Lubricate The oil must form a fluid film between highly loaded moving parts.
Act as a Hydraulic Medium The oil doesn’t have this job in all applications, but it is not uncommon for the lubricating oil to be part of the hydraulic system.
Act As a Coolant The oil must remove heat generated both Inside and outside the machine.
Carry Away Contaminants The oil often becomes contaminated In the process of carrying contaminants to the filter. Contamination is the major reason oils “wear out” and must be changed. Contaminants can come from both internal and external sources.
Protect Against Wear The oil must protect highly loaded parts which can wear when the fluid film is very thin (boundary Lubrication).
Protect Against Corrosion The oil must protect precision parts made of various metals which are vulnerable to rust and corrosion.
Protect Against Deposits The oil is designed to resist the formation of deposits (like sludge and varnish), which can accumulate in the lubricating system and interfere with the oil’s ability to lubricate.
Resist Aeration and Foaming The oil is designed to resist aeration and foaming and prevent serious malfunctions which can result from these conditions.

As can be seen, we expect a lot from industrial lubricants. Industrial lubricants have to accomplish many tasks in any given application. Some of these tasks-lubricating, cooling, carrying away contaminants, and (if necessary) acting as a hydraulic medium- can be performed by a well selected base oil. The other functions involve properties not normally possessed by mineral base oils. They can, though, be provided adding additives to the oil.
Antiwear and extreme pressure (EP) additives
These provide load-carrying capacity where needed and prevent scuffing of moving parts under conditions of boundary (thin film) lubrication
These control oil oxidation, rust, and corrosion."
Dispersants and detergents:
These control deposit formation throughout the system.
Special purpose additives:
These additives are put in the oil to meet the needs of specific applications. Examples are pour point depressants, bactericides, emulsifiers, demulsifiers, tackiness agents, friction reducers, foam inhibitors, mist suppressors, and viscosity index improvers.

An oil designed for one application can’t be expected to work well in another situation. It is extremely important that the correct oil be used in a given machine at all times.

There are two basic causes:
• It becomes contaminated from internal and external sources.
• It loses some of the lubricating properties provided by additives, during service.

Contamination is the major reason oils “wear out” and must be changed. External contaminants-dirt, water, corrosive fluids, etc. – are the prime offenders, but wear metals from the machine also contribute to the problem. Normally, contaminants collect slowly in the oil. In mild service where contaminants are removed periodically of continuously, lubricants can last for many, many years.
Accidents happen, though. And in certain types of service, contamination can build quickly to a dangerous level.
They seem to come from everywhere. No system can keep these contaminants completely at bay, but regular and careful maintenance sure helps.
One frequent culprit is airborne dust.
One can understand ingress of airborne dust into open systems. It is harder to see how it finds its way into closed or semi closed systems. Lack of air filters on vents or storage tanks, or air filters in poor repair, can be a source of entry for airborne dust. Hydraulic systems that breathe a lot due to oil surging in and out of the equipment are good candidates for airborne dust contamination. This problem is compounded in dusty environments, like those around construction machinery, and in mining and ore milling operations.
Faulty seals are another point of entry for dust and dirt. Exposed piston rods on hydraulic rams are very vulnerable. This of course, wears down seals further, and makes further contamination with dust even easier.
Other solids that are a part of the local environment will enter in the same manner. Mill scale can be a contaminant in steel mill circulating oils. Paper fly is common problem in paper mill systems. Machining and grinding operations provide lots of metals debris which can foul a lubrication or hydraulic system. Dust, core sand, or sandblasting sand can be left in new equipment, and although elaborate flushing procedures are often used before the new equipment is put in service, getting this debris dislodged from every nook and cranny can be an almost impossible job.
Of course, the most worrisome metal particles in a lubrication system are those created by internal wear. They’re real danger signals that something is wrong with the machine or lubricant. It is essential to pinpoint the problem and correct it at the first sign.

Rust is another solid contaminant that we can do without. It is also a signal that something is wrong. Water and oxygen have combined with iron or steel to produce rust. This could have happened long before oil was charged to the system, or it could be caused by corrosive process liquids. Like the other solids mentioned, it’s abrasive and, like them, makes poor addition to a lubricant.

Soot is a solid contaminant that is common in engine crankcase lubricants. It’s less common in industrial systems, but the base oil can be thermally cracked and carbonaceous matter formed if a journal should overheat or if a heat transfer oil is subjected to excessive surface temperatures at the primary heat source.
" Along the same lines, oil oxidation can form sludge and varnish –they’re not abrasive, but they can gum up close-tolerance parts such as controllers, hydraulic valves, governors, and small oil passageways. Additives designed to minimize oxidation help control this problem.
The other solids mentioned can be, and usually are removed from the oil by, filters, centrifuges, or other purification systems. Unfortunately, such protection devices are usually some distance from the source of the contaminants. So by the time they are captured, they’ve already done some damage to fine finishes and close tolerances of machine elements.
Water is undoubtedly the number one culprit. We all know the old saying, “Oil and water don’t mix.” But it gets more complicated when it is lubricating oil and water.
• Water weakens the lubricating film between moving surfaces,
• Water is a necessary ingredient in forming one of a machine’s worst enemies-rust,
• Water can interfere with the action of some additives,
• Water and oil sometimes mix only too well- they form emulsions which complicate purification
What’s been said of water is also true of aqueous process fluids: they can readily form emulsions, and they’re often more corrosive and more prone to cause rust and interfere with the lubrication process than water alone.
When other lubricants besides the circulating oil are used in a system, they also become potential contaminants. Whether or not they are harmful depends on the system and the type and amount of contaminant.
For example, roll oils could contaminate the circulating oil in a steel mill. Cutting oils could contaminate hydraulic and circulating oil in a machining operation. In both cases emulsion problem could occur.
Grease in support bearings can leak into main oil sumps or gear cases causing foaming or plugging of lines.
Not all contaminants leak in from the outside. Oil oxidation products can form within the lubricant.
Unfortunately, they are not removed by filtration or normal purification systems. The acidity of the oil increases, and metals get corroded. As stated earlier, oxidation inhibitors can minimize this problem.
Other potential contaminants are oil supplements. Some additives are acidic in nature & others are basic. If these are mixed in systems where water is present, insoluble gels can form and cause serious trouble.
Oils for specific services are carefully formulated and extensively tested before being put on the market. Oil companies make sure that the oil meets the requirements of the equipment as well as industry standards. No supplementary additives are needed. They can interfere with the action of the additive in the oil and hurt the oil’s performance.
Foaming and aeration are serious problems in industrial systems, and both are caused by gaseous contaminants.
Foam is a mixture of air in oil with thousands of air bubbles, each surrounded by its own oily skin, floating on the surface of the oil. Fortunately, some of the bubbles break. But those which don’t continue to pile up on the surface, if the foam rises so high that it reaches the overflow vent of the oil reservoir, it’s possible for the entire oil charge to be lost through the vent, resulting in a lubrication failure.
Pure oils do not foam, but some of the additives used to compound an oil actually change the oil’s properties and promote the formation of stable foam.
Air-pulled in through leaking pump glands or poorly designed reservoir return lines, or mixed into the oil by high speed pumps- can be the source of the problem.
Foaming can be aggravated by water and particulate matter contamination. Examples of the latter are dust, detergents, soaps and salts; but many other finely divided, oil insoluble solids or liquids can also cause problems. What happens is that the foam is stabilized by these materials-the bubbles can’t just pop and disappear.
Aeration is closely related to foaming. Tiny bubbles of air accumulate in the oil below the surface when the oil is thrashed around by bearings, couplings, gears, or an oil return stream.
Normally most of these bubbles escape, but when they cannot, aeration becomes a serious problem. Rapid release of air is an important property of hydraulic and circulating oils. The rate at which air is released from the oil is affected by:
• The oil’s viscosity
• System pressure
• Contaminants
• Additives

Slow release can reduce a lubricant’s efficiency. Aeration can cause hydraulic controls to become spongy and unreliable (the oil-air mixture is compressible). Pumps-especially the centrifugal type used in some circulating systems –can lose pressure, or even lose suction altogether. Excessive aeration has been implicated in turbine thrust-bearing failures.
Many additives are sacrificial-they are consumed or chemically changed while doing their jobs. When they are used up, or are sufficiently changed so that they no longer function, they must be replenished by make-up oil; or all the oil must be replaced by a fresh charge.
Some additives are temperature sensitive. They may evaporate or break down if the oil is operated above the temperature the system was designed for. In addition, heat and contamination and loss of additive effectiveness can damage the base oil itself, so that even oil without additives can reach the end of its useful life.

Oxidation of an oil occurs when oxygen chemically combines with oil molecules. The hotter the oil and the greater the exposure to air, the faster oxidation proceeds.
The compounds formed in the early stages of oxidation are not, in themselves, harmful. But further oxidation converts these initial oxidation products into acids which attack and corrode metals. The metal salts formed as a result of this action, and the products of oil oxidation themselves, are catalysts that speed up further oil oxidation and becomes a chain reaction.

Oxidation inhibitors can reduce the rate of oxygen attack;
but, unfortunately, they can’t stop it entirely.
Such inhibitors interfere with the chain reaction
effect but in the process are themselves chemically changed or consumed.

Oil additives can also oxidize, and their performance suffers as a result.

Continued oxidation then yields products that are less and less oil like and which eventually become insoluble in the bulk oil. Varnish and sludge result, and these can deposit on machine parts. Oil viscosity and acid content increase as the oxidation progresses, and the oil darkens in colour.

The rust-inhibiting properties of a lubricating oil are commonly enhanced by the use of special anti-rust additives.
Some of these function by neutralizing the acids formed by oxidation. This type of additive gets used up while doing the job. Others are surface-active chemical molecules that function by forming protective films on iron surfaces. Part of the molecule is attracted to a metal surface or to water, while the rest of the molecule remains soluble in the oil. The barriers formed by these molecules prevent water and oxygen from reaching the surface of the metal, and, thus, prevent the formation of rust.

These surface-active rust inhibitors are also sacrificial, but they are used up in different ways.
• The rust inhibitor can be attracted to solid contaminants Such as dust, dirt, wear metals and oxidation products. Removal of these contaminants from the oil system results in removal of some of the rust inhibitor.
• The rust inhibitor can be attracted to liquid contaminants & when circulating oil is continuously purged of the water and process fluids, some of the rust inhibitor can be lost.
• The rust inhibitor can be oxidized; they then tend to become more soluble in water. This further increases the loss of inhibitor in wet systems where liquid contaminants are continuously purged.
If there are such losses, the oil will eventually fail to give the rust protection it possessed originally.

Viscosity is one of the most important properties of a machine lubricant. It is a measure of the resistance to flow, or how “thick” or “thin” an oil is.
In parts of the machinery where oil films between moving parts are thick-that is, where there is no contact between metal surfaces-load-carrying ability is governed by the oil’s viscosity. Loss of load carrying ability due to a change in the oil’s viscosity can be a problem with mineral oils which do not contain additives.
Viscosity change is an important measure of used oil condition. An increase in viscosity over the fresh oil can indicate that oxidation is far advanced, or that the oil is contaminated with dirt or water. Seldom does the viscosity of an industrial oil decrease with use. If it does, it would suggest contamination with a solvent or a lower-viscosity oil, or shearing of the additives called “viscosity index improvers” if the lubricant is a multigrade oil.
Wear of metal surfaces under boundary lubrication conditions can occur if the additives put in the oil to prevent such damage are used up or removed. Boundary lubrication occurs in parts of some machinery where pressures (loads) are high or where high temperatures thin the oil to the point where metal-to-metal contact occurs. (An example would be a hydraulic vane pump, where vane pressures are high.)

The lubricant’s ability to prevent wear under such conditions is improved by antiwear additives such as zinc dialkyl dithiophosphate (ZDTP). This agent forms a thin inorganic film of zinc compounds, which protect the underlying metal by shearing readily without welding or causing other damage. This film is considered “non-sacrificial,” because it does not use molecules of the iron surface to form its protective barrier. It is a deposit which bounds to underlying surface. Sacrificial films, such as iron oxides, iron phosphates, and iron sulfide are also formed, but they appear to be of secondary importance to the deposit mechanism.
They are of primary importance, however, in the functioning of some “extreme pressure” (EP) additives; such as are used in Automotive API service classification GL4 and GL5 gear lubricants, and also in AGMA EP-type lubricants. The lubricant’s ability to prevent galling and seizing of metal surfaces under these very high pressure conditions is improved by the use of these additives. Some of these additives, such as those containing sulfur-phosphorus compounds, function by chemically reacting with metal surfaces. The iron films formed have lower shear strengths than the base metal so that if surface-to-surface contact occurs they can “sacrifice” themselves rather than letting the base metal be damaged.
As this surface film is worn away, it is replaced by further reaction of the exposed metal with the chemical additives. This is a slow process under normal circumstances. However, when the conditions are severe, EP additives can be used up or degraded. When this happens, the lubricant will fail to protect with the same effectiveness as when new.

A different kind of EP agent forms a protective film by adhering to the metal surfaces, mainly just where it is needed. The additives of this type used in industrial oils contain borate. Borate-containing additives prevent damage by holding the surfaces apart, not by providing a metal salt which is sheared away. As such borate also behaves as an antiwear agent and friction reducer.
Since this type of EP additive doesn’t from sacrificial films, it doesn’t get used up in the usual manner. However, its nature makes it possible to lose the agent from the oil.
Borate is a water-soluble substance which is in the oil as a fine dispersion. Contact with water (in sufficient quantities and at a certain temperature) can break the dispersion and cause the borate to become depleted. Prolonged centrifuging at high rates can also remove some of the borate physically. In either case of oil must then be changed to regain EP protection.
Dispersant additives are used in heavy-duty, high temperature industrial lubricants where water separation is not a problem. Examples of such lubricants are power transmission fluids, heat transfer oils, and paper mill circulating oil. Dispersants control deposits formed by contaminants and by oil oxidation.
Such additives are surface active agents, like the rust inhibitors; but their chemical make-up is different. They function by forming “envelopes” around contaminants and oxidation products such as varnish and sludge.

They keep this debris suspended in a very fine state of dispersion in the oil. As long as this state can be maintained, varnish and sludge won’t deposit on machine parts.
Continued oxidation can overload a dispersant. The envelopes are spread too thin and gradually allow the fine particles to join with other particles to form larger agglomerates. When these can no longer remain suspended, they’ll drop from the oil and from deposits.

This is yet another example of the sacrificial nature of additives. They are put in lubricants to do specific jobs. They do these jobs, but they can’t last forever. They’ve got to be replenished through fresh make-up oil, or all the oil must be replaced by a fresh charge.


Such systems are used in many applications where large quantities of lubricant are needed for lubrication and cooling. Steel mills and paper mills are examples of tough services where oil oxidation and contamination can be major problems.
In steel mills, high speeds, high temperature, high pressures, and lots of chances for oil aeration all lead to oil oxidation. This is often accelerated by gross contamination from water, dirt, and mill scale. Other lubricants can get in through leaky seals to contaminate the oil system further. Rolling oil and roll cooling water sprays add to the problem.
Paper mills have some similar conditions. Bearings on the steam-heated drier rolls run hot, and oxidation can be a problem. Water, processing liquids, dirt and fibrous paper fly are sources of oil contamination.

Compressors come in all sizes, shapes and designs. (See COMPRESSOR LUBRICATION section in KNOWLEDGE CENTRE)

Transformer and circuit breakers require insulating oils to insulate and cool the internal windings. They’re usually long life products, but in time the oil oxidizes. It also picks up small amounts of water as the oil reservoir breathes air. Both oxidation products and water reduce insulating properties. The inevitable formation of oil insoluble oxidation products leads to the fouling of cooling surfaces and plugging of oil ducts.
In the case of circuit breakers, arcing occurs, and the oil in the path of the arc can be carbonized. In transformers, provisions are often made to remove water by filtration or vacuum dehydration. This helps to restore the insulating properties of the oil, but eventually the buildup of oxidation products degrades the oil. Then it’s time for the oil to be reclaimed or replaced.
As with other oils, the sensory tests can tell us much. A smell of ozone or a burned smell in transformer and circuit breaker oil can warn us that things are not right in the equipment.

The lubrication of food machinery presents some special problems. Modern food machinery lubricants are made from highly purified materials that meet the U.S. Department of Agriculture H1 Classification for lubricants that have incidental food contact.
Food machinery is usually designed to minimize the leakage of lubricant into the food being processed. Seals are designed so that any leakage will be of the processed fluids into the lubricant. This is fine for the food, but moisture and corrosive fruit juices can be hard on the lubricants.
In addition to the problem of moisture and fluids from the processed food, these oils are subject to the same problems as the hydraulic oils and gear oils. One redeeming factor, however, is that food machinery is often overhauled between the end of one canning season and beginning of the next. This provides an opportunity for cleaning up equipment and removing contaminants from the oil.

Gear sets, like compressors, come in all sizes, shapes, and designs. Lubricants range from heavy, asphaltic products to turbine and hydraulic oils to special lubricants fortified with EP and friction-reducing additives.
All gears transmit power. There is friction at the mating gear teeth, and this generates heat. Gears operating continuously under heavy load require lots of oil for cooling, and coolers for cooling the oil. Oil in this service can oxidize, and this will increase the oil viscosity. Higher viscosity means more friction loss and even higher temperatures. Deposits form. Coolers foul. In long-service gear sets, oil viscosity monitoring is an important check point. Whether outside contamination is a problem depends on gear case design and environment. (See GEARS section in KNOWLEDGE CENTRS)

Heat transfer fluids are formulated with special oxidation inhibitors and detergents. They are designed to operate at temperatures upto 300C for long periods of time. Oxidation can be a problem at these high temperatures. Most modern systems have expansion chambers blanketed with nitrogen or with flue gas to reduce the oil’s contact with air.
In spite of these and other design precautions. Oxidation and/or cracking can take place with inevitable formation of deposits. Deposits lead to poor heat transfer, this can cause overfiring of the furnace with energy loss and even higher skin temperatures. Hot spots can develop where deposits insulate heating surfaces, and the oil molecules can crack and carbonize. The lowering of the oil’s flash point due to cracking, an increase in viscosity, or an increase in insolubles are warning signs for this service.

Modern hydraulic oils can contain rust and oxidation inhibitors, additives to control foaming and air-entrainment, AW additives. Internal clearances in modern hydraulic system components are extremely small and hence particulate contamination is a serious issue. (See HYDRAULIC SYSTEMS section in KNOWLEDGE CENTRE)

Refrigeration compressors work on a variety of refrigerant gases and require differing prpoerties. As usual, high compressor temperatures and contaminants are the root causes of oil deterioration; but here the gas being compressed can create problems. Ammonia and Freon are two of the most widely used types of refrigerant gases. Air can get into ammonia systems and, in time, will oxidize the lubricant. Oxidation products react with the ammonia refrigerant to form very heavy, oil-insoluble sludge. Freon is quite thermally stable, but both air and water must be excluded from Freon systems because they react with this type of refrigerant to produce corrosive by-product. The result is internal lubricant contamination and such problems as “copper plating” of steel.

Most turbine installations use circulatory systems to provide large quantities of clean oil for bearing cooling and lubrication



Oxidation Control Inhibitors used up stopping attack on oil and additives Oil viscosity increases; deposits from. Acids corrode metals.
Rust Inhibition Inhibitors used up protecting iron surfaces.  
Load carrying Additives consumed by reaction with metal surface, or removed by water Oil can’t continue to protect against scuffing.
Dispersancy Dispersant becomes overloaded with liquid and solid contaminants. Solids (Varnish, sludge) form.
Dirt Dirt comes from everywhere Promotes wear; taxes lubricant properties.
Wear metals A sign of an unhealthy machine Shortened machine life unless corrected.
Rust Oxygen, water, and iron have interacted. Contributes to wear.
Carbonaceous Matter The oil has been overheated Deposits clog oil passageways.
Sludge and Varnish Oxidation products have become insoluble. Deposits from on machine parts and control valves.
Water and Process Fluids A sign of leaky seals and condensation Affects lubricant efficiency; promotes oil deterioration.
Oxidation products Excessive system temperature; drain interval too long. Forerunner of more solid debris.
Other Lubricating oils Misapplication of oils on hand Can alter desirable properties of system lubricant.
Lubricant supplements Usually added by well-meaning servicemen. Can alter desirable properties of system lubricant.