Fastener
A fastener is a mechanical hardware device that joins or affixes two or more objects together, typically in a removable manner to enable assembly, disassembly, or maintenance without permanent deformation of the joined components.[1] Common examples include screws, bolts paired with nuts, and rivets, which function by generating clamping forces through threads or deformation to resist separation under load.[2] Screw threads, a core principle underlying many threaded fasteners, originated around 400 BCE with the work of Archytas of Tarentum, who applied the helical mechanism for mechanical advantage in early devices.[3] In engineering and manufacturing, fasteners are indispensable for creating non-permanent joints that maintain structural integrity, accommodate thermal expansion, and allow for vibration resistance, thereby preventing failures that could arise from welding or adhesives in high-stress applications.[4] Standardized specifications, such as those from the ASME, define dimensions, materials, and performance criteria to ensure reliability across industries like aerospace, automotive, and construction.[5]
Fundamentals
Definition and Mechanical Principles
A fastener is a hardware device that mechanically joins or affixes two or more components together, typically by generating internal forces such as axial tension in the fastener itself, compressive clamp load on the joined members, and frictional resistance to shear.[6] These devices enable non-permanent assemblies that can be installed and removed using tools, relying on precise control of these forces to achieve joint stability without deformation or separation under load.[7] The core mechanical principle involves inducing a preload, defined as the initial tensile force in the fastener that creates a corresponding compressive force clamping the joint components. This preload is generated by applying torque to the fastener or nut, where the torque-preload relationship approximates T = K × D × F_i, with T as torque, K as the nut factor (typically 0.1–0.2, incorporating thread and head friction coefficients), D as nominal diameter, and F_i as preload; friction dissipates approximately 80–90% of the input torque as heat rather than axial force, necessitating calibrated tools for accuracy.[8][9] In tension-dominated joints, the preload ensures the fastener remains in tension while external separating loads are countered by this stored energy; in shear, it generates frictional grip exceeding the shear force (μ × clamp load, where μ is the friction coefficient, often 0.1–0.3 for lubricated steel interfaces), preventing slippage without relying solely on the fastener's shear strength.[8] Fasteners operate within the elastic portion of their stress-strain curve, where stress σ relates to strain ε by Hooke's law, σ = E × ε (E ≈ 200 GPa for steel), allowing reversible elongation and preload retention up to the yield point.[10] Insufficient preload—below 50–75% of the fastener's proof load, depending on joint design—causes partial load transfer to the fastener, inducing cyclic stress variations that accelerate fatigue; conversely, excessive preload risks yielding and permanent set, reducing effective clamp load over time.[11] Thus, joint integrity demands quantifiable clamp load verification (e.g., via strain gauges or ultrasonics) over torque-based approximations, as variations in friction or embedding can alter outcomes by 20–30% without direct measurement.[8][9]Load-Bearing and Joining Mechanisms
Fasteners sustain loads through specific mechanical interactions that distribute forces across their cross-sections and interfaces. In tensile loading, a fastener like a bolt experiences axial stress calculated as σ = P / A_t, where P is the applied tensile force and A_t is the tensile stress area of the threaded section, ensuring the material's yield strength is not exceeded to prevent elongation or failure. Shear loads induce transverse forces, with average shear stress τ = F / A for single shear or τ = F / (2A) in double shear configurations common in bolted lap joints, where the doubled effective area halves the stress compared to single shear.[12][13] Preload in threaded fasteners generates clamping force that compresses joined parts, approximating uniform pressure distribution under ideal stiffness conditions and primarily resisting external tensile loads via friction at the interface rather than direct bolt tension increase.[8] This preload, often set to 70% of the bolt's proof load, minimizes joint separation and enhances fatigue resistance by keeping the bolt in compression-dominant cycling.[14] Fatigue arises from cyclic loading, where repeated stress amplitudes propagate cracks, particularly if preload is insufficient; empirical testing shows fatigue strength reduced by over 50% under pure tension versus combined shear-tension in bolts.[15] Vibrational loads promote loosening through mechanisms like transverse slip at threads, eroding preload unless mitigated by locking features, as verified in dynamic testing standards.[16] Mechanical fasteners, such as bolts, enable semi-permanent joining via reversible threading, allowing disassembly for inspection and maintenance, unlike permanent methods like rivets—which deform to interlock materials—or welds, which fuse via melting and cannot be undone without destructive cutting.[17] Rivets bear shear directly through their shank without preload, suitable for high-vibration but lacking reusability, while bolts' clamping permits torque verification post-assembly, reducing hidden failure risks in critical applications.[18] Adhesives and welds offer continuous bonds but degrade under thermal cycling or require specialized equipment for reversal, making mechanical options preferable where empirical data prioritizes serviceability over initial bond strength.[19] Exceeding recommended torque risks yielding, where plastic deformation in the bolt shank or threads reduces effective preload and initiates fatigue cracks, as laboratory tests demonstrate increased hardness and fracture susceptibility beyond yield torque.[20] Standards testing, including torque-to-failure protocols, reveals that overtorquing by 20-30% can halve fatigue life by shifting the stress-strain curve into nonlinear regimes, underscoring that optimal strength balances preload without overload, not maximal material rating.[21][22] This causal link, derived from fracture mechanics analysis, counters assumptions of indefinite scalability in fastener strength, as excessive clamping induces compressive yielding in parts or galling, per NASA design criteria.[23]Historical Development
Origins in Antiquity
The earliest known metal fasteners date to the Bronze Age, with archaeological finds including gold and bronze dress fasteners, pins, and brooches used for securing garments and accessories. For instance, a gold dress fastener from approximately 1000 BCE was discovered in Staffordshire, England, exemplifying early precision crafting for functional joining.[24] Similarly, bronze spectacle fasteners—formed from coiled wire into paired loops for fastening cloaks or straps—appear in European Bronze Age contexts around 1500–1000 BCE, often found in hoards alongside tools and jewelry.[25] These artifacts, produced via hand-forging and casting, demonstrate rudimentary load-bearing capabilities but suffered from inconsistencies in dimensions and metallurgy, resulting in variable tensile strength and fit.[26] The invention of the screw thread, a pivotal advancement in mechanical fastening, is traditionally attributed to Archytas of Tarentum around 400 BCE, who applied it in wooden presses for extracting olive oil and wine.[27] This helical mechanism enabled greater clamping force through rotation, building on lever principles and foreshadowing later devices like Archimedes' screw for water lifting in the 3rd century BCE.[28] Early screws were primarily wooden, carved with chisels or saws, limiting their use to low-precision applications such as agricultural presses; metal variants emerged sporadically but remained rare due to forging challenges.[29] Pre-industrial production methods, reliant on manual labor without gauges or dies, precluded standardization, as evidenced by mismatched threads and fittings in surviving ancient assemblies from Greek and Roman sites.[30] This variability often led to unreliable joints, with empirical analysis of artifacts showing irregular pitches and diameters that hindered interchangeability and contributed to failures under load.[31] Such limitations confined fasteners to custom, site-specific roles rather than scalable engineering.Industrial Advancements and Standardization
The Industrial Revolution, commencing around 1760 in Britain, introduced machine tools that enabled the shift from hand-forged to machine-cut screws and bolts, facilitating mass production and greater uniformity in fasteners.[27] Early innovations included J. and W. Wyatt's factory process for producing screw threads in 1760, which marked the onset of industrialized fastener manufacturing.[27] By the late 18th century, Henry Maudslay's development of the screw-cutting lathe in 1797 allowed for precise thread generation, reducing variability that had previously caused assembly inconsistencies in machinery.[32] These advancements directly contributed to efficiency gains by minimizing production time and material waste, as machine-cut threads could be replicated consistently across batches. Prior to standardization, screw threads varied widely, rendering fasteners non-interchangeable and complicating repairs in expanding industrial applications like railways and steam engines.[28] In 1841, British engineer Sir Joseph Whitworth proposed the British Standard Whitworth (BSW) thread form, featuring a 55-degree flank angle and rounded roots, as the first national standard to ensure reliable mating between screws and nuts.[33][34] This uniform profile addressed causal issues of poor fit and stripping, promoting durability under load. In the United States, William Sellers independently advanced a 60-degree V-thread system in 1864, which simplified machining for mechanics and became the basis for the United States Standard, easing adoption in American manufacturing.[35][28] Standardized threads enabled interchangeable parts, revolutionizing assembly processes by allowing components from different producers to fit seamlessly, which reduced downtime in factories through faster repairs and scaled production.[28] Accompanying developments included standardized nuts for achieving controlled preload via torque application, which mitigated loosening from machinery vibration, and washers to distribute clamping forces evenly, as integrated into early 19th-century designs for bolted joints in engines and structures.[35] These causal links to reliability lowered failure rates in dynamic environments, with empirical adoption in sectors like railroads demonstrating reduced maintenance intervals compared to pre-standard era custom fittings.[27] Overall, such standardization underpinned economic efficiencies, supporting the proliferation of assembly lines and contributing to the era's manufacturing output growth.[36]Post-Industrial Innovations
Following World War II, advancements in metallurgical engineering led to the widespread adoption of high-strength low-alloy (HSLA) steels in fasteners, which provided superior tensile strengths compared to pre-war carbon steels, typically achieving yields exceeding 50 ksi while maintaining ductility for welding and forming.[37] These materials, developed through microalloying with elements like niobium and vanadium, enabled engineers to design lighter structural components in automotive and machinery applications without compromising load-bearing capacity, as evidenced by empirical testing showing reduced weight-to-strength ratios in post-1950s vehicle frames.[37] Heat treatments such as quenching and tempering further enhanced these properties, pushing ultimate tensile strengths to 100-150 ksi in alloyed fastener grades by the 1960s, allowing for thinner sections and improved fuel efficiency in industrial designs.[38] In the late 20th century, the International Organization for Standardization (ISO) promulgated metric fastener standards, such as ISO 898 for mechanical properties, which harmonized thread profiles and dimensions across global manufacturers starting from the 1970s, thereby reducing compatibility issues in international supply chains.[39] This shift from imperial (inch-based) to metric systems facilitated smoother trade and assembly in multinational projects, though legacy imperial infrastructure in regions like the United States created transitional challenges, including dual-inventory costs and retooling expenses estimated in billions during the 1980s metrication efforts.[40] Precision engineering techniques, including computer numerical control (CNC) machining introduced in the 1950s and refined thereafter, enabled tighter tolerances—down to microns—in fastener production, improving joint reliability under dynamic loads.[41] Military applications drove innovations in vibration-resistant fasteners, particularly self-locking nuts and prevailing torque designs, which underwent rigorous testing in high-vibration environments like aircraft and weaponry during the Cold War era, demonstrating superior resistance to loosening compared to standard nuts under cyclic stresses exceeding 10g.[42] These mechanisms, often incorporating deformed threads or nylon inserts, were validated through combat hardware deployments, where failure rates dropped by orders of magnitude relative to peacetime lab assumptions, influencing civilian sectors such as automotive engines for enhanced durability.[43]Classification and Types
Threaded Fasteners
Threaded fasteners predominate in reversible mechanical joints owing to their capacity to induce controlled axial preload, which clamps mating surfaces to resist shear, tension, and fatigue under dynamic loads. This preload, generated by torque application, creates compressive forces that exceed joint separation thresholds in empirical tests, with properly tensioned assemblies demonstrating up to 90% of the fastener's ultimate tensile strength in clamped load capacity before failure.[44][45] Unlike permanent methods, their threaded engagement permits disassembly without material damage, facilitating maintenance and reuse, a critical advantage in applications like automotive and machinery where empirical loosening studies highlight vibration resistance via sustained clamp force.[46] Bolts feature external threads along their shank and a head for torque application, typically inserted through aligned clearance holes in components and secured by a mating nut to develop joint preload.[47] Screws differ by engaging directly into tapped holes or self-tapping into untapped materials, eliminating the need for a separate nut and enabling compact, self-contained fastening.[48] Nuts provide internal threads to pair with bolts, distributing load across the joint face via washers if needed. The torque-preload relationship approximates T = K F d, where T is applied torque, F is achieved preload force, d is nominal diameter, and K is the nut factor (typically 0.10 to 0.20 depending on lubrication and thread condition), allowing predictable tensioning for joint integrity.[49][9] Head designs optimize tool compatibility and torque transmission: hexagonal heads suit open-end wrenches for high-torque manual assembly, while recessed Phillips crosses enable self-centering in power drivers but prone to cam-out under overload. Torx (star-pattern) drives outperform slotted and Phillips in torque transfer efficiency, sustaining 25-50% higher applied torque before stripping due to radial contact geometry that minimizes slippage, as evidenced in comparative stripping torque tests.[50][51] Specialized variants include studs, which are headless rods threaded at one or both ends for embedded or adjustable positioning in confined spaces, and set screws, fully threaded headless fasteners driven into tapped holes to exert radial pressure against shafts or components. Set screws minimize misalignment stresses by locking rotational elements, with cup-point styles providing highest compressive force and vibration resistance in assembly retention evaluations.[52][53][54]Non-Threaded and Permanent Fasteners
Non-threaded permanent fasteners, such as rivets, dowel pins, and structural adhesives, form irreversible joints by means of deformation, interference fit, or chemical bonding, making them suitable for applications emphasizing shear resistance and assembly efficiency over disassembly. Unlike threaded options, these avoid helical engagement, reducing points of stress concentration that can initiate fatigue cracks under cyclic loading, though they sacrifice inspectability for failure analysis since joints cannot be non-destructively separated. Empirical testing reveals rivets exhibit superior vibration resistance, maintaining clamp without loosening, as deformation distributes loads more uniformly than threads.[55] Rivets, typically comprising a cylindrical shank expanded via hammering or pressing to form a second head, excel in shear-heavy scenarios where materials must resist lateral forces without rotation. Lap-shear tests on riveted aluminum connections demonstrate ultimate shear capacities exceeding 20 kN for 4.8 mm diameter rivets before deformation yields, with post-installation deformation curves showing progressive ductility up to 10-15% elongation prior to failure, contrasting threaded fasteners' notch sensitivity that accelerates fatigue under equivalent cyclic shear.[56] Solid rivets in single shear maintain strengths around 300-400 MPa, with minimal reduction from initial hot-driven installation, prioritizing bulk deformation over localized thread stripping.[57] Dowel pins provide permanent alignment and shear transfer through press-fits into pre-drilled holes, undergoing controlled plastic deformation to embed without threading. In double-shear configurations, load capacity scales with diameter to the 1.5 power and substrate modulus of rupture to the 0.6 power, yielding ultimate strengths of 50-100 kN for 10 mm steel pins in metallic joints before shear plane rupture, though exceeding yield (around 400 MPa for alloy steel) introduces permanent set that enhances friction lock but limits reusability.[58] Experimental dowel-pinned granite-steel assemblies confirm pins yield plastically at 80-90% of ultimate load, deforming 5-10% prior to substrate cracking, offering predictable failure modes for overload protection.[59] Structural adhesives, curing via polymerization to create molecular bonds, serve as non-mechanical permanents but exhibit time-dependent creep under sustained shear or tension, with strain rates amplified in humid conditions due to plasticizer migration and hydrolysis weakening cross-links. Epoxy adhesives under 20-30% of tensile strength at 55% relative humidity display creep strains of 1-5% after 1000 hours, escalating to 10-20% at 80-90% RH from moisture-induced chain scission, undermining claims of indefinite load-bearing without empirical validation of environmental controls.[60][61] Hygrothermal aging further accelerates this, with polyurethane joints showing 2-3 times higher creep compliance after 5000 hours at 40°C and 85% RH compared to dry benchmarks.[62] Permanent fasteners yield assembly cost reductions of 30-50% in high-volume production through simplified tooling and elimination of torque sequencing, as seen in transitions from rivets to adhesives in aerospace panels, but incur elevated lifecycle expenses in repair-prone contexts due to destructive removal necessitating full joint replacement.[63] This permanence hinders causal failure attribution, as non-destructive evaluation is infeasible without bond severance, contrasting removable alternatives' allowance for iterative inspection and preload verification to preempt cascading defects.[64]Specialized Variants
Self-locking fasteners incorporate mechanisms such as nylon inserts or deformed threads to generate prevailing torque, resisting loosening in vibration-prone environments through frictional resistance that maintains preload under cyclic loading. Nylon-insert variants, standardized under ISO 10511, deform against mating threads to provide locking action suitable for moderate temperatures up to approximately 250°C, with empirical tests demonstrating retention of clamp load beyond 2,000 vibration cycles before significant loss in controlled setups.[65][66] Prevailing torque designs, which exhibit off-torque values exceeding installation torque, prevent complete detachment under transverse vibration; studies indicate that higher initial prevailing torque levels correlate with reduced axial relaxation and no full loosening in dynamic tests simulating machinery operation.[67] Blind fasteners, including rivets and bolts installable from one side, enable secure joining in access-restricted assemblies, particularly in aviation where bilateral access is infeasible, such as in fuselage panels or internal structures. These variants expand or lock upon setting to distribute load evenly, minimizing deformation risks in thin materials, and are verified through aerospace protocols to lower foreign object debris (FOD) incidence by avoiding detachable components during vibration exposure.[68][69] Captive fasteners extend this by integrating retention features like springs or threads that tether the element to the host assembly post-installation, reducing loose part hazards in maintenance scenarios; guidelines from the National Aerospace FOD Prevention Initiative endorse self-retaining types in critical zones like fuel systems to prevent migration under operational stresses.[70][71] Masonry anchors, such as expansion or screw types embedded in mortar joints or blocks, prioritize pull-out resistance quantified via standardized tensile testing rather than installation simplicity, ensuring anchorage against withdrawal forces from structural loads or seismic events. ASTM E754 specifies procedures for measuring extraction resistance of embedded ties and anchors, typically yielding metrics in kN based on embedment depth and mortar shear strength, with values often exceeding 5 kN for qualified installations in medium-duty brickwork.[72] Complementary ASTM C1892 evaluates overall tensile and shear capacities in masonry assemblages, confirming that pull-out failure modes dominate in low-strength substrates, necessitating empirical validation over theoretical projections for site-specific efficacy.[73]Materials and Properties
Primary Materials and Their Characteristics
Steel fasteners, primarily composed of carbon, alloy, and stainless variants, constitute the majority of mechanical fasteners due to their favorable balance of high tensile and yield strengths relative to density. Carbon steels, often low- to medium-carbon grades, exhibit yield strengths ranging from 55 ksi for Grade 2 bolts to 92 ksi for Grade 5, enabling robust load-bearing in general applications, though they require coatings like zinc plating to mitigate corrosion susceptibility in moist environments.[74] Alloy steels, such as those in SAE Grade 8 specifications, achieve minimum yield strengths of 130 ksi through quenching and tempering processes, providing superior fatigue resistance and clamping force in high-stress assemblies, with a density of approximately 7.85 g/cm³ yielding a specific strength competitive for structural uses.[74] Stainless steels, alloyed with at least 10.5% chromium to form a passive oxide layer that inhibits corrosion via electrochemical passivation, offer yield strengths around 30 ksi for common AISI 304 grades, prioritizing durability in oxidative or chloride-exposed conditions over peak mechanical strength.[75]| Material Type | Typical Yield Strength (ksi) | Density (g/cm³) | Key Characteristics |
|---|---|---|---|
| Carbon Steel (Grade 5) | 92 | 7.85 | High ductility, cost-effective for moderate loads; prone to rust without protection.[74] |
| Alloy Steel (Grade 8) | 130 | 7.85 | Exceptional tensile capacity (150 ksi min); suited for dynamic loads but susceptible to hydrogen embrittlement.[74] |
| Stainless Steel (304) | 30 | 8.0 | Chromium-induced corrosion resistance; lower strength limits use in non-critical joints.[75] |