Proof load refers to an externally applied axial load. Under such load, the nut shall show no signs of thread stripping or fracture. For testing purposes, the proof load is a specified value; for design applications, the proof load represents the maximum allowable limit.
During testing, the proof load is applied to the test nut via an axially loaded test bolt or machine spindle. After load removal, the nut shall be freely removable from the bolt or spindle. This requirement is critical, as it verifies that thread deformation is insufficient to cause seizure of engaged threads. Tests indicate that thread seizure tends to occur when the applied load reaches approximately 95% of the thread stripping load.
In most nut strength grade standards, proof load is expressed in terms of stress in pounds per square inch (psi). To calculate the actual proof load in pounds, multiply the proof stress (psi) by the bearing area.
An important note: Outside the common stress range, the nut proof load has no direct correlation with the specified externally threaded fasteners.
Conical Proof Load
Conical proof load is an externally applied axial load that a nut must withstand without thread stripping or inner wall cracking. In the test, the nut is assembled onto a hardened 120° conical washer. The conical washer generates strong radial expansion force on the nut inner wall when axial load is applied. If a nut fails this test, the failure mode is usually inner wall cracking rather than thread stripping.
Nuts complying with ASTM A194 for high-temperature service shall undergo the conical proof load test. Furthermore, standards defining load limits for discontinuous nut surfaces mandate this test for slotted nuts. A passing result confirms that seams on the nut do not impair its service function.
The conical proof load of a nut can be calculated by multiplying its specified axial proof load by a coefficient ranging from 0.3 to 1D, where D denotes the nominal nut diameter in inches.
The conical proof load test is currently the most effective method to identify non-conforming nuts and will remain a mandatory verification requirement until superior inspection technologies become available.
Hardness
Hardness is an important mechanical property of nuts. Not only is hardness testing convenient to perform, more importantly, it is currently the only acceptable criterion for evaluating nuts whose proof load exceeds the capability of conventional tensile testing. Unlike externally threaded fasteners, there exists no definable correlation between hardness and strength for steel nuts or nuts made from other metals. This is attributed to the dominant influence of nut geometry on its load-bearing capacity.
Generally, nuts with a specified proof load below 120,000 psi may be subjected to proof load testing. Nuts of higher strength must be verified against a specified minimum hardness value. Additionally, nut strength grade specifications require hardness testing for all sizes and grades to ensure hardness meets or exceeds the specified minimum value. This requirement is especially vital for heat-treated nuts, confirming that proper processing has been implemented to eliminate potential embrittlement.
