US3754430A

US3754430A - Method and apparatus for expanding tubes

Info

Publication number: US3754430A
Application number: US00236313A
Authority: US
Inventors: F Halstead
Original assignee: HALSTEAD IND Inc
Current assignee: HALSTEAD IND Inc
Priority date: 1972-03-20
Filing date: 1972-03-20
Publication date: 1973-08-28
Anticipated expiration: 1990-08-28

Abstract

Method and apparatus for expanding tubing, particularly metallic tubing having a U-shaped bend therein, wherein a plug is inserted into one end of the tubing and forced therethrough by fluid pressure. In order to facilitate passage of the plug around the U-shaped bend, the plug is provided with generally rounded (e.g., spheroidal) ends and a recessed central portion having a generally concave cross section. In this manner, the plug, under the influence of fluid pressure, can pass through straight sections of the conduit and can easily pass around a U-shaped bend therein without becoming jammed in the tube and without materially distorting the circular cross-sectional shape of the tube.

Description

United States Patent [1 1 Halstead METHOD AND APPARATUS FOR EXPANDING TUBES [75] inventor: Fred E. Halstead, Waynne, Ark.

[73] Assignee: Halstead Industries, Inc., Felienople,

[22] Filed: Mar. 20, 1972 [2]] Appl. No.: 236,313

[56] References Cited UNITED STATES PATENTS 942,184 12/ 1909 Persons 29/523 1,818,593 8/1931 Walworth 113/118 2,914,839 12/1959 Schwenger 72/370 Aug. 28, 1973 Attorney-Thomas H. Murray et a1.

[57] ABSTRACT Method and apparatus for expanding tubing, particularly metallic tubing having a U-shaped bend therein, wherein a plug is inserted into one end of the tubing and forced therethrough by fluid pressure. in order to facilitate passage of the plug around the U-shaped bend, the plug is provided with generally rounded (e.g., spheroidal) ends and a recessed central portion having a generally concave cross section. 1n this manner, the plug, under the influence of fluid pressure, can pass through straight sections of the conduit and can easily pass around a U-shaped bend therein without becoming jammed in the tube and without materially distorting the circular cross-sectional shape of the tube.

8 Claims, 8 Drawing Figures Patented Aug. 28; 1973 3,754,430

2 Sheets-Sheet 1 FIG. 24 /4 'lNTEG/PAL 24 BEA/D5 l2 l8 l4 I? F/G'. 2.

k1 l 1 i 12/ l -*1" Ii: ii TUBE METHOD AND APPARATUS FOR EXPANDING TUBES BACKGROUND OF THE INVENTION While not limited thereto, the present invention is particularly adapted for use in the manufacture of heat transfer devices, such as radiators, condensers and the like wherein a coil of tubing carrying a heat transfer medium passes through openings in a plurality of fins. In an extended surface coil of this type, the fins must be brought into close contact with the coil for the purpose of conducting heat from the fins to the tube wall, or vice versa.

Methods employed in the past to achieve this close fin-to-tube contact included solder or zinc dipping, winding or pressing fins tightly onto the tubes, or expanding the tube from within and forcing it against a collar at the root of each fin. This latter method involves stressing the material from which the tube is formed beyond its elastic limit such that a permanent deformation takes place.

In the manufacture of heat exchangers of this type wherein the tube is expanded, a plurality of U-shaped hairpin tubes are passed through openings in the fins such that adjacent, parallel leg portions are connected at one end through an integral U-shaped hairpin bend while the other ends of the leg portions are open. Each hairpin tube is then expanded such that its diameter increases and its outer periphery is forced into snugabutting relationship with the openings in the fins while the 180 bends in the U-shaped hairpin tubes remain substantially unexpanded. Following the expanding operation, U-shaped couplings or return bends are brazed to the open ends of the leg portions in order to provide a continuous back and forth path for the flow of a heat transfer medium through the coil.

The expanding operation can be achieved by simply pressurizing each U-shaped tube with a hydraulic fluid. The difficulty with this'method, however, is that the tubing becomes work-hardened when it is bent. Consequently, the area of the tubing adjacent the bend is harder than the remaining tubing and does not expand as readily. As a result, the tubing may not expand into close abutting contact with the fins at the end of the leg portions adjacent the bend. In order to overcome this problem, attempts have been made to force spheres or balls through the tubing under hydraulic pressure. It has been found, however, that a sphere will become lodged in and will not pass around the U-shaped hairpin bend at one end of the tubing.

SUMMARY OF THE INVENTION- In accordance with the present invention, expansion of tubing used in heat exchangers is accomplished by means of a specially-shaped, generally cylindrical metallic plug which is driven through the tube by application of hydraulic pressure. In order to successfully force the plug around the U-shaped hairpin bend in the tubing, it has been found that it must be provided with generally spheroidal or rounded ends and a recessed central portion having a generally concave cross section. Furthermore, the radius of the concave median portion must be at least equal to or greater than the inner radius of the hairpin bend. It has been found that whereas a sphere cannot be forced around a U-shaped hairpin bend in tubing under the influence of hydraulic pressure, a plug of the type described herein can effectively and uniformly expand the straight-line sections of the tube but not the U-shaped hairpin bend connecting them, thus avoiding unnecessary friction and possible binding while passing through the bent section. The plug, in passing around the U-shaped hairpin bend, distorts it slightly; however this is unimportant since no fins are attached to the tubing at this point.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic illustration of a heat exchanger which can be fabricated in accordance with the principles of the invention;

FIG. 2 is an enlarged view of the tubing extending through the heat exchanger of FIG. 1, showing the manner in which it is expanded into contact with flanges on fins;

FIG. 3 is an elevational cross-sectional view of a U- shaped hairpin bend in tubing, showing the plug of the invention passing therethrough;

FIGS. 4A and 48 comprise cross-sectional views showing the configuration of .the tube along line IV-IV before and after the plug of the invention passes therethrough;

FIGS. SA and 5B are cross-sectional views taken substantially along line V-V of FIG. 3 showing the crosssectional configuration of the tube along line V-V before and after the plug of the invention passes therethrough; and

FIG. 6 is a cross-sectional view of a fixture for inserting the plug of the invention into one end of the tubing and for it forcing therethrough under hydraulic pressure.

With reference now to the drawings, and particularly to FIGS. 1 and 2, there is shown a finned heat exchanger 10, such as a condensing or evaporating coil used in air-conditioning equipment. It comprises a plurality of parallel fins 12 preferably formed from aluminum, copper or some other metal of high heat conductivity characteristics. Provided in the fins 12, as best shown in FIG. 2, are flanged openings 14, the flanges 15 being long enough to accurately space the fins one from the other. Received within the openings 14 are the opposite,

parallel leg portions

16 and 18 of a number of generally U-shaped hairpin tubes 20. In this respect, the

leg portions

16 and 18 are connected at adjacent ends by an integral hairpin bend 22; while the

opposite ends

24 and 26 are open. As shown, a number of hairpin tubes 20 are passed through aligned openings in the fins 12 with the outer diameter of the

leg portions

16 and 18 being slightly less than the diameters of the openings 14 in the fins 12.

In order to complete the assembly and provide good heat conduction between the tubing and the fins 12, it is then necessary to expand the individual U-shaped hairpin tubes 20 so as to increase the diameters of the

leg portions

16 and 18. After the expanding operation, U-shaped elbows or return bends 28 are brazed or otherwise fastened to the end of the lower leg 18 of each U-shaped hairpin tube 20 and the upper leg 16 of the next succeeding tube in order to provide a continuous coil providing a serpentine path for a heat exchange medium passing through the heat exchanger.

As was mentioned above, it has been common in the past to expand the U-shaped hairpin tubes 20 by simply pressurizing them. However, due to the fact that the tubes are formed from a single length of tubing which is bent as at 22, the bent portion 22 becomes workhardened; and this work-hardening effect will extend for a distance along the

straight leg portions

16 and 18. As a result, when a hairpin tube of this type is pressurized to expand it, the softer portions in the

straight leg sections

16 and 18 will expand more readily than the work-hardened portions adjacent the bend 22; and if the pressure is increased to the point where the workhardened portion will expand into contact with the inner periphery of the openings 14 in the fins 12, the pressure may burst or rupture the softer portions of the tube. Furthermore, expanding tubes in this manner by pressurizing them is a time-consuming and relatively costly process.

It is also known that a straight length of tubing can be expanded by forcing a ball or projectile through it under the influence of fluid pressure. When an attempt is made to force a sphere through tubes such as those shown in FIG. 1, however, it is found that the sphere becomes lodged within the U-shaped hairpin bend 22. Accordingly, the use of a sphere for expanding hairpin tubes such as those shown in FIG. 1 is an unacceptable procedure.

In accordance with the present invention, U-shaped tubes such as those shown in FIG. 1 are expanded by means of the shaped plug 30 shown in FIG. 3. This plug is driven through the hairpin tube by means of hydraulic pressure in a manner hereinafter described. The shape of the plug 30 is designed to facilitate propelling it through the tube with a minimum of friction while it is performing the expanding function. This shape is further designed to allow the plug to travel through 180 turns in the tube with a minimum of friction and distortion. In FIG. 3, the plug 30 is shown after it has traveled halfway through a 180 hairpin or U-shaped turn. The

ends

32 and 34 of the plug 30 are rounded as shown, with the rounded surfaces being defined by a radius R, having a center point 36, above the center point of the radius of the 180 turn. More precisely, the

ends

32 and 34 comprise truncated spheroids. Intermediate the ends 32 is a concave central portion 40 which, in cross section, is defined by a radius R whose center point coincides with the center point 38 of the [80 bend. However, it is only necessary that the radius R, be at least equal to or greater than that of the radius of the lower, inner peripheral surface of the bend. With this configuration, the plug 30 will engage the inner periphery of the tube at narrow

circular surfaces

42 and 44 as it travels through the straight-

line leg portions

16 and 18. The

surfaces

42 and 44 are formed at the intersections of concave portion 40 with

ends

32 and 34 and-comprise very narrow, essentially flat areas extending around the plug 30. As the plug passes around the 180 bend 22, it will reach a point midway around the bend where the surface defined by the concave portion 40 engages the inner periphery of the tube. In contrast to a sphere, for example, the plug shown in FIG. 3 will pass around the 180 turn. The diameter of the

circular surfaces

42 and 44 is, of course, greater than the inner periphery of the tube before expanding; and the inner diameter of the tube will approach the diameter of the

circular surfaces

42 and 44 after the plug passes therethrough.

FIG. 4A shows the cross section of the tube at the 180 hairpin turn before the plug 30 passes therethrough; whereas FIG. 4B shows the cross-sectional configuration after the plug is forced through the tube. Note that the cross section is somewhat distorted in FIG. 48 after the plug passes therethrough. There is no expansion of the tube in a direction perpendicular to its axis; however there is some deformation of the 180 turn in the plane of the turn. This deformation is caused by a slight eccentricity in the profile of the plug as shown in FIG. 3. Without this novel profiled, it is not possible to force the plug through a 180 turn in the tube. Since it is not intended to attach fins to the U- shaped ends of the coil, the design of the plug is such as to enable it to glide through the hairpin turn 22 without expanding the hairpin significantly. The straight sections of the hairpin tube, however, are expanded uniformly as shown in FIGS. 5A and 5B which represent the cross section of the tubing before and after the plug 30 passes therethrough.

If it is assumed that the plug 30 is traveling around the bend in the direction of arrow 31 in FIG. 3, and that the plug is in the midway position shown in FIG. 3, the straight tube section to the right will have expanded to an inner diameter D2 since the plug has already passed through it. The straight tube section to the left, however, still has its original inner diameter D1. Diameter D2 is substantially equal to the diameters of

circular surfaces

42 and 44. Note that as the plug passes around the bend, the diameters of

surfaces

42 and 44 are not parallel to the radius of the bend, but at angles thereto, which accounts for the fact that the tube is not expanded in the turn in the same manner as it is in the straight tube portions.

A fixture for accurately feeding the plug of FIG. 3 into the open end of the tube and for forcing it through the tube under hydraulic pressure is shown in FIG. 6. The end 26 of a U-shaped tube, for example, is expanded as at 50 and is engaged by serrated gripper jaws 52. The jaws 52, which are circumferentially spaced around the tube end 26, are carried within a barrel 54 having a forward end provided with converging side walls 56. The side walls 56 engage corresponding sloped surfaces on the gripper jaws 52 such that when the jaws 52 are forced forwardly (i.e., to the right in FIG. 6), they converge and grip the end of the tube 26. The plug 30 is carried between the jaws 52 and is adapted to engage a seat 58 on the end of a piston or plunger 60.

Surrounding the piston 60 is a second cylindrical piston 62 having an enlarged forward end 64 provided with a seal 66. The enlarged end 64, in turn, is adapted to slide within cylinder 68 and is provided at its forward end with an annular flange 70 which fits into cooperating notches'in the gripper jaws 52. With this arrangement, forward movement of the piston 62 will also force the jaws 52 forwardly, causing them to converge and grip the enlarged portion 50 of the tube end 26. Hydraulic fluid, such as oil or water under pressure, is adapted to be connected to the fixture of FIG. 6 through fitting 72 from whence it passes into the interior of the hollow cylinder 62.

In the operation of the device of FIG. 6, the jaws 52 will normally be retracted, as will the cylinder 62. With the jaws thus retracted, a plug 30 is inserted into the forward opening of the fixture until it rests against the seat 58. Thereafter, the forward end of the fixture is positioned over the enlarged portion 50 of tube end 26. Following this operation, suitable valving, not shown,

forces fluid under pressure through the fitting 72 and into the interior of piston 62. In so doing, the pressure acting on the enlarged diameter end 64 of piston 62 forces it forwardly, as well as the jaws 52 which engage and grip the tube end 26. Immediately thereafter, fluid pressure acting on the piston 60 drives it forwardly, thereby forcing the plug 30 into the interior of the tube end 26. A seal 73 is provided between an enlarged diameter portion of piston 60 and the hollow interior of piston 62. It will be noted that the piston 60 is provided with a central bore 74. Hence, fluid will pass through this bore 74 and force the plug 30 through the U- shaped tube after the tube end is engaged and the plug inserted. The fluid used to force the plug 30 through the tube drains after the plug passes through the tube and is recycled. The force on the plug may be enough to cause it to travel some distance after it leaves the other open end of the tube. A suitable screen or basket is provided to intercept all plugs so that they can be recycled and used repeatedly.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In the method for expanding the diameter of a hairpin tube having a generally U-shaped bend therein, the steps of inserting a generally cylindrical plug into one end of the tube with the ends of the plug being rounded and the central portion of the plug having an annular recess of concave cross-sectional configuration, and forcing a fluid into one end of said tube after said plug has been inserted therein to thereby force the plug through the hairpin tube including the U-shaped bend therein' 2. The method of claim 1 wherein said plug slides within said tube on annular surfaces defined between said rounded ends and said central portion of concave cross-sectional configuration.

3. The method of claim 1 wherein said central por tion of concave cross-sectional configuration is defined by a radius substantially equal to or greater than the inner radius of said U-shaped bend as expanded.

4. A generally cylindrical plug adapted to be forced under hydraulic pressure through hairpin tubing having a generally U-shaped bend therein, the ends of the plug being rounded and the central portion of the plug having an annular recess of concave cross-sectional configuration which permits said plug to pass around said U- shaped bend.

5. The plug of claim 4 wherein said plug is provided with circular surfaces between its rounded ends and said central portion of concave cross-sectional configuration, said plug sliding on said circular surfaces as it passes through the interior of the tubing.

6. The plug of claim 5 wherein said circular surfaces have diameters greater than the diameter of the tube before expansion.

7. A fixture for inserting a plug into one end of tubing and for forcing it through the tubing under fluid pressure, comprising an outer tubular member carrying within the interior thereof converging jaws adapted to engage and grip the end of said tubing, means for causing said jaws to converge and grip the end of said tubing, piston means carried within said converging jaws and adapted to force a plug also positioned between the jaws into the end of said tubing, an opening extending through said piston means along the axis thereof, and means for forcing fluid under pressure through said opening for forcing said plug through the tubing after it has been forced into said end of the tubing.

8. In the method for expanding a hairpin tube having a U-shaped bend therein, which comprises forcing a plug through the tube with the plug being shaped such that the straight leg portions of the hairpin tube are expanded in diameter, while the diameter of the U- shaped hairpin bend therein remains substantially unchanged.

Claims

1. In the method for expanding the diameter of a hairpin tube having a generally U-shaped bend therein, the steps of inserting a generally cylindrical plug into one end of the tube with the ends of the plug being rounded and the central portion of the plug having an annular recess of concave cross-sectional configuration, and forcing a fluid into one end of said tube after said plug has been inserted therein to thereby force the plug through the hairpin tube including the U-shaped bend therein.

2. The method of claim 1 wherein said plug slides within said tube on annular surfaces defined between said rounded ends and said central portion of concave cross-sectional configuration.

3. The method of claim 1 wherein said central portion of concave cross-sectional configuration is defined by a radius substantially equal to or greater than the inner radius of said U-shaped bend as expanded.

4. A generally cylindrical plug adapted to be forced under hydraulic pressure through hairpin tubing having a generally U-shaped bend therein, the ends of the plug being rounded and the central portion of the plug having an annular recess of concave cross-sectional configuration which permits said plug to pass around said U-shaped bend.

8. In the method for expanding a hairpin tube having a U-shaped bend therein, which comprises forcing a plug through the tube with the plug being shaped such that the straight leg portions of the hairpin tube are expanded in diameter, while the diameter of the U-shaped hairpin bend therein remains substantially unchanged.