Tightening Process Of High-strength Bolt Connection Pair

The high-strength bolts are tightened by the torque method, that is, on the basis of a series of process tests, the tightening of the high-strength bolts is completed in two stages: initial tightening and final tightening. Both initial tightening and final tightening use fixed torque electric wrenches. , The wrenches for initial and final tightening are strictly separated.

Before the initial screwing, the installation should be tightened with ordinary bolts, so that the splicing seams must be closely attached, and the gap between the board layers is inserted between the board layers with a 0.3mm feeler gauge, and the depth shall not be greater than 20mm.

Initial tightening: After the high-strength bolts are inserted into the bolt holes and checked, they should be tightened with an ordinary manual wrench with a ratchet. For M30 high-strength bolts, use a calibrated electric wrench to tighten the bolts to 50% of the final tightening torque; for M24 high-strength bolts, use an electric wrench to tighten the bolts to 50% of the final tightening torque to make splicing The board is tightly attached to prevent under-tightening of the bolts due to the influence of the bolt group.

For the high-strength bolts that have been initially screwed, immediately use a whiteboard marker to draw a straight line on the nut, washer and board surface to prevent repeated screwing, omissions and easy inspection.

5. Final tightening: the final tightening torque value of each batch of high-strength bolts is determined by the following formula

M=K×N×d

Where: M—the final tightening torque value of high-strength bolts (Nm);

N—The construction pre-tension value of high-strength bolts (KN); (according to 1.1 times the design pre-tension)

K—average value of torque coefficient of each batch of high-strength bolts; (value based on site reinspection data)

d—Nominal diameter of high-strength bolts (mm).

The design pre-tension of high-strength bolts is in accordance with the provisions of the "Code for Design of Railway Bridges and Culverts", considering that the loss of pre-tension is about 10%, so the construction pre-tension of high-strength bolts is 1.1 times the design pre-tension.

6. The final tightening is completed by a calibrated electric fixed torque wrench. For bolts that cannot be used with electric tools on individual parts, use a calibrated digital display inspection wrench to complete the final tightening.

In the process of tightening high-strength bolts, the bolt heads and washers must not rotate with the nut, so be careful when tightening. If you find that the bolts and washers are rotating, replace the high-strength bolts and repeat the process of initial and final tightening. The replaced high-strength bolts shall be returned to the warehouse and returned to the factory for treatment. They can be used only after passing the inspection and it is strictly prohibited to use them again.

In the process of tightening the high-strength bolts, the manual ringing wrench must be used evenly, and no impact force shall be used; when the electric torque wrench is started, it must run continuously and smoothly until the electric wrench automatically stops. After finishing the tightening of a high-strength bolt and the electric wrench stops automatically, it is strictly forbidden to start the electric wrench again to tighten the bolt to prevent over-tightening. After the first batch of 50% or 75% of the high-strength bolts on the same gusset plate is finally screwed, the other 50% or 25% of the bolt holes can be punched out in batches, and the high-strength bolts should be installed immediately, and the initial tightening and Operating procedures for final screwing.

9. Whether in the initial and final stages, or when using different wrenches, the tightening sequence from the center of the bolt group to the surroundings should be followed.

10. Electric wrenches should use an independent power supply or a regulated power supply during construction to reduce voltage fluctuations and affect the stability of the torque output of the electric wrench.

11. If various wrenches are abnormal or malfunctions during use, the on-site construction personnel shall not deal with them without authorization. The construction should be stopped immediately, and the wrenches should be handed over to the high-strength bolt test team for inspection and repair, and the malfunctions should be reported so that corresponding measures can be made on the site. The bolts are processed.