Serrated washers (DIN 6797, DIN 6906)
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.
- Design and Function
Toothed washers are mechanical screw locking elements whose primary function was to prevent the spontaneous loosening of screw connections under dynamic stresses such as vibrations, shocks, or alternating loads. According to the current state of the art, they do not provide a locking effect against self-loosening; therefore, spring rings are considered unsuitable. Tooth washers should not be used to compensate for slackening effects, as the necessary residual spring effect—e.g., as tested per DIN 267 Part 26—is not sufficiently present. Serrated washers are also used to create electrical contacts, for example, in coated parts. Serrated washers are completely unsuitable for this purpose due to the lack of residual spring force in combination with the undefined penetration of the material notches into the conductive mating surface.
- Mode of Operation
- Spring Effect (Axial Preload)
When the screw is tightened, the fan-shaped washer undergoes elastic deformation and generates an additional axial preload force. This increases the frictional engagement between the screw head and the mating surface, which can delay the loosening of the joint.
- Wedge or locking effect
The edges of the fan-shaped washer create a locking effect during relative rotational movement of the screw, which is intended to prevent spontaneous back-turning through mechanical jamming.
- Application Limits and Typical Uses
Material and Strength Limitations
Toothed washers are primarily used in mechanical screw connections or electrical contact glands with low property classes (max. 5.8) as well as with screws made of spring steel or A2 stainless steel. When using spring rings made of A4 stainless steel, the spring effect is further limited due to reduced elasticity. Spring washers are completely unsuitable for electrically conductive connections.
- Advantages of toothed washers
- Inexpensive
- Disadvantages of toothed washers
- no slackening effect whatsoever and a dangerous placebo effect, meaning that slackening and self-acting loosening by rotation can occur despite the unsuitable locking measures.
- Surfaces of the mating parts are damaged
- Due to the uneven friction coefficients that occur during the tightening process, the assembly preload forces are dispersed, so that they may not be reached or exceeded.
- No protective effect in the event of short-term overloading
- No defined contact areas for earthing and equipotential bonding screw connections. Increased risk of corrosion combined with inadequate spring effect.
- Normative Classification
The standards mentioned were removed in 2004 because the locking effect of toothed washers is considered insufficient. Toothed washers are therefore no longer considered state-of-the-art. Their use may potentially give rise to liability issues, which is why a legal review is recommended prior to use.
- Design Notes
The described mechanisms of action do not result in the desired locking effects with toothed washers in accordance with current norms and technical standards. The combined use of toothed washers with flat washers results not only in limited effectiveness but also in additional settlement issues and is therefore not recommended.
Their use must be critically evaluated from both technical and legal perspectives. Due to their limited locking effect, toothed washers are or should be replaced by modern, more effective screw-locking elements, such as the multifunctional NSK screw-locking system. For NSK washers, types E and EL are suitable for electrical contact glands, and types S, M, and L are suitable for mechanical screw connections, depending on the property class of the screws or the preload force.
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.
Slackening
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.
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.
Electrical Contact Glands
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.
