IRES²2

4BackgroundResult : In-situ observation

Ball joint for automobile [1]In-situ observation 1 : Behavior of grease

Rotation

Used as a connection parts in (Fig.1)
Steering system device
Suspension system device
Consist of steel ball (ball stud), resin (bearing seat)and grease as lubricant. (Fig. 2)
Lubrication mechanism have to be understood.
Reciprocating sliding motion
Metallic material against resin material
Lubrication by grease (thickener & base oil)

Hertziancontact area

Sliding direction of the pin specimen

Oscillation

Ball stud(Metal)

Starved area

GreaseFig. 1 Use position of ball joint [2](shown in blue)

Issues in improving the ball joint

3mm

Immediately after the start of the movement of the pin specimen

Ball joints are used in various temperature atmosphere from -30 to 80 . The viscosityof the base oils (PAO: poly- -olefin) are changed by changing temperature that causes
the change of lubricating behavior of grease in ball joints.
Consider using a grease consisting of silicone oil, which has low temperaturedependence of viscosity, as a base oil.

Bearing seat(Resin)

Fig. 2 Schematic figure of ball joint

Purpose of this research : Investigating the effects of different base oils on the lubricating behavior of greases

Approximately 15 minutes after the start of the movement of the pin specimen

Samples

Grease held between
pin and disk

Fig. 8 Behavior of Gre_PAO around the pin specimen

Two types of greases (Gre_PAO, Gre_Si) and their base oils (Oil_PAO, Oil_Si) were tested as lubricant. (Table 1)
Gre_PAO and Gre_Si consist of the same thickener, lithium stearate, and the base oil is different from PAO (poly-alpha olefin)
and dimethyl silicone oil, respectively.
Silicone oil is well known as the oil which has small temperature dependence of viscosity.
For observing the behavior of thickener in grease by fluorescent method, 0.01% coumarin was mixed into the grease (Gre_PAO*
and Gre_Si* ).
Table 1 Samples
GreaseLubricant

Gre_PAO
Thickener
Base oilComponent

Gre_PAO*: Lithium stearate: PAO

Immediately after the start of the movement of the pin specimen

Oil
Gre_SiThickener
Base oil

Gre_Si*

Oil_PAO

Oil_SiPAODimethylsilicone oil

1,240

7,570

: Lithium stearate: Dimethyl silicone oil

Penetration

280290Kinematic viscosity(40 ) mm2/s

1,240

6,050

Approximately 15 minutes after the start of the movement of the pin specimen
Fig. 9 Behavior of Gre_Si around the pin specimen

Appearance

Fluorescenttreatment

Note

Fluorescenttreatment

ExperimentBase oil of
Gre_PAO

Base oil of
Gre_Si

Load (500N)

Friction test

Reciprocating sliding
Pin-on-disk friction test (Fig. 3) was carried out in reciprocating(1.3Hz, 3.0mm)
sliding. As a combination of resin and metal in the ball joint, Pin (POM)
polyoxymethylene (POM) pin (Fig. 4) and chromium molybdenum
steel (SCM440) disk were used as specimens. Grease and oil weresupplied so that the contact area was sufficiently immersed. TheLubricant
normal load was 500N, considering the actual contact pressure of
the ball joints (100MPa). The reciprocating frequency and stroke
Disk (SCM440)
length was 1.3Hz and 3.0mm, respectively. Number of total
Fig. 3 Friction test
Fig. 4 Pin specimen
reciprocation was 2,500.

In-situ observation
In order to visualize and observe the contact area, a glass disk was used instead of the metal disk in both in-situ observation.
In-situ observation 1 (Observation of the behavior of grease by high speed camera, Fig. 5)
The purpose of this observation is to understand how the grease existing in and around the sliding area behaves due toreciprocating sliding. Fixed-area observation against glass specimen is conducted, and the contact area moves within theobservation image as the pin specimen moves. Normal load is 2N, and other experimental conditions are the same as those of thefriction test.
In-situ observation 2 (Observation of the behavior of thickener by fluorescent method, Fig. 6)
The purpose of this observation is to understand how the thickener of grease existing in and around the contact area behaves due
to reciprocating sliding by using confocal laser scanning microscope (CLSM). Fixed-point observation against pin specimen isconducted, and the contact area is stationary in the observation image, and the behavior of the thickener around and inside thecontact circle is observed along with the movement of the glass specimen. Although the normal load is as small as 5N, the contact
surface pressure is 100MPa, which is equivalent to the contact pressure of the actual Ball Joint, because the tip radius of the POM
pin is as small as 2mm.
Objective lensHigh speed camera

Glass disk

Reciprocating slide(Pin moved)

Glass disk
set to XY stage

Load

Pin specimen
set to load unit

Although there was retained grease in the gap between the pin
and disk specimens as shown in Gre_PAO, it was not supplied tothe contact area, but only moved following the movement of thepin specimen. This behavior of grease is considered to be one of
the causes of the rapid increase in the friction coefficient inGre_Si.

In-situ observation 2 : Behavior of thickener
Statical observation of thickeners
In both greases, fluorescent by thickeners[3] are shown as green bright area. (Fig.
10) In Gre_PAO*, fine and dispersed thickener were observed. On the other
hand, uneven distribution of thickener were observed in Gre_Si *.
Dynamical observation of thickeners
It was observed that the thickener is stretched out as the pin specimen is pressed. (Fig.11) The brightness inside of the contact area of
Gre_PAO * was darker than Gre_Si * even though around contact area was brighter. This means that there were fewer thickeners of
Gre_PAO * in the contact area.
In both greases, island-like thickener remains in the contact area during sliding. It is considered that the lumps of this thickener werecaptured by the small dents present on the surface of the pin specimen. It can be seen that the thickener gradually decreases from thecontact area as the number of reciprocations increases. In Gre_PAO *, it is difficult for the thickener dragged out from the contact
area due to sliding to return to the contact area during reversal sliding. On the other hand, Gre_Si * tends to retain the thickener in thecontact area more than Gre_PAO *. Such differences in behavior of thickener are thought to be affected by differences in theseparability of the base oil from thickener, and differences in adhesiveness of the thickener to the glass or POM.
200 m
100 m
Fine and dispersed thickeners

Gre_PAO *Gre_Si *CLSMIn contrast, the results for grease lubrication differed from the trends expected from the results for oil lubrication. Gre_Si showed alower coefficient of friction than Gre_PAO at the beginning of friction, and this trend was similar to that of oil lubrication.
However, after 200 cycles, the friction coefficient for Gre_Si increased and showed unstable fluctuations. On the other hand,
Gre_PAO maintained a relatively low value, although the coefficient of friction gradually increased.
0.25

Oil_Si

Gre_PAO

Gre_Si

0.20

0.04

0.05
0012502500 01250

BeforeslidingStart
pressing

1Loading
1N

10Loading
5N

40

Gre_Si *Thickeners pressed by the pin specimen

Fig. 10 Statical observation of thickeners

2500

Number of cycles

Number of cycles

Fig. 7 Friction coefficient

Thickeners in contact area

Fig. 11 Dynamical observation of thickeners

Summary
In this research, the effects of difference of the base oil of grease on lubrication were investigated by friction test and in-situ observation.
Oil_Si showed lower friction coefficient than Oil_PAO, however, the greases composed of each base oil showed the opposite result,
Gre_Si showed higher friction coefficient than Gre_PAO.
Gre_PAO and Gre_Si around the contact area showed different behaviors during sliding, and this difference was considered to greatly
affect the ability of grease supply to the contact area.
Differences in the behavior of thickeners in the contact area lubricated with Gre_PAO* and Gre_Si* were observed, and thisdifference was considered to affect lubrication properties.

References

0.10
0.02

0Beforepressing

Uneven distribution of thickeners

0.15

0.03

0.01

Gre_Si *Gre_PAO*Fig. 6 In-situ observation 2The graph (Fig. 7) show the results of friction tests using two types of base oils and greases composed of these base oils aslubricants. All friction tests were performed three times. Although the viscosity of Oil_Si is 6 times higher than that of Oil_PAO,
the friction coefficient of Oil_Si was lower than that of Oil_PAO. This suggests that this lubrication condition is in a mixed
lubrication mode, and that the higher viscosity oil prevents direct contact of specimens.

0.05

Gre_PAO *LoadResult : Friction test

Oil_PAO

Approximately 15 minutes after the start of the sliding, it wasobserved that grease was retained in the gap between the pin
and disk specimens. This grease formed a wing-like shape(encircled in green) and was supplied forward the contact areain the moving direction.

GreasePinspecimen
(POM)

0.06

Starved area of grease such as observed in Gre_PAO wasobserved and its area was larger than that in Gre_PAO.

Incident light
Reciprocating slide(Glass moved)

PropFig. 5 In-situ observation 1

Features of behavior of Gre_Si (Fig.9)

Immediately after the pin specimen began to slide, a starved
area of grease was observed behind the contact area in themoving direction.

Objective lens

XY stage

Greases

Features of behavior of Gre_PAO (Fig. 8)[1] M. Suzuki, T. Shirai : Technical Trends and Tribology of Chassis Ball Joint,
Journal of Japanese Society of Tribology, 54, 9(2009) 604-609[2] SOMIC ISHIKAWA INC. home page < http://www.somic.co.jp >[3] K. Maruyama, T. Nawata, T. Ando, Y. Takeichi, M. Suzuki, R. Morita, K.
Arakawa : Observation of behavior of thikener at grease lubicated friction
interface by fluorescence method, International Tribology Conference Sendai2019