Lubrication (continued) ____________________________________________________________________________
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Base oil:
Petroleum oils, silicone or synthetic diester/tetraester oil or
fluorinated oils are mainly used as the base oil for grease.
Although the lubricating properties of grease depend mainly on those
of its base oil, the thickener used also has an effect. In general,
low viscosity base oils are more suitable for high speed, low
temperature applications and high viscosity base oils are more
suitable for high temperature, heavy load applications.
| BASE OIL |
TEMPERATURE RANGE |
OTHER
PROPERTIES |
| Petroleum |
-25oF to
250oF
(-32o to
121oC) |
Excellent load capacities (high film
stregth). Lower viscosity = high speed (dN) capability. |
| Diester/tetraester |
-65oF to
350oF
(-54o to
177oC) |
Wide temperature range. Moderate film
strength. Oxidation resistant. |
| Silicone |
-100oF to
400oF
(-73o to
204oC) |
Widest temperature range. Less film
strength than diester/tetraester. Tends to migrate giving less
rust prevention. |
| Fluorinated |
-30oF to
550oF
(-34o to
288oC) |
Stable at high temperature. Chemically
inert. Ultra low vapor pressure (10-9 to
10-11 torr) gives long life. Can be viscous at low
temperatures. |
Thickener: Grease thickeners can
be grouped into several families. Types used include compounds of
metallic soaps (lithium, sodium), inorganic (silica gel, bentonite),
heat resistant organic (urea, flouric) - or a mixture of several
types. Important grease characteristics influenced by the choice of
thickener are the dropping point (temperature at which the
grease becomes sufficiently fluid to drip, affecting
high-temperature performance), and the water solubility. In
particular, sodium soap emulsifies with high humidity and cannot be
used as a thickener when moisture is present.
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| GREASE
CHARACTERISTICS |
Channeling: If a
grease stays in place after it has been displaced by the balls
during initial rotation, it is referred to as a channeling grease. This property is useful to reduce torque. A non-channeling grease will tend to migrate back into the
raceways, which can promote better lubrication in heavy load
applications.
Dropping point: The temperature
at which a grease heated in a specified small container becomes
sufficiently fluid to drip. Dropping points for common greases are
listed in the table of commonly available lubricants. Above the
dropping point, a grease will lose its essential
characteristics.
Consistency: The
consistency of a grease is characterized by its penetration, which is the depth in 1/10th millimeters that
a standard cone sinks into the grease under prescribed conditions.
High numbers indicate softer grease, since the cone has sunk
further. Grease may change consistency once it is worked (kneaded),
so penetration levels are referred to as worked or unworked. If a grease has a uniform consistency, then there
will only be a small spread between the worked and unworked values. Channeling greases tend to be thicker, so
will have lower penetration values. Specific penetration values are
available from grease manufacturers (ranging from approximately 100
to 400 x 1/10th mm).
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