Properties of Lubricating Oil

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Composition of lubricating oils

Lubricating oil fractions extracted from crude oil are a widely varying mixture of straight and branched chain paraffinic, napthenic aromatic hydrocarbons having boiling points ranging from about 302^o to 593^oC. Some specialty lubricants may have boiling point extremes of 177 and 815^oC. The choice of grade of lubricating oil base is determined by the expected use.

General capabilities expected from an engine lubricant
• Dispersivity or capacity to the cold parts of an engine clean
• Detergency or capacity to keep hot parts of an engine clean
• Thermal strength or capacity to withstand temperature changes
• Anti-oxidant or capacity to resist the action of oxygen
• Anti-wear or capacity to contain wear
• Anti-scuffing or capacity to preserve oil film even in the presence of high pressures
• Alkalinity reserve or capacity to neutralise acids formed during combustion or other sources thereby preventing corrosive wear
• Demulsibility or capacity to separate contaminants
• Resistance to hydrolysis or capacity to withstand the action of water which can affect additives
• Pumpability
• Centrifugibility and filterability or capacity to separate insoluble elements
• Anti-rust,corrosive and anti-foam are just some of the other properties required

Properties ideal for bearings
• Soluble for high speed fluid film hydrodynamic lubrication, hence, low viscosity with reduced oil film friction.
• moderate bearing loads
• improved heat transfer behavior
• corrosion protection
• cooling
• low friction
• good low temperature viscosity
• good high temperature viscosity

Properties ideal for gear case
• high film strength to prevent metal to metal contact. Hence, high viscosity adhesive to resist sliding and centrifugal forces
• corrosion protection
• cooling
• reduces friction
• good low tempo viscosity
• good high tempo viscosity
  The thicker the oil film the greater the cushioning against shocks. Also less tendency for pit formation by hydraulic action in cracks,
• sound damping properties with cushioning effects
• antifoam properties

Turbine oil
Compromise between above two requirements
• Generally a good quality refined mineral oil derived from paraffanic base stock used with various additives including EP additives for highly loaded gearing.
• Anti-foaming properties important

Additives

Improvements in lubricating oil over the last twenty years have come about almost entirely from the use of additives.

These are added for three main reasons;
1. to protect the lubricant in service by limiting the chemical change and deterioration
2. To protect the mechanism from harmful combustion products and malfunctioning lubricating oil
3. To improve existing physical properties and to create new beneficial characteristics in the oil

Typical additives are; Barium, calcium, phosphorus, Sulphur, chlorine, zinc, oxidation inhibitor-increases oil and machinery life, decreases sludge and varnish on metal parts

Corrosion inhibitor-protects against chemical attack of alloy bearings and metal surfaces.

Antiwear improvers-protects rubbing surfaces operating with this film boundary lubrication. One such antiwear ( and oxidation inhibitor) chemical is Zinc dithiophosphate or ZDDP

Detergent-tend to neutralise the deposits before formation under high temperature and pressure conditions, or as a result of using a fuel with high sulphur content. The principle detergents are soaps and alkaline metals, usually calcium ( often referred to as 'matallo-organic compounds'). They are usually ash forming and spent additive will contribute to the insolubles loading of a used oil. It should be noted that additives which do not burn cleanly without ash tend to be avoided for use with Cylinder Lubricating Oils.

Dispersant-used to disperse or suspend the deposits forming contaminants. Typical dispersants, such as polyesters and benzlamides, are usually clean burning. The molecules have a polar charge at one end which attracts and holds the deposits

Alkaline agents-neutralises acids, htese form the TBN of the oil and includes additives such as the above dispersants and detergents. An excess of acid neutralising alkalis are present in the oil and these help to keep parts clean. Failure to keep an oil alkaline can lead to damage to bearings due to acidic attack as well as increased liner wear.

Rust inhibitors-

Pour point depressants-improves low temperature viscosity

Oiliness agent-reduces friction seizure point and wear rates

EP additives-increases film strength and load carrying capability

Antifoam agents-prevents stable bubble formation

Viscosity Improvers-an additive that improves the viscosity index of the oil. I.e. reduces the effect of temerpature of=n the viscosity of the oil. Shear stability property is measured indicating the effect of high rates of shar on the VI improver as the improver molecules are broken down into smaller molecules

Metal deactivators-prevent catalytic effects of metal

Antiseptic-bactericide.

Oxidation

Oxidation degrades the lube oil producing sludges, varnishes and resins. Presence of moisture, and some metals particularly copper tend to act as a catalyst. Once oxidation starts, deterioration of the properties of the oil is rapid.

Recharging

When recharging no more than 10 % of the working charge should be topped up due to heavy sludgeing that can occur due to the heavy precipitation of the sludge.

EP additive oils

Can assist in healing of damaged gear surfaces but should be used as a temporary measure only due to risk of side effects

Emulsification

This occurs due to water contamination; also, contamination with grease, fatty oils, varnish, paint and rust preventers containing fatty products can also promote emulsification. The presence of an emulsion can be detected by a general cloudiness of the sample. Salt water emulsifies very easily and should be avoided.
Water entrained in the oil supplied to a journal bearing can lead to loss of oil wedge, rub and failure.
Fresh water contamination whilst not in itself dangerous can lead to rusting. The iron oxides catalyses the oil to form sludge's. The additives in the oil can leach out to change the water into an electrolyte.
Salt water contamination is very serious as it causes tin oxide corrosion, and also leads to electrochemical attack on the tin matrix in the white metal. The sea water act as then electrolyte.
A major problem of water within a lub oil is where the mix enters a bearing, here it is possible for the water to be adiabatically heated causing it to flash off collapsing the oil wedge.

Stresses on Lube oil

The main stresses experienced by Lube oils in diesel engines operating on heavy fuel oils are expressed as follows Acid Stress- Caused by sulphuric and oxidation acids. This leads to increased corrosive wear, deposits, reduced Base Number and shorter oil life.Rapid depletion of the BN is the clearest sign of oil stress
Thermal/Oxidative stress-This caused by elevated temperatures leading to increased rates of thermal/oxidative breakdown of lubricant and fuel. This leads to increased levels of deposits, sludges, corrosive wear of bearing material, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corosion on the piston.
Asphaltene Stress-This caused by fuel contamination of the lube oil and can lead to increased levels of deposits, sludges, lacquers, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corosion on the piston