Properties of Lubricating Oils

Properties of Lubricating Oils

Lubricating oil as such has limited characteristics based on the type of Oil, whose quality is established by the refining processes.

 Additives are added to make up for the special service requirements the oil is intended to function.

Properties of Lubricating oils can be broadly classified follows:

General properties of base oils

a.    Viscosity.

Viscosity of an oil is a measure of the oil’s resistance to shear, more commonly known as resistance to flow.

Lubricating oil is considered as a series of fluid layers superimposed on each other, the viscosity of the oil is a measure of the resistance to flow between the individual layers.

A high viscosity implies a high resistance to flow while a low viscosity indicates a low resistance to flow. Viscosity varies inversely with temperature and is directly variable by pressure

Higher pressure causes the viscosity to increase, and subsequently the load-carrying capacity of the oil also increases.

This property enables use of thin oils to lubricate heavy machinery. The load carrying capacity also increases as operating speed of the lubricated machinery is increased.

Two methods for measuring viscosity are

(1)Shear. When viscosity is determined by directly measuring shear stress and shear rate, it is expressed in centipoise (cP) and is referred to as the absolute or dynamic viscosity. It is more common to use kinematic viscosity, (absolute viscosity divided by the density of the oil being tested). Kinematic viscosity is expressed in centistokes (cSt).

Viscosity in centistokes is conventionally given at two standard temperatures: 40 °C and 100 °C

(2) Time. Another method used to determine oil viscosity measures the time required for an oil sample to flow through a standard orifice at a standard temperature.

The units of viscosity can be expressed as centipoise (cP), centistokes (cST), or Saybolt Universal Seconds (SUS), depending on the actual test method used to measure the viscosity.

b.    Viscosity index.

The viscosity index, commonly designated VI, is an arbitrary numbering scale that indicates the changes in oil viscosity with changes in temperature.

Low VI - below 35;

 Medium VI - 35 to 80;

High VI - 80 to 110;

Very high VI - above 110.

A high viscosity index indicates small oil viscosity changes with temperature

 Therefore, a fluid that has a high viscosity index can be expected to undergo very little change in viscosity with temperature extremes and is considered to have a stable viscosity

 Viscosity index of an oil is crucial when selecting a lubricant for an application, and is especially critical in extremely hot or cold climates.

Failure to use oil with the proper viscosity index when temperature extremes are expected may result in poor lubrication and equipment failure.

 Paraffinic oils are rated at 38 °C .

 Naphthenic oils are rated at-18 °C. Proper selection of petroleum stocks and additives can produce oils with a very good VI.

c.    Pour point.

The pour point is the lowest temperature at which oil will flow. This property is crucial for oils that must flow at low temperatures. A commonly used rule of thumb when selecting oils is to ensure that the pour point is at least 10 °C lower than the lowest anticipated ambient temperature.

d.    Cloud point.

The cloud point is the temperature at which dissolved solids in the oil, such as paraffin wax, begin to form and separate from the oil. Oils must be maintained at temperatures above the cloud point to prevent clogging of filters.

e.    Flash point and fire point.

The flash point is the lowest temperature, to which a lubricant must be heated before its vapor, when mixed with air, will ignite but not continue to burn.

The fire point is the temperature at which lubricant combustion will be sustained.

The flash and fire points are useful in determining a lubricant’s volatility and fire resistance.

 A lubricant exhibiting a flash point significantly lower than normal will be suspected of contamination with a volatile product.

The fire point for a lubricant is usually 8 to 10 percent above the flash point.

 The flash point and fire point should not be confused with the auto-ignition temperature of a lubricant, which is the temperature at which a lubricant will ignite spontaneously without an external ignition source.

f. Acid number or neutralization number. The acid or neutralization number is a measure of the amount of potassium hydroxide required to neutralize the acid contained in a lubricant. Acids are formed as oils oxidize with age and service. The acid number for an oil sample is indicative of the age of the oil and can be used to determine when the oil must be changed.

Service Characteristics based on additives

General

Additives improve the service performance of base oils. Although the overall performance of oil can be improved by introducing additives, a poor quality oil cannot be converted into a premium quality oil by introducing additives.

 There are limits to the amount of additives that can be introduced to improve performance. Above this, the benefits are minimal or may provide no gains in performance and may be detrimental to the metals and oil.

Function of Additives

a) Protecting lubricated surfaces. These additives coat the lubricated surfaces and prevent wear or rust.

b) Improving performance. They make the oil perform in a desired manner for specific applications. Examples are Viscosity index improvers and antifoaming agents.

c) Protecting the base lubricant. Antioxidants reduce the tendency of oil to oxidize and form sludge and acids.

Types of Additives

Surface Additives

The primary purpose of surface additives is to protect lubricated surfaces. These additives coat the lubricated surfaces to prevent wear or rust.

a. Rust inhibitors. Rust and corrosion inhibitors are added to most industrial lubricants to minimize rusting of metal parts

As oil ages it develops polar compounds. Presence of moisture or water in oil (though they mix poorly) that exists in suspension or bottom (based on agitation), tends to emulsify in combination with polar compounds. This when adheres to metal surfaces, forms rust due to free oxygen present.

Inhibitors form a surface film that prevents water from making contact with metal parts. This is accomplished by making the oil adhere better or by emulsifying the water if it is in a low concentration.

Emulsifiers Form emulsions; either water-in-oil or oil-in-water, according to type of oil.

b. Corrosion inhibitors. Corrosion inhibitors suppress oxidation and prevent formation of acids.

These inhibitors form a protective film on metal surfaces and are used primarily in internal combustion engines to protect alloy bearings and other metals from corrosion.

Antifoam additives Reduce surface foam. Antioxidants Reduce oxidation. Various types are: oxidation inhibitors, retarders; anticatalyst metal deactivators, metal passivators.

Detergents Reduce or prevent deposits formed at high temperatures, and wash all into the oil sump. e.g., in internal combustion engines.

Dispersant Prevent deposition of sludge by dispersing a finely divided suspension of the insoluble material formed at low temperature, enabling removal by filtering or centrifugation.

 Detergents and dispersant are used primarily in internal combustion engines to keep metal surfaces clean by preventing deposition of oxidation products.

c. Extreme pressure (EP) additives. They react with the metal surfaces to form compounds that have lower shear strength than the metal. The reaction is initiated by increased temperature caused by pressure between asperities on wearing surfaces. The reaction creates a protective coating at the specific points where protection is required. This coating reduces friction, wear and scoring.

Base additives Neutralize contaminating strong acids formed by combustion of          fuels or by decomposition of active EP additives

Oiliness enhancers Reduce friction under boundary lubrication conditions; increase load-carrying capacity where limited by temperature rise by formation of mainly organic surface films.

Extreme pressure Prevent scuffing of rubbing surfaces under severe operating conditions, e.g., heavy shock load, by formation of a mainly inorganic surface film.

Antiwear agents. Additives that cause an oil to resist wear by coating the metal surfaces are called antiwear agents. Molecules of the antiwear compound are polar and attach (adsorb) themselves to metal surfaces or react mildly with the metal. When boundary lubrication conditions (direct contact between metal asperities) occur, such as in starting and stopping of machinery, these molecules resist removal more than ordinary oil molecules

Tackiness agents. In some cases, oils must adhere to surfaces extremely well. Adding polymers composed of long-chain molecules or aluminum soaps of long-chain fatty acids increases the tackiness or adhesiveness of oils.

Compounded oil.

A small amount of animal fat or vegetable oil added to a mineral oil will reduce the coefficient of friction without affecting the viscosity. The ability of oil to provide a lower coefficient of friction at a given viscosity is often called lubricity.

Fatty oil is added to obtain this quality of oiliness, the lubricant is called a compounded oil. Fatty oil adheres to metal more strongly than mineral oil and provides a protective film. Compounded oils are generally used in worm gears.

Performance-Enhancing Additives

These additives improve the performance of lubricants.

a. Pour-point depressants. An oil's pour point is the temperature at which the oil ceases to flow under the influence of gravity.

 In cold weather, oil with a high pour point makes machinery startup difficult or impossible.

The stiffness of cold oil is due to paraffin waxes that tend to form crystal structures.

Pour-point depressants reduce the size and cohesiveness of the crystal structures, resulting in reduced pour point and increased flow at reduced temperatures.

b. Viscosity index (VI) improvers. The viscosity index is an indicator of the change in viscosity as the temperature is changed. The higher the VI, the less the viscosity of an oil changes for a given temperature change.

 Viscosity index improvers are used to limit the rate of change of viscosity with temperature, when heated; the improvers enable the oil viscosity to increase within the limited range permitted by the type and concentration of the additive.

c. Emulsifiers. Soluble oils require emulsifiers to promote rapid mixing of oil and water and to form stable emulsions. Soluble oils are used as lubricants and coolants for cutting, grinding, and drilling applications in machine shops.

d. Demulsifiers. Demulsifiers promote separation of oil and water in lubricants exposed to water.

 Lubricant Protective Additives

Lubricant protective additives are employed to protect the lubricant instead of the equipment.

a. Oxidation inhibitors. Oxidation inhibitors reduce the quantity of oxygen reacting with oil by forming inactive soluble compounds and by passivating metal-bearing surfaces to retard the oxidation rate. Oxidation inhibitors are consumed as the oil ages.

Lack of this inhibitor produces acids, sludge, and varnish that foul or damage metal parts.

At temperatures above 82 ºC the oxidation rate is doubled for every 6.67 ºC rise in temperature.

Oxidation of hydrocarbons is a complex chemical process and depends on the nature of the oil.

b. Foam inhibitors. Foam inhibitors such as silicone polymers or poly acrylates; modify the surface properties reducing air or other gases admixture in oil, by way of releasing them.

Lubricant producers do not disclose which compounds are used to enhance the lubricant quality, but only specify the generic function such as anti-wear, EP agents, or oxidation inhibitors. No two brands have the same chemicals used to attain the purpose.

Hence compatibility charts to be cross referenced while using different lubricants. Stratification of additives can result if the new charge added to existing charge exceeds 10 percent in volume, even though oils are compatible.