A self-locking screw-nut assembly is provided characterized in that the screw (2) has one or more median threads (4) of reduced diameter, whereas the nut (1) is designed to be deformed locally by an external action at the level of these reduced diameter threads after tightening to the desired torque, so that the displaced metal fills the gap (7) left free between the inner thread of the nut (1) and said reduced diameter threads (4) of the screw (2).

FIELD OF THE INVENTION 
The present invention relates to a self-locking screwnut assembly, namely a 
fixing assembly having a torque resisting unscrewing. 
BACKGROUND OF THE INVENTION 
Self-locking fixing systems known at present are generally based on a 
permanent radial deformation of the nut before mounting, which produces a 
torque resisting unscrewing by friction on the thread of the screw. Such 
systems have however drawbacks. 
In fact, the friction is manifested during tightening and causes a 
resistant torque which introduces a considerable dispersion factor in the 
relationship between the mounting torque and the effective tension in the 
assembly. Moreover, the locking torque against unscrewing drops under the 
action of the first tightening, so that in order to guarantee a sufficient 
locking torque in service, it is necessary to attain a level of 
deformation of the nut which induces a relatively high torque resisting 
tightening. 
It will finally be noted that, to the extent that there is no veritable 
locking of the nut on the screw, possible unscrewing during service, for 
example under the action of heavy vibrations, may not be completely 
impossible. 
SUMMARY OF THE INVENTION 
The main purpose of the present invention is then to overcome these 
drawbacks and, for this, it provides a screw-nut assembly which is 
essentially characterized in that the screw has one or more median threads 
of reduced diameter, whereas the nut is designed so as to be deformed 
locally by an external action at the level of these threads of reduced 
diameter after tightening to the desired torque, so that the displaced 
metal fills the gap left free between the inner thread of the nut and the 
threads of reduced diameter of the screw. 
With this arrangement, the nut may be freely mounted on the screw, so with 
a well defined tightening torque, and its subsequent local deformation 
causes locking of the nut on the screw making the system truly 
self-locking without the action of a high torque appreciably greater than 
the tightening torque. 
In a particular embodiment of the invention, the nut includes an external 
protuberance whose compression by means of an appropriate tool causes the 
displacement of metal required at the level of the threads of reduced 
diameter of the screw. 
Preferably, the screw is formed by laminating a piece provided beforehand 
with an annular groove. 
Thus, the lamination operation causes, at the level of the annular groove, 
the formation of a thread incompletely filled at its apex and which is 
therefore of reduced diameter with respect to the other threads formed.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first of all to FIG. 1, a nut 1 is shown mounted on a screw 2 for 
maintaining in position a part 3 to be assembled. According to the 
invention, screw 2 is provided at 4 with a median thread whose diameter at 
the apex is substantially less than that of the other threads situated on 
each side. 
Nut 1 may thus be freely mounted on screw 2, until its base 5 comes into 
contact with the piece 3 to be assembled. After tightening the nut to the 
desired torque, an appropriate tool, possibly formed by the tightening 
tool itself causes, for example by means of a ball such as 6, a local 
plastic deformation of the external surface of the nut at the level of the 
reduced diameter thread 4 of screw 2. Thus, the displaced metal fills the 
gap 7 left free between the inner thread of the nut and the reduced 
diameter thread of the screw, as illustrated in FIG. 2. The nut is thus 
perfectly locked on the screw and can no longer be unscrewed without the 
action of a high torque, capable of causing the metal of the nut to come 
back to its initial position under the effect of the increase in diameter 
of the thread. 
In the embodiment shown in FIGS. 3 and 4, nut 1 is provided with an 
external protuberance 8, whose compression by means of a tool 9 in the 
form of a die, causes, at the level of the reduced diameter thread 4, the 
necessary displacement of metal for locking the nut on the screw. 
The median thread or threads of reduced diameter may naturally be obtained 
by machining with a tool cutting the top of the threads in the adequate 
zone of the screw. 
Even more economically, a reduced diameter thread may be obtained directly 
during manufacture of the screw by the rolling process or lamination. To 
this end, and as shown in FIG. 5, a blank is used such as 10 which is 
previously provided with an annular groove 11. The lamination operation 
causes, at the level of this annular groove 11, the formation of a thread 
incompletely filled at its apex because of the smaller volume of metal to 
be displaced, whereas the other threads situated on each side have a 
greater diameter at the apex because of complete filling of the rolling 
tool.