The Use of Two Nuts to Prevent Self Loosening

7/29/2019 The Use of Two Nuts to Prevent Self Loosening

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The Use o f Two Nuts toPrevent Se l f Loosen ing

Many types of old machinery have twonuts on the bolts. A thin nut is frequently

used in these applications. Sometimesthe thin nut can be observed below thestandard thickness nut and on otherinstallations, it’s on top. Although it may

seem counter-intuitive, the thin nutshould go next to the joint and not beput on last. In other applications, forexample on column attachments, twostandard thickness nuts are frequentlyused.

In this article the effectiveness of this locking method isinvestigated and the tightening procedure that should be usedif effective locking is to be achieved. The use of two plain nuts goes back at least 150 years basedupon observation of historic machinery. Tightening one nutdown and then simply tightening another nut on top of itachieves little locking effect. A specific procedure needs to befollowed if locking is to be achieved. When a thin and thick nutare used, it may be thought that the thick nut should go nextto the joint since this would take the entire load. However, by

placing the thin nut on first, when the thick nut is tightened ontop 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% to50% of the overall tightening torque. The second(thick) nut is then placed on the bolt and the thinnut 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 theprocedure has on forces present between the nutsand in the bolt.

When the thick nut is tightened onto the thin nut,as the load increases, the load is lifted from thepressure flanks of the thin nut. As tighteningcontinues a point is reached when the bolt threadtouches the top flanks of the thin nut. At this point

F3 = F2. Continuing to tighten the top nut resultsin 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 inresisting self loosening is due to the way thethreads are jammed together (hence the term jamnut being frequently used for the thin nut). Sincethe 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 betweenthe bolt and nut threads must occur. It is this jamming action that is the secret of the two-nut

method.

In order toachieve theappropriate

bolt preloadprior to thethreads jamming it is necessary to tighten the smaller nut.The greater the grip length of the joint, thegreater is the extension needed to achieve a givenpreload and hence the higher the initial load thatmust be sustained by the small nut. Although theaxial backlash can be calculated for giventolerance conditions of the nut and bolt threads,

there can be a factor of 10 difference between theminimum and maximum values. Such variationmakes it difficult to establish the correctpreloading of the small nut. As a result, thebottom 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 theprinciples that have been outlined above are followed. Small (jam nuts) are frequentlyused since there is no need to have a full height nut on the bottom since the threads donot carry the load. An advantage of a thin nut in this application is that a greateramount of axial backlash will be provided for a given tolerance class.

The two videos shown below presents the results of a Junker fastener vibration testperformed on the two arrangements that a thick and thin nut on can be arranged. Thetests 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 withM10 nuts and bolts. The results are illustrated in the graph below. With the small nut ontop, both nuts can be observed to rotate together and can subsequently comecompletely loose. The results are slightly better than is normally observed with a singleplain nut. With the small nut next to the joint, some relaxation occurs but not asignificant amount of self-loosening . The performance of the two-nut method, whenproperly 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 andrequires a degree of skill and is hence unlikely to make a major comeback on newmachinery any time soon.



View Video with the Thick Nut Next to the Joint

View Video with the Thin Nut Next to the Joint

A word o f warn ing

Sometimes there are reasons other than self-loosening why two nuts may be used. Twonuts can be used to reduce the risk of thread stripping especially if the threads are

galvanised. Hence there may be a good reason to use two nuts i.e. to ensure that thefull strength of the bolt is developed rather than a premature failure from threadstripping. If there is a significant amount of fastener extension that would be developedi.e. when a long bolt is used, then using the thin nut on the bottom can beinappropriate. In such circumstances the bolt extension could be such that the thin nutwould sustain thread stripping due to the extension exceeding the thread clearanceavailable in the thin nut. Hence the warning that placing the thin nut next to joint is notappropriate in all circumstances. Tightening the thin nut on top of the thick nut tends not to work since there is minimalbolt extension existing in the threads between two nuts. Hence they prone to looseningas a result of embedding between the nut surfaces even under an axial load situationwhen self-loosening (i.e. nut rotation) would tend not to occur.

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The Use of Two Nuts to Prevent Self Loosening