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Development of Hydraulic Controlled Bending Technology

for Bent Pipe of Plant without Thickness Reduction

Ken Ichiryu*

*Mechatronics Laboratory, Kikuchi Seisakusho Company

2161-12, Miyama-cho, Hachioji, Tokyo, Japan

(E-mail: ichiryu@kikuchiseisakusho.co.jp)

ABSTRACT

This new pipe bending method was adopted as a strategic fundamental support program of government. In

this program, original bending machine was manufactured using induction heating and tested 100A pipe

bending for obtaining condition of pipe thickness without reduction. In this bending machine, because of

accurate pipe thrust control is required, hydraulic servo control was successfully applied. Until now, we have

been concerned with three dimensional cold bending machine using parallel link head. For large pipe

application such as diameter of 50-60mm, hydraulic servo control was introduced. But more large size

application for chemical, petroleum and power generation plant, hot induction heated bending is required. In

the year of 2000-2008, new hot bending machine was developed by Miyasaka and Sato in Hachioji Branch

Campus of KOGAKUIN University[1]. The bending machine is based on the new concept of without

reduction of pipe thickness by applying axial compression thrust force.

KEYWORDS

Oil-hydraulic servo, Pipe Bending machine, Hot bending, High-frequency heating

OUTLINE OF NEWLY DEVELOPED

BENDING MACHINE

Piping system of chemical, atomic power plant

and so forth are composed of enormous number of

welded pipe and elbow combination.

This study is aimed to replace this welded

elbow-straight pipe combination by new bent pipe

produced by local induction heating under

compressive axial load.

This new bending principle is disclosed in

Japanese patent #2010-131649.

Feature of the patent is to make it possible to

determine neutral position of bending independent of

pipe bending radius.

That is, by this invention, for example, if to assign

neutral position toward the pipe outer or inner

position, total section of pipe becomes under

compression or tension stress, respectively.

Conceptual diagram is shown in Figure 1, where

pipe thrust force is produced by the differential

movement of pulley and carriage controlled by the

velocity difference of pull and push wire rope.

In Figure 2, pipe bent state at induction heated

zone is shown.

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Fig. 1 Principle of new bending method

Fig. 2 Pipe bent state at induction heating zone

compression   load Wf 

compressionload We

annular heating zone 

heating coil 

Induction heating coil

hydraulic actuator for pull 

hydraulic actuator for push 

carriage 

pipe to bend 

pipe clamp 

pulley

Pull wirePush wire for braking

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Applying fW force by pull cylinder and gW

braking force by push cylinder, pipe heated zone is

bent under axial load to create circular profile.

Figure 3 is a model diagram of hydraulic thrust

control system. In Figure 4, detail diagram of pulley

part is shown.

Designating fV as pull relative velocity to the

carriage and gV as push backward velocity to the

carriage, carriage relative velocity or pipe feed

velocity is given as follows.

2gf

p

VVV

(1)

Position where pulley rotational velocity relative to

the pipe is zero, corresponds to the neutral position

of pipe bending.

If defining gf VV / and pulley radius pR ,

pRX / is given as follows,

1

1

pR

X(2)

For accurate control of X position, it is necessary

to determine σ value precisely. So that

electro-hydraulic flow control using servo valve is

attempted for cylinder velocity fV and gV

determination. Velocity diagram including gradation

control is shown in Figure 5.

Fig. 4 Detail of Pulley Part

Fig. 3 Hydraulic Control System Model of Axial Thrust Force

carriage 

Induction heater 

G Cylinder

F CylinderPipe 

X R

Vf

Vg

O Vp

Pipe sensor 

Servo motor 

SERVO VALVE

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Fig. 5 Example of Velocity Diagram

OUTLINE OF EXPERIMENTAL APPARATUS

Bending machine is composed of A) machine base

B) pipe clamp C) cylinder support box, F, G

cylinders and pipe to bend D) pulley that is driven by

wire rope coupled with F, G cylinders E) pulley

support F) 3 axis induction heating coil moving

table.

Incremental position sensor is incorporated in F, G

cylinder rod end.

Bird eye view of bending machine is shown in

Figure 6.

Hydraulic system is consisted of three components,

such as hydraulic cylinder, hydraulic unit and servo

pack as shown in Figure 3.

Main function of hydraulic system is accurate

velocity control of fV and gV of F, G cylinder,

respectively.

Both cylinder specifications are Φ200×Φ300×3000

stroke and 30 ton output force.

Fig. 6 Bird Eye View of Assembled Bending Machine

Coil Velocity Coil Velocity 

Start of Bending  End of Bending Length of Pipe

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Controller is composed of FA-PC/windows attached

to SPX motion controller. Main function is accurate

control of fV and gV .

Controller block diagram is shown in Figure 7.

Three input signals shown in Figure 5 are generated

by four inputs of F, G cylinder position, pipe

position and pulley rotational angle.

High frequency heating device is composed of high

frequency oscillator, transformer and heating coil.

Bending system is classified in base part, bending

main body composed of cylinder and pulley,

transformer table and stroke measurement part, are

shown in Table 1.

Fig. 7 Control block diagram

Bender main

body

Base composed of main frame and end frame L=10350mm, W=2760mm, H=430mm

Carriage two piece bolt structure (3425×1600ｍm) F, G cylinder are clamped in side wall

Pulley Diameter Φ2010 driven by wire coupled with F, G cylinder

Transformer

table

3 axis drive

3 axis motor

X-Y-Z 3 axis movable Z=±80mm Y, Z=±10mm

Z axis: AC servo motor X,Y; AC motor

Position sensing pulse sensor

25μm pitch

F, G cylinder

Stroke 3000mm ERGO JAPAN

F  servo  valve  flow 

command 

G servo valve flow 

command 

Coil movement 

command 

Pipe position 

Digital scale 400mm 

Pulley rotational 

angle 

F cylinder 

tape 

G cylinder 

tape 

 

 

Controll

er 

Display 

Table1 Bending machine specification 

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EXPERIMENT OF STEEL PIPE

At first, experiment of 1.33DR, so called, elbow

bending was conducted using 50KVA high

frequency power source. 1.33 DR means DR /

=1.33, where R: radius of bending of pipe center,

D: diameter of pipe.

90 degree bend of 5 samples was tried. Used pipe

is STPG 370-E material, nominal outer diameter

114.3 mm, nominal thickness 8.6 mm. Bending

radius is 152.4mm. Bending velocity is set as

0.75mm/s.

As shown in Figure 8, thickness decrease of

tension side becomes small by giving strong

compression force to the pipe, putting neutral axis

outward by velocity ratio gf VV / large.

Measured values in Figures 8 and 9 are minimum

thickness and mean thickness of measured domain,

respectively.

Actual bent profile is shown in Figures 10 and 11,

respectively. In the former case, no wrinkles were

observed. But, in the latter case wrinkle was

observed in the outlet domain.

If large compression force is imposed aiming zero

thickness reduction, danger of wrinkle is difficult to

avoid. Therefore, to allow a little reduction of

thickness at the tension side is considered practical.

If we consider pipe thickness variation, no thinning,

reduction-less bending is not always effective.

By the thinning of bent pipe, plant designer is

forced to select pipe thickness of one size up.

Problem is how to avoid this situation. We could

obtain practical solution by the optimum selection

of velocity ratio σ ＝ gf VV / .

Fig. 8 Relationship between velocity ratio

and thickness (minimum value comparison )

s

comparison )

Fig. 9 Relationship between velocity ratio

and thickness (mean value comparison )

Fig. 10 Right angle bent profile ,

Velocity ratio 1.53 thickness variation -2.7%

Velocity ratio Vf/Vg 

Thickness

Variation 

(%) 

Thickness

Variation 

(%) 

decrease of thickness 

Velocity ratio Vf/Vg 

Mean value 

Mean value 

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Fig. 11 Right angle bent profile velocity ratio

1.53 thickness variation -3.0%

CONCLUSIONS

New bending method without thickness reduction

is realized by the joint work of T. Satoh and K.

Miyasaka [1] under the sponsorship of Japanese

government fund. This machine is hot bending

machine of high frequency induction heating.

Performance of axial thrust force control was

confirmed effective to prevent thinning of bent pipe.

By the experiment of 100A pipe, nearly thickness

reduction-less bending was realized.

This bending method is anticipated to contribute to

the large scale plant construction by the economical

pipe usage.

In the end of this paper, I express my sincere thanks

to Y. Ishikura for his cooperation of experiment and

also I. Kikuchi of president of Kikuchi Company for

his support of this development.

REFERENCES

1 K. Miyasaka; Dieless pipe bending by high

frequency induction heating, Journal of Japan society

for technology of Plasticity, 2010, 51-591

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