Interrupted screw

An interrupted screw, also known as an interrupted thread, is a mechanical fastening device featuring helical threads that are periodically interrupted by longitudinal grooves or channels, enabling the screw to engage or disengage from a mating component with only a partial rotation rather than a full turn.^[1] This design allows for rapid assembly and disassembly while maintaining secure locking under high pressure, distinguishing it from continuous-thread screws that require multiple rotations.^[1] The interrupted screw is most prominently employed in the breech mechanisms of artillery and large-caliber firearms, where a breechblock with interrupted threads mates to a stationary breech ring to seal the gun chamber against propellant gases.^[1] In this application, the mechanism typically involves two primary motions: a rotational lock via the interrupted threads (often a quarter-turn or less, depending on the number of thread sectors) and a hinging action to open or close the breech, facilitating quick reloading in separate-loading ammunition systems.^[1] Common variants include single-cut threads, conical-taper designs like the Bofors system, and multi-stepped threads such as the Welin breech, which enhance strength by increasing the effective thread engagement length and distributing stress uniformly across the outer circumference.^[1] These mechanisms are actuated manually via levers or crankshafts, or semiautomatically using recoil energy, and incorporate safety interlocks to prevent firing unless fully locked.^[1] Historically, the interrupted screw concept was devised in France around 1845 and emerged more broadly in the mid-19th century amid efforts to improve breech-loading artillery over slower muzzle-loaders, with American inventor Benjamin Chambers receiving a U.S. patent for a slotted-screw breech design in 1849.^[2]^[3] French engineers subsequently refined the system, leading to widespread adoption in European militaries by the 1870s, as demonstrated in Prussian rifled breechloaders during the Franco-Prussian War (1870–1871).^[2] In the United States, the design was integrated into field artillery following congressional mandates for breech-loading tests in 1872, culminating in its standardization for the 3.2-inch steel field gun in 1881 under Chief of Ordnance Stephen Vincent Benét, which achieved muzzle velocities up to 1,629 feet per second in 1884 trials.^[2] By the early 20th century, modifications like the Gerdom interrupted-screw breech (adopted in 1897) enabled rapid-fire capabilities, as seen in the U.S. 3-inch Field Gun Model of 1902, marking a shift toward more efficient and powerful cannon systems.^[2] Despite their advantages in speed and strength, these mechanisms require precise machining to ensure proper alignment and gas sealing, often via obturator pads, and remain a cornerstone of modern large-caliber weapon design due to their balance of simplicity and robustness.^[1]

History

Invention and Early Development

The interrupted screw mechanism emerged in the mid-19th century as a critical innovation for breech-loading artillery, enabling faster and more secure closure of the gun's rear end compared to earlier designs. The first documented patent for this device was issued to American inventor Benjamin Chambers on July 31, 1849, describing a movable breech for fire-arms that utilized sectional (interrupted) screw threads to allow quick engagement and disengagement with a partial rotation of the breech plug.^[4] Chambers' design featured alternating threaded and unthreaded segments on both the barrel's interior and the breech piece, permitting the plug to slide into place and lock securely without requiring multiple full turns, thus facilitating rapid loading and swabbing of the bore.^[4] Following Chambers' patent, the concept was acquired by the French military and refined by engineers under General Antoine Treuille de Beaulieu, who adapted the crude American prototype into a more practical system for artillery use by the early 1850s.^[5] These initial French iterations retained the basic interrupted thread configuration on the breech plug but incorporated improvements for better alignment and gas sealing, addressing early issues with leakage in high-pressure environments.^[5] The primary aim of these early interrupted screw designs was to support swift breech closure in artillery, allowing gunners to reload and fire more rapidly than with muzzle-loading systems or fully threaded plugs, which demanded time-consuming rotations. Primitive prototypes, such as those tested in experimental breech-loaders, often employed simple two- or three-segment interruptions on wrought-iron gun barrels, demonstrating feasibility in controlled trials but revealing challenges like thread wear and imperfect obturation.^[4]^[5] This invention occurred amid the pivotal transition from muzzle-loading to breech-loading artillery in the 1840s and 1850s, spurred by evolving military tactics that emphasized rate of fire during engagements. The Crimean War (1853–1856) highlighted the need for faster artillery loading, accelerating the broader transition from muzzle-loaders to breech-loaders despite reliability concerns. Early testing of interrupted screw-equipped guns during this period laid the groundwork for later advancements, such as the Welin breech block introduced in the 1890s.^[5]

Key Innovations and Adoption

By the 1870s, the design saw widespread adoption in European militaries, including Prussian rifled breechloaders during the Franco-Prussian War (1870–1871).^[2] In the United States, the design was integrated into field artillery following congressional mandates for breech-loading tests in 1872, culminating in its standardization for the 3.2-inch steel field gun in 1881.^[2] A major advancement came in 1889-1890 when Swedish engineer Axel Welin, working for Thorsten Nordenfelt in London, invented the stepped, multi-start interrupted thread system known as the Welin breech block, which enabled quarter-turn locking for faster operation in large-caliber artillery.^[6]^[7] The timeline of adoption traces from experimental breech-loaders in the mid-19th century to implementations in Armstrong guns starting in the 1880s, to full standardization in 20th-century naval guns across major powers, reflecting iterative improvements in thread engagement and obturation.^[8] The design's adoption accelerated in the late 19th and early 20th centuries; the U.S. Navy integrated the Welin screw breech plug into all new guns above 4 inches in caliber starting in 1899, following post-Spanish-American War evaluations that highlighted its efficiency for heavy naval ordnance.^[9] The British Royal Navy adopted Welin breeches in the early 1900s, incorporating them into pure-couple mechanisms for battleship main batteries to support rapid firing rates.^[10] During World War I, the Welin breech played a key role in artillery systems, becoming standard for Army weapons of 155 mm bore or larger due to its reliable sealing and compatibility with separate-loading ammunition, enabling sustained barrages in major offensives.^[1] Further evolution included the development of sliding-block variants in the early 20th century, where interrupted threads were combined with horizontal or vertical sliding motions to reduce rotational effort and integrate semiautomatic operation powered by recoil energy, as seen in designs for field howitzers.^[1]

Design Principles

Basic Mechanism

The interrupted screw mechanism operates on the principle of alternating threaded and unthreaded segments on both the screw and its mating component, allowing for rapid engagement and disengagement through a partial rotation rather than multiple full turns. This design divides the thread into multiple sectors, typically six to twelve, where the interruptions—clearance channels or gaps—enable the threaded portions to align and interlock with minimal angular movement, often as little as one-sixth to one-quarter of a full revolution. The core advantage lies in mimicking the strength of a continuous thread while drastically reducing assembly time, making it suitable for applications requiring quick handling under pressure.^[1] In operation, the process begins with aligning the unthreaded interruptions of the breechblock (or screw) with the corresponding gaps in the receiving thread, permitting axial insertion without resistance. Once aligned, a short rotational motion—facilitated by manual levers, gears, or automated systems—engages the threaded sectors, drawing the components together to form a secure closure. As pressure builds, such as from propellant gases, the engaged threads lock firmly due to frictional forces, ensuring a gas-tight seal without further rotation. Disengagement reverses this sequence: the block is rotated to realign interruptions, allowing withdrawal. This step-by-step engagement distributes the load evenly across the active thread sectors, preventing stress concentrations and enhancing overall durability.^[1] Compared to traditional continuous threads, which require several full rotations to achieve secure fastening and thus prolong exposure to high-pressure environments, the interrupted screw minimizes rotation to a fraction of a turn, enabling faster operation while maintaining comparable axial strength through the collective engagement of multiple thread segments. The physics of load distribution in this mechanism relies on shear and compressive forces transmitted uniformly across the interrupted threads, analyzed via models of thick-plate stress to confirm that the design withstands longitudinal forces effectively despite the partial circumferential contact.^[1]

Thread Configurations

Interrupted screw threads feature geometric interruptions that divide the continuous helical thread into discrete segments separated by equal gaps, enabling quick engagement and disengagement. Standard configurations typically employ six to twelve thread segments with corresponding gaps, where the number of sectors determines the rotation angle required for locking— for instance, six sectors necessitate a 60° turn for full engagement.^[1] These designs commonly use single-start threads.^[1] The Welin screw represents a specialized interrupted thread design characterized by stepped diameters across 2 to 3 levels, which promote progressive engagement starting from the largest diameter and advancing to smaller ones.^[1] This stepped geometry increases the effective contact area under load, achieving up to 75% thread engagement in optimized implementations and enhancing overall strength and sealing performance.^[1] Threads often incorporate a 20-degree pressure angle to extend service life by distributing shear stresses more evenly, with maximum shear reaching approximately 1.4 times the average value.^[1] Manufacturing interrupted screws demands precision to maintain structural integrity, typically involving special lathes or milling machines to cut the interruptions accurately while preserving thread uniformity.^[1] High-strength alloy steels, such as AISI E4340 or 4140/4340 hardened to Rockwell C 34-40, are standard materials to ensure durability under high pressures.^[1] Variations include buttress thread profiles for high-load scenarios, which provide superior axial load resistance due to their asymmetrical shape.^[1] Strict tolerances, such as head clearances of 0.010 to 0.025 inches, are critical to minimize misalignment and ensure reliable interlocking.^[1]

Applications

Military Uses

The interrupted screw mechanism has been a cornerstone of military artillery design, particularly in breech-loading systems that facilitate rapid reloading under combat conditions. Developed for large-caliber guns using separate-loading ammunition with bagged charges, it allows the breech block to be opened and closed with fewer turns compared to continuous threads, enabling sustained fire rates essential for field and naval artillery. This design excels in transmitting high chamber pressures while providing secure sealing against propellant gases through effective obturation.^[1] In naval applications, interrupted screw breeches, often employing the Welin stepped-thread variant, were widely adopted in 20th-century bag guns for battleships and cruisers, supporting quick partial rotations to enhance reloading speed during engagements. For instance, the British 15-inch/42 Mark I gun and the U.S. Navy's 16-inch/50 Mark 7 (used on Iowa-class battleships during World War II) incorporated this mechanism to handle the demands of high-pressure naval combat. Similarly, in field artillery, systems like the U.S. 8-inch Howitzer M2A2 and the 155 mm Self-Propelled Howitzer M126A1 (mounted on the M109 vehicle) utilized semiautomatic interrupted-screw breeches that harness recoil energy for rotation and pivoting, allowing crews to maintain fire while minimizing manual effort. The 175 mm Self-Propelled Gun M113A1 further exemplifies this, featuring a Welin two-stepped-thread breech tested to 75,000 psi for reliable performance in mobile artillery roles.^[1]^[10] Tank and self-propelled gun cannons have also integrated interrupted screw designs for rapid maintenance and reloading in armored warfare, evolving from 19th-century smoothbore adaptations to high-pressure 20th-century systems. The interrupted screw's strength and uniform stress distribution make it ideal for transmitting longitudinal forces in these environments, as seen in the 155 mm Howitzer XM199, which adheres to military specifications for threaded connections in tank-mounted applications. Post-World War II advancements shifted toward semiautomatic variants, improving rate of fire and reliability in dynamic battlefield scenarios. In firearms, interrupted threads appear in historical contexts, such as barrel attachments on early machine guns from World War I and II eras.^[1]^[1]

Civilian and Industrial Uses

In standard diving suits, interrupted screws secure the helmet to the breastplate, enabling a quick quarter-turn attachment and detachment for emergency release during underwater operations. This mechanism, featuring segmented threads that align for rapid engagement, incorporates a leather gasket in the breastplate recess to ensure a watertight seal while allowing tenders to swiftly remove the helmet if needed.^[11]^[12] In industrial machinery, interrupted screw presses facilitate dewatering processes in food processing, such as extracting juice from fruits or separating solids from fish byproducts. Vincent Corporation's designs employ interrupted flighting on the screw, which halts material flow to build pressure and promote agitation, resulting in higher liquid yield and consistent cake formation without jamming on variable feeds. These presses, often with tapered screws and perforated screens, handle slippery or fibrous materials like orange peel and fish waste efficiently.^[13] Interrupted screws serve as quick-release fasteners in general engineering applications, including connectors, valves, and modular assemblies where rapid assembly and disassembly are required. For instance, quarter-turn locks using interrupted threads, such as M39x3.0 configurations, enable tool-free securing in prototypes and equipment housings by aligning segments for a 90-degree rotation lock.^[14] Patents from the 1950s advanced interrupted screw applications in industrial contexts, such as US2850782A for quick-make-and-break fasteners interconnecting overlapping work sections like telescoping tubes or caps on containers. These innovations extended to tooling and high-pressure systems, where segmented threads provide secure yet releasable joints under load, as seen in engineering closures for pressure testing.^[15]^[16]

Advantages and Limitations

Advantages

The interrupted screw mechanism provides significant operational efficiency through its quarter-turn or fractional locking action, which requires only about 27.5 degrees of rotation for full engagement, drastically reducing assembly and disassembly times compared to continuous threads that demand multiple full rotations. This design is particularly suited for rapid tasks in high-pressure environments, such as artillery loading, where semiautomatic operation leverages recoil energy and counterbalance springs to enhance the overall firing cycle rate. Modern systems, such as the M109 howitzer, continue to employ this mechanism for efficient reloading in sustained fire scenarios.^[17]^[1] In terms of structural performance, the interrupted screw excels in strength and stress distribution, with engaged threads covering approximately 75% of the surface area to bear uniform longitudinal loads, achieving near-equivalent strength to fully threaded systems despite the interruptions. The stepped thread configurations distribute shear stresses across multiple sectors, limiting maximum shear to about 1.4 times the average and minimizing localized wear under high pressures up to 60,000 psi.^[17]^[1]^[7] Ease of use is another key benefit, as the minimal rotation simplifies actuation in confined spaces and requires lower torque for engagement than traditional screws, facilitated by efficient crankshaft and gear systems that reduce operator effort. This makes it ideal for quick-firing applications, where the breech block can be swung open via a pivot for swift access.^[1]^[7] The mechanism also demonstrates superior durability in dynamic settings, such as artillery recoil, where integrated locks prevent unintended rotation under vibration and pressure, while balanced stress distribution and robust materials like hardened AISI E4340 steel ensure long-term reliability, with life estimates exceeding 9 million rounds in high-stress scenarios.^[1]

Limitations and Weaknesses

One significant limitation of the interrupted screw design is the reduced contact area between threads in basic configurations, where interruptions in single-cut threads remove up to 50% of the threading, limiting engagement to half the block surface and thereby decreasing overall shear strength under extreme loads. However, stepped-thread variants like the Welin design mitigate this by increasing engagement to up to 75%, better distributing stresses. This reduction in holding power can still lead to higher stress concentrations, with the tooth nearest the load experiencing up to 140% of the average shear stress, potentially compromising structural integrity in high-pressure applications like artillery breeches.^[1] Manufacturing the interrupted screw introduces notable complexity, as the design demands precise machining of stepped threads and complex breechblock surfaces, which increases production costs and heightens the risk of defects such as misalignment or tolerance deviations.^[1] For instance, configurations like the Welin stepped-thread type require advanced fabrication processes for thread sectors and clearance channels, making it inherently difficult to produce without specialized equipment, and any imprecision can degrade performance in operational settings.^[1] The presence of gaps and interruptions in the threading makes the mechanism particularly vulnerable to debris accumulation, where contaminants like sand, dust, or mud can become trapped, causing jamming, abrasion, or uneven stress distribution in contaminated environments.^[1] Such issues necessitate frequent inspections for obstructions like bag fragments before loading.^[1] Historically, early interrupted screw designs were prone to gas leakage due to incomplete sealing under propellant pressures, often requiring auxiliary systems like the DeBange obturator to achieve a gas-tight closure, though these mitigations were not always reliable and could fail under extreme conditions, leading to erosion of threads or excessive propellant gas escape.^[1]^[17] Modern iterations continue to rely on such seals, but vulnerabilities persist, as obturator pads must endure pressures from 8,000 to 60,000 psi, and any design flaw can result in sealing failures that compromise the mechanism's safety and efficiency.^[1]