Glossary

An eccentric locknut is a type of locknut designed with an off-center (eccentric) collar or ring that creates a wedging action when tightened onto a bolt or threaded stud. Unlike a standard nut, the eccentric feature causes the threads of the nut to press tightly against the bolt’s threads, generating friction that resists loosening from vibration or dynamic loads.

The locking action does not rely on additional components like washers or adhesives—it’s built directly into the nut’s geometry. Once installed, the eccentric section of the nut slightly distorts the thread fit, increasing resistance to rotation. This makes it reusable and reliable in high-vibration environments.

Where you’ll see them

Automotive and heavy machinery where vibration is constant.

Railway and aerospace applications where safety and security are critical.

Industrial equipment where repeated maintenance and reassembly occur, since the nut maintains its locking ability over multiple uses.

Eddy Current Testing (ECT) is a non-destructive testing (NDT) method that relies on electromagnetic induction to detect flaws in conductive materials. A probe containing a coil is energized with alternating current, which generates a changing magnetic field. When the probe is brought near a conductive material, this field induces circulating electrical currents—called eddy currents—at the surface. These eddy currents in turn produce their own opposing magnetic fields, which alter the electrical impedance of the probe’s coil. If the material is free from defects, the eddy currents flow in a predictable way. However, when there is a discontinuity such as a crack, corrosion pit, or thinning of the material, the flow of eddy currents is disrupted, and the resulting change in coil response is measured and displayed on an instrument.

ECT is widely used in aerospace, power generation, and manufacturing industries for detecting cracks in aircraft skins, measuring tube wall thickness in heat exchangers, inspecting welds, and monitoring corrosion under paint or thin coatings. It has several advantages: it is fast, highly sensitive to small surface-breaking flaws, requires little or no surface preparation, and can even detect defects through thin coatings. Unlike ultrasonic testing, it does not require couplants, and unlike radiographic testing, it does not involve radiation. However, it does have limitations: it only works on electrically conductive materials, its penetration depth is shallow (best for surface and near-surface defects), and accurate interpretation depends heavily on the skill of the operator and proper calibration with reference standards. Despite these constraints, ECT remains an essential inspection method wherever precision, speed, and sensitivity are required.

Elastic deformation is the temporary change in shape or size of a material that occurs when an external force or stress is applied, but the material returns to its original shape once the force is removed. It happens within the material’s elastic limit, meaning the internal atomic bonds are stretched or compressed but not permanently rearranged. The relationship between stress and strain in this region follows Hooke’s Law, which states that the deformation is directly proportional to the applied load.

In simpler terms, when a force is applied to a bolt, spring, or metal rod, it may elongate or compress slightly, but once the load is removed, it returns to its initial length and form. This behavior occurs because the atomic structure of the material only experiences reversible displacements—the bonds between atoms act like tiny springs that stretch and recover.

Elastic deformation is a critical concept in engineering and materials science because it defines the safe working range of a material. Designers and engineers ensure that fasteners, beams, and machine components operate below their elastic limit to prevent permanent deformation or failure. When the stress exceeds this limit, the material enters the plastic deformation stage, where changes become irreversible.

In summary, elastic deformation is a reversible, proportional response to stress, allowing materials to absorb energy and return to their original form once the load is released. It represents the phase before permanent, plastic deformation begins.

The elastic limit is the maximum stress a material can withstand and still return to its original shape once the load is removed. Below this point, deformation is fully elastic—atoms in the material’s crystal lattice are displaced slightly but return to their original positions when the stress is released. If the stress goes beyond the elastic limit, the material enters plastic deformation, where the changes in shape are permanent and the material will not fully recover its original dimensions.

The elastic limit is closely related to, but not always identical to, the yield strength, which marks the onset of measurable permanent deformation. It is a critical property in the design of fasteners, springs, and structural components, because keeping stresses within the elastic range ensures a part can repeatedly bear loads without lasting damage. Exceeding the elastic limit can cause permanent distortion or failure, especially in applications where tight tolerances, fatigue resistance, or safety are essential.

An elastic stop nut (often called a nyloc nut or nylon insert locknut) is a type of locknut that incorporates a non-metallic insert, usually made of nylon, at the top of the nut. When the nut is tightened onto a bolt or threaded stud, the threads of the bolt cut into the nylon insert, creating friction and resistance that prevents the nut from loosening due to vibration or dynamic loads.

Unlike ordinary nuts that may back off under vibration, the elastic insert keeps constant pressure on the threads. This locking feature does not damage the bolt threads and allows the nut to be reused several times, though the locking ability decreases after repeated installations.

Elastic stop nuts are widely used in aerospace, automotive, machinery, and industrial applications where vibration resistance is critical. They are especially common in assemblies that experience continuous motion or dynamic stress, such as engines, airframes, and equipment frames.

Electrical Discharge Machining (EDM) is a non-traditional machining process that removes material from a conductive workpiece using controlled electrical discharges, or sparks. Instead of relying on physical contact to cut, EDM erodes the material by creating rapid, repetitive sparks between an electrode and the workpiece, both of which are submerged in a dielectric fluid. This makes EDM uniquely capable of machining extremely hard materials and producing complex, precise shapes that conventional machining methods cannot easily achieve.

The process works by placing a tool electrode, often made of graphite, copper, or brass, into a dielectric fluid along with the conductive workpiece. A voltage is applied between them, and as they are brought close, a spark discharge jumps across the gap. This discharge generates intense localized heat—ranging from 8,000 to 12,000 °C—that melts and vaporizes a tiny portion of the workpiece. The dielectric fluid cools the area, flushes away debris, and prevents continuous arcing. Because the spark frequency, duration, and position can be carefully controlled, EDM is able to achieve high precision in material removal.

There are several main types of EDM. Die-Sinking EDM (or Ram EDM) uses a shaped electrode that is pressed into the workpiece to create a cavity of the same form, which is commonly used for molds and dies. Wire EDM relies on a thin, continuously fed brass wire electrode to cut through a workpiece in a manner similar to a bandsaw, producing fine and accurate cuts. Hole Drilling EDM is specialized for making very small, deep holes by using tubular electrodes.

The applications of EDM are widespread. In tool and die making, it is invaluable for producing molds, dies, and punches with intricate details. In the aerospace and automotive industries, EDM is used to machine hard alloys, turbine blades, and engine components. The medical field employs EDM to create precise parts from biocompatible metals like titanium. It is also essential in micro-machining, where it produces tiny, complex parts for electronics. Additionally, EDM is well-suited for shaping very hard materials such as carbide, hardened steel, and exotic alloys.

EDM offers several advantages. It can machine very hard and brittle materials that are otherwise difficult to process. It allows the creation of complex shapes and fine details with high precision. Because there is no direct contact between the tool and the workpiece, there are no cutting forces, making it ideal for delicate or thin features that could distort under stress.

However, EDM also has limitations. It only works on electrically conductive materials, so non-conductive materials cannot be machined with this method. It is slower than conventional machining when removing large amounts of material. Electrodes wear down over time and must be replaced, adding to costs. Finally, depending on the application, the surface finish may require additional post-processing to meet requirements.

An electrically applied finish used to add corrosion protection to large or complex parts such as truck frames, food and beverage containers, tractors, and other industrial equipment. Sometimes referred to as a coating, this process is more similar to electroplating. Parts are dipped into a tank filled with a water-based solution containing coating materials. An electric current is then applied, causing particles to bond evenly to the submerged surface. After application, parts are typically cured in an oven through a process called cross-linking, which helps bond the coating to the base material. 

Appearance - The defining characteristic of an E-Coat is its exceptionally uniform and consistent appearance. Thanks to the electrical application process, the coating thoroughly covers all surfaces, including complex geometries and internal areas, without the risk of runs, drips, or sags.

A manufacturing process that uses an electric current to deposit a thin, adherent layer of a desired metal onto the surface of a fastener. This metallic coating is applied to enhance various properties, including corrosion resistance, wear resistance, hardness, conductivity, and appearance. Common electroplated finishes for fasteners include zinc, nickel, chrome, and cadmium.

Video: Why is Plating Important for Industrial Fasteners?

Electropolishing is an electrochemical finishing process used to smooth, polish, and clean the surface of metal parts. Instead of removing material by mechanical grinding or sanding, it uses an electrical current in combination with a chemical electrolyte to dissolve a very thin layer of the metal’s surface. This process levels out microscopic peaks, reduces surface roughness, and leaves behind a bright, reflective, and clean finish.

During electropolishing, the metal part is connected as the positively charged anode and immersed in a specially formulated acid bath. A cathode (negatively charged electrode) is also placed in the solution. When electrical current passes through, the microscopic high points on the surface dissolve faster than the low points. As a result, the surface becomes smoother and more uniform at the microscopic level.

Electropolishing provides several advantages: it improves corrosion resistance by removing surface contaminants and embedded particles, reduces the risk of stress corrosion cracking, makes surfaces easier to clean, and enhances the appearance of the part with a shiny finish. It is often used for stainless steel, titanium, aluminum, and other metals in industries such as medical device manufacturing, food processing, aerospace, and pharmaceuticals—where extremely clean, smooth, and passivated surfaces are critical.

Embedment relaxation (also called bedding-in or joint settlement) is the early loss of clamp load in a bolted joint that occurs as microscopic surface high spots—asperities—flatten under preload; after tightening, the nut/washer bearing surfaces and the thread flanks contact only at scattered peaks, and as those peaks plastically flow, coatings or paint cold-flow, and burrs seat, the joint stack becomes slightly thinner, which shortens the bolt and reduces its tensile preload, and this drop can be approximated with the inline relation “the clamp-force loss is ΔF ≈ k_bolt·Δδ_embed,” where k_bolt is the bolt’s axial stiffness and Δδ_embed is the amount of settlement. It is largely a one-time effect that unfolds over minutes to hours (or a few early load cycles) and is distinct from stress relaxation/creep (time-dependent loss at elevated temperature) and from self-loosening (rotation under vibration), though all three can interact.

The magnitude is small in thickness but meaningful in force: for a typical M12 class 10.9 bolt with k_bolt ≈ 80 kN/mm, a settlement of about 0.05 mm produces roughly 4 kN of preload loss—often a few percent up to ~10% of initial preload in steel-on-steel joints, and potentially higher with soft materials like aluminum or plastics, rough finishes, squishy coatings/paint, burrs, or multiple interfaces. Most settlement occurs under the nut or bolt head/washer (bearing surface seating and coating compression), at the thread flanks (asperity flattening and micro-yield), and between the joined parts if surfaces are rough, non-parallel, or include gaskets or shims. This matters because lower clamp load increases the risk of joint slip, micro-movement, fretting, and vibratory loosening, and it also skews torque-to-tension expectations during rework.

You can reduce embedment relaxation by controlling contact surfaces (machine or grind bearing faces, deburr, avoid thick soft paint where parts seat, and use hardened flat washers), choosing finishes and coatings that stabilize friction without excessive compressible thickness (e.g., phosphate-and-oil, MoS₂, PTFE, zinc-flake systems), and using a tightening strategy that accommodates seating (for example, a snug-pause-final sequence or torque-angle/torque-to-yield methods for better tension control, with a targeted retorque after initial seating to recover preload). Design choices that raise joint stiffness relative to bolt stiffness—shorter grip length, larger under-head bearing area, stiffer clamped members—also help, and verification tools such as load-indicating washers (DTIs), direct-tension bolts, or ultrasonic elongation confirm that the final clamp load remains after bedding-in. Lubrication won’t eliminate embedment itself, but consistent lubrication trims friction scatter so the achieved preload is closer to target despite the inevitable settlement.

The fastener endurance limit is the maximum stress a fastener can endure for an essentially infinite number of load cycles without failing from fatigue. It represents the stress threshold below which a fastener can be repeatedly loaded and unloaded without developing cracks. This property is especially important for bolts, studs, and screws used in engines, turbines, machinery, and bridges, where vibration and cyclic loading are common. If stresses remain below the endurance limit, the fastener can theoretically last indefinitely; if stresses exceed it, fatigue failure can occur even at levels far below the tensile strength.

As an example: Consider a medium-carbon steel bolt with an ultimate tensile strength (UTS) of 800 MPa. The endurance limit for steels is typically around 40–50% of UTS. In this case, the endurance limit would be approximately 320–400 MPa. If the bolt is subjected to a cyclic tensile stress of 300 MPa, it should survive millions of cycles without failure. But if the applied cyclic stress is 450 MPa, the bolt may eventually fail from fatigue even though 450 MPa is well below the 800 MPa tensile strength. Factors like surface finish, corrosion, thread design, and preload can reduce the effective endurance limit in real-world applications.

An EPDM washer is a type of washer made from EPDM rubber, a synthetic rubber material known for its excellent resistance to weathering, UV rays, ozone, and environmental elements. EPDM stands for ethylene propylene diene monomer, which is a type of durable and flexible elastomer.

Epoxy is a corrosion-resistant coating applied to fasteners using a dip-spin process. Known for its durability and strong barrier properties, epoxy can offer up to 1,000 hours of salt spray protection and helps prevent degradation from UV exposure, moisture, oxidation, chemicals, and general wear. The primary drawback of epoxy coatings is their thickness, which can sometimes interfere with thread fit, especially in precision applications.

Appearance - Epoxy coatings typically have a thick, matte finish and may appear in a range of colors depending on the formulation, most commonly in shades of gray, black, or brown.

An expansion bolt is a specific type of expansion anchor system that comes preassembled with both the anchor sleeve and the bolt. When you tighten the bolt, it pulls a cone into the sleeve, forcing it to expand against the hole walls. Expansion bolts are common in heavy-duty structural applications like securing machinery, columns, or safety-critical components into concrete or masonry.

Key point: An expansion bolt is essentially a bolt integrated with an expansion anchor mechanism — it’s an all-in-one fastener.

An external retaining ring—also called an external circlip or snap ring—is a spring-steel ring that snaps into a groove machined around the outside of a shaft. Once seated, it forms a shoulder that blocks axial movement of parts such as bearings, gears, spacers, and pulleys. “External” indicates it fits around a shaft; the counterpart is an internal retaining ring that seats inside a bore.

During installation, the ring is elastically expanded and then springs back into the shaft groove. Thrust loads are transferred from the retained parts through the ring to the groove wall or shoulder. Most rings are made from carbon spring steel or stainless steel, with finishes like phosphate or zinc for corrosion protection. Sizing is based on shaft diameter, and groove width and depth follow standards such as DIN 471 and ASME B18.27 for external rings.

Common styles include the standard circlip with lug holes, installed with snap-ring pliers; the E-ring, which pushes radially into a groove for quick installation; the spiral (spiral-wound) ring, which winds into the groove to provide 360° contact without lugs; and the constant-section (C-ring), which has a uniform rectangular cross-section suited to high loads and impact.

For installation, use proper pliers or applicators and avoid over-expanding the ring. Many stamped circlips have a sharp “punch” edge and a rounded “die” edge—orient the sharp edge toward the primary thrust direction for the best retention. Always verify groove dimensions and shaft chamfers, and select material and finish appropriate to the environment and load.

A helical ridge formed on the outside surface of a cylindrical part, such as a bolt or screw. External threads are designed to mate with internal threads, allowing the fastener to be securely screwed into nuts, tapped holes, or other threaded components.

Externally Threaded Inserts are fastener components designed to provide a strong, reusable threaded connection inside a softer or weaker material, such as plastic, wood, or soft metals. They have external threads on the outside surface that allow them to be securely screwed or pressed into a pre-drilled hole in the base material, creating a durable mounting point.

A metal-forming process where material is pushed through a shaped die to create a specific, continuous profile. In fastener manufacturing, it's used to produce raw material like wire, or to form internal features such as holes within parts.

An eye bolt is a type of fastener that features a loop (or "eye") at one end and a threaded shaft on the other. It’s used primarily as a point for attaching ropes, cables, chains, or hooks to lift, secure, or support loads.

An Eye Lag Screw (also called an Eye Lag Bolt) is a heavy-duty fastener that combines features of an eye bolt and a lag screw. It has a threaded screw shaft designed to be driven directly into wood or other materials without a nut, and a closed loop ("eye") at the head.