US2996100A - Method and apparatus for bending pipe to short radh with minimum thinning of the outer wall thickness at the bend - Google Patents

Description

Aug. 15, 1 1 w. L. NEWHALL ETAL 2,995,100

METHOD AND APPARATUS FOR BENDING PIPE T0 SHORT RADII WITH MINIMUM THINNING OF THE OUTER WALL THICKNESS AT THE BEND 2 Sheets-Sheet 1 Filed April 8, 1957 INVENTOR.

WELROSE L. NEWHALL. SAMUEL B.'FLOYD,JR.

ATTORNEYS.

Aug. 15, 1961 2,996,100

. L. NEWHALL ETAL METHOD AND APPARATUS FOR BENDING PIPE T0 SHORT RADII WITH MINIMUM THINNING OF THE OUTER WALL THICKNESS AT THE BEND 2 Sheets-Sheet 2 Filed April 8, 1957 INVENTOR.

WELROSE L. NEWHALL. SAMUEL B, FLOYD, JR.

MAMLQW ATTORNEYS.

United atesPflwfl i z 996 100 METHOD AND APPArzATUs FOR BENDING PIPE T snonr RADII WITH MINIMUM T G HINNIN gNHE ()UTER WALL THICKNESS AT THE Welrose L. Newhall, Coraopolis, and Samuel B. Floyd, In, Wexford, Pa., assignors to Dravo Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 8, 1957, Ser. No. 651,448

14 Claims. (Cl. 153-40) bent pipe and tubing have introduced increasingly difiicult problems for the manufacturers of piping for the equipment used in the practice of such processes. In order to reduce space requirements of the piping of such equipment, smaller minimum radii of bends are required without reduction of pipe strength due to thinning of the outer wall of the pipe or tube section being bent.

In the practice of the present invention in order to obtain uniformity between bent pipe sections of the same size, accurately machined forming dies of well known types are used and a complete description and mode of operation of such dies are not, therefore, deemed necessary for a complete disclosure of the invention. Such dies are mounted on a suitable bed or machine frame and powered for the purpose'in any well known manner.

The herein described invention may be practiced with such known types of forming dies wherein there is an axial movement of the pipe during bending. The invention is also applicable to both hot and'cold bending of the pipes.

2,996,100 Pat nt 19,61

reduce or entirely eliminate any thinning that might occur due to the bending operation.

These and other objects of the invention will be made apparentas the description proceeds.

Referring now to the drawings, FIG. 1 shows a length of pipe positioned at the start of the bending operation in a conventional type of rotating forming die imparting axial movement to the pipe during bending, a reinforcing strap of the invention applied to the outer face of the pipe section to bebent; i

. FIG. 2 is a view similar to FIG. 1 of the drawings showing the rotating forming and pressure dies in position at the end of the bending operation;

FIG. 3 is a view similar to FIG. 1 showing a modification of the invention wherein the reinforcing strap is clamped at one end to the pipe and at the other end connected with a hydraulic cylinder mounted on the pipe whereby the pipe may be pre-stressed before bending to further reduce thinning of the metal in the outer wall of the pipe at the bend;

FIG. 4 is a view similar to FIG. 3 showing the position of the apparatus at the end of the bending operation;

FIG. 5 of the drawings shows a further modification of the invention of FIG. 3 wherein one end of the pipe is clampedv directly to the bed of machine preventing axial movement of the pipe, and the conventional forming and pressure dies move relative to the pipe which is prestressed by means of a chain bar clamped at opposite ends to the pipe as shown;

FIG. 6 is a view similar to FIG. 5 showing the relative position of the dies and the chain bar at the end of the bending operation; i

FIG. 7 shows a still further modification of the invention wherein the pipe section to be bent is in fixed relation to .the bed of the machine, a flexible mandrel is mounted within the pipe for pre-stressing same before bending,

As it is well known in the art that when a given section of pipe is bent, the length of the pipe on the inner side of the bend is shorter than the length of the pipe at the outer side of the bend and the neutral axis of the bent section has moved from a position mid-way between the Walls of the pipe to a new position closer to the inner side of the bend and has thinned or stretched at the outer walls of the pipe at the bend has thickened at the inner side of the bend and has thinner or stretched at the outer side of the bend. It is also 'well known that the stretching or elongation of the metal inthe outer side of the bend is not uniform, in that it is greater at an intermediate portion than at the beginning and at the end of the bent portion. In the present invention we are concerned with reduction of the inside radius of the pipe bend without exceeding a maximum thinning of the wall at the outside diameter of the bend greater than that permitted by standard mill tolerances for the unbent pipe.

It is one of the objects of this invention to provide a process and apparatus for bending pipe and tubing to minimum radii substantially smaller than is now possible form of mandrel which is usable with both hot and cold bending of the tubing or pipe.

A further object of the invention is to combine with the aforementioned standard type of forming dies, suitable apparatus imposing compressive forces on the wall of the pipe at the outer face of the pipe bend in order to p 'such'stress being applied by any suitable means such as a hydraulic cylinder clamped to the. bed of the machine as shown and the rotating die with supporting shoe. and pressure die moving relative to the pipe in forming the bend;

FIG. 8' is a view similar to FIG. 7 of the drawings showing the position of the apparatus at the conclusion of the bending operation.

Referring now in detail to FIGS. 1 and 2 of the drawings, the arrangement is generally that of a pipe bending machine having a bed portion (not shown) upon which are mounted a rotating forming die 2, a clamping die 3, support shoe 4 and pressure die 5. The forming die has a peripheral substantially semi-circular face with the portion 6 thereof of an inside radius equal to that of the radius of the inside of the pipe bend to be formed. The pipe 7 to be bent is received within the recessed semicircular face of the forming die in the usual manner. The forming die 2 rotates on the fixed pivot 1 and is power driven in any well known manner. The support shoe 4 is disposed, upon said bed portion, forwardly of the rotating die 2 and has a semi-circular recessed face for reception of the pipe 7. The support shoe is normally mounted in fixed alignment with the bearing face of the rotating die 2 for engagement with and support of the pipe 7 as it moves past the shoe. The clamping die 3 is mounted in any suitable manner upon the rotating forming die 2 for rotation therewith and for movement into and out of clamping engagement with the pipe 7. The clamping die 3 likewise has an arcuate face 9 for engagement with the pipe 7 in clamping it to the forming die face 10 so that the forming die, clamping die and pipe move as a unit about the shaft 1 of the forming die. The pressure die 5 is mounted on the bed of the machine in any suitable manner for movement towards and away from bearing engagement with the pipe 7. pressure 'and die.

die also has a recessed arcuate face 11 providing bearing support for the pipe moving therepast in the usual manner. Mounted on the machine forwardly of the support shoe 4 and pressure die 5 may be a pair of rolls 12 and 14, the roll 12 being usually mounted on a fixed shaft 13 and the roll 14 mounted on a shaft 15 adapted for moveinent towards and away from the pipe 7. The rolls .12 and 14 are power driven and rotate in a maner which, when clamping the pipe between them, advance the pipe towards the support shoe 4 and pressure die 5 to over come the frictional resistance thereof. The constructions so far described with regard to FIGS. 1 and 2 constitute a well known form of pipe bending machine.

Within the pipe 7 may be mounted a suitable mandrel having a head portion 16, a stem portion 17 and an end portion 18 for attachment to any suitable support of the machine bed (not shown). The mandrel is used priinarily for bending thin Wall sections of pipe and tubing and usually is not necessary for heavier walled pipe and tubing. Where it is desired to heat the pipe before bending, as hereinafter described, a heating means may be substituted for or combined with the mandrel. One form of means for adding mass to the metal of the pipe wall at the outer face of the bend is shown as a strap 19 mounted on the pipe 7 and positioned to overlie the outer face of the pipe. Such a strap is preferably formed of metal having a greater tensile strength than the metal of the pipe and may be secured thereto by any suitable means such as the welds 20 at opposite ends of the strap. The strap is in continuous engagement with the adjacent wall of the pipe and bends with the pipe as it passes through pressure die 5 in bending about the rotating forming die 2. Instead of a strap, a bar, rod or other section may be used and may be clamped or otherwise positioned on the pipe. The pressure die 5 has a suitable opening 21 formed therein to receive the strap 19 and the clamping die likewise has a suitable opening 22 therein for the same purpose.

Referring now in detail to FIGS. 3 and 4 of the drawings wherein is shown a modification of the invention, the apparatus again comprises a suitable frame for a pipe bending machine (not shown), upon which is mounted a rotating forming die 50 upon a shaft 51 secured in a suitable manner to said machine frame. A clamping die 52 is suitably mounted on the forming die 50 for clamping engagement with the pipe. A support shoe 53 and a pressure die 54 are mounted on the machine forwardly of the forming die. The structure so far described is the same as in FIG. 1. A pipe 55 is engaged by the forming 57 disposed above and below the pipe. These drive rolls "are for the purpose of advancing the pipe through the support shoe and pressure die under suitable compression to overcome the frictional resistance of said shoe Within the pipe may be mounted a suitable mandrel comprising a head portion 58, stem portion 59 and suitably supported on the machine frame aforesaid for the intended purpose.

Mounted on one end of the pipe 55, forwardly of the drive rolls, a sufficient distance to prevent engagement therewith during formation of the required pipe bend,

'is a suitable hydraulic cylinder 60 connected to the pipe in any suitable manner as by the clamp supporting bracket 61. Within the cylinder 60 is a suitable piston head (not shown) and suitable hydraulic connections may be applied to the cylinder for providing reciprocal movement to the piston head within the cylinder in a well known manner. Secured to the piston head and extending through the cylinder is a piston stem 62. The piston stem of the strap 64 is secured to the pipe in any suitable manner. As here shown the strap is clamped to the pipe by the clamping die 52 and the strap provided with an offset end portion 65 preventing relative movement between the strap and clamp. Suitable guides 66 and 67 may be mounted on the machine frame. The guide 66 is positioned to maintain the strap 64 in engagement with the pipe 55 and the guide 67 is positioned to maintain the end of the strap in axial alignment with the cylinder piston stem 62. The strap 64 is preferably formed of a high tensile strength material which bends with the pipe during the formation of the pipe bend. The strap 64 is positioned to be on the outer face of the pipe bend and through suitable manipulation of the pressure within the hydraulic cylinder may be tensioned so as to pre-stress the pipe before bending. The cylinder, through suitably controlled pressure, will maintain any desired load on the strap 64 during the bending operation. It will be obvious that a chain type strap such as shown in FIG. 5 of the drawings may be substituted for the tint bar strap here shown. By reason of the strap 64 being mounted wholly upon the pipe and moving therewith no additional stresses are imposed upon the drive roll 56 when advancing the pipe 54 against the frictional resistance of shoe 53 and die 54 during rotation of the forming die 50.

Referring now in detail to FIGS. 5 and 6 of the draw ing there is shown a further modification of the invention of FIGURE 1, wherein there is no axial movement of the pipe relative to the bed of the bending machine during formation of the desired pipe bend. As is shown in FIGS. 5 and 6 one end the pipe is secured to the bed of the bending machine (not shown) by means of the adjustable clamp 101. The rotating forming die 102 is pivotally mounted upon the bed of said machine by means of a pivot 103 and is power driven in any well known manner. The support shoe 104 is rigidly mounted on the rotating forming die 102 and the pressure die 105 is adjustably mounted on the rotating forming die 102 all for rotation therewith. The forming die 102, support shoe 104 and pressure die 105 have suitable surfaces thereon, as previously described, for receiving the pipe and engaging therewith to provide movement between said members and the pipe during formation of the bend.

The means for adding mass to the outer face of the pipe bend is shown herein as a chain bar comprising the end bar portion 107. intermediate connected link portion 108 and a second end bar portion 109. The adjustable clamp 101 has a suitable opening therein for reception of the end bar portion 107 and may be retained therein by any suitable means such as the offset end portion 110. The opposite end of the chain bar portion 109 may be secured to the adjacent end of the pipe by any suitable means such as the adjustable clamp 111 having a depending hook portion 112 for engagement with the eye 113 formed on the end of the bar portion 109. The portions 107 and 109 lie against straight portions of the pipe and the flexible connected link portion 108 overlies the length of the pipe forming the outer face of the bend. The pressure shoe 105, like the adjustable clamp 101, has a suitable opening therein for reception of the portion 109 of the chain bar passing therethrough and aids in holding the portion 109 and portion 108 in engagement with the pipe during the bending operation. It will be obvious that instead of the chain bar strap as shown a steel strap as in FIG. 3, or any other suitable section may be used.

In FIGS. 5 and 6 of the drawings the support shoe 104 and the pressure die 105 are shown as being mounted for concurrent rotation with the forming die 102 and are indicated as being adapted for inclusion of heating means. Such heating means are described in greater detail in the description of FIGS. 7 and 8.

Referring now to FIGS. 7 and 8 of the drawings, a further modification of the invention is shown wherein thinning of the outer pipe wall during bending is restricted by maintaining an axial compressive fforce'on the bending pipe by means disposed within the pipe instead of externally as in the previous figures. A pipe 155 is held in fixed relation on the bed of the pipe bending machine by means of an adjustable clamp 150. A rotatable forming die 151 is mounted on advance of the clamp 150 by means of the shaft 152 secured to the bed (not shown) of the forming machine and is power driven as in FIGS. 1 to 6. Rigidly mounted on a forming die 151 is the support shoe 153. A pressure die 154 is adjustably mounted on the die 151 for concurrent rotation therewith. Disposed within the pipe 155 is a flexible mandrel 156 having a fixed head portion disposed in advance of the support shoe and pressure die. Such head portion of the mandrel comprises the two shoes 157 disposed in contact with the inner walls of the pipe and provided with wedge shaped opposing faces. A member 158 connects the shoes 157 and is provided with end faces corresponding to the slope of the adjacent faces of the member 157. Member 158 is movable longitudinally of members 157 to spread and maintain them in frictional engagement with the adjacent inner walls of the pipe 155. Connected with the mandrel head portion 158 and extending rearwardly therefrom is a steam portion' 159 which extends through the end of the pipe 155. Such extension is provided with a threaded end portion 160 for connection through the coupling member 161, with the piston rod 162 of the hydraulic cylinder 163. The piston rod 162 is connected to a piston (not shown) mounted within cylinder 163, and said cylinder 163 is provided with suitable hydraulic connections to effect reciprocal movement of the piston within the cylinder. Intermediate the ends of mandrel shaft 159 and extending for at least that portion of the length of the pipe '155 in which the bend is to be formed, are a plurality of spaced members 164 provided with arcuate peripheral faces and pivotally connected together by the link members 165 so as to conform to the pipe as it bends and prevents crushing of the walls thereof. By means of suitable pressure imposed within cylinder 163 upon its contained piston, the piston stem 162 may be retracted within the cylinder imposing a compressive stress in the walls of the pipe which subtracts from the tension stresses imposed during bending in the member forming the outer wall to resist stretching or thinning of the pipe in this area. Obviously therefore, in order to provide sufficient length of bending pipe in the outer wall of the pipe bend additional thick- (wing of the pipe at the. inner side of the bend results and in order to prevent wrinkling or crushing at said inside of the bend the use of the mandrel is very desirable. Due to such greater thickening of the pipe wall on the inner face of the bend it is desirable to provide heat on the inner face of the bend to assist in the upsetting of the metal in that area. One method of providing heat is to mount an oxy-fuel burner in the portion 166 of the support shoe and extend suitable fuel connection lines thereto. To assist in the escape of gases, ports 167 and 168 may be formed in the support shoe and pressure die respectively. By reason of the intense heat provided by an oxy-fuel burner, the width of the flame measured longitudinally of the pipe need only be approximately the thickness of the adjacent pipe wall before bending and the flame be applied to about one-half the periphery of the pipe at the inner face of the bend. Preferably a water cooling device should be mounted rearwardly of the burner for chilling each increment of pipe after it is bent and the heater moves to an adjacent increment. One such method of applying a heating and water cooling burner is shown in Linden et a1. Patent 2,433,055. Obviously any desired modification can be made in the construction of the burner of the patent for adapting it to the present form of apparatus. A modified form of induction heater may also be provided if it is desirable to provide electrical heating of the bending pipe. As shown in FIGS of the drawing, a right angled bend has been formed in the pipe, obviously by suitablyproportioning the apparatus this bend could readily be extended to or more.

Referring now to the operation of the apparatus as shown in FIGS. 1 to 6 inclusive it will be noted that three different methods or means are disclosed for adding mass to the outer surface of the pipe at the outer side of the bend, namely the strap of FIG. 1, the flexible chain bar of FIG. 5 and the strap of FIG. 3 which may be maintained in uniform or variable tension throughout the entire pipe bending operation. It will be readily apparent that the chain bar of FIG. 5 may be substituted for the straps of FIGS. 1 and 3 and that either the strap or chain bar can be of any desired cross section.

Referring now to FIGS. 1 and 2, it will be noted that the strap 19 overlying the outer face of the pipe wall forming the outer surface of the bend is temporarily secured thereto as by the welds 20 and is maintained in contact with the adjacent wall of the pipe between the clamping die 3 and the pressure die 5 throughout the bending operation. The resultant effect is to add mass to the outer wall of the pipe causing the neutral bending axis of the cross section through the pipe at the strap 19 to move in a direction towards the strap 19 or the adjacent wall of the pipe. In cold bending of the pipe 7 the strap 19, if of metal having the same tensile strength as that of the metal in the pipe and being under greater stress than the adjacent wall of the pipe, would tend to stretch or thin more than the metal of the adjacent wall of the pipe thereby reducing thinning in the pipe wall in the area in contact with the strap 19. When however the strap 19 is made of metal having a higher tensile strength than the metal of the pipe, the strap 19 because of its greater yield strength further restricts the thinning or stretching of the adjacent outer pipe wall to an extent represented by its increased yield strength. The drive rolls 12 and 1 4, when powered to impose suflicient additional compression on the pipe 7 to overcome the frictional resistance of the pipe and strap moving through the pressure die and support shoe, further reduces tension in the outer wall of the pipe and-correspondingly restricting thinning of the metal therein. When however heat is applied to the inner surfaces of the bend, requiring less force to upset the metal therein, correspondingly less tension is placed on the outer bending Wall of the pipe resulting in a reduction of the thinning of the metal therein.

Referring now to FIG. 3 of the drawing, the strap 64 therein comprises an improvement over the strap 19 of FIG. 1 in that the strap 64 may be initially tensioned prior to bending so as to initially place the adjacent wall of the pipe in compression thereby better enabling it to resist stretching with corresponding less thinning during the bending operation. By means of suitable back pressure in the cylinder 60 of FIG. 3 any stretching of the strap 64 can be taken up by the cylinder so as to maintain a uniform tension on the pipe. Recognizing however that the greatest thinning stress is imposed upon that portion of the outer wall of the pipe which is intermediate the ends of the length of the bend, the pressure in the hydraulic cylinder 60 can be varied so as to increase the tension on the strap 64 during formation of the intermediate portion of the bend thereby further resisting thinning of the metal therein. As in FIG. 1, the metal of the pipe wall at the inner face of the .bend is constantly thickening during the bending operation and any resistance in the metal to this action correspondingly increases the stress in the outer wall of the pipe. Therefore, the greatest benefits are obtained from the apparatus when the inner face of the pipe bend is heated by means mentioned heretofore thereby avoiding wrinkling on the inner face of the bend and facilitating upsetting of the metal in this area of the bend. The maximum effectiveness of the apparatus in FIG. 3, as in FIG. 1, during either hot or cold bending is obtained where the metal forming strap 64 has a greater tensile strength than the metal of the adjacent pipe wall.

The operation of the apparatus shown in FIG. is similar to that shown in FIGS. 1 and 3 and differs therefrom by reason of the chain connected intermediate portion of the chain bar secured to the outer face of the bending pipe. Whether the chain bar is attached to the pipe as shown in FIG. 5 or is tensioned as in FIG. 3, the hinged intermediate portion of the chain bar readily conforms to the bending pipe and the stresses in this portion of the chain bar are restricted to {primarily straight tension such as would result through force applied by the cylinder of FIG. 3 rather than combined bending and tension stresses.

Referring now to the apparatus disclosed in FIGS. 7 and 8 of the drawings, the mandrel is placed in tension by the force applied through cylinder 163 causing corresponding compressive forces in both walls or pipe or tube 155 which are resisted by clamp f) securing the pipe to the bed or frame of the bending machine. This method varies from that disclosed in FIGS. 1 to 6 inclusive wherein a force is applied over only one side of the pipe. The forces applied to the pipe in FIGS. 7 and 8 may be constant or varied as those applied to the pipe by the apparatus of FIGS. 3 and 4. In any event the compressive force applied to the inner wall of the heated pipe aids in upsetting the metal of the pipe wall during bending. Correspondingly the applied forces on the outer wall of the bend resist thinning of the metal therein as aforesaid. The use of hydraulic instead of pneumatic pressure is preferred due to the uniformity of force obtained. It will be obvious however that other means for tensioning the strap in FIG. 3 or the mandrel shank 159 in FIG. 7 may be utilized, such as mechanical means instead of hydraulic means.

In the case of FIGS. 1 to 6 inclusive the mass added to the outer wall of the pipe is of course temporary and is intended to be removed after the pipe bend is formed. The obvious advantages of the chain bar over the solid bar are readily apparent in that no permanent deformation of the chain bar takes place during the bending operation as takes place in the strap of FIGS. 1 and 3. It will be equally obvious that a mandrel may be used with FIGS. 1 to 6 inclusive where desired and heat may be concurrently applied or not as desired.

Having thus described the invention and indicating several modifications of apparatus suitable in practicing the method of pipe bending which is the subject of the invention it will be obvious to those skilled in the art that many changes in the exact details of construction of the apparatus will suggest themselves which may be made without departing from the spirit of the invention as disclosed. For these reasons it is to be understood that the exact details of construction disclosed are for the purposes of illustration and not limitation except as made necessary by the scope of the appended claims.

We claim:

I. A method of restricting thinning of the metal in the outer wall of a hollow pipe during formation of a bend therein comprising the steps of applying against the outer face of the outer Wall of the pipe a temporary metal section of greater tensile strength than the metal of the pipe to provide the outer face of the bend to be formed, fixing opposite ends of the metal section to the pipe, simultaneously and progressively bending the pipe and the added metal section over a forming surface from one end of the area to be bent to the other while maintaining compressive forces parallel to the axis on the inner and outer faces of the bending pipe and tension parallel to the axis in the outer face of the bending outer metal section.

2. The method as defined in claim 1 wherein heat is applied to the face of the inner pipe bend during such progressive bending thereof.

3. The method as defined in claim 1 wherein the added metal section is stressed during bending to impose added compressive forces on the outer wall of the bending pipe in contact with said metd section.

4. The method as defined in claim 3 wherein the stress applied to the added metal section is variable during the bending operation.

5. A method of restricting thinning of the metal in the outer wall of the hollow pipe during formation of a bend therein, comprising imposing a temporary addi tional metal mass upon the outer surface of the outer wall of the pipe over the entire area to be bent, retain-i ing said metal mass in contact with and fixed relation to the pipe, simultaneously and progressively fonning a bend in the pipe and said mass by pulling same over a forming surface, then removing the said additional metal mass from the pipe. V

6. The method as defined in claim 5 wherein said metal mass is stressed to place the adjacent pipe wall in compression during bending.

7. The method as defined in claim 6 wherein the inner face of the forming bend of the pipe is heated to reduce tension stresses imposed in the pipe wall adjacent the outer face of the bending pipe.

8. Apparatus for forming pipe bends including a rotating forming die for engaging the pipe during bending, a pressure die slidably engaging the pipe of the bend in advance of the forming die and a clamp for fixed engagement with the rotating die and the pipe adjacent one end thereof, the combination of a metal member positioned upon and in continuous contact with the wall of the pipe disposed at the outer side of the pipe bend when forming. one end of said member being secured between said clamp and the adjacent pipe, and means connecting the opposite end of the metal member to the said pipe wall in advance of the pressure die.

9. Apparatus as defined in claim 8 wherein the said means connecting the metal member with the pipe is adjustable for imposing tension stresses in the member before the pipe bend is formed.

10. Apparatus as defined in claim 9 wherein the said means comprises a fluid pressure actuated cylinder removably mounted on the pipe, a reciprocating piston in said cylinder overlying said pipe outer wall, a connection between the adjacent ends of the metal member and said piston, and guide means associated with said metal member and pipe retaining said member in alignment with said pipe wall.

11. Apparatus as defined in claim 8 wherein the tensile strength of the said metal member is greater than that of the bending pipe wall in contact therewith.

12. Apparatus as defined in claim 8 wherein the metal member is a chain bar comprising a connected link portion engaged with the pipe in the area forming the bend and rigid end portions connected with the pipe.

13. Apparatus as defined in claim 8 wherein the pressure die has associated therewith means for heating the pipe at the inner side of the bend in the area first engaging the rotating forming die.

14. A method of bending pipe comprising the steps of, supporting the pipe in fixed position adjacent one end of the area thereof to be bent, applying a compresive force to the entire internal area of the pipe to be bent, maintaining said applied force while bending the pipe over a rotating bend forming surface moving relative thereto, and supporting the inner pipe walls to maintain a constant inside diameter in the bending pipe.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENTS 328,868 Great Britain May 8, 1930 1 0 11 h M 1 1932 658,010 rm y r- 1938 197; 045 K358033113? s f zs, 1934 545,454 Great Britain M y 7 942 2:433:055 Linden Dec. 23, 1947 729,216 Germany 11, 1942 2 24, Horwitz 1 1954 5 638,150 Great Britain -M y 1950' Brueckner 3, FOREIGN PATENTS American Machinist, reprint dated July 24, 1950, 2,153 Great Britain I an. 28, 1907 McGraw-Hill Publishing Company 1110., New York, NY.

380,094 Germany Sept. 1, 1923 10 page 2, FIG. 4.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2, 996 100 August 15, 1961 Welrose L. Newhall et aln It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column l line 43. for "bend and has thinned or stretched at the outer" read bend. As a consequence the metal in the 3 line 45 for "thinner" read thinned column 4 line 31 after "end" insert of column 5 line 24L for "steam" read stem ERNEST W. SWIDER Signed and sealed this 26th day of December 1961.

(SEAL) Attest:

DAVID L. LADD Commissioner of Patents Attesting Officer USCOMM-DC-

Images