Double Nuts — Do They Work?
Let’s compare a typical assembly using one nut and a bolt against the use of double nuts.
When the nut is rotated in the counterclockwise direction, it moves toward the bolt head. Each full rotation (360°) moves the nut by a distance equal to the thread pitch distance. Once the nut contacts the clamp surface and the components being clamped are fully pulled together, additional rotation of the nut stretches the bolt in the area shown in figure 1. The amount of bolt stretch is mostly dependent on the clamp distance and is typically quite small.
- Information and how much the bolt stretches is available here: Bolt Stretch
- The amount of nut rotation needed to stretch the bolt and create the proper clamp force is outlined here: Rotation Angle
This stretch and resulting clamp force are critical to the function of the bolted joint.
For applications that experience either heavy load vibrations, micro vibrations, or extreme load cycling, a Security Locknut (pictured to the right) is often used to prevent joint loosening and failure. The patented lock ring on the Security Locknut isolates the load and locking function. The Security Locknut is designed so the nut takes the load and vibration and the patented lock ring helps prevent nut rotation. Read how in our post How Does the Security Locknut Work.
When a double nut is used in an attempt to prevent nut loosening and loss of clamp force, the top nut is tightened against the bottom nut (figure 2). With zero distance between the two nuts, the tightening energy bends the threads in the bolt around the top nut instead of stretching the bolt. The resulting thread damage is shown in figure 3. With bent threads, the top nut can easily loosen with just a very slight amount of rotation caused by impact or vibration. This is one reason why a bolted joint assembly with a double nut arrangement can often fail in high vibration or impact environments.
Tightening a second nut onto a bolt is often used for the visual effect of a “locknut,” despite providing little-to-no extra resistance to vibration loosening.
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