Removable threaded fastener with locking plate

The present invention provides a threaded insert assembly with a locking plate having a non-circular shape and a non-serrated outer edge instead of a conventional circular shaped locking ring having a circular serrated outer edge. The non-circular locking plate is shaped to engage mating edges of a correspondingly shaped recess formed in the anchoring structure so as prevent the insert from rotating out of the anchoring structure once the insert is locked into the plate. The threaded insert is secured to the locking plate by interdigitated serrations swaged into engagement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to fastening assemblies such as threaded inserts 
with anti-rotational locking devices used to receive threaded fasteners, 
more particularly, it relates to such assemblies having a non-circular 
anti-rotation locking plate used to secure the threaded insert. 
2. Description of Related Art 
The present invention relates to fasteners in general and more particularly 
to anchoring fasteners with lock rings that are preferably removably 
threaded in an anchoring structure and which typically cooperates with 
another fastener to hold other structures to the anchoring structure. 
Unscrewing of the threaded anchoring fastener is typically prevented by a 
circular locking ring having serrations around its periphery which is 
swaged into a suitably adapted recess in the anchoring structure. The 
threads of the insert and stud can be of hardened material, much harder 
than the anchoring structure. Examples of threaded anchoring fasteners are 
threaded inserts and studs. 
Inserts installed in a workpiece provide a means of attachment of an object 
to the anchoring structure. Inserts are used when the stress requirements 
are greater than are provided by the anchoring structure. An insert can 
distribute the stress over a much greater area and can be harder material 
than would be provided by the anchoring structure alone. Inserts are also 
used to provide positive locking of the anchoring fastener in the 
anchoring structure by means of a locking device or feature contained 
within the insert. An example of an insert is a plug-like object having 
external threads to bit into or cooperate with the anchoring structure 
material. The insert also has internal threads for receiving the male 
threads of a cooperating piece. 
Studs in contrast provide an anchor for attachment of objects to a 
anchoring structure. A stud secures to a anchoring structure with male 
threads and itself provides male threads for cooperation with a female 
threaded nut or other fastening means. 
Threaded inserts of the type formed of a relatively hard material such an 
INCONEL 718 and adapted to be anchored in an anchoring structure of softer 
material such as aluminum have been widely used in such applications as 
gas turbine engine structures and similar structures. These threaded 
inserts typically are adapted to be threaded into a bore in the anchoring 
structure and then locked into position by means of a generally circular 
serrated locking ring which is adapted to mate with the threaded insert 
and to be forced into the surrounding anchoring structure such as by 
swaging. This causes cold flow of the anchoring structure material so as 
to secure the locking ring and thereby the insert against rotation in the 
anchoring structure. The serrations provide a means for allowing the 
forcing of the locking ring into the surrounding anchoring structure 
material by cold flow and also allow for relatively great torsional 
strength between the locking ring and the threaded insert. The cold flow 
of the anchoring structure material firmly secures the locking ring in 
position to prevent if rom becoming dislodged. 
A major disadvantage of the serrated locking ring is that the serrations 
can result in unacceptable stress risers in he parent material and may 
become failure initiation sites and initiate failure in the anchoring 
structure if the parent material is highly loaded. Often the advantages of 
threaded inserts cannot be realized because the stress risers caused by 
the serrated locking ring is unacceptable. Furthermore, designs which 
propose to use keys or serrations that are preferentially oriented out of 
the primary stress field may result in failures if they are inadvertently 
installed in the wrong orientation or if the direction of the stress field 
changes. 
Another drawback to these arrangements exists in that no practical, easily 
implemented technique is possible for removing an insert so as to allow 
replacement without damaging the surrounding material of the anchoring 
structure. The serrations of the locking ring on the new insert may not be 
firmly secured due to the presence of the old impressions left by the old 
locking ring of the original insert. Thus, drilling out of the old bore 
and replacement with a larger size insert would be necessary. 
Therefore, the object of the present invention is to provide such an 
anti-rotational locking ring and threaded insert assembly which lowers 
stress risers caused by serrated edge locking rings and is easily 
removable thereby providing for easily replaceable threaded inserts. 
SUMMARY OF THE INVENTION 
The present invention provides a threaded insert assembly with a locking 
plate having a non-circular planform shape and a non-serrated outer edge 
instead of a conventional circular shaped locking ring having a circular 
serrated outer edge. The non-circular locking plate is shaped such that it 
will engage mating edges of a correspondingly shaped recess formed in the 
anchoring structure so as to not allow the insert to rotate out of the 
anchoring structure once the insert is locked into the plate. The threaded 
insert is locked to the locking plate by serrations or locking keys. The 
preferred embodiment provides an upper portion of the insert having 
external serrations that can be swaged into the inner diameter of a center 
aperture in the locking plate. The swaging drives the outer serrations of 
the insert into locking engagement with inner serrations on the inner 
diameter of the center aperture of the locking plate. 
One embodiment provides the locking plate with a non-circular planform 
shape to minimize a stress riser created by installing it into the 
anchoring structure. Shaping the locking plate so as to avoid sharp 
corners, notches or edges in any direction helps reduce stress in the 
anchoring structure. Among the shapes contemplated by the present 
invention are square, rectangular, triangular, elliptical, and the 
preferred racetrack shape. Another embodiment provides a means to press 
fit the plate into the anchoring structure after the insert is threaded 
in. 
ADVANTAGES 
The present invention provides a threaded insert assembly which lowers 
stresses in and increases the life of the anchoring structure into which 
the insert is installed by locking the insert to the plate and using the 
plate to anti-rotationally lock the insert into the anchoring structure. 
Any stress risers associated with a conventional locking ring having keys 
or serrations are located in the lowly loaded locking plate rather than in 
the highly loaded anchoring structure.

DETAILED DESCRIPTION OF THE INVENTION 
Illustrated in FIG. 1 is an element 4 mounted to an anchoring structure 6 
by a fastener in the form of a threaded screw 8. The threaded screw 8 is 
threaded into insert internal threads 12 of the threaded insert 10 and the 
externally threaded insert 10 is threaded into threaded bore 14 in the 
anchoring structure 6 by insert external threads 16. The threaded insert 
10 includes an upper portion 11 that has external serrations 18 disposed 
about its outer diameter OD above the insert external threads 16. 
Surrounding the external serrations 18 is a non-circular shallow recess 28 
disposed in the anchoring structure 6 and shaped to receive an 
anti-rotational locking plate 22. 
Referring to FIG. 2, the locking plate 22 has internal serrations 20 about 
an inner diameter ID of a center aperture 19 which provides a 
anti-rotational locking means to lock the locking plate 22 to the threaded 
insert 10. The external serrations 18 are designed to be aligned with the 
internal serrations 20 and then together with the upper portion 11 swaged 
onto and anti-rotationally secured to the locking plate 22. Equivalent 
means of securing the locking plate 22 to the threaded inset 10 are 
contemplated by the present invention including having mating keys instead 
of serrations or serrations on just one of th either the ID or OD of the 
locking plate or threaded insert 10 respectively. 
The preferred embodiment illustrated in FIG. 2 provides that the inner 
diameter ID of the locking plate 22 is sufficiently larger than the outer 
diameter OD of the threaded insert 10 such that the external serrations 18 
and the internal serrations 20 do not interfere with each other as the 
threaded insert 10 is threaded into the bore 14 after the locking plate 
has been placed in the recess 28. The serrations of either the locking 
plate or the insert may then be aligned so as to be disposed between the 
serrations of the other element by threading the insert in or out of the 
bore 14. Alternatively the inner diameter ID of the locking plate 22 may 
be smaller than the outer diameter OD of the threaded insert 10 and the 
threaded insert must be threaded into the bore before the locking plate is 
placed into the recess 28. Alignment in either case may be done by 
threading the threaded insert 10 into or out of the threaded bore 14 just 
a bit so that the serrations align. This alternative embodiment provides 
an additional feature and benefit that the swaging operation is not 
required to secure the insert to the locking plate to prevent rotation 
therebetween. 
FIGS. 4 and 5 illustrate the mated serrations after the upper portion 11 of 
the threaded insert 10 has been swaged outward forcing its external 
serrations 18 between and in locking engagement with the internal 
serrations 20 of the locking plate 22. An annular chamfer 27 is provided 
at the interface between recess 28 and the threaded bore 14 to provide 
room for the upper portion 11 to be cold flowed and deformed outward 
during the swaging operation. Alternatively if no internal serrations 20 
are used on locking plate 22 then the external serrations 18 would bite 
into and secure themselves to the locking plate by a cold flow process. 
The shape of the locking plate 22 is illustrated as a racetrack having two 
semi-circular ends 30 and straight legs 31 therebetween. This type of 
shape is very economical to produce. An elliptical shape may also be used 
having its major axis generally aligned parallel to the direction of 
predetermined tension forces T illustrated in FIG. 4. Such a shape 
minimizes local maximum stresses which occur at high stress area S. Which 
ever shape is used the major axis of the planform shape is preferably 
aligned parallel to the direction of predetermined tension forces T as 
illustrated in FIG. 4. A major axis defined, for the purpose of this 
patent, as a line between the distal ends of the planform shape such as 
the major axis of an ellipse or the centerline between the distal ends of 
a semi-circular ends of a racetrack shape. FIGS. 6a, 6b, and 6c illustrate 
other alternative planiform shapes such as triangular, square, and 
rectangular respectively. In accordance with the present invention the 
locking plate 22 has outer edges E that are smooth and continuous and have 
no sharp discontinuities such as serrations or key slots. The edges E also 
have smoothly curved corners C to avoid stress concentrations. 
While the preferred embodiment of the present invention has been described 
fully in order to explain its principles, it is understood that various 
modifications or alterations may be made to the preferred embodiment 
without departing from the scope of the invention as set forth in the 
appended claims.