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Description  |
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BACKGROUND OF INVENTION
This invention relates generally to self locking fasteners and, more
particularly, to a fastener having a novel and improved locking load
surface which embeds into a mating part and provides means to lock against
accidental loosening of the fastener.
PRIOR ART
Self locking fasteners having saw tooth type teeth which embed into a
mating surface to prevent accidental loosening are well known. Examples of
such fasteners are described in the U.S. Pat. Nos. 1,009,801, 1,298,863,
2,959,204, 3,078,899, 3,255,797, 3,373,631, 3,389,734, and 3,605,845.
The Rigot Pat. No. 2,959,204, assigned to the assignee of the present
invention, provides a radial bearing surface and peripheral teeth which
extend pass the plane of the bearing surface. In such devices, the bearing
surface tends to limit the penetration of the teeth to a maximum depth
substantially equal to the distance the teeth project beyond such planes.
The Gutshall Pat. No. 3,389,734 describes a self locking screw in which an
annulus of inclined teeth extend radially inward from a relatively narrow
axially extending flange. When such fastener is tightened against the
mating part, the teeth and flange embed into the mating surface. The
Gutschall device does not provide any load bearing surface substantially
normal to the axis of the screw.
The Junker Pat. No. 3,605,845 discloses several embodiments of self-locking
fasteners. In such embodiment, teeth which are inclined or radial extend
from the shank or threads of the fastener to an outer load bearing surface
having substantial radial width. In the illustrated embodiments, the
bearing surfaces are inclined back from the bearing plane. Such structure
provides teeth of excessive length, the inner extremities of which tend to
have insufficient radius to provide significant locking functions.
Further, the relatively large area of the bearing surface tends to limit
the penetration of the teeth required for good locking.
SUMMARY OF INVENTION
In accordance with the present invention, a novel and improved locking and
load bearing structure is provided in which the teeth have edges extending
radially along a bearing plane perpendicular to the axis of the fastener.
The teeth are bounded at the outer extremity by relatively narrow axially
extending skirt having an axial extremity in such bearing plane. A bearing
surface extends inwardly from the teeth and also provides an axial
extremity in the bearing plane.
When a fastener incorporating this invention is tightened against a mating
part, the extremity of the outer flange, the edges of the teeth and the
bearing surface initially engage the mating surface. As the fastener is
tightened, the surfaces progressively penetrate into the mating surface
causing material of the mating surface to be displaced into the recesses
between the teeth. As tightening progresses, the area of engagement of the
bearing surface rapidly increases to prevent excessive penetration of the
locking system into the mating surface. With this structure, excessive
forces are not required to obtain proper penetration since the effected
area of the bearing surface is relatively small initially, and increases
as the penetration progresses. The outer flange functions to provide a
peripheral seal around the locking projections to prevent moisture or
other contaminants from penetrating under the head into the zone at the
locking projections where any surface finish on the mating part is
distroyed. Further, because the outer flange is circular, it does not
cause any chipping of the protective coating which might exist beyond the
engaged area. Still further, the flange provides support for the outer
ends of the saw tooth projections. Similarly, the inner bearing surface
provides a seal with the mating part within the zone engaged by the
locking projections and provides support for such locking projections.
In the preferred embodiment, the inner wall and outer ends of the recesses
are inclined to eliminate sharp corners at the ends of the recess forming
projections on the tooling. Consequently, good tool life is obtained with
the tools required to form the screws in accordance with this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation of a flange screw in accordance with this
invention.
FIG. 2 is an end view of the screw illustrated in FIG. 1 illustrating the
locking system provided on the underside head of the screw.
FIG. 2b is an enlarged fragmentary view similar to FIG. 2 illustrating the
details of the structure of the locking system at greater scale.
FIG. 3 is a enlarged centerline section taken generally along 3--3 of FIG.
2; and,
FIG. 4 is an enlarged fragmentary perspective view illustrating a portion
of the locking system.
DETAILED DESCRIPTION OF THE DRAWINGS
The present invention is illustrated as applied to a flange screw. However,
in its broader aspects, this invention can be also applied to other types
of screws, bolts and nuts. Therefore, when the term "threaded fastener" is
used herein, it is intended that it encompass the various types of
threaded fasteners to which this invention can be applied and is not
intended to be limited to flange screws per se.
Referring to the drawings, FIG. 1 illustrates a flanged screw incorporating
the present invention. Such screw includes a threaded shank 10 and a head
11 providing a flange 12 and a hexagonal wrenching portion 13. The load
faces 14 on the underside of the flange 12 is adapted to engage a mating
part when the screw is threaded into place and is provided with a locking
system to prevent accidental loosening of the screw.
Referring to FIG. 2, the locking system includes an annular bearing surface
16 which extends from the shank 10 to an annulus of locking surfaces 17
consisting of a plurality of circumferentially arranged radially extending
saw tooth shaped projections 18. Positioned around the locking surfaces 17
is an axially extending skirt 19.
Referring now to FIGS. 2b through 4, the bearing surface 16 extends
radially out from the shank 10 from an inner extremity at 21 to an outer
extremity at 22 where it joins with the inner ends of the saw tooth shaped
projections 18. The outer extremity 22 of the bearing surface 16
constitutes an annular edge extending around the threaded shank 10
concentric with the central axis 23 of the fastener and located in a
bearing plane 24 which is perpendicular to the axis 23. The bearing
surface is preferably inclined back from the bearing plane 24 as it
extends inwardly from the outer extremity for reasons more fully described
below.
The projections 18 each include a ramp surface 26 which extends from an
edge 27 to an intersection 28 with a locking surface 29 of an adjacent
tooth projection. The locking surfaces 29 extend from the edge 27 to the
associated intersection 28 and extend along radial planes containing the
axis 23. The ramp surfaces 26 on the other hand, extend in a peripheral
inclined manner back from the edge line 27 contained in the plane 24 so
that the ramp surface of each projection 18 in cooperation with the
locking surface 29 of the next adjacent projection cooperate to define a
recess 31 extending radially of the fastener from an inner wall 32 to an
outer wall 33. Preferably, the inner wall 32 is a portion of a cone and is
included radially outward as it extends back from the plane 24 by an angle
D. Similarly, the walls 33 are also portions of a cone which extend
radially inward as they extend back from a plane 24 and form with the
plane 24 an angle C.
The skirt 19 is formed partially by the inclined walls 33 and an outwardly
extending opposed inclined wall 34 which intersect at 36 in the plane 24.
The line of intersection 36 preferably extends along the plane 24 as does
the circular line established by the outer extremity 22 bearing surface
and the edges 27 of the projections.
When a fastener is installed against a mating part schematically
represented at 41 in FIG. 3 having a surface 42 perpendicular to the axis
23 of the fastener, the surface 42 is engaged substantially simultaneously
by two concentric lines formed by the intersection 36, the outer extremity
22 of the bearing surface and by each of the edges 27 of the projections
18. As the fastener is tightened in against the surface 42 the pressure
developed along the lines of engagement reaches values sufficiently high
to cause the locking surface to commence to penetrate into the surface 42
and cause displacement of the material of the mating part 41. Such
penetration can cause either cold flow, bending or other forms of
displacement of the material of the mating part. As the penetration
continues, the portions of the ramp surfaces 26 adjacent to the edges 27
progressively contact the material of the mating part along with the
portions of the bearing surface 16 adjacent to the outer extremity 22.
Similarly, the material of the mating part engages portions of the
surfaces 33 and the surface 34. Because the ramp surfaces are inclined
with respect to the tightening rotation, they function as camming surfaces
to allow continued tightening rotation of the fastener without excessive
buildup in resistance to such continued rotation.
Continued penetration rapidly increases the effective area of the bearing
surface and along with the additional engagement of the ramp surfaces 26
with the material of the mating part limits, the degree of penetration of
the locking system into the mating part.
In order to insure that the proper amount of penetration occurs to provide
the self locking function described below, it is important that excessive
bearing surface areas to not come into play to excessively limit the
penetration. For this reason, the skirt 19 is formed with a relatively
sharp edge and the tooth surfaces 33 and 34 extend back from the bearing
plane 24 at angles A and C which are substantially large angles. On the
other hand, the bearing surface 16 preferably extends back from the plane
24 by a relatively small angle. In the illustrated embodiment, the angle A
is preferably no less than about 30 degrees, the angle B is preferably at
least 8 degrees. The angle C is preferably a maximum of about 45 degrees
and the angle D is preferably about 75 degrees. Since the angles A and C
have relatively large values, the outer flange 19 does not provide any
substantial area of bearing surface as the penetration of the locking
surfaces continues. However, the relatively small angle B ensures that
sufficient areas provided along the bearing surface to limit the
penetration and prevent excessive penetrations during the tightening of
the fastener.
It has been found in actual practice if excessive bearing surface is
provided, insufficient penetration tends to occur. For this reason, the
location of the bearing surface 16 inward from the locking projections 18
where the radius is relatively small and correspondingly, the area of
engagement is not excessive provides improved results for controlled
penetration. Further, the use of a relatively sharp skirt 19 adjacent to
the periphery where the radius is large, insures that excessive bearing
areas do not develop to prevent adequate penetration of the locking
projections. The outer flange, however, penetrates into the surface 42 as
the fastener is tightened and provides a full seal therewith to prevent
the entry of contaminants or moisture which can cause corrosion.
Similarly, the bearing surface 16 provides an inner seal which prevents
any possibility of moisture penetration from inner side of the locking
engagement. It should be understood that the engagement of the locking
projections with the surface 42 and the penetration of the projections
into such surface destroys any protective coatings which might have been
applied to the mating surface 42 and would provide bare material which
could cause excessive corrosion if moisture could penetrate into the zone.
Also because the skirt surface 34 is conical, there is no tendency for
chipping to occur beyond the area of engagement. A similar situation
exists with respect to the bearing surface 16 because the area of
engagement with the mating part is also conical.
With the illustrated structure, the teeth are supported at both ends and
therefor have substantial strength. Also, the inclination of the two
surfaces 32 and 33 eliminates the need for sharp corners on the tooling
used to form the parts, and improves the tool life.
As the fastener is tightened causing penetration of the locking system into
the surface 42, material is displaced into the recesses between the teeth
and functions to mechanically interfer with the locking surfaces 29 to
prevent unintentional loosening of the fastener. With the present
invention, the initial torque to loosen the fastener is higher than the
tightening torque and can have a breakaway loosening torque in the order
of 11/2 times the tightening torque involved. Further, a given fastener in
accordance with the present invention can be used with mating parts of
substantially different hardnesses and can be tightened to substantially
different clamping forces while still providing proper self locking
characteristics.
Although a preferred embodiment of this invention is illustrated, it should
be understood that various modifications are rearrangements of parts may
be resorted to without departing from the scope of the invention disclosed
and claimed herein.
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