What are the Common Tightening Methods for Hexagonal Bolts?

I. Torque Control Method

1.Manual Torque Wrench This is the most basic method. During operation, set the corresponding torque value on the torque wrench according to the specification of the hexagonal bolt and the required pretightening force. For instance, for an M10 hexagonal bolt on general industrial equipment with a medium tightening requirement for the joint, a torque of 3050 N·m may need to be set. The operator turns the torque wrench manually. When the set torque is reached, the wrench emits a “click” sound, indicating that the specified tightening level has been achieved. This method is simple and easy to implement. It is suitable for scenarios that do not require extremely high precision of pretightening force, such as fastening ordinary mechanical housings and furniture assembly.

2.Electric Torque Wrench Electric torque wrenches are widely adopted in mass production or applications demanding high tightening precision, such as automobile manufacturing production lines. They deliver precisely controlled torque output. With preprogrammed torque values, electric torque wrenches can fasten hexagonal bolts to the required torque quickly and accurately. As an example, the tightening of cylinder head bolts for automobile engines requires a torque accuracy within ±5%, which electric torque wrenches can satisfactorily meet. Featuring high working efficiency, they greatly shorten tightening time and ensure consistent tightening quality.

II. Rotation Angle Control Method

With this method, the hexagonal bolt is first tightened to a low torque value (known as the initial torque). The initial torque is generally used to eliminate gaps between the bolt and connected components and ensure close contact between the bolt head or nut and the surface of the connected parts. Afterwards, the bolt is rotated by a certain angle to reach the final tightened state. For example, in some steel structures with highstrength bolted connections, a certain torque is first applied to bring the bolts into contact, followed by a further rotation of 90°120°. This method achieves more accurate control of bolt pretightening force. It is especially applicable to joints subjected to large tensile and shear loads, including steel structure connections of largescale bridges and critical joints of heavyduty machinery.

III. Marking Method

Punch marks are made on the bolt head and the surface of the connected component. Tighten the bolt preliminarily to misalign the marks by a certain distance, and then keep tightening the bolt until the marks line up again. Relatively straightforward yet low in precision, this method is mainly used for joints with low requirements on pretightening force and simple working conditions, such as assembly of simple farm tools and temporarily erected simple supports.

IV. Hydraulic Tensioning Method (for Largesize Hexagonal Bolts)

The hydraulic tensioning method is applied to large hexagonal bolts with a diameter greater than 50 mm used in huge structures including large ships and offshore oil platforms. A hydraulic tensioner is used to apply axial tensile force to elongate the bolt, and the pretightening force is controlled by calculating the bolt elongation. Take the foundation bolt tightening of ship main engines as an example. The hydraulic tensioner stretches the bolt to the specified elongation, and the nut is then tightened. After hydraulic pressure is released, the bolt generates accurate pretightening force. This method can effectively guarantee the reliability and uniformity of large bolted connections.