Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No. 742006
Use of oil is effective to decrease the friction
coefficient. Until now, NTN had been attempting to
lower the friction coefficient for sliding bearings by
using an oil dispersion technique with a polymer alloy,
for example. 3)
However, conventional techniques had limitations
because the addition of an excessive amount of oil
can lead to deterioration in the mechanical strength
and formability of a sliding material. Therefore, we
developed a technique that adopted porous silica as
an oil retaining material (oil-impregnated silica).
Thereby, we attempted to increase the content of oil
included in the resin while maintaining good
mechanical characteristics and formability of the
resultant sliding resin material. 4)
Compositions and frictional characteristics of
materials prepared by blending different proportions of
oil-impregnated silica into ultrahigh-molecular-weight
polyethylene (UHMWPE) are summarized in Table 1
and Fig. 3. Compared with the material blended with
oil alone 2, the material blended with oil-impregnated
silica 3 exhibited a lower dynamic friction coefficient
though its oil content was equal to that of material 2.
With an increase in the amount of oil-impregnated
silica (material 4), the dynamic friction coefficient
further decreased, reaching a level lower than a third
the level obtained with a PTFE-based sliding material.
Having undergone the test, the sliding surface of
material 4 was inspected, and the oil-impregnated
Table. 1 Composition of test specimens for sliding tests
Material
1
2
3
4
Composition (vol%)
UHMWPE (100)
UHMWPE (85)Silicone oil (15)
UHMWPE (82)Oil-impregnated silica (18)
UHMWPE (70)Oil-impregnated silica (30)
1:5 porous silica to silicone oil ratio
silica exposed on the sliding surface was found as a
result (Fig. 4). In addition, the static oil seepage from
material 4 at room temperature and 15 MPa conditions
was measured. As shown in Fig. 5, the oil seepage
increased as the time elapsed and reached 6% in 100
hours. Assuming that only the oil near the surface layer
seeped out this level of leakage is too great.
From these findings, we estimated the low friction
mechanism of this material as shown in Fig. 6. In
other words, we believe the following theory about the
mechanism. Since the oil-impregnated silica serves as
the load bearing point, solid-to-solid contact is
inhibited, making it easier for an oil film to form.
Seepage wt%
Fig. 4 Surface of the material 4 after sliding test
8.00
6.00
4.00
2.00
Pressure: 15 MPa
Room temperature
Oil-impregnated
silica
10m
0.00
0
20 40 60 80 100
Time h
Fig. 5 Relationship between loading time and oozed oil
amount from the material 4
Dynamic friction coefficient
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Two test runs
UHMWPE
100%
UHMWPE
15%
silicone oil
Test rig: oscillating ring-on-disk
Specific pressure: 15MPa
Velocity: 0.1m/s
Counterpart material: SUS304 (Ra 0.1m)
Characteristic to be measured:
dynamic friction coefficient 5 minutes later
Silicone oil added
Porous silica added
Increased amount added
UHMWPE
3% porous silica
15% silicone oil
UHMWPE
5% porous silica
25% silicone oil
PTFE
Oil film
Resin material
Transfer of oil
Counterpart material
Oil-impregnated
porous silica
Operating mechanism of oil-impregnated silica (estimated)
Load-bearing point – decrease in true contact area
Oil supply source – decrease in shear strength
Low
Fig. 3 Friction properties of the materials filled with oilimpregnated-silica
Fig. 6 Estimated mechanism of low friction of the
developed material
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