US20060112558A1

US20060112558A1 - Process of making variable wall thickness tubing

Info

Publication number: US20060112558A1
Application number: US10/999,177
Authority: US
Inventors: Patrick Lorenz; Robert Johnson; Karl Klankowski; Curtis Hoehn; Robert Sugg
Original assignee: CRS Holdings LLC
Current assignee: CRS Holdings LLC
Priority date: 2004-11-29
Filing date: 2004-11-29
Publication date: 2006-06-01

Abstract

A method for manufacturing a tube with varying wall thickness includes the step of drawing a tube over a generally cylindrical mandrel having an outer diameter that fluctuates along the length of the mandrel between a first diameter and a second diameter. The tube is drawn over the mandrel to form a tube wall having a thickness that fluctuates along the length of the tube between a first wall thickness and a second wall thickness. After the tube is drawn over the mandrel, the mandrel is removed from the tube. The outer surface of the tube may be finished to form a substantially uniform outer diameter along the length of the tube.

Description

FIELD OF THE INVENTION

The present invention relates to methods for manufacturing small diameter tubes, and more specifically to methods of forming small diameter tubes having varying wall thicknesses.

BACKGROUND

In the present state of the art, small diameter tubes and hoses are manufactured for use in a variety of industrial and commercial applications, including air conditioners and radiators. Many of these tubes are manufactured with a varying inner diameter to achieve certain properties, including tube flexibility. For example, some manufacturing methods form tubes with an annular profile having multiple corrugations impressed into the tube. Other manufacturing methods form tubes with a single helical corrugation along the length of the tube. These manufacturing methods are capable of varying the inner diameter of tubes, but also create a varying outer diameter on the tube. In addition, these methods typically form tubes with a relatively constant wall thickness. Therefore, present methods for forming tubes leave much to be desired, especially in those applications that require tubes having constant outer diameters and variable wall thicknesses.

SUMMARY OF THE INVENTION

A method for manufacturing a variable wall thickness tube includes the step of drawing a tube over a generally cylindrical mandrel having an outer diameter that fluctuates along the length of the mandrel between a first diameter and a second diameter. The tube is drawn over the mandrel to form a tube wall having a thickness that fluctuates along the length of the tube between a first wall thickness and a second wall thickness. After the tube is drawn over the mandrel, the mandrel is removed from the tube. The outer surface of the tube may be finished to form a substantially uniform outer diameter along the length of the tube.

DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following description will be better understood when read in conjunction with the drawing FIGURE which is a block flow diagram of a process for making a variable-wall thickness tube in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, a method 10 for manufacturing a variable wall thickness tube in accordance with the present invention is shown. The method 10 may be used to manufacture variable wall thickness tubes for use in a variety of applications. For purposes of this description, the method 10 will be described in the context of heat exchanger tubes, with the understanding that the method of the present invention may be used to manufacture tubes for use in a variety of applications.
The method 10 is designed to produce a tube having a substantially uniform outer diameter and a non-uniform inner diameter that fluctuates along the length of the tube. The uniform outer diameter and non-uniform inner diameter provide a tube wall that varies in thickness along the length of the tube. The inner diameter of the tube fluctuates along the length of the tube between a minimum inner diameter and a maximum inner diameter. The tube wall has a maximum wall thickness, or T_max, at those sections where the inner diameter is at its minimum. Conversely, the tube wall has a minimum wall thickness, or T_min, at those sections where the inner diameter is at its maximum. In this arrangement, the tube profile can be divided into a first group of wall sections having wall thickness T_max, and a second group of wall sections having wall thickness T_min. Wall sections having thickness T_minprovide substantial flexibility in the tube wall and permit the tube to flex. The presence of thinner wall sections also reduces the weight of the tube. Wall sections with wall thickness T_maxare more rigid than sections having thickness T_minand substantially prevent the tube from buckling when the tube is flexed.
In addition to providing flexibility without buckling, the non-uniform inner diameter provides beneficial fluid flow and heat transfer characteristics. In straight-walled tubes, pressurized fluid flow tends to be more or less laminar, with fluid remaining generally in the same location relative to the tube cross-section as it passes through the tube. Fluid toward the outer circumference of the tube remains near the outer circumference, and fluid near the center of the tube remains near the center. Since heat passes through the tube wall, the fluid near the outer circumference of the tube has a higher rate of heat transfer than the fluid at the center axis, resulting in a temperature gradient that decreases outwardly from the center of the tube. This temperature gradient often limits the overall transfer of heat from the fluid, since the higher temperature fluid at the center of the tube remains insulated. In tubes with non-uniform inner diameters, the varying inner diameter facilitates turbulent flow characteristics. In turbulent flow, the fluid is continuously mixed, limiting the development of temperature gradients within the fluid. The non-uniform inner diameter also increases the effective surface area available for heat transfer. Moreover, the turbulent flow characteristics provided by the non-uniform inner diameter can provide a scouring effect and limit the accumulation of solids in the tube.
The method 10 can be applied in several ways to form a variable wall thickness tube, and the arrangement of sections having wall thicknesses T_maxand T_minwill vary depending on the desired properties of the tube. The inner contour of the tube can be formed with a variety of geometric configurations, including curved annular projections, flat annular projections, or a combination of curved and flat projections. The projections may be spaced apart at regular intervals, or varying intervals.
The appropriate wall thickness of the tube is dependent on the selected outer diameter of the tube and the physical properties of the material selected in manufacturing the tube. The ratio of wall thickness to outside diameter preferably varies from about 0.03 to 0.10. That is, the ratio of wall thickness to outside diameter is preferably about 0.03 where the tube has a minimum wall thickness, and about 0.10 where the tube has a maximum wall thickness. Tubes with lower or higher ratios may also be used with satisfactory results. The outer diameter of the tube may be selected based on the particular application of the tube. Based on the ratio stated above, a tube having an outside diameter of 0.1 inch preferably has a minimum wall thickness of about 0.003 inches and a maximum wall thickness of about 0.01 inches. More preferably, a tube having an outside diameter of 0.1 inch has a maximum wall thickness of about 0.005 inches.
The relative spacing and dimensions of annular projections along the inner wall of the tube may vary considerably without departing from the scope of the invention. For instance, the axial width of the projections may be a few hundredths of an inch, or several inches.
A shaped mandrel rod is used to form the variable wall-thickness tube. Tubing is drawn through a die and over the shaped mandrel rod to create the variable wall thickness tube. In general, the inner contour of the tube is the inverse of the exterior contour of the shaped mandrel. The mandrel is preferably cylindrical with an outer diameter that fluctuates between a minimum outer diameter and a maximum outer diameter. The mandrel may have a variety of shapes adapted to form a tube wall with a fluctuating inner diameter. For example, to form a series of rounded grooves on the inner surface of the tube, a mandrel is selected having a series of rounded circumferential ridges that extend radially outwardly from the mandrel. Preferably, the mandrel rod is sufficiently long to form multiple tubes in a single draw operation. In particular, the mandrel rod preferably includes multiple segments, with each segment designed to form one tube unit. Once the tubing is drawn over the mandrel rod, individual tube units can be cut from the drawn tubing and trimmed to finished lengths. The mandrel rod segments are spaced apart from one another so that each tube unit is separated by a short distance. In this arrangement, a small amount of excess tubing is provided on each end of the tube units to allow the tube ends to be trimmed to a finished length.
Referring to the drawing, the method 10 is illustrated in block diagram form, with individual steps designated generally by reference numbers 100-900. In step 100, a length of tubing, or “starting tube” is prepared. The tube may be formed from a variety of alloy materials, including but not limited to materials comprising stainless steel, titanium or cobalt-chrome alloy. The tubes may be prepared by bending and welding strips of alloy. Strips having a thickness of between 0.010-0.025 inches have been used successfully in forming tubing. Where seamless tubing is required, the tubes may be prepared by gun drilling round bar stock. The starting tube diameter will vary depending on the desired application. The method steps that follow have been applied to starting tubes having an outer diameter of 0.25 inches.
Once the starting tube is formed, the outer diameter of the tube is reduced in step 200 by cold drawing the tube through a first die. The first die is a round outer die configured to reduce the tube to an intermediate outer diameter. Typically, the tube is prepared for cold drawing by pointing the end of the tube and lubricating the outer surface of the tube. The end of the tube is pointed so that the outer diameter of the tube end is slightly smaller than the diameter of the draw die. In this way, the tube end can be gripped and fed easily into the die. The outer surface of the starting tube is then coated with a suitable coating or lubricant to permit the tube to be passed smoothly through the die, and to prevent damage to the tube caused by friction forces. Once the starting tube is prepared, the tube is cold drawn through a die to decrease the outer diameter to a predetermined intermediate diameter. Following cold drawing, the tube is cleaned with an alkaline cleaner or other suitable cleaning solution to remove the coating or lubricant on the tube.
Once the tube is cold drawn through the die and cleaned, the tube may be annealed, as shown in step 300. Annealing is done to reduce internal stresses created during cold drawing, soften the tube and provide sufficient ductility in the tube material to permit further drawing, if desired. Preferably, the tube is annealed at a temperature of about 1,900° F.-2,250° F. for about 1-2 minutes after drawing the tube.
If the intermediate outer diameter of the tube is not achieved after cold drawing and annealing the tube, the cold drawing step 200 and annealing step 300 may be repeated. Once the intermediate outer diameter is achieved, the tube is cold drawn through a second die in step 400. The second die is a round outer die having a smaller diameter than the first die used in step 200. The tube is drawn through the second die and over a shaped mandrel to shape the inner surface of the tube. Prior to drawing, the tube may be pointed as necessary, and the inner and outer surface may be lubricated with a suitable coating or lubricant to permit the tube to be passed smoothly through the die, and to prevent damage to the tube caused by friction forces. The mandrel is positioned in the tube, and the tube and mandrel are fed through the second die. As the tube and mandrel pass through the second die, the tube wall flows into a shape conforming with the exterior contour of the mandrel.
When the tube and mandrel are passed through the second die, the tube wall flows into the spaces between the outer die and the mandrel to form an inner wall with a shape that is inverse to the exterior of the mandrel. The drawn tube and mandrel combination are then processed to separate the mandrel from the shaped tube in step 500. In the separating step 500, the diameter of the tube wall is expanded to provide clearance between the mandrel and inner tube wall, allowing the mandrel to be removed from the tube. The extent of tube wall expansion varies, but is typically sufficient to increase the inner diameter of the tube so that the minimum inside diameter of the tube exceeds the maximum outer diameter of the mandrel. One method to accomplish this is to run the tube and mandrel combination through a roll type tube straightener. The flexing of the tube and mandrel in the straightening process increases the tube diameter, providing sufficient clearance between the inner surface of the tube and the outer surface of the mandrel to allow extraction or removal of the mandrel from the tube. After the tube and mandrel are separated, the tube is cleaned to remove the coating or lubricant from the inner and outer surfaces of the tube. If desired, the tube is then annealed in step 600 in the same general manner described in step 300.
As the tube is drawn with the mandrel through the second die, the outer surface of the tube may or may not develop ridges or depressions in response to material flow over the surface of the mandrel. If ridges or depressions develop, the tube may be passed through a finishing step to provide a smooth exterior with a uniform outer diameter along the length of the tube. The exterior surface may be finished in a variety of ways. As shown in the drawing, the tube is sink drawn through a third die in step 700 to provide a smooth outer surface. Prior to sink drawing, the tube is pointed as necessary and lubricated. The tube is then passed through the third die. The third die is a round outer die with a diameter slightly smaller than the diameter of the second outer die. The third die is configured to reduce the tube to a desired finished diameter. The tube may be sink drawn in one or more passes to provide the desired finish. After the outer surface is finished and cleaned to remove any coating or lubricant, the tube may be annealed in step 800. The tube may be straightened as necessary and cut transversely to desired lengths in step 900.
In many cases, tube sections with thickness T_minare not rigid enough to withstand compressive forces exerted on the tube by gripping tools and other implements. If the ends of the finished tube have thickness T_min, the ends may buckle or pinch closed when being handled. Therefore, it may be desirable to shape the inner surface of the tube so that the ends of the tube have a larger wall thickness that extends for a short length along each end of the tube. In this way, the ends of the tube will be relatively rigid and resistant to buckling when handled by tools.
The tube may be manufactured in different lengths, and the length of the workpiece during manufacturing will vary depending on many factors, including the desired application of the end product, and the manufacturing equipment used to form the tube. If circumstances permit, the tube is preferably cold drawn in long lengths to form multiple tubes on one length of tubing. By processing tubes in longer lengths, the manufacturer reduces the number of times that the tubing must be pointed, lubricated, drawn and annealed. This can significantly reduce the time and effort expended in producing the desired quantity of finished tubes.
The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the following claims.

Claims

1. A method for manufacturing a variable-wall thickness tube, said method comprising the steps of:

A. forming a generally cylindrical tube;

B. drawing the tube through a first die to reduce the outer diameter of the tube to an intermediate outer diameter;

C. placing the tube over a mandrel having an outer diameter that fluctuates along the length of the mandrel between a first diameter and a second diameter;

D. drawing the tube and mandrel through a second die to form a tube wall having a thickness that fluctuates along the length of the tube between a first wall thickness and a second wall thickness; and

E. removing the mandrel from the tube.

2. The method of claim 1, comprising the step of finishing the outer surface of the tube so that the tube has a substantially uniform outer finished diameter.

3. The method of claim 2, wherein the step of finishing the outer surface comprises the step of tube sinking.

4. The method of claim 2, comprising the steps of straightening and cutting the tube to a predetermined length after the outer surface finishing step.

5. The method of claim 1, wherein the step of forming the generally cylindrical tube comprises the step of bending a metal strip having a pair of side edges to form a cylinder and welding the side edges together.

6. The method of claim 1, wherein the step of forming the generally cylindrical tube comprises the step of drilling a bore through the center of a piece of bar stock.

7. The method of claim 1, comprising the step of annealing the tube after the outer surface finishing step.

8. The method of claim 1, wherein the step of removing the mandrel from the tube comprises the step of running the mandrel and the tube through a roll type tube straightener to provide clearance between the tube and the mandrel and allow extraction of the mandrel from the tubing.

9. A method for manufacturing a plurality of variable wall thickness tubes, said method comprising the steps of:

A. drawing a length of tubing through a first die;

B. placing the tubing over a mandrel rod comprising a variable diameter section and a uniform diameter section abutting said variable diameter section;

C. drawing the tubing and mandrel rod through a second die to form a first tubing section and a second tubing section, said first tubing section having a fluctuating inner diameter, and said second tubing section having a uniform inner diameter;

D. removing the mandrel from the tubing; and

E. cutting the tubing transversely through each of said second tubing sections to form a plurality of tubes.

10. The method of claim 9 comprising the step of finishing the outer surface of the tubing so that the tubing has a substantially uniform outer diameter.

11. The method of claim 10 comprising the step of removing the tubing from the mandrel after drawing the tubing and mandrel through the second die and prior to the finishing step.

12. The method of claim 10, wherein the step of finishing the outer surface comprises the step of sink drawing the tubing.

13. The method of claim 9, wherein the mandrel rod has a second variable diameter section abutting said uniform diameter section, and a second uniform diameter section abutting the second variable diameter section.

14. The method of claim 9, wherein the step of removing the mandrel from the tubing comprises the step of running the mandrel and the tubing through a roll type tube straightener to provide clearance between the tubing and the mandrel and allow extraction of the mandrel from the tubing.

15. A method for manufacturing a plurality of variable wall thickness tubes, said method comprising the steps of:

A. drawing a piece of tubing through a first die to reduce the outer diameter of the tubing to an intermediate outer diameter;

B. placing the tubing over a mandrel rod comprising:

a plurality of first sections having a uniform rod diameter; and

a plurality of second sections arranged alternately with said first sections, each of said second sections having a rod diameter that fluctuates between a first rod diameter and a second rod diameter;

C. drawing the tubing and mandrel rod through a second die to form:

a plurality of first tubing sections, each of said first tubing sections having an inner diameter that fluctuates between a first inner diameter and a second inner diameter that is less than the first inner diameter; and

a plurality of second tubing sections, each of said second tubing sections having a uniform inner diameter that is less than the first inner diameter of the first tubing sections;

D. removing the mandrel from the tubing; and

E. cutting the tubing through each of said second tubing sections to form a plurality of tubes.

16. The method of claim 15 comprising the step of finishing the outer surface of the tubing so that the tubing has a substantially uniform outer diameter.

17. A method for manufacturing a variable-wall thickness tube, said method comprising the steps of:

A. placing a generally cylindrical tube over a mandrel having an outer diameter that fluctuates along the length of the mandrel between a first diameter and a second diameter;

B. drawing the tube and mandrel through a die to form a tube wall having a thickness that fluctuates along the length of the tube between a first wall thickness and a second wall thickness; and

C. removing the mandrel from the tube.

18. The method of claim 17 comprising the step of finishing the outer surface of the tube so that the tube has a substantially uniform outer finished diameter.