Threaded element for use as an insert

A threaded element for forming a threaded profile in the wall of a bore formed in a material and for insertion into the bore so as to reinforce the bore. The threaded element includes a cylindrical stem having an external thread having a plurality of separate tapping lobes for cutting the threaded profile in the wall of the bore. The stem has an unthreaded radius smaller than the unthreaded radius of the bore and a threaded radius greater than the unthreaded radius of the bore. A recess is defined between each adjacent pair of tapping lobes. Each tapping lobe is defined by two cutting arcs formed at its opposite ends and a convex escape-gap arc is located intermediate the two cutting arcs. Each cutting arc has a convex configuration relative to the center of the stem, and the apex of each of the cutting arcs defines the maximum thread radius of the threaded profile, as measured from the center of the stem. Each escape-gap arc is convex and has an escape radius that is smaller than the aforementioned maximum thread radius but greater than the minimum thread radius of the threaded profile, as measured from the center of the stem.

BACKGROUND AND SUMMARY OF THE INVENTION 
1. Technical Field 
This invention relates to a threaded element for forming a threaded profile 
in the wall of a bore formed in a material, such as plastic, or a part, 
such as a zinc or aluminum alloy diecasting part, and for insertion into 
the bore so as to reinforce the bore. Threaded elements of this nature are 
used in products such as bicycles and electrical home appliances, and in 
various other industrial fields. 
2. Prior Art 
Frequently a bolt which is inserted into a threaded bore in a plastic 
material, or a zinc or aluminum alloy diecasting part is inadequately 
tight in light of the deficiency in strength of the aforementioned type of 
material or part when compared with steel. This fact makes the threaded 
profile of the threaded bore more likely to become ineffective or damaged. 
In such cases, it is, therefore, advantageous to install a threaded 
element within the threaded bore. A typical one of such kind is disclosed 
in U.S. Pat. No. 3,200,691 to Neuschotz, which has the same shape at the 
ends, and thus is advantageously installable in bidirectional way. When 
installed in a bore of a somewhat greater inner diameter than 
specifications dictate, however, the force with which the threaded element 
is retained against removal from the bore, hereinafter referred to as a 
pull-out strength, falls remarkably. This constitutes a significant 
problem with the threaded element of U.S. Pat. No. 3,200,691. 
OBJECTS AND FEATURES OF THE INVENTION 
It is, therefore, the principal object of the invention to provide a 
threaded insert element which not only has the same cross-sectional 
configuration at its ends, thereby advantageously having not only 
bidirectional installability as disclosed in U.S. Pat. No. 3,200,691 but 
also being free from the aforesaid defect that the pull-out strength 
becomes sharply lower with only a little improperly larger inner diameter 
of the bore. This problem has been solved by virtue of the feature of the 
present invention that the cross-sectional configuration of the threaded 
element has basically a symmetrical closed line consisting of alternately 
convex and concave portions with the same interval and every other concave 
portion thereof is replaced by a circular arc about the axis of said 
threaded element, the radius thereof being a little shorter than the 
distance to the peak of said convex curvature. That is to say, each of the 
threads of the threaded element is characterized by three separate tapping 
lobes oriented along a radial axis of the threaded element. Each such 
tapping lobe is defined by two convex cutting arcs formed on opposite ends 
of each of the lobes and a convex escape-gap arc situated intermediate the 
cutting arcs. The apex of each of the cutting arcs defines to the maximum 
radius of the thread, while the radius of the escape-gap arc is a little 
smaller than the aforementioned maximum radius. The escape-gap arcs 
replace selected concave arcs located intermediate selected convex cutting 
arcs of the threaded element of U.S. Pat. No. 3,200,691. 
The threaded element according to the invention can be installed by 
screwing it into a unthreaded bore previously formed in a material or part 
with the result that the threaded element forms a threaded profile in the 
wall of the bore and is thereafter retained in the bore. The threaded 
element of the present invention tends to have a greater pull-out strength 
than previous threaded elements in that the particular cross-sectional 
configuration of its threads increases the engagement area or encroachment 
area between the threaded element and the wall of the bore as compared to 
other threaded elements. That is, it increases the cross-sectional area of 
the threaded element that protrudes into the wall of the bore and, 
consequently, into the threaded profile formed by the threaded element.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates an elevational section view of an embodiment of threaded 
element 20 according to the invention when installed in an initially 
unthreaded bore 21 of a connectable material or part 22. 
The threaded element 20 according to the invention is held on an 
installation tool by engagement of an internal thread 29 of the element 20 
with an external thread of the tool, as shown in FIG. 2. The element 20 is 
then screwed into the bore 21 previously formed in the material or part 22 
such that the element 20 taps the wall 31 of the bore 21 through the 
cutting action of an external threads 25 of the threaded element 20. 
As understood from FIG. 2, the threaded element 20 has an internal thread 
29 of standard dimensions through the center, inner chamfers 34, 34' and 
outer chamfers 30, 30' at the upper and lower ends, respectively, all 
having the same shape. 
FIG. 4 shows the cross-sectional configuration of the threaded element 20 
along its threads 25 according to the present invention. It has three 
diameters 1-7, 3-9 and 5-11 of 2R which extend between opposite peaks or 
apexes 27 of convex cutting arcs and intersect at the transverse axis of 
the threaded element 20 (where R is maximum radius relative to the radial 
axis of the threaded element 20) and three minimum diameters 2-8, 4-10, 
and 6-12 which similarly intersect the aforementioned transverse axis but 
extend between the peak of a convex escape-gap arc and an opposing concave 
surface that corresponds to the curvature of the bottom 26 of the external 
thread 25. As shown in FIG. 4, every other concave curvature as 
represented by a broken line in the prior art is replaced by a convex 
escape-gap arc of radius R.sub.1 which is a little (.DELTA.r) shorter than 
the maximum radius R. 
According to the invention, each external thread 25 of the threaded element 
20 has the aforementioned cross-sectional configuration along itself such 
that each thread 25 forms three tapping lobes as shown in FIG. 6. The 
threaded element 20 of the present invention can be installed with little 
interference with the surface of the bore 21, with substantially the same 
torque as that of threaded elements of prior art, and with an increased 
sectional encroachment area on the wall 31 of the bore 21. 
The invention will now be described more fully be reference to FIG. 5, 
which illustrates, on an enlarged scale, a cross-sectional view of about 
one-half of a cross section of the threaded element 20. 
The following is a list of symbols referred to in FIG. 5 and their 
corresponding definitions. 
R=OC: Maximum radius from the transverse axis "0" of the threaded element 
to a peak "C" 
R.sub.1 =OD: A little shorter radius than R 
R.sub.3 =OA: Unthreaded radius of the bore 21 
R.sub.3 '=OA': A little longer unthreaded radius than R.sub.3 ' 
R.sub.4 =PA: Radius of the convex curvature 
.alpha.: .angle.AOC, .alpha.': .angle.BOP, .alpha.": .angle.A'OC 
.beta.: .angle.OAP, .beta.': .angle.OBP, .beta.": .angle.OA'p 
A, E, F: Intersections of the circumferences of R.sub.3 and R.sub.4 
A', E', F': Intersections of the circumferences of R.sub.4 and R.sub.3 ' 
C, H: Intersections of the circumferences of R and R.sub.4 
B, D, G: Intersections of the circumferences of R.sub.1 and R.sub.4 
Now, the sectional area that the cross section of the prior art threaded 
element shown in FIG. 3 encroaches on the wall 31 of the bore 21 is equal 
to six times the sectional area ABCDE. Letting area ABCDE be equal to 
A.sub.1, then: 
EQU S=1/2(R.sub.3 +R.sub.4 +R-R.sub.4)=1/2(R.sub.3 +R) 
EQU S'=1/2(R.sub.1 +R.sub.4 +R-R.sub.4)=1/2(R.sub.1 +R) 
EQU S'=1/2(R.sub.1 +R.sub.4 +R-R.sub.4)=1/2(R.sub.3 +R) 
##STR1## 
where 
S.sub.1 : Area of .DELTA.AOP, 
S.sub.2 : Area of .DELTA.BOP, 
S.sub.3 : Area of .DELTA.A'OP 
It therefore follows that 
##EQU1## 
As described above, in the prior art, it is 6 times of A.sub.1 in section 
that encroaches on the wall of the bore. 
Likewise, the sectional area of the prior art threaded element of FIG. 3 
that encroaches upon the wall 31 of the bore 21 where the bore is of a 
little greater radius R.sub.3 ' is equal to six times the sectional area 
A'BCDE'. That is, let sectional area A'BCDE' be A.sub.2, then: 
##EQU2## 
A.sub.1 and A.sub.2 are areas defined by the circular arcs and, therefore, 
a small enlargement of the inner diameter of the bore 21 results in sharp 
reduction of the cross-sectional encroachment area, and thus in a 
markedly-lowered pull-out strength. Defining B.sub.1 and B.sub.2 to be 
areas CE'F'H and CDGH of which the radius is a little (.DELTA.r) shorter 
than the maximum radius R, respectively, 
##EQU3## 
Letting C be area DE'F'G, C=B.sub.1 -B.sub.2, it follows that the increased 
area is C.times.3. Consequently, the encroachment area is increased by 
C.times.3 in that there are three convex escape-gap arcs according to the 
present invention. It will, therefore, be appreciated that the 
cross-sectional encroachment area of the prior art threaded element on the 
wall 31 of bore 21 is 6A.sub.1 and that of the threaded element 20 
according to the invention is 6A.sub.2 +3C when the bore 21 has a somewhat 
increased radius, R.sub.3 '. 
Comparative list of individual parameters of a threaded element M6 is as 
follows: 
##EQU4## 
From the foregoing, it will be apparent that the area A.sub.1 .times.6 
which is equal to 2.274 is only equal to 58.9% of the area A.sub.1 
.times.6 which is equal to 3.858. Consequently, an increase of 0.1 mm 
(i.e. R.sub.3 '-R.sub.3) in the radius of the bore 21 is accompanied by 
marked reduction of the cross-sectional area, and as the result, the 
pull-out strength drops greatly. It therefore is indispensable for the 
solution of this problem to prevent the encroachment area from getting 
sharply low compared with A.sub.1 .times.6. In view of this, the 
cross-sectional configuration of the threaded element 20 according to the 
invention has an encroachment area of A.sub.2 .times.6+C.times.3=5.241 
mm.sup.2, which is 1.36 times "A.sub.1 .times.6'. Thus, the threaded 
element 20 of the present invention has an adequately increased pull-out 
strength. 
The function of the threaded element 20 according to the invention will now 
be described. It is screwed downwardly into a bore 21 of a material or 
part such that its external threads 25 form a threaded profile in the wall 
31 of the bore 21, as shown in FIG. 1. FIG. 6 shows that the external 
threads 25 each have three separate tapping lobes each of which consists 
of two convex cutting arcs formed on opposite ends of the lobes and a 
convex escape-gap arc intermediate the two convex cutting arcs. As shown 
in FIG. 7, the external thread peaks 27, 28 of the external threads 25, 
which correspond to the apex of the convex cutting arcs, cut an internal 
thread 32,33 on the wall 31 of the bore 21. As shown in FIGS. 2 and 7-9, a 
reference numeral 26 designates the bottom of the external thread 25 of 
the threaded element 20. FIG. 8 illustrates the threaded profile 32, 33 
formed on the wall 31 of the bore 21 through engagement of the external 
threads 25 of the threaded element 20 with the wall 31 is affected with 
contraction in the course of the subsequent tapping. That is, the threaded 
profile tends to contract during intervals at which only the bottom 26 of 
the external thread 25 is in alignment with the threaded profile cut by 
the external thread 25. FIG. 9, which is a cross-sectional view taken line 
9--9 of FIG. 6, depicts an escape gap formed by the bottom 32 of the 
internal thread 32,33 and the apex of the external thread peak 27,28 of 
the external thread 25 of the threaded element 20. The escape gap permits 
the threaded element 20 to be screwed into the bore 21 with less 
resistance. As shown by the force vectors in FIG. 9, the escape gap also 
permits the threaded profile to contract somewhat during the course of the 
tapping of the wall 31 of the bore 21 by external threads 25 of the 
threaded element 20. This tends to prevent the threaded element 20 from 
loosening its threaded engagement with the wall 31 of the bore 21 due to 
vibration and otherwise. Besides, the cross-sectional configuration of the 
external thread 25 of the threaded element 20, formed as shown in FIG. 6, 
permits an axial load on the external thread 25 to be imposed on the ridge 
of the external thread 25 of the threaded element 20 to be and not on the 
groove. This results in elastic deformation caused by the axial load where 
the internal thread 32, 33 of the wall 31 of the bore 21 mates with the 
external thread 25 of the threaded element 20 and on the other hand 
remaining unaffected by the axial load where it mates with the recess of 
the external thread 25. Thus, a small dislocation is produced at the 
boundary between the peak and the recess which tends to prevent loosening 
of the threaded element 20 due to vibration. 
The cross-sectional configuration according to the present invention can be 
easily obtained by drawing a bar stock through a die, and mass-production 
of the threaded element 20 according to the invention can be made with 
ease by an automatic machine. 
Advantages of the invention 
The threaded element 20 for use as an insert according to the invention can 
be employed for forming a reinforced threaded bore of a connectable 
material or part, substantially without lowering of the pull-out strength, 
so far as variation in diameter of the bore is within about 0.2 mm. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is an elevational view of a threaded element according to the 
invention illustrated as installed in a bore of a connectable material or 
part; 
FIG. 2 is a front elevational view of the same threaded element, of which 
the left half is shown in cross-section; 
FIG. 3 is a cross-sectional view of a prior art threaded elements; 
FIG. 4 is a cross-sectional view of the threaded element according to the 
invention in an enlarged scale; FIG. 5 is an illustration for comparison 
in cross-sectional configuration between the threaded elements of the 
prior art and the present invention; 
FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 1; 
FIG. 7 is a cross-sectional, fragmentary view taken along line 7--7 of FIG. 
6; 
FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 6; and 
FIG. 9 is a cross-sectional view taken along line 9--9 of FIG. 6. 
Although the invention has been described in detail with reference to the 
presently preferred embodiment, those of ordinary skill in the art will 
appreciate that various modifications can be made without departing from 
the invention. Accordingly, the invention is defined only by the following 
claims: