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Experimental study on splashing characteristics of magnetic fluid in a magnetic fluid seal
Journal of Magnetism and Magnetic Materials 122 (1993) 420 423 North-Holland
Ai I"
Experimental study on splashing characteristics of magnetic fluid in a magnetic fluid seal Yasunaga
M i t s u y a ~, K e n j i T o m i t a " a n d M a s a y u k i H o s o y a b
" Dept. o1 Electronic-Mechanical Engineering, Nagoya Unit,ersity, Chikusa-ku, Nagoya 464-01, Japan :' Material Technology Laboratoo', Nihon Seiko Co. Ltd., Kugenuma-simmei, Fu/isawa, Kanagawa 251, Japan
The splashing characteristics of a magnetic fluid spindle seal designed tk~rmagnetic disk storage use werc experimentally studied in terms of the meniscus spread and configuration formed around the seal gap. The critical rotational speed at which splashing of the magnetic fluid is initiated is found to be uniquely determined by the spread of the meniscus and the magnetic flux density.
1. Introduction M a g n e t i c fluid seals are a d v a n t a g e o u s in that they realize a p e r f e c t seal with no actual c o n t a c t b e t w e e n the sliding surfaces. A successful application of the seals is the s p i n d l e seal for m a g n e t i c disk drive use e n g i n e e r e d to p r o t e c t t h e m from dust and moisture. F r o m the viewpoints of reliability and life of the seal action, a g r e a t a m o u n t of fluid is c o n s i d e r e d d e s i r a b l e for injection into the gap b e t w e e n the yoke and spindle. However, any excessive fluid is f o r c e d to splash from the gap, a n d this s u b s e q u e n t splashing b e c o m e s a serious p r o b l e m as the r o t a t i o n a l s p e e d of the disk drive increases. In this p a p e r , the initiation c o n d i t i o n s for splashing are e x p e r i m e n t a l l y investigated in c o n n e c t i o n with the meniscus s h a p e of the m a g n e t i c fluid f o r m e d a r o u n d the gap.
tom one is a four-stage seal that is c a p a b l e of w i t h s t a n d i n g h i g h e r p r e s s u r e c o m p a r e d with the e x p e r i m e n t a l one. T h e inside hole is c o n n e c t e d to a p r e s s u r e r e g u l a t o r a n d a p r e s s u r e gauge. Each of the test seals c o m p r i s e s two d i s k - s h a p e d yokes and a ring m a g n e t , and thus forms a two-stage seal. T h e yoke has a 40-rnm inside d i a m e t e r and a
M~>nilo]
]'}xp~'l im~'l: al ma!Zh.li, tl~fid .~(,;d
",
:gha[l [>i~,~~,
~'~
/l
l,;tb~'l
/
2. Experimental apparatus and procedure A s c h e m a t i c d i a g r a m of the e x p e r i m e n t a l app a r a t u s is illustrated in fig. 1. Being s u p p o r t e d by two ball bearings, a cylindrical r o t o r having a hole inside it is driven vertically by a belt. T h e hole is s e a l e d by two kinds of m a g n e t i c fluid seals at b o t h ends. T h e t o p one, i n t e n d e d for e x p e r i m e n tal p u r p o s e s , is a two-stage seal and is exchangeable to facilitate e x p e r i m e n t a t i o n , while the bot-
/
] ~}l,~l/>',l'lt>
__J]__ Correspondence to." Prof. Y. Mitsuya, Dept. of Electronic-
Mechanical Engineering, Nagoya University, Chikusa-ku, Nagoya 464-(11, Japan.
]Fig. ]. Schematic diagram of experimenlal apparatus.
113/14-8853/93/$06.00 ~ 1993 - Elsevier Science Publishers B.V. All rights reserved
421
Y Mitsuya / Splashing characteristics of magnetic fluid in a seal
Microdispenser
t-"~:I
~U
v---t
-
Shaft ~ piece ~
= Yokes Magnet
c.~
I
"1
LI"
"1
1 mm Yoke surface (Height)
v539.4 o4o Fig. 2. Filling-in of magnetic fluid. 1.5-mm thickness. T h e radial clearance between the yoke and spindle measures 0.3 + 0.05 mm. Fairly larger (one and half times larger) clearance was selected to facilitate experimentation compared with the value obtainable in practical use. It should be noted that the test seal rotates together with the cylinder. Magnetic flux inside the gap was designed to be 0.2, 0.3 or 0.4 T. Magnetic fluid having a 200 cp viscosity (at 20°C) and a 0.018 T magnetization (at 75.4 A / m ) was selected since it is the same as that for magnetic disk drive use. Before initiating the experiment, a prescribed a m o u n t of fluid is injected onto only one gap of the two-stage seal and is allowed to form a meniscus a r o u n d the gap as d e m o n s t r a t e d in fig. 2. This is done to enhance experimental accuracy especially with respect to the relationship between the injected a m o u n t of fluid and meniscus spread. A microscope, light source and the meniscus were arranged in a straight line so that the meniscus serves as a light shield and as a surface on which the light is reflected. A laser was selected as the light source to obtain a parallel beam. T h e meniscus surface was then clearly observed by microscope.
3. Experimental results Figure 3 presents representative photos demonstrating the typical meniscus shape with mag-
"-cJ
~D ¢
r/~
r"
CD
"-cJ
1 mm Yoke surface (Height)
(D e.)
,
1 mm
Yoke surface (Height) Fig. 3. Typical meniscus shape; ordinate indicates spindle surface, abscissa yoke surface (rotational speed: n = 0 rpm; amount of injected fluid: q = 100 p.l). Magnetic flux density Bg = 0.2 T (top), 0.3 T (middle), 0.4 T (bottom).
Y. Mitsuya / Splashing characteristics of'magnetic [luid in a seal
422
• .3.p Inm.A(l
2.0
-30
• b_
• E]
-.~ 1.0
q
/x.
[]
/x
o • []
30
85
•
115
o
•
:
•
[]
'U)
q
pl
2.0
•
• E
Z
%
A
E3
~
'2x
~.
o
o
Fto~ational ~l)(,(,(t,
((.2 1'
4
1500 n
•
O
O
e
0.0
1000
,2
ii
0.0
500
•
I~,~
•
ii
1 1.5
(,
30
1.0 J39 = (1.2 r
lZl
s5
l
3O
•
.2
E1
l/llil:\(
500
1000
l/ot;ltioaal spu,,d,
rt)m
=
1500 n
I'I/lli
(a) Meniscus width. (b) Meniscus height. Fig. 4. Meniscus width or height versus rotational speed relationships.
netic flux density inside the gap as a parameter. In the photos, the bright white brush strokes bridging the lower left corner exhibit the interface surface between the magnetic fluid and air. The vertical fine white brush stroke designates the spindle surface. The yoke surface is arranged to coincide with the horizontal scale line. The surface tension effect evidently appears close to the spindle and yoke surfaces. From the comparison between the results from three kinds of magnetic flux densities, where Bg = 0.2, 0.3 and 0.4 T, the interface shape is found to be affected not only by the surface tension but also by the magnetic flux. The contour of the magnetic flux density inside the corner is known to comprise concentric quarter circles having a center at the cross point of the two surfaces. The interface line is found to curve convexly with increasing magnetic flux density so as to follow the contour. The meniscus spread is measured by its height, h, and width, w, that is, the meniscus distance along the yoke surface and spindle surface, respectively. Figure 4 presents the relationships between the meniscus width or height and the rational speed with fluid amount and pressure difference as parameters, where the pressure difference means outside pressure minus inside pressure. The meniscus width is found to monotonously decrease with an increasing rotational speed. This
is because the centrifugal force acts to flatten the meniscus. As for the meniscus height, it is interesting to note that there are two kinds of trends depending on the pressure difference. One is an increase in the height with an increasing rotational speed for positive and null pressure differences, and the other is a decrease in the height for a negative pressure difference. The reason for the decrease in height is thought to be that, for the negative pressure difference, the rise of the meniscus at the inside yoke might be larger than that at the outside such that the centrifugal force
t; [-]
~2.0
[q
X\ idrh
, D (-;,
L2,~
d -~c= 1.0
Ks
O
iJ
,7'0 c o %~
L
,
j[YlY~
f]
[] /f,~ = 0.2 i
0.0
L
500
i
I
1000
i
i
i
1500
( ' r i t i c a l i~iidi~ili;i[ ~t>t,'d. /6
I
2000 IIml
Fig. 5. Relationship between Meniscus width or height and critical speed wherein splashing begins for a magnetic flux density of Bg = 0.2 T.
Y. Mitsuya / Splashing characteristics of magnetic fluid in a seal
•
~•
mmmm
2.0
~•
mm
• (IN,IfI 0
•
•
•
B9 T 0.2
•
0.3
•
0.4
•
• Mm•
m m
1.0
T
0.0
I
i
I
t
1000 2000 Crilica] totatiott;'d spe(~ct, ,'~, i-pill
Fig. 6. Relationship between Meniscus height and critical speed wherein splashing begins with magnetic flux density as a parameter.
serves to attract the fluid toward the inner direction. The experiments outlined above were performed until splashing began. Therefore, the highest speeds appearing in the figures designated critical speeds just before initiation of splashing. Figure 5 summarizes the relationship between the critical speeds, n c, and meniscus width, w or height, h. These results include whole data obtained under the conditions of fluid amounts ranging from 85 to 145 ~1 and pressure differences ranging from - 3 0 to 30 mmAq. In such a wide range of differences, the critical speeds lie on nearly a unique curve similar to a hyperbola. This confirms the facts that the initiation conditions uniquely depend on the meniscus height or width no matter what the difference is
423
in fluid amount and pressure, and that the larger is the meniscus spread, the lower is the critical speed. The same experiments were performed for the values of magnetic flux density, where B e = 0.2, 0.3 and 0.4 T. Each result is superimposed in fig. 6 with the focus on the meniscus height, h, versus critical speed, n c, relationship. A quasi-inverse proportion relationship between h and n c is obtained for every case. From the comparison, it is found that, with an increasing Be, the critical speed tends to shift to a higher region. It should be noted that the increase in B e from 0.2 to 0.4 T considerably increased the critical speed nearly two times.
4. Summary Splashing characteristics of the magnetic fluid spindle seal for magnetic disk storage use were experimentally studied. The meniscus spread and configuration formed around the seal gap were measured in terms of the injected amount of magnetic fluid, pressure difference, magnetic flux density and rotational speed. Initiation of splashing of magnetic fluid is found to be uniquely determined by the spread of the meniscus for a fixed magnetic flux density. It should be noted that, with increasing flux density from 0.2 to 0.4 T, the critical speed is considerably increased nearly two times. Large magnetic flux, little fluid and small pressure difference are desirable owing to the small meniscus spread.
Ai I"
Experimental study on splashing characteristics of magnetic fluid in a magnetic fluid seal Yasunaga
M i t s u y a ~, K e n j i T o m i t a " a n d M a s a y u k i H o s o y a b
" Dept. o1 Electronic-Mechanical Engineering, Nagoya Unit,ersity, Chikusa-ku, Nagoya 464-01, Japan :' Material Technology Laboratoo', Nihon Seiko Co. Ltd., Kugenuma-simmei, Fu/isawa, Kanagawa 251, Japan
The splashing characteristics of a magnetic fluid spindle seal designed tk~rmagnetic disk storage use werc experimentally studied in terms of the meniscus spread and configuration formed around the seal gap. The critical rotational speed at which splashing of the magnetic fluid is initiated is found to be uniquely determined by the spread of the meniscus and the magnetic flux density.
1. Introduction M a g n e t i c fluid seals are a d v a n t a g e o u s in that they realize a p e r f e c t seal with no actual c o n t a c t b e t w e e n the sliding surfaces. A successful application of the seals is the s p i n d l e seal for m a g n e t i c disk drive use e n g i n e e r e d to p r o t e c t t h e m from dust and moisture. F r o m the viewpoints of reliability and life of the seal action, a g r e a t a m o u n t of fluid is c o n s i d e r e d d e s i r a b l e for injection into the gap b e t w e e n the yoke and spindle. However, any excessive fluid is f o r c e d to splash from the gap, a n d this s u b s e q u e n t splashing b e c o m e s a serious p r o b l e m as the r o t a t i o n a l s p e e d of the disk drive increases. In this p a p e r , the initiation c o n d i t i o n s for splashing are e x p e r i m e n t a l l y investigated in c o n n e c t i o n with the meniscus s h a p e of the m a g n e t i c fluid f o r m e d a r o u n d the gap.
tom one is a four-stage seal that is c a p a b l e of w i t h s t a n d i n g h i g h e r p r e s s u r e c o m p a r e d with the e x p e r i m e n t a l one. T h e inside hole is c o n n e c t e d to a p r e s s u r e r e g u l a t o r a n d a p r e s s u r e gauge. Each of the test seals c o m p r i s e s two d i s k - s h a p e d yokes and a ring m a g n e t , and thus forms a two-stage seal. T h e yoke has a 40-rnm inside d i a m e t e r and a
M~>nilo]
]'}xp~'l im~'l: al ma!Zh.li, tl~fid .~(,;d
",
:gha[l [>i~,~~,
~'~
/l
l,;tb~'l
/
2. Experimental apparatus and procedure A s c h e m a t i c d i a g r a m of the e x p e r i m e n t a l app a r a t u s is illustrated in fig. 1. Being s u p p o r t e d by two ball bearings, a cylindrical r o t o r having a hole inside it is driven vertically by a belt. T h e hole is s e a l e d by two kinds of m a g n e t i c fluid seals at b o t h ends. T h e t o p one, i n t e n d e d for e x p e r i m e n tal p u r p o s e s , is a two-stage seal and is exchangeable to facilitate e x p e r i m e n t a t i o n , while the bot-
/
] ~}l,~l/>',l'lt>
__J]__ Correspondence to." Prof. Y. Mitsuya, Dept. of Electronic-
Mechanical Engineering, Nagoya University, Chikusa-ku, Nagoya 464-(11, Japan.
]Fig. ]. Schematic diagram of experimenlal apparatus.
113/14-8853/93/$06.00 ~ 1993 - Elsevier Science Publishers B.V. All rights reserved
421
Y Mitsuya / Splashing characteristics of magnetic fluid in a seal
Microdispenser
t-"~:I
~U
v---t
-
Shaft ~ piece ~
= Yokes Magnet
c.~
I
"1
LI"
"1
1 mm Yoke surface (Height)
v539.4 o4o Fig. 2. Filling-in of magnetic fluid. 1.5-mm thickness. T h e radial clearance between the yoke and spindle measures 0.3 + 0.05 mm. Fairly larger (one and half times larger) clearance was selected to facilitate experimentation compared with the value obtainable in practical use. It should be noted that the test seal rotates together with the cylinder. Magnetic flux inside the gap was designed to be 0.2, 0.3 or 0.4 T. Magnetic fluid having a 200 cp viscosity (at 20°C) and a 0.018 T magnetization (at 75.4 A / m ) was selected since it is the same as that for magnetic disk drive use. Before initiating the experiment, a prescribed a m o u n t of fluid is injected onto only one gap of the two-stage seal and is allowed to form a meniscus a r o u n d the gap as d e m o n s t r a t e d in fig. 2. This is done to enhance experimental accuracy especially with respect to the relationship between the injected a m o u n t of fluid and meniscus spread. A microscope, light source and the meniscus were arranged in a straight line so that the meniscus serves as a light shield and as a surface on which the light is reflected. A laser was selected as the light source to obtain a parallel beam. T h e meniscus surface was then clearly observed by microscope.
3. Experimental results Figure 3 presents representative photos demonstrating the typical meniscus shape with mag-
"-cJ
~D ¢
r/~
r"
CD
"-cJ
1 mm Yoke surface (Height)
(D e.)
,
1 mm
Yoke surface (Height) Fig. 3. Typical meniscus shape; ordinate indicates spindle surface, abscissa yoke surface (rotational speed: n = 0 rpm; amount of injected fluid: q = 100 p.l). Magnetic flux density Bg = 0.2 T (top), 0.3 T (middle), 0.4 T (bottom).
Y. Mitsuya / Splashing characteristics of'magnetic [luid in a seal
422
• .3.p Inm.A(l
2.0
-30
• b_
• E]
-.~ 1.0
q
/x.
[]
/x
o • []
30
85
•
115
o
•
:
•
[]
'U)
q
pl
2.0
•
• E
Z
%
A
E3
~
'2x
~.
o
o
Fto~ational ~l)(,(,(t,
((.2 1'
4
1500 n
•
O
O
e
0.0
1000
,2
ii
0.0
500
•
I~,~
•
ii
1 1.5
(,
30
1.0 J39 = (1.2 r
lZl
s5
l
3O
•
.2
E1
l/llil:\(
500
1000
l/ot;ltioaal spu,,d,
rt)m
=
1500 n
I'I/lli
(a) Meniscus width. (b) Meniscus height. Fig. 4. Meniscus width or height versus rotational speed relationships.
netic flux density inside the gap as a parameter. In the photos, the bright white brush strokes bridging the lower left corner exhibit the interface surface between the magnetic fluid and air. The vertical fine white brush stroke designates the spindle surface. The yoke surface is arranged to coincide with the horizontal scale line. The surface tension effect evidently appears close to the spindle and yoke surfaces. From the comparison between the results from three kinds of magnetic flux densities, where Bg = 0.2, 0.3 and 0.4 T, the interface shape is found to be affected not only by the surface tension but also by the magnetic flux. The contour of the magnetic flux density inside the corner is known to comprise concentric quarter circles having a center at the cross point of the two surfaces. The interface line is found to curve convexly with increasing magnetic flux density so as to follow the contour. The meniscus spread is measured by its height, h, and width, w, that is, the meniscus distance along the yoke surface and spindle surface, respectively. Figure 4 presents the relationships between the meniscus width or height and the rational speed with fluid amount and pressure difference as parameters, where the pressure difference means outside pressure minus inside pressure. The meniscus width is found to monotonously decrease with an increasing rotational speed. This
is because the centrifugal force acts to flatten the meniscus. As for the meniscus height, it is interesting to note that there are two kinds of trends depending on the pressure difference. One is an increase in the height with an increasing rotational speed for positive and null pressure differences, and the other is a decrease in the height for a negative pressure difference. The reason for the decrease in height is thought to be that, for the negative pressure difference, the rise of the meniscus at the inside yoke might be larger than that at the outside such that the centrifugal force
t; [-]
~2.0
[q
X\ idrh
, D (-;,
L2,~
d -~c= 1.0
Ks
O
iJ
,7'0 c o %~
L
,
j[YlY~
f]
[] /f,~ = 0.2 i
0.0
L
500
i
I
1000
i
i
i
1500
( ' r i t i c a l i~iidi~ili;i[ ~t>t,'d. /6
I
2000 IIml
Fig. 5. Relationship between Meniscus width or height and critical speed wherein splashing begins for a magnetic flux density of Bg = 0.2 T.
Y. Mitsuya / Splashing characteristics of magnetic fluid in a seal
•
~•
mmmm
2.0
~•
mm
• (IN,IfI 0
•
•
•
B9 T 0.2
•
0.3
•
0.4
•
• Mm•
m m
1.0
T
0.0
I
i
I
t
1000 2000 Crilica] totatiott;'d spe(~ct, ,'~, i-pill
Fig. 6. Relationship between Meniscus height and critical speed wherein splashing begins with magnetic flux density as a parameter.
serves to attract the fluid toward the inner direction. The experiments outlined above were performed until splashing began. Therefore, the highest speeds appearing in the figures designated critical speeds just before initiation of splashing. Figure 5 summarizes the relationship between the critical speeds, n c, and meniscus width, w or height, h. These results include whole data obtained under the conditions of fluid amounts ranging from 85 to 145 ~1 and pressure differences ranging from - 3 0 to 30 mmAq. In such a wide range of differences, the critical speeds lie on nearly a unique curve similar to a hyperbola. This confirms the facts that the initiation conditions uniquely depend on the meniscus height or width no matter what the difference is
423
in fluid amount and pressure, and that the larger is the meniscus spread, the lower is the critical speed. The same experiments were performed for the values of magnetic flux density, where B e = 0.2, 0.3 and 0.4 T. Each result is superimposed in fig. 6 with the focus on the meniscus height, h, versus critical speed, n c, relationship. A quasi-inverse proportion relationship between h and n c is obtained for every case. From the comparison, it is found that, with an increasing Be, the critical speed tends to shift to a higher region. It should be noted that the increase in B e from 0.2 to 0.4 T considerably increased the critical speed nearly two times.
4. Summary Splashing characteristics of the magnetic fluid spindle seal for magnetic disk storage use were experimentally studied. The meniscus spread and configuration formed around the seal gap were measured in terms of the injected amount of magnetic fluid, pressure difference, magnetic flux density and rotational speed. Initiation of splashing of magnetic fluid is found to be uniquely determined by the spread of the meniscus for a fixed magnetic flux density. It should be noted that, with increasing flux density from 0.2 to 0.4 T, the critical speed is considerably increased nearly two times. Large magnetic flux, little fluid and small pressure difference are desirable owing to the small meniscus spread.