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Magring sealed ball bearing
Journal of Magnetism and Magnetic Materials 65 (1987) 389-392 North-Holland, Amsterdam
MAGIUNG
SEALED
389
BALL BEARING
A. HIRAO Minebea Co. Ltd., Miyota, Kitmaky
Nagano, 389-02, Japan
and F.D. EZEKIEL Servoflo Corporation,
75 Allen St., Lexington,
MA. 02173, USA
A one piece, magnetic liquid shaft seal installed between the ball bearing races provides a hermetic dynamic seal. This paper discusses the advantages of such a sealed ball bearing and includes experimental data dealing with differential pressure capability, effect of mixing of magnetic liquid with grease on bearing noise, etc.
1. Introduction
Magnetic liquid shaft seals have been extensively and successfully used in hard disc drives to prevent ball bearing grease particles from entering the magnetic disc and head areas. These seals are uniquely suited to this application for their hermetic sealing capability, zero static friction, low dynamic drag, high speed capability, long life, no wear, etc. With the advent of smaller and thinner disc drives, the designer is under constant pressure to reduce the distance between the ball bearings and thus sacrifice shaft stiffness. Any axial space becomes a premium. Thus it becomes desirable to include the magnetic liquid seal within the ball bearing itself. This paper discusses the use and design of a magring sealed bearing.
tion molded and can be press fitted in the housing without any adhesive for statically sealing the OD. It can be made as thin as one mm. These characteristics of the magring seal make it particularly attractive for inclusion in a ball bearing. The magring seal is generally secured to the ID of the outer race of the ball bearing, while magnetic liquid is retained in the gap between the ID of the seal and the OD of the inner race. The entire sealing function is accomplished with just one mechanical part. Three to four pieces may be necessary to achieve the same function by using
2. Product description
A magring seal is a one piece ring, usually made out of ferrite-filled plastic, in the form of an axially magnetized permanent magnet. It is designed to retain magnetic liquid between its ID and a rotating shaft. Since this seal has no pole pieces, it can be used with a magnetic as well as a non-magnetic shaft or housing material. It is usually injec030~8853/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Inner shield Magring seal
Material Outer race-SAE 52100 I nner race-SAE 52100 Ball -SAE 52100 Retamer -AlSl 304 I nner shield-A IS I 304 Magrmg seal-Strontium Ferrite + Nylon 12 Fig. 1. Cross section of magring sealed ball bearing.
390
A. Hirao, F. D. Ezekiel / Magring sealed bail bearing
more conventional magnetic liquid seal designs. Fig. 1 shows a magring seal installed in a ball bearing. A magring sealed ball bearing offers several advantages. To the designers, it means the elimination of an external seal with the associated installation and charging procedures; a stiffer shaft as a result of placing the ball bearings further apart; an option of applying axial forces to the inner or outer bearing races for preload; and simpler over-all design. From a production point of view, it means one less part to purchase; inventory and assemble; .the elimination of the use of adhesive to secure the seal to the housing; and less over-all cost.
* +
24’C
Dimensions
(mm)
Volume of magnetic liquid
3. Test results and discussions Fig. 2. Differential
In order to achieve the desired results, the width of a standard ball bearing has to be increased slightly in order to accomodate the magring seal. General test results show that a magring seal can perform well in a high precision ball bearing. Virtually no mixing between the magnetic liquid and the bearing grease has been observed. Also the bearing noise, which is one of the critical characteristics for the disc drive application, did not increase with the introduction of the seal. Fig. 2 shows the measured differential pressure capability of a$ magring sealed ball bearing as a function of the volume of magnetic liquid introduced. For most disc drive applications, a dif-_ ferential pressure of 100 to 200 mm of water column is adequate. The effect of intentional mixing of magnetic liquid with bearing grease on the resulting differential pressure has been examined. No significant effect is observed up to 10% mixture. Fig. 3 shows the effect of intentional mixing of magnetic liquid with various types of commonly used bearing greases on bearing noise level. No deteriorating effect on noise as a result of adding up to 3 microliter of magnetic liquid has been observed. Fig. 4 shows a measurement of magnetic flux leakage around a magring sealed ball bearing at various axial positions. At a distance of 15 mm
R-1650 R-2812
( pl )
pressure capability vs. volume of magnetic liquid introduced.
from the bearing the flwc was less than 3 G. This level is acceptable for most magnetically sensitive applications. It should be pointed out that the magnetic flux levels are usually altered when the bearings are installed in the disc drive housing which includes the electric drive motor. Extensive
,50-
Rotation
4 Q?
Test
40
rpm
R-1650
ANDOK
-
BEACON 325
Ltthium whetic
!RUkTEMP
Lithium Synthetic ester 011
-
0
1800
-c-
I(
0
speed
bea ring
C
Mmeral oil Sodium complex ester oil
I 2 1 Volume of magnetic liquid mixed with grease
3
(~1)
Fig. 3. Effect of mixing of magnetic liquid with bearing-grease on noise level.
A. Hirao, F. D. Ezekiel
--D* +
391
::I,5
R-1650
Z=lOmm z-15mm Z=X)mm Z=X)mm
e
/ Magring sealed ball bearing
_ a, %?
u -8 Ditterential
0
Fig.
> 2
4. Magnetic
4
6 Distance
8 Y
0 (mm
flux leakage around bearing.
5000 Rotation
) magring
sealed
ball
study has been made on the magring seal geometry in order to optimize the differential pressure capability without sacrificing the high precision bearing performance. Fig. 5 shows diffrential pressures developed by magring seals having various OD’s and thickness but with fixed ID’s_ This
Fig. 6. Torque,
\ 150 mmHn0
Magnetic liquid : Base oil Synthetic hjdru carbon IrwlOXide Magnetic particle 150-200 cps Viscosity Magnetic saturation 3OOGaus.S Shaft : SAE 52100 Housing SAE 52100 3
2 ot Magring
-10
% Ir
20000
1 pressure vs.
study is important from the point of view of maintaining a certain stiffness of the outer race so that it will not be distorted by the press fitting of the magring seal. Fig. 6 shows some of the ~ynan$ characterjstics of a magring sealed ball’ beariqg. They z+re dynamic drag vs yotatior)d sp@, differential pressure ys speed @d tepperz+tuyg rise in @?e
burjqg bpqipg
rota-
tion. The increase in torque and iemperature are minimal up to 10000 rpm without any observed adverse effects. The differential pressure is fairly stable over a wide range of sped. A seal failure however, tends to be ab&pt wd cap$eq s#teyiqg of the magnetic liquid on the side face of the seal. It is to be noted that the dynamic torque due to the seal is considerably less than the calculated value based on magnetic liquid viscosity data. This is probably due to an area reduction effect in the share domain of the liquid caused by the centrifugal force.
100 mmHzO
1 Thickness
15000 ( rRm
temperature rise and differential rotational speed.
vjciuity qf qpagjF)etk l#@
\
10000 Speed
pressure capabillty (20°C)
4. Conclusion
(mm)
Fig. 5. Differential pressure developed by magring various OD and thickness.
seal of
A one piece, axially magnetized seal installed between ball bearing races can succesfully provide
392
A. Hirao, F.D. Ezekiel / Magring sealed ball bearing
a hermetic dynamic seal with a magnetic liquid. This concept will be applicable to most contamination sensitive applications using ball bearings. It may be concluded that a magring
sealed ball bearing offers a promising approach where clean and smooth running features are required.
MAGIUNG
SEALED
389
BALL BEARING
A. HIRAO Minebea Co. Ltd., Miyota, Kitmaky
Nagano, 389-02, Japan
and F.D. EZEKIEL Servoflo Corporation,
75 Allen St., Lexington,
MA. 02173, USA
A one piece, magnetic liquid shaft seal installed between the ball bearing races provides a hermetic dynamic seal. This paper discusses the advantages of such a sealed ball bearing and includes experimental data dealing with differential pressure capability, effect of mixing of magnetic liquid with grease on bearing noise, etc.
1. Introduction
Magnetic liquid shaft seals have been extensively and successfully used in hard disc drives to prevent ball bearing grease particles from entering the magnetic disc and head areas. These seals are uniquely suited to this application for their hermetic sealing capability, zero static friction, low dynamic drag, high speed capability, long life, no wear, etc. With the advent of smaller and thinner disc drives, the designer is under constant pressure to reduce the distance between the ball bearings and thus sacrifice shaft stiffness. Any axial space becomes a premium. Thus it becomes desirable to include the magnetic liquid seal within the ball bearing itself. This paper discusses the use and design of a magring sealed bearing.
tion molded and can be press fitted in the housing without any adhesive for statically sealing the OD. It can be made as thin as one mm. These characteristics of the magring seal make it particularly attractive for inclusion in a ball bearing. The magring seal is generally secured to the ID of the outer race of the ball bearing, while magnetic liquid is retained in the gap between the ID of the seal and the OD of the inner race. The entire sealing function is accomplished with just one mechanical part. Three to four pieces may be necessary to achieve the same function by using
2. Product description
A magring seal is a one piece ring, usually made out of ferrite-filled plastic, in the form of an axially magnetized permanent magnet. It is designed to retain magnetic liquid between its ID and a rotating shaft. Since this seal has no pole pieces, it can be used with a magnetic as well as a non-magnetic shaft or housing material. It is usually injec030~8853/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Inner shield Magring seal
Material Outer race-SAE 52100 I nner race-SAE 52100 Ball -SAE 52100 Retamer -AlSl 304 I nner shield-A IS I 304 Magrmg seal-Strontium Ferrite + Nylon 12 Fig. 1. Cross section of magring sealed ball bearing.
390
A. Hirao, F. D. Ezekiel / Magring sealed bail bearing
more conventional magnetic liquid seal designs. Fig. 1 shows a magring seal installed in a ball bearing. A magring sealed ball bearing offers several advantages. To the designers, it means the elimination of an external seal with the associated installation and charging procedures; a stiffer shaft as a result of placing the ball bearings further apart; an option of applying axial forces to the inner or outer bearing races for preload; and simpler over-all design. From a production point of view, it means one less part to purchase; inventory and assemble; .the elimination of the use of adhesive to secure the seal to the housing; and less over-all cost.
* +
24’C
Dimensions
(mm)
Volume of magnetic liquid
3. Test results and discussions Fig. 2. Differential
In order to achieve the desired results, the width of a standard ball bearing has to be increased slightly in order to accomodate the magring seal. General test results show that a magring seal can perform well in a high precision ball bearing. Virtually no mixing between the magnetic liquid and the bearing grease has been observed. Also the bearing noise, which is one of the critical characteristics for the disc drive application, did not increase with the introduction of the seal. Fig. 2 shows the measured differential pressure capability of a$ magring sealed ball bearing as a function of the volume of magnetic liquid introduced. For most disc drive applications, a dif-_ ferential pressure of 100 to 200 mm of water column is adequate. The effect of intentional mixing of magnetic liquid with bearing grease on the resulting differential pressure has been examined. No significant effect is observed up to 10% mixture. Fig. 3 shows the effect of intentional mixing of magnetic liquid with various types of commonly used bearing greases on bearing noise level. No deteriorating effect on noise as a result of adding up to 3 microliter of magnetic liquid has been observed. Fig. 4 shows a measurement of magnetic flux leakage around a magring sealed ball bearing at various axial positions. At a distance of 15 mm
R-1650 R-2812
( pl )
pressure capability vs. volume of magnetic liquid introduced.
from the bearing the flwc was less than 3 G. This level is acceptable for most magnetically sensitive applications. It should be pointed out that the magnetic flux levels are usually altered when the bearings are installed in the disc drive housing which includes the electric drive motor. Extensive
,50-
Rotation
4 Q?
Test
40
rpm
R-1650
ANDOK
-
BEACON 325
Ltthium whetic
!RUkTEMP
Lithium Synthetic ester 011
-
0
1800
-c-
I(
0
speed
bea ring
C
Mmeral oil Sodium complex ester oil
I 2 1 Volume of magnetic liquid mixed with grease
3
(~1)
Fig. 3. Effect of mixing of magnetic liquid with bearing-grease on noise level.
A. Hirao, F. D. Ezekiel
--D* +
391
::I,5
R-1650
Z=lOmm z-15mm Z=X)mm Z=X)mm
e
/ Magring sealed ball bearing
_ a, %?
u -8 Ditterential
0
Fig.
> 2
4. Magnetic
4
6 Distance
8 Y
0 (mm
flux leakage around bearing.
5000 Rotation
) magring
sealed
ball
study has been made on the magring seal geometry in order to optimize the differential pressure capability without sacrificing the high precision bearing performance. Fig. 5 shows diffrential pressures developed by magring seals having various OD’s and thickness but with fixed ID’s_ This
Fig. 6. Torque,
\ 150 mmHn0
Magnetic liquid : Base oil Synthetic hjdru carbon IrwlOXide Magnetic particle 150-200 cps Viscosity Magnetic saturation 3OOGaus.S Shaft : SAE 52100 Housing SAE 52100 3
2 ot Magring
-10
% Ir
20000
1 pressure vs.
study is important from the point of view of maintaining a certain stiffness of the outer race so that it will not be distorted by the press fitting of the magring seal. Fig. 6 shows some of the ~ynan$ characterjstics of a magring sealed ball’ beariqg. They z+re dynamic drag vs yotatior)d sp@, differential pressure ys speed @d tepperz+tuyg rise in @?e
burjqg bpqipg
rota-
tion. The increase in torque and iemperature are minimal up to 10000 rpm without any observed adverse effects. The differential pressure is fairly stable over a wide range of sped. A seal failure however, tends to be ab&pt wd cap$eq s#teyiqg of the magnetic liquid on the side face of the seal. It is to be noted that the dynamic torque due to the seal is considerably less than the calculated value based on magnetic liquid viscosity data. This is probably due to an area reduction effect in the share domain of the liquid caused by the centrifugal force.
100 mmHzO
1 Thickness
15000 ( rRm
temperature rise and differential rotational speed.
vjciuity qf qpagjF)etk l#@
\
10000 Speed
pressure capabillty (20°C)
4. Conclusion
(mm)
Fig. 5. Differential pressure developed by magring various OD and thickness.
seal of
A one piece, axially magnetized seal installed between ball bearing races can succesfully provide
392
A. Hirao, F.D. Ezekiel / Magring sealed ball bearing
a hermetic dynamic seal with a magnetic liquid. This concept will be applicable to most contamination sensitive applications using ball bearings. It may be concluded that a magring
sealed ball bearing offers a promising approach where clean and smooth running features are required.