The diameter, material and length of bolts in the same bolt set shall be the same.5) in order to prevent the bolt from bearing additional axial load or bending load, on the one hand, try to ensure that the load is outside the load center in the structure, on the other hand, in the process of making sure that the connecting parts, bolt head, nut support surface is flat, and at the same time with the central axis of the bolt orthogonal. When bolted on rough surfaces such as castings and forgings, the connection of screw head or nut shall be made into countersunk head or convex head