Why is Double Head Screw Nut Better?

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.

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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.

In order to achieve the appropriate bolt preload prior to the threads jamming it is necessary to tighten the smaller nut. The greater the grip length of the joint, the greater is the extension needed to achieve a given preload and hence the higher the initial load that must be sustained by the small nut. Although the axial backlash can be calculated for given tolerance conditions of the nut and bolt threads, there can be a factor of 10 difference between the minimum and maximum values. Such variation makes it difficult to establish the correct preloading of the small nut. As a result, the bottom nut is tightened to a simple percentage (i.e. 25% to 50% of the overall torque value). Two full height nuts can be used if the principles that have been outlined above are followed. Small (jam nuts) are frequently used since there is no need to have a full height nut on the bottom since the threads do not carry the load. An advantage of a thin nut in this application is that a greater amount of axial backlash will be provided for a given tolerance class.

On occasions, two thin, or jam nuts used together can be observed, one placed on top of the other. If the nuts are tightened to any extent, this is a poor and undesireable practice. It may be believed that the strength of two thin nuts used in this way is greater than that of a single normal nut. This is true if the two thin nuts are on top of each other and a proof load test is completed on both simultaneously, but not true if the nuts are tightened, which is the norm. What can happen is that when the first nut is tightened, some partial thread stripping can occur, often of the bolt due to the small length of engagement that the thin nut provides. This may not be readily detectable at the time of assembly. The second nut is then tightened on top of the first. Because of the partial stripping that's occurred, plastic deformation in the threads occurs after the tightening is completed. This results in a loss of preload and the joint potentially coming loose. If two nuts are desired to be used, for whatever reason, use of two normal height nuts would avoid the thread stripping issue.

The two videos shown below presents the results of a Junker fastener vibration test performed on the two arrangements that a thick and thin nut on can be arranged. The tests were conducted to investigate the effectiveness of the two-nut method in terms of resistance to self-loosening. A Junker transverse vibration test machine was used with M10 nuts and bolts. The results are illustrated in the graph below. With the small nut on top, both nuts can be observed to rotate together and can subsequently come completely loose. The results are slightly better than is normally observed with a single plain nut. With the small nut next to the joint, some relaxation occurs but not a significant amount of self-loosening . The performance of the two-nut method, when properly applied, provides a superior locking capability when compared to many so-called lock nuts. The proper application of the two-nut method is time intensive and requires a degree of skill and is hence unlikely to make a major comeback on new machinery any time soon.

The first video shows a test in which the normal height nut is tightened onto the joint first followed by the thin nut being tightened on top.

In this second test shown below, the thin nut was placed next to the joint and the normal height nut tightened on top.