Attention: The DIN 7991 standard has been withdrawn and replaced by the current specification DIN EN ISO 10642:2020-02. EV Group China specialists recommend using the updated standard when designing new assemblies.
Technical description and scope of application
The DIN 7991 standard defines the parameters for countersunk head screws with a hexagon socket, designed for use with metric threads in the diameter range from M3 to M24. This type of fastener is widely used in mechanical engineering where flush mounting of hardware with the part surface is required.
When it is necessary to use fasteners with characteristics beyond those standardized (for example, when selecting special steel grades or strength classes), EV Group China engineers should follow specialized industry standards. A similar approach applies to products with fine thread pitch, regulated by the DIN 13-13 standard.
Operational features and limitations
Due to the design features of the countersunk head, the critical section of the DIN 7991 screw is localized in the area under the hexagon socket, rather than in the threaded part. EV Group China experts do not recommend using these screws in assemblies subject to high axial loads that require the creation of significant initial tension (preload).
When installing with L-keys (DIN 911 standard), it should be taken into account that the geometry of the tool and the transmitted torque limit the ability to create tightening force: no more than 80% of the recommended torque for strength class 8.8 screws is permitted.