Benoit Millet, NOF Metal Coatings Europe SA
The primary function of a threaded fastener is to ensure a specified clamp load. To that extent, fastener coatings are designed to not only provide corrosion protection, but also to result in the right assembly characteristics, in this way ensuring that the applied torque brings the required clamping force and a secured safe joint. These demands are assessed by through the process of testing friction in a controlled environment. The specifications define parameters to verify characteristics such as torque or clamping force.
Multi-tightening testing can be a part of these test specifications, especially for wheel screws testing where it is an important feature.
To secure that the required clamping force will always be reached, the test specifications define the range of a friction window that on one hand guarantees a minimum clamp load, and on the other hand avoids overloading the screw. Most of the OEMs request this against several types of material like e-coat, aluminum and steel, all for the same screw with one coating system only.