10 Methods for Bending Pipes You Should Know

Do you know how many methods can be used to bend pipes?

In this article, we have introduced 10 different methods of bending pipes, and you will benefit a lot after reading them.

Let’s dive into it.

10 Methods for Bending Pipes You Should Know 1

Conventional mandrel free bending of pipes

Conventional mandrel free bending refers to the non filling bending method commonly used in production under room temperature.

The main bending die and principle are shown in the figure below.

It includes stretch bending, press bending, bypass bending, push bending, roll bending processes.

10 Methods for Bending Pipes You Should Know 2

Main bending die and bending principle

Related reading: The Ultimate Guide to Pipe Bender

1. Tensile bending

On the basis of pure bending moment, tube bending can be realized by adding an axial tension, which can generally be divided into tension bending, rotary traction bending, etc.

All stretch bending processes have a common feature.

Because the additional tensile stress counteracts a part of tangential compressive stress on the concave side during pure bending, the wall thickness and wrinkling of the bend can be suppressed, but inevitably, the possibility of wall thickness thinning or even cracking on the convex side of the bend will be increased.

(1) Stretch bending

The following figure shows the simplified working principle of a common stretch bending forming.

During tube bending, the tube blank will pass through three horizontal support rollers and be fixed on the rotary arm connected with the runner with a chuck.

When the rotary arm rotates, it will drive the tube blank to produce bending deformation.

During stretch bending forming, the radius of curvature of the bend can be changed by adjusting the distance between the support roll and the chuck and the axis of the runner.

If a heating device is installed before the tube blank enters the bending zone, such as the medium frequency induction current to preheat the tube blank, its plastic deformation ability can be improved and the heating stretch bending forming can be realized.

10 Methods for Bending Pipes You Should Know 3

Simplified working principle of four common stretch bending forming

(2) Slewing traction bending

As shown in the figure below, for rotary traction bending, each working part of the bending die has a groove that matches the shape of the pipe.

The length of the cross section curve is slightly less than the semicircle of the pipe blank. It is used to clamp or press the pipe blank when bending.

Replacing the rotary bending die can change the bending radius.

The tube blank is pressed against the rotary bending die that can rotate around the axis by the traction die and the clamping block.

The traction die rotates on the circular arc track with a fixed radius with the axis of the rotary bending die, which drives the tube blank to rotate with the rotary bending die under the radial pressure and tangential friction of the clamping block to achieve bending forming.

Due to the increase of the tangential stretch deformation of the tube blank on the convex side of bending, it has the property of stretch bending.

At present, the working principle of most pipe bending equipment and CNC pipe bending machine in production is similar to rotary traction bending.

10 Methods for Bending Pipes You Should Know 4

CNC pipe bender rotary traction bending

2. Compression bending

The compression bending corresponds to the tension bending, which is the tube bending formed by the joint action of additional or derived axial thrust and bending moment.

The additional axial thrust can reduce or counteract the tangential tension on the convex side of the bend, thus preventing the wall thickness of the bend from thinning or cracking.

At the same time, due to the increase of the tangential compression stress on the concave side of the bend, the pipe wall will increase or even wrinkle.

Tube compression bending is mainly divided into: compression bending, rotary compression bending and axial force bending.

(1) Press Bending

As shown in the figure below, pipe bending is similar to sheet V-bending, and can be used to bend and shape medium and small diameter elbow products with straight pipe segments.

The bending die and support roller are provided with a working groove with the same diameter as the pipe and slightly smaller than the semicircle.

The bending die, which is consolidated with the push rod and has a certain bending radius, pushes the pipe blank out, and rolls two bending forming between the support rollers on both sides.

Replacing the bending die can change the bending radius of the pipe, and the size of the two bending internal angles is determined by the pushing out stroke of the bending die.

The tube bending production efficiency is high, and the die adjustment is simple.

The disadvantage is that the bending force applied during bending is concentrated between the two support rollers, and the initial contact between the tube blank and the bending die is easy to distort the travel section, which affects the bending quality.

10 Methods for Bending Pipes You Should Know 5

(2) Rotary compression bending

Rotary compression bending is similar to rotary traction bending in form, but the bending deformation mechanism of pipes is different.

As shown in the figure below, when bending, the sliding die or roller exerts radial pressure on the tube blank while rotating around the axis of the fixed bending die, forcing the straight tube blank to gradually stick to the groove surface of the fixed bending die under the tangential friction to bend and form.

Different from the rotary traction bending of tubes, the bending deformation zone is generated between the sliding die and the fixed bending die, and the surface of the convex tube blank is always subject to the combined action of radial pressure and tangential friction, which to some extent can relatively reduce the tangential tensile deformation of the convex tube blank.

10 Methods for Bending Pipes You Should Know 6

(3) Axial force push bending

As shown in the figure below, the mold consists of two halves centered by the bending plane.

Under the axial thrust of the die, the tube blank is forced to be bent and formed by pressing into the mold cavity after passing through the guide sleeve.

The bending deformation process of the tube blank in the mold is complex. In addition to being subjected to the bending torque, it is also subjected to the axial thrust and the friction force opposite to its running direction.

Different from ordinary bending, the neutral layer of the pipe wall strain may move to the outside of the bending, which alleviates the thinning of the outside wall.

Generally, the relative wall thickness of pipe pushing bending shall be greater than 0.06 to avoid wrinkling or twisting inside the elbow.

In order to reduce friction, it is often necessary to lubricate the tube blank or bending mold cavity.

For thin-walled elbow, push bending with core is usually adopted to avoid buckling and wrinkling.

10 Methods for Bending Pipes You Should Know 7
Schematic Diagram of Pipe Mold Axial Push Bending

3. Other common tube bending methods without mandrel

In addition to the methods shown above, there are detour bending, roll bending, etc.

Most of these methods are difficult to meet the strict bending accuracy and bending quality, so they are usually used to bend pipe fittings with low requirements for shape and size accuracy.

(1) Tube bending forming

Pipe bending is a typical bending process, which is generally divided into manual bending and pipe bender bending.

As shown in the figure below, bypass bending is similar to rotary compression bending, but rolling friction occurs between the working groove of the roller and the surface of the tube blank.

When bending, one end of the tube blank is clamped in the clamping head of the bending die, the side pressing wheel presses it against the groove surface of the bending die and rotates around the bending die, and the clamping head rotates with the tube blank.

10 Methods for Bending Pipes You Should Know 8
Principle of tube bending

(2) Tube roll forming

Tube roll bending is usually used for thick wall tube bending, as shown in the following figure.

Place the tube blank between three or four or more rollers.

Each roller has a groove with an inner diameter slightly larger than the outer diameter of the tube and a section circumference slightly smaller than the semicircle of the tube blank.

Each roller rotates and moves in different directions, so that the tube blank is rolled to form a certain bending shape.

Multi roll bending can improve the bending accuracy of tubes, reduce the distortion of bending cross section, and improve the degree of uniform bending deformation of tubes.

It can sometimes be used for thin-walled tube bending.

10 Methods for Bending Pipes You Should Know 9
Schematic Diagram of Tube Roll Bending Forming

(a) Schematic diagram of asymmetric three roll bending
(b) Schematic diagram of side roll swing four roll bending

Tube filling bending

Tube filling bending, also known as bending with mandrel, is a common bending process for medium and large diameter thin-walled tubes.

In order to reduce or eliminate the defects such as flattening of cross section or collapse and wrinkling of pipe wall when bending such pipes, the method of bending forming by filling various mandrels or fillers into the blank to be bent as supports can be called filling bending.

1. Mandrel bending

Mandrel bending is a bending process that strongly controls the cross section shape of elbows.

The difference between mandrel bending and ordinary mandrel-free bending is that the mandrel is pre placed in the tube blank when bending.

The mandrels or mandrel rod used in actual production are divided into two categories: rigid mandrels and solid flexible mandrels.

The rigid rods include round head mandrel, spoon shaped mandrel and bull horn mandrel.

Flexible mandrels include single section flexible mandrels and multi section flexible mandrels.

10 Methods for Bending Pipes You Should Know 10
Pipe bending diagram with mandrel

2. Solid particle filling push bending forming

Solid particle filling and push bending forming is commonly used to bend thin-walled bending members or elbows with relatively small wall thickness.

The particles used as fillers are gradually changed from traditional quartz sand to resin particles or metal beads.

Before the push bending forming, a certain number of large diameter balls slightly smaller than the outer view of the tube blank are placed in the bending die to prevent the bead filler from slipping, and then fill the tube blank with small diameter beads.

During push bending forming, the slider of the press drives the push bending punch to extrude the bead filler in the tube blank.

Strictly speaking, any solid particle can not be regarded as a continuous medium, and it is difficult to approximate its flow deformation to a constant volume under pressure.

In addition, the hard steel ball may cause defects such as indentation on the inner wall of the elbow.

10 Methods for Bending Pipes You Should Know 11
Schematic Diagram of Pipe Solid Particle Filling and Bending

3. Liquid filling bending

Filling and bending method of pipe with liquid as filler.

At present, the relatively mature processes include pipe punching hydraulic bending and pipe punching and feeding push bending.

Although liquid filling bending has solved some shortcomings of solid particle filling bending, medium sealing is a technological difficulty.

10 Methods for Bending Pipes You Should Know 12
Working principle of tube hydroforming

4. Cure bending with liquid filler

The curable liquid is injected into the tube blank when it is in liquid state.

After the two ends are closed, the liquid substance is solidified and filled and bent as an integral filler.

Solidified filler materials include water, rosin, low melting point alloy and various resins.

Heating bending

10 Methods for Bending Pipes You Should Know 13

Heating bending is a traditional plastic processing method.

Heating is used to soften the tube blank and enhance the plastic deformation ability.

When the shape of pipe components is not suitable for cold bending, heating bending can be considered.

In general, the bending process usually combines heating, bending and cooling.

The commonly used methods are: overall heating filling bending, overall heating immersion bending, local medium frequency induction heating bending, laser bending, etc.

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