Why is it loose after the bolt is tightened? There are two reasons for loose bolts: spontaneous loosening and material deformation. Spontaneous loosening is due to frictional countervailing due to the transverse force on the connection exceeding the preload

Why is it loose after the bolt is tightened?

There are two reasons for loose bolts: spontaneous loosening and material deformation. Spontaneous loosening is due to frictional resistance caused by the lateral force on the connection exceeding the preload, while material deformation is due to settlement, creep and relaxation. Testing fastener anti-loosening under real-world conditions is the only practical way to be safe. To prevent loosening, slippage between connecting parts needs to be eliminated or lowered below critical levels, which can be achieved by increasing preload, increasing friction or reducing cyclic loads.