Precautions for the use of self-locking safety nuts

Precautions for the use of self-locking safety nuts
1. Control the quality and design of self-locking parts and mating parts within the applicable standard thread specifications.

2. According to the instructions in the installation data, counterbore the female butt joint or chamfer the end of the male butt joint.

3. The mating parts must be clean and free of all foreign matter.

4. It is not recommended to use self-locking elements in mating parts with keyways, grooves, cross holes or other thread shape interruptions, which will adversely affect the locking torque performance and reusability.

5. The manufacture of some commercial strip fasteners may produce very small metal burrs at the mating ends of the longitudinal grooves. These parts can typically be used in 90% of self-locking applications. If you specify that the part is burr-free (this term is defined in the Glossary), Long-Lok will deburr the ends.

6. Extensive independent test data demonstrate that the tensile strength of Long-Lok tape and grain fasteners is not adversely affected by the milled or drilled holes used to accommodate their respective locking elements when properly used. Dimensions 1/4 inch in diameter or smaller may show very little reduction directly in the grooved or drilled area. However, similar test data show that there is still a high tensile strength margin. All parts still meet minimum specification requirements. The torsional strength of these parts is only affected at sizes smaller than #8, and then only when tightened past yield strength. No loss of size in shear strength; no change in high shear bolt grip. For strip parts, the ends of the insertion slots should be rounded to prevent “notch effect” or stress rise.

7. The maximum torsional torque (the ubiquitous “make-on” torque) is greatest when initially installing self-locking assemblies with mating threaded forms. This is due to the thread form being machined into the locking features, as well as the surface roughness of the polished threads. The second and third installations continue these operations to a lesser extent. The torque value produced through subsequent repeated cycles will gradually decrease to an extent that will depend on the wear factor of the mating material and the resilience of the locking feature used.

9. For parts with external threads, the size of the tapping drill should form 65% to 75% of the theoretical thread height in the mating hole. All holes should be drilled with thread cutting rather than threading taps to ensure quality mating threads and better general purpose torque performance.

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