The Use of Two Nuts to Prevent Self-Loosening Intro

In this article, Bill Eccles of Bolt Science investigates a locking method that has been in use for over 150 years but is frequently applied incorrectly: the two-nut system. The method works — but only when assembled in the right sequence, with the right torque applied to each nut. Done wrong, it provides little to no locking benefit.

Two nuts on a bolt looks like a simple solution. The assumption is usually that tightening one nut against another creates a mechanical lock. That part is true, but the sequence and torque split matter enormously, and the common instinct about which nut goes where is exactly backwards. Getting this wrong is widespread enough that the article treats it as the starting point.

What the article covers:

• Why the thin nut goes next to the joint — The counterintuitive core of the method. Common practice puts the thin nut on top, but this produces almost no locking effect. The correct assembly places the thin (jam) nut nearest the joint, tightened first to 25–50% of the final torque value. The thick nut is then tightened on top to full torque while the thin nut is held against rotation. This sequence is what produces the thread-jamming action the method depends on.
• The thread-jamming mechanism — When assembled correctly, the bolt threads are simultaneously in contact with the top flank of the thin nut and the bottom flank of the thick nut, placing the bolt thread in compression between them. This eliminates the thread clearance that self-loosening requires. Relative thread movement between bolt and nut becomes mechanically impossible, which is what distinguishes this method from locking devices that merely add friction.
• Junker test performance — M10 two-nut assemblies were tested on a Junker transverse vibration machine under identical conditions. With the thin nut on top (incorrect assembly), both nuts rotated together and the joint came completely loose. With the thin nut next to the joint (correct assembly), some initial relaxation occurred but no significant self-loosening followed. The correctly assembled two-nut configuration outperformed many commercially available lock nuts.
• Practical limitations — The method is skill-dependent. Achieving the correct preload in the thin nut before the thick nut is applied requires care, and the axial backlash in the thread system means the thin nut preload can vary by a factor of 10 between minimum and maximum values for a given tolerance class. This variability makes the method difficult to control in high-volume assembly environments, which limits its practicality on new production machinery despite its strong locking performance when correctly applied.
• When two full-height nuts are used — In applications such as column attachments where two standard-thickness nuts are used, the same assembly principles apply. The nut nearest the joint still carries the role of the jam nut and must be tightened first.

The two-nut method offers superior locking performance compared to many proprietary solutions when correctly applied, but that “when correctly applied” qualifier carries real weight. This article provides the mechanical explanation needed to understand both why it works and why it so often doesn’t.

About Bolt Science 

Bolt Science was founded in 1992 to provide independent technical expertise in bolted joint technology. The company offers bolted joint analysis software, consulting and problem-solving services, fastener and joint testing, and training on bolting technology. Their client list includes many of the world’s major engineering organizations. Learn more at boltscience.com.