Calculation Method And Precautions For High-strength Bolts

Bolts made of high-strength steel or requiring significant preload can be referred to as high-strength bolts. High strength bolts are commonly used for connecting bridges, steel rails, high-voltage and ultra-high voltage equipment. The fracture of this type of bolt is mostly brittle. High strength bolts applied to ultra-high pressure equipment require significant prestressing to ensure the sealing of the container.

The pre tension of high-strength bolts is achieved by tightening the nuts. Generally, torque method, angle method, and torsion shear method are used.

(1) Torque method. Using a specially designed wrench that can directly display torque, apply torque based on the relationship between pre measured torque and bolt tension to achieve the predetermined pre tension.

(2) Angle method. It is divided into two steps: initial tightening and final tightening. The initial tightening is to use an ordinary wrench to make the connected components tightly fit each other. The final tightening is to start from the initial tightening position and use a powerful wrench to rotate the nut according to the final tightening angle determined by the bolt diameter and plate thickness. When the nut is tightened to the predetermined angle value, the bolt tension reaches the required pre tension value.

(3) Torsional shear method. Twist off the plum blossom head at the end of the bolt. First, tighten the bolts initially, and then use the two sockets of a special electric wrench to cover the nut and the end of the bolt's plum blossom head. During operation, the large socket is turned forward to apply tightening torque, while the small socket is tightened with counter torque. After the bolts are tightened, then unscrew the plum blossom head along the rear groove. Due to the fact that the depth of the groove is determined by the relationship between the final tightening torque and the pre tension, the specified pre tension value is reached when the plum blossom head is unscrewed with the bolt.

Quality considerations

(1) The surface rust, oil stains, burrs on the bolt hole wall, welding burrs, etc. should be cleaned up.

(2) After treatment of the contact friction surface, it is necessary to meet the specified scratch resistance coefficient requirements. The high-strength bolts used should have matching nuts and washers, which should be used according to the matching requirements and should not be interchanged.

(3) When installing the friction surface of the treated components, it is not allowed to be contaminated with oil, soil, and other debris.

(4) During installation, the friction surface of the component should be kept dry and should not be operated in the rain.

(5) Strictly inspect and correct the deformation of the connected steel plates before installation.

(6) During installation, it is prohibited to hammer into the bolts to prevent damage to the bolt threads.

(7) An electric wrench that is regularly tested during use to ensure torque accuracy and operate in the correct tightening sequence.

Main safety technical measures

(1) The opening size of the Adjustable spanner shall be consistent with the size of the nut, and the small wrench shall not be used to add a sleeve. When working at heights, a dead wrench should be used. If a live wrench is used, a rope should be used to secure it, and people should fasten their safety belts.

(2) When assembling the connecting bolts of steel components, it is strictly prohibited to use hands to insert the connection surface or touch the screw holes. When removing and placing the pad iron plate, fingers should be placed on both sides of the pad iron plate.