Process And Apparatus For Forming Tubes With Spiral Corrugations

Abstract

A spirally corrugated heat exchange tube is formed from a smooth-walled tube. The smooth-walled tube is drawn over a rotating mandrel between spirally fluted rotary dies. As the tube rotates with the mandrel, the newly corrugated portion is drawn away from the mandrel and dies, and a smooth-walled portion is advanced, forming continuous spiral corrugations along the length of the tube.

Description

BACKGROUND OF THE INVENTION

Corrugated heat exchange tubes are in general use for promoting heat transfer in fluid evaporation and condensation systems. Linear corrugations, aligned with the longitudinal tube axis, are typically employed on these tubes. Circular and spiral corrugations, having more desirable operating characteristics for some applications, are also employed where their increased cost is warranted. Spiral corrugations are particularly desirable because of their comparatively high strength for a given wall thickness, and for the spiral pattern of fluid flow resulting on their outer surfaces. Prior to this invention, however, the difficulty and cost of fabricating spirally corrugated heat exchange tubes has impeded their general acceptance.

Prior techniques for fabricating longitudinally corrugated tubes are generally unsuitable for fabricating spirally corrugated tubes. One prior method shown by H. A. Greis et al. in U.S. Pat. No. 3,407,638 employs a fluted mandrel inserted within a smooth-bored tube. The mandrel occupies the entire length of the tube on which corrugations are formed. Rotating fluted dies press the tube wall inward between the mandrel flutes, forming longitudinal corrugations. The tube is removed after corrugation by sliding it over the mandrel. It is apparent that with spiral corrugations on the tube, this final step of separating the mandrel and tube is difficult, if not impossible. In addition, the difficulty and expense of fabricating a full-length spirally fluted mandrel preclude the economic use of Greis et al. process for forming spirally corrugated tubes.

A method and apparatus using a relatively short spiral mandrel for spirally corrugating a malleable tube is shown in U.S. Pat. No. 2,757,706, issued to J. M. Johnston. In the Johnston process the tube is held against rotation as a spiral mandrel rotates within the bore of the tube and fluted dies rotate around the outer wall. For holding the tube against rotation, as required by the Johnston process, the tube sidewalls are deformed from a circular to a square cross section prior to corrugation. The process and apparatus are not only inordinately complex, but also are potentially damaging to the tube when the sidewalls are deformed.

Magneforming is another method available in the prior art for forming spirally corrugated tubes. For Magneforming spiral corrugations, a spirally fluted mandrel is inserted within a smooth-walled tube. Eddy-currents are electrically induced in the tube, creating forces which draw the tube into close conformity with the spiral flutes on the mandrel. In many instances a Magneforming process of this type is unsuitable for forming spiral corrugations because the process is primarily restricted to metals of low resistivity such as copper and aluminum, and to tubes having a relatively thin-wall dimension. This invention was made in order to overcome these deficiencies of the prior art.

SUMMARY OF THE INVENTION

This invention is a process and apparatus for forming spiral corrugations on smooth-walled heat exchange tubes. To form the corrugations, a short, spirally fluted mandrel is inserted within a tube and positioned between a pair of spirally fluted rotary dies, so that the tube is sandwiched between the mandrel and dies. While the mandrel is rotated, pressure is exerted on the rotary dies to deform the tube inward around the teeth of the mandrel. As the mandrel, tube, and dies rotate in synchronism, a ring of spiral corrugations is formed on the tube.

The length of the spiral corrugations formed as the tube rotates is governed by the length of the mandrel and dies. Because the mandrel must be removed from the tube when corrugation is completed, the mandrel is just long enough to function properly, without binding rigidly within the tube. Long spiral corrugations along the length of the tubes are ultimately formed by drawing the tube between the dies and mandrel as they rotate. When the corrugating process is completed, the mandrel which has already passed through a substantial portion of the tube is easily removed.

Therefore, one object of this invention is a process for forming a spirally corrugated tube by drawing a smooth-walled tube over a longitudinally fixed mandrel as the tube and mandrel rotate together.

Another object of this invention is a tube-drawing engine for forming a spirally corrugated tube by drawing a smooth-walled tube over a longitudinally fixed mandrel as the tube and mandrel rotate together.

These and other objects of the invention are described in greater detail in the following specification and drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a plan view of a tube-forming engine, showing a partially corrugated tube in operating position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

On the tube-forming engine 10 shown in the sole figure, a spirally fluted, cylindrical mandrel 12 rotates between a horizontally aligned pair of spirally fluted rotary dies 14. The mandrel 12 extends with a sliding fit into the smooth bore of a malleable tube, or workpiece 16, which is operated upon by the tube-forming engine 10 to form a spirally corrugated tube. The fluted dies 14 abut two diametrically opposite exterior areas of the tube wall, sandwiching the tube 16 between the mandrel 12 and a die on each side. When the dies 14 press against the rotating mandrel with sufficient pressure, teeth 18 on the dies effectively engage teeth 20 on the mandrel, through the intermediate tube wall, causing the mandrel, tube, and dies to rotate synchronously. The mandrel teeth 20 are smaller than the teeth 18 on the dies 14 by an amount equal to the wall-thickness of the tube, so that as the tube and mandrel rotate between the dies, spiral corrugations are formed on a short length of the tube. By rotating the tube in this way, and simultaneously moving it across the rotating mandrel and dies, longer spiral corrugations are formed along the length of the tube.

Torque for rotating the spirally fluted mandrel 12 is supplied by a spindly 24 of a lathe (not shown) through an intermediate arbor 26. A standard collet 28 anchors the arbor 26 in axial alignment with the spindle rotation axis and prevents any longitudinal displacement of the arbor or mandrel 12.

Each fluted die 14 rotates freely on a concentric axle 30 within a U-shaped frame 32. For radial movement of the die 14 relative to the tube 16 a rectangular cross-sectioned shaft 34 at one end of the frame 32 slides within a complementarily shaped rectangular cavity in a fixed bearing block 36. The die is moved radially relative to the tube 16, as indicated by double-ended arrows, by turning a threaded portion 38 of a shaft 40 in a threaded hole through the rectangular shaft 34. Radial shoulders 42 and 44 on the shaft 40 abut opposite sides of a cavity end wall 46, preventing longitudinal translation of the shaft as it rotates. When the shaft is rotated in one direction by turning a handle 48, the die 14 advances toward the tube 16. When the shaft is rotated in the opposite direction the die withdraws. In operation, with the mandrel 12 turning and a tube 16 in position the opposing shafts 40 are turned simultaneously, applying uniform pressure in opposite directions until the die and mandrel teeth seat with the tube between them.

After the mandrel and die teeth have seated, forming a narrow corrugated area on the tube 16, longitudinal force is applied to continuously draw an unworked, smooth-walled portion of the tube between the synchronously rotating mandrel and dies. For this purpose a drawing arm 50 firmly grips the tube 16 for longitudinal displacement, while allowing free axial rotation of the tube with mandrel 12. The tube is gripped on the drawing arm 50 by a standard collet 52, wedged against an internally tapered sleeve 54 at one end, and screwed to a threaded sleeve 56 at the other end. Between the threaded sleeve 56 and the tapered sleeve 54, the collet is firmly anchored to the inner race 58 of a longitudinal thrust bearing 60. The outer race 62 of the bearing 60 is anchored to a carriage extension 64 for movement of the drawing arm 50 and tube 16 with the carriage (not shown) of the lathe upon which the spindle 24 is mounted.

As the lathe spindle 24 turns, an automatic feed slowly draws the carriage longitudinally along the lathe bed in either direction shown by the double-ended arrow through the drawing arm 50. Longitudinal force is transmitted from the lathe carriage to the tube 16 through bearing balls 66, riding in concave circular recesses in the inner and outer bearing races 58 and 62. While the thrust bearing 60 transmits longitudinal force, the balls 66 simultaneously permit free relative rotation of the inner and outer races, allowing the tube 16 to rotate with the mandrel 12 as it passes over the arbor 26. Advancing through the dies 14, the tube also rotates relative to the longitudinally fixed mandrel in response to the spiral flutes which mesh them. Bearing blocks 68 support the tube 16 at convenient intervals, preventing damage or distortion of the simultaneously translating and rotating tube.

Because the mandrel 12 contacts only a relatively small interior surface area of the tube 16, a minimum amount of force effectively draws the tube over the mandrel. As corrugating progresses the tube moves across the mandrel, so that the mandrel remains meshed with only a small portion of the tube when the corrugating process is completed. In this way, the disadvantages or prior tube-forming engines with mandrels extending through an entire tube, namely, tight meshing between the mandrel and tube, and removal of the entire mandrel only after corrugating is completed, are eliminated by this invention without resort to the complex procedures and apparatus adopted by the prior art.

While the preferred embodiment of the invention has been shown and described, modifications within the scope of this disclosure are expected for adapting the invention to diverse tube-forming environments. The tube-forming engine need not include a lathe, as other machines for performing a similar function are equally suitable. Equivalent drawing arms 50 and thrust bearings 60 will be apparent to the skilled worker in the art for performing the function described, as will alternate apparatus for advancing the fluted dies 14 toward and away from the tube 16. The arbor 26 need not pass through the drawing arm 50 or the thrust bearing 60, since the tube 16 can be originally positioned over the arbor and drawn off in a direction away from the spindle 24 where a carriage with sufficient travel is available. While a single pair of fluted rotary dies have been shown, one or any number of dies can be used equivalently. For these reasons, this invention should be limited only by the scope of the following claims.

Claims

What is claimed is:

1. A method for corrugating a malleable tube comprising the steps of:

inserting a fluted, cylindrical mandrel into the tube with the longitudinal axes of the mandrel and tube parallel and substantially colinear the mandrel being short in relation to the length of the tube, and having a maximum diameter closely approximating the internal bore diameter of the tube,

rotating the fluted mandrel about its longitudinal axis while laterally pressing one or more complementarily fluted rotary dies against the exterior surface of the tube in the longitudinal position occupied by the fluted mandrel, so that corrugations are formed on a short length of the tube,

applying force distinct from that exerted by the mandrel and fluted dies to progressively move the tube longitudinally over the mandrel, while simultaneously allowing the tube to rotate with the mandrel and one or more fluted dies, so that corrugations are formed on a length of the tube longer than the length of the mandrel.

2. A method, as claimed in claim 1, for corrugating a malleable tube by employing a spirally fluted mandrel for forming spiral corrugations, in which the step of moving further comprises:

applying force independent of the mandrel and fluted dies to progressively move the tube longitudinally over the mandrel, while simultaneously allowing the tube to rotate both with, and relative to the mandrel, so that spiral corrugations are formed on a length of the tube longer than the length of the mandrel.

3. A method, as claimed in claim 2, for corrugating a malleable tube, in which the step of moving further comprises:

pulling the tube across the mandrel.

4. A method, as claimed in claim 2, for corrugating a malleable tube, in which the step of moving further comprises:

pushing the tube across the mandrel.

5. An apparatus for corrugating a malleable tube comprising:

a fluted, cylindrical mandrel, having a relatively short length in relation to the length of the tube, and a maximum diameter closely approximating the internal bore diameter of the tube,

an arbor, having a smaller diameter than the mandrel, joined to the mandrel at one end, with the longitudinal axes of the mandrel and arbor parallel and substantially colinear,

means for rotating the tube, mandrel and arbor about their substantially colinear longitudinal axis,

one or more fluted rotary dies positioned adjacent to the mandrel for synchronous rotation with the mandrel about an axis parallel to the colinear longitudinal axis as the mandrel rotates with the wall of the tube sandwiched between the mandrel and the one or more dies, the flutes on both the mandrel and the one or more dies having complementary shapes for forming corrugations on the tube, and

means distinct from the mandrel and rotary dies for progressively moving the tube across the mandrel, while simultaneously allowing free rotation of the tube with the mandrel and die.

6. An apparatus, as claimed in claim 5, for corrugating a malleable tube in which:

the fluted, cylindrical mandrel and the one or more fluted rotary dies have complementarily shaped spiral flutes for spirally corrugating the tube.

7. An apparatus, as claimed in claim 5, for corrugating a malleable tube in which the means for progressively moving the tube across the mandrel includes:

a thrust bearing

means rigidly connecting the tube to an internal race of the thrust bearing for free rotation with the internal race, and

means acting upon an external race of the thrust bearing for moving the bearing and tube in a direction parallel to the longitudinal axis of the tube,

whereby the tube is free to rotate about the longitudinal axis as it is moved in a direction parallel to the longitudinal axis.

8. An apparatus, as claimed in claim 6, for corrugating a malleable tube in which the means for simultaneously moving the tube across the mandrel includes:

a thrust bearing,

means rigidly connecting the tube to an internal race of the thrust bearing for free rotation with the internal race, and

means acting upon an external race of the thrust bearing for moving the bearing and tube in a direction parallel to the longitudinal axis of the tube,

whereby the tube is free to rotate about the longitudinal axis as it is moved in a direction parallel to the longitudinal axis.

9. An apparatus, as claimed in claim 6, in which:

the means for progressively moving the tube includes an opening of sufficient diameter in alignment with the axis of rotation of the arbor and tube, so that the means for progressively moving is able to draw the tube over the arbor as the tube is moved in the direction parallel to the longitudinal axis.

10. An apparatus, as claimed in 7, in which:

the means for progressively moving the tube includes an opening of sufficient diameter in alignment with the axis of rotation of the arbor and tube, so that the means for progressively moving is able to draw the tube over the arbor as the tube is moved in the direction parallel to the longitudinal axis

11. An apparatus, as claimed in claim 8, in which:

the means for progressively moving the tube includes an opening of sufficient diameter in alignment with the axis of rotation of the arbor and tube, so that the means for progressively moving is able to draw the tube over the arbor as the tube is moved in the direction parallel to the longitudinal axis.