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Many types of old machinery have two nuts on the bolts. A thin nut is frequently used in these applications. Sometimes the thin nut can be observed below the standard thickness nut and on other installations, it’s on top. Although it may seem counter-intuitive, the thin nut should go next to the joint and not be put on last. In other applications, for example on column attachments, two standard thickness nuts are frequently used.
In this article the effectiveness of this locking method is investigated and the tightening procedure that should be used if effective locking is to be achieved.
The use of two plain nuts goes back at least 150 years based upon observation of historic machinery. Tightening one nut down and then simply tightening another nut on top of it achieves little locking effect. A specific procedure needs to be followed if locking is to be achieved. When a thin and thick nut are used, it may be thought that the thick nut should go next to the joint since this would take the entire load. However, by placing the thin nut on first, when the thick nut is tightened on top of it, the load on the threads of the thin nut are relieved of their load.
The thin nut should be placed on the bolt first. This nut is typically tightened to between 25% to 50% of the overall tightening torque. The second (thick) nut is then placed on the bolt and the thin nut held to prevent rotation by a spanner whilst the thick nut is tightened to the full torque value. The series of diagrams show the effect that the procedure has on forces present between the nuts and in the bolt.
When the thick nut is tightened onto the thin nut, as the load increases, the load is lifted from the pressure flanks of the thin nut. As tightening continues a point is reached when the bolt thread touches the top flanks of the thin nut. At this point F3 = F2. Continuing to tighten the top nut results in the jamming of the threads leading to F3 > F2. If tightening is continued, the force between the two nuts will continue to increase. If the thick nut is overtightened, there is the risk of thread stripping or the tensile fracture of the bolt between the two nuts.
The reason why the two nut system is effective in resisting self loosening is due to the way the threads are jammed together (hence the term jam nut being frequently used for the thin nut). Since the bolt thread is in contact with the top flank of the small nut and the bottom flank of the top nut, relative thread movement is not possible. For self-loosening to occur, relative movement between the bolt and nut threads must occur. It is this jamming action that is the secret of the two-nut method.