On the vibration in wet friction systems using paper-based materials

Transient Processes in Tribology G. Dalmaz et al. (Editors) 9 2004 Elsevier B.V. All fights reserved

133

On the Vibration in Wet Friction Systems Using Paper-Based Materials Yuzuru SAMBONGI ~ ([email protected]), Takao SHIBUYA

a

and Shin MORISHITA b

~ Dynax Corporation: 6-302, Kashiwabara, Tomakomai, Hokkaido, 059-1362, Japan, b Yokohama National University: 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan The vibration in a developed wet friction system with paper-based material was investigated. As a result, torsional vibration of the input shaft occurred during engagement. In addition, two components of separator plate vibration were observed in the circumferential direction and normal direction of the friction surface. The effects of lubricating oil and surface roughness of the separator plate on friction and vibration characteristics were discussed. Moreover, it was found that separator plate vibration occurred periodically during the stick-slip motion of the input-shaft.

1. I N T R O D U C T I O N Wet friction systems have been widely applied to diverse machines such as automatic transmissions (AM') of automobiles and brake systems of heavy-duty vehicles [1]. In these systems, useless vibration or noise called "shudder" or "squeal" occasionally occurs during the engagement process. The vibration caused by the friction force has been often explained as one kind of self-excited vibration caused by friction. In this case, when the friction force increases with decreasing relative sliding speed, this friction characteristic is generally expressed as negative damping [2]. Although many experimental and theoretical studies have been conducted [3-8] to solve the vibration problems observed in these friction systems, the vibration problems still remain unsolved. In order to avoid the useless vibration or noise in these systems, it is important to understand the vibration phenomena itself. This paper is the first report in which the characteristics of the vibration in a wet friction system were experimentally clarified in detail.

shows the cut-model of a typical AM" for passenger cars [10]. In this system, the friction force is generated by the interaction of the friction material and the separator plate (S/P) used as the mating surface of the friction material in lubricating oil during an engagement. The friction plate (F/P) bonded with friction material, S/P and lubricating oil are considered to have a significant influence on the friction characteristics [ 11-13]. Fig.2 shows the appearance of the F/P and S/P. Fig.3 shows the surface structure of the paper-based friction material as a SEM image. The "paper" for the friction material contains cellulose fiber, synthetic fiber as the main skeletal structure, solid lubricant such as a graphite, friction modifier such as a diatomaceous earth and other ingredients [1]. This "paper" is impregnated with a thermosetting resin. It can retain enough porosity even after impregnation with resin.

2. W E T F R I C T I O N SYSTEMS Wet friction systems which function in oillubricated conditions are widely used in various devices and machines. For a long time, various kinds of wet friction material have been developed for them [9]. Today, paper-based friction materials are used for almost all A/Ts for passenger cars. Fig.1

Fig.1 Typical A/T (cut-model)

134 Motor

lk

Input shaft Torque meter F/P fixed to hub E E S/P dipped in oil bath

Load cell

Hydraulic lifter

800 (mm) Fig.3 SEM image of paper-based friction material surface

Fig.4 Schematic of testing apparatus

3. E X P E R I M E N T A L

r. . . .

i

Accelerometer

3.l.Testing apparatus To investigate the vibration and friction phenomena, the testing apparatus including an actual F/P and S/P in addition to incidental parts was developed by the authors [14]. A schematic of the testing apparatus is shown in Fig.4. A pair of a rotating F/P and a stationary S/P was set up in the testing apparatus as the testing specimen. The former was fixed to a hub located at the bottom end of the input shaft and the latter was fixed inside the oil bath filled with lubricating oil. The temperature of the lubricating oil was kept at a constant value by an electrical heater equipped in the oil bath. The S/P was pressed normally against the F/P by using hydraulic pressure. In this experimental study, the vibration that occurred around the F/P and S/P was directly measured by using accelerometers. These accelerometers were attached to the bottom end of the input shaft and also to the S/P as shown in Fig.5. Torque was measured by a torque meter installed in the input shaft. The transient vibration of acceleration and torque in addition to their correlation were investigated.

F/P

J

Accelerometer

i/

S/P Oil bath Fig.5 Location of accelerometers

3.2.Test conditions In this study, experiments were conducted under two conditions for motor operation. One was decreasing speed mode and the other was constant speed mode. For each case, the temperature of the lubricating oil was set at 80 ~ Applied normal load ranged in values up to 12kN. The test conditions are shown in Tablel and Table 2.

135 (f)

3.3.Specimen Both of the S/P and the friction material of the F/P as testing materials are standard production parts, and the friction material is paper-based. Sizes of these specimens are shown in Table 3. Three kinds of lubricating oils named Oil-a, b and c were used. The diameter of the part indicated by " * " of the input shaft in Fig.4 can be changed to 30mm or 40mm.

~. 1600

=

~

results

0

125

100--~0 /5 [s]

Normal load [kN]

2 "~ 12

;"

! i "

o

~z [

o"

~ =

-1600

i

100-

,---,

i

Ii L

0

_,oo.~ "" -200200_ @ -300100_ -~----1----.r r

=

i

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,1

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--~- . . . . ~ ....

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ill

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ii

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.........

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. . . . .

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Time [s]

';

(a) Torque (b) Motor speed (c) Torsional vibration of input shaft (d) In-plane vibration of S/P (e) Out-of-plane vibration of S/P

Fig.6 Typical vibration phenomenon under the decreasing speed test mode during engagement = ~ & ~

~

,....,

,.~

~

Motor speed [rpm]

i-i

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~, ""

Table 1 Test conditions: Decreasing speed test mode

I

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Fig.6 shows typical phenomena observed with Oil-a under the decreasing speed test mode. Here, the part of the input shaft indicated by " * " in Fig.4 was 40mm in diameter. During the decrease in motor speed with respect to time (shown in Fig.6 (b)) at 5kN of normal load, torsional vibration of the input shaft occurred (c). Its amplitude began to increase during the latter half of the engagement. In-plane vibration of the S/P occurred circumferentially as shown in Fig.6 (d). Moreover, out-of-plane vibration of the S/P occurred in the normal direction of the friction surface. S/P vibration occurred immediately before the motor speed reached 0rpm. In this engagement process, the torque (a) increased with decreasing motor speed. Fig.7 shows the detailed time history of input shaft torsional vibration (c) near the vertical line (f) and (g) in Fig.6. The amplitude of vibration increased gradually from line (f) to (g) maintaining a constant frequency.

i',

E

4.RESULTS AND DISCUSSIONS 4.l.Test

(g)

-i .......... [ ........ 1:- ...... ~....... ;; ..... ! ...... ~, ....

500 250 0 -250

"7~ ~ -500 2.300 2.302 2.304 2.306 2.308 2.310 O

Lubricating oil temp. [~

Table 2 Test conditions: Constant speed test mode ,,,,,

Motor speed [rpm] Normal load [kN]

Time [s] (i) Near the vertical line (f) in Fig.6

80

[ ]

Lubricating; oil temp. [~

5 ~ 50 5 ~ 12 ..

80

Table 3 Sizes of specimens

. Friction material 1 0 uter diameter [mm] I Inner Friction material thickness [ m m | ........... S/P thickness [mm] ]

.. 115 95 0.4 3.5

= ~-~ 500 ~

250

"~ ~

0

~ ,,,,q

.

.

.

.

.

" '"

'~ ~ -250 "5

~ -500

o "-9

4.400 4.402 4.404 4.406 4.408 4.410 Time [s] (ii) Near the vertical line (g) in Fig.6

Fig.7 Detail of input shaft torsional vibration

136 Fig.8 shows typical phenomena under the constant speed mode. In this case, motor speed was 14rpm, normal load was 12kN with Oil-a, and the part of the input shaft indicated by " * " in Fig.4 was 40mm in diameter. Torsional vibration of the input shaft began to occur about 0.8 sec. after torque was generated as shown in Fig.8 (b). Its amplitude kept increasing for about 0.4 sec. and stopped increasing about 1.2 sec. after torque was generated. In-plane vibration and out-of-plane vibration of the S/P occurred almost simultaneously with the stoppage of input shaft vibration increase as shown in (c) and (d) of Fig.8. From these results, torsional vibration of the input shaft and in-plane/out-of-plane vibration of the S/P are considered to occur independently from motor driving modes shown in Tables 1 and 2 in this wet friction system. 4 . 2 . F r e q u e n c y c h a r a c t e r i s t i c s of v i b r a t i o n s

Frequency characteristics of each vibration were considered separately in two stages of engagement. ........

%"80 ,=

One was the early stage of engagement. During this stage, the amplitude of the input shaft torsional vibration increased. The other was the latter stage of engagement. In this stage, S/P vibration occurred on a large scale. Fig.9 shows frequency characteristics of each vibration near the vertical line (f) indicated in Fig.6 as the early stage. In Fig.9, the strongest frequency components of three kinds of vibrations were almost the same in this stage. In addition, these strongest frequency components were changed by the diameter of the input shaft, namely 725Hz at 30ram diameter and 850Hz at 40mm. Other experiments showed that the thickness of the S/P had no relation with these characteristics. 600

' '

n.., 6.

400

30m~

E ~

200

t__.__a

i ..................

n

800

~ ,.--, -~

....

0

0

n

500

o

o

2.0

'

1.5

30ram

-

n_

I

,

1000 1500 Frequency [Hz]

2000

'l

9

a

9

I

9

40mm

fi~ 1.0 -800

1~

OJ

500""I

0.5 _L _~_ ~ , , - L _

0.0

0

-looo

1000

"G"

500

-1500 J

J

.....

__

In

_

_,,,...

1000 1500 Frequency [Hz]

2000

(ii) In-plane vibration of S/P ,--,

<

-500

"~

1000

'~

'

(i) Torsional vibration of input shaft

+01 . 0=

"n

........ ~.....:...... ::....

< ,d~

9

,[

~. ~

n 2 m m

40 = o

" 9

2.01

& 1.5

.............................~"~..............:

0i

(a) Torque

t

i

i

i

;

Time [s]

(b) Torsional vibration of input shaft (c) In-plane vibration of S/P (d) Out-of-plane vibration of S/P Fig.8 Typical vibration phenomenon under the constant speed test mode Motor speed: 14rpm, Normal load: 12 kN

-E

'

n

30mm

1.0

9

/

I 40m]"m

!

"

_.i

__ _.__

"i

0.5 0.0

-"

0

n.

500

.

.

.

1000 1500 Frequency [Hz]

2000

(iii) Out-of-plane vibration of S/P Fig.9 Frequency characteristics of vibrations at early stage of engagement

137 Fig.10 shows the frequency characteristics of each vibration in the lower speed region as the latter stage. In this figure, only the results from using a 40mm diameter for the part of the input shaft indicated b y " * "in Fig.4 are shown. In Fig.10, frequency characteristic of input shaft vibration in the torsional direction was almost the same as the results in the early stage shown in Fig.9. On the other hand, frequency characteristics of in-plane vibration and out-of-plane vibration of the S/P were different from the results shown in Fig.9. In this stage, many higher-order components of frequency appeared in S/P vibrations. As shown above, frequency characteristics of vibrations that occurred in this wet friction system were clarified. Considering these vibrations, input shaft torsional vibration is suggested to be a 6. E

1000 100. 10 1

'

I

'

I

'

I

principal vibration in the tested system. In addition, in-plane and out-of-plane vibrations of the S/P are considered as the secondary vibrations induced by the input shaft vibration in this case. 4.3.Effects of normal load to e a c h v i b r a t i o n Fig.ll shows the effects of normal load on the input shaft and S/P vibrations observed under the decreasing speed test mode. Although these vibrations showed dispersions for each normal load condition, larger vibrations tended to occur under the higher normal load condition. Test results obtained under the constant speed mode resembled the results described above. From these results, normal load is considered an effective parameter for vibration magnitude.

'

~ ~ 14001., , . "~" 000 ~ ~ 800 V ~. ~ 600

~

0.1 0.01

~_. ~ ~.~

'

10

. =-

100

1000

10000

100000

,

o.1 0.01 0.001

i

9

t

10 (ii)

100

,

i

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i

9

9

i

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t

3000 .o.. & 2500 ~ % 2000 o~ ~.~E 1500 1000 500 o

1000 10000 Frequency [Hz] In-plane vibration of S/P .

i

0.1

0.001 1000

10000

"1

t~ I

' . I . ! 9

2

14

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''""i [~

n

,

0

= "--K 3000 9- & 2500 % 2000 1500 e.) E 9 ~ 1000 • 500 o 0

0.01 100

, [~

I i ~

0

~,,, 2

i

i

,

i

4 6 8 10 12 Normal load [kN]

14

(ii) In-plane vibration of S/P 9

1

10

!

""

u

100000

o

'E

o

i " I ;

4 6 8 10 12 Normal load [kN] (i) Torsional vibration of input shaft

,

100

200

0

Frequency [Hz] (i) Torsional vibration of input shaft 100

i 4oo I

i.

100000

Frequency [Hz]

9 9

I

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9

I

n

I

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i

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9 9

0

2

4 6 8 10 Normal load [kN]

12

(iii) Out-of-plane vibration of S/P

(iii) Out-of-plane vibration of S/P

Fig.10 Frequency characteristics of vibrations at latter stage of engagement

Fig.11 Effects of normal load to each vibration

14

138 6.0 5.0 " 4.0 3.0 L 2.0 1.0 0.0

4.4. Effects of friction characteristics on input shaft vibration

In this section, the relationship between torque characteristics appearing as being dependent on relative sliding speed (Torque-V slope, hereafter) and increasing rate of input shaft vibration were considered. The envelopes of vibration measured as acceleration were approximated as an exponential function shown as equation (1).

d2x/dt 2 = A "exp(1/ r "0

i

9

I

"

0

I

'

o with Oil-a " 9 with Oil-b " A wit.h Oil-c "

~]lt

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

-1.0

I

,

-1500

I

,

-1000

,

|

-500

,

0

500

Torque-V s lope [N-m/(m/s)] Fig.12 Relationship between Torque-V slope and 1/z" vibration of input shaft are discussed. Several kinds of surface-finished S/Ps with different surface roughness were prepared and tested. The surface roughness and typical surface profile of the prepared S/Ps are shown in Table 4. These S/Ps were tested under the decreasing speed test mode shown in Table 1 with Oil-a. The results for the variations of input shaft vibration with applied S/Ps for each normal load are shown in Fig.13. Table 4 Surface roughness and profile of tested S/Ps Typical profile

i RoughneS s k

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.

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--

Ra =

0.03

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[urn]

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.

.

.

j 200[/a,m] . . . . . .....i.i .... - ' , ,,

Ra =

0.30 [~zm]

Ra

c

4.5. Effects of surface roughness of S/P on input shaft vibration

It is known that the surface roughness of the S/Ps has some effects on the friction characteristics to some degree in wet friction systems. Especially at the higher relative sliding speed region, these effects are known to be very large as described in a previous report by the authors [13]. In this section, effects of the surface roughness of the S/Ps on friction characteristics and torsional

"

~

(1)

Here, d2x/dt 2, A and t refer to the acceleration of the input shaft, amplitude and time, respectively. I/z" is a time constant deduced from equation (1). Fig.12 shows the relationship between Torque-V slope and 1/z-" for each lubricating oil. In Fig.12, the negative value of Torque-V slope means that torque increases with decreasing relative sliding speed, and vice versa. From these results, in the case that the Torque-V slope is negative and its absolute value is larger than a specified one, a rapid increase of input shaft vibration is expected to occur. On the other hand, a positive Torque-V slope excited no vibrations. In Fig.12, lubricating oil is considered to give large effects to the Torque-V slope and increasing rate of input shaft vibration. In the case of Oil-a, the Torque-V slope was negative and its value was greater than the other oils. In addition, the rate of increase for input shaft vibration was faster compared with the others. For the case of Oil-b, the Torque-V slope was positive and no vibration occurred. Oil-c showed intermediate characteristics relative to the other two cases. In spite of using the same friction material and S/P, the use of different lubricating oils led to different results. These results suggest that the lubricating oil is a very important factor for the friction and vibration characteristics in wet friction systems.

I

o

-

0.34 [/lm] 9

........

d i

,, ,,,

,,,,

Ra = 1.34

i [~m] ...... j........... !

e

Ra -

1.67 [~1 m] !

,

,

..,

....

...

...

.

139 =

800

'

I'

~ a

"

I

"

I

'

"'I

E 600 " - - O - - - b .

9~ .-~ 400 o "~, 200 ~.=

_

.... E} ....

I

"

I

"

500

I

E g

0/~

c

--I--- d

'"

_

/r

I

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I

"

I

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I

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,,,

|

,

|

,

i

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I

,

| ,,'~T-I

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0

~...."-..."

& -5oo

"_--"

O

-1000, 6 = -1500

0 0

9

2

4 6 8 10 Normal load [kN]

12

14

0

0

e

0.05 0.1 0.15 0.2 0.25 0.3 0.35 Motor speed [m/s]

Fig.13 Effects of S/P surface roughness for input shaft vibration with normal load

Fig.14 Effects of S/P surface roughness for Torque-V slope with relative sliding speed

In Fig.13, the surface roughness of the S/Ps can be considered to affect the magnitude of input shaft vibration. In the case of S/P-a and c, the accelerations induced by vibration were larger than those in the case of other S/Ps at various normal load conditions. For all tested S/Ps, larger vibrations occurred under the higher normal load test conditions. The Torque-V slope was changed by the S/Ps as shown in Fig.14. In particular, at a lower speed range, large differences between five Torque-V slopes appeared. S/P-a and c showed larger negative slopes than the others. On the other hand, little differences were found at a higher speed range for the Torque-V slope. From these results, the surface roughness of the S/Ps can be considered to affect the friction and vibration characteristics. Judging from the viewpoint of the design, S/P surface configuration must be recognized as one of the important parameters for friction and vibration characteristics as well as lubricating oils. Though it was clarified that larger negative Torque-V slopes tend to induce larger vibrations in this wet friction system, how S/P surface roughness changes friction characteristics as shown in Fig.14 has not been clarified yet. Considering the surface roughness and test results shown in Table 4 and Fig.14 respectively, it is obvious that the surface roughness of the S/P alone is insufficient to explain these results. To comprehend the relationship between S/P surface configuration and friction characteristics has to be carried out for solving the vibration problems.

The relative sliding speed of the rotating F/P and stationary S/P is considered the calculated velocity. Fig.15 shows an example of the calculated velocity of the input shaft with S/P vibrations obtained under the decreasing speed test condition. The amplitude of velocity oscillation gradually increased with decreasing average speed. In the lower average speed range, the minimum velocity was almost 0m/s. At this moment, the amplitude stopped increasing and began to decrease while maintaining the periodical motion. During this state, in-plane and out-of-plane vibration of the S/P became noticeable.

4.6. Input shaft and S/P motion state Input shaft velocity including oscillation was calculated from the acceleration and motor speed.

0.6 o =3,--, .8 ~ . ~ 0.4 ~--~ E o o.2 9=- = . ~ 0.0 -0.2 = ,---,

2000

9

9

l

"

|

9

.

9

._ E >

~

~ "q

10000

-1000 -2000

i

I

2000

9

9

,i

1000~_ .= ~ ~ -1000

i

~I

>

-2000"

I

' ,....

9

I

,

1

1.5

0

Time [s] Fig.15 Input shaft velocity and vibration of S/P

140 This input shaft periodic motion can be considered as a stick-slip motion as the point of velocity periodically reached 0m/s. Fig.16 shows the details of S/P vibration during stick-slip motion of the input shaft. At the moment the input shaft motion turned from a stick phase to a slip phase, S/P vibration occurred. Both the in-plane and out-of-plane vibration was generated almost at the same time. In-plane vibration of the S/P was observed as a free oscillation with damping. Frequency of out-of-plane vibration of the S/P was higher than one of in-plane vibration.

REFERENCES

1.

2. 3.

4. 5.

CONCLUSION

The vibration induced by friction force in a wet friction system was investigated. As a result, several points were made clear as follows. (1) In-plane and out-of-plane vibrations of the S/P as well as the torsional vibration were observed during the engagement process of F/P and S/P. (2) Torque-V slope as the friction characteristics affects the rate of increase for input shaft torsional vibration. (3) Lubricating oil and S/P surface roughness affect the friction and vibration characteristics. (4) Vibration of the S/P occurred during the stick-slip motion of the input shaft and in-plane vibration was a free oscillation with damping.

5.

6.

7.

8.

9. 0.3

-

0.2 0.1 0.0 -0.1

10. 11.

600 4O0 ~ ~ ~'~'~ 2

.

.

.

.

.

~

:00

12.

9~

-400 -600 ~ 600 = o ~-' 400 ~ : ~. . .~. 2000 '

13.

~" ~ - 2 0 0

9 "7~ ~ -400 -600

I

I

0

0.002

Time [s] Fig.16 Detail of input shaft velocity and vibration of S/P

14.

S. Ikawa and K. Ito et al., Trends of Friction Materials Developments, Proc. of the International Symposium of TVF, '98 Yokohama, (1998) 24. J. P. Den Hartog, Mechanical Vibrations, McGraw-Hill Book Company, (1956). R.A.C. Fosberry and Z. Holubecki, An Investigation of The Cause and Nature of Brake Squeal, M.I.R.A. Report No.1955/2, (1955). R.A.C. Fosberry and Z. Holubecki, Some Experiments on The Prevention of Brake Squeal, M.I.R.A. Report No. 1957/1, (1957). R.A.C. Fosberry and Z. Holubecki, Third Report on Squeal of Drum Brakes, M.I.R.A. Report No. 1957/3, (1957). R.A.C. Fosberry and Z. Holubecki, Interim Report on Disc Brake Squeal, M.I.R.A. Report No. 1959/4, (1959). Y. Hattori and T. Kato, Theory of Frictional Vibration in Wet Clutches Considering Poroelastic Properties of Paper-Based Facing, ASME J. of Tribology Vo1.118 (1996) 520. M.L. Haviland, M.C. Goodwin and J.J. Rodgers, Friction Characteristics of ControlledSlip Differential Lubricants, (1966) SAE Paper 660778. F . A . Lloyd and M. A. DiPino, Advances in Wet Friction Materials 75 Years of Progress, (1980) SAE Paper 800977. ATZ WORLD WIDE, Friedr. Vieweg & Sohn Verlagsgesellschaft mbH, Vol.9, Sept. (2001) T. Matsumoto, A Study of the Influence of Porosity and Resiliency of a Paper-Based Friction Material on the Friction Characteristics and Heat Resistance of the Material, (1993) SAE Paper 932924. T. Kugimiya and J. Mitsui et al., Development of Automatic Transmission Fluid for Slip-Controlled Lock-Up Clutch Systems, (1995) SAE Paper 952348. Y. Sambongi, K. Ito and T. Shibuya, The Effect of Surface Configuration of the Separator Plate on the Friction Characteristics of Wet Clutch, Proc. of the ITC Nagasaki, (2000) Vol.3 1961. Y. Sambongi, T. Shibuya and S. Morishita et al., On the Mechanism of Noise due to Frictional Vibration in Wet Friction Systems, Proc. of Kanto Branch Conference of JSME, (2002) 169 (in Japanese).