Schnorr washer
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.
- Definition and Construction
Schnorr washers are one-piece, mechanically acting screw locking elements whose primary function is to prevent self-acting loosening by rotation under dynamic loads.
They are made of hardened spring steel or stainless steel with a truncated cone-shaped geometry and radial, asymmetrical teeth on the top and bottom surfaces. The steep tooth flank faces the direction of loosening.
- Functionality and Operating Principles
- Form-fit action:
During tightening, the radial teeth anchor themselves into the mating surfaces and, through plastic deformation,
- Force-fit action:
The spring-like shape of the washer provides a limited, elastic effect; however, it is not suitable for preventing loosening.
- Applications and Limitations
Schnorr washers are suitable for:
- Mechanical connections up to property class 10.9
- Applications involving dynamic transverse loads and vibrations
- Safety-critical applications (with appropriate manufacturer testing)
Not suitable for:
- Increasing elastic resilience or slip resistance
- Clamping parts with low strength
- Electrical screw connections (e.g., contact, earthing, or equipotential bonding screw connections)
- Advantages
- Good protection against loosening
- Consistent coefficient of friction
- One-piece design, cost-effective, short assembly time
- Disadvantages
- No protection against loosening due to embedding
- Increased settlement and creep due to incomplete engagement of the teeth
- Risk of surface damage (plastic indentation)
- Scratches on the outer surfaces
- No defined residual spring effect according to DIN 267-26
- Normative Classification
Schnorr washers do not meet the requirements of DIN 267-26 for tension washers.
For use in accordance with DIN EN 17976 (e.g., in rail vehicle construction), a tested locking effect per DIN 25201-4 Annex B is required—which may be provided depending on the manufacturer.
- Design Notes and Recommendations
Schnorr washers provide reliable protection against loosening due to rotation but are not suitable for preventing loosening.
Future-proof solutions include multifunctional locking systems such as NSK washers. These combine form-fit and force-fit locking, increase elastic resilience, and improve the connection’s resistance to slippage. The NSK-L (for steel) and NSK-N (for stainless steel) variants are recommended.
Back to the glossary overview
NSK Washers
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.
DIN 267-26
"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.
Loosening
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.
Elastic Resilience
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.
Slip resistance
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.
Electrical Screw Connections
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.<
Embedding
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.
DIN EN 17976
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.
DIN 25201
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.
