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Description  |
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2. Field of the Invention
The present invention relates to a waterproof nut, particularly a
resin-coated waterproof nut which may be used, for example, inside a water
storage tank, and more particularly to a waterproof nut which may be
employed in conditions in which it comes into contact with water or in
which a liquid that may produce condensation is present, and which may
also be used for a joint or the like in piping.
3. Description of the Related Art
One type of known water storage tank is constituted by panel units which
are tightly joined together by means of nuts and bolts which are screwed
and tightened thereon. Since the nuts and bolts employed in a
high-humidity atmosphere, as in the case of a water storage tank, must be
resistant to corrosion, it is general practice to subject such nuts and
bolts to plating or chromating, or to make them of a stainless steel
material or a synthetic resin material. These anticorrosion treatments,
however, cannot prevent corrosion from chlorine gas generated from the
water contained in the tank, and even nuts and bolts which are made of a
stainless steel material are corroded by this chlorine gas. Nuts and bolts
which are made of a synthetic resin material have insufficient mechanical
strength for obtaining the desired level of performance, and are therefore
unsatisfactory for industrial purposes.
U.S. Pat. No. 4,397,437 to Madej discloses a nut of a beam clamp wherein
the nut is encapsulated by a pressure molding process with
polyvinylchloride. The polyvinylchloride coating surrounding the nut forms
a cap having an elongated body portion which is open at one end. This
patent discloses a resilient annular flange 78 that is part of a clamping
member rather than a nut. The annular flange 78 of the clamping member
contacts the outer surface of the face of the nut body. This cannot be
construed as a washer. Moreover, the nut of this patent has no space or
groove to receive a washer. Madej patent does not teach an interference
fit or a tight fit between an elongated hole formed in a synthetic resin
cap-shaped portion and a bolt.
U.S. Pat. No. 4,316,690 to Voller teaches placement of a tapered lip on the
underside of the head of a bolt so that the tapered lip would mesh with
and seal the threads where the bolt meets a nut. This patent discloses a
tapered portion of the head of a bolt rather than a nut. That tapered
portion is designed to seal the point of engagement between the bolt and a
nut by meshing with the threads. The arrangement of this patent does not
enable a flexible packing material, such as a washer, for sealing to be
accurately and effectively positioned when a nut is tightened onto a bolt.
A plastic-headed fastener assembly is disclosed in Bettini et al. U.S. Pat.
No. 4,373,842. In this patent, an elastic washer is used to protect the
under surface of the head of a fastener which is not a nut from moisture
and corrosion.
The washer of this patent engages the underside of the head of the fastener
but does not engage a synthetic resin outer layer on the head of the
fastener. Rather, it engages a pair of flanges 38 as they project from the
head of the fastener. This patent relate to a self-drilling and
self-tapping fastener which passes through successive panels, and is
unrelated to any technology in which a nut is fastened to a bolt.
There is a known method in which, after panel units have been fastened
together by nuts and bolts, the nut and bolt assemblies are externally
covered with caps made of a resin. This method, however, involves
troublesome operations, and has the disadvantage that the caps may come
off the nut and bolt assemblies during use.
To overcome these disadvantages, the applicant of the present invention has
already proposed an improved waterproof nut (see Japanese Utility Model
Laid-Open No. 99998/1984). The structure of this waterproof nut is shown
in FIG. 10. More specifically, a cap nut body 20 which has a cap-shaped
portion 21 is employed. The surface of the cap-shaped portion 21 is
machined so that projections and recesses are formed thereon, and is then
coated with a synthetic resin layer 22. This proposed prior art has
already been widely used as a nut for, in particular, water storage tanks.
This type of nut, however, necessitates a complicated manufacturing
process, which leads to an increase in the production cost. More
specifically, the formation of the cap nut body 20 requires much more time
and labor than ordinary nut bodies, so that the production cost of the
former can be at least ten times that of the latter.
The necessity of providing the cap-shaped portion 21 on the nut body 20
will be explained below.
The nut of the above-described type is generally produced by setting the
cap nut body 20 in a mold and charging a synthetic resin into the mold. In
this case, if an ordinary nut body with no cap-shaped portion 21 is
employed, a core which has the same configuration as that of the bolt
which is to be screwed into the completed nut is screwed into the nut
body, and the nut body is set in the mold in this state. The synthetic
resin is then charged, and, after the resin has hardened, the core is
unscrewed from the nut body which is still in the mold.
In this process, it is impossible to prevent the charged synthetic resin
(liquid) from entering the small space defined between the nut body and
the threaded portion of the core, but any resin in that small space makes
it extremely difficult to unscrew the core from the nut body. In order to
solve this problem, the cap-shaped portion 21 is formed on the nut body
20, and the synthetic resin is thereby prevented from entering the small
space.
In addition, since the inner diameter of the synthetic resin portion is
inevitably reduced by mold shrinkage, the thread engagement area between
the nut and a bolt may become so tight that the nut cannot be smoothly
screwed onto the bolt, which means that the working efficiency of the
fastening operation is greatly reduced, and it is not possible to obtain
the required clamping force.
SUMMARY OF THE INVENTION
In view of the above-described circumstances, it is an object of the
present invention to provide a waterproof nut coated with a synthetic
resin, by employing an ordinary nut body.
The present invention provides a waterproof nut comprising a nut body and a
synthetic resin layer which covers the nut body, the resin layer covering
both the nut body and the front surface thereof as well as defining a
cap-shaped portion over the rear surface thereof, the cap-shaped portion
accommodating the free end of a bolt which is screwed into the nut.
According to the present invention, a core in the form of a bolt is screwed
into a nut body from the front surface thereof in such a manner that the
free end of the core projects from the rear surface of the nut body.
In this case, the nut body has an inner diameter which is 0.05 to 0.4 mm
larger than the nominal diameter thereof, and the threaded portion of the
core has an outer diameter which is 0.02 to 0.1 mm larger than the nominal
diameter of the nut body. The engaged nut body and the core are set in a
mold which has a section for forming a synthetic resin outer layer which
covers the outer periphery of the nut body, another section for forming a
front surface covering layer which covers the front surface of the nut
body and still another section for forming a cap-shaped portion which
covers the rear surface of the nut body. Synthetic resin is then charged
into the mold in a state such that the temperature of the nut body and the
core is lower than the ordinary molding temperature, and the resin is
hardened. Thereafter, the core is unscrewed from the nut body, and a
cap-shaped portion is formed over the rear surface of the nut body.
One of the features of the waterproof nut, according to the present
invention, resides in the arrangement wherein the nut body is covered with
a synthetic resin layer evenly over the front surface thereof. The
synthetic resin layer portion covering the front surface prevents the nut
body from being separated or removed from the synthetic resin layer when
the nut is tightened. This resin layer portion may have a characteristic
configuration, such as described later. Another feature of the waterproof
nut resides in the cap-shaped portion of a synthetic resin which covers
the rear surface of the nut body, the cap-shaped portion being of a size
which enables it to accommodate the free end of a bolt which is screwed
into the nut. The outer periphery of the cap-shaped portion may be made
cylindrical or it may be polygonal, e.g., hexagonal, so that this portion
can be held when the nut is tightened.
It is generally considered that the temperature of the mold and the nut
body, etc., should be set at a relatively high level during the process of
charging the synthetic resin into the mold. With the present invention,
therefore, the temperature of the mold itself and the nut body, etc., can
generally be about 70.degree. to 100.degree. C. if using a resin such as a
polyamide resin (nylon 6,6), although this temperature depends upon the
kind of synthetic resin employed. However, according to the present
invention, the temperature of the nut body and the core is restricted to
within a temperature range lower than that mentioned above, and the
molding process is carried out at about 5.degree. to 45.degree. C.
In addition, it is generally recommended that, when molding synthetic resin
by injection molding or other similar molding process, the mold and the
core should be at a temperature which is as high as possible, from the
overall viewpoint of the quality and external appearance of the product.
This is because it is necessary to smoothen the flow and hardening rate of
the resin charged into the mold. However, the method according to the
present invention makes good use of this practice. More specifically,
since it is not preferable that the charged resin flows through the gap
between the nut body and the core, the mold and the core are held at a
relatively low temperature so that the resin is prevented from entering
the gap.
Experiments have shown that when a nylon resin was employed and the
temperature of the nut body was set at 80.degree. C., a relatively large
amount of resin entered the gap between the nut body and the core, and it
was therefore difficult to unscrew the core from the nut body. According
to the present invention, however, the core and the nut body are held
within the aforementioned relatively low temperature range, so that only
an extremely small amount of resin is able to enter the gap, thus
facilitating the unscrewing of the core. In addition, the outer diameter
of the core and the inner diameter of the nut body are specified as
appropriate, to allow for shrinkage of the resin.
It is preferable that the external thread ridge formed on the bolt which is
screwed into the waterproof nut and the internal thread ridge formed on
the cap-shaped portion of the nut should be in contact with each other to
a certain extent, to increase the mechanical strength of the nut and bolt
assembly with tight or interference fit. The dimension by which the thread
ridges are in contact should be set to 0.4 to 1.4 mm preferably 0.7 to 1.2
mm, in order to obtain an increased mechanical strength and excellent
thread engagement.
From the above-described point of view, the dimensions of the core should
be selected so that the dimension of the internal thread ridge of the
cap-shaped portion is within the aforementioned range. The outer periphery
of the cap-shaped portion may be cylindrical, but it is preferably
polygonal, e.g., hexagonal, so that this portion can be held when the nut
is tightened.
The kind of synthetic resin employed by the present invention is not
necessarily limited; it is possible to select any kind of resin which
satisfies the strength requirements (for example, when the nut is
tightened). Examples include various kinds of synthetic resin, such as
polyamide, ester, ether, sulfone and sulfide resins, and more
particularly, nylon 6,6, nylon 6, polyethylene terephthalate, polybutylene
terephthalate, polycarbonate, polyphenylene oxide, polyoxymethylene,
polyether sulfone, polysulfone, polyphenylene sulfide, etc. However, the
present invention is, of course, not necessarily limited to the
above-mentioned resins.
The selected synthetic resin is melted to form a liquid and is charged into
the mold. During this time, the mold and the core are held at an
appropriate temperature which depends upon the resin employed. That
temperature is generally 70.degree. to 100.degree. C. for a polyamide
resin, 60.degree. to 100.degree. C. for an ester resin, 70.degree. to
100.degree. C. for an ether resin, 140.degree. to 180.degree. C. for a
sulfone resin, or 110.degree. to 150.degree. C. for a sulfide resin. In
the present invention, however, it is necessary to charge the resin in a
state wherein the core and the nut body are held at a temperature which is
considerably lower than the corresponding temperature range mentioned
above. It is preferable that the temperature should be 50.degree. C. or
less (preferably 25.degree. to 45.degree. C.) for a polyamide resin,
60.degree. C. or less (preferably 25.degree. to 45.degree. C.) for an
ester resin, 55.degree. C. or less (preferably 25.degree. to 45.degree.
C.) for an ether resin, 110.degree. C. or less (preferably 25.degree. to
100.degree. C.) for a sulfone resin, or 110.degree. C. or less (preferably
5.degree. to 100.degree. C.) for a sulfide resin.
Of the above-mentioned resins, a polyamide resin (a nylon resin) is most
preferable from the overall viewpoint of availability, cost and strength.
In addition, it is possible to mix a filler or the like into the resin
employed, according to need. For example, glass fibers, carbon fibers,
milled fibers, or aramid fibers, may be employed.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will become more apparent from the following description of the
preferred embodiments thereof, taken in conjunction with the accompanying
drawings, in which like reference numerals denote like elements, and in
which:
FIG. 1 is a partially-cutaway side view of one embodiment of the waterproof
nut according to the present invention;
FIG. 2 is a side view of the waterproof nut shown in FIG. 1, as viewed from
the right-hand side thereof;
FIG. 3 is a side view of the waterproof nut shown in FIG. 1, as viewed from
the left-hand side thereof;
FIG. 4 is a partially-cutaway side view of another embodiment of the
waterproof nut according to the present invention;
FIG. 5 shows a modification of the waterproof nut according to the present
invention which has a ring-shaped groove in the front surface thereof;
FIG. 6 is a side view of the waterproof nut shown in FIG. 5, as viewed from
the right-hand side thereof;
FIGS. 7(a) and 7(b) are sectional views showing washers which may be
engaged with the waterproof nuts shown in FIGS. 5 and 1, respectively;
FIG. 8 is a sectional view of a mold, a core and a nut body which are set
in the mold, showing an example of the method of manufacturing a
waterproof nut according to the present invention;
FIG. 9 is an enlarged sectional view of a essential portion of the engaged
core and the nut body shown in FIG. 8;
FIG. 10 is a partially-cutaway side view of a conventional waterproof nut;
FIG. 11 is a graph showing the contact among and torque strength of the
waterproof nut;
FIG. 12 is a graph showing the initial clamping torque and return torque in
a result of a comparison test between the present invention and a
Hard-lock nut;
FIG. 13 is an enlarged sectional view of the waterproof nut of the present
invention and a bolt applied thereto;
FIG. 14 is an enlarged sectional partial view showing relation of engaging
a metal nut and a cap-shaped portion of the water-proof nut of the present
invention with the bolt; and
FIG. 15 is an illustration of each structure of samples of conventional
nuts and bolts, and the waterproof nut of the present invention referred
in Table 3 of the specification in which torque strength thereof is
compared with each other.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIGS. 1 to 3 which show in combination a waterproof nut
A in accordance with one embodiment of the present invention, a nut body 1
is coated with a nylon resin layer 2. The nut body 1 has a hexagonal outer
periphery, and an internal thread 3 is cut in the inner peripheral surface
of the nut body 1. The nylon resin layer 2 is provided so as to form a
front surface covering layer or lip 4 which covers the front surface of
the nut body 1 and a cap-shaped portion 5 which covers the rear surface of
the nut body 1.
The cap-shaped portion 5 is of a size which enables it to accommodate the
free end of a bolt when screwed thereinto. The inner peripheral surface of
the cap-shaped portion 5 may be provided with an internal thread which is
contiguous with the internal thread 3 cut in the inner peripheral surface
of the nut body 1. The outer periphery of the cap-shaped portion 5 may be
shaped so that it can be held when the nut is tightened. In the
illustrated example, a multiplicity of small projections 6 having a
triangular cross-section are formed on the outer periphery of the
cap-shaped portion 5.
The inner end of the lip 4 preferably defines a step 7 as shown in FIG. 1,
or the thickness of the lpi 4 may be gradually reduced toward the internal
thread 3 so as to provide a tapered portion 8 as shown in FIG. 4, in order
to accurately and effectively position a packing material (not shown)
which may be used when the nut is screwed onto a bolt. If neither the step
7 nor the tapered portion 8 is provided, when the nut is tightened, the
packing material may be pressed outwardly to damage the external
appearance considerably.
As shown in the drawings, each outer diameter of the nut body 1 and small
projection portion 6, that is, the cap-shaped portion 5 is equivalent each
other, accordingly, any fastening tool such as a spanner, socket wrench or
the like which is applied to the nut body 1 can be used in common to put
on the cap-shaped portion 5 to fasten the waterproof nut of the present
invention.
FIGS. 5 and 6 show in combination a nut A which has a ring-shaped groove 12
formed in the front surface of the lip 4. A washer B which is shown in
FIG. 7(a) is engaged with the groove 12. The washer B has a ring-shaped
projection 14 which is fitted into the groove 12, so that the nut A and
the washer B are connected together in one unit and are prevented from
separating from each other. The outer diameter of the projection 14 is
preferably made slightly larger than the largest diameter of the groove
12.
The washer B shown in FIG. 7(b) is provided with a projection 16 which is
fitted to the step 7 of the nut A shown in FIG. 1. This washer B is
snap-fitted into the nut A in the same manner as in the case of the washer
B shown in FIG. 7(a).
In the illustrated example, the nut body 1 is an M12 hexagon nut, in which:
the diameter R of the largest-diameter portion, including the nylon resin
layer 2, is 30 mm; the inner diameter r of the lip 4 is 14 mm; the length
L of the portion whose diameter is R is 7 mm; and the thickness W of the
lip 4 is 1 to 3 mm. The substantial diameter of the cap-shaped portion 5
which covers the rear surface of the nut body 1 is 18.9 mm, and twelve
projections 6 having a triangular cross-section are continuously formed on
the outer periphery of the cap-shaped portion 5, the height of the
projections 6 being 1 mm.
The depth d of the cap-shaped portion 5 is 15 mm, and the inner peripheral
surface of the cap-shaped portion 5 is provided with an internal thread
which is contiguous with the internal thread formed on the inner
peripheral surface of the nut body 1. The overall length L.sub.o of the
waterproof nut A is 27 to 29 mm. These dimensions, as a matter of course,
differ depending upon conditions in which the nut A is used, and
performance requirements, and the kind and thickness of resin employed and
the type of nut body may be selected as desired in accordance with the use
conditions and performance requirements.
FIGS. 8 and 9 are sectional views which show in combination the positional
relationship between a mold 32, a nut body 30 and a core 31 in
manufacturing the waterproof nut according to the present invention. A
synthetic resin, e.g., nylon 6,6, is heated at about 270.degree. C. to
280.degree. C. and thereby melted in advance and is charged into the mold
32 through a small bore 35 provided therein. The outer diameter of the
core 31 must be strictly determined, since it strongly influences the
clamping property of the waterproof nut completed. Since the synthetic
resin shrinks after it has been molded, the outer diameter of the core 31
is set so as to be 0.02 to 0.1 mm larger than the nominal diameter of the
nut body 30, preferably 0.04 to 0.06 mm larger than the latter in order
that the amount of contact between the internal thread ridge of the
cap-shaped portion formed of the synthetic resin and the external thread
ridge of the bold which is screwed into the nut is within a desired range.
The nut body 30 is mainly made of a metal. Since it is screwed onto the
core 31 in the form of a bolt, the nut body 30 is over-tapped so that the
inner diameter thereof is slightly larger than the nominal diameter by
about 0.05 to 0.4 mm in general, preferably about 0.1 to 0.3 mm. However,
if the nut body 30 is excessively over-tapped, it undesirably becomes easy
for the synthetic resin to enter the gap between the engaged core 31 and
the nut body 30 when the resin is charged.
As to the kind of synthetic resin which is to be charged, it is preferable
to employ a nylon resin among the aforementioned resins from the viewpoint
of availability and strength. In addition, a filler or the like may be
added according to need, and the mixing ratio of a filler is preferably 10
to 60%, more preferably 30 to 50%, from the overall viewpoint of
reinforcing effect, external appearance and moldability.
In a practical example of the method of manufacturing the waterproof nut
according to the present invention, a core 31 having a length of 25 mm and
an outer diameter of 12.05 mm (the nominal diameter+0.05 mm) was employed,
and the inner peripheral surface of the nut body 30 was over-tapped so
that the inner diameter thereof was 0.2 mm larger than the nominal
diameter, the nut body 30 having been subjected to deposition so that the
deposit thickness is 20 .mu.m.
The core 31 was inserted into a cavity 34 in the mold 32 which had the same
configuration as the external shape of the waterproof nut, and the core 31
was fixedly set. Then, nylon 6,6 (containing 40% of short glass fibers)
was injected into the cavity 34 by an injection molding machine (not
shown).
According to experiments, when the temperature of the mold 32, the core 31,
etc., was 80.degree. C. or more, the resin undesirably entered and
completely filled up the small space 33 between the nut body 30 and the
core 31, so that it was impossible to unscrew the core 31 from the nut
body 30. However, it was possible to prevent the resin from entering the
small space 33 by charging the resin in a state wherein the temperature of
the mold 32 and the core 31 was controlled so as to be 50.degree. C. or
less. When the mold temperature was 45.degree. C. or less, it was
particularly easy to unscrew the core 31 from the nut body 30.
The strength of the waterproof nut thus obtained was measured by variously
changing the amount of contact between the internal thread ridge of the
cap-shaped portion and the external thread ridge of the bolt screwed into
the nut. More specifically, waterproof nuts which were different from each
other in terms of the thread diameter of the cap-shaped portion were
obtained by properly selecting the diameter of the threaded portion of the
core 31, and then a marketing M12 bolt which had been subjected to
electroplating was screwed into each of the waterproof nuts. Table 1 shows
the thread ridge contact amount and the breaking torque strength of each
of the waterproof nuts. Examples 1 to 3 allowed the bolt to be smoothly
and effectively screwed thereinto, but Example 4 was slightly tight for
the bolt as compared with Examples 1 to 3. However, all of them had a
satisfactory high strength and were able to endure easily an ordinary
clamping operation with an impact wrench. Example for comparison had a
contact amount of 0 mm, which means that the bolt was in thread engagement
with the nut body alone. In consequence, the breaking torque strength is
extremely low, and this waterproof nut cannot be practically used.
TABLE 1
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Example
Example Example Example Example
for
1 2 3 4 comparison
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Contact
0.5 0.8 0.9 1.2 0
amount
(mm)
Breaking
600 900 1100 720 350
torque
strength
(kg .multidot. cm)
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Table 2 shows the results of a vibration test carried out on the waterproof
nut of the present invention. In this test, after members to be joined had
been fastened together with an axial force of 1.5 ton, sine-wave
vibrations with an amplitude of .+-.1.0 mm were applied in a direction
perpendicular to the axial direction of the nut and bolt assembly, and the
residual axial force was examined after the nut and bolt assembly had been
vibrated 1,000 times.
TABLE 2
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Examples for comparison
Example 5
SUS 304
Titanium SS 41 Resin nut and bolt
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Residual axial
0.05 0 0.35 0.93
force
Q.sub.z (ton)
Residual axial
3 0 23 62
force ratio (bolt
Q.sub.z /Q.sub.0 (%)
broken)
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*The head of the bolt employed in Example 5 is coated with nylon.
As will be clearly understood from Table 2, the nut and bolt assembly which
includes the waterproof nut according to the present invention is less
than the metallic nut and bolt assemblies in terms of the degree of
looseness brought about by the vibration and has a higher residual axial
force ratio than those of the latter.
The resin-coated waterproof nut of the present invention has the same
torque strength as a normal metallic nut. Prior resin coated nuts could
not provide the same torque strength as metallic nuts. The present
invention requires a metallic nut body having internal threads, a layer of
synthetic resin covering the periphery and front surface of the nut body
and having a thickness of at least 1 mm at the front surface in a
preferred embodiment. The nut also has a cap-shaped portion integrally
connected with the layer of synthetic resin and disposed over the rear
surface of the nut. The cap-shaped portion has an internal thread which is
contiguous with the internal thread of the nut body so as to provide an
interference fit with a bolt. And, an outer part of the capshaped portion
is formed in order to engage a tool for tightening the nut.
The high torque strength of the nut of the present invention is achieved:
by the contact between the internal thread ridge of the cap-shaped portion
and the thread ridge of a bolt being screwed into the nut, and by the
resin covering on the front surface of the nut. When the amount of contact
is within the claimed range the deformation of the cap-shaped portion is
limited. Therefore, the cap-shaped portion will not break from the
rotational force of a tool on its outer periphery.
Table 3 below compares the torque strength of the claimed nut with
conventional nuts and illustrates the results obtained by the claimed
invention.
TABLE 3
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Torque
Sample Strength
No. Kind Material (kg .multidot. cm)
Broken Mode
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1. metal SS41 900- Bolt is
1,100 broken.
2. all Nylon 200-300 Screw
resin 6,6 thread is
broken.
3. metal SS41 + 300-500 Metal
and Nylon insert
resin 6,6 comes off.
4. An Example SS41 + 900- Bolt is
used in Nylon 1,100 broken.
the present
invention
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The metallic nut (Sample No. 1) shown in the Table 3 has a high torque
strength and, therefore, the bolt to be screwed to the metallic nut is
broken.
Also, in the all resin nut (Sample No. 2), the torque strength is as low as
1/3-1/5 of the metallic nut. Therefore, the all resin nut can not be
applied for construction.
In addition, the resin and metal nut (Sample No. 3) is produced by
inserting the metallic nut into resin material, but the torque strength of
the nut is low, so that the metallic nut (insert) comes off from the resin
cover, when the nut is revolved by a tool such as a spanner. Therefore,
the nut (Sample No. 3) can not be applied for construction as a waterproof
nut having a high torque strength (kg. cm).
It can be readily seen that the present invention, Sample 4., has the same
high torque strength as a metallic nut normally used in construction. It
can also be readily seen that a resin nut and a resin coated metallic nut
of the prior art are damaged at a much lower torque strength. Only the nut
of the present invention has the desired strength combined with the
waterproof quality that is required for use in water storage tanks and in
pipes.
The torque strength of the nut is assisted by a resin covering on the front
surface of the nut. The resin covering on the front surface of the nut
prevents the nut from becoming separated from the cap-shaped portion at
low torque strength, as occurred with sample 3 above.
Table 4 shows the relationship between the thickness of the front surface
covering layer and the torque strength.
TABLE 4
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Thickness Torque Strength
Sample No.
(mm) (kg .multidot. cm)
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