Attempts to facilitate low volume production of soft aluminium cups with large drawing ratios by...

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Journal of Mechanical Working Technology, 2 (1979) 357--366 357 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands ATTEMPTS TO FACILITATE LOW VOLUME PRODUCTION OF SOFT ALUMINIUM CUPS WITH LARGE DRAWING RATIOS BY DEEP DRAWING BASED ON MASLENNIKOV'S TECHNIQUE K. YAMAGUCHI, N. TAKAKURA and M. FUKUDA Department of Mechanical Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto (Japan) (Received October 12, 1978) Industrial Summary When producing cylindrical cups by conventional deep drawing with metal tools, the maximum first-stage drawing ratio seldom exceeds 2.2. For a larger drawing ratio, therefore, re-drawing in one or more stages is necessary. Since a punch and die are required for each stage, tooling costs increase relatively as the total drawing ratio increases and as the number of products decreases. Two methods, based on Maslennikov's process, are investigated to facilitate low volume production of cylindrical cups with very large drawing ratios. Firstly, a conventional direct re-drawing is carried out as a subsequent operation to increase the total drawing ratio and to correct the shape and dimensions of cups produced by Maslennikov's process. Secondly, a modified form of Maslennikov's process employing a metal punch is examined and the addi- tional effects resulting from the introduction of the punch are discussed. When these methods are applied to soft aluminium sheets of 1 mm thickness, a fully- drawn cup with an extremely large drawing ratio can be produced using only one metal punch and one or two dies throughout the drawing process. The process does not seem to be too slow for the production of cups with drawing ratios of less than about 6. The shape, dimensional accuracy and surface finish of the cups produced are also adequate for practical use. Introduction The principle of the so-called punchless deep drawing of cylindrical cups was first proposed by N.A. Maslennikov [1] in 1956, The uniqueness of this technique is in the method of employing a rubber ring instead of a metal punch. The deformation of the blank is caused by the frictional force devel- oped at the interface between the ring and the blank, Since a metal punch is not used, a longitudinal tensile stress is not induced at the cup wall throughout the drawing process. This is of help in achieving an extremely large drawing ratio. Maslennikov stated that the maximum drawing ratio achieved by this process was 7.0 for aluminium sheets. A few years later, however, other

Transcript of Attempts to facilitate low volume production of soft aluminium cups with large drawing ratios by...

Page 1: Attempts to facilitate low volume production of soft aluminium cups with large drawing ratios by deep drawing based on Maslennikov's technique

Journal of Mechanical Working Technology, 2 (1979) 357--366 357 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

A T T E M P T S T O F A C I L I T A T E L O W V O L U M E P R O D U C T I O N O F S O F T A L U M I N I U M C U P S W I T H L A R G E D R A W I N G R A T I O S B Y D E E P D R A W I N G B A S E D O N M A S L E N N I K O V ' S T E C H N I Q U E

K. YAMAGUCHI, N. TAKAKURA and M. FUKUDA

Department of Mechanical Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto (Japan)

(Received October 12, 1978)

I n d u s t r i a l S u m m a r y

When producing cylindrical cups by conventional deep drawing with metal tools, the maximum first-stage drawing ratio seldom exceeds 2.2. For a larger drawing ratio, therefore, re-drawing in one or more stages is necessary. Since a punch and die are required for each stage, tooling costs increase relatively as the total drawing ratio increases and as the number of products decreases.

Two methods, based on Maslennikov's process, are investigated to facilitate low volume product ion of cylindrical cups with very large drawing ratios. Firstly, a conventional direct re-drawing is carried out as a subsequent operation to increase the total drawing ratio and to correct the shape and dimensions of cups produced by Maslennikov's process. Secondly, a modified form of Maslennikov's process employing a metal punch is examined and the addi- tional effects resulting from the introduction of the punch are discussed.

When these methods are applied to soft aluminium sheets of 1 mm thickness, a fully- drawn cup with an extremely large drawing ratio can be produced using only one metal punch and one or two dies throughout the drawing process. The process does not seem to be too slow for the product ion of cups with drawing ratios of less than about 6. The shape, dimensional accuracy and surface finish of the cups produced are also adequate for practical use.

I n t r o d u c t i o n

T h e p r i n c i p l e o f t h e s o - c a l l e d p u n c h l e s s d e e p d r a w i n g o f c y l i n d r i c a l c u p s was f i r s t p r o p o s e d b y N . A . M a s l e n n i k o v [1 ] in 1 9 5 6 , T h e u n i q u e n e s s o f t h i s t e c h n i q u e is in t h e m e t h o d o f e m p l o y i n g a r u b b e r r i ng i n s t e a d o f a m e t a l p u n c h . T h e d e f o r m a t i o n o f t h e b l a n k is c a u s e d b y t h e f r i c t i o n a l f o r c e deve l - o p e d a t t h e i n t e r f a c e b e t w e e n t h e r ing a n d t h e b l a n k , S ince a m e t a l p u n c h is n o t u s e d , a l o n g i t u d i n a l t e n s i l e s t ress is n o t i n d u c e d a t t h e c u p wal l t h r o u g h o u t t h e d r a w i n g p r o c e s s . Th i s is o f h e l p in a c h i e v i n g an e x t r e m e l y la rge d r a w i n g r a t i o .

M a s l e n n i k o v s t a t e d t h a t t h e m a x i m u m d r a w i n g r a t i o a c h i e v e d b y t h i s p r o c e s s was 7 .0 f o r a l u m i n i u m shee t s . A f e w y e a r s l a t e r , h o w e v e r , o t h e r

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investigators [2] carried out experiments and concluded that it was impos- sible to realize the large drawing ratio claimed by Maslennikov. Since then, few investigations and practical applications have been made of this drawing process.

In recent years, polyurethane elastomers have been frequently used as a part of the tooling in various press practices. F.L. Derweesh and P.B. Mellor | 3] used two grades of polyurethane for the drawing ring and discussed the process variables of Maslennikov's technique in detail. They obtained success- ful soft aluminium cups with a drawing ratio of 6.0. M. Fukuda and his co- workers [4,5] also used polyurethane rings and examined the relationship between drawing conditions and drawing limits. They achieved the remarkable drawing ratio of 12.0 for soft aluminium blanks of 1 mm thickness using draw- ing operations with intermediate anneals. They also made an analysis [6] of the drawing mechanism and determined the most effective drawing conditions.

From these investigations, the feasibility of the Maslennikov process has been confirmed and also the productivity has been considerably increased. However, a main disadvantage still remains in the uncertainty of the shape anti dimensional accuracy of the drawn cup. Because of the absence of a metal punch, the shape at the bot tom of the cup varies widely and the diameter and thickness of the cup also vary from portion to portion, due to the formation of so-called nodes [4] at the cup wall.

To put the Maslennikov process into practice, it is necessary to overcome the undesirable aspects described above. For this purpose, a conventional re- drawing with ironing is carried out here as a subsequent operation to correct the shape and dimensions of the cup drawn by the Maslennikov process, and also a modified form of the Maslennikov process employing a metal punch is examined. These ideas were briefly suggested by Maslennikov, but the details of the drawing conditions for these methods and the effectiveness of correc- tive drawing on the quality of the cup have been virtually unexamined in previous work.

Main features of Maslennikov's process

Drawing mechanism The principle of Maslennikov's process can be understood from Fig. l~

which shows a simplified cross-section through the tooling. When a load is ap- plied to the die, the polyurethane ring is compressed and deforms radially in- wards. Provided that the frictional characteristics at the interface between the ring and the blank are satisfactory, the flange of the blank can be appreciably dragged towards the die opening by the radial movement of the ring. After the load is removed the polyurethane ring recovers its original dimensions. If the operations of loading and unloading are repeated, therefore, a fully drawn cup with an extremely large drawing ratio can be finally obtained, using only one metal die throughout the drawing process. This is the main advantage of Maslennikov's process.

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In the present experiments, soft aluminium sheets of I mm thickness were used, to provide a blank material to which Maslennikov's process can be suc- cessfully applied. The dimensions of dies and polyurethane rings* used are listed in Table 1. The outer diameters of the blank, ring and die were standard- ized at 120 mm. The ring-blank interface was dusted with powdered rosin, which has a high coefficient of friction [ 7]. A polyethylene film coated with Johnson's wax was used as a lubricant at the blank-die interface.

Figure 2(a) shows deformation aspects of the polyurethane ring and the blank at an arbitrary stage of drawing. When the ring is compressed from the initial thickness To to T, the outer radius of the blank contracts from r a to r0. At this moment, the radial velocity distributions of the ring and the blank can be expressed as shown in Fig. 2(b). The radial velocity of the ring ~]R increases from zero at the container wall to a certain magnitude at the die opening, while the blank has a velocity distribution determined by the displacement of the outer radius ( r a - r 0 ) . As a result, these two curves intersect at a point in the flange portion**. This point has a similar role to the neutral point in the up- setting of hollow circular disks.

~ RAM

~ BLANK

~O,5<T><>I ~POLYURETNANE RING ~--CONTAINER

(a)

• r~ t

(b) == RAL RADIUS

Fn F a

FLANGE RADIUS Fig. 1. Schematic arrangement of the tooling for the Maslennikov process.

Fig. 2. (a) Frictional forces acting on the blank surfaces. (b) Appearance of a neutral radius r n due to difference in radial velocities of polyurethane ring UR and blank UB-

The direction of the frictional forces acting at the interface between the ring and the blank changes at the neutral radius rn. The frictional force acting on the inside of the neutral radius contributes to the drawing deformation of the blank, but that on the outside acts to prevent it. Consequently, the radial tensile stress in the flange of the blank often has its maximum value at the

*The optimum combination of a die and polyurethane ring has been discussed in [4]. **The radial velocities of the polyurethane ring UR and the blank UB can be calculated from the condition of volume constancy. Putting ~/R equal to ~]B, a neutral radius can be determined. Derivation of these equations is described in detail in [6 ].

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TABLE 1

Dimensions of dies and polyurethane rings used in the Maslennikov process

Die

Hole Profile diameter (ram) radius (turn)

20, 25, 30, 5 35, 40, 50, 60

Urethane ring

Hole Thickness Durometer diameter (mm) (ram) hardness

20, 30, 40, 50 6, 10 85

neutral radius. When this maximum stress exceeds the fracture strength of the blank material, circumferential cracks [4] occur in the flange of the blank. This supports the existence of the neutral radius, which characterizes the drawing mechanism of Maslennikov's process.

Drawing conditions to increase productivity In the investigations repor ted to date, frictional agents such as powdered

rosin or Crestex paper have been used over the whole interface between the ring and the blank. However, f rom the drawing mechanism discussed above, it is clear that such a technique does not always provide the opt imum condit ions to carry out the drawing effectively. To increase the deformat ion of the blank, it is of importance to remove or to decrease the frictional resistance which exists on the outer side of the neutral radius.

In order to decrease the frictional resistance, it was decided to apply powdered rosin only partially on the contac t surface between the poly~ urethane ring and the blank. Figure 3 shows the effect of this method o~ the number of strokes required to produce fully drawn cups with a drawing ratio of 4. The current drawing ratio was here defined as (ra - (r0 - rd) }/rd, where ra and r0 are the initial and current outer radii of the blank, respectively, and rd is the radius of the die opening. When r0 reduces to r d the expression gives the normal drawing ratio. A rosined diameter of 120 mm corresponds to the case when powdered rosin is used over the whole of the ring surface, i t should be noted that the number of strokes required diminishes f rom ten for a rosined diameter of 120 mm to five for a rosined diameter of 90 mm. It has also been confirmed that a similar beneficial effect can be achieved by the use of a blank with a diameter smaller than that of the ring [ 6].

It has been said that one of the main disadvantages of Maslennikov's process is the low product ivi ty which results f rom the drawing mechanism based on the frictional force. Indeed, it has been found from previous analysis [6] that the work contributing to the deformat ion of the blank is about 20--25% of the work performed by the frictional force. This means that most of the work performed is t ransformed into slip between the ring and the blank. However, it

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should be emphasized that when the Maslennikov process is carried out under the drawing conditions proposed above, the productivity can be considerably increased, compared with that for the drawing conditions which have been used to date (see Fig. 3). Figure 4 shows the number of strokes required to produce fully drawn cups with various drawing ratios. It is seen that cups with drawing ratios of less than about 5 can be obtained in seven strokes, although for achieving a larger drawing ratio the number of strokes increases markedly. Thus, when the use of the Maslennikov process is confined to drawing ratios of below 5, the process would not appear to be too slow for practical application. It is thought, therefore, that the remaining problem to be overcome will be how to increase the quality of the cup produced by Maslennikov's process.

o

~ 3

~2

- ROSINEDDIA, / ) _ / ~

/ / ~4 ~ 120 m~

DIA, 2 BLANK 12Dram

DIE OPENING 30m~ v j ~ j ~ I ~ I , J 1

0 2 4 6 8 10

NUHBER OF STROKES

j ~ J /

BLANK DIA, 120mm

ROSINED DIA, 90 mm

J t I J i i ~ [

I0 20 30 40

NUMBER OF STROKES

Fig. 3. Effect of rosined diameter on the number of strokes required to produce fully drawn cups with a drawing ratio of 4. Polyurethane ring: internal diameter 40 mm, thick- hess 10 ram, compression ratio 40%.

Fig. 4. Number of strokes zequired to produce fully drawn cups with various drawing ratios. Polyurethane ring: thickness 10 ram, compression ratio 40%.

Re-drawing as subsequent operation

As a subsequent operation to correct the shape and dimensions of cups pro- duced by the Maslennikov process, a conventional re-drawing with ironing was carried out using a metal punch and die. Johnson's wax was used as a lubricant at the interface between the die and the cup. The ironing ratio was defined as a = ( t o - C ) / t o × 100%, where to and C are the initial blank thickness and clearance between the punch and die, respectively. A positive value of a means that the ironing is carried out simultaneously with the re-drawing.

Figures 5 and 6 show diameters and thicknesses of cups after a corrective drawing. In this case, a die opening of 35 mm was used for Maslennikov's process and a punch of 25 mm diameter was used for the subsequent re- drawing. For re-drawing without ironing (a = -50%} corrective effects are hard- ly expected and for excessive ironing (a = 40%) a fracture occurred at the wall

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E

27

3 5 -

\ MASLENNIKOV PROCESS

3 4 - -

28

IRONING RAIIO

- - ~ "~ ~ = - S O %

~ 2 0 Z ~ 3 0 " ,

26 _ _ L i I _ _ ~ _ _

20 40 60 80

(BOTTOM) LENGTH ALONG CUP WALl, ~,~

0,2 ~ .........

- 0 , 2

- 0 , 4

!

WALL F~SLENNIKOV PROCESS

~I×-×~×~- "×j× D~-50 %

J _____L '~ ...... J ........ 20 40 bO 80 100

LENGIH FROM CENTRE OF CUP BOIIOM, ~ r i

Fig. 5. E f f e c t o f correct ive re-drawing wi th ironing on the d imens ional accuracy o i cups.

Fig. 6. Ef fec t o f correct ive re-drawing wi th ironing on the th ickness strains in ful ly drawn cups.

of the partially re-drawn cup. However, it is seen that when an ironing ratio in the range from 0--30% is chosen, both the thickness and the diameter become almost uniform over the whole of the cup wall. Since the wall thickness of the cup produced by Maslennikov's process has been already increased, the sub- stantive ironing ratio is somewhat larger than the nominal one defined above. Thus, the corrective effect appears even for an ironing ratio of zero. The shape at the bottom of the cup, of course, fits the punch profile with good accuracy and the surface finish at the cup wall is also satisfactory (the surface roughness expressed as a centre line average is below 0.3 pm).

Figure 7 shows Vickers hardnesses of re-drawn cups. The dotted line corre- sponds to the initial hardness of the soft aluminium blank. It is found that the work hardening is considerable at the cup wall and increases as the ironing ratio increases.

The subsequent re-drawing with ironing is useful not only for increasing the quality of a cup but also for increasing the total drawing ratio. Figure 8 shows the total drawing ratio achieved by a re-drawing after the Maslennikov process. The drawing ratios of cups produced by the Maslennikov process, i.e. the first- stage drawing ratios, were 2.5, 3.0, 3.4 and 4.0, and the limiting re-drawing ratios for these cups were about 1.5 regardless of the first-stage drawing ratios. Thus, the total drawing ratio for each cup becomes 3.75, 4.5, 5.1 and 6.0, re- spectively. To achieve a total drawing ratio of 6.0, for instance, the number of strokes required was five in the Maslennikov process, with subsequent re- drawing in one stage. To achieve the same drawing ratio using only the Maslennikov process, however, the number of strokes required increases con- siderably, as shown in Fig. 4.

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60

50

40

ua

20

10

WALL

f f ~ o % o o ~ /o 2 ~ ~" ~. ~"

HAR NES • × o ' f / D S OF BLANK

I x ~SLENNIKOV PROCESS 1 • 0C= -50 % i o (3(= 0 %

o (3(,= 30 %

20 40 60

LENGTH FROM CENTRE OF CUP BOTTOM, rnm

o

~ 4

2 80

~'~"'CONVENT[ONAL

TWO-STAGE DRAWING

J I i

2 3 4

FIRST-STAGE DRAWING RATIO

Fig. 7. Vickers hardness of cups after corrective re-drawing with ironing.

Fig. 8. Total drawing ratio achieved by re-drawing in o n e stage after the Maslennikov process (a = 0%).

For comparison, the results for conventional two-stage drawing are also shown in Fig. 8. In this case, the maximum first-stage drawing ratio was 1.9 and the limiting re-drawing ratio was about 1.5. Thus, the maximum total drawing ratio achieved becomes 2.85.

Punch-aided Maslennikov process

A similar corrective effect to the subsequent re-drawing discussed above can also be realized by introducing a metal punch into Maslennikov's device. In this method, however, a problem arises with respect to the compression ratio of the polyurethane ring. Because of the introduction of the metal punch, the compression ratio of the ring which can be adopted is confined to a value somewhat less than that in the Maslennikov process itself. This leads to an in- crease in the number of strokes required for the production of a fully drawn cup.

To improve this defect the drawing device shown in Fig. 9 was used in the present experiments. A so-called double-die with different hole diameters was employed to make the radial movement of the ring as large as possible. By the use of the double-die the maximum compression ratio can be increased from 20% (for a single die) to 30%, under which the drawing proceeds acceptably. The dimensions of the punch, dies and polyurethane ring used are listed in Table 2.

When the polyurethane ring is compressed by the press ram, the movement of the punch is simultaneously adjusted by die cushion pins so as to exert a constant force on the blank. In the early stage of drawing, the deformation of the blank is caused by both the applied punch load and the frictional force developed at the ring-blank interface. After the punch reaches the second die, re-drawing can be performed consecutively.

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[ - f DEFORMED BLANK

J- FIRST DIE ~.~ !

._~ ~_] ~ ~ - ~ POLYURET HANE RING ~- CONTAINEP ' ~3

~ ~ PUNCH L ~ ~ Y y ~ ~ _ ' . . ) , a

/@- i

q $ ,i ' ¢

'4UMBER OF STROKEC

Fig. 9. Schematic ~'angement of the tooling for the modified form of the Maslennikov process employing a metal punch.

Fig. 10. Number of strokes required to produce fully-drawn cups with various drawing ratios. Compression ratio of polyurethane ring: 30%.

TABLE2

Dimensions of tools used in the modified form of the Maslennikov process

Punch Die Urethane ring

Diameter Profile Hole Profile Hole Thickness Durometer (mm) radius (mm) diameter radius diameter (ram) hardness

(ram) (mm) (mm)

26 5 First die: 35, 5 50 6,5 85 37.5, 40

Second die: 30

In the final stage, where the flange of the partially drawn cup is comple te ly dragged in to the first die, the process reduces to convent iona l re-drawing with metal tools. Thus, in this m e t h o d of drawing, a fully drawn cup with adequate qual i ty can be p r o d u c e d using only one device t h r o u g h o u t the drawing process. This is the main advantage of the Maslennikov process as modi f ied with the aid of a metal punch.

In this process, the magni tude o f the applied punch load is one of the im- po r t an t fac tors which govern the success or failure of the process. The n u m b e r of s t rokes required to ob ta in a fully drawn cup decreases as the applied punch load increases. However , for an excessive punch load, f racture of the cup oc- curs at the punch profi le por t ion . The upper limit of the punch load can be

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estimated approximately from the tensile strength o B of the blank material, Pmax = ~d to OB, where d and to are the punch diameter and the initial thick- ness of the blank, respectively. The maximum punch load calculated for the drawing condit ion adopted here (d = 26 mm, to = 1 mm and a B = 82.4 MPa) is about 6.73 kN and was nearly equal to the fracture load of 6.86 kN found in the experiments.

The number of strokes is also affected by the diameter ratio of the first and second die, D~/D2, which has a similar meaning to the re-drawing ratio. The number of strokes decreases with increasing diameter ratio. However, when the value of D~/D2 exceeds a certain magnitude, puckering or fracture tends to oc- cur and, therefore, the diameter ratio adopted was limited to about 1.25.

Figure 10 shows the increase in cup height with the number of strokes. The punch load and the diameter ratio of the first to second die are 5.88 kN and 1.25, respectively. It is found that the height of the cup increases rapidly at the final stage, where the process reduces to conventional re-drawing. It is also seen that the number of strokes required for producing a fully drawn cup in- creases as the total drawing ratio increases. For instance, the number of strokes to achieve a drawing ratio of 4 is eight. As the compression ratio of the ring was restricted to 30%, the number of strokes required for this process slightly increases compared with that for the ordinary Maslennikov process (see Fig. 4). However, it should be emphasized that a very deep cup with good quality can be produced using simple tooling throughout the drawing process.

Typical cups produced are shown in Fig. 11, together with a cup having a drawing ratio of 6.0, which was achieved by an additional re-drawing after the Maslennikov process.

Fig. 11. (a), (b), (c): Typical cups produced by the punch-aided Maslennikov process. (d): Cup produced by additional re-drawing with ironing in one stage after the Maslennikov process. Drawing ratio: (a) 3.0; (b) 3.33; (c) 4.0; (d) 6.0.

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Conclusions

In an a t tempt to facilitate low volume production of cylindrical cups with large drawing ratios, deep drawings based on Maslennikov's process were made using soft aluminium sheets. For a modified Maslennikov process in which a metal punch is used together with a polyurethane ring, the quality of the cup produced is good enough for practical use but the number of strokes required to obtain a fully drawn cup is rather large. Thus, from the practical point of view, it seems to be bet ter to carry out a conventional re-drawing with ironing after the Maslennikov process. In this case, the use of the Maslennikov process should be restricted to drawing ratios of up to about 5, since the number of strokes increases considerably for a larger drawing ratio. A subsequent re- drawing contr ibutes not only to increase the quality of the cup, but also to in° crease the productivity. To achieve a drawing ratio of 6.0, for instance, the number of strokes required is five for the Maslennikov process with subsequent re-drawing in one stage, which is probably acceptable for practical application. The shape, dimensional accuracy and surface finish of the cup produced are also adequate.

Acknowledgement

The authors would like to thank Professor P.B. Mellor of the School of Mechanical Engineering, University of Bradford, for his valuable discussion.

References

• /

1 N.A. Maslenmkov, Vestnik Maschinostroyenia, 36 (1956) 56. 2 H. Hermans and F.H.R.F. Vermeulen, Metalworking Production, October 11 (1961) 69. 3 F.L. Derweesh and P.B. Mellor, in S.A. Tobias and F. Koenigsberger (Eds.), Proc. 10th

Int. Mach. Tool Des. Res. Conf., Manchester, 1969, Pergamon, Oxford, 1970, p. 499. 4 M. Fukuda, K. Yamaguchi and K. Takayaraa, Bull. JSME, 15 (1972) 401. 5 M. Fukuda, K. Yamaguchi and K. Takayama, Bull. JSME, 15 (1972) 554. 6 M. Fukuda, K. Yamaguchi and T. Nishikoji, Bull. JSME, 17 (1974) 1513. 7 M. Fukuda and K. Yamaguchi, Bull. JSME, 17 (1974) 157.