Effect of Water Absorption on the Clamp Load of Plastic Nuts

The use of nuts made of engineering plastics offers a high potential due to corrosion resistance, cost-effectiveness and a high strength-to-weight ratio of the plastics. However, relaxation of the plastic leads to a continuously decreasing clamp load of the joint. The remaining clamp load strongly depends on the climatic ambient conditions (especially humidity), as thermoplastics absorb moisture, which changes the relaxation behavior. To quantify the impact of humidity on the joint, clamp load measurements were conducted using load cells. Within a Design of Experiments (DoE), variations in the glass fiber content of the plastic, the screwing depth, and the moisture content were explored. The research was divided into two parts, such that a humidity exposure of the nuts before the tightening and a humidity exposure after the tightening were considered. In the case of the humidity exposure before tightening a reduced relaxation behavior for wet conditioned nuts can be stated. A rising glass fiber content leads to a reduced, absolute amount of relaxation. When the screwing depth is reduced, the relaxation reduces in a linear manner, which can be explained by the reduced tightening torques. In case of humidity exposure after tightening comparable losses of the clamp load, independent of the moisture content, are measured. It is shown, that the investigation period is too short to measure differences. Hence, it can be derived, that an interim change in air humidity does not affect the remaining clamp load.