Using Two Nuts to Avoid Self-Loosening on Bolts: Is it Effective?

May 9, 2017

Double-nutted bolts use a "jamming" nut to solve fastener self-loosening problems. Take a look at one of these double clamped bolts. This is an anchoring practice that screws the thinner jam nut first, then the primary, load bearing nut is applied. It's a well-recognized application of a fundamental engineering principle, this two nut approach, but is it effective? Is this a reliable nut locking solution?

A Jamming Review

A bolt jam aid is a second nut, one that's used before the main locking nut. It's typically half as wide as the larger hexagonal fastener aid, yet its fastener-supporting credentials are proven. Basically, it's a preloading mechanism. Tightened by a wrench (spanner), it locks against a surface, forms the initial fastener joint, and provides approximately fifty-percent of the required support torque. At this point, the main nut is screwed down. It clamps against the thin jam nut and creates a chain of interlocking forces. There are opposing forces between the two mated nuts, jointed forces between each nut, and torqued anchoring forces holding the threaded bolt rod to the threaded nuts. Importantly, the spanner holds the thinner nut as that larger hex fitting is screwed down.

Evaluating Self-Loosening Capabilities

As the slender nut passes its load over to its larger companion piece, the leading edges of the two mobile nuts lock together until they cancel each other's directed energy output. In effect, the equal and opposite loads carried by each nut are absolutely equalised and subtracted. From a practical standpoint, the seemingly counterintuitive engineering principle has initially placed the load on the jam nut, but the main nut has assimilated that load while also creating a self-locking mechanism, one that's based on the equal but opposing forces generated by the two nuts. Again, the presence of that spanner is important, for the slender nut must not be allowed to rotate or add torque to the clamping action used here.

When the single half-width nut is locked down first, it initiates the self-locking mechanism. Held in place, the slender semi-torqued hex fastener is further locked down by a primary, much thicker nut, a hex fitting that equalises the forces generated by the unfinished joint. This is indeed a very effective means of avoiding self-loosening issues when plain bolts and nuts are being used by a skilled craftsman. Of course, the quality of the nuts and bolts used in this self-anchoring endeavour must also be assured. Otherwise, a substandard fitting could slip a thread, or the thread could be stripped.

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