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June 2008 Newsletter
The Nuts & Bolts of It

The other day the writer was asked how Bolts and Nuts are tested, and thought it may be of interest to readers. Many believe a hardness test is all that is required. Not so!

Bolt & Nut Testing

There are two standard tests for Bolts; Proof Load Test; and Wedge Tensile Test, and for Nuts the Proof Load Test.

Bolt Proof Load Test
Bolt Proof Load is an axial tensile load which the Bolt must withstand without evidence of permanent set. The Bolt is screwed into a tapped attachment, with six threads exposed, and the Head supported on a parallel collar.

The required Proof Load is then applied. An accurate measurement of the Bolt length is taken before and after. The difference cannot exceed 12.5 microns, or 0.0005 inches. This is an allowance for measurement error.
Proof Loads are set approximately 5% less than the expected minimum Yield strength.

This proves the Bolt will behave “elastically” (without permanent stretch) under Proof Load.

Bolt Wedge Tensile Test
The Bolt is assembled as for the Proof Load test, but with the substitution of a “wedge” under the head.

The Bolt is then loaded until it fractures. Obviously the Load must be above the specified minimum. The position of the fracture is particularly important. The fracture must occur in the threads or the plain shank – but not at the head/shank junction. The wedge introduces flexural stresses as well as axial stresses in the body of the Bolt – particularly right under the head. Use of the wedge demonstrates ductility, and Heading quality.

Nut Proof Load Test
The Proof Load test measures the load a nut will take before stripping. The Nut is screwed on to a hardened mandrel, and an axial Load equal to the required Proof Load is applied.

Proof Loads of Nuts are set at or above the minimum Tensile Strength of the Bolts for which they are intended. The Nut must withstand this load, without failure by rupture or stripping, and be easily removable from the mandrel after the load is released.

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