106
The use of two nuts to prevent
self loosening
By Bill Eccles, Bolt Science
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 on top of a standard thickness nut (figure 1) and on other installations, it’s next to the joint, under the thick nut (figure 2).
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 I investigate the effectiveness of this
locking method and discuss the tightening procedure that should be used if effective locking is to be achieved.
thin nut, as the load increases, the load is
Figure 1 - lifted from the pressure flanks of the thin nut.
Thin nut on top As tightening continues a point is reached
of a thick nut. 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. This is shown in
figure 4. 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.
Figure 2 -
Thin nut placed Figure 3 -
next to the joint.
Small nut is placed on the bolt - The first
stage involves tightening the small nut
The reason why the two nut system is
onto the bolt. The small nut is typically
effective in resisting self loosening is due to
tightened to between 25% to 50% of the
the way the threads are jammed together
The use of two
overall torque value. This torque generates
(hence the term jam nut being frequently used
plain nuts goes back at for the thin nut). Since the bolt thread is in
least 150 years based upon observation of
a force of F1 in the bolt that the nut reacts
contact with the top flank of the small nut and
historic machinery. Tightening one nut down
onto the joint.
the bottom flank of the top nut, relative thread
and then simply tightening another nut on top movement is not possible. This is illustrated in
of it achieves little locking effect. A specific figure 5. For self-loosening to occur, relative
procedure needs to be followed if locking is to movement between the bolt and nut threads
be achieved. When a thin and thick nut are must occur. It is this jamming action that is the
used, it may be thought that the thick nut secret of the two-nut method.
should go next to the joint since this would In order to achieve the appropriate bolt
take the entire load. However, by placing the preload prior to the threads jamming it is
thin nut on first, when the thick nut is necessary to tighten the smaller nut. The
tightened on top of it, the load on the threads greater the grip length of the joint, the greater
of the thin nut are relieved of their load. What is the extension needed to achieve a given
follows is an explanation as to why such a preload and hence the higher the initial load
procedure is effective in resisting that must be sustained by the small nut.
self-loosening, whereas just tightening a thin Although the axial backlash can be calculated
nut on top of a thick nut is not effective. for given tolerance conditions of the nut and
The thin nut should be placed on the bolt bolt threads, there can be a factor of 10
first. This nut is typically tightened to between difference between the minimum and
25% to 50% of the overall tightening torque. maximum values. Such variation makes it
The second (thick) nut is then placed on the
Figure 4 -
difficult to establish the correct preloading of
bolt and the thin nut held to prevent rotation
Thick nut is placed over the thin nut - The
the small nut. As a result, the bottom nut is
by a spanner whilst the thick nut is tightened
next stage involves holding the small nut
tightened to a simple percentage (i.e. 25% to
to the full torque value. A series of diagrams,
with a spanner to prevent its rotation
50% of the overall torque value). Two full
figures 3 to 5 show the effect that the
whilst tightening the thick nut on top of it
height nuts can be used if the principles that
procedure has on forces present between the
to the full torque value. This generates a
have been outlined above are followed. Small
nuts and in the bolt. force F2 in the bolt that is reacted through (jam nuts) are frequently used since there is
When the thick nut is tightened onto the the small nut and into the joint. no need to have a full height nut on the
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