Profile Rings
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.
- Definition, Purpose, and Suitability
Profile rings (also known as locking rings) are mechanical screw-locking elements designed to prevent spontaneous loosening under dynamic loads. They consist of profiled wire and resemble spring rings.
They do not provide an effective locking function according to 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.
- Principles of Operation and Functioning
- Spring Effect:
When tightened, the elastic deformation generates an axial preload force that temporarily increases frictional locking.
- Wedge/locking effect:
The open ring shape, in combination with the profiling, creates a slight clamping force during relative rotational movement. This is intended to mechanically prevent back-turning; however, its effectiveness cannot be validated.
- Application Limits
- Can only be used with screws up to property class 5.8
- Restrictions for A4 stainless steel due to reduced elasticity
- Not suitable for electrically conductive connections
- Combination with flat washers results in additional interface surfaces → increased settling and deterioration of locking properties
- Advantages
- Low cost
- Disadvantages
- No standard-compliant locking effect; placebo effect
- Damage to mating surfaces
- No protective effect during short-term overloading
- Dispersion of assembly force due to uneven friction
- Increased settlement when used in combination with flat washers
- Standard Status
Profile and locking edge rings are no longer permitted under DIN EN 17976, as they do not meet the required residual spring force specified in DIN 267-26.
The previously valid approval (e.g., according to DIN 25201-3 for electrical applications) is obsolete; there is no current standard. Their use is therefore no longer considered state-of-the-art.
- Conclusion and Recommendation
The retaining effect is insufficient and cannot be substantiated by standards.
Their use, especially in safety-critical areas such as rail vehicle construction, is not recommended.
Modern alternatives such as the NSK locking system offer a combined spring and locking effect as well as compliance with standards. Suitable variants are:
- NSK-E / NSK-EL for electrical connections
- NSK-S / M / L depending on screw strength for mechanical connections
Back to the glossary overview
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.
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.
Electrically Conductive Connections
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.
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.
