‘Settling’ is used to describe in general terms the levelling out of surface unevenness in screw head and nut contact surfaces and in nuts, screw thread flanks and the joints of braced parts. Because a levelling out of the surface unevenness already occurs when tightening the screw, the degree of settling is generally less than the size of the unevenness would lead one to assume. After fitting a threaded connection, loss of preload force can set in as a result of settling even at room temperature.
Planning for settling in advance
Particularly critical connections include, for instance, those made by wheel bolts. The intention here is that the connection should hold the wheel securely in place, but also be possible to be undone again. At the same time, however, any unintentional loosening has definitely got to be avoided. The connection has to be designed in such a way that settling is calculated into it and other influences such as vibrations and changes in temperature are also taken into account.
Micro-layer systems have hardly impact on settling behaviour.
The amounts of settling to be planned for in advance are essentially dependent on several different factors. In addition to the strength of the braced parts, others that play a role include the unevenness of the surfaces where the parts engage, the level of surface pressure, the type and degree of loading and the temperature. It is also necessary to taken into account the elastic flexibility of the parts used, above all when using plastic or rubber elements, but also when applying very thick coatings. The influence of surface systems such as zinc flake technology or electroplating on the preload force of threaded fittings is marginal and can be ignored. Sometimes, however, in the automotive industry for example, maximum coating thicknesses under the head of 25 µm are being demanded, which in practice are barely achievable.
Detailed information on calculating the settling behaviour of screws is contained in VDI guideline 2230 2015-11. There is more on the influence of surface coatings on settling behaviour here