Thread relaxation is one of the main causes of bolt […]
Thread relaxation is one of the main causes of bolt loosening. Thread loosening can lead to loss of preload, and can be divided into three categories: the relaxation caused by the settlement of the thread; with the surface relaxation and gradually caused by thread relaxation; or temperature changes caused by thread relaxation (creep).
The occurrence of subsidence is related to surface roughness and irregularity. After the preload is applied, the surface of the clamped part will produce a pressure, causing the irregular construction of the threaded surface to begin to flatten. When the irregular surface is completely flattened, this part becomes loose and the preload starts to decrease. If the degree of pre-tightening is sufficiently large, the bolted connections may be loosened. And low preload will increase the risk of fatigue damage.
If the material is pressed for some time the degree of continuous improvement, there will be relaxation, resulting in further loss of preload. Polymers, composites, copper and other soft metals and other materials often appear this situation.
Creep refers to the slow movement of solid material or permanent deformation under the influence of external pressure. Creep occurs when the material is subjected to a pressure that is less than the yield strength of the material for a long time. If the material is heated for a long period of time and the temperature is close to the recrystallization temperature, the creep is more severe. The creep situation always doubles as the ambient temperature increases.
Today, more bolted connections are exposed to more extreme environments such as high speed, high temperature and high load conditions. If the parts are also used "old" steel production, will be very cumbersome. To reduce the total weight, today's parts are made of synthetic materials, plastic and aluminum, which is the reason for the increasing number of thread relaxation problems.http://www.lwhardware.com/