Fundamentals of Lubrication
Tribology and Friction
Tribology is the science and technology of friction, lubrication, and wear. It is a term to describe the study of interacting moving surfaces and encompasses aspects of physics, chemistry, metallurgy, material science, mechanical engineering, chemical engineering and applied mechanics.
Friction is the resistance to relative motion between two bodies in contact. Studies estimate that as much as 1/3 of all usable energy is lost to friction and wear. Reactive maintenance due to high friction and wear conditions costs three (3) times that of planned maintenance and proper lubrication. In addition to energy waste, friction and wear greatly affect equipment reliability, maintainability, safety, service, life, and environmental factors. One of the primary functions of a lubricant is to reduce friction. By reducing friction, you reduce the energy required to perform work.
Two types of friction (dry friction and fluid friction) and two modes of friction (sliding and rolling) will be discussed below.
Dry friction is the impacting of asperities between two surfaces in contact without a lubricant film for protection. Although a metal surface may appear smooth to the naked eye, it actually consists of many peaks or high spots called asperities. Figure 1 shows how the asperities of two surfaces come into contact to create friction. The asperities block free movement of the surfaces, requiring a force to overcome them called the frictional force. Even highly polished materials will show asperities under magnification. As shown in Figures 2 and 3, a precision watch gear and a highly machined surface both show asperities and significant surface roughness upon high magnification.
The real contact area between two surfaces may be much lower than the calculated area, because only the area of the asperity peaks is in contact, not the entire surface (Figure 4). This means that localized loading at the asperity contact areas can be extremely high.
Real Contact Area (shaded areas) < Nominal Area (a x b)
Fluid friction is the resistance created within the fluid film when the molecules collide or rub against each other. It generally occurs when too much lubricant is present (i.e. over-lubrication) or the viscosity is too high for the conditions. A good example is lubrication of a bearing. Bearings are usually best lubricated by packing grease in about 30-50% of the available space within the housing. If the housing were to be packed 100% full, the grease would have no place to go as the rollers move through it, thus creating fluid friction within the grease. Dry friction is typically much higher than fluid friction, but fluid friction should not be overlooked as it can lead to heat, viscosity loss, and increased wear rates.
Sliding friction is the force that resists sliding motion. Applications with sliding friction include slideways, screw threads, compressors, chains, hydraulic pumps, hydraulic valves, hydraulic cylinders, plain journal bearings, worm gears (high sliding), hypoid gears (high sliding), and other gear types (low to moderate sliding). As shown in Figure 5, the applied force must be greater than the frictional force for the block to move. Lubrication is designed to reduce this frictional force which in turn reduces the applied force (energy) needed to perform work (move the block).
Coefficient of static friction in sliding applications is determined by the force required to move an object divided by the weight of the object (Figure 6). This unit-less measure will vary depending on the types of materials in contact (Figure 7) and degree of lubrication (if any) present. One of the goals of lubrication is to reduce the friction and thus the applied force needed to perform work.
|MATERIALS IN CONTACT||COEFFICIENT OF FRICTION (μ)|
|BRONZE ON BRONZE||0.20|
|HARD STEEL ON HARD STEEL||0.42|
|MILD STEEL ON MILD STEEL||0.57|
|HARD STEEL ON BABBITT||0.34|
|MILD STEEL ON BRONZE||0.34|
Sliding wear particles are a unique shape that can be identified under microscopic examination (Figure 8). This can be helpful in troubleshooting activities to identify the area of highest wear.
Rolling friction is the force that resists rolling motion. Ball and roller bearings are good examples of rolling friction along with many gear types (rolling at the pitch line). As shown in Figure 9, the applied force must be greater than the frictional force in order to get the ball rolling.
Rolling friction is obviously much lower than sliding friction which is why balls or rollers are used in bearings to achieve smooth rotary motion. Even with this lower friction, metal-to-metal contact can be very detrimental in bearings, so good lubrication is required for long life and efficient operation.
Rolling wear particles are distinct from sliding wear particles, appearing as pitting on the metal surface and in a round or spherical shape under magnification (Figure 10).
It is important to understand the difference between a “Lubricant” and “Lubrication”.
A lubricant is a substance that helps control friction and wear.
Lubrication is the science of reducing friction by the application of a suitable substance between the rubbing surfaces of bodies having relative motion.
In many cases, you can have a “lubrication” problem that may not be associated with the “lubricant” at all. The Lubrication Rule below shows that the correct lubricant is only one piece of the lubrication puzzle.
Improper lubrication may mean any of the following:
- Too much or too little lubricant is being used
- The wrong lubricant type, wrong viscosity, or poor quality lubricant is being used
- The lubricant is not getting to the correct surfaces
- The lubricant is not getting there at the right time, meaning the lubrication frequency is incorrect
Functions of a Lubricant
Lubricants serve a variety of functions to help the overall performance of the equipment. They do this by forming a thin layer between the work surfaces in order to prevent contact with each other. The main purposes of lubrication are:
1. Reduce Friction
Energy losses are proportional to the amount of friction encountered. By reducing friction, you reduce energy required to perform work. Studies have shown that overcoming frictional forces accounts for 30% of all energy consumed.
2. Control Wear
WEAR = The removal or transfer of material. By reducing wear, equipment life is extended and the quality of production is maintained longer. Wear exists in many forms:
3. Prevent Corrosion
Stopping corrosion can eliminate premature failure of equipment, especially those due to a loss of critical tolerances or accelerated abrasive wear. Many types of corrosion exist:
4. Cool Surfaces / Carry Away Heat
Cooling helps to maintain working tolerances and extend oil life. It also reduces the chance for catastrophic wear and allows equipment to be run at higher loads and faster speeds.
5. Seal Out / Carry Away Contaminants
Greases work well to seal out contaminants. Oils work well to carry away contaminants, so they can be settled or filtered out. Reducing contamination lowers the amount of abrasive wear within a component and increases the lubricant and equipment service life. Contamination in the form of liquid or solid will be a catalyst for oxidation of the oil and will reduce the lubricant life.
6. Dampen Shock
The oil film must remain intact during load changes to prevent metal-to-metal contact. By acting as a cushion, the high loads imposed by shock loading conditions can be reduced thus extending equipment life. Vibration and noise are also reduced.
Lubricant Film and the Lubrication Regimes
The fundamental goal of lubrication is to develop a lubricant film between moving surfaces to prevent metal-to-metal contact. To generate a load-carrying oil layer, the formation of a converging, wedge-shaped pressure film is necessary (Figure 11). As the tilted pad is moved across the stationary surface, lubricant is forced to the inlet resulting in the pressure distribution as shown. This increased pressure provides the necessary force to raise the tilting pad off the stationary surface. If the speed and viscosity are great enough and/or the load small enough, hydrodynamic lubrication will result, meaning the surfaces are fully separated by the oil film and no contact occurs.
A full fluid film separating the surfaces is not always possible, so the following lubrication regimes may exist depending upon actual conditions:
Regime I: Hydrodynamic Lubrication
- Named for hydro (fluid) and dynamic (motion)
- Complete separation of surfaces by a fluid film
- In theory, no wear occurs
Regime I: Hydrodynamic Lubrication
h >> R
h = height of lubricant film
R = height of asperities(surface peaks)
Regime II: Mixed Film Lubrication
- Separation of most surface peaks
- Some surface peaks come in contact so wear occurs
- Most common lubrication regime
Regime II: Mixed Film Lubrication
h ≈ 0
h = height of lubricant film
R = height of asperities (surface peaks)
Regime III: Boundary Lubrication
- Many surface peaks come in contact
- Must rely on EP / solid film additives
- Wear accelerates
Regime III: Boundary Lubrication
h → 0
h = height of lubricant film
These regimes can occur in the same application as conditions change. For example, at rest, a plain journal bearing will have zero speed and thus will have full metal-to-metal contact putting it in Regime III Boundary Lubrication. As the shaft of the bearing begins to turn, an oil film will begin to generate and move it into Regime II Mixed Film Lubrication. Once the bearing is up to full operating speed and temperature, it will ideally be in Regime I Hydrodynamic Lubrication. This is contingent on the ideal conditions being met regarding speed, load, temperature, and oil viscosity. In hydrodynamic lubrication, the only friction present is due to fluid friction, since the metal surfaces do not come in contact.
The various conditions that determine the oil film thickness and thus lubrication regime can be demonstrated by the Stribeck Curve (Figure 13).
As viscosity & velocity decrease and load increases, we move to the left on the curve showing an increase in friction, a reduction of oil film, and eventually boundary lubrication conditions prevail. As viscosity & velocity increase and load decreases, we move to the right on the curve showing that the coefficient of friction is reduced, a larger oil film is created, and eventually hydrodynamic lubrication is reached. If viscosity and/or velocity continue to increase, fluid friction will prevail which explains the upward trend of the curve at the end showing an increase in friction even with full hydrodynamic lubrication.
Types of Wear
One of the key purposes of lubrication is to reduce and/or control wear. Wear may come in many forms as shown here:
Adherence and welding causing material loss and/or transfer.
#1 - Metal surfaces have a natural affinity for each other and can adhere
#2 - Under pressure, local temps of the metal can be extremely high. Welding of the two surfaces will occur. This is called “cold welding” since the bulk of the metal remains cold.
#3 - As the two surfaces move relative to each other, the cold weld shears causing metal removal. Wear debris is released and goes on to create abrasive wear, thus increasing the overall wear rate.
Abrasive medium causing physical damage.
Abrasive material may be:
- Wear debris
- Atmospheric contamination
- Solid by-product of oxidation
- Contamination due to handling
Loose and embedded particles create significant wear within a system. This wear creates small particles that go on to cause more abrasive wear, thus compounding the problem.
Surface deterioration resulting from chemical attack. Stopping corrosion can eliminate premature failure of equipment, especially that which is due to a loss of critical tolerances.
Corrosive wear may be caused by:
- Water (most common)
- Process chemicals
- Oil oxidation by-products
Cyclic stresses causing fracture or breakage.
Fatigue life: A properly run and lubricated part will eventually fail due to fatigue (which should take a long time). Premature fatigue may be the result of overloaded equipment or improper lubrication.
Deterioration (oxidation) of surfaces caused by vibration. Also called “Fretting Corrosion” because of the red oxides produced. Fretting is common on the inner ring of bearings where they meet the shaft.
If the proper interference fit does not occur between the bearing and shaft, vibration can occur and lead to the fretting corrosion seen here.
Removal of surface material by impinging particles. Erosive wear is important especially in hydraulic and pneumatic systems. Hydraulic valves are highly susceptible with the valve edges being most prone to erosive wear.
Under high pressures or high fluid velocities, particles are forced against metal surfaces, causing metal removal. Again, we see that wear debris is produced which will accelerate wear rates.
Not all wear is considered bad. The running-in period of newly installed parts will actually increase the real contact area of the metal surface. Running-in helps with hot spots, work hardening, and contact area improvement.
Destructive wear modes can be minimized through proper lubrication (per the Lubrication Rule), appropriate lubricant additives, sound maintenance practices, and good contamination control techniques.
Base Oil Types
Lubricants are made up of base oils which are typically 90+% of the final formulation and additives (which will be discussed in the next section).
Most lubricants use mineral (petroleum) base oils developed by refining naturally occurring crude oil. Crude oil is a mixture of many hydrocarbons with the composition dependent on the source and location of the crude. It varies by viscosity, color, odor, and chemical make-up. All are some degree of mixtures of paraffinic, naphthenic, aromatics, and asphalt, but most crude oils will be more concentrated in one component.
As crude oil comes out of the ground, it consists of complex compounds of hydrocarbons and impurities. The refining process removes the impurities and separates the various fractions from the crude oil. This primary separation takes place in distillation towers (Figure 14). As the crude oil is heated, the lighter (lower boiling point) components will rise to the top. The heaviest ones will remain in the bottom of the tower. Crude oil products include: asphalt, very heavy base oils, engine base oils, industrial base oils, kerosene, diesel fuel, gasoline, and jet fuels.
A typical barrel of crude oil (42 US gallons) contains less than 3%lubricating base oil, as a majority of it ends up as fuels (Figure 15).
This base oil is further refined to improve its quality. The American Petroleum Institute (API) has categorized base oils into five main categories (Figure 16). Groups I, II, and III are petroleum refined paraffinic base stocks, group IV is synthetic PAO, and group V is for all other base oils that do not fit in the other categories.
|GROUP I||SOLVENT REFINED MINERAL OIL|
|GROUP II||HYDRO-PROCESSED MINERAL OIL|
|GROUP III||HYDRO-CRACKED / ISOMERIZED MINERAL OIL|
|GROUP IV||ALL POLYALPHAOLEFINS|
|GROUP V||ANYTHING NOT COVERED IN GROUPS I-IV|
Paraffinic mineral oils
Paraffinic base oils are derived from paraffinic crude oil by distillation and solvent separation techniques. They may be further refined into higher quality oils as shown in Figure 16 above. The final properties of the base oil are determined by the type of crude and the refinement methods employed. The overall strengths and weaknesses of paraffinic oils are shown in Figure 19 below.
Naphthenic Mineral Oils
Naphthenic base oils are derived from naphthenic crude oil and have a cyclic chemical structure. This type of base stock is less expensive and has some limitations such as poor thermal and oxidative stability and lower VI. Strengths include low pour point, good solubility, and low carbon residue.
Synthetic base oils
Synthetic oils are man-made oils formed by a chemical reaction process. The base fluid’s molecular structure is planned and controlled; therefore the properties are very predictable. Synthetics account for a small % of the market due primarily to their higher cost, but they continue to gain ground on mineral oils, as engine oil and other industry specifications become stricter. The main benefits and limitations of synthetics are listed in Figure 21 below. These will vary by synthetic type.
|GREATER OXIDATIVE STABILITY (LONG LIFE)||HIGHER PURCHASING COST|
|WIDER OPERATING TEMPERATURE RANGE||POTENTIAL INCOMPATIBILITY WITH SEALS|
|LOWER POUR POINT||POTENTIAL INCOMPATIBILITY WITH OTHER FLUIDS|
|HIGHER FLASH AND FIRE POINTS||POSSIBLE STORAGE AND HANDLING CHALLENGES|
|REDUCED DEPOSIT FORMATION||HIGHER DISPOSAL COST|
|IMPROVED FILM STRENGTH||FLUID MISAPPLICATION POTENTIAL|
The major classes of synthetics include:
|DESCRIPTION||KEY BENEFITS||KEY DRAWBACKS|
|A.K.A. SYNTHETIC HYDROCARBONS (SHC) HAVE SIMILAR PROPERTIES TO PARAFFINIC MINERAL OILS EXHIBIT THE BEST PROPERTIES OF A PARAFFINIC OIL WITHOUT THE DRAWBACKS||EXCELLENT COMPATIBILITY WITH SEALS AND MINERAL OILS HIGH VISCOSITY INDEX GOOD OXIDATIVE AND THERMAL STABILITY LOW POUR POINTS||LIMITED ADDITIVE SOLUBILITY AND RESPONSE HIGHER COST THAN MINERAL OIL|
|CONSIST OF DIESTERS, POLYOL ESTERS, AND PHOSPHATE ESTERS DIESTERS HAVE EXCELLENT SOLUBILITY PHOSPHATE & POLYOL ESTERS ARE USED AS FIRE-RESISTANT HYDRAULIC FLUIDS||HIGH OXIDATION STABILITY GOOD ANTI-WEAR EXCELLENT ADDITIVE SOLUBILITY AND RESIDUE CONTROL LOW VAPOR PRESSURE||POOR SEAL AND PAINT COMPATIBILITY PHOSPHATE ESTERS HAVE LOW VIS HIGHER COST THAN MINERAL OIL|
|PAG (POLYALKYLENEGLYCOL)||VARIOUS FORMS INCLUDING POLYETHERS, POLYGLYCOL ETHERS, POLYETHER POLYOLS MOST ARE MISCIBLE WITH WATER MOST HAVE POOR COMPATIBILITY WITH OTHER OILS||EXCELLENT LUBRICITY (VERY LOW COEFFICIENT OF FRICTION) VERY HIGH VISCOSITY INDEX (UP TO 300+) GOOD ADDITIVE SOLUBILITY||POOR COMPATIBILITY WITH OTHER OILS WATER SOLUBILITY VERY HIGH COST|
PolyAlphaOlefin (PAO) is created by polymerizing a simple olefin and is considered a group IV base oil (Figure 16). They are the most common synthetic base oils due to their all-around excellent properties and lower cost as compared to other synthetics.
Organic esters are considered the highest quality base oils and are more expensive than PAOs to produce. They are incredibly stable at very high temperature ranges. They electro-chemically bond with metals for a continuous lubrication film at both low and high temperatures.
PolyAlkyleneGlycols (PAG) have a variety of uses as industrial oils due to their extremely high VI and excellent lubricity. They can often consolidate multiple oil viscosity grades and be used year-round in very cold, outside applications. The low coefficient of friction makes them especially useful for worm gear applications which have very high sliding friction to overcome.
Group III oils (Figure 16) are also considered synthetic molecules due to the high degree of processing required to produce them. They are commonly used in synthetic engine oils and are slowly gaining more use in industrial applications.
As a general rule of thumb, fluids should always be considered incompatible until tested in the laboratory. Many consequences of incompatibility can occur such as fluid thickening, fluid thinning, additive dropout, and residue formation. Figure 22 below provides a very general guideline for fluid compatibility of base oils.
|POLYOL & DIESTER||-|
- = COMPATIBLE
- = INCOMPATIBLE (DETAILED FLUSH NEEDED)
When converting from one fluid type to another, the level of flush needed is dependent on fluid compatibility:
- Some old oil may be left behind with little or no detrimental effects.
- Level of flush should be based on a goal of removing as much spent fluid as possible while considering the time/cost commitment
- The more old oil left, the greater the detrimental effect on the new oil due to oxidation by-products and contamination
- Old fluid must be removed as much as possible using physical means to remove from reservoirs, filters, lines, valves, cylinders, etc.
- Full, detailed flush is required to get any trapped residual fluid
- Time/cost are fixed
Lubricant Characteristics and Additives
As we learned in the previous section, base oils are a large portion of the finished lubricant and each has its own properties. To further develop the properties needed for a fully formulated oil, additives are used extensively. While additives can’t overcome a poor-quality base oil, they can improve upon an already good one. In this section, the most important lubricant characteristics will be discussed along with the additives used to improve them. Some of the key lubricant physical properties include:
- Viscosity Index
- Pour Point
- Flash & Fire Points
- Oxidation Resistance
- Anti-Wear & EP characteristics
- Corrosion Resistance
- Foam Control
Viscosity is a measure of a fluid’s resistance to flow and is often referred to as the “thickness” of the oil. It is typically the most important property of any oil as it plays an important role in the development of a proper oil film (see Figure 13, Stribeck Curve). Low viscosity oils are used for applications with higher speeds, lighter loads, and lower temperatures. Conversely, high viscosity oils are used with lower speeds, higher loads, and higher temperatures. The viscosity of an oil changes based on temperature, with viscosity dropping as temperature increases (and vice versa) as shown in Figure 23.
The main viscosity classification for industrial oils is the ISO system (Figure 24), which uses an oil’s kinematic viscosity at 40˚C (104˚F) measured in centistokes (cSt) (or mm2/s) to determine its corresponding viscosity grade. Industrial equipment manufacturers use this system to specify the viscosity grade required, i.e. ISO 46 anti-wear hydraulic oil.
Other classification systems as shown in Figure 25 include AGMA which is related to the gear industry, SAE which is related to the automotive industry, and SUS which is an older system previously used for industrial oils (and is still referenced for older equipment).
|ISO INTERNATIONAL ORGANIZATION FOR STANDARDIZATION||MOST COMMON SYSTEM FOR INDUSTRIAL OILS.||ISO 46 HYDRAULIC OIL|
|AGMA AMERICAN GEAR MANUFACTURERS ASSOCIATION||SOMETIMES USED TO DESCRIBE GEAR OIL VISCOSITY.||AGMA 6 EP GEAR OIL|
|SAE SOCIETY OF AUTOMOTIVE ENGINEERS||MEASUREMENT SYSTEM FOR AUTOMOTIVE ENGINE AND DRIVETRAIN OILS.||SAE 5W-30 MOTOR OIL|
|SUS (OR SSU) SAYBOLT UNIVERSAL SECONDS||INDUSTRIAL VISCOSITY CLASSIFICATION SYSTEM SOMETIMES REFERENCED ON OLDER EQUIPMENT.||500 SUS CIRCULATING OIL|
Not all viscosity systems can be directly converted to each other, since they each have their own requirements and ranges, but general comparisons can be made using the viscosity chart below (Figure 26).
Viscosity Index (VI)
As previously discussed, an oil will drop in viscosity as its temperature goes up and increase in viscosity as its temperature goes down, but each oil has a different rate of viscosity rise and fall with temperature. This rate of change is described by the oil’s viscosity index. In a viscosity-temperature chart such as the one in Figure 27, a high VI oil will have a flatter line, because its viscosity does not change as fast with temperature. A low VI oil will have a steeper line, because its viscosity changes more rapidly with temperature. VI is determined by measuring the viscosity in centistokes at 40˚C and 100˚C and calculated using a formula based on its slope. Viscosity Index is an empirical number meaning that it has no unit of measure.
Typical viscosity index of various oils:
- Naphthenic Mineral Oil ---> 40
- Paraffinic Mineral Oil ---> 90-100
- PAO Synthetic ---> 140-180
- PAG Synthetic ---> Up to 350
Synthetic oils typically have a naturally higher VI than mineral oils, but polymer additives are available to boost the VI of mineral oils for wide temperature applications. At low temperatures, the polymer chains remain compact and offer little resistance to flow (Figure 28) thus lowering the viscosity. As the temperature increases, the chains start to uncoil and interfere with each other; this restricts oil from flowing and increases the viscosity.
The pour point of an oil is the depressed temperature at which it no longer flows (Figure 29). This is not considered the freezing point, as it has not necessarily turned to a solid, but simply the temperature at which the viscosity has increased enough to prevent flow. Knowing the pour point is primarily useful for cold temperature situations, particularly at start-up when the oil is cold and not moving.
Rule of Thumb: The lowest useful temperature of a fluid lubricant is a minimum 15˚F to 20˚F above its pour point, at system start-up.
Pour point is highly dependent on:
- Viscosity - lower viscosity oils will have lower pour points
- Oil type
- – Paraffinic mineral oils have higher wax content leading to higher pour points
- – Naphthenic mineral oils have lower wax content and lower pour points
- – Synthetic oils can vary but typically have lower pour points than equivalent viscosity paraffinic mineral oils, primarily due to their higher VI
Pour point depressant additives are available which interfere with the wax crystallization at low temperatures and allow for better flow. These additives are frequently used in higher viscosity oils and oils developed for cold weather service.
Flash and Fire Points
The flash and fire points of a lubricant are determined in a lab by heating up the oil and passing a flame repeatedly over the top in the vapor space above it.
Flash Point = The temperature at which the oil vapor will ignite but not continue to burn for more than five seconds.
Fire Point = The temperature at which the oil vapor ignites and continues to burn for more than five seconds.
These measures are generally used by health & safety professionals and maintenance personnel to assess the oil’s risk factor in various applications and conditions within the facility.
Oxidation is the reaction of oxygen with the oil, which occurs naturally to break down the oil. It is generally a slow reaction but is accelerated primarily by heat, but also by catalysts such as water, metals (copper, zinc), entrained air, and dirt. The by-products of this reaction can be quite detrimental to the system (Figures 30 and 31). Oxidation of an oil can be tracked by measuring the weak acid level, known as Total Acid Number or TAN. Over time, the TAN will increase as the oxidation increases.
Our rule of thumb suggests the rate of oxidation increases significantly over 150˚F for mineral oils. For example, if an oil at 150˚F lasts for 1 year, that same oil will only last 6 months at 168˚F. For this reason, oils running consistently higher than 150˚F may benefit from the use of premium mineral oils or synthetics.
The oxidation resistance of an oil can be measured in several ways, but the two most common tests are:
TOST – Turbine Oil Stability Test
- Oxygen passes through the oil which is held at 95˚C until TAN reaches 2.0
- Test duration is generally long (months to even years)
- Result reported in “hours”
- Conventional hydraulic oils last 2000-5000 hours
- Castrol Tribol HM 943 lasts 18,000+ hours in this test (over 2 years!)
RPVOT – Rotary Pressure Vessel Oxidation Test
- Oil is pressurized with oxygen and held at 150˚C until a specified pressure drop is reached
- Much shorter test than TOST (~1-2 days); result reported in “minutes”
- Can be used to test remaining oxidation resistance of used oils (common for turbine oils)
Oxidation resistance is important for most oils but especially those in long-term and high temperature service, such as turbine oils, hydraulic oils, gear oils, and compressor oils. Synthetic oils generally have higher oxidation resistance and/or produce less egregious by-products, but this varies by type of synthetic.
Oxidation inhibitors work to slow oxidation by decomposing radicals, terminating free radicals, and/or neutralizing oxidation by-products. Basically, they get rid of open areas on the lubricant molecule that are potential reaction points for oxygen. Amine and phenol-based additives are often used to increase an oil’s oxidation resistance.
Anti-Wear and EP Characteristics
Anti-wear (AW) is the ability of a lubricant to prevent wear due to metal-to-metal contact from friction generated through sliding or rolling motion.
Extreme Pressure (EP) is the ability of a lubricant to prevent scuffing, scoring and seizure as load are increased. This is also referred to as “Anti-Scuff” especially by the American Gear Manufacturers Association (AGMA).
AW and EP additives perform in a similar manner. They react with the metal surface, under the right conditions of load and heat, to form a protective, sacrificial layer to prevent metal-to-metal contact. The key difference is that AW additives tend to be milder in reaction than EP additives.
Several lab tests are available for measuring the AW and EP characteristics of oils:
Four-ball EP / Four-ball AW
- These tests are the most common for assessing AW and EP
- Designed primarily for gear oils but used more frequently to assess the properties of other oil types such as hydraulic oils
- An older test with low repeatability
Falex Pin and Vee
- An infrequently used test but can provide good results
Various hydraulic pumps tests (Denison, Eaton Vickers) are used to determine the AW performance of hydraulic oils
The most commonly used AW/EP additives are compounds formed from zinc, phosphorus, sulfur, and calcium. Older additives may still use antimony, chlorine, and lead but these are generally considered health & safety concerns. AW/EP additives require specific temperatures in order to react with the metal surface (Figure 32). This is not the temperature of the bulk oil but the localized temperature that is created when the asperities rub.
Under hydrodynamic lubrication conditions, the metal surfaces do not touch and therefore AW & EP additives are not reactive and remain undisturbed in the oil. In mixed-film and boundary lubrication conditions, heat and load from metal contact removes the natural oxide layer and causes the additives to react with the metal surface (Figure 33). The additives sacrifice themselves to provide a protective layer which prevents wear.
Solid lubricants can provide additional protection from wear by imparting a non-reactive, protective layer to the metal surface. This layer is physically applied as the surfaces rub and is non-sacrificial (Figure 34).
Common solid lubricants include:
- Molybdenum Disulfide (Moly)
Moly is used extensively to provide extra EP protection to oils and greases, especially in heavily loaded, shock loaded, and high sliding applications. The structure of the molecule and the way it works is shown in Figure 35 below.
Resistance to rust, corrosion, and chemical attack is an important characteristic for most oils to have. Corrosion inhibitors are commonly added to most industrial oils to ensure they provide this protection, given foreseen and unforeseen conditions. Protection of both ferrous components and copper based metals ensures a variety of equipment will not sustain corrosive wear.
Several lab tests are available to measure corrosion resistance including:
- Copper Corrosion
- – Copper strip is immersed in the oil
- – The degree of corrosion over a given time period is measured from 1a (no corrosion) to 4c (heavy corrosion) as shown in Figure 36
- – Most industrial lubricating oils are formulated to achieve a 1b or better rating
- – Oils with aggressive additives, such as active sulfur, will be a 4c and not suitable for contact with yellow metals
- Rust Test (ASTM D 665 A & B)
- – Steel test rod is placed in oil with 10% water at 140˚F for 24 hours
- – Pass / Fail test – any corrosion causes a "fail"
- – Test A uses distilled water
- – Test B uses salt water
- Emcor Rust Test for Grease
- – Bearings are partially immersed in water and operate in a predetermined sequence of running (80 RPM) and stopping for one week
- – At the end of the test, the raceway of the outer ring is inspected for rust
- – Ratings are 0 (no corrosion) to 4 (heavy corrosion)
- Corrosion Properties of Grease (ASTM D 1743)
- – Tapered roller bearings are packed with grease
- – Bearings are exposed to water and stored for 48 hours in 100% humidity
- – Failure is determined by any corrosion spot 1mm or larger
Corrosion inhibitors work in two main ways: 1) neutralize any acids present, and/or 2) form a protective layer on the metal surface to prevent contact with water and acids (Figure 37).
Air will naturally be forced into an oil as it is agitated and circulated within a system. The oil should be properly formulated to prevent excess foam from creating problems (Figure 38). When a lubricant foams, it reduces the ability to lubricate and transfer heat. Foam can be a housekeeping issue and cause excess oil usage, increased oxidation, pump cavitation, and slow or erratic system response.
Foam is most commonly tested using ASTM D-892 method as follows:
- Oil sample of 200 mL is added to a graduated cylinder
- A diffuser stone is immersed in the oil
- Air is pumped through the stone continuously for 5 min
- The amount of foam (in mL) after the 5 min blowing period is recorded
- – This is referred to as the foaming "tendency"
- The amount of foam (in mL) remaining after a 10 min settling time is recorded
- – This is referred to as the foaming "stability"
- Three sequences are run:
- – Sequence I at 75˚F
- – Sequence II at 200˚F
- – Sequence III at 75˚F (repeat of Seq I)
Anti-foam additives are surface active, preventing air from being trapped within the oil structure. Even with proper formulation, a lubricant can foam due to excess small fines or system issues forcing too much air into the oil as shown in Figure 39.