Page 44 - needle bearings

Lubrication
NTN

8. Lubrication

The purpose of lubricating bearings is to form a thin oil
film on the rolling and sliding surfaces and to thereby
prevent metal to metal direct contact. Furthermore,
lubricating rolling bearings has the following effects.

(1) Reduction of friction and wear
(2) Discharge of friction heat
(3) Further extension of bearing life
(4) Rusting prevention
(5) Prevention of foreign matter invasion
For achieving these lubrication effects it is necessary to
select a good quality lubricant, to remove dust from the
lubricant, and to design a proper seal structure for
prevention of lubricant leak as well as to adopt the Fig. 8.1 An example of bearing unit with grease sector and
lubrication method suitable to the respective actual grease valve
operating conditions.
Rolling bearings are mostly lubricated with grease or
oil, but solid lubricants such as molybdenum disulfide, 8.1.1 Types and characteristics of grease
graphite, etc. are used for special application. Lubrication oil is composed of a lubrication base oil (ex.
Further, the comparative data of grease lubrication and mineral oil base or a synthetic oil base) held with a
oil lubrication given in Table 8.1 can be utilized as the thickener and various additives added thereto. The
selection guide. properties of grease is determined by the kinds and
combination of base oil, thickener and additives.
Table 8.2 shows the general types of grease and their
characteristics. Even the grease of same type is
Table 8.1 Characteristic comparison of grease lubrication with
oil lubrication significantly different in the performance depending on
Lubrication method Grease the grease brand selected. In selecting a grease brand,
Comparative items lubrication Oil lubrication therefore, it is necessary to verify the property data
submitted from the grease manufacturer.
Handling ˕ ˚
Reliability ˓ ˕ 8.1.2 Base oil
˓ Mineral oil or synthetic oils such as diester oil, and
Cooling effect ʷ
(Recirculation needed)
synthetic oil such as silicone oil, fluorocarbon oil, etc. are
Seal structure ˓ ˚ used as the base oil for grease.
Power loss ˓ ˓ The lubrication performance of grease is mainly
determined by the lubrication performance of the base oil.
Environmental pollution ˓ ˚ In general, a grease composed of low viscosity base oil is
High speed running of bearing ʷ ˓ excellent in low temperature characteristic and high
speed performance and, on the other hand, a grease
˕ɿ Extraordinarily advantageous ˓ɿAdvantageous composed of high viscosity base oil is excellent in high
˚ɿ Fairly advantageous ʷɿDisadvantageous
load characteristic.
8.1.3 Thickener
8.1 Grease lubrication Thickener is blended and diffused in base oil to hold
Grease lubrication is the simplest lubrication method. grease in semi-solid form. And various thickeners are
This method enabling to simplify design of the seal available for use as follows; thickener composed of
structure is broadly used. metallic soap such as lithium, sodium or calcium, etc.,
Important points in this lubrication method are to select thickener composed of inorganic material such as silica
an optimum grease and to fill it securely in a bearing. gel, bentonite, etc., and non-soap base thickener
Particularly where the cage is guided by the inner ring or composed of organic material such as polyurea,
outer ring of bearing, good care must be exercised so the fluorocarbon, etc.
guide surface is fully greased throughout its entire area. The grease characteristics such as critical operating
Furthermore, for grease replenishment it is desirable to temperature, mechanical stability, durability, etc. are
provide a grease sector and a grease valve, etc. as a mainly determined by the kind of thickener used. In
grease replenisher. Fig. 8.1 illustrates an example of general, the sodium soap base grease is inferior to others
bearing unit with grease sector and grease valve. in water resistance. Non-metallic soap base thickener
composed of bentone, polyurea, etc. is excellent in high
temperature characteristic.

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