Composite dual tubing

The present invention relates to a steel tubing and more particularly a composite dual tubing which can be suitably used for an exhaust pipe capable of effectively decreasing the radiated noise from the exhaust pipe of automobile. Several known dual tubings are effective occasionally for the reduction of radiated noise, but their effects are drastically reduced at high temperature. A metallic dual tubing of the present invention comprises an inner tube (10) and an outer tube (20) and is, characterized in that the metallic dual tubing is provided with an intermediate layer (18) comprising combustible material filled between the inner tube (10) and the outer tube (20). This layer may be single layer consisting of the combustible material or of the mixture of such material with a heat-resistant material. The intermediate layer may be a dual layer of the combustible material and the heat-resistant material.

The present invention relates to a steel tubing and more particularly a 
composite dual tubing which can be suitably used for an exhaust pipe 
capable of effectively decreasing the radiated noise from the exhaust pipe 
of automobile. 
In the automobiles, an exhaust pipe connects the exhaust manifold to the 
muffler and allows the exhaust gas having a high temperature and a 
pulsating pressure to pass therethrough. The exhaust pipe radiates 
therearound noise, which is generated by the mechanical vibration of an 
internal combustion engine and then transmitted via a flange and the like, 
as well as the noise which is generated by the vibration of the exhaust 
pipe itself due to the pulsating exhaust gas. The automobile noises are 
divided into a group of a power source sounds and a group of travelling 
sounds. The former group is caused by the engine, the radiator fan, the 
exhaust gas and the like. The latter group is caused by the rotational 
contact of tyres with the road, the windage of an automobile body and the 
like. It is considered that the radiated noise from an exhaust pipe 
amounts to about 14% of the power source sounds. Research and development 
of steel tubing for the exhaust pipe proceed, therefore, so as to decrease 
the radiated noise of the exhaust pipe and hence to provide automobiles 
with low noises. The material of the exhaust pipe has been devised and the 
so developed ferritic stainless steel tubing for the exhaust pipe radiates 
reduced noise as compared with the radiated noise in the case of a carbon 
steel exhaust pipe. The reduction effect of radiated sound is, however, 
not satisfactory and the exhaust pipe becomes expensive in the ferritic 
stainless steel exhaust pipe. 
Most conventional exhaust pipes are made of a single steel tube and their 
soundproof effect is not appreciable, if any. The structure of the exhaust 
pipe has therefore been devised so as to decrease the radiated noise of 
such pipes. An example of the so devised and developed exhaust pipes is to 
use a dual tubing which may be also referred to as a double wall tubing. 
The prior art is now described with reference to FIGS. 1 through 6.

When the single tubing is enclosed by the sound absorbing material or the 
sound insulating material, the radiated noise can obviously be decreased 
due to such enclosure as compared with the radiated noise from the exhaust 
pipe of conventional single tubing. Referring to FIGS. 1 through 3, 
examples utilizing the noise decreasing effect of the enclosure made of 
the sound absorbing or insulating material are illustrated. In FIGS. 1 and 
2, the reference numerals 10, 12 and 14 designate an inner tube, the sound 
absorbing or insulating material, such as asbestos and glass fiber, and a 
cover of an exhaust pipe, respectively. The cover 14 in FIG. 1 is embodied 
as a sheet and is elastic, while the cover in FIG. 2 is embodied as an 
armor like member. In order to satisfactorily decrease the noise by the 
composite dual tubings shown in FIGS. 1 and 2, the sound absorbing or 
insulating material 12 must be very thick, so that the whole tubings 
become disadvantageously bulky. In addition, it is complicated to attach 
the sound absorbing or insulating material around the inner tube 10, 
because the exhaust pipe is curved. Namely, when the attachment of sound 
absorbing or insulating material is carried out by a winding method, the 
material must be manually wound around the inner tube and therefore this 
method is not likely to be adapted in the mass production of composite 
dual tubings. As shown in FIG. 3, a method other than manual winding can 
be employed for attaching the sound absorbing or insulating material 
around the inner tube 16. In the method illustrated in FIG. 3, five half 
sections 16 of the sound absorbing or insulating material are shown. A 
pair of the upper and lower half sections are mounted on the inner tube 10 
from both sides of the inner tube and then secured to each other by screws 
at dot points of a flange of these sections, as shown in FIG. 3. The 
workability of this method is higher than the winding method of the sound 
absorbing or insulating material. However, in the method illustrated in 
FIG. 3, half sections 16 must be provided in straight, curved or various 
shapes adapted to the shape of an exhaust pipe and must be selectively 
attached to the respective regions of the exhaust pipe. This is a still 
complicated work of the method illustrated in FIG. 3. The dual tubings are 
advantageous in this regard, because they can be mounted as exhaust pipes 
as simply as the single wall tubings. The dual tubings shown in FIGS. 4 
and 5 are composite tubings with an insert 18 between the inner tube 10 
and the outer tube 20. The composite dual tubing shown in FIG. 5 is 
different from that shown in FIG. 4 in the fact that the inner tube 10 and 
the outer tube 20 are directly in contact and secured or welded to one 
another at the portion A. The tubing shown in FIG. 6 is a dual tubing with 
a gap 22 which separates the inner tube 10 from the outer tube 20, and 
both tubes 10, 20 are secured to one another through portions B, C at the 
ends of the tubing. The dual tubings shown in FIGS. 5 and 6 turned out by 
the test by the present applicant to be effective occasionally for the 
reduction of radiated noise. However, such effect is drastically decreased 
when heating the dual tubings to a high temperature. The reduction effect 
of radiated noise by the dual tubing shown in FIG. 6 is only slightly 
thermally influenced, however, its reduction effect is not remarkable in 
the first place. In addition, since the inner and outer tubes must be 
secured to one another at the ends thereof or at an appropriate 
intermediate point between the ends, so as to prevent the inner tube from 
falling out of the outer tube during the tube handling, the working of 
tubes is disadvantageously complicated. Furthermore, when the dual tubing 
is subjected to bending, the inner and outer tubes are deformed so that 
the gap 22 may not be assured. In order to eliminate such problem, sand 
and the like are usually filled in the gap and then withdrawn from the gap 
after working of the tubes, which is a very complicated method. In this 
regard, the composite dual tubings shown in FIGS. 4 and 5 comprise the 
insert 18, which is filled between the tubes, and thus can be subjected to 
bending without a danger of causing direct contact between the inner and 
outer tubes. However, as stated above, these composite dual tubings 
involve a problem in a drastic reduction of the radiated sound-reduction 
effect at a high temperature. 
It is, therefore, an object of the present invention, to improve the known 
tubings, so that the radiated noise from the composite dual tubing can be 
kept at a low level at normal temperature and especially at high 
temperature. The composite dual tubing according to the present invention 
must be manufactured by a simple and reliable method. 
In accordance with the objects of the present invention, there is provided 
a metallic dual tubing comprising an inner tube and an outer tube, 
characterized in that the metallic dual tubing is provided with an 
intermediate layer comprising combustible material filled between the 
inner tube and the outer tube. 
The preferred embodiments of the present invention are hereinafter 
described with reference to FIGS. 7 through 10. 
Referring to FIG. 7, a preferred embodiment of the composite dual tubing 
according to the present invention is illustrated. In FIG. 7, the insert 
18 is made of combustible material and filled between the inner tube 10 
and the outer tube 20. The insert 18 is hereinafter referred to as the 
combustible material layer. The combustible material layer 18 preferably 
has a thickness of from about 0.1 to 0.5 mm. 
Referring to FIG. 8 is illustrated in another preferred embodiment, wherein 
the intermediate layer is a dual layer consisting of a combustible 
material layer and a heat-resistant material layer. The heat-resistant 
material layer is denoted by the reference numeral 18a. 
The combustible material layer 18 (FIGS. 7 and 8) consists of the material 
combustible at a temperature, to which, for example, the exhaust pipe is 
heated. For the tubing as the exhaust pipe, such materials as synthesized 
resin, and an appropriately treated paper and pulp, which are combustible 
at a temperature in the range of from 600.degree. to 800.degree. C., can 
be used. The heat-resistant material layer 18a consists of a material not 
burnt away at a temperature to which, for example, the exhaust pipe is 
heated. For the tubing to be used as the exhaust pipe such materials as 
glass cloth and asbestos can be used. The heat-resistant material layer 
18a has preperably a thickness of from 0.1 to 0.5 mm. The combustible 
material layer 18 (FIG. 8) has preferably thickness of from 0.1 to 0.5 mm. 
The composite dual tubings shown in FIGS. 7 and 8 may be shipped from the 
tubing manufacturer with or without the combustible material layer 18. 
Namely, the composite dual tubings, in which the combustible material is 
not yet burnt away, may be shipped and then subjected by a purchaser to 
bending, cutting and securing flanges around the outer tube thereby 
adapting the composite dual tubings to the machine and apparatus, to which 
such tubings are attached. The composite dual tubings are then heated and 
the combustible material layer 18 is burnt away, when the machine and 
apparatus is put into operation. The smoke and other vapors generated by 
heating can be discharged or emitted via the end of the composite dual 
tubing attached to the machine and apparatus by means of the flanges 
formed on the outer tube, at which end the combustible material layer is 
or has been exposed. The combustible material layer may be intentionally 
burnt by a burner at an appropriate stage after bending. After burning 
away the combustible material layer, a gap is formed between the inner 
tube 10 and the outer tube 20 in the composite dual tubing shown in FIG. 
7, while the space between the inner and outer tubes is filled with the 
heat-resistant material layer 18a in the composite dual tubing shown in 
FIG. 8. The gap formed between the inner and outer tubes of the composite 
dual tubing shown in FIG. 7 is however difficult to detect with the naked 
eye, because the dimensions of the gap corresponds to the thickness of the 
combustible material intermediate layer and is very small, e.g. from 0.1 
to 0.5 mm. In the composite dual tubing shown in FIG. 8, the 
heat-resistant material layer 18a spreads somewhat upon the burning away 
of the combustible material layer 18. 
Referring to FIG. 9, another preferred embodiment of the composite dual 
tubing is illustrated and this tubing is also provided with an 
intermediate layer comprising combustible material. In this preferred 
embodiment, the intermediate layer consists of a mixture of heat-resistant 
material with the combustible material. The intermediate layer mentioned 
above is denoted in FIG. 9 by 18b and is hereinafter referred to as the 
intermediate layer mixture 18b. The proportion of the combustible material 
to the intermediate layer mixture 18b is from 50 to 60% by volume. The 
heat-resistant material may be an inorganic material, such as rock wool, 
and the combustible material may be resin, such as polyethylene, acryl and 
nylon. The intermediate layer mixture 18b is, for example, a mixture of 
pulp with either glass cloth, ceramic fiber or potassium titanate and is 
in the form of a sheet, in which the glass cloth and the like are 
agglomerated by means of the pulp. The intermediate layer mixture 18b may 
have a thickness of from about 0.2 to 1.0 mm. The combustible material of 
the intermediate layer mixture 18b is burnt away upon heating, when, for 
example, a high temperature exhaust gas flows through the composite dual 
tubing used for the exhaust pipe of automobiles. After heating the 
heat-resistant material is left unburned. 
A method for manufacturing the composite dual tubing according to the 
present invention is now described. 
Referring to FIG. 10, the manufacturing method according to the electric 
tube-welding technique is illustrated. The manufacturing plant is provided 
with uncoilers 30 and 32 of strips for the outer and inner tubes, 
respectively, an uncoiler 34 of the intermediate layer, guide rolls 36, 
forming rolls 38, a high frequency-power source 40, a welding coil 42, 
squeezing rolls 44, a welding assembly 46 for a continuous supply of the 
strips, and a looper 48 which can supply the strips even during welding. 
The strips for the inner and outer tubes are drawn out from the uncoilers 
32 and 30, respectively. The material for intermediate layer is drawn out 
from the uncoiler 34 and is fed into the space between these strips. The 
strips and the material for the intermediate layer are bent into the 
tubular form by the forming rolls 38 and subjected to a high frequency 
induction-heating by means of the welding coil 42. The high frequency 
induction-heated steel edges are bonded or welded, while the strips are 
being pressed by the squeezing rolls 44. The strips are welded as 
illustrated in FIG. 11. The reference numeral 50 designates an impedor for 
decreasing the wattless current, made of ferrite and the like. The 
composite dual tubing as manufactured by the continuous method as 
illustrated in FIGS. 10 and 11 and welded at 24 may have the cross section 
as shown in FIG. 12. 
The thickness of the intermediate layer (the combustible material layer, 
the heat-resistant material layer or the intermediate layer mixture) is 
the thickness of the layer not before but after manufacture of the 
composite dual tubings. When the combustible for heat-resistant material 
is compressed during the manufacture of composite dual tubings, the 
thickness of this material must be so selected that the required thickness 
can be obtained in the completed tubings. The thicknesses of the inner and 
outer tubes may be equal to one another. However, the outer tube should be 
of larger thickness than the inner tube, considering the fact that flanges 
are welded on the outer tube. 
Not only the methods described above but any optional method for 
manufacturing the composite tubings can be utilized for manufacturing the 
composite dual tubing according to the present invention. 
The present invention is further explained by way of Examples. 
EXAMPLE 1 
In the table below illustrated are the results of testing the steel tubings 
of according to the present invention numbered as Nos. 4 through 13 and 
other steel tubings acting as comparative samples numbered as Nos. 1, 2, 
and 3. The steel tubings were dual tubings except that the steel tubing of 
sample No. 1 was a singe or single wall tubing with a diameter of 65 mm 
and wall thickness of 2.4 mm. The diameter of the dual tubings according 
to Sample Nos. 2 through 11 was 65 mm and the wall thickness of the inner 
and outer tubes of these dual tubings was 1.2 mm. The diameter of the dual 
tubings according to Sample Nos. 12 and 13 was 89.1 mm, and the wall 
thicknesses of the outer and inner tubes were 1.6 mm and 1.0 mm, 
respectively. 
The test for measuring the radiated noise from the tubings was carried out 
as follows. The tubings having a length of 1000 mm were held at their ends 
by a carrying member comprising a chuck and a pivot. The steel balls were 
placed in the tubings and struck the inner wall of the tubings, while the 
tubings were caused to rotate by means of the supporting member. The 
radiated sound was measured by a sound level meter located near the 
tubings. The test was carried out at room temperature indicated in the 
table by "As Roll" and at elevated temperature of 600.degree. C., 
700.degree. C. and 800.degree. C. 
TABLE 1 
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Intermediate Layer 
Sound Pressure Level by 
Thickness 
Internally Vibrating (dB) 
No. Material (s) 
(mm) As Roll 
600.degree. C. 
700.degree. C. 
800.degree. C. 
__________________________________________________________________________ 
1 (comparative) 
-- -- 105 104 104 104.5 
2 (comparative) 
none -- 95.5 97 96 98 
3 (comparative) 
Glass Cloth 
0.2 91.5 98 97 97 
4 (invention) 
Paper 0.2 94.5 90 89 91 
5 (invention) 
Synthetic Resin 
0.4 98 90 90 90.5 
Glass Cloth 
6 (invention) 
Synthetic Resin 
0.4 91 87 86 86 
Glass Cloth 
0.2 
7 (invention) 
Synthetic Resin 
0.4 97.5 96 95.5 
97 
Glass Cloth 
0.7 
8 (invention) 
Paper 0.6 100 95.5 
96 98 
Glass Cloth 
0.2 
9 (invention) 
Paper 0.2 90 86 86 86.5 
Ceramic Fiber 
0.5 
10 (invention) 
Paper 0.06 96 94 94.5 
94 
11 (invention) 
Synthetic Resin 
0.05 98 96.5 
95 94 
12 (invention) 
Synthetic Resin 
0.2 92 90.5 
90.5 
91 
Glass Cloth 
0.2 
13 (invention) 
Synthetic Resin 
0.1 92.5 92 91 92 
Glass Cloth 
0.2 
__________________________________________________________________________ 
As is apparent from the table above, the noise from the single tubing of 
Sample 1 was the highest. Sample 2 for the comparison purpose was not 
provided with an intermediate material. The noise as "As Roll" of Sample 2 
was 95.5 dB, and the noise at elevated temperatures became higher. This is 
believed to be because the inner and outer tubes were in contact with one 
another at some parts thereof. 
In Sample No. 3 the intermediate layer consisted exclusively of 
heat-resistant material and was inserted in the annular space between the 
inner and outer tubes so that 80% of the circumferential length was filled 
by the intermediate layer. The sound pressure level was low at "As Roll" 
but became high at 600.degree. C. and higher. 
The sound pressure level of Samples Nos. 4 and 5 according to the present 
invention was high at "As Roll" but was reduced to the level of from 89 to 
91 dB at elevated temperatures where the combustible materials, i.e. paper 
and synthetic resin, were burnt away. The sound pressure level of Sample 
No. 6 according to the present invention was low at both "As Roll" and 
elevated temperatures. 
In Sample Nos. 7 and 8, the thickness of the combustible intermediate layer 
was thicker than the preferable thickness, and the sound pressure level 
was high but was lower than in the single tubing (Sample No. 1). 
Sample No. 9 of the present invention exhibited the best low 
noise-radiation property. 
The above described results can be summarized as follows. 
The sound pressure level is high at elevated temperatures, when the 
intermediate layer is heat-resistant and thus fills densely the annular 
gap (Sample No. 3). Even when the intermediate layer comprises the 
combustible material, a too thin layer of such material (Sample Nos. 10 
and 11) and a too thick layer (No. 8) are not most desirable. Good results 
are obtained by using the intermediate layer comprising the combustible 
material and heat-resistant materials, and by selecting the thickness of 
the material layer(s) in an appropriate range. Samples according to the 
present invention were characterized as compared with the other Samples by 
low radiated noise at elevated temperatures according to the 
characteristic of the present invention was very significant since the 
exhaust pipe flowing a high temperature gas therethrough was heated to a 
high temperature of from 600.degree. to 800.degree. C. 
EXAMPLE 2 
The procedure of Example 1 was repeated to test the composite dual tubings 
with the intermediate layer mixture. The diameter of the composite dual 
tubings of Sample Nos. 14 through 19 given in Table 2 was 65 mm and the 
wall thickness of the inner and outer tubes was 1.2 mm. The diameter of 
the composite dual tubings of Samples Nos. 20 through 25 was 89.1 mm and 
the thicknesses of the outer and inner tubes were 1.6 mm and 1.0 mm, 
respectively. 
TABLE 2 
__________________________________________________________________________ 
Sound Pressure 
Intermediate Layer Mixture 
Level by 
Thickness of 
Internally 
Heat Resistant 
Combustible 
Layer Vibrating (dB) 
Sample Nos. 
Material 
Material 
(mm) As Roll 
600.degree. C. 
700.degree. C. 
800.degree. C. 
__________________________________________________________________________ 
1 (Comparative) 
none none -- 105 104 104 104.5 
2 (Comparative) 
none none -- 95.5 97 96 98 
14 (Comparative) 
Glath Cloth 
Polyester: 20% 
0.2 91.5 98 97 97 
15 (Comparative) 
Ceramic Fiber 
Paper: 30% 
0.5 90.5 95.5 
95 95.5 
16 (Invention) 
Ceramic Fiber 
Paper: 50% 
0.5 91.5 92 92 91.5 
17 (Invention) 
Ceramic Fiber 
Paper: 60% 
0.5 92 89 87 87.5 
18 (Invention) 
Asbestos 
Polyester: 60% 
0.7 95 87 85 85 
19 (Invention) 
KTiO.sub.2 
Paper: 60% 
0.25 96 87 87 88 
20 (Invention) 
Ceramic Fiber 
Paper: 50% 
1.0 97 93 94.5 
93.5 
21 (Invention) 
Ceramic Fiber 
Paper: 50% 
0.5 95 86 85 87 
22 (Comparative) 
Ceramic Fiber 
Paper: 30% 
0.5 91 96 96.5 
96 
23 (Comparative) 
Ceramic Fiber 
Paper: 50% 
0.1 92 95.5 
95 96.5 
24 (Comparative) 
Ceramic Fiber 
Paper: 50% 
1.2 91 94.5 
95 97 
25 (Invention) 
Ceramic Fiber 
Polyester: 60% 
0.5 94 88 88 88 
__________________________________________________________________________ 
In Table 2, above, Sample Nos. 1 and 2 of Example 1 an given so as to 
facilitate the comparison of the composite dual tubings having the 
intermediate layer mixture with the single tubing (No. 1) and the dual 
tubing (No. 2). 
In Sample Nos. 14 and 15, the noise reduction property was good at "As 
Roll" but inferior at elevated temperatures. This was because the 
proportion of the combustible material to the heat-resistant material was 
low. Sample Nos. 16 through 21 and 25, in which the amount of the 
combustible material was from 50 to 60% by volume, radiated a low level of 
noise at elevated temperatures, where the combustible material was burnt 
away, although the noise reduction property at "As Roll" was inferior in 
Sample Nos. 19 and 20. Although in Sample No. 23, the amount of the 
combustible material was 50%, the noise increased at elevated 
temperatures. This was because the intermediate layer mixture was thin. 
The noise of Sample No. 23 at elevated temperatures was high. This was 
because the intermediate layer mixture was thin. 
The low noise radiation of the composite dual tubing according to the 
embodiments shown in FIGS. 7 and 8 can be explained as follows. The 
heat-resistant material layer and the gap generated as a result of burning 
away the combustible material have a function of the vibration attenuator 
or vibration damper and attenuates or damps the vibration of the inner and 
outer tubes. Namely, when the inner and outer tubes vibrate, the 
heat-resistant layer is caused to slide or frictionally be displaced with 
respect to the tubes, with the consequence that the vibration of the tubes 
is damped. In addition, when the inner and outer tubes vibrate, the gap 
generated as a result of burning away of the combustible material is 
caused to expand or shrink, with the consequence that the vibration of the 
tubes is damped. The heat-resistant layer and the gap mentioned above do 
not only have the sound absorbing and sound insulation function as the 
conventional tubings shown in FIGS. 1 through 3 but also have the sound 
damping effect which effectively decreases the radiated noise at elevated 
temperatures. In addition, since the vibration of the tubings is 
suppressed, it is possible to suppress the noise, which results from 
vibration of the exhaust pipe due to the pulsating exhaust gas as well as 
the noise, which results from the engine vibration transmitted to and 
radiated from the exhaust pipe. The vibration damping by the 
heat-resistant layer is realized by deformability of the layer. If the 
heat-resistant intermediate layer were too compactly inserted between the 
inner and outer tubes under pressure to deform, the inner and outer tubes 
would be integrally combined with one another and thus would not be 
different from single tubing from the point of view of vibration. This 
could elucidate the reason why the composite dual tubings shown in FIGS. 4 
and 5 do not exhibit a low noise radiation property. In this regard, the 
combustible material intermediate layer is burnt away when the composite 
dual tubing is heated at, for example, 600.degree. to 800.degree. C. using 
the tubing as the exhaust pipe, which results in the loosely filled and 
thus deformable state of the heat-resistant material layer in the annular 
gap. Since the gap is created by burning away the combustible material, in 
the case where the intermediate layer consists only of the combustible 
material, and also since the heat-resistant material layer is loosely 
filled in the annular space, the inner and outer tubes seem to vibrate 
independently of each other. In addition, the inner and outer tubes may be 
in contact with each other during the vibration in the former case. As a 
result of the independent vibration and the contact between the inner and 
outer tubes the vibration damping seems to be satisfactory. 
If the intermediate layer is very thin, the vibration of the composite dual 
tubings seems to be equivalent to that of the dual tubings which are 
provided with no intermediate layer. On the other hand, when the 
intermediate layer is very thick, the inner and outer tubes are so 
isolated that they are vibrated independently of each other, which seems 
to result in non satisfactory vibration-damping. The composite dual tubing 
with the intermediate layer may be subjected to bending, without resulting 
in direct contact between the tubes which contact would prevent the 
independent vibration of the inner and outer tubes. Since the combustible 
material is burnt away after bending, direct contact is not caused by the 
bending. 
The low noise-radiation of the composite dual tubing shown in FIG. 9 can be 
explained by the vibration damping explained with regard to the composite 
dual tubings shown in FIGS. 7 and 8. The combustible material of the 
intermediate layer mixture is burnt away due to heating by, for example, 
the exhaust gas of automobiles, and therefore the intermediate layer is 
provided with a deformability or is in a loosely filled state. 
The intermediate layer mixture may therefore be strongly compressed between 
the inner and outer tubes during the manufacturing of the tubings, because 
the heat-resistant material of this mixture is later filled loosely 
between the inner and outer tubes. 
The composite dual tubing according to the present invention can be used 
for not only the exhaust tabe but also any vibration-damping purpose.