Glossary

Friction in bolted joints occurs at the threaded and head contact surfaces, and its coefficient of friction has a significant impact on the achievable preload force. Targeted friction control with defined coefficient of friction ranges, as well as suitable bolt locking elements such as NSK® washers or wedge washers, is essential for ensuring assembly quality and operational safety. Coefficients of friction below 0.05 are critical, as they can negate self-locking and promote spontaneous loosening of the joint; therefore, elements with undefined friction behavior should be avoided.

Corrosion protection for screw locking elements is more than just a material or a coating. What matters most is the tested condition of the finished fastener—including its surface, coefficient of friction, spring effect, and locking function.

teckentrup SLI offers three versions optimized for corrosion protection:
zinc-lamellar-coated spring steel / C60 for cost-effective applications requiring high mechanical load-bearing capacity, A4 stainless steel / NIRO / 1.4401 for particularly demanding corrosive environments, as well as zinc-nickel coatings for connections with electrical conductivity.

Creeping is a time-dependent, plastic, and irreversible deformation that occurs during the relaxation phase of a bolted joint under constant stress as a result of dislocation movements in the material. It occurs under long-term loading, particularly when the time-dependent strength or fatigue strength is exceeded. In bolted joints, creeping leads to a loss of preload over time.

DIN EN 17976:2025-03 fully replaces the national standard DIN 25201 and standardizes design, safety, and functional requirements for bolted joints in rail vehicle construction across Europe. For the first time, the new standard defines specific requirements for residual spring effect during slackening—in addition to the already detailed requirements for protection against self-acting loosening by rotation—which is central to lightweight construction and operational safety. See also Eisenbahningenieur Technical Report.

"Mechanical fasteners—Technical delivery conditions—Part 26: Spring steel locking washers for bolted joints" specifies requirements for conical spring steel locking washers designed to compensate for a loss of preload due to embedding or creeping. It specifies materials, hardness (420–490 HV), surface finishes, test methods (e.g., embedding, endurance, and spring force tests), as well as packaging and marking. The standard ensures the quality and functional reliability of the washers.

DIN 6796 defines lock washers as friction-lock spring elements for screws in property classes 8.8 through 10.9. They compensate for settlement through elastic restoring force; however, they do not prevent the screw from loosening under transverse loads and are suitable only for axially loaded connections.

DIN EN 17976:2025-03 defines new Europe-wide standards for screw connections in rail vehicles, replacing the previous DIN 25201. The focus is on a dual-security approach to prevent slackening and unscrewing. Outdated components such as spring rings are not permitted. For electrical glands, only single securing elements are allowed. Based on the defined properties of spring elements, elastic resilience and slip resistance can be increased. See also Eisenbahningenieur Technical Report.

Disc springs are conical spring elements made of hardened spring steel that generate a high axial restoring force through elastic deformation. They compensate for settlement, relaxation, and thermal linear expansion in bolted joints and are used under dynamic or thermal loads. They are not suitable for electrically conductive connections or as anti-rotation devices. Advantages include high preload, lifetime, and an adjustable spring characteristic curve; disadvantages include, among other things, the reduced compression cone, susceptibility to corrosion, and high design complexity. They comply with standards such as DIN EN 16983 and VDI 2230. For multifunctional requirements, NSK washers are a superior alternative.

Earthing and equipotential bonding screw connections enable direct current transfer through the screw when insulating layers such as paint or oxide prevent direct contact between conductors. Electrical conductivity is enhanced by special contact discs, while mechanical requirements—such as embedding and relaxation behavior—must be taken into account through an elastic design. Due to the lack of standards, the design of such connections is practice-oriented and follows proven recommendations, e.g., from *Elektropraktiker* or from manufacturers such as Rittal. In such cases, electrical conductivity is specifically established or supported by contact discs—e.g., of the NSK®-K type.

Elastic resilience has a significant influence on the preload force and sliding resistance of bolted joints. It is determined by the screw, the backing surfaces, and suitable spring elements. Lock washers conforming to DIN 6796 or NSK washers enhance this effect and reduce loss of preload, especially with short clamping lengths. Unsuitable elements are excluded; variants that comply with standards can be verified by measuring the residual spring force. See also Railway Engineer Technical Report.

Electrical contact glands transmit current via the clamped contact surfaces of the conductor materials, not via the screw. They require stable contact pressure and a minimal number of interface surfaces. NSK-E/EL washers offer elastic resilience, anti-loosening protection, and safety in accordance with standards—ideal for modern applications. See also NSK-E Technical Report.

Electrical screw connections ensure the transfer of current and mechanical clamping. A distinction is made between indirect current transmission via electrical contact glands and  directly current-conducting connections, known as electrically conductive screw connections. Settlement behavior, corrosion, and thermal influences require a targeted design using NSK washers. See also NSK-E Technical Report.<

Electrically conductive screw connections ensure low-resistance transfer of current while providing mechanical strength, even through insulating layers. The NSK-K contact washer combines electrical conductivity, spring action, and anti-loosening protection—ideal for earthing and equipotential bonding. See also NSK-E Technical Report.

Material combinations that should be considered in electrical engineering when assembling components with screws.

During the settling phase, the preload of a bolted joint initially decreases due to embedding effects. This “embedding” occurs through the plastic leveling of surface roughness in the contact areas, the loaded flanks of the mated threads, and other interface surfaces. The amount of set depends on the number of interface surfaces and the surface roughness. The settling process begins as early as the assembly stage and is completed within a few hours.

Screw locking elements enhance the functionality of screw connections by performing general tasks such as pressure distribution and protection of mating surfaces, compensation for slackening effects, and prevention of self-acting loosening by rotation. These functions are achieved through various designs, with multifunctional elements such as NSK® washers combining all three areas of effect in a single component. Especially with short clamping lengths, they increase structural resilience and are therefore becoming increasingly important from a technical standpoint.

The five-phase model describes the bolted joint not as a static structural element, but as a time-dependent, mechanically elastic system whose preload force F_Vim undergoes continuous changes throughout its life cycle.

Locking rings (profiled rings) are screw-locking elements made of profiled wire that offer limited protection against self-loosening. They do not provide an effective locking function according to the current state of the art (DIN EN 17976). The required residual spring force according to DIN 267-26 is not achieved, which is why they are considered unsuitable. Using them with flat washers leads to additional settling. Modern alternatives such as NSK washers are technically superior and comply with standards.

Locking-edge washers are disc-spring-like, one-piece securing elements made of spring steel or stainless steel with radial locking edges that prevent loosening through a purely mechanical form fit. They do not increase elastic resilience and are suitable only for dynamically loaded mechanical connections. Locking-edge washers are not suitable for electrically conductive connections (electrical potential equalisation and grounding screw connections) or electrical contact glands. NSK washers represent the multifunctional, standards-compliant further development.

Self-acting loosening by rotation refers to the rotational movement of a screw or nut without an external torque load, resulting from the loss of preload and anti-slip strength, preceded by slackening and triggered by dynamic transverse loads. The self-loosening mechanism proceeds in several phases—beginning with relative rotational movements and bending stress (Phase 1), through thread slippage and head tilting (Phase 2), to rotation caused by stored torsion (Phase 3), and finally to complete loosening due to loss of friction (Phase 4). To ensure safety, securing elements such as anti-loosening elements or multifunctional locking elements such as NSK® washers are required; in addition to preventing loosening, these elements also increase slip resistance and resilience, thereby improving overall resilience—especially with short clamping lengths.

Multifunctional screw locks, such as NSK® washers, combine anti-rotation and anti-loosening protection in a single component, increase resilience, slip resistance, and robustness, thereby improving the efficiency and resilience of the entire structure. They also enable new design freedoms, particularly with short clamping lengths, and will replace traditional individual solutions for preventing loosening or unscrewing in the future.

Nordlock washers consist of two washers with wedge surfaces on the inside and radial serrations on the outside. Nordlock wedge-locking washers secure bolted joints against loosening under dynamic loads through a combination of form fit and friction. According to DIN EN 17976, they are not suitable for securing bolted joints against slackening and are not suitable for electrical connections of any kind.

NSK washers are one-piece, multifunctional screw-locking elements that provide both a force-fit and a form-fit to prevent slackening and unscrewing. They increase elastic resilience and anti-seize properties; up to property class 12.9, they are also suitable for electrically conductive connections (earthing & electrical potential equalisation) as well as electrical contact glands, and comply with various standards, such as DIN EN 17976, DIN 25201, DIN 50343, and DIN 267 Part 26; they are considered state-of-the-art and offer assembly-friendly, standards-compliant, and cost- and weight-efficient solutions.

Profile rings (locking edge rings) are screw-locking elements made of profiled wire that offer limited protection against self-loosening. They do not provide an effective locking function in accordance with the current state of the art (DIN EN 17976). The required residual spring force specified in DIN 267-26 is not achieved, which is why they are considered unsuitable. Using them with flat washers leads to additional settling. Modern alternatives such as NSK washers are technically superior and comply with standards.

Profile washers are one-piece screw locking elements made of hardened spring steel with radial teeth that provide a form-fit anti-loosening mechanism under dynamic loads. They offer limited spring action but do not prevent loosening. Their use is limited to applications up to property class 10.9. Disadvantages include potential surface damage, a spring travel that does not comply with standards (DIN 267-26), and unsuitability for electrical connections. A future-proof alternative: multifunctional NSK washers.

During the relaxation phase, relaxation—defined as a time-dependent reduction in stress at constant elongation—leads to a gradual loss of preload force. This is caused by microplasticity or viscoelastic flow at contact surfaces. This impairs sliding resistance and function, which is why, according to VDI 2230, suitable materials and designs are required.

Schnorr washers are one-piece screw locking elements made of hardened spring steel with radial teeth, which provide a positive-lock anti-rotation mechanism under dynamic loads. They offer limited elasticity but do not prevent loosening. Their use is limited to applications up to property class 10.9. Disadvantages include potential surface damage, a spring travel that does not comply with standards (DIN 267-26), and unsuitability for electrical connections. Future-proof alternative: multifunctional NSK washers.

Screw locking elements enhance the functionality of screw connections by performing general tasks such as pressure distribution and protection of mating surfaces, compensation for slackening effects, and prevention of self-acting loosening by rotation. These functions are achieved through various designs, with multifunctional elements such as NSK® washers combining all three areas of functionality in a single component. Especially in the case of short clamping lengths, they increase structural resilience and are therefore becoming increasingly important from a technical standpoint.

Tooth washers conforming to DIN 6797/6906 provide neither sufficient spring force nor a standard-compliant locking effect to prevent self-acting loosening by rotation. The required residual spring force specified in DIN 267-26 is not achieved. Their use is limited to screws ≤ 5.8; they are unsuitable for electrical connections. The standards were removed in 2004. Due to potential surface damage and placebo effects, modern, standard-compliant securing elements such as NSK washers are preferred.

First, the slackening phase occurs in the slackening mechanism of bolted joints. During slackening—or in the slackening phase of bolted joints—the preload force decreases due to embedding, relaxation, and creeping. The sliding resistance is maintained, so that operational forces continue to be transmitted through clamping. Embedding occurs through the plastic leveling of rough surfaces immediately after assembly. Relaxation and creeping lead to further force reduction over the long term. Screw connections can be secured against slackening using special screw locking elements or multifunctional screw locking elements.

Slip resistance refers to the maximum shear force that a screw connection can transmit before components begin to slip relative to one another. Especially with short clamping lengths, the use of spring-loaded elements such as tension washers (DIN 6796) or NSK washers significantly increases resistance to slippage. In addition to functional advantages, these elements also support lightweight design goals and CO₂ reduction. Evaluation is performed, for example, in accordance with VDI 2230 Part 1. See also Eisenbahningenieur Technical Report.

Slotted screw connections, or screw connections with special hole geometries, significantly alter the mechanical behavior of the joint due to their segmented contact area; they negatively affect surface pressure, pressure distribution, and elastic resilience, while simultaneously promoting embedding, creeping, and loosening effects. Notches on the hole flanks and the absence of lateral screw guidance are particularly critical, which is why it is necessary to evaluate additional factors such as contact area, deflection, and resistance to loosening. Since no standard specifications exist, tested securing elements such as NSK L, NSK B, and NSK EL are recommended for mechanical and electrical applications.

The loosening of bolted joints occurs in two phases as a result of embedding, creeping, relaxation, or dynamic transverse loads: first, slackening due to a loss of preload, followed, if necessary, by self-acting loosening by rotation. This reduces the clamping force and compromises the connection’s force-fit or form-fit integrity. To increase operational safety, enhancing elastic resilience is essential, as it significantly reduces the risk of loosening. Pure anti-loosening devices carry a residual risk of bolt failure; multifunctional securing elements such as NSK washers offer combined protection against slackening and self-loosening, differentiated by property classes and application, thereby increasing the resilience of the bolted joint.

Screw connections with special hole geometries significantly alter the mechanical behavior of the joint due to their segmented contact area, negatively affecting surface pressure, pressure distribution, and elastic resilience while simultaneously promoting settlement, creeping, and loosening effects. Notches on the hole flanks and the absence of lateral screw guidance are particularly critical, which is why it is necessary to evaluate additional factors such as contact area, deflection, and resistance to loosening. Since no standard specifications exist, tested securing elements such as NSK L, NSK B, and NSK EL are recommended for mechanical and electrical applications.

Spring rings conforming to DIN 127, 128, 6905, and 7980 do not adequately prevent the spontaneous loosening of bolted connections and are considered technically unsuitable under current standards. The required residual spring force specified in DIN 267-26 is not achieved, which can lead to placebo effects, unreliable preload, and surface damage. Their use is no longer permitted by standards. It is recommended to replace them with modern securing elements such as the NSK washer.

Spring washers conforming to DIN 137/DIN 6904 provide only limited spring forces that are not effective over the long term for preventing loosening and experience failure when used to prevent unscrewing. Due to their lack of effectiveness, the withdrawal of the standards (since 2004), and potential liability risks, they are considered technically obsolete. Their use is no longer state-of-the-art and is not recommended. Spring washers are now being replaced by more effective securing elements, such as NSK washers.

Increasing corrosion loads require robust verification tests on the actual fastener. Stainless steel components are often evaluated solely based on the corrosivity classes specified in ISO 12944, a standard for coatings that can serve only as a general guide for stainless steel.

Tension washers are conical spring elements made of spring steel that undergo elastic deformation when tightened, generating a residual spring force that compensates for a loss of preload due to embedding or relaxation. They provide a force-fit lock against loosening but are unsuitable as anti-rotation locks. They are suitable for screws rated ≥ 8.8 under axial loads. Restrictions apply in the case of transverse loads or soft supports. Alternative: multifunctional NSK washers.

TREP washers are pre-assembled disc spring assemblies (3L/4L) with high spring force designed to compensate for settlement loss, relaxation, and thermal effects. They consist of stacked springs and, where necessary, flat washers, connected by a sleeve. Used in bolted connections subject to dynamic loads; not suitable for electrically conductive connections. Disadvantages: large installation space, weight, limited property classes. NSK washers offer a more compact alternative.

Wedge washers (DIN 6798, 6907) are mechanical securing elements with limited spring and locking action. They do not provide standard-compliant protection against loosening, and the required residual spring force specified in DIN 267-26 is not achieved. Their use is permitted only for screws with a strength class of ≤ 5.8; they are unsuitable for electrical connections. The standards were removed in 2004. Modern alternatives, such as NSK washers, are technically superior and comply with standards.

Wedge lock washers consist of two washers with wedge surfaces on the inside and radial serrations on the outside. Wedge lock washers secure screw connections against loosening under dynamic loads through both form fit and friction.
According to DIN EN 17976, they are not suitable for securing screw connections against slackening and are not suitable for electrical connections of any kind.