Page 122 - Rollingbearings

B.4 Lubrication
Lubrication systems Viscosity and viscosity index The pour point of an oil is deined as the
lowest temperature at which a lubricant will
Continuous lubrication can be achieved via The required viscosity is primarily given by low, but it must not be used as a functional
single point or multi point automatic lubrica- the lubrication condition κ, at the expected limit when selecting oil type If the tempera-
tors, e g SKF’s SYSTEM 24 or SYSTEM operating temperature, evaluated as ture is above but near the pour point, the
MultiPoint described in Lubrication condition – the vis- viscosity is still very high, which may impair
Centralized lubrication systems, such as cosity ratio, κ, page 102. The viscosity index, pumping, iltering, and other characteristics
SKF MonoFlex, SKF ProFlex, SKF DuoFlex, VI, is the measure of how the oil viscosity The thickness of the hydrodynamic ilm is
SKF MultiFlex (table 8) and Lincoln Centro changes with temperature. VI is a part of the determined, in part, by the viscosity index
Matic, Quicklub and Dual Line can reliably selection process, in particular for applications (VI) and the pressure-viscosity coeficient
deliver grease in a wide range of quantities that operate in a large temperature range. For most mineral oil based lubricants, the
For additional information about Oils with a VI of at least 95 are recommended pressure-viscosity coeficient is similar, and
SKF lubrication systems, refer to you can use the generic values obtained
skf com/lubrication from literature However, for synthetic oils,
Oil type the effect on viscosity to increasing pressure
is determined by the chemical structure of
There are two broad categories of oil types its base stock As a result, there is consider-
Selecting a – mineral and synthetic – with the following able variation in pressure-viscosity coefi-
types of synthetic oils available:
cients for different types of synthetic base
suitable oil • polyalphaoleins (PAO) stocks
Because of the differences in the viscosity
• esters index and pressure-viscosity coeficient, the
Oil selection criteria • polyglycols (PAG) formation of a hydrodynamic lubricant ilm,
when using a synthetic oil, may differ from
When you select a lubricating oil, the most Choice of oil type is mainly determined by that of a mineral oil with the same viscosity
important parameters are the viscosity and the temperature range in which the applica- Regarding the lubrication condition for
viscosity index, the temperature stability tion is expected to operate mineral and synthetic oils, the combined
(which inluences the choice of oil type) and effect of the viscosity index and the
the additive package (EP/AW and corrosion • Mineral oils are generally favoured as the pressure-viscosity coeficient normally
protection) that its the operating conditions lubricant for rolling bearings cancel each other out
for the application • Synthetic oils should be considered for are summarized in table 9. For additional
The properties of the different oil types
operational temperatures above 90 °C
Lubrication (195 °F) because of their improved thermal information about synthetic oils, contact the
lubricant supplier
and oxidation resistance, or below –40 °C
(–40 °F) because of their better properties
Oils, and in particular synthetic oils, may
B.4 at low temperatures interact with such things as seals, paint or
water in a different way than mineral oils, so
such effects, as well as miscibility, must be
investigated

Diagram 6
The SKF trafic light concept – standard greases
Thickener type Base oil Temperature range
–50 0 50 100 150 200 250 °C

Lithium soap Mineral
Lithium soap Diester

Lithium complex Mineral
Lithium complex Polyalphaolein
Calcium complex Mineral
Aluminium complex Mineral
Polyurea Mineral
–60 30 120 210 300 390 480 °F

120