Meaning
Mechanical settlement in bolted joints over time causes a gradual reduction in the clamping force that holds an assembly together. This issue, known as fastener torque relaxation, occurs as micro-roughness on the mating surfaces flattens under the initial load. It is an important factor in maintaining the structural and electrical integrity of electronic enclosures and battery modules.
The relaxation process typically stabilizes within the first few days after assembly.
Compression Analysis
Initial compression forces cause both elastic and plastic deformation of the mating surfaces and the fastener threads. This initial yielding reduces the tension in the bolt even without external loads. The material properties of the housing and the screw determine the rate of relaxation.
Environmental Influence
Thermal cycling accelerates the loss of clamping force due to the mismatch in thermal expansion coefficients between the metal fastener and the plastic housing. As the temperature rises, the materials expand at different rates, increasing the stress on the joint. When the temperature drops, the joint relaxes further than before.
This cycle can eventually lead to loose fasteners and compromised ingress protection or electrical bonding.
Mitigation Strategy
Using spring washers or thread-locking compounds helps maintain the necessary clamping force over the life of the product. This prevents the fastener from backing out under vibration. It ensures the mechanical integrity of the enclosure remains intact.
