What is a Bolt Tightening sequence
Bolt tightening sequences are when an arrange of bolts are tightened in a pattern to the required torque. The sequence the bolts are tightened is a symmetrical pattern to reduce the amount of pre-load lost in a bolt, as a result of elastic interactions. Torque is normally applied to a bolt using a torque wrench.
Pre-load in a bolt results in the bolt stretching and creates bolt tension. Pre-load is an equal and opposite force to the compressive force induced on the materials in the bolted joint. Therefore, it is essential for maintaining safety in bolted joints.
Why Are Bolt Tightening Sequences Important
The reason we use a bolt tightening sequence is to reduce the loss of pre-load in the bolt. If pre-load is lost, the bolt will relax, potentially leading to detrimental failure. A loss of preload can result from differences in thermal expansions between the bolt and the joint, along with other mechanisms.
The image below shows the elastic interactions when a bolt is tightened. The top picture shows bolt one and three torqued and bolt two not. The bottom image shows bolt two being tightened and the affect this has on the surrounding material. Although bolt one and three have not been loosened there has been a loss of pre-load in the bolt. When bolt two is tightened it directly clamps the material beneath it, however also introduces more clamp on the material beneath bolt one and three.
When to use a bolt tightening sequence
Bolt tightening sequences are used on safety-critical and crucial joints of systems that use multiple bolts. A commonplace to see a bolt tightening sequence is in on bolted flanged joints.
Example
The image below shows an example of how a flanged joint may be secured using a bolt tightening sequence. Ideally, during the process, the bolts will be progressively tightened. This means going around the sequences a couple of times until the desired torque is achieved..
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