Composite dual tubing

A steel tubing and more particularly a composite dual tubing which can be suitably used for an exhaust pipe capable of effectively reducing the radiated noise from the exhaust pipe of automobiles. A metallic dual tubing of the present invention comprises: lugs (20a); lugs (20a) and heat-resistant filler material (24); or, the spacer member (16) and the heat-resistant filler member (17).

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 reducing the radiated noise from the exhaust pipe 
of automobiles. 
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 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 of about 14% of the power source sounds. Research and development 
of steel tubings for the exhaust pipe proceeds, therefore, so as to reduce 
the radiated noise of the exhaust pipe and hence to provide automobiles 
with lower 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 
using the carbon steel exhaust pipes. The reduction effect of radiated 
noise 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 
pipes 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 3.

Referring to FIGS. 1 and 2, examples utilizing the noise reducing effect of 
the enclosure made of the sound-absorbing and heat-insulating material are 
illustrated. In FIGS. 1 and 2, the reference numerals 10, 18, and 20 
designate an inner tube, the spacer member having a good sound-absorbing 
and heat-insulating property, such as asbestos and glass fiber, and an 
outer tube, respectively. In order to satisfactorily reduce the noise by 
the composite dual tubings shown in FIGS. 1 and 2, the sound-absorbing the 
heat-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 and heat-insulating material around the inner tube 10, 
because the exhaust pipe is curved. Namely, when the attachment of 
sound-absorbing and heat-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. The composite dual tubing shown in FIG. 2 is 
different from that shown in FIG. 1 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. 3 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 and C at 
the ends of the tubing. The dual tubings shown in FIGS. 1 and 2 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 of the dual tubing shown in FIG. 3 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. 
It is, therefore, an object of the present invention, to improve the known 
tubing, 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 inner tube and the outer tube are in contact or 
welded with each other at a part thereof, and further at least one stripe 
of lug is formed on at least one of the outer surface of the inner tube 
and the inner surface of the outer tube. The at least one stripe of lug 
acts as a spacer between the inner and outer tubes. 
There is also provided a metallic tubing comprising an inner tube, an outer 
tube and an intermediate layer inserted between these tubes, characterized 
in that said intermediate layer consists of a narrower spacer member and a 
wider heat-resistant filler member, said spacer member being in the form 
of a strip having a thickness of from about 0.2 to 0.5 mm, and extending 
in the longitudinal direction of the tubing, and said heat-resistant 
filler member consisting of material filled between the inner and outer 
tubes and having a thickness of from about 0.1 to 0.5 mm. 
The preferred embodiments of the present invention are hereinafter 
described with reference to FIGS. 4 through 6. 
Referring to FIG. 4, a preferred embodiment of the dual tubing with the lug 
according to the present invention is illustrated. In FIG. 4, three 
stripes of the lugs 20a are formed on the inner surface of the outer tube 
20 and extend in the longitudinal direction of the inner and outer tubes 
10 and 20, respectively. The inner tube 10 and the outer tube 20 are in 
contact with or welded to each other at the part thereof indicated by the 
symbol A. The lugs 20a are therefore formed at the half section of the 
dual tubing opposite to the half section where the tubes are in contact or 
welded. The lugs 20a form the spacers defining the gap 22 between the 
inner and outer tubes. The lugs 20a has a pointed cross sectional shape. 
Referring to FIG. 5, a heat-resistant filler material 24 is disposed in the 
gap 22. Although the heat-resistant filler material is disposed entirely 
in the gap 22, it may be disposed at least partly, for example only 
between three lugs 20a in the gap 22. 
The lugs 20a (FIGS. 4 and 5) have preferably a height of from about 0.1 to 
0.5 mm. The number of the lugs 20a is not limited to three stripes but may 
be optional. A number of stripes of lugs may be arranged circumferentially 
and separately from one another. The lugs 20a (FIGS. 4 and 5) may be 
provided on the outer surface of the inner tube 10 or on both the outer 
and inner surfaces of the inner and outer tubes 10 and 20, respectively. 
As the heat-resistant filler material 24, such inorganic heat-resistant 
materials as, glass cloth, asbestos, ceramic wool and rock wool can be 
used. The diameter of the dual tubings is optional. 
Referring to FIG. 6, a preferred embodiment of the dual tubing with the 
intermediate layer is illustrated. Between the inner tube 10 and the outer 
tube 20, the heat-resistant filler member 17 and the spacer member 16 are 
inserted. The heat-resistant filler member completely surrounds the inner 
tube 10 and consists of a heat-resistant material not damaged due to 
burning at a temperature, to which, for example, an exhaust pipe is 
heated. Such heat-resistant material is, for example, glass cloth, 
asbestos or ceramic fiber. The heat-resistant filler member 17 is filled 
in the annular space between the inner and outer tubes. In the process of 
filling the heat-resistant filler member should be compressed in such an 
appropriate degree as to not cause disappearance of the porosities, 
cavities or channels. 
The spacer member 16 must be able to resist against the compression force 
which is applied to the dual tubing during the tube bending and the 
manufacturing of tubing. Provided that the spacer member 16 is resistant 
against the compression, the spacer member 16 may be organic or inorganic 
and also heat-resistant or combustible at a temperature, to which, for 
example, the exhaust pipe is heated. The width of the spacer member 16 is 
considerably smaller than the width of the heat-resistant filler member 17 
or the circumferential length of the annular space between the inner and 
outer tubes. The spacer member 16 is therefore in the form of a strip 
extending in the longitudinal direction of the dual tubing. The spacer 
member 16 is a single strip or track in the drawing. However, a plurality 
of separated spacer members may be arranged. The thickness of the spacer 
member 16 is from 0.2 to 0.5 mm in the state where the spacer member is 
inserted and pressed between the inner and outer tubes. The thickness of 
the heat-resistant filler member 17 is from about 0.1 to 0.5 mm in the 
state where this member is inserted and pressed between the inner and 
outer tubes. The diameter of the composite dual tubing basically exerts no 
influence on how the thickness of the members 16, 17 mentioned above 
should be adjusted. 
The inner tube 10 and outer tube 20 (FIGS. 4 through 6) may have the same 
wall-thickness. The wall-thickness of the outer tube 20 is preferably 
greater than that of the inner tube 10, in light of the fact that a flange 
may be welded to the outer tube. 
Methods for manufacturing the composite dual tubing according to the 
present invention are now described. 
Referring to FIG. 7, the manufacturing method of the dual tubing as shown 
in FIG. 5 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 
heat-resistant filler material, such as a glass cloth, 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 provides a supply of the strips even during 
welding. One or both of the strips are preliminarily provided with a 
stripe or stripes of lugs on the surface of one strip opposite to the 
other strip or the opposite surfaces of the strips. The strips for the 
inner and outer tubes are drawn out from the uncoilers 32 and 30, 
respectively. The glass cloth is drawn out from the uncoiler 34 and is fed 
into the space between these strips. The strips and the glass cloth 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 edges of the strips are bonded or welded at a 
common bead, while the strips are being pressed by the squeezing rolls 44. 
The dual tubing shown in FIG. 4 can be manufactured by utilizing the 
process illustrated in FIG. 7. Obviously, the uncoiler 34 is not 
necessary. 
The composite dual tubing shown in FIG. 6 can be manufactured by the tube 
reduction method. The heat-resistant filler member 17 is disposed around 
the outer circumference of the inner tube 10, and namely the inner tube 10 
is enclosed by the heat-resistant filler member 17 having a width equal to 
the circumference of the inner tube 10 (FIG. 6). Alternately, the 
heat-resistant filler member may be wound around the inner tube 10, when 
the heat-resistant filler member 17 is in the form of tapes. The spacer 
member 16 is bonded on an appropriate position of the heat-resistant 
filler member 17. The inner tube 10 with both members 16 and 17 is 
inserted into the outer tube 20 (FIG. 6). And the diameter of the outer 
tube 20 is decreased by the tube reduction, thereby tightly pressing the 
outer tube 20 to the spacer membe 16 and the heat-resistant filler member 
17. During the tube reduction, the heat-resistant filler member 17 is 
strongly compressed between the inner tube 10 and the spacer member 16, 
but, this member 17 is kept relatively loosely filled between the inner 
tube 10 and the outer tube 20 where the spacer member 16 is not situated. 
The dual tubings shown in FIGS. 4 through 6 are subjected to bending, when 
they are used as the curved exhaust pipe. It is possible to prevent during 
bending by means of the lugs 20a (FIGS. 4 and 5) contact between the inner 
and outer tubes. When a number of separated lugs 20a are provided, the gap 
22 is not reduced appreciably by the bending. In addition, it is possible 
to prevent by means of the heat-resistant filler member 17 (FIG. 6), which 
covers entirely or substantially entirely the inner tube 10, contact 
between the inner and outer tubes liable to be caused by the bending. This 
results in a low noise-radiation property. 
The term of intermediate layer used hereinafter collectively designates the 
spacer member 18 (FIGS. 1 and 2), the spacer member 16, the heat-resistant 
filler member 17, (FIG. 6) and heat-resistant filler material 24 (FIG. 5). 
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 Table 1, below, illustrated are the results of testing the steel tubings 
according to the present invention numbered as Nos. 5 through 10 and other 
steel tubings acting as comparative samples numbered as Nos. 1 through 4, 
11 and 12. The steel tubings were dual tubings except that the steel 
tubing of Sample No. 1 was a single 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 7 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. 8 through 12 was 89.1 mm, and 
the wall thicknesses of the outer and inner tubes were 1.6 mm and 1.0 mm, 
respectively. The symbol GC in Table 1 was the 0.2 mm thick glass cloth. 
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 clamp and a pivot. Some steel balls were 
placed on the inner wall of the tubings and struck against the inner wall 
of the tubings, while the tubings were rotated by means of the supporting 
member. The radiated nose was measured by a sound level meter located near 
the tubings. This method is so called an internal vibration method. The 
test was carried out at room temperature, the tubings provided for the 
test being indicated in the table by "As Formed", and at elevated 
temperatures of 600.degree. C. 700.degree. C. and 800.degree. C. for 5 
hours, respectively. 
__________________________________________________________________________ 
Sound Pressure Level by 
Lugs Internally Vibrating (dB) 
Height Filler 
As 
No. (mm) 
Stripes 
Material 
Formed 
600.degree. C. 
700.degree. C. 
800.degree. C. 
__________________________________________________________________________ 
1 (comparative) 
-- -- -- 105 104 104 104.5 
2 (comparative) 
-- -- -- 95.5 97 96 98 
3 (comparative) 
-- -- GC 91.5 98 97 97 
4 (comparative) 
0.05 
3 -- 94 96.5 
96 97 
5 (invention) 
0.1 3 -- 93 94 93.5 
94 
6 (invention) 
0.2 1 -- 92.5 93 92 92 
7 (invention) 
0.2 3 -- 91.5 92 92 93.5 
8 (invention) 
0.4 1 -- 92 92.5 
92 93.5 
9 (invention) 
0.2 3 GC 90 90.5 
91 91 
10 (invention) 
0.2 5 GC 88 88.5 
88 89 
11 (comparative) 
0.6 1 -- 94.5 95.5 
95 96 
12 (comparative) 
0.6 1 GC 93 95 95.5 
95 
__________________________________________________________________________ 
As is apparent from the table above, the noise from the single tubing of 
Sample 1 was the highest. Sample 2 for comparison purposes was provided 
with neither lugs nor filler material. The noise at "As Formed" of Sample 
2 was 95.5 dB and higher than acceptable level of 95 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. 
Sample No. 3 for the comparison purposes was not provided with lugs but 
comprised the 0.2 mm thick glass cloth densely filled between the inner 
and outer tubes. The noise was low at room temperature but high at 
elevated temperatures in Sample No. 3. 
Sample Nos. 5 through 10 according to the present invention were 
characterized as compared with the other Samples by low radiated noise at 
both room and elevated temperatures. 
Although Sample Nos. 4, 11 and 12 for the comparison purposes were provided 
with the lugs, the radiation noise was high. The lug(s) were low in Sample 
No. 4 and high in Sample Nos. 11 and 12. 
The low noise radiation at elevated temperatures according to the 
characteristic of the present invention is very significant since the pipe 
flowing the exhaust gas therethrough is heated to a high temperature of 
from 600.degree. to 800.degree. C. 
EXAMPLE 2 
The procedure of Example 1 was repeated in order to test the noise 
radiation property of the dual tubings with the spacer member. 
The dimensions of the steel tubes of Sample Nos. 13 through 16 were the 
same as in Sample Nos. 2 through 7. The dimensions of steel tubes of 
Sample Nos. 17 through 21 were the same as in Sample Nos. 8 through 12. 
The test results and structure of the tubings are explained with reference 
to Table 2. 
__________________________________________________________________________ 
Intermediate Layer 
Heat-Resistant Sound-Pressure Level by 
Filler Member Spacer Member Internally Vibrating (dB) 
Thickness 
Filler Dimension 
Spacer As 
No. Sample No. 
(mm) Material 
(mm) Material Formed 
600.degree. C. 
700.degree. 
800.degree. 
__________________________________________________________________________ 
C. 
1 (Comparative) 
-- -- -- -- 105 104 104 104.5 
2 (Comparative) 
-- -- -- -- 95.5 97 96 98 
13 (Comparative) 
0.2 Glass Cloth 
-- -- 91.5 98 97 97 
14 (Invention) 
0.2 Glass Cloth 
10 w .times. 0.2 t 
Glass Cloth 
91 91.5 91 92 
15 (Invention) 
0.2 Glass Cloth 
10 w .times. 0.5 t 
Metallic Mesh 
92 92.5 91.5 
91 
16 (Invention) 
0.5 Ceramic Fiber 
10 w .times. 0.2 t 
Steel Foil 92 93 92.5 
94 
17 (Invention) 
0.5 Ceramic Fiber 
0.5 Three Wires 
92.5 92.5 92 92 
(diameter) 
18 (Invention) 
0.2 Glass Cloth 
10 w .times. 0.5 t 
Polyester 91.5 89 89 89 
19 (Comparative) 
0.2 Glass Cloth 
10 w .times. 1.0 t 
Metallic Mesh 
92.5 95.5 95 96 
20 (Comparative) 
0.5 Ceramic Fiber 
10 w .times. 0.7 t 
Glass Cloth 
92.5 95 96 95.5 
21 (Comparative) 
0.2 Glass Cloth 
10 w .times. 0.1 t 
Three Ceramic Fibers 
93 97 96.5 
98 
__________________________________________________________________________ 
Note: w -- width; and, t -- thickness 
In Table 2, the results of Sample Nos. 1 and 2 are reproduced so as to 
facilitate the comparison of these results with those of dual tubings with 
the spacer member. 
In Sample No. 13 a 0.2 mm thick glass cloth was inserted between the inner 
and outer tubes around essentially the entire circumference. The noise 
reduction property of Sample No. 13 was good at normal temperature, but 
the radiated sound became high at elevated temperatures. 
In Sample Nos. 14 through 18 of the present invention, the 0.2 mm thick 
glass cloth (Sample Nos. 14, 15 and 18) or the 0.5 mm thick ceramic fiber 
(Sample Nos. 16 and 17) was inserted between the inner and outer tubes 
around the entire circumference as the heat-resistant filler member. In 
addition, between the heat-resistant filler member and the outer tube 
inserted were the glass cloth (Sample No. 14), the metallic mesh (Sample 
No. 15), the steel foil (Sample No. 16), the three wires (Sample No. 17), 
and the polyester (Sample No. 18), having the dimension given in Table 2. 
The noise radiation of Sample Nos. 14 through 17 was good both at normal 
temperature and elevated temperatures. The noise-reduction property of 
Sample Nos. 19 through 21 was poor, because the thickness of the insert 
member was too large in Sample Nos. 19 and 20 and was too small in Sample 
No. 21. 
The low noise radiation of the dual tubing according to the present 
invention can be explained as follows. 
The lug or intermediate layer according to the present invention has a 
function of the vibration attenuator or the vibration damper and 
attenuates or damps the vibration of the inner and outer tubes. Namely, 
when the inner and outer tubes vibrate, the lug or intermediate layer is 
caused to slide or frictionally be displaced with respect to the tube(s), 
with the consequence that the vibration of the tubes is damped. The lug or 
intermediate layer does not only have the sound absorbing and insulation 
function as the intermediate layer in the conventional tubings shown in 
FIGS. 1 and 2 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 
both noise which results from the pulsating exhaust gas and noise which 
results from the vibration of the exhaust pipe due to the engine vibration 
transmitted to and radiated from the exhaust pipe. 
The vibration damping by the intermediate layer is realized by 
deformability of the layer. If the 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 dual tubings shown in 
FIGS. 1 and 2 and having the densely filled intermediate layer exhibit a 
relatively low noise-radiation property at normal temperature but radiates 
high level of noise at elevated temperatures. The conventional 
intermediate layer expands at elevated temperatures and is compressed 
strongly by the inner and outer tubes, with the consequence that the thus 
closely combined tubes behave integrally during the vibration. In this 
case, the intermediate layer does not accomplish the vibration damping. 
According to the research by the present inventor, the filling state of 
the intermediate layer is significant for the vibration damping. The 
intermediate layer should be relatively loosely filled in the annular gap, 
so that the inner and outer tubes vibrate independently upon each other 
and their vibration is damped. The intermediate layer according to the 
present invention can be very thin as compared with the conventional 
intermediate layer which has not the vibration damping function but the 
sound absorbing and sound insulating function. The intermediate layer 
according to the present invention is for example 1 mm or less. The dual 
tubing of the present invention is therefore hardly distinguishable from 
the single or single wall tubing from the appearance. 
The desirable filling state of the intermediate layer is realized by the 
lug(s) in the dual tubing shown in FIG. 5. This lug(s) ensures the space 
between the inner and outer tubes and prevents the intermediate layer from 
being strongly compressed by the inner and outer tubes during 
manufacturing of the tube. In any tube-manufacturing processes the 
compression force is applied to the intermediate layer, but the lug(s) can 
prevent the intermediate layer from being densely filled between the 
tubes. 
The desirable filling state of the heat-resistant filler member 17 is 
realized by the spacer member 16 in the dual tubing shown in FIG. 6. If, 
however, the spacer member 16 is very thick, the filling state of the 
heat-resistant filler member 17 is too loose to prevent the independent 
vibration of the inner and outer tubes. The inner and outer tubes thus 
vibrate without vibration damping. If the spacer member is very thin, the 
heat-resistant filler member 17 is strongly compressed, which seems to 
result in integral behavior of both tubes during the vibration. The spacer 
member may be combustible. In this case, the spacer member should be left 
unburnt at the tube production and bending of the tubes. Although the 
spacer member, which is combustible, is burnt away at a high temperature, 
to which, for example, the exhaust pipe is heated, the heat-resistant 
filler member 17 is kept from being strongly compressed due to the 
compression force exerted on the tubes during their production and the 
heat-resistant filler member 17 satisfactorily retains the vibration 
damping effect. 
The vibration damping is also realized by the lugs shown in FIG. 4, namely, 
without the intermediate layer. If the lugs are very low, the inner and 
outer tubes are very close to each other and both tubes behave like an 
integral tubing during the vibration. On the other hand, if the lugs are 
very high, the inner and outer tubes vibrate indipendently of each other 
without vibration damping. The lug(s) should have a height of from about 
0.1 to 0.5 mm.