Bolt torque is the amount of rotational force that is applied to a bolt to create clamp force. Clamp force is the amount of pressures that is applied to the two components that the bolt is holding together. Clamp force is the force that holds the components together and prevents them from coming apart.
If too little bolt torque is applied, the vibration that exists in many component can cause the bolt to become loose, which can lead to the components falling apart. If you apply too much bolt torque to a bolt, the bolt can be stretched past the yield point of the bolt. A bolt that is stretched past it’s yield point will not return to its original shape.
How to Tighten Bolts Correctly
Therefore, using a torque chart for bolts are necessary to ensure that the bolts are tightened to the correct amount of torque. A torque chart provides the values of torque that should be applied to bolts of a given diameter and property class. The property class of a bolt is the number that is located on the bolt head that indicate the strength of the bolt.
For instance, a bolt with a property class of 8.8 is of standard strength, but a bolt with a property class of 12.9 is of high strength and is made of alloy steel. The property class of bolts must be identified because each property class of bolts require a different amount of bolt torque in order to create the correct clamp force. Many bolts use the metric system to denote their size and strength, as well as use property classes to indicate the strength of those bolts.
The use of lubricants, such as machine oil, anti-seize compounds, and grease changes the amount of bolt torque that is required to achieve the desired clamp force. When you use lubricants in the bolt, the friction between the threads of the bolt is reduced. Therefore, the amount of torque that is applied to the bolt must be reduced if the bolt is to be lubricated.
If too much torque is applied to a lubricated bolt, the bolt may snap due to the over-application of clamp force. For instance, dry threads have a high level of friction between the components of the bolt, but lubricated threads have a low level of friction between those components. If dry torque values are applied to a lubricated bolt, the bolt will be over-tightened and may break.
The tool that is used to apply bolt torque must be accurate. Click-type torque wrenches are among the most common tools technicians use. Click-type torque wrenches will click when the desired bolt torque is reached.
Dial-type torque wrenches have a needle that can be turned to the desired bolt torque. Digital torque adapters display the bolt torque value on a screen. Some bolts, known as stretch bolts, require the use of an angle wrench.
Angle wrenches are used to tighten the stretch bolt to the proper amount of torque, then turning the bolt to a specific number of degrees. Additionally, bolt wrenches should of been calibrated regularly to ensure that they provide accurate bolt torque values. The sequence of tightening bolts is also important.
For instance, bolts on a flange or a cylinder head should be tightened in a star pattern. A star pattern implies that the bolts are tightened in a crisscross pattern rather than in a circle. Using a star pattern for tightening bolts will distribute the clamp force even across the components, which will prevent them from warping.
Additionally, bolts are also tightened in several stages. For instance, bolts can be tightened to 30% of the specified torque, 70% of the specified torque, then the full amount of torque. Lastly, different materials require different amounts of bolt torque.
For instance, aluminum components are softer than steel components, so a softer material will require less bolt torque. If too much bolt torque is applied to an aluminum component, the threads may be stripped. Additionally, the use of stainless steel components can create a phenomenon known as galling.
Galling occurs when the threads of two stainless steel bolts stick together, so to prevent this phenomenon, the bolts should be lubricated heavily. Additionally, care should also be taken when tightening bolts to prevent cross-threading the bolt, which will ruin the bolt and prevent it from achieving clamp force. Finally, the threads of bolts should be cleaned with a wire brush prior to applying clamp force and tightening the bolt.
