Test Methods For The Mechanical Properties Of Anti-loosening Nuts

The proof load is an externally applied axial load; under this load, the nut must exhibit no signs of thread stripping or fracture. In the context of testing, the proof load serves as a specific numerical value; however, in the context of design applications, the proof load represents a maximum limit.

 

During testing, the proof load is applied to the nut under evaluation via a test bolt or machine mandrel subjected to axial loading. Once the load is removed, the self-locking nut must be capable of being freely removed from the bolt or mandrel. This latter requirement is particularly noteworthy, as it confirms that the degree of thread distortion is insufficient to cause the mating threads to seize together. Tests have demonstrated that thread seizure can occur when the applied external load reaches 95% of the load level required to induce thread stripping.

 

In most standards governing the strength grades of self-locking nuts, the proof load is expressed in terms of "stress"-specifically, in pounds per square inch (psi). To calculate the actual proof load in pounds, the proof stress (psi) is multiplied by the stress area.

 

A crucial point to note is that, apart from its relationship to the stress area, the proof load bears no direct correlation to the specific external-thread fasteners with which the nut is intended to be paired.

 

The tapered proof load is an external axial load that a nut must withstand; during the application of this load, the self-locking nut must exhibit no signs of thread stripping or internal wall cracking. In the test procedure, the self-locking nut is mounted onto a hardened, tapered washer featuring a 120° angle. The purpose of the tapered washer is to generate a strong outward radial force that causes the nut's internal walls to expand when subjected to axial loading. If a nut fails this test, the failure is typically attributed to internal wall cracking rather than thread stripping.

 

Nuts intended for high-temperature environments-as covered under ASTM A194-are all subject to the tapered proof load test. Additionally, standards governing the integrity of discontinuous surfaces on self-locking nuts specifically mandate that nuts featuring seams undergo the tapered proof load test. Passing this test signifies that any seams present on the self-locking nut have not reached a severity level that would compromise the nut's functional performance.