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Bolt Preload

30th Nov 2023

What is Bolt Preload?

The tension created when a fastener is tightened – when a nut is screwed onto a bolt when holding two components together - is called bolt preload. The optimal preload in a bolt assembly ensures that the working load is effectively distributed among the installation materials, preventing the bolt from bearing the entire load. Preload in bolts prevents the joint from loosening under vibrational or dynamic loading conditions, ensuring the integrity and longevity of bolted joints.

The importance of bolt preload in connections cannot be overstated. In a bolted connection, ensuring that the bolts are fully tightened is crucial for maintaining stability in the assembly. By distributing the force across the materials, bolt preload reduces the strain on the assembly and enhances its overall strength. Bolt preload becomes particularly significant in demanding applications where bolt failures are more likely to occur.

Without bolt preload all the weight would be solely supported by the bolts, greatly increasing the risk of failure. Introducing preload allows for fewer bolts to be used as support plates play a role in bearing the load. However, it is essential to ensure that the working load remains lower than the bolt preload to prevent failure of both support plates and bolts themselves.

How to Calculate Bolt Preload

Now that we know how important bolt preload is, let’s take a look at various ways preload bolt calculation is determined.

  • Achieve Optimal Torque Using a Torque Wrench - While not an exact measure, reaching the optimal torque can indicate that the bolt is close to the correct preload. However, keep in mind that the torque value can be influenced by the material it is being fastened against. A rougher material will increase friction, resulting in higher torque but lower preload tension. Conversely, a smoother material will have the opposite effect. This method provides a decent estimate based on the material.
  • Reach Accurate Preload by Tightening until Snug and Adding "X" Degrees - Another approach is to snugly tighten the nut and then torque it a certain number of degrees, such as 90 degrees. To implement this method, spin the nut onto the assembly until it is snug against the material. Then, attach a wrench and rotate it a quarter of a turn (90 degrees). Although less reliable, this method provides a fairly accurate bolt preload measurement.
  • Ensure Proper Compression with Preload Indicating Washers - Preload indicating washers are designed to spin until a specific load is applied to them. Once the washer stops free-spinning, it signifies that the correct preload has been achieved. This straightforward method provides a reliable way to determine compression.
  • Observe Preload Using Silicon Direct Tension Indicating Washers - Similar to the direct tension-indicating washers, silicon direct tension indicating washers have recesses filled with silicon paste. As the nut tightens, the silicon starts to emerge from the sides of the washer. To determine optimal bolt preload, count the number of recesses on the washer minus one, and ensure that this many recesses are exposed. For example, if there are 6 recesses, at least 5 must be visible to indicate that preload has been achieved.
  • Verify Preload with Direct Tension Indicating Washers - Direct tension indicating washers feature bumps that flatten when the preload is reached. A feeler gauge can then be used to ensure that the bolt shaft is no more than 50% accessible under the washer, ideally less. This method provides an accurate measurement of preload.

Explore these methods to confidently determine bolt preload calculation and ensure optimal performance.

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