METHODS FOR TIGHTENING THREADED BOLTS

Threaded fasteners mechanically create non-permanent joints between parts. They are easy to install and are commonly used as compared to non-threaded fasteners. The suitability of a bolt-tightening method for the job depends upon the needs of the assembly and the budget. In industrial bolting, it is important to make sure that bolts and joints are fastened properly to bear the heavy-duty loads and jobs.

Why you should avoid uncontrolled tightening:

Controlled bolting can avoid delays in operations and ensure safety-on-site. Supervised tool tensioning is required to choose required fastening devices for bolting to measure and apply force to joints consistently. Quality of screw joint is influenced by the tightening method used. To choose a suitable fastening method, it is important to understand how the preload is generated. Generally, the preload is assumed to be equal and opposite of the clamping force. Inappropriate tightening methods or improper tools can lead to unwanted fastening failures.

Bolt Tightening Methods:

• Torque tightening

Torque tightening is the most frequently used method for fastening threaded bolts. In this method, the application of torque to a nut is accurately placed so that a bolt can hold its load. The torque should be never too high otherwise, it may damage or even break the bolt. It is advisable to use a properly calibrated mechanical torque wrench to ensure that the right amount of tension is applied to a bolt.

• Hydraulic tensioning

While using large bolts, torqueing may not be feasible, as a better alternative hydraulic bolting can be used. In hydraulic tensioning, the tools are used to stretch the bolt by generating the right tension force to tighten the bolt for assembly. It is famously used in industries as a bolt stretch method.

Hydraulic tools such as stud tensioners remove the load from the stud and stretch it. The load gets transferred to the bolting joint by clamping force. After the stud has been stretched, the nut is rundown to the joint. The load on the stud is removed, which results in a clamping force transferred to the now bolted joint. Hydraulic tensioning is more complex than torque tightening, but it is highly precise and fast.

• Mechanical Tensioning 

Another method other than hydraulic fastening is mechanical tensioning. It helps to achieve a higher bolt load than torqueing and hydraulic bolt tensioning. It is best suited for assemblies where high bolt yield strength is required. Due to high yield strength, it is easy to eliminate any springing back or torsional stress.

• Yield-point-controlled tightening method

For yield control tightening the yield point of the fastener is detected to measure the combined tension and torsion. For accuracy, the bolted joint must be dimensioned, it helps the bolt to be tightened into the plastic zone and the joined parts do not yield under the thread or in the bolt head It is advisable to preload the fastener to the yield point of the bolt material to reach the maximum clamp force possible from each fastener.

• Induction Heating

Heat is applied to the bolt and due to the thermal expansion characteristics of fasteners it expands. The bolt contracts after cooling down and then constricted to achieve the preload. The nut is heated under direct heat. This method is comparatively slow and not recommended for smaller bolts.

•  Angle control tightening

Also known as “turn-of-the-nut”, angle control tightening is another fundamental method used for assembly. This method is based on the principle of tension control. The angle control method is suitable where the joints have consistent stiffness and variable friction. It is very important to determine the right angle. With the help of power wrenches, the fastener is tightened to its elastic limit, and then further turned by a particular degree beyond this point.

• Tension indicating methods

These are suggestive methods that involve the measurement of bolt tension by using load indicating bolts and washers. It is also important to calculate the length change of the fastener and reach a proper preload. This process requires to reach desired tension that can be achieved only by appropriate tool tensioning. With advances in bolting technology, the introduction of tension indicating fasteners are far ahead of the ordinary bolts through establishing and maintaining the specified clamp force on the joint.

Content Sources :- ABSGroup