and the taper and allow the breech block to be hung on a swinging hinge mechanism that allows the block to swing entirely out of the way for open access to the breech for the crew and any type of ammunition and loading systems from bagged powder charges and projectiles with no cartridge case to standard cartridge-type ammunition which is identical in form and function to small arms cartridge ammunition of the rimless variety the overwhelming majority of the time due to the necessity of headspacing the cartridge cases on their shoulders and "preloading" them with the closed and locked breech block to safely handle the very high pressures and velocities smokeless, rimless and bottlenecked cartridges generate during firing. The breech block upon initial closing when the ammunition and gun are properly designed, constructed and assembled will never physically contact the chamber/breech of the gun anywhere but on the threads themselves and upon closing on a loaded cartridge case will be perfectly aligned with the threads while resting flat and lightly on the rear face of the fully chambered and seated case.
The result is zero headspace with the case fully into the chamber, the block is fully into the breech and the threads precisely aligned ready to engage with block rotation providing the final compression and clamping of the cartridge block into the chamber and breech. The breech, which is screwed into the breech and the process of screwing it into the breech are not only not "interrupted", they are greatly accelerated and abbreviated by reducing what would take one full revolution of the breech plug per thread simply to screw it firmly against the case head while leaving it "loose" in the breech to a fraction of one revolution once the block "drops" to the bottom of the threaded breech bore just as the bolt of a rifle or shotgun sliding directly and completely into its receiver in a straight line into a "threaded" receiver upon bottoming out against a cartridge case depending on the design and at most requires approximately 100 degrees of rotation to as few as 60 degrees of rotation if equipped with 3 or 6 locking lugs.
See also
Notes
{{reflist}
External links
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