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Paper XVIII(ii) Study on fundamental characteristics of rotating lip-type oil seals
553
Paper XVlll(ii)
Study on fundamental characteristics of rotating lip-type oil seals Masanori Ogata, Takuzo Fujii and Yorikazu Shimotsuma
The puipose of this paper is to investigate the friction and lubrication conditions within the sealing zone of lip-type oil seals. The friction coefficient, oil film breakdown ratio, and dynamic lip motion are simultaneously measured from extreme low-speed of 0.003 m/s ( 1 rpm ) to high-speed of 18 m/s ( 7000 rpm ). Up to the present, it has been accepted that the lubrication conditions within the sealing zone are subject to fluid film lubrication. However, experimental results show that they vary from dry friction in the extreme low-speed region to fluid film lubrication in the high-speed region through boundary and elastohydrodynamic lubrication conditions. 1 INTRODUCTION
It has been explained that the lubrication conditions within the sealing zone are subect mainly to fluid film lubrication. From the results measured for the friction coefficient, Hirano and Ishiwata (1) found that the friction coefficient f was proportiona to the characteristic term (Q*U/P~)~)~ which is equivalent to the bearing modulus. Furthermore, they verified this theoretically applying foil bearing theory by Blok et.al (2). On the other hand, Jagger and Walker (3) defined that the friction coefficient f was roportional to the terms of (rl*U)1/3 and Pa-119 of contact pressure based on the theory for elastohydrodynamic lubrication. In the present paper, not only mesurement of the friction coefficient, but also the oil film breakdown ratio (4) are used as methods for quantities estimation of the lubrication condition to observe the seal surface in datail. The results suggest that the friction and lubrication conditions within the sealing zone of lip-type oil seals cannot probably be explained by one of the lubrication theories alone. 2
APPARATUS
The lip-type oil seals used in this study are shown in Figure 1. They have a metal case embedded and a garter spring. Their Case width is 12.5 mm and outer diameter is 72.0 nun. The bore diameter before installation is 48.2 mm at room temparature, and after installation with a standardized shaft of 50 mm in diameter, the lip is given the interference of 1.8 nun in diameter. The contact band width b generated in contact with the shaft expands to 1.43 mm, and the radial load P, amounts to 326 N. To electrically observe the lubrication within conditions the sealing zone, the seal is given electric conductivity adding carbon black in larger quantities than usual to the NBR. The resistance for unit volume is 528 Q/m. The thermocouples are set to the lip whose position is 1.0 mm just under the lip edge. They are used to measure the body and surface
LIP w;E Fig.1
Sectional view of lip-type oil seal used for test
temeratures. Fur hermore. he strain gauges are'attached to the outside and sealing side of the seal to observe lip motions. A test seal is mounted to the head part of the experimental apparatus shown in Figure 2. The main shaft is driven by a positively infinite variable gear changer from 1 to 7000 rpm. A main shaft of 50.0 mm in diameter is used. The type of sealed lubricating oil is turbine oil of viscosity 56 cSt at 40 OC characteristically. It is filled in the sealing side, and circulated by the oil pump. The temperature of the lubricating oil is controlled to 30 OC at the position of the lip
554
TEMPERATI JRE RECORDER
STRAIN METER
D I G I T A L MEMORY AND SYNCHRONIZED OSCILLOGRAPH
LUbR
I
I
RC C I R C U I T
Fig. 2 Head part of seal lubrication tester type I
surface. The friction torque Tf of the lip at the sealing zone is measured by the load cell connected to the housing. The friction coefficient f of the seal lip at the sealing zone can be expressed as equation (1) employing the radial load P, and the shaft diameter d. f = Tf/ ( Pr.d/2 ) (1) The oil film breakdown ratio is measured by a direct current circuit comprising a resistor, capacitor, shaft, and seal. The charging characteristic of the capacitor differs from cases where the shaft is perfectly contacted o r occasionally contacted with the seal lip. Thereupon, the time elapsed when the voltage of the capacitor reached a certain threshold value are added to t, and ti respectively, the oil film breakdown ratio E is defined by equation 2 E = (t,/tl).100 (2) The oil film breakdown ratio E takes a value of 100 percent when the oil film is perfectly broken the shaft contacts the seal lip as usual, meanwhile, E takes a value of almost zero percent when the shaft and the seal lip are separated by a sufficiently thick oil film. This method is an original development ( 4 ) .
.
3 RESULTS and DISCUSSIONS Simultaneously measured friction and lubrication conditions and dynamic motion of the seal lip within the sealing zone are shown in Figures 3 and 4 . For example, please look at Figure 3 ( S ) .
The figure in the upper row, waveform Waveform @ indicates that an oil film between the shaft and lip was formed and broken. Voltage Vf at zero in order of magnitude indicates a condition where the oil film is broken down, and on the other hand, the voltage Vf at 1 indicates that the oil film is formed. Waveform @ shows the friction torque Tf of the seal lip. Waveforms @ and in the figure in the middle row, show circumferential and radial motions of the seal lip respectively. Waveform @ in the figure in the lower row, shows resultant motion obtained by synchronizing @ with The friction and lubrication conditions within the sealing zone and the temperatures of the seal lip are shown in Figure 5. They are measured when the shaft speed was changed at random from 1 rpm (0.0026 m/s) in the extremely low-speed range to 7000 rpm (18 m/s) in the high-speed range. The friction coefficient f and the oil film breakdown ratio E are obtained from waveformes and @ in Figures 3 and 4 employing equations (1) and (2). And the symbols of S,A,B,C, and D in Figures 3, 4 and 5 indicate the same points of measurement, In the extremely low-speed region from 1 to 5 rpm, the friction coefficient takes a value of 0.08 to 0.07. And the oil film breakdown ratio indicates 80 to 90%, so an effective oil film is scarcely existent. Both values show a contact. Therefore, the possible cause
0, shows shaft rotation.
0
0.
0
555
for this region is dry friction. When point S of shaft speed 1 rpm and point A are investigated clearly at Fig. 3 , oil film breakdown indicates instantaneons breakdown, friction torque waveform @ fluctuates abruptly, and dynamic lip motion waveform @ indicates stick-slip. In the low-speed region from 5 to 50 rpm, the friction coefficient decreases from 0.08 to the minimum value of 0.025. On the other hand, the oil film breakdown ratio decreases from 80 to 40%, though, this does not yet indicate the minimum value. In general, the oil film breakdown ratio also corresponds to the phenomenon of friction coefficient. This difference in phenomenon is explained by waveform @ of the lip motion. The displacement 6 , of data B which indicates the motion in the circumferential direction is smaller than that of data A . This means that the followability of the lip in circumferential direction at data B is inferior to data A. For this reason, although the friction coefficient decreases, the asperities of the shaft and the seal lip easily contact each other, indicating that a high film breakdown ratio is the possible cause. This region is assumed to be the boundary lubrication. In the medium-speed region from 50 to 1000 rpm, the friction coefficient increases to 0.12. The value is a little larger than the 1 rpm value in the extremely low-speed region. As the oil
0
- - - - - --
SEPARATION : Vf = 1 V
'm
film breakdown ratio decreases further from 40%, it takes the minimum value of 2% at 1000 rpm. One of the reasons for these phenomena can be explained by the increase of viscous resistance in the oil film which is formed thicker by the speed effect. The other reason is that the lip motion in radial direction at data C is larger than that of B, therefore the tendency stated above is also explained by the effect of inducement of an oil film to the sealing zone caused by the fluctuation of the seal lip. This region is probably subject to the fluid film lubrication. In the high-speed region from 1000 to 7000 rpm, the friction coefficient fluctuates, however, its mean value indicates 0.12 approximately. Then, the oil film breakdown ratio increases from the minimum value of 2% to 10% with fluctuation. This tendency is probably caused by a temperature of 5 " C in the body of the seal lip slightly higher than the temperature of the surface where the temperature of the lubricating oil is controlled to 3CE2 " C . In this region, although the oil film is formed by the speed effect, the viscosity decreases, and the followability of the seal lip to shaft rotation is increased due to heat generation in the seal lip. When these two factors occur simultaneously, the micro asperities on the surfaces of the shaft and the seal lip do not easily contact with each other and also the oil film is broken. Thereby, the waveform @ of lip motion is obviously different up to this point in the elliptical motion
r
1
> Ic.
>
n
0
r
r
CONTACT : Vf = 0 V
+ . -
I
Tf
r+
'r+
LIP
(S)
1 rpm
Fig. 3 Friction and lubrications within the sealing zone, and motion of a seal lip
556
2 sfdiv
P----l
250 msfdiv
p-----l
W
W
r+
(B) 50 rpm
p d
2.5 msfdiv
(C)
Fig. 4
lC00 rpm
(D) 6000 rpm
Friction and lubrications within the sealing zone, and motion of a seal lip
557
0
50
0
CD W
tlL
40
z IQ
cr W
30
a E W I-
20
cc 1
V
U LL
LL
1 n
c (
SHAFT SPEED Fig. 5
N rpm
Effect of shaft speed on friction coefficient and oil film breakdown ratio
558
tendency that it demostrates. The area of the ellipse indicates the energy consumed by lip motion. It can be easily assumed that energy causes the temperature within the sealing zone to rise high and the viscosity of the lubricating oil to fall. Besides, in the high-speed region, it has been confirmed that the oil film breakdown ratio increases remarkably when the temperature of the lubricating oil is not controlled to a constant value (4). From the above discussion, the oil film thickness within the sealing zone is investigated as follows. The oil film thickness is calculated substituting the operation conditions of the seal for lubrication theories. They are, h~ in case the viscous fluid formed between plain-parallel surfaces is subject to Newton's rule, hg-F by Hirano (1) applying the theory of Blok (2) for foil bearing to a rigid body with surface roughness, and hD by Dowson (5) in consideration for elastic deformation, line contact, and viscosity change by pressure in theory of elastohydrodynamic lubrication. The equation used for the calculations is given in the appendix.
The results are shown in Figure 6 . Hereupon, the operation factor is a characteristic number equivalent to the bearing modulus. The oil film formed between the shaft and the seal lip becomes thicker accordingly as the operation factor increases. By the theoretical equations, the oil film thickness of hg-H and hD take values of 0.67 and 0.7 in gradient, respectively. On the other hand,where the gradient of hN should become 1, it shows fluctuation owing to the variation in the ratio of the shaft speed to the friction coefficient. The film thickness exists in the range from 0.001 pm of hN to 10 pm of hD in order of magnitude. However, from the results measured for the surface roughness of the shaft and the lip shown in Table 1, the oil film thickness between data B of 50 rpm and the data D to almost the maximum speed of 7000 rpm is considered to be reasonable. They are from 0.22 pm in Rrms of the seal lip to 2.04 vm in Rp of the shaft. As mentioned in Figure 5, in these regions the oil film is formed positively by the effect of
10
10 10
10-
.
A
A hNewton
lo-"
I
10-
LO-'
-
0 hBlok-Hirano (1,2)
A
10-
OPERATION FACTOR
rl*U/P1
Fig. 6 Oil film thickness calculated by theories employing experimental conditions speed, and the phenomena whereby the friction coefficient increases by the resistance.of viscosity and the oil film breakdown ratio decreases, are observed. However, in the limited operation factor within the range from to the minimum oil film thickness is measured as 0.05 Um experimentally (4). This value corresponds with the oil film Table 1.
Surface roughness in shaft rotating direction [urn].
Roughness
Shaft
Seal lip
Rmax Rm Ra Rrms
2.04 0.67 1.37 1.28 0.25 0.31
1.89 0.13 1.76 0.91 0.22 0.30
thickness at data S of 1 rpm in the extreme low-speed range calculated by the theory of Dowson et,al. The relation between the friction coefficient, oil film breakdown ratio and oil film thickness, oil film parameter are shown in Figure 7. Here, the oil film parameter indicates the ratio of the oil film thickness to the resultant values of Rrms employing the shaft and the seal lip shown in Table 1. General tendencies are similar to that discussed in Fig. 5. However, if we look at the turning point, the friction coefficient is divided into four regions, and the oil film breakdown ratio is divided into three regions. The difference of the number of regions and turning points indicates a characteristic of the measuring method. Namely, the friction coefficient probably expresses the region A through B where the lubrication condition changes from dry friction to boundary lubrication involving a thin fluid film.
559
10
10
i n
10
10
10
1 0 -3
10
1 0 -2
100
101 h prn
OIL FILM THICKNESS
I 1 0 -3
I
1 1 1 1
1 0 -2
I
I
I l l
I
1 0 -I
I
Ill
100
1
102
I I I
I
I I l l
I
lo1
OIL FILM PARAMETER A
Fig. 7
Relation between friction coefficient, oil film breakdown ratio and oil film thickness, oil film parameter
I I l l
560
On the other hard, the oil film breakdown ratio seisitirely indicates111where microscopic contact between the two surfaces existed in fluid film lubrication. According to Johnson's theory (6), shown as a broken line in Fig. 7, it is said that the oil film breaks down more than 90% under the value 1 of oil film parameter A , and in the value 3 to 4 of A , the oil film does not break to the extent that the two surfaces are almost separated. With regard to this, the theory is partially overlapped with film thickness hD in high region of oil film breakdown ratio E and with hB-D in low-region of E . To look again at Fig. 6 for this, the oil film thickness h~ is fit for data B and b n e a r b y to data C. Therefore, the oil film thickness within the sealing zone is assumed to exist from 0.1 to 1 urn. Thereupon, the elastohydrodynamic lubrication is applicable for the transient region from low speed to medium speed. 4
CONCLUSIONS
The friction coefficient, oil film breakdown ratio within the sealing zone and the motion of the seal lip are investigated by changing the shaft speed under the constant temperature of the lubricating oil. The results show that the friction and lubrication conditions within the sealing zone changed depending on the shaft speed. This can be explained by, dry friction in extremely low-speed, are boundary lubrication in low-speed. In the transient region from low-speed to medium speed, elastohydrodynamic lubrication is assumed. In medium-speed regions and higher, the results can be explained by fluid film lubrication including partial boundary oil film. Particularly, the contact of micro asperities which break down the fluid film and are considered as the core of seizure are detected in the high-speed region. This suggests the probrem of lubrication when the lip seals are used in further high speed. To determine the oil film thickness width the sealing zone precisely practically and theoretically, however, they are assumed to exist from 0.1 to 1 m approximately. This suggests the necessity for application of the elastohydrodynamic lubrication theory of thin film considering surface roughness or for the starved lubrication theory. 5 ACKNOWLEDGMENT We would like to express our gratitude to Professor Koichi Sugimoto, fh-.Heihachiro Inoue and Mr. Yasushi Atago of Kansai University and Professor Andere Deruyttere, Jacques Paters, Raymond Snoeys, and Hendrick Van Brussel of Katholieke Universiteit Leuven for their kind advices. And also, we would like to thank Mr. Yasuo Shimoji of Koyo Co., Ltd. and Mr. Masanori Nakatani of Honda Co., Ltd. for their good cooperation in the experiments. References
(1)
Hirano, F and Ishiwata, H. 'The lubricating condition of a seal lip', Proc. Instn. Mech. Engrs., 1965-66, 180-Pt.3B. 138-147.
BLOK, H. and VAN ROSSUM, J,J. 'The foil bearing - a new departure in hydrodynamic lubrication', Lubic. Engug.,b1953, 9, 316-. JAGGER, E.T. and WALKER, P.T. 'Further studies of the lubrication of synthetic rubber rotary shaft seals', Proc, Instn. Mech. Engrs., 1966-67, 181-Pt.2, 191-204. OGATA, M., KITADA, F., FUJII, T., and SHIMOTSUMA, Y. 'Studies of lip-type oil seal - friction and lubrication conditions within the sealing zone', JSLE. Intern. Edit., 1983, No.4, 135-142. DOWSON, D. and HIGGINSON, G.R. 'Elastohydrodynamic lubrication' , 1966 (Pergamon Press, New York), 187-212 JOHNSON, K.L., GREENWOOD, J.A., and POON, S.Y., 'A simple theory of asperity contact in elastohydrodynamic lubrication', Wear, 1972, 2, 91-108. Appendix Equations used for calculations of oil film thickness are shown as follows: By Newton's law,
By Dowson et,al. (5) modified from experiment, h, = 2.65.R.GO.54.U0.7.~0.13. Where, A Area influenced by friction F Friction force G Material parameter in EHL h a x Maximum height in surface roughness hi Difference in surface roughness shaft and seal lip Pi Radial load per unit contact band width R Equivalent radius of curvature U Speed parameter in EHL W Load parameter in EHL 11 Viscosity coefficient X Interval of peak to peak in surface roughness
m
2
N
m
m
N/m m N*s/m2 m
Paper XVlll(ii)
Study on fundamental characteristics of rotating lip-type oil seals Masanori Ogata, Takuzo Fujii and Yorikazu Shimotsuma
The puipose of this paper is to investigate the friction and lubrication conditions within the sealing zone of lip-type oil seals. The friction coefficient, oil film breakdown ratio, and dynamic lip motion are simultaneously measured from extreme low-speed of 0.003 m/s ( 1 rpm ) to high-speed of 18 m/s ( 7000 rpm ). Up to the present, it has been accepted that the lubrication conditions within the sealing zone are subject to fluid film lubrication. However, experimental results show that they vary from dry friction in the extreme low-speed region to fluid film lubrication in the high-speed region through boundary and elastohydrodynamic lubrication conditions. 1 INTRODUCTION
It has been explained that the lubrication conditions within the sealing zone are subect mainly to fluid film lubrication. From the results measured for the friction coefficient, Hirano and Ishiwata (1) found that the friction coefficient f was proportiona to the characteristic term (Q*U/P~)~)~ which is equivalent to the bearing modulus. Furthermore, they verified this theoretically applying foil bearing theory by Blok et.al (2). On the other hand, Jagger and Walker (3) defined that the friction coefficient f was roportional to the terms of (rl*U)1/3 and Pa-119 of contact pressure based on the theory for elastohydrodynamic lubrication. In the present paper, not only mesurement of the friction coefficient, but also the oil film breakdown ratio (4) are used as methods for quantities estimation of the lubrication condition to observe the seal surface in datail. The results suggest that the friction and lubrication conditions within the sealing zone of lip-type oil seals cannot probably be explained by one of the lubrication theories alone. 2
APPARATUS
The lip-type oil seals used in this study are shown in Figure 1. They have a metal case embedded and a garter spring. Their Case width is 12.5 mm and outer diameter is 72.0 nun. The bore diameter before installation is 48.2 mm at room temparature, and after installation with a standardized shaft of 50 mm in diameter, the lip is given the interference of 1.8 nun in diameter. The contact band width b generated in contact with the shaft expands to 1.43 mm, and the radial load P, amounts to 326 N. To electrically observe the lubrication within conditions the sealing zone, the seal is given electric conductivity adding carbon black in larger quantities than usual to the NBR. The resistance for unit volume is 528 Q/m. The thermocouples are set to the lip whose position is 1.0 mm just under the lip edge. They are used to measure the body and surface
LIP w;E Fig.1
Sectional view of lip-type oil seal used for test
temeratures. Fur hermore. he strain gauges are'attached to the outside and sealing side of the seal to observe lip motions. A test seal is mounted to the head part of the experimental apparatus shown in Figure 2. The main shaft is driven by a positively infinite variable gear changer from 1 to 7000 rpm. A main shaft of 50.0 mm in diameter is used. The type of sealed lubricating oil is turbine oil of viscosity 56 cSt at 40 OC characteristically. It is filled in the sealing side, and circulated by the oil pump. The temperature of the lubricating oil is controlled to 30 OC at the position of the lip
554
TEMPERATI JRE RECORDER
STRAIN METER
D I G I T A L MEMORY AND SYNCHRONIZED OSCILLOGRAPH
LUbR
I
I
RC C I R C U I T
Fig. 2 Head part of seal lubrication tester type I
surface. The friction torque Tf of the lip at the sealing zone is measured by the load cell connected to the housing. The friction coefficient f of the seal lip at the sealing zone can be expressed as equation (1) employing the radial load P, and the shaft diameter d. f = Tf/ ( Pr.d/2 ) (1) The oil film breakdown ratio is measured by a direct current circuit comprising a resistor, capacitor, shaft, and seal. The charging characteristic of the capacitor differs from cases where the shaft is perfectly contacted o r occasionally contacted with the seal lip. Thereupon, the time elapsed when the voltage of the capacitor reached a certain threshold value are added to t, and ti respectively, the oil film breakdown ratio E is defined by equation 2 E = (t,/tl).100 (2) The oil film breakdown ratio E takes a value of 100 percent when the oil film is perfectly broken the shaft contacts the seal lip as usual, meanwhile, E takes a value of almost zero percent when the shaft and the seal lip are separated by a sufficiently thick oil film. This method is an original development ( 4 ) .
.
3 RESULTS and DISCUSSIONS Simultaneously measured friction and lubrication conditions and dynamic motion of the seal lip within the sealing zone are shown in Figures 3 and 4 . For example, please look at Figure 3 ( S ) .
The figure in the upper row, waveform Waveform @ indicates that an oil film between the shaft and lip was formed and broken. Voltage Vf at zero in order of magnitude indicates a condition where the oil film is broken down, and on the other hand, the voltage Vf at 1 indicates that the oil film is formed. Waveform @ shows the friction torque Tf of the seal lip. Waveforms @ and in the figure in the middle row, show circumferential and radial motions of the seal lip respectively. Waveform @ in the figure in the lower row, shows resultant motion obtained by synchronizing @ with The friction and lubrication conditions within the sealing zone and the temperatures of the seal lip are shown in Figure 5. They are measured when the shaft speed was changed at random from 1 rpm (0.0026 m/s) in the extremely low-speed range to 7000 rpm (18 m/s) in the high-speed range. The friction coefficient f and the oil film breakdown ratio E are obtained from waveformes and @ in Figures 3 and 4 employing equations (1) and (2). And the symbols of S,A,B,C, and D in Figures 3, 4 and 5 indicate the same points of measurement, In the extremely low-speed region from 1 to 5 rpm, the friction coefficient takes a value of 0.08 to 0.07. And the oil film breakdown ratio indicates 80 to 90%, so an effective oil film is scarcely existent. Both values show a contact. Therefore, the possible cause
0, shows shaft rotation.
0
0.
0
555
for this region is dry friction. When point S of shaft speed 1 rpm and point A are investigated clearly at Fig. 3 , oil film breakdown indicates instantaneons breakdown, friction torque waveform @ fluctuates abruptly, and dynamic lip motion waveform @ indicates stick-slip. In the low-speed region from 5 to 50 rpm, the friction coefficient decreases from 0.08 to the minimum value of 0.025. On the other hand, the oil film breakdown ratio decreases from 80 to 40%, though, this does not yet indicate the minimum value. In general, the oil film breakdown ratio also corresponds to the phenomenon of friction coefficient. This difference in phenomenon is explained by waveform @ of the lip motion. The displacement 6 , of data B which indicates the motion in the circumferential direction is smaller than that of data A . This means that the followability of the lip in circumferential direction at data B is inferior to data A. For this reason, although the friction coefficient decreases, the asperities of the shaft and the seal lip easily contact each other, indicating that a high film breakdown ratio is the possible cause. This region is assumed to be the boundary lubrication. In the medium-speed region from 50 to 1000 rpm, the friction coefficient increases to 0.12. The value is a little larger than the 1 rpm value in the extremely low-speed region. As the oil
0
- - - - - --
SEPARATION : Vf = 1 V
'm
film breakdown ratio decreases further from 40%, it takes the minimum value of 2% at 1000 rpm. One of the reasons for these phenomena can be explained by the increase of viscous resistance in the oil film which is formed thicker by the speed effect. The other reason is that the lip motion in radial direction at data C is larger than that of B, therefore the tendency stated above is also explained by the effect of inducement of an oil film to the sealing zone caused by the fluctuation of the seal lip. This region is probably subject to the fluid film lubrication. In the high-speed region from 1000 to 7000 rpm, the friction coefficient fluctuates, however, its mean value indicates 0.12 approximately. Then, the oil film breakdown ratio increases from the minimum value of 2% to 10% with fluctuation. This tendency is probably caused by a temperature of 5 " C in the body of the seal lip slightly higher than the temperature of the surface where the temperature of the lubricating oil is controlled to 3CE2 " C . In this region, although the oil film is formed by the speed effect, the viscosity decreases, and the followability of the seal lip to shaft rotation is increased due to heat generation in the seal lip. When these two factors occur simultaneously, the micro asperities on the surfaces of the shaft and the seal lip do not easily contact with each other and also the oil film is broken. Thereby, the waveform @ of lip motion is obviously different up to this point in the elliptical motion
r
1
> Ic.
>
n
0
r
r
CONTACT : Vf = 0 V
+ . -
I
Tf
r+
'r+
LIP
(S)
1 rpm
Fig. 3 Friction and lubrications within the sealing zone, and motion of a seal lip
556
2 sfdiv
P----l
250 msfdiv
p-----l
W
W
r+
(B) 50 rpm
p d
2.5 msfdiv
(C)
Fig. 4
lC00 rpm
(D) 6000 rpm
Friction and lubrications within the sealing zone, and motion of a seal lip
557
0
50
0
CD W
tlL
40
z IQ
cr W
30
a E W I-
20
cc 1
V
U LL
LL
1 n
c (
SHAFT SPEED Fig. 5
N rpm
Effect of shaft speed on friction coefficient and oil film breakdown ratio
558
tendency that it demostrates. The area of the ellipse indicates the energy consumed by lip motion. It can be easily assumed that energy causes the temperature within the sealing zone to rise high and the viscosity of the lubricating oil to fall. Besides, in the high-speed region, it has been confirmed that the oil film breakdown ratio increases remarkably when the temperature of the lubricating oil is not controlled to a constant value (4). From the above discussion, the oil film thickness within the sealing zone is investigated as follows. The oil film thickness is calculated substituting the operation conditions of the seal for lubrication theories. They are, h~ in case the viscous fluid formed between plain-parallel surfaces is subject to Newton's rule, hg-F by Hirano (1) applying the theory of Blok (2) for foil bearing to a rigid body with surface roughness, and hD by Dowson (5) in consideration for elastic deformation, line contact, and viscosity change by pressure in theory of elastohydrodynamic lubrication. The equation used for the calculations is given in the appendix.
The results are shown in Figure 6 . Hereupon, the operation factor is a characteristic number equivalent to the bearing modulus. The oil film formed between the shaft and the seal lip becomes thicker accordingly as the operation factor increases. By the theoretical equations, the oil film thickness of hg-H and hD take values of 0.67 and 0.7 in gradient, respectively. On the other hand,where the gradient of hN should become 1, it shows fluctuation owing to the variation in the ratio of the shaft speed to the friction coefficient. The film thickness exists in the range from 0.001 pm of hN to 10 pm of hD in order of magnitude. However, from the results measured for the surface roughness of the shaft and the lip shown in Table 1, the oil film thickness between data B of 50 rpm and the data D to almost the maximum speed of 7000 rpm is considered to be reasonable. They are from 0.22 pm in Rrms of the seal lip to 2.04 vm in Rp of the shaft. As mentioned in Figure 5, in these regions the oil film is formed positively by the effect of
10
10 10
10-
.
A
A hNewton
lo-"
I
10-
LO-'
-
0 hBlok-Hirano (1,2)
A
10-
OPERATION FACTOR
rl*U/P1
Fig. 6 Oil film thickness calculated by theories employing experimental conditions speed, and the phenomena whereby the friction coefficient increases by the resistance.of viscosity and the oil film breakdown ratio decreases, are observed. However, in the limited operation factor within the range from to the minimum oil film thickness is measured as 0.05 Um experimentally (4). This value corresponds with the oil film Table 1.
Surface roughness in shaft rotating direction [urn].
Roughness
Shaft
Seal lip
Rmax Rm Ra Rrms
2.04 0.67 1.37 1.28 0.25 0.31
1.89 0.13 1.76 0.91 0.22 0.30
thickness at data S of 1 rpm in the extreme low-speed range calculated by the theory of Dowson et,al. The relation between the friction coefficient, oil film breakdown ratio and oil film thickness, oil film parameter are shown in Figure 7. Here, the oil film parameter indicates the ratio of the oil film thickness to the resultant values of Rrms employing the shaft and the seal lip shown in Table 1. General tendencies are similar to that discussed in Fig. 5. However, if we look at the turning point, the friction coefficient is divided into four regions, and the oil film breakdown ratio is divided into three regions. The difference of the number of regions and turning points indicates a characteristic of the measuring method. Namely, the friction coefficient probably expresses the region A through B where the lubrication condition changes from dry friction to boundary lubrication involving a thin fluid film.
559
10
10
i n
10
10
10
1 0 -3
10
1 0 -2
100
101 h prn
OIL FILM THICKNESS
I 1 0 -3
I
1 1 1 1
1 0 -2
I
I
I l l
I
1 0 -I
I
Ill
100
1
102
I I I
I
I I l l
I
lo1
OIL FILM PARAMETER A
Fig. 7
Relation between friction coefficient, oil film breakdown ratio and oil film thickness, oil film parameter
I I l l
560
On the other hard, the oil film breakdown ratio seisitirely indicates111where microscopic contact between the two surfaces existed in fluid film lubrication. According to Johnson's theory (6), shown as a broken line in Fig. 7, it is said that the oil film breaks down more than 90% under the value 1 of oil film parameter A , and in the value 3 to 4 of A , the oil film does not break to the extent that the two surfaces are almost separated. With regard to this, the theory is partially overlapped with film thickness hD in high region of oil film breakdown ratio E and with hB-D in low-region of E . To look again at Fig. 6 for this, the oil film thickness h~ is fit for data B and b n e a r b y to data C. Therefore, the oil film thickness within the sealing zone is assumed to exist from 0.1 to 1 urn. Thereupon, the elastohydrodynamic lubrication is applicable for the transient region from low speed to medium speed. 4
CONCLUSIONS
The friction coefficient, oil film breakdown ratio within the sealing zone and the motion of the seal lip are investigated by changing the shaft speed under the constant temperature of the lubricating oil. The results show that the friction and lubrication conditions within the sealing zone changed depending on the shaft speed. This can be explained by, dry friction in extremely low-speed, are boundary lubrication in low-speed. In the transient region from low-speed to medium speed, elastohydrodynamic lubrication is assumed. In medium-speed regions and higher, the results can be explained by fluid film lubrication including partial boundary oil film. Particularly, the contact of micro asperities which break down the fluid film and are considered as the core of seizure are detected in the high-speed region. This suggests the probrem of lubrication when the lip seals are used in further high speed. To determine the oil film thickness width the sealing zone precisely practically and theoretically, however, they are assumed to exist from 0.1 to 1 m approximately. This suggests the necessity for application of the elastohydrodynamic lubrication theory of thin film considering surface roughness or for the starved lubrication theory. 5 ACKNOWLEDGMENT We would like to express our gratitude to Professor Koichi Sugimoto, fh-.Heihachiro Inoue and Mr. Yasushi Atago of Kansai University and Professor Andere Deruyttere, Jacques Paters, Raymond Snoeys, and Hendrick Van Brussel of Katholieke Universiteit Leuven for their kind advices. And also, we would like to thank Mr. Yasuo Shimoji of Koyo Co., Ltd. and Mr. Masanori Nakatani of Honda Co., Ltd. for their good cooperation in the experiments. References
(1)
Hirano, F and Ishiwata, H. 'The lubricating condition of a seal lip', Proc. Instn. Mech. Engrs., 1965-66, 180-Pt.3B. 138-147.
BLOK, H. and VAN ROSSUM, J,J. 'The foil bearing - a new departure in hydrodynamic lubrication', Lubic. Engug.,b1953, 9, 316-. JAGGER, E.T. and WALKER, P.T. 'Further studies of the lubrication of synthetic rubber rotary shaft seals', Proc, Instn. Mech. Engrs., 1966-67, 181-Pt.2, 191-204. OGATA, M., KITADA, F., FUJII, T., and SHIMOTSUMA, Y. 'Studies of lip-type oil seal - friction and lubrication conditions within the sealing zone', JSLE. Intern. Edit., 1983, No.4, 135-142. DOWSON, D. and HIGGINSON, G.R. 'Elastohydrodynamic lubrication' , 1966 (Pergamon Press, New York), 187-212 JOHNSON, K.L., GREENWOOD, J.A., and POON, S.Y., 'A simple theory of asperity contact in elastohydrodynamic lubrication', Wear, 1972, 2, 91-108. Appendix Equations used for calculations of oil film thickness are shown as follows: By Newton's law,
By Dowson et,al. (5) modified from experiment, h, = 2.65.R.GO.54.U0.7.~0.13. Where, A Area influenced by friction F Friction force G Material parameter in EHL h a x Maximum height in surface roughness hi Difference in surface roughness shaft and seal lip Pi Radial load per unit contact band width R Equivalent radius of curvature U Speed parameter in EHL W Load parameter in EHL 11 Viscosity coefficient X Interval of peak to peak in surface roughness
m
2
N
m
m
N/m m N*s/m2 m