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Rotary forming for the straightening of tubing
Journalof
ELSEVIER
Rotary Ken-ichi
Journal o f Materials Processing Technology 48 (1995) 135-141
Forming Kawai,
Hakoto
for
the
Satake,
Straightening Yukio
Inoue
and
of Haruhiko
Materials Processing Technology
Tubing Sugita
Department of Hechanical Engineering and Haterials Science, Faculty of Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama, 240 Japan
A new straighterfing method using rotary forming with three rollers is proposed. A h e l i c a l r o l l i n g mill w i t h t h r e e r o l l e r s i s u s e d i n t h e e x p e r i m e n t . The prebend aluminum seamless pipes are straightened under various working conditions. It is clarified that an attainable straightening ratio of the product can be determined by the pitch and the nominal reduction in r a d i u s , a n d t h e r e e x i s t s a minimum l e n g t h of r o l l e r s to g u a r a n t e e the selected straightening ratio. The curved shape can be straightened u p t o 90% under optimum working conditions. It is also clarified that there exists an maximum p i t c h to g u a r a n t e e a c e r t a i n s t r a i g h t e n i n g ratio and the axial feed rate can be selected arbitrarily in practice.
1. INTRODUCTION S e a m l e s s p i p e s , s u c h a s oil c o u n try tubular goods, are mainly m a n u f a c t u r e d b y s e a m l e s s p i p e mills s u c h a s p l u g mill a n d m a n d r e l rail/. A solid round billet is uniformly heated in a furnace and pierced by a piercer. Then a hollow shell is roll-reduced in outside diameter and wall t h i c k n e s s . The rolled tubing is simultaneously burnished inside and outside by a reeler and t h e n s i z e d i n t h e s i z i n g roll. A common fault in manufacturing of seamless pipes is that the product is not straight. The curved tubing is effectively straightened by a rotary straightener but the curved s h a p e n e a r b o t h e n d s of t u b i n g is not corrected completely by the straightener [1]. In order to reduce the cropping l o s s of b o t h e n d s of t u b i n g it is important to develop a straightening method for the curved shape
near both ends of tubing. In the present paper, the authors propose a straightening method using rotary forming with three rollers.
2. EXPERIMENTAL PROCEDURE In order to straighten the curved s h a p e n e a r t h e e n d of t u b i n g , a h e l i c a l r o l l i n g mill w i t h t h r e e r o l l e r s , d e s i g n e d b y Yokai e t al. [2], was used in the experiment. In the experiment without feed angle, the axes of rollers are parallel t o t h e c e n t e r line of t h e b l a n k holding axis and the product is pulled forward with a constant velocity by a pulling mechanism. On t h e o t h e r h a n d , m t h e e x p e r i ment with a certain feed angle, the tubing is fed automatically in the axial direction of the product due to the contact between rollers and the tubing. It is necessary to prepare a mandrel or guide rollers
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 6 4 3 - F
136
K. Kawai et al. / Journal o f Materials Processing Technology 48 (1995) 135-141
Roller aoOre,
(a) Front
(b)
Side
view
view
Fig. i Schematic illustration of straightening using a helical rolling mill w i t h t h r e e r o l l e r s . in front of the product in order to maintain a steady feed of the tubing in the latter case. Schematic i l l u s t r a t i o n s of the straightening of tubing using rotary forming with three r o l l e r s a r e s h o w n i n F i g . 1, for the latter case. The curved shape near both ends of seamless pipe appears during piercing process but it is not straightened completely by a rotary straightener. Since the straightening method proposed by the present authors is a rotary forming process with the reduction in radius of the product by three rollers, the outer diameter of the p i p e will r e d u c e slightly and the roundness of the product will g r o w worse. This process should be inserted after piercing and before sizing in the present manufacturing l i n e o f s e a m l e s s p i p e s , t h e n i t will be a hot forming process. The material used in the experi-
ment was commercially aluminum seamless pipe, JIS All00-TD, with t h e i n i t i a l o u t e r d i a m e t e r o f 35 ram, wall thickness o f i mm a n d l e n g t h o f 500 mm. T h e o u t e r d i a m e t e r a n d the wall thickness of s e a m l e s s p i p e used in the experiment was selected as about 1/10 size of the actual large diameter seamless pipes whose curved shape near both ends causes trouble in industry. Assuming the Swift-type stress-strain relationship for this material, the flow s t r e s s in the uniaxiai tension is given by (~ = 136(0.002 + E p) 0.0493 MPa. Before the straightening experiment, pipe specimen was so bent as to give a curved shape whose radius of curvature i s e i t h e r 60 mm o r 180 ram. The straightening experiment was carried out at room temperature without any lubricant after degreasing the surfaces of s p e c i m e n and rollers. While the nominal reduction in radius was defined by t h e o u t e r d i a m e t e r of s p e c i m e n a n d the gap of rollers, the actual reduction in radius measured from outer diameters before and after straightening, except for the experiment by convex rollers, was less than 0.25% at the most. Three types of roller, flat roller, concave roller and convex roller were used in the experiment. Since it was clarified that the longer roller gives the higher straightening ratio in the preliminary straightening experiment without feed angle, the l e n g t h of r o l l e r s w a s d e t e r m i n e d a s to be maximum supported by the roller-holding attachment. The length and the outer diameter of f l a t r o l l e r a r e 72 mm a n d 60 ram. The radii of curvature of c o n c a v e r o l l e r a n d c o n v e x r o l l e r a r e 675 mm a n d 69.8 ram, r e s p e c t i v e l y . The straightening ratio is defined
K. Kawai et aL / Journal of Materials Processing Technology 48 (1995) 135-141
b y (ho - h)/ho, where ho a n d h are values of b e n d displacement before and after straightening, respectively. The pitch, which plays an important role in rotary forming, is defined as the feed in the axial direction per revolution of specimen. Three or more specimens were used for every working condition in the experiment.
3. R E S U L T S
AND DISCUSSION
3.1. Straightening without feed angle In order to investigate the basic characteristics of the straightening using rotary forming, the straightening experiment without feed angle was carried using flat rollers. In this experiment, the product was pulled forward at a constant velocity and various pitches were realized by the combination of rotationai s p e e d of rollers and pulling velocity of the product. 100
I
80 o
60 ,~
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~ . --.
40
Prebend o 3ram • 6ram ""EF ....................
-~,
I
Bend radius: 60 mm Ruduction: 4% Fiat r 0 ] ] e r
[]
20
~o 1'0
2'0 3'0 Pitch (mm/rev)
4'0 ' 50
Fig. 2 Relationship between t h e straightening ratio and the pitch in the straightening method without feed angle.
137
The relationship between the straightening ratio and the pitch at 4% n o m i n a l r e d u c t i o n i n r a d i u s i s s h o w n i n F i g . 2. The pitch is an important working condition in the proposed straightening method as well as in other rotary forming, and smaller pitch than 5 mm/rev should be selected in order to obtain high straightening ratio. While t h e value of straightening ratio varies dependent upon the a m o u n t o f p r e b e n d , i t may b e c o n sidered that the qualitative relationship between the straightening ratio and the pitch is independent upon the amount of prebend. The amount of prebend was selected as 6 mm i n t h e e x p e r i m e n t w i t h f e e d angle in the following section, b a s e d on t h i s r e s u l t s h o w n in Fig. 2. 3.2. S t r a i g h t e n i n g w i t h f e e d a n g l e The straightening method without feed angle described in the preceding section needs the feeding m e c h a n i s m in t h e axial d i r e c t i o n of tubing. When we observe the deforming zone by rotary compression due to rollers, the top end of tubing needs a feeding mechanism like extrusion and the bottom end of tubing needs one like drawing. Since the tubing is rotating continuously during process, the feeding mechanism may need special devices. If r o l l e r s can be d r i v e n in planetary motion, the tubing does not need rotation round its own axis but the feeding mechanism should be introduced in this case. On t h e o t h e r h a n d , t h e t u b i n g c a n be f e d a u t o m a t i c a l l y in the straightening m e t h o d with feed angle. Thus, the straightening method with feed angle may be introduced easily in the present manufacturing line of seamless pipes.
K. Kawai et aL / Journal of Materials Processing Technology 48 (1995) 135-141
138
(a) Rotational speed of rollers It is clarified that the pitch plays an important role in the straightening of tubhlg from Fig. 2, but there are numerous working conditions to realize the same pitch. Using the feed angle, the pitch, that is axial feed of the product per revolution of tubing, in mm/rev, is determined uniquely by the feed angle but the axial feed rate of product, in ram/s, can be controlled arbitrarily by the rotational speed of rollers. When the working conditions except for the rotational speed of rollers were fixed, the straightening ratio was independent upon the rotational s p e e d of rollers, as s h o w n in Fig. 3. This result implies that a constant straightening ratio can be attained w h e n the value of pitch is controlled b y adjusting the feed angle. T h e axiai feed rate of the product, in ram/s, can be selected arbitrarily in accordance with the 100
~~
~
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actual velocity in the manufacturing line of seamless pipes. In the following experiment, the rotational speed of rollers was selected at a constant value of 44 rpm, based on the results in Fig. 3. (b) Radius of curvature of prebend pipe While the amount u f prebend is selected as a certain value, the radius of curvature of prebend pipe can be determined arbitrarily. The effect of the radius of curvature of the prebend pipe on the straightening ratio was not significant and the prebend pipes with radius of curvature o f 60 mm w e r e used in the following experiment. (c) Diameter of mandrel When the mandrel is used in order to maintain a steady feed of tubing, the diameter of mandrel or the clearance between the mandrel and the inner diameter of the
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Pitch: 12,6 mm/rev Bend radius: t80 mm Mandrel diameter: 31.8 mm Flat r o l l e r 60
100
Rotational speed of r o l l e r s
=
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~
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P i t c h (mm/rev) o 6. g [] i6.1
<> 21,6 27. 5
F]at r o l l e r 44 rprn
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Fig. 3 Effect of the rotational speed of rollers on the straightening ratio at a constant pitch of 12.6 mm/rev in the straightening method with feed angle.
Reduction (5/0) Fig. 4 Relationship between the straightening ratio and the nominal reduction in radius for various pitches in the straightening method with feed angle.
K. Kawai et al. / Journal of Materials Processing Technology 48 (1995) 135-141
product affects the straightening ratio. The straighteiting ratio by the mandrel with the outer diameter o f 31.8 mm w a s h i g h e r t h a n t h a t b y the mandrel with the outer diameter o f 30.8 ram, t h u s t h e m a n d r e l w i t h t h e o u t e r d i a m e t e r o f 31.8 mm w a s used in the following experiment. (d) N o m i n a l r e d u c t i o n i n r a d i u s The effect of the nominal reduction in radius on the straightening ratio by flat rollers is shown in F i g . 4. While t h e v a l u e s o f t h e straightening ratio vary dependent upon the pitch, the linear relationship between the straightening ratio and the nominal reduction in r a d i u s c a n b e s e e n h t F i g . 4. This result implies that the nominal reduction in radius should be selected as large as possible in order to obtain high straightening ratio.
139
(e) P i t c h As m e n t i o n e d b e f o r e , t h e v a l u e of pitch is determined uniquely by the feed angle in the straightening method with feed angle. The effect of the pitch on the straightening ratio by flat rollers is shown in F i g . 5. A s n o t e d i n F i g . 2, t h e range of pitch to give high straightening ratio is limited in the stralghtenhag method without feed angle. On t h e o t h e r h a n d , t h e h i g h straightening ratio can be attained below a certain value of pitch in the straightening method with feed angle and the smaller pitch saturates the value of the straightening r a t i o , a s i n F i g . 5. It implies that we c a n s e l e c t a p i t c h in a wide range in order to obtain a certain value of straightening ratio in practice.
(f) Shape of r o l l e r s I n t h e e x p e r i m e n t of t h e e f f e c t o f the shape of rollers on the
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Fig. 5 Effect of the pitch o n the straightening ratio f o r v a r i o u s nominal reductions in radius in the straightening method with feed angle.
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.
.
.
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Fig. 6 The attainable maximum straightening ratio and the limiting nominal reduction in radius without buckling by flat rollers in the straightening method with feed angle.
140
I~ Kawai et al. / Journal of Materials Processing Technology 48 (1995) 135-141
straightening ratio for a constant pitch of 27.5 mm/rev, it was clarified that concave rollers give higher straightening ratio than flat rollers. On the other hand, though the actual reduction m radius by convex rollers comes to about 1%, the straightening ratio of the product was extremely low and it was clarified that convex rollers are not applicable to the straighte n i n g of c u r v e d t u b i n g .
(g) Maximum Straightening ratio and limiting reduction ratio It is clarified that the larger nominal reduction in radius gives the higher straightening ratio of seamless pipe but the increase of t h e n o m i n a l r e d u c t i o n i n r a d i u s will cause the buckling of tubing. Thus, there exist limiting values in the straightening ratio and the reduction in radius. Fig. 6 shows the maximum straightening ratio attainable and the limiting nominal reduction in radius without any buckling by flat rollers. When t h e value of pitch is large, the limiting nominal reduction in radius for buckling increases but the attainable straightening ratio is less than that for the smaller pitch. On t h e other hand, when the value of p i t c h is small, t h e limiting nominal reduction in radius for buckling decreases but the attainable straightening ratio is high. In other words, though the nominal reduction is not so large, high straightening r a t i o can be a t t a i n e d in the latter case. Furthermore, higher straightening r a t i o t h a n 90% can be obtained by selecting the p i t c h b e l o w 16.1 m m / r e v and the value of attainable straightening ratio is saturated. The similar tendency to Fig. 6 can be seen in F i g . 5. T h e s e r e s u l t s i m p l y , i f we selected a certain value of
straightening ratio, the actual feed rate in mm/s can be increased by selecting a pitch as large as possible it guarantees the selecting straightening ratio and a rotational speed of rollers as high as possible in practice. (h) C o n t a c t l e n g t h a n d n u m b e r o f bending-unbending In the straightening method with feed angle, the changeable working c o n d i t i o n s a r e t h e s h a p e of r o l l e r s , the feed angle and the nominal reduction in radius, and the pitch which plays an important role in rotary forming is determined uniquely by the feed angle. Though the rolling force and torque dependent upon the working conditions affect the straightening r a t i o o f t h e p r o d u c t , w e will d i s cuss here the contact length and t h e n u m b e r of b e n d i n g - u n b e n d i n g determined by the geometrical contact condition between the tubing and rollers. Assuming the shape of rollers, the feed angle and
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Reducti0n 02.0% ~2.57 % ~2,86 %
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Contact l e n g t h ' I
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64 mm __
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4'0 8'0 120 Number of bending-unbending
Fig. 7 Effect of the number of bending-unbending on the straightening ratio in the straightening method with feed angle.
I~ Kawai et al. / Journal of Materials Processing Technology 48 (1995) 135-141
the nominal reduction in radius, the shape of contact area can be evaluated geometrically and the contact length along the blank holding axis can be calculated. Furthermore, the number of bending-unbending in the roller gap during the process can be calculated by the value of pitch. A typical effect of the number of b e n d i n g - u n b e n d i n g on the straightening ratio for the case of c o n t a c t length of 64 m m is shown a s in Fig. 7 When the number of b e n d i n g - u n b e n d i n g comes to a c e r t a i n value, the straightening ratio is saturated as in Fig. 7. O n the other hand, if w e select the pitch a n d the n u m b e r of b e n d i n g - u n b e n d i n g as fixed values, the contact length saturates the straightening ratio. T h e s e results imply that there exist a limiting contact length and a limiting number of b e n d i n g - u n b e n d i n g to give a certain straightening ratio. It m a y be stated that a n attainable straightening ratio can be obtained b y selecting the feed angle a n d the nominal reduction in radius, a n d a m i n i m u m length of rollers exists to g u a r a n t e e the selected straightening ratio.
4. CONCLUSIONS A straightening method of the curved shape near both ends of tubing using rotary forming with three rollers was proposed. The effect of straighteuing method proposed here on the straightening r a t i o of t h e p r o d u c t was i n v e s t i g a t ed experimentally in detail. The conclusions can be summa-
141
rized as follows: (1) T h e s t r a i g h t e n i n g method with feed angle is superior to one without feed angle in order to introduce in the present manufacturing line of seamless pipes. (2) A n a t t a i n a b l e s t r a i g h t e n i n g ratio can be determined by the nominal reduction in radius and the pitch which is determined uniquely by the feed angle. (3) T h e r e e x i s t a maximum p i t c h a n d a minimum l e n g t h of r o l l e r s to guarantee a certain straightening ratio of the product. (4) T h e c u r v e d s h a p e o f t u b i n g c a n be straightened u p t o 90% u n d e r optimum working conditions. (5) T h e a x i a l f e e d rate of the product can be selected arbitrarily in accordance with the actual velocity in the manufacturing line of seamless pipes.
ACKNOWLEDGEMENTS The authors would like to express their sincere thanks t o D r . M. Hayama, Professor Emeritus of Yokohama National University, for his helpful suggestions. They also w i s h t o t h a n k NKK C o r p o r a t i o n f o r financial support to a part of the experiment.
REFERENCES 1. J a p a n Society for Technology of Plasticity, Straightening of Metal Products, Corona Publishi n g , T o k y o , 1992. 2. M. Y o k a i a n d H. K u d o , J. J p n . Soc. T e c h . P l a s t . , 10{1969)627.
ELSEVIER
Rotary Ken-ichi
Journal o f Materials Processing Technology 48 (1995) 135-141
Forming Kawai,
Hakoto
for
the
Satake,
Straightening Yukio
Inoue
and
of Haruhiko
Materials Processing Technology
Tubing Sugita
Department of Hechanical Engineering and Haterials Science, Faculty of Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama, 240 Japan
A new straighterfing method using rotary forming with three rollers is proposed. A h e l i c a l r o l l i n g mill w i t h t h r e e r o l l e r s i s u s e d i n t h e e x p e r i m e n t . The prebend aluminum seamless pipes are straightened under various working conditions. It is clarified that an attainable straightening ratio of the product can be determined by the pitch and the nominal reduction in r a d i u s , a n d t h e r e e x i s t s a minimum l e n g t h of r o l l e r s to g u a r a n t e e the selected straightening ratio. The curved shape can be straightened u p t o 90% under optimum working conditions. It is also clarified that there exists an maximum p i t c h to g u a r a n t e e a c e r t a i n s t r a i g h t e n i n g ratio and the axial feed rate can be selected arbitrarily in practice.
1. INTRODUCTION S e a m l e s s p i p e s , s u c h a s oil c o u n try tubular goods, are mainly m a n u f a c t u r e d b y s e a m l e s s p i p e mills s u c h a s p l u g mill a n d m a n d r e l rail/. A solid round billet is uniformly heated in a furnace and pierced by a piercer. Then a hollow shell is roll-reduced in outside diameter and wall t h i c k n e s s . The rolled tubing is simultaneously burnished inside and outside by a reeler and t h e n s i z e d i n t h e s i z i n g roll. A common fault in manufacturing of seamless pipes is that the product is not straight. The curved tubing is effectively straightened by a rotary straightener but the curved s h a p e n e a r b o t h e n d s of t u b i n g is not corrected completely by the straightener [1]. In order to reduce the cropping l o s s of b o t h e n d s of t u b i n g it is important to develop a straightening method for the curved shape
near both ends of tubing. In the present paper, the authors propose a straightening method using rotary forming with three rollers.
2. EXPERIMENTAL PROCEDURE In order to straighten the curved s h a p e n e a r t h e e n d of t u b i n g , a h e l i c a l r o l l i n g mill w i t h t h r e e r o l l e r s , d e s i g n e d b y Yokai e t al. [2], was used in the experiment. In the experiment without feed angle, the axes of rollers are parallel t o t h e c e n t e r line of t h e b l a n k holding axis and the product is pulled forward with a constant velocity by a pulling mechanism. On t h e o t h e r h a n d , m t h e e x p e r i ment with a certain feed angle, the tubing is fed automatically in the axial direction of the product due to the contact between rollers and the tubing. It is necessary to prepare a mandrel or guide rollers
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 6 4 3 - F
136
K. Kawai et al. / Journal o f Materials Processing Technology 48 (1995) 135-141
Roller aoOre,
(a) Front
(b)
Side
view
view
Fig. i Schematic illustration of straightening using a helical rolling mill w i t h t h r e e r o l l e r s . in front of the product in order to maintain a steady feed of the tubing in the latter case. Schematic i l l u s t r a t i o n s of the straightening of tubing using rotary forming with three r o l l e r s a r e s h o w n i n F i g . 1, for the latter case. The curved shape near both ends of seamless pipe appears during piercing process but it is not straightened completely by a rotary straightener. Since the straightening method proposed by the present authors is a rotary forming process with the reduction in radius of the product by three rollers, the outer diameter of the p i p e will r e d u c e slightly and the roundness of the product will g r o w worse. This process should be inserted after piercing and before sizing in the present manufacturing l i n e o f s e a m l e s s p i p e s , t h e n i t will be a hot forming process. The material used in the experi-
ment was commercially aluminum seamless pipe, JIS All00-TD, with t h e i n i t i a l o u t e r d i a m e t e r o f 35 ram, wall thickness o f i mm a n d l e n g t h o f 500 mm. T h e o u t e r d i a m e t e r a n d the wall thickness of s e a m l e s s p i p e used in the experiment was selected as about 1/10 size of the actual large diameter seamless pipes whose curved shape near both ends causes trouble in industry. Assuming the Swift-type stress-strain relationship for this material, the flow s t r e s s in the uniaxiai tension is given by (~ = 136(0.002 + E p) 0.0493 MPa. Before the straightening experiment, pipe specimen was so bent as to give a curved shape whose radius of curvature i s e i t h e r 60 mm o r 180 ram. The straightening experiment was carried out at room temperature without any lubricant after degreasing the surfaces of s p e c i m e n and rollers. While the nominal reduction in radius was defined by t h e o u t e r d i a m e t e r of s p e c i m e n a n d the gap of rollers, the actual reduction in radius measured from outer diameters before and after straightening, except for the experiment by convex rollers, was less than 0.25% at the most. Three types of roller, flat roller, concave roller and convex roller were used in the experiment. Since it was clarified that the longer roller gives the higher straightening ratio in the preliminary straightening experiment without feed angle, the l e n g t h of r o l l e r s w a s d e t e r m i n e d a s to be maximum supported by the roller-holding attachment. The length and the outer diameter of f l a t r o l l e r a r e 72 mm a n d 60 ram. The radii of curvature of c o n c a v e r o l l e r a n d c o n v e x r o l l e r a r e 675 mm a n d 69.8 ram, r e s p e c t i v e l y . The straightening ratio is defined
K. Kawai et aL / Journal of Materials Processing Technology 48 (1995) 135-141
b y (ho - h)/ho, where ho a n d h are values of b e n d displacement before and after straightening, respectively. The pitch, which plays an important role in rotary forming, is defined as the feed in the axial direction per revolution of specimen. Three or more specimens were used for every working condition in the experiment.
3. R E S U L T S
AND DISCUSSION
3.1. Straightening without feed angle In order to investigate the basic characteristics of the straightening using rotary forming, the straightening experiment without feed angle was carried using flat rollers. In this experiment, the product was pulled forward at a constant velocity and various pitches were realized by the combination of rotationai s p e e d of rollers and pulling velocity of the product. 100
I
80 o
60 ,~
L
~ . --.
40
Prebend o 3ram • 6ram ""EF ....................
-~,
I
Bend radius: 60 mm Ruduction: 4% Fiat r 0 ] ] e r
[]
20
~o 1'0
2'0 3'0 Pitch (mm/rev)
4'0 ' 50
Fig. 2 Relationship between t h e straightening ratio and the pitch in the straightening method without feed angle.
137
The relationship between the straightening ratio and the pitch at 4% n o m i n a l r e d u c t i o n i n r a d i u s i s s h o w n i n F i g . 2. The pitch is an important working condition in the proposed straightening method as well as in other rotary forming, and smaller pitch than 5 mm/rev should be selected in order to obtain high straightening ratio. While t h e value of straightening ratio varies dependent upon the a m o u n t o f p r e b e n d , i t may b e c o n sidered that the qualitative relationship between the straightening ratio and the pitch is independent upon the amount of prebend. The amount of prebend was selected as 6 mm i n t h e e x p e r i m e n t w i t h f e e d angle in the following section, b a s e d on t h i s r e s u l t s h o w n in Fig. 2. 3.2. S t r a i g h t e n i n g w i t h f e e d a n g l e The straightening method without feed angle described in the preceding section needs the feeding m e c h a n i s m in t h e axial d i r e c t i o n of tubing. When we observe the deforming zone by rotary compression due to rollers, the top end of tubing needs a feeding mechanism like extrusion and the bottom end of tubing needs one like drawing. Since the tubing is rotating continuously during process, the feeding mechanism may need special devices. If r o l l e r s can be d r i v e n in planetary motion, the tubing does not need rotation round its own axis but the feeding mechanism should be introduced in this case. On t h e o t h e r h a n d , t h e t u b i n g c a n be f e d a u t o m a t i c a l l y in the straightening m e t h o d with feed angle. Thus, the straightening method with feed angle may be introduced easily in the present manufacturing line of seamless pipes.
K. Kawai et aL / Journal of Materials Processing Technology 48 (1995) 135-141
138
(a) Rotational speed of rollers It is clarified that the pitch plays an important role in the straightening of tubhlg from Fig. 2, but there are numerous working conditions to realize the same pitch. Using the feed angle, the pitch, that is axial feed of the product per revolution of tubing, in mm/rev, is determined uniquely by the feed angle but the axial feed rate of product, in ram/s, can be controlled arbitrarily by the rotational speed of rollers. When the working conditions except for the rotational speed of rollers were fixed, the straightening ratio was independent upon the rotational s p e e d of rollers, as s h o w n in Fig. 3. This result implies that a constant straightening ratio can be attained w h e n the value of pitch is controlled b y adjusting the feed angle. T h e axiai feed rate of the product, in ram/s, can be selected arbitrarily in accordance with the 100
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actual velocity in the manufacturing line of seamless pipes. In the following experiment, the rotational speed of rollers was selected at a constant value of 44 rpm, based on the results in Fig. 3. (b) Radius of curvature of prebend pipe While the amount u f prebend is selected as a certain value, the radius of curvature of prebend pipe can be determined arbitrarily. The effect of the radius of curvature of the prebend pipe on the straightening ratio was not significant and the prebend pipes with radius of curvature o f 60 mm w e r e used in the following experiment. (c) Diameter of mandrel When the mandrel is used in order to maintain a steady feed of tubing, the diameter of mandrel or the clearance between the mandrel and the inner diameter of the
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Reduction (5/0) Fig. 4 Relationship between the straightening ratio and the nominal reduction in radius for various pitches in the straightening method with feed angle.
K. Kawai et al. / Journal of Materials Processing Technology 48 (1995) 135-141
product affects the straightening ratio. The straighteiting ratio by the mandrel with the outer diameter o f 31.8 mm w a s h i g h e r t h a n t h a t b y the mandrel with the outer diameter o f 30.8 ram, t h u s t h e m a n d r e l w i t h t h e o u t e r d i a m e t e r o f 31.8 mm w a s used in the following experiment. (d) N o m i n a l r e d u c t i o n i n r a d i u s The effect of the nominal reduction in radius on the straightening ratio by flat rollers is shown in F i g . 4. While t h e v a l u e s o f t h e straightening ratio vary dependent upon the pitch, the linear relationship between the straightening ratio and the nominal reduction in r a d i u s c a n b e s e e n h t F i g . 4. This result implies that the nominal reduction in radius should be selected as large as possible in order to obtain high straightening ratio.
139
(e) P i t c h As m e n t i o n e d b e f o r e , t h e v a l u e of pitch is determined uniquely by the feed angle in the straightening method with feed angle. The effect of the pitch on the straightening ratio by flat rollers is shown in F i g . 5. A s n o t e d i n F i g . 2, t h e range of pitch to give high straightening ratio is limited in the stralghtenhag method without feed angle. On t h e o t h e r h a n d , t h e h i g h straightening ratio can be attained below a certain value of pitch in the straightening method with feed angle and the smaller pitch saturates the value of the straightening r a t i o , a s i n F i g . 5. It implies that we c a n s e l e c t a p i t c h in a wide range in order to obtain a certain value of straightening ratio in practice.
(f) Shape of r o l l e r s I n t h e e x p e r i m e n t of t h e e f f e c t o f the shape of rollers on the
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140
I~ Kawai et al. / Journal of Materials Processing Technology 48 (1995) 135-141
straightening ratio for a constant pitch of 27.5 mm/rev, it was clarified that concave rollers give higher straightening ratio than flat rollers. On the other hand, though the actual reduction m radius by convex rollers comes to about 1%, the straightening ratio of the product was extremely low and it was clarified that convex rollers are not applicable to the straighte n i n g of c u r v e d t u b i n g .
(g) Maximum Straightening ratio and limiting reduction ratio It is clarified that the larger nominal reduction in radius gives the higher straightening ratio of seamless pipe but the increase of t h e n o m i n a l r e d u c t i o n i n r a d i u s will cause the buckling of tubing. Thus, there exist limiting values in the straightening ratio and the reduction in radius. Fig. 6 shows the maximum straightening ratio attainable and the limiting nominal reduction in radius without any buckling by flat rollers. When t h e value of pitch is large, the limiting nominal reduction in radius for buckling increases but the attainable straightening ratio is less than that for the smaller pitch. On t h e other hand, when the value of p i t c h is small, t h e limiting nominal reduction in radius for buckling decreases but the attainable straightening ratio is high. In other words, though the nominal reduction is not so large, high straightening r a t i o can be a t t a i n e d in the latter case. Furthermore, higher straightening r a t i o t h a n 90% can be obtained by selecting the p i t c h b e l o w 16.1 m m / r e v and the value of attainable straightening ratio is saturated. The similar tendency to Fig. 6 can be seen in F i g . 5. T h e s e r e s u l t s i m p l y , i f we selected a certain value of
straightening ratio, the actual feed rate in mm/s can be increased by selecting a pitch as large as possible it guarantees the selecting straightening ratio and a rotational speed of rollers as high as possible in practice. (h) C o n t a c t l e n g t h a n d n u m b e r o f bending-unbending In the straightening method with feed angle, the changeable working c o n d i t i o n s a r e t h e s h a p e of r o l l e r s , the feed angle and the nominal reduction in radius, and the pitch which plays an important role in rotary forming is determined uniquely by the feed angle. Though the rolling force and torque dependent upon the working conditions affect the straightening r a t i o o f t h e p r o d u c t , w e will d i s cuss here the contact length and t h e n u m b e r of b e n d i n g - u n b e n d i n g determined by the geometrical contact condition between the tubing and rollers. Assuming the shape of rollers, the feed angle and
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I~ Kawai et al. / Journal of Materials Processing Technology 48 (1995) 135-141
the nominal reduction in radius, the shape of contact area can be evaluated geometrically and the contact length along the blank holding axis can be calculated. Furthermore, the number of bending-unbending in the roller gap during the process can be calculated by the value of pitch. A typical effect of the number of b e n d i n g - u n b e n d i n g on the straightening ratio for the case of c o n t a c t length of 64 m m is shown a s in Fig. 7 When the number of b e n d i n g - u n b e n d i n g comes to a c e r t a i n value, the straightening ratio is saturated as in Fig. 7. O n the other hand, if w e select the pitch a n d the n u m b e r of b e n d i n g - u n b e n d i n g as fixed values, the contact length saturates the straightening ratio. T h e s e results imply that there exist a limiting contact length and a limiting number of b e n d i n g - u n b e n d i n g to give a certain straightening ratio. It m a y be stated that a n attainable straightening ratio can be obtained b y selecting the feed angle a n d the nominal reduction in radius, a n d a m i n i m u m length of rollers exists to g u a r a n t e e the selected straightening ratio.
4. CONCLUSIONS A straightening method of the curved shape near both ends of tubing using rotary forming with three rollers was proposed. The effect of straighteuing method proposed here on the straightening r a t i o of t h e p r o d u c t was i n v e s t i g a t ed experimentally in detail. The conclusions can be summa-
141
rized as follows: (1) T h e s t r a i g h t e n i n g method with feed angle is superior to one without feed angle in order to introduce in the present manufacturing line of seamless pipes. (2) A n a t t a i n a b l e s t r a i g h t e n i n g ratio can be determined by the nominal reduction in radius and the pitch which is determined uniquely by the feed angle. (3) T h e r e e x i s t a maximum p i t c h a n d a minimum l e n g t h of r o l l e r s to guarantee a certain straightening ratio of the product. (4) T h e c u r v e d s h a p e o f t u b i n g c a n be straightened u p t o 90% u n d e r optimum working conditions. (5) T h e a x i a l f e e d rate of the product can be selected arbitrarily in accordance with the actual velocity in the manufacturing line of seamless pipes.
ACKNOWLEDGEMENTS The authors would like to express their sincere thanks t o D r . M. Hayama, Professor Emeritus of Yokohama National University, for his helpful suggestions. They also w i s h t o t h a n k NKK C o r p o r a t i o n f o r financial support to a part of the experiment.
REFERENCES 1. J a p a n Society for Technology of Plasticity, Straightening of Metal Products, Corona Publishi n g , T o k y o , 1992. 2. M. Y o k a i a n d H. K u d o , J. J p n . Soc. T e c h . P l a s t . , 10{1969)627.