Answer: a very small but important amount.
In previous posts we discussed how a bolted joint works, how clamp force is calculated, and how to determine torque required to achieve the clamp force. This month we’ll present more details about the workings of a bolted joint and why bolt stretch is important to understand.
To get clamp force on a bolted joint assembly, the bolt is stretched just a small amount. The solid steel shaft of the bolt acts like a very strong tension spring as illustrated below. The spring tension created when the nut is rotated, and the bolt is stretched, creates the clamp force.
It is important to understand this because, if this small amount of stretch is lost, the joint will fail.
For example, a 3/4″ UNC grade 5 bolt with a 3” clamp distance, fully tightened, only stretches the bolt approximately 0.006”. This stretch creates 21,300 lbs clamping force.
In most components that are bolted together, after tightening, the components settle in or embed in each other, and the clamp distance along with the bolt stretch distance is reduced. This is especially an issue if the bolted components have rough finished surfaces, softer materials, or flex under load.
In the example above, if just 0.006” of clamp distance is lost due to these issues, the clamp force is reduced to zero and the joint will begin to fail.
To calculate the amount of stretch for any bolting situation, the following formulas are commonly used:
Click below to download a simple Excel spreadsheet that makes it fast and easy to calculate the bolt stretch along with clamp load and installation torque. All you need to enter is the bolt size, thread pitch, proof load (as shown in the table), and clamp distance.
Calculate bolt stretch:
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