How to tighten your bolts and which tools you should use to achieve the perfect seal is not as seamless and straightforward as we would like. Various factors come into play in these situations, such as bolt grade, size, and even application type.
Tightening torques in this sense can therefore become a minefield.
However, it remains vital to determine the optimum tightening torque for each application as every bolted joint is unique.
Bolt tensioning safety is crucial – but what is torque?
Torque is a rotating force measuring force multiplied by distance. Where distance equals the length of the wrench, and the force is the force you apply.
What does a correctly tightened bolt look like?
When a gasket spiral is tightened correctly, it will stretch like a rigid spring, connecting two surfaces securely and as long as the two loads do not exceed each other, then the components/two mating surfaces will not pull apart.
As you continue to rotate the bolt, the stretching and tightening will continue; however, how much tension you require depends on several factors.
- Bolt diameter is an important area to acknowledge as it will take more force to tighten a ¾ – 10 bolt compared to a 318-16 bolt.
- Bolt grade– depending on the material’s strength, it will take more force to stretch an SAE Grade 8 bolt than it does to stretch an SAE Grade 5, for example.
- Nut factor– also known as the friction coefficient, this element considers the bolting surface, i.e., those that are hard, smooth, and sleek require less torque than those that are softer, rougher, and stickier.
Taking these factors into account and combined will allow you to opt for the correct torque tightening level.
Note: The torque tightening value is only valid if all the application aspects remain constant, and each application should be evaluated on its own to determine the optimum torque value.
Determining the required torque
This, as we’ve mentioned, does depend on the spiral wound gasket’s material properties and its overall purpose.
However, we know that achieving proper torque is essential as if they are under torqued, bolts will deform and won’t provide a solid clamping force. Alternatively, an over-torqued bolt will simply break and give no protection.
The bolt tightening sequence
As the most well-known and common sequence, a Star pattern is typically applied to all types of gasket materials, particularly to spiral wound gaskets UK.
This sequence is then applied when torquing a bolted flange in assembly.
Of course, gasket type, arrangement of flange connections, etc., still all determine which tightening sequence will be most suitable (we’ll look at some others later in this post).
Ultimately, the aim is not to damage the gasket in the process of torquing.
The Star procedure
Following a star pattern, each flange bolt goes through a process of tightening, which starts at applying between 20-30% force, and then following the same pattern again going over these bolts applying 50-7-% force value, before finally using a rotational pass, stopping when the bolt stops turning. For spiral wound gaskets, typically two passes will be required to ensure optimum torque.
Other sequences to note:
Modified star bolting pattern – an advance on the original star pattern, this sequence is much quicker when you’re working with applications with 20+ bolts; however, it still follows the same star sequence moving from one bolt to the next, applying varying levels of force value.
Quadrant pattern – a more structured sequencing approach, the quadrant sequence avoids the crisscrossing, moving instead a bolt over only after you’ve completed the first sequence.
Circular pattern – this sequence is suitable only for solid/hard gaskets such as Kammprofiles, making this sequence unsuitable for spiral wound gaskets.
As the leading supplier of Spiral Wound Gaskets in Leeds, we carry a large stock volume when it comes to gaskets and bolts. To find out more, make sure to check out our pages online or call us on 01535 274 776; we’d be happy to help.
Reference video: Nord-Lock Group