Page 310 - The Tribology Handbook

c2 Mineral oils

Thermal decomposition ADDITIVE OILS
Mineral oils are also relatively stable to thermal decom- Plain mineral oils are used in many units and systems
position in the absence of oxygen, but at temperatures for the lubrication of bearings, gears and other mechan-
over about 330°C, dependent on time, mineral oils will isms where their oxidation stability, operating temperature
decompose into fragments, some of which polymerise to range, ability to prevent wear, etc., are adequate. Now-
form hard insoluble products. adays, however, the requirements are often greater than
plain oils are able to provide, and special chemicals or
additives are ‘added’ to many oils to improve their pro-
Table 2.7 Thermal decomposition products perties. The functions required of these ‘additives’ gives
them their common names listed in Table 2.9.
Product Effect
Light hydrocarbons Flash point is reduced; viscosity is
reduced Table 2.9 Types of additives

Carbonaceous residues Hard deposits on heater surfaces
reduce flow rates and accentu- Main tvpc Function and sub-lypes
ate overheating
Acid Neutralise contaminating strong acids
neutralisers formed, for example, by combustion
Some additives are more liable to thermal decom- of high sulphur fuels or, less often, by
position than the base oils, e.g. extreme pressure additives; decomposition of active EP additives
and surface temperature may have to be limited to tem-
peratures as low as 130°C. Anti-foam Reduces surface foam
Anti-oxidants Reduce oxidation. Various types are:
oxidation inhibitors, retarders; anti-
catalyst metal deactivators, metal pass-
ivators
Con$amination Anti-rust Reduces rusting of ferrous surfaces swept
by oil
Contamination is probably the most common reason for
changing an oil. Contaminants may be classified as shown Anti-wear Reduce wear and prevent scuffing of
in Table 2.8. agents rubbing surfaces under steady load
operating conditions; the nature of the
film is uncertain
Table 2.8 Contaminants
Corrosion Type (a) reduces corrosion of lead; type
inhibitors (b) reduces corrosion of cuprous metals
7ype Example
Detergents Reduce or prevent deposits formed at
Gaseous Air, ammonia
high temperatures, e.g. in ic engines
Liquid Water, oil of another type or viscosity grade Prevent deposition of sludge by dispersing
or both Dispersants
a finely divided suspension of the in-
soluble material formed at low tem-
Solid Fuel soot, road dust, fly ash, wear products
perature
Where appropriate, oils are formulated to cope with Emulsifier Forms emulsions; either water-in-oil or
likely contaminants, for example turbine oils are designed oil-in-water according to type
to separate water and air rapidly, diesel engine oils are Extreme Prevents scuffing of rubbing surfaces
designed to suspend fuel soot in harmless finely divided pressure under severe operating conditions, e.g.
form and to neutralise acids formed from combustion of heavy shock load, by formation of a
the fuel. mainly inorganic surface film
Solid contaminants may be coptrolled by appropriate
filtering or centrifuging or both. Limits depend on the Oiliness Reduces friction under boundary lubrica-
abrasiveness of the contaminant and the sensitivity of the tion conditions ; increases load-carrying
system. capacity where limited by temperature
rise by formation of mainly organic
surface films
Pour point Reduces pour point of paraffinic oils
depressant
Oil life Tackiness Reduces loss of oil by gravity, e.g. from
Summarising the data given under the headings Oxida- vertical sliding surfaces, or by centri-
tion and Thermal decomposition, above, Figure 2.5 gives an fugal force
indication of the time/temperature limits imposed by Viscosity index Reduce the decrease in viscosity due tc
thermal and oxidation stability on the life of a well-refined improvers increase of temperature
HVI paraffinic oil.
C2.6