A pipe-forming machine

Technical Field The invention relates to a pipe-forming machine for forming long helical pipes with a fixed diameter by continuously winding strips made of synthetic resin or the like, into a helical shape.

Background Art

In recent years, pipes buried in the ground such as sewage pipes that deteriorate with age are rehabilitated with a lining of synthetic resin. For this purpose, a pipe-forming machine for the manufacture of pipes by continuously winding strips of synthetic resin in a helical fashion has been used. The pipe-forming machine is arranged to face one end of a buried pipe to be rehabilitated, so that a helical pipe that is formed within the pipe-forming machine is directly introduced into the buried pipe. By using a pipe-forming machine of this type, efficiency in rehabilitating the aged pipes is much increased. Moreover, since the strips are transported to their destination in the form of a wound coil, it is not necessary to transport bulky, finished pipes of synthetic resin to their destination, resulting in a reduction of transportation cost.

Japanese Patent Publication No. 62-13170 discloses this type of pipe-forming machine, in which a plurality of rollers convey the strips while regulating the outer diameter of helical pipes to be formed from the strips. By use of this machine, it is also

possible to obtain helical pipes with different diameters by changing the speed of conveying the helical pipes formed from the strips and the speed of conveying the strips to be wound into a helical shape. Japanese Laid-Open Patent Application No. 62-20987 also discloses a pipe-forming machine of the type mentioned above in which the strips, being conveyed along the inner faces πf hfi hnrifid pipes, are wound into a helical shape to form helical pipes with a regulated outer diameter.

As described above, these pipe-forming machines are designed so that the outer diameter of the helical pipes can be regulated while the strips are being wound into a helical shape. When stress in the opposite direction to the flow of conveyance is applied to the strips while helical pipes are continuously formed from strips, the strips can be forced out from between the rollers. Such a stress is liable to bear on the strips while long helical pipes are being manufactured, so that it is not easy to form long helical pipes with a fixed diameter.

Japanese Patent Publication No. 51-13503 discloses a machine for manufacturing helical hoses in which a soft material extruded from an extruding die is wound into a helical shape in an overlapping way by a plurality of rollers. In the process, the rollers rotate while being kept in contact with the inner surfaces of the helical pipes being formed. However, it is impossible to employ strips made of a rigid material in this machine and to wind them into a helical shape, so that helical pipes to be used for rehabilitating

buried pipes cannot be manufactured according to this machine.

Disclosure of the Invention

The pipe-forming machine for forming helical pipes by winding strips into a helical shape of the invention, which overcomes the above-mentioned and various other disadvantages and deficiencies of the prior art, comprises a plurality of following rollers which are rotatable and arranged at a fixed helical angle on the circular area of a virtual cylinder; at least one of first and second pairs of driving rollers which hold and convey the strips therebetween in such a manner that the following rollers rotate while being kept in contact with the inner surfaces of the strips; and a pair of tension rollers which hold the strips therebetween so that the strips can be conveyed to the following rollers and which cooperate with the driving rollers to keep the strips in tension in such a manner that the following rollers rotate while being kept in contact with the inner surfaces of the strips.

In a preferred embodiment, one driving roller forms the first pair, and one driving pair forms the second pair, and the following rollers are arranged at a fixed helical angle on the circular area of a virtual cylinder, these driving rollers and following rollers constituting a mandrel portion.

In a preferred embodiment, the following rollers and driving rollers constituting the mandrel portion has a diameter which is greater in the middle

portion than in the end portions in the direction of its axis.

In a preferred embodiment, the first pair of driving rollers are positioned so that the strip is conveyed by the following rollers constituting the lower half of the mandrel portion in such a manner that these following rollers rotate while being in contact with the inner surfaces of the strips between the pair of tension rollers and the first pair of driving rollers.

In a preferred embodiment, the second pair of driving rollers which hold and convey the strips from the first pair of driving rollers therebetween and the first pair of driving rollers are symmetrically positioned with respect to the axis of the mandrel portion.

^{in a} preferred embodiment, the strips from the first pair of driving rollers are kept in tension by the second driving rollers so that the strips are conveyed in such a manner that these following rollers constituting the upper half of the mandrel portion rotate while being kept in contact with the inner surfaces of the strips.

In a preferred embodiment, one driving roller forming the second pair is pierced by the same axis as pierces one following roller constituting the mandrel portion.

In a preferred embodiment, the mandrel portion is detachably attached to a given frame.

In a preferred embodiment, one driving roller forming the first pair and one driving roller forming the second pair are attached to the frame so that they can come nearer to or go further away from the other driving rollers constituting the mandrel portion, respectively.

In a preferred embodiment, the driving rollers attached to the frame are respectively rotated by hydraulic motors.

In a preferred embodiment, the two driving rollers forming the first pair or second pair are rotated in reverse directions to each other by a pair of gears which mesh with the corresponding driving rollers forming the first pair or second pair when the gears approach the driving rollers.

Thus, the invention described herein makes possible the objectives of ( 1 ) providing a pipe- forming machine with great propulsion power with which long helical pipes with a fixed diameter can be easily manufactured; (2) providing a pip-forming machine which can be operated easily without complicated and troublesome controlling; and ( 3 ) providing a pipe- forming machine with which helical pipes with different diameters can be easily manufactured.

Brief Description of the Drawings

This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows:

Figure 1 is a partial sectional front view of a pipe-forming machine of the invention.

Figure 2 is a partial sectional side view of a pipe-forming machine of the invention.

Figure 3 is a plane view showing the principal portion of a pipe-forming machine of the invention.

Figure 4 is a cross-sectional view of a strip to be used in the pipe-forming machine.

Figure 5 is a schematic view showing the configuration of a roller for forming pipes.

Figure 6a is a perspective view of a roller for forming pipes and Figure 6b is a cross-sectional view of a roller for forming pipes taken along its axis.

Best Mode for Carrying Out the Invention

Example

As shown in Figures 1 and 2, a pipe-forming machine of the invention comprises a rectangular

parallelepiped frame 10 and a mandrel portion 20 that is detachably mounted on the frame 10. The frame 10, comprising a top plate 11, a bottom plate 12, side plates 13 and 13, and a back plate 14, has the shape of an oblong rectangular parallelepiped, the front side of which is open. The mandrel portion 20 is introduced into and mounted on the frame 10 from its front side. The individual side plates 13 are slidably attached to the top plate 11 and the bottom plate 12 through oblong holes by bolts, so that the individual side plates 13 can come closer to and go further away from each other.

The mandrel portion 20 is rotatably supported by an axis 15 and the axis 15 is supported at one end by the back plate 14 in a substantially horizontal position.

The mandrel portion 20 is constituted by a plurality of rollers 21 for forming pipes. The forming rollers 21 comprise following rollers 21a, which are rotatably attached to roller axes 22, and a pair of driving rollers 21b, which are pierced by the roller axes 22 in a fixed way. Front-side and back-side supporting plates 23 and 24 with a disk shape are spaced at a fixed interval and the roller axes 22 for supporting the following rollers 21a are bridged therebetween. In the space between the centers of the disk-shaped supporting plates 23 and 24, there is provided a connecting pipe 26 that penetrates the back- side supporting plate 24, and an oblong connecting plate 25 is attached to the end portion of the connecting pipe 26 projecting from the back-side supporting plate 24. Cut-out portions 24a with a shape

of a circular arc are formed symmetrically on the back¬ side supporting plate 24 with respect to the center of the disk-shaped plate 24, and the connecting plate 25 is positioned so that its end portions face the cut-out portions 24a. Between the individual end portions of the connecting plate 25 and the front-side supporting plate 23, the roller axes 22a are disposed. One of the roller axes 22a positioned at the profile supply side pierces both the following roller 21a and the driving roller 21b. That is, the back-side half of the roller axis 22a positioned at the profile supply side pierces the following roller 21a and the front-side half of the roller axis positioned at the profile supply side pierces the driving roller 21b. The other of the roller axes 22a pierces only the driving roller 21b from end to end. The outer surfaces of the circular area of the driving rollers 21b are made of, for example, rigid polyurethane rubber. Both of the roller axes 22a pierce the driving rollers 21b, and the back- side ends of the roller axes 22a pierce gears 27 (See Figure 3).

The roller axes 22 and 22a are arranged at a fixed helical angle in the circular area of a virtual cylinder.

. The mandrel portion 20 with the above- mentioned structure is attached to the inside of the frame 10 in such a manner that the supporting axis 15 is introduced into the connecting pipe 26 from the side of the connecting plate 25 and so that the driving rollers 2lb are positioned on the edge portions of a virtual plane within the mandrel 20. When the

connecting plate 25 is brought onto the back-side plate 14 of the frame 10, the supporting axis 15 penetrates the connecting pipe 26 and then a stopper 15a is attached to the tip of the supporting axis 15, so that the whole mandrel portion 20 is fixed to the supporting axis 15. Thereafter, strips 40 made of synthetic resin are wound around all the forming rollers 21 of the mandrel portion 20 to form a helical shape, in such a manner that the forming rollers 21 rotate while being kept in contact with the inner surfaces of the strips.

The two driving rollers 16a that form pairs with the two driving rollers 21b constituting the mandrel portion 20 are attached to the two side plates 13. The individual driving rollers 16a are pierced by and fixed to the corresponding roller axes 16b disposed within rectangular parallelepiped housings 17a. Each of the housings 17a is positioned so as to be in parallel with its corresponding driving roller 16a. Around the outer surfaces of the circular areas of the driving rollers 16a, a plurality of circular fins 16σ projecting in the radial direction are disposed at fixed intervals. As described below, the fins 16c are to be fit into the grooves between projections 42 formed on the back of the strips 40. The outer surfaces of the circular areas of the driving rollers 16a positioned between the fins 16c are previously knurled.

The back-side ends of the individual roller axes 16b are connected to the output shafts of hydraulic motors l6d supported by the housings 17a, and

by the operation of the hydraulic motors 16d, the driving rollers 16a are made to rotate. Between the hydraulic motors 16d and driving rollers 16a, gears 16e are pierced by the roller axes 16b. As mentioned above, the driving rollers 16a form pairs with the driving rollers 21b constituting the mandrel portion 20, and the gears 16e pierced by the same axes as pierce the driving rollers 16a are designed to mesh with the gears 27 pierced by the same axes as pierce the driving rollers 21b.

The bottom faces of the housings 17a are attached to the lower end portion of the supporting plates 17b, which are disposed on the inner surfaces of the individual side plates 13. The housings 17a can be shifted over a virtual plane at right angles to the supporting plates 17b. The upper faces of the housings 17a are attached to piston rods 17e of hydraulic cylinders 17d by means of connectors 17c. The hydraulic cylinders 17d are supported by the supporting plates 17b so that the piston rods 17e extend in the direction of the mandrel portion 20. Therefore, when the piston rods 17e of the hydraulic cylinders 17d extend toward the mandrel portion 20, the housings 17a shift around the respective portions at which the housings 17a are supported by the lower end portion of the supporting plates 17b and come nearer to the mandrel portion 20 disposed within the frame 10. However, when the piston rods 17e retract, the housings 17a move further away from the mandrel portion 20. By the access of the housings 17a to the mandrel 20, the driving rollers 16a attached to the housings 17a are brought nearer to the corresponding

driving rollers 21b of the mandrel portion 20, so that the strip 40 is held between a pair of driving rollers consisting of the driving rollers 16a and 21b. By the access of the driving rollers 16a to the driving rollers 21b constituting the mandrel portion 20, the gears 16e pierced by the roller axes 16b come to mesh with the gears 27 pierced by the roller axes 22a, so that the driving rollers 16a disposed within the frame 10 are rotated by the hydraulic motors 16d, giving rise to the rotation of the driving rollers 21b at the side of the mandrel portion 20 and the conveyance of the strips 40. The strips 40, being held between the driving rollers 16a and 21b, are conveyed in such a manner that the following rollers 21a rotate while being kept in contact with the inner surfaces of the strips 40.

The supporting plates 17b for supporting the hydraulic cylinders 17d and the housings 17a are attached to the side plates 13 so that the supporting plates can be shifted over the side plates 13, and consequently, the driving rollers 16a supported by the housings 17a are made to slant by the shifting of the supporting plate 17b. The driving roller 16a can be positioned in parallel to the driving roller 21b disposed at a fixed helical angle in the mandrel portion 20, after the shift of the supporting plate 17b.

The upper end portion of one of the supporting plates 17b is bent toward the inside of the frame 10, and a supporter 17f that penetrates the upper plate 11 of the frame 10 is attached to the bent

portion of the supporting plate 17b. The supporter 17f supports a casing 18a, in which a driving tension roller 18b and a following tension roller 18c are disposed. The driving tension roller 18b is held in a position parallel to and above the driving roller 16a attached to the lower portion of the supporting plate 17b for supporting the tension roller 18b, and the following tension roller 18c is rotatably held in a position parallel to the driving tension roller 18b. The driving tension roller 18b is connected to the output shaft of the hydraulic motor 18d supported by the casing 18a, to be rotated by the operation of the hydraulic motor 18d. The following tension roller 18c is supported by the hydraulic cylinder 18f held by the casing 18a, so that the following tension roller 18c can come nearer to and go further away from the driving tension roller 18b.

The strips 40 made of synthetic resin are conveyed between the pair of tension rollers. When the following tension roller 18c comes nearer to the driving tension 18b, the strips 40 are held between the pair of tension rollers and then sent .downward to the gap between the driving roller 16a and the following roller 21a, which are positioned below the pair of tension rollers.

Around the outer surface of the circular area of the driving tension roller 18b, circular fins 18g projecting in the radial direction are disposed at fixed intervals. As described below, the fins 18g are to be fit into the grooves between the projections 42 formed on the back of the strips 40. The outer

surfaces of the driving tension roller 18b between the fins 18g are previously knurled.

On the upper face of the casing 18a, there is disposed an encoder 19a for detecting the rotation of the driving tension roller 18b. The rotation of the driving tension roller 18b is transmitted to the encoder 19a by means of a pair of sprockets 19b and 19c and a chain 19d round the sprockets 19b and 19c.

The pipe-forming machine of the invention forms helical pipes from the strips 40 with the cross- sectional structure shown in Figure 4, for example. The strip 40 has a sheet portion 41 with a smooth surface, and there are a plurality of projections 42, 42, ... on the back of the sheet portion 41 at fixed intervals, lined up in the direction of the width of the strip. Each projection 42 has a T-shaped cross- section. At one edge portion in the direction of the width of the surface of the sheet portion 41, there are meshing projections 43 that have a ball-shaped tip, which are arranged in parallel with the other projections 42. At the tops of the T-shaped projections 42 that are adjacent to the meshing projections 43, stoppers 42a that bend toward the meshing projections 43 are provided. The other edge portion in the direction of the width of the sheet portion 41 is arranged so that an indentation 44 that meshes with one edge portion 41a of the sheet portion 41 with the meshing projections 43 can be formed, and thus placed in an outward position with respect to the surface of the sheet portion 41 by the thickness of the sheet portion 41. This portion is

formed with a meshing groove 45 that is curved and protrudes outwardly so that the meshing projection 43 can mesh with the meshing groove 45. The curved area formed by this meshing groove 45 has a projection 45a that projects outwardly and that is T-shaped in cross- section. On the edge in the direction of the width of the sheet portion 41 with the meshing groove 45, there is formed a rib 46 that extends slantingly outwards. The rib 46 is formed so that its top will engage with the curved stopping portion 42a of the projection 42 that is adjacent to the meshing projection 43 when the adjacent meshing projection 43 and the meshing groove 45 are fit together by winding of the said strip 40 into a helical shape. In fitting the meshing projection 43 into the meshing groove 45, an adhesive agent is applied to the inner surface of the meshing groove 45 of the said strip 40.

By use of the pipe-forming machine of the invention with the structure mentioned above, inner helical pipes with which buried pipes are to be rehabilitated are formed as follows. First, the mandrel portion 20 with an outer diameter corresponding to the diameter of the helical pipe to be manufactured is attached to the frame 10 on the ground. In the mandrel portion 20, the forming pipes are arranged at a fixed helical angle. Then, the supporting plates 17b to which the driving rollers 16a attached to the frame 10 are attached are shifted over the side plates 13 to a position in which the driving rollers 16a are in parallel with the corresponding driving rollers 21b constituting the mandrel portion 20, and then fixed. The pair of tension

rollers 18b and 18c attached to one supporting plate 17b are slanted with the shifting of the supporting plate 17b, to be in parallel with the driving roller 16a positioned below.

Then, the following tension roller 18c is taken further away from the driving tension roller 18b by control of the hydraulic cylinder 18f, and the driving rollers 16a within the frame 10 are also taken further away from the corresponding driving rollers 21b constituting the mandrel portion 20 by control of the hydraulic cylinders 17d. Then, one tip of the strip 40 of synthetic resin to be formed into helical pipes is inserted from above through the gap between the pair of tension rollers 18b and 18c, which are separated from each other, and the pair of tension rollers 18b and 18c are again brought closer to each other by control of the hydraulic cylinder 18f, followed by the operation of the hydraulic motor 18d to rotate the driving tension roller 18b. Thus, the strip 40 is guided to the gap between the driving roller 16a and the following roller 21a positioned below the pair of tension rollers 18b and 18c while being held between the pair of tension rollers 18b and 18c, which are separated from each other. After the tip of the strip 40 is passed through the gap between the driving roller 16a and the following roller 21b, the driving roller 16a is brought nearer to the following roller 21a and the driving roller 21b by control of the hydraulic cylinder 17d, so that the gear 16e pierced by the driving roller 16a comes to mesh with the gear 27 pierced by the roller axis 22a and the rotatory driving force of the driving roller 16a driven by the

hydraulic motor 16d is transmitted to the roller axis 22a, giving rise to the rotation or the driving roller 21b.

In this way, the strip 40 is conveyed while being held between the driving roller 16a and the following roller 21a« The strip 40 is guided by hand to be put round a plurality of pipe-forming rollers 21 (driving rollers 21a) constituting the lower half of the mandrel portion 20, and then conveyed to the other pair of driving rollers 16a and 21b. When the tip of the strip 40 is passed through the gap between the pair of driving rollers 16a and 21b, the hydraulic cylinder 17d is controlled so that the pair of driving rollers 16a and 21b come nearer to each other. By the access of the driving roller 16a to the driving roller 21b, the gear (not shown) pierced by the driving roller axis 16b comes to mesh with the gear (not shown) pierced by the roller axis 22a, so that the rotatory driving force of the driving roller 16a being driven by the hydraulic motor (not shown) is transmitted to the roller axis 22a, giving rise to the rotation of the driving roller 21b. In this case, it is necessary to position the fins 18g and 16c disposed around the outer surfaces of the circular areas of the driving tension roller 18b and the driving rollers 16a so as to be fit into the grooves between the projections 42 and 43 formed on the back of the strip 40.

Thus, the strip 40 put round the pipe-forming rollers 21 (following rollers 21a) constituting the lower half of the mandrel portion 20 is conveyed while being held between the pair of driving rollers 16a and

21b. The strip 40 is then put round a plurality of pipe-forming rollers 21 (following rollers 21a) constituting the upper half of the mandrel portion 20, to be conveyed to the first pair of driving rollers 16a and 21b, which are positioned below the pair of tension rollers 18b and 18σ and which are holding the strip 40 therebetween. In the vicinity of the following roller 21a and the driving roller 21b facing to each other, the meshing project 43 of the rolled portion of the strip 40 is fit into the meshing groove 45 of the newly fed portion of the strip 40, and the foregoing process is repeated to form helical pipes by continuously rotating the driving rollers.

After winding of the strip 40 two or three times into a helical shape, the hydraulic motors 16d and 18d are stopped and then the whole pipe-forming machine is installed inside a manhole to which the end portion of the buried pipe to be rehabilitated is connected. When the inlet of the manhole is not sufficiently large, the pair of tension rollers 18b and 18c are put apart by controlling the hydraulic cylinder 18f, and the supporter 17f attached to the casing 18a is detached from the supporting plate 17b, so that the frame 10 can be put into the manhole with one of its side plates facing downward, and then the pipe-forming machine can be reconstructed inside the manhole.

In this way, the pipe-forming machine is set at a predetermined position and the hydraulic motors 16d and 18d are again operated, so that the strip 40 paid out from a coil on the ground is wound

into a helical shape and then issued directly into the buried pipe to be rehabilitated from the front side of the frame 10 as a helical pipe.

The first pair of driving rollers 16a and 21b and the driving roller 16a disposed below the tension rollers 18b and 18σ are symmetrically positioned with respect to the supporting axis 15, and the velocity of conveying the strip 40 by the first pair of driving rollers is set to be higher than that of conveying the strip 40 by the pair of tension rollers 18b and 18c. Accordingly, the strip 40 extending from the pair of tension rollers 18b and 18c to the first pair of driving rollers 16a and 21b is in tension, so that the strip 40 is forcibly pressed to the following rollers positioned between the pair of tension rollers 18b and 18c and the first pair of driving rollers 16a and 21b, to be curved into a circular arc without fail. The second pair of driving rollers consists of the driving roller 16a disposed below the pair of tension rollers 18b and 18c and the driving roller 21b constituting the mandrel portion 20, and the velocity of conveying the strip 40 by the second pair of driving rollers 16a and 21b is set to be higher than that of conveying the strip 40 by the first pair of driving rollers 16a and 21b. Accordingly, the strip 40 extending from the first pair of driving rollers 16a and 21b to the second pair of driving rollers 16a and 21b is in tension, so that the strip 40 is forcibly pressed to the following rollers 21a which are positioned between the first and second pairs of driving rollers and which constitute the upper half of the mandrel portion 20.

Here, the correlation among the velocities of conveying the strip 40 by the pair of tension rollers 18b and 18c, by the second pair of driving rollers 16a and 21b, and by the first pair of driving rollers 16a and 21b is described. With the pipe- forming machine of the invention, the velocities of conveying the strip 40 by the three pairs of rollers are different from each other, as shown above. Consequently, the strip 40 is in tension resulting from the differences in the conveyance velocities while being conveyed. When the driving rollers have an equal diameter, the rotation frequencies of the individual hydraulic motors functioning as power sources are set at different ratios. For example, when the rotation frequency of the hydraulic motor 16d driving the first pair of driving rollers 16a and 21b is set at 10, the rotation frequency of the hydraulic motor (not shown) driving the second pair of driving rollers 16a and 21b and the rotation frequency of the hydraulic, motor 18d driving the driving tension rollers 18b are preferably set at 9 and 4 to 5, respectively, so that the strip 40 is at the tension of the desired strength while being conveyed and forcibly put around the mandrel portion 20 to form a helical pipe with a fixed diameter.

Moreover, it is preferable that a second- speed system with which the formation of pipes can be conducted at either high speed or low speed or else a multiple-speed system for forming pipes is applied to the pipe-forming machine of the invention. In this case also, the rotation frequencies of the motors are maintained at a fixed ratio, as shown above.

The encoder 19a is used to detect and display the rotation of the driving tension roller 18b, and the worker recognizes the actual speed of pipe-formation according to the display of the encoder 19a.

The pair of driving rollers and the pair of tension rollers of the pipe-forming machine of the invention are not limited to the aforementioned example, but various other embodiments are applicable thereto, as follows.

For example, it is possible to use a pipe- forming machine that does not have the first pair of driving rollers 16a and 21b. In this case, the portion of the strip 40 extending from the pair of tension rollers 18b and 18c to the pair of driving rollers 16a and 21b positioned below the pair of tension rollers 18b and 18c around the mandrel portion 20 is in tension, so that the whole strip 40 is forcibly pressed to all the following rollers constituting the mandrel portion at once, which is different from the above- mentioned case in which the strip 40 is first pressed to the following rollers constituting the lower half of the mandrel portion and then pressed to the following rollers constituting the upper half of the mandrel portion, in the presence of the first pair of driving rollers.

It is also possible to use a pipe-forming machine in which only the following roller 21a is positioned below the pair of tension rollers 18b and 18c for the meshing of the strip 40, and in which the

pair of driving rollers 16a and 21b are positioned symmetrically with the following roller 21a with respect to the supporting axis 15. In this case, the portion of the strip 40 extending from the pair of tension rollers 18b and 18c to the pair of driving rollers 16a and 21b halfway around the lower part of the mandrel portion 20 is in tension, and although the strip 40 is forcibly pressed only to the following rollers 21a constituting the lower half of the mandrel portion 20 at first, it is eventually pressed to all the rollers constituting the mandrel portion 20.

The pipe-forming rollers 21 consisting of the following rollers 21a and driving rollers 21b of the mandrel portion 20 are arranged at a fixed helical angle on the circular area of a virtual cylinder. When the pipe-forming rollers 21 in which the outer surfaces of the circular areas are in parallel to their axes; that is, the pipe-forming rollers 21 with a uniform circular section are arranged at a helical angle on the circular area of a virtual cylinder, and another virtual cylinder with an irregular shape in which the circular section is smaller in the middle portion than in either end portions is newly con- stituted by the pipe-forming rollers 21. When the strip 40 is wound around the pipe-forming rollers 21 to form helical pipes, great resistance is to be applied to the strip 40 while the strip 40, being wound into a helical shape, is moving in the direction of the axis of the virtual cylinder, because helical pipes formed from strip 40 cannot easily come off from the pipe- forming rollers 21 at their ends.

In order to solve the above-mentioned problem, the pipe-forming machine of the invention uses the following rollers 21a and the driving rollers 21b in which the circular section is greater in the middle portion than in either end portions. Accordingly, even when the pipe-forming rollers 21 are arranged at a helical angle on the circular area of a virtual cylinder, another virtual cylinder newly constituted by the pipe-forming rollers 21 has a fixed diameter in the direction of its axis, resulting in smooth conveyance of the finished helical pipes from the mandrel portion 20 constituted by all the following rollers 21a. Both the driving roller 21b constituting the second pair of driving rollers and the following rollers are pierced by the same axis and have the same configuration as that of all the other pipe-forming rollers.

The configuration of the pipe-forming rollers 21 is determined by the diameter of helical pipes to be formed and the width of the strip. Figure 5 is a view schematically showing the configuration of the pipe-forming rollers 21, in which the x axis is the axis of the pipe-forming rollers 21 and the y axis is the line at right angles to the x axis through the center of the pipe-forming rollers 21. The pipe- forming roller 21 is symmetrical with respect to the y axis, so that its diameter is the greatest at the y axis and becomes smaller with the distance from the y axis toward both ends of the pipe-forming roller 21. The difference in length between the diameter of the pipe-forming roller at a certain point on the x axis (shown by the y axis) and the greatest diameter of the

pipe-forming roller, \y, is given by the following equation.

(i)

wherein D is the inner diameter of the helical pipe, and W is the width of the strip.

Equation 1 indicates that the outer diameter of another virtual cylinder newly constituted by the pipe-forming rollers is uniform in the direction of its axis even when the pipe-forming rollers are arranged at a given helical angle on the circular area of a virtual cylinder, provided that the pipe-forming rollers 21 have diameters that are the greatest in the middle portion thereof and that become smaller with distance from the middle portion toward the end portions.

Figure 6b shows a cross-section taken along the axis of the pipe-forming roller 21, in which the configuration of the outer. sur ace of the pipe-forming roller 21 is substantially an circular arc with diameter R. The diameter R is obtained from the

following equations.

W

B tan 0 = R tanσc (3)

B 0L = tan ^-1 ( tan Θ ) (4)

D

wherein D is the outer diameter of the virtual cylinder 30 constituted by the pipe- forming roller 21 (the inner diameter of the helical pipe to be manufactured); B is the length of the cylinder 30; 6 is the helical angle of the pipe-forming roller 21 (the angle made by a line on the circular area of the cylinder 30 parallel to its axis and a line through t e σontaot point of pipo- forming rollers 21 and the cylinder 30 parallel to the axis of the pipe-forming roller 21); w is the width of the strip; and -X is the angle of torsion of the pipe- forming roller 21 (the deviation in the center of one end face from the other of

the pipe-forming roller 21, shown as the central angle at one end face of the cylinder 30).

Equation 5 is obtained from the equations 2 to 4.

+ Δv ² 4

R = (5)

2- Δy

wherein Δy is the difference in length between the greatest diameter (in the middle portion) and the smallest diameter (at the ends) of the pipe-forming roller 21.

When the Δy is eliminated from equation 5, R is given by the following equation.

(6)

In this way, when the pipe-forming rollers are formed so as to constitute a virtual cylinder with a circular area that is an circular arc with the radius R in cross-section, which is represented by the factors D, B and W, the outer diameter of the virtual cylinder constituted by the pipe-forming rollers becomes uniform.

Table 1 shows the relationship among the inner diameter D of helical pipes (equal to the outer diameter of the virtual cylinder constituted by the pipe-forming rollers), the maximum max Δy of the difference in length between the greatest outer diameter and the smallest outer diameter of the pipe- forming roller, and the radius of the circular arc regulating the configuration of the circular area of

- -

the pipe-forming roller, at the time when the length of each of the pipe-forming roller is 280 mm and the width of a strip is 95 mm.

Table 1

(mm)

In this way, strips can be easily formed into helical pipes with a fixed diameter by use of the pipe- forming machine of the invention in which a plurality of following rollers are arranged at a fixed helical angle on the circular area of a virtual cylinder and the strips are in tension to be wound around the following rollers into a helical shape in such a manner that the following rollers rotate while being kept in contact with the inner surfaces of the strips. The

pipe-forming machine of the invention can be easily operated, because the rotation frequencies of the driving rollers are previously set so that the strips are kept in tension, and it is not necessary to control the rotation frequencies of the driving rollers while forming pipes. When the diameter of helical pipes is to be changed, the mandrel portion is replaced by another mandrel portion. With the pipe-forming machine of the invention, it is possible to form helical pipes by regulating the inner diameter of helical pipes, so that the diameter of helical pipes is maintained at a - fixed measurement even under great driving force.

It is understood that various other modifica- tions will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.