An Introduction to Lubricant Types. Classification and Applications
Introduction
The main function of a lubricant is to reduce the friction between the moving surfaces and so facilitate motion. Its second most important function is to remove heat generated in the equipment being lubricated, such as piston engine, enclosed gears and machine tools. It has also to remove away debris from the contact area.
To understand how a lubricant function it is necessary to know
something about the nature of the surfaces. Even the most carefully finished metallic surface is not truly flat but has a certain sub-microscopic roughness something like sandpaper on a much smaller scale. When two dry surfaces are in contact, the asperities tend to interlock and resist any effort to slide one surfaces over the other surfaces. This resistance is called the friction and before sliding takes place sufficient forces must be applied to overcome it.
The
main project of lubrication is to replace this solid friction between the two
interlocking surfaces by the much lower internal friction in a film of
lubricant maintained between them and keeping them apart so that the asperities
no longer touch. The viscosity is the measure of its internal friction.
Lubricant oil can be produced by modern methods of refining from crude. they may distillates or residues derived from vacuum distillation of primary distillate with a boiling range above that of gas oil
Lubricant oil can be produced by modern methods of refining from crude. they may distillates or residues derived from vacuum distillation of primary distillate with a boiling range above that of gas oil
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| Fig 1 |
1- Tribology
the science and technology dealing with the design, lubrication, friction, and wear of interacting surfaces in relative motion
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| Fig 2 |
Function of lubricant
• Reduce Friction
• Minimize Wear (Keep Moving Surfaces Apart)
• Cool Parts (Carry Away Heat)
• Prevent Corrosion
• Disperse Combustion Products (e.g., Soot)
• Act as a Sealant
• Transmit Power
Why lubricate
• Lubrication is key when sliding (area) contact is present.
• Lubricants are used to reduce friction
• wear by preventing metal to metal contact
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| Fig 3 |
The modern period of lubrication began with the work of Osborne Reynolds (1842-1912). Reynold's research was concerned with shafts rotating in bearings and cases this show in Fig 4 .When a lubricant was applied to the shaft, Reynolds found that a rotating shaft pulled a converging wedge of lubricant between the shaft and the bearing. He also noted that as the shaft gained velocity, the liquid flowed between the two surfaces at a greater rate. This, because the lubricant is viscous, produces a liquid pressure in the lubricant wedge that is sufficient to keep the two surfaces separated. Under ideal conditions, Reynolds showed that this liquid pressure was great enough to prevent direct contact between the metal surfaces. Fig.4 taking a plain journal bearing as example, Fig.6 which is known as Stribeck curve summarizes the lubrication regimes by describing the relationship between speed, load, oil viscosity, oil film thickness, and friction.
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| Fig 4 |
In this graph, the coefficient of friction is plotted against the expression ZN/P (sometimes referred to as the Hersey number)
Where
ZN/P = (oil viscosity × shaft speed) /bearing pressure
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| Fig 5 |
there are three distinct zones separated by points A and B. At B the oil film is just thick enough to ensure that there is no contact between asperities on the shaft and bearing surfaces. Smoother surfaces shift B to the left, while at point A the oil film thickness reduces virtually to nil. Zone 2, between A and B is known as the zone of mixed lubrication. Mixed film lubrication is unstable at which increase in lubrication temperature causes further increases in lubrication temperature.
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| Fig 6 |
1-1 Hydrodynamic lubrication
Basically, lubrication is governed by one of two principles: hydrodynamic lubrication and boundary lubrication. In the former, a continuous full-fluid film separates the sliding surfaces.In the latter, the oil film is not sufficient to prevent metal-to-metal contact. Hydrodynamic lubrication is the more common, and it is applicable 2to nearly all types of continuous sliding action where extreme pressures are not involved. Whether the sliding occurs on flat surfaces, as it does in most thrust bearings, or whether the surfaces are cylindrical, as in the case of journal (plain or sleeve) bearings, the principle is essentially the same.It would be reasonable to suppose that, when one part slides on another, the protective oil film
between them would be scraped away. Except under some conditions of reciprocating motion, this is not necessarily true at all. With the proper design, in fact, this very sliding motion constitutes the means of creating and maintaining that film.
1-2 Boundary lubrication
The oil film has become so thin in Zone 1 that there is no hydrodynamic contribution and only boundary lubrication which is defined by Campbell in 1969 as the lubrication by a liquid under
conditions where the solid surfaces are so close together that appreciable contact between opposing asperities is possible. The friction and wear in boundary lubrication are determined predominantly by interaction between the solids and between the solids and the liquid. The bulk flow properties of the liquid play little or no part in the friction and wear behavior.
1-3 Extreme Pressure (EP) Lubrication
Definition. AW agents are effective only up to a maximum temperature of about 250 EC (480 EF).
Unusually heavy loading will cause the oil temperature to increase beyond the effective range of the
antiwear protection. When the load limit is exceeded, the pressure becomes too great and asperities make contact with greater force. Instead of sliding, asperities along the wear surfaces experience shearing, removing the lubricant and the oxide coating. Under these conditions the coefficient of friction is greatly increased, and the temperature rises to a damaging level.
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| Fig 7 |
1-4 Elastohydrodynamic (EHD) Lubrication
a. Definition of EHD lubrication. The lubrication principles applied to rolling bodies, such as ball or
roller bearings, is known as elastohydrodynamic (EHD) lubrication
2- Classification of Lubricants
2- 1 Lubricants can be broadly classified. On the basis of their physical state, as follows:
A good lubricating oil must have-
i) low vapour pressure (high B.P.)
ii) adequate (enough) viscosity for particular service conditions.
iii) low freezing point.
iv) high oxidation, resistance
v) heat stability
vi) non corrosive property
vii) stability at the operating temperatures.
2- 1 Lubricants can be broadly classified. On the basis of their physical state, as follows:
A good lubricating oil must have-
i) low vapour pressure (high B.P.)
ii) adequate (enough) viscosity for particular service conditions.
iii) low freezing point.
iv) high oxidation, resistance
v) heat stability
vi) non corrosive property
vii) stability at the operating temperatures.
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| Fig 8 |
2-1-1 Liquid lubricants
2-1-1-1 Animal & Vegetable oils
Soluble vegetable oils are used as coolants and cutting tool lubricants in metal machining. The vegetable oils used have a high flash point. This is essential since high temperatures can be generated
at the tip of a cutting tool. When mixed with water, the oil takes on a low viscosity milky appearance. It can be delivered under pressure to the cutting tool and assist in the clearance of metal particles. This is of particular importance in deep-hole drilling operations.
Castor oil has been a popular additive to the lubricating oil used for the highly tuned engines in motor cycle and formula car racing. Indeed, it gives its name to a popular brand of motor oil. When used, the exhaust gases have a distinctive smell that immediately identifies the additive.
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| Table 1 |
Also vegetable oils as lubricants is likely to increase due to environmental and government requirements and is becoming increasingly important.
2-1-1-2 Mineral oils
Mineral oils are hydrocarbons. That is to say that they are principally made up of complex molecules composed of hydrogen and carbon. The chemists who specialise in this field refer to the hydrocarbon types as paraffins, naphthenes and aromatics. Mineral oils for different applications contain different proportions of these compounds. This affects their viscosity and the way that itchanges under the effects of temperature and pressure.
Traditionally, lubricating oils have been grouped according to their different uses and viscosity values as shown in Table 2.
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| Table 2 |
Within the different types shown in Table 2, lubricating oils are graded according to the change in viscosity which takes place with a rise in temperature. This is indicated by the viscosity index number that has been allocated to the oil. It can range from zero to over one hundred as shown in Table 3.
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| Table 3 |
The higher the value, the less is the effect of temperature rise. A value of 100+indicates that the viscosity of the oil is not much affected by temperature change
Table 4 shows the typical viscosity and viscosity index values for the different types of lubricating together with the likely percentage composition of paraffins, naphthenes and aromatic hydrocarbon molecules which might be expected. You might notice that within each group, the high viscosity index oils, which are least affected by temperature change, have the highest percentages of paraffin compounds. Also within each group, the oils with the lowest viscosity have the highest percentages of aromatic compounds. It might thus be stated as a general rule, that aromatic compounds lower the viscosity of an oil whilst paraffin compounds help it to retain its viscosity as the temperature rises.
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| Table 4 |
2-1-1-3 synthetic oils
Plain mineral oils are suitable for applications such as the lubrication of bearings, gears and slide-ways where the operating temperature is relatively low and the service environment does not contain substances that will readily contaminate or degrade the lubricant. Typical contaminants are air (which is unavoidable), ammonia, water, oil of another grade, soot, dust and wear particles from the lubricated components. Special chemicals are often added to plain mineral oils to improve their properties and prolong their life. Table .5 lists some of the more common additive types.
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| Table 5 |
Polymers are sometimes added to mineral oils to enhance their properties. These are often referred to as synthetic oils although it is the mineral oil that forms the bulk of the lubricant. Various types of ester, silicone and other polymers are added, mainly to increase the viscosity index and wear resistance of the oil.
Some of the more expensive multi-grade motor oils contain polymers. They appear to be much thinner than the less expensive engine oils at normal temperature but undergo a much smaller change of viscosity as the temperature rises. This makes cold starting easier and gives improved circulation of the lubricant during the warming-up period. At running temperatures their viscosity is similar to that of the less expensive multi-grades but they are purported to have better wear resistant properties.
2-1-1-4 Blended Oils
No single oil serves as the most satisfactory lubricant for many of the modern machineries. Typical properties of petroleum oils can be improved by using Specific additives. These are also called blended oils. Fatty acids, organic compound Glycols.
It is done to reduce pour point, improve viscosity, increase oiliness, resist oxidation, reduce corrosion & improve color.
2-1-2 Semi Solid lubricants
he most important Semi-solid lubricants are greases, vaselines, wares & other compounds of oil & fats. These are called Semi-solid because they are neither solids nor liquid at ordinary temperature.
A grease consists of a lubricating fluid which contains a thickening agent. It may be defined as a semi-solid lubricant. The lubricating fluid may be a mineral oil or a polymer liquid and the thickening agent may be a soap or a clay. Table 6 lists some of the more common thickeners. The main advantage of a grease is that it stays where it is applied and is less likely to be displaced by pressure or centrifugal action than oil. It can act as both a lubricant and a seal, preventing the entry of water, abrasive grit and other contaminants to the lubricated surfaces.
Grease is classified by penetration number and by type of soap or other thickener. Penetration classifications have been established by NLGI , ASTM D 217 and D 1403 are the standards for performing penetration tests.
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| Table 6 |
i) For high load, low speed, intermittent operation, sudden jerks etc.
ii) In bearing & gears at high temperature.
iii) as sealing agents in bearing.
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| Table 7 |
2-1-2-1 Calcium – based greases or Cup greases
These are emulsion of petroleum oils with calcium soaps.
Calcium hydroxide + hot oil+ 𝑈𝑛𝑑𝑒𝑟/𝐴𝑔𝑖𝑡𝑎𝑡𝑖𝑜𝑛 Ca-based grease.
They are cheapest, most commonly used & good water resistance & used upto 800 C because above 800 C oil & soap beings to separate out.
2-1-2-2 Soda-based greases
These are petroleum oil thickened by mixing sodium soaps. They are poor water resistance because sodium soap is soluble in water. However they can be used up to 175 ֠C . They are suitable for use in ball bearings
2-1-2-3 Lithium based greases
These are petroleum oils thickened by mixing lithium soaps. They are water resistance & are suitable for use at low temp (15 ֠C) only.
2-1-2-4 Axle grease
Ca(OH)2 + Fatty Acid (Resin) + Filler (Tale & mica) ---- >> Axle grease.
They are water resistance & suitable for less delicate equipment working under high loads & at low speeds.
They are cheapest, most commonly used & good water resistance & used upto 800 C because above 800 C oil & soap beings to separate out.
2-1-2-2 Soda-based greases
These are petroleum oil thickened by mixing sodium soaps. They are poor water resistance because sodium soap is soluble in water. However they can be used up to 175 ֠C . They are suitable for use in ball bearings
2-1-2-3 Lithium based greases
These are petroleum oils thickened by mixing lithium soaps. They are water resistance & are suitable for use at low temp (15 ֠C) only.
2-1-2-4 Axle grease
Ca(OH)2 + Fatty Acid (Resin) + Filler (Tale & mica) ---- >> Axle grease.
They are water resistance & suitable for less delicate equipment working under high loads & at low speeds.
2-1-3 Solid lubricants
There are many applications, such as at very low and very high working temperatures, where lubricating oils and greases are unsuitable. In such cases the use of a solid lubricant can reduce wear without adversely affecting the contact surfaces or the working environment. Three of the most common solid lubricants are : graphite, molybdenum disulphide and polytetrafluoroethylene, which is better known as PTFE. You will be quite familiar with non-stick cooking utensils that are coated with PTFE. Lead, gold and silver are also used as dry lubricants for aerospace applications but their use is expensive.
Solid lubricants may be applied to the contact surfaces as a dry powder and you might recall that cast iron is self-lubricating because it already contains graphite flakes. They may be mixed with a resin and sprayed on the surfaces to form a bonded coating, or they may be used as an additive to oils and greases. Molybdenum disulphide, whose chemical formula is MoS2, has been used in this way by car owners for a number of years. It can be purchased from motor accessory dealers in small tins as a suspension in mineral oil. Solid lubricants can also be added to molten metal in the forming process so that when solidified, the metal is impregnated with particles of the solid lubricant. Graphite is often added to phosphor-bronze in this way to improve its qualities as a bearing material.
1. When heavy machinery is to be operated at high speed & moderate load or at very high load & low speed.
2. When the machine parts are not easily accessible
3. When the machine is at high working temp & press. & hence under such conditions combustible lubricants are unsuitable.
2-1-3-1 Graphite –
Carbon atom in a network of hexagons. Each C atom bonded by only three covalent bonds.
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| Fig 8 |
The distance of fourth carbon is almost double, due to which this fourth valency atom is Hexible & keeps moving about, thereby weakening the bonds between different layers.
As a result it is soft & has a lubricating property.
Ti is non- inflammable, soapy to touch + not oxidized in the presence of air below 375 ֠C. It can be used upto much higher temp in the absence of air. It can be used in powdered forms or as suspension – in oil or water.
i) The suspension of graphite in oil is known as oil dag & it is used in I.C. Engines.
ii) The suspension of graphite in water is known as aquaday & it is used in food processing industry.
iii) Graphite is also mired with greases to form graphite greases which are sued at high temp.
Uses -In air compressors, lathes, railway truck joints, cast iron bearing, etc.
2-1-3-2 Molybdenum disulphide –
Molybdenum disulphide has a sandwich, like structure in which a layer of Mo atoms lies between two layers of S atom. Poor inter-laminar attraction is responsible for low shear strength in a direction Parallel to the layers.
It possesses very low coefficient of friction & is stable in air up to 400 ֠C. Its fine powder may be sprinkled on surface sliding at high velocities. When it falls low spots in malal surfaces forming.
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| Fig 9 |
It is also used along with colverus & in greases is known as molykotes. used in automotive & truck chassis.
Besides the more important graphite & molybdenum disulphite, the other substances like Teflon, Soapstone, talc, mica, etc, are also used as a solid lubricants.
Polytetrafluoroethylene (PTFE) lubricant, often known as Teflon, is a high-performance, dry lubricant that reduces friction and wear by leaving a clean, non-staining film that resists dirt, dust, and oil. It is safe for metal, plastic, rubber, and glass, making it ideal for chains, locks, gears, and machinery
2-1-4 Compressed gases
Compressed air and inert gases such as carbon dioxide have a very low viscosity compared to oils and greases. With compressed gas bearings, the contact surfaces are separated by a thin cushion of gas which offers very little resistance to motion. The main disadvantage
is the cost.
A gas delivery system is required and the stationary outer surface of a journal bearing or lower stationary slide-way, must be machined with evenly spaced exit orifices to provide a dry and clean gas cushion. The usual operating pressures are 2 to 5 bar with higher pressures up to 10 bar being used for
heavy duty applications. The bearing surfaces may be coated with a solid lubricant to guard against dry running should the air supply fail.
Gas bearings are well-suited to high speed applications. They can operate at a speed up to 300 000 rpm and at temperatures well outside the range of oils and greases. A further advantage is that they do not allow dirt or moisture to enter the bearing and are surprisingly rigid. You might think that the air cushion could be easily be penetrated or displaced by high contact pressures and shock loads.
heavy duty applications. The bearing surfaces may be coated with a solid lubricant to guard against dry running should the air supply fail.
Gas bearings are well-suited to high speed applications. They can operate at a speed up to 300 000 rpm and at temperatures well outside the range of oils and greases. A further advantage is that they do not allow dirt or moisture to enter the bearing and are surprisingly rigid. You might think that the air cushion could be easily be penetrated or displaced by high contact pressures and shock loads.
This is not the case, however, as gas bearings have a self-correcting property. Shaft displacement causes the air gap to increase on one side and the pressure in that region to fall. At the same time, the air gap on the opposite side decreases and the pressure rises, forcing the shaft back to the central position.
2-2 Classification by viscosity grade.
Classification according to viscosity is the most prevalent method of describing oils, and the most common classification systems are those of the SAE, AGMA, and
ISO. Each organization uses a different kinematic viscosity range numbering system..
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| Fig 10 |
The most widely encountered systems are those of the following organizations:
! SAE (Society of Automotive Engineers) table 8
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| Table 8 |
! API
(American Petroleum Institute) table 9
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| Table 9 |
! AGMA
(American Gear Manufacturers Association), table 10
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| Table 10 |
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| Table 11 |
1- ! NLGI (National Lubricating Grease Institute) table 12
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| Table 12 |
2-3 Classification by additives.
WHY ADDITIVES?
TO PROTECT THE METAL SURFACES
• TO IMPROVE LUBRICANT PERFORMANCE
• TO EXTEND LUBRICANT SERVICE LIFE
2-3-1 SURFACE PROTECT
-Rust inhibitor
-Corrosion inhibitor
-Anti‐wear
-Extreme pressure
-Dispersant
-Detergent
-Tackifier
2-3-2 LUBE ENHANCER
-Antioxidant
-Anti-formant
-Pour Point depressant
-Vi improver
-Friction modifier
-Emulsifier
-De-emulsifier
Oils are classified according to the additives included in the oil to enhance its performance properties as follows:
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| Table 13 |
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| Table 14 |
Mixing lubricants that are designated as antiwear, extreme pressure or rust and oxidation lubricants can cause issues with the adsorption of the films. While most lubricants exhibit all of these properties, the chemistry used to achieve them is different based on the designation.
The adsorptive films do not form during the initial introduction of oil but build up over time. When a conflicting film type is introduced into a system, it interferes with current film and begins creating anew film type that may not meet the needs of the application.
For component applications where specific films are required, mixing of lubricant additive packages can have disastrous effects.
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| Table 15 |
2-4 Classification according to use.
This system of classification arises because refining additives and type of petroleum (paraffinic or naphthenic) may be varied to provide desirable qualities for a given application. Some of the more common uses are:
- Compressor oils (air, refrigerant).
- Engine oils (automotive, aircraft, marine, commercial).
- Quench oils (used in metal working).
- Cutting oils (coolants for metal cutting).
- Turbine oils.
- Gear oils.
- Insulating oils (transformers and circuit breakers).
- Way oils.
- Wire rope lubricants.
- Chain lubricants.
- Hydraulic oils.
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| Table 16 |
Transformer Oil
Transformer oil, also known as insulating oil, is a highly refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. The primary purpose of transformer oil is to insulate, suppress corona and arcing, and serve as a coolant. The oil helps to prevent the oxidation of the transformer’s cellulose insulation.
Types of Transformer Oil
There are two primary types of transformer oil: mineral oil and synthetic oil. Each type has its own set of characteristics, advantages, and disadvantages.
Mineral Oil
Mineral oil is the most commonly used type of transformer oil. It is derived from refining crude oil and is widely preferred due to its cost-effectiveness and excellent insulating properties. Mineral oils are further classified into two categories: naphthenic and paraffinic.
*Naphthenic Mineral Oil: Known for its high solubility and stability, naphthenic oil has a low pour point, making it suitable for use in cold environments. However, it has a higher tendency to form sludge compared to paraffinic oil.
*Paraffinic Mineral Oil: This type of oil has a high pour point but better oxidation stability. It is less soluble than naphthenic oil but is preferred in warmer climates due to its higher boiling point and lower evaporation rate.
Synthetic Oil
Synthetic transformer oil is manufactured through various chemical processes, offering superior performance in several aspects compared to mineral oil. Synthetic oils include silicone-based oils, ester-based oils, and others.
Silicone Oil:
Silicone oil has excellent thermal stability and electrical insulation properties. It is highly resistant to oxidation and fire, making it ideal for use in high-risk environments. However, it is significantly more expensive than mineral oil.
Ester Oil:
Ester-based oils are biodegradable and offer excellent insulation and cooling properties. Natural esters (vegetable oils) and synthetic esters both fall into this category. These oils are increasingly preferred for their environmental benefits and fire safety characteristics.
•Viscosity changes inversely with temperature * i.e., As temperature increases, oil becomes thinner
•Change in Viscosity is NOT linear
VISCOSITY AND TEMPERATURE
• Lubricant Viscosity Decreases Dramatically With Increasing Temperature (Log‐Log Relationship)
• Viscosity Index (V.I.) is a Measure of the “Viscosity‐Temperature Relationship” for an Oil
• Multigrade Oils Have Higher V.I.’s Than Single Grades
• KIENMATIC VISCOSITY
Measure of fluid’s resistance to flow due to gravity
• Derived from the time taken for a lubricant to travel through a capillary tube
• Measurement – Stoke (St) = 1 cm² / second• Typically reported as centistoke – (cSt) = 1 mm² / second
3-2 Viscosity index
The most frequently used method for comparing the variation of viscosity with temperature between different oils by calculation of dimensionless numbers, known as the viscosity index
(VI). The kinematic viscosity of the sample is measured at two different temperatures (40°C, 100°C) and the viscosity compared with an empirical reference scale. VI is used as a convenient measure of the degree of aromatics removal during the base oil manufacturing process, but comparison of VI of different oil samples is only realistic if they are derived from the same distillate feedstock
Viscosity Index (VI) is a measurement of the rate of change of viscosity over a range of temperatures. In simple terms: it measures how fast the oil thickens up as it gets colder or how fast it thins out as it gets hotter.
With Most lubricants, the higher the VI the better LUBRICANT
3-3 Low temperature properties.
When a sample of oil is cooled, its viscosity increases in a predictable manner until wax crystals start to form. The matrix of wax crystals becomes sufficiently dense with further cooling to cause an apparent solidification of the oil. Although the solidified oil does not pour under the influence of gravity, it can move if sufficient force is applied. Further decrease in temperature cause more wax to form, increasing the complexity of the wax/oil matrix. Many lubricating oils have to be capable of flow at low temperatures, and a number of properties should be measured.
3-4 cloud point
It is the temperature at which the first sign of wax formation can be detected. A sample of oil is warmed sufficiently to be fluid and clear. It is then cooled at a specified rate. The temperature at which haziness is first observed is recorded as the cloud point, the ASTM D2500/IP 219 test. The oil sample must be free of water because it interferers with the test.
3-5 pour point
It is the lowest temperature at which the sample of the sample of oil can make to flow by gravity alone. The oil is warmed and then cooled at a specified rate. The test jar is removed from the cooling bath at intervals to see if the sample is still mobile. The procedure is repeated until movement of the oil doesnot occur, ASTM D 97/IP 15. the pour point is the last temperature before the movement ceases, not the temperature at which solidification occurs. This is an important property of diesel fuels as well as lubricant base oils. High- Viscosity oils may cease to flow at low temperatures because their viscosity becomes too high rather than because of wax formation. In these cases, the pour point will be higher than the cloud point.
3-6 High temperature properties.
The high temperature properties of oil are governed by distillation or boiling range characteristics of the oil.
3-7 volatility
It is important because it is an indication of the tendency of oil to be lost in service by vaporization.
3-8 flash point
It is important for oil from a safety point of view because it is the lowest temperature at which auto-ignition of the vapour occur above the heated oil sample. Different methods are used, ASTM D 92, D93, and it is essential to know which equipment has been used when comparing results.
3-9 Acid number or neutralization number.
3. Terminology: What Do These Things Mean?
To make correct lubrication choices, you must have an understanding of the terminology involved. Commonly used terms include the following:
* Viscosity — This describes how thick or resistant to flow the oil or grease is. Higher viscosity means higher flow resistance.
* Kinematic viscosity — A simple pour test can be used to offer a visual illustration of viscosity. Lube technicians can provide a demonstration of kinematic viscosity on the shop floor, because it’s easy to do and takes very little time.
* Weight — This also relates to how viscous an oil is or how easily it flows at a specific temperature.
* NLGI consistency — The consistency indicates how hard or soft a grease is. The numbers range from 000 (like cooking oil) to 6 (like cheddar cheese).
* Runout — This term refers to the ability of a grease or oil to resist higher temperatures, which tend to make lubricants less viscous.
* Shear stability — This is the resistance of an oil to a change in viscosity caused by mechanical stress. * Metal on metal — A condition that every equipment owner should avoid like the plague, this describes a situation in which no lubricant is left in a bearing or other application
4. Ease of Use
Not all oils are applied the same way. The method of application will depend on your particular equipment. For some situations, such as an easy-to-reach hinge, an aerosol lubricant may suffice. A grease may be easily applied by hand to an accessible gearbox. However, for hard-to-reach locations, an automatic dispenser that only needs to be refilled every six months can be a great option. For chains that require regular lubing, a continuous dispenser might be best. If you are purchasing lubricants from a reputable vendor, consult with one of their specialists to determine what method will be most beneficial.
5. Longevity
Lubricants have an optimal lifespan, and once it’s over, they need to be replaced. Failure to do so can result in runout, metal-on-metal contact, destroyed bearings and other issues leading to downtime and higher expenses. The problem is that it’s not always obvious when this is about to occur.
One important way to track a lubricant’s lifespan is by monitoring the operating temperature of your equipment. Excess heat destroys lubricants. Every rise in temperature of 10 degrees C (18 degrees F) above 65 degrees C (150 degrees F) will cut the lubricant’s service life in half. This means a lubricant that would normally last one month at 150 degrees F will last only two weeks at 168 degrees F, one week at 186 degrees F, and just three or four days at 204 degrees F before needing to be replaced or rejuvenated. After that, it will cease to do its job, essentially offering little or no protection even if lubricant levels appear high.
Heat tracking is best done with digital calibration tools, which can offer thermal imaging as well as vibration analysis. It’s also recommended to monitor your lubrication intervals. Some digital systems make this simple, but at smaller companies where records are still kept by hand, it’s all too easy to let this slip. Don’t make that mistake. At least one person on your staff should be in charge of overseeing lubrication, including making sure that accurate records are being kept.
Finally, if you’re not familiar with the concept of predictive maintenance (PdM), it’s highly recommended that you take a few minutes to research it. This trend in maintenance scheduling is proving to be highly effective at saving companies money while keeping equipment functioning at optimum levels.
6. Cost
As with most things in life, you get what you pay for. A food-processing plant may opt to use mineral oil on transport chains because it’s cheap and food-safe. However, plain mineral oil has a number of disadvantages that outweigh the low cost. For instance, this particular oil may have poor runout characteristics, attract contaminants to form an abrasive paste or drip to create an unsafe working environment. Managers who choose this option will soon discover that their savings are wiped out by the cost of downtime resulting from using inferior-quality lubricants, not to mention the potential of workplace accidents.
To calculate the real cost of a lubricant, don’t just look at the price tag on the container. Take that number and compare it to the cost of downtime or replacement, loss of product due to halted production or contamination, employee hours, and other factors that may be an issue for your company. If you just spent $2 million on a new production line, paying a few hundred dollars a month for a high-quality lubricant should seem like a no-brainer if it means the machine will function better and its lifespan will be increased.
In conclusion, while all production facilities depend on lubricants to continue operating, remember that not all lubricants are created equal. It can be a serious mistake to simply purchase a cheap lubricant without considering the factors mentioned above. Breakdowns are expensive and can have a snowball effect. Smart businesses spend as much time thinking about lubrication as they do about the types of equipment they purchase.
Roller bearing oil selection guide figure 22
Newton's Law of Viscous Flow
A surface of area A is moving with the linear velocity V on a film of lubricant as shown in
Figure 17. The thickness of the lubricant is s and the deforming force acting on the film is F. The layers of the fluid in contact with the moving surface have the velocity v = V and the layers of the fluid in contact with the fixed surface have the velocity v = 0.
Newton's law of viscous flow states that the shear stress r in a fluid is proportional to the rate of change of the velocity v with respect to the distance y from the fixed surface
where µ is a constant, the absolute viscosity, or the dynamic viscosity. The derivate әν is the rate of change of velocity with distance and represents the rate of shear, or the velocity gradient. Thicker oils have a higher viscosity value causing relatively higher shear stesses at the same shear rate. For a constant velocity gradient Eq. 1 can be written as:
The unit of viscosity µ, for U.S. Customary units, is pound-second per square inch, (Ib-s/in²), or reyn (from Osborne Reynolds).
In SI units the viscosity is expressed as newton-seconds per square meter, (N-s/m²), or pascal-second (Pa-s).
The conversion factor between the two is the same as for stress:
1 reyn = 1 lb • s/in² = 6890 N • s/m² = 6890 Pa • s.
The reyn and pascal-second are such large units that microreyn (µreyn) and millipascal second (mPas) are more conmionly used. The former standard metric unit of viscosity was the poise (shortening of Jean Louis Marie Poiseuille, French physician and physiologist).
One centipoise, cp, is equal to one millipascal-second 1 cp = 1 mPa • s.
Dynamic viscosities are usually measured under high shear conditions, for example, the cylinder viscometer in which the viscous shear torque is measured between two cylinders.
The kinematic viscosity is defined as absolute viscosity, µ, divided by mass density, p
The units for kinematic viscosity are lengths/time, as cm²/s, which is the Stoke (St). Using SI units:
Liquid viscosities are determined by measuring the time required for a given quantity of the liquid to flow by gravity through a precision opening.
Viscosity Index Calculation
To calculate the Viscosity Index, we use the ASTM D2270 method, which involves the following steps:
1. Measure the Kinematic Viscosity at 40°C (KV40): This is the lubricant’s viscosity at a standard temperature of 40°C. The unit is usually centistokes (cSt).
2. Measure the Kinematic Viscosity at 100°C (KV100): Similarly, measure the viscosity at 100°C. This provides insight into how the viscosity changes with a higher temperature.
3. Determine L and H:
· L is the viscosity at 40°C of a reference oil with the same viscosity at 100°C as the lubricant in question but with a VI of 0.
· H is the viscosity at 40°C of another reference oil with the same viscosity at 100°C as the lubricant in question but with a VI of 1004. Use the VI Calculation Formula:
- For lubricants with a KV100 less than or equal to 70 cSt, the formula is:
- For lubricants with a KV100 greater than 70 cSt, the formula is slightly adjusted to account for higher viscosity oils:
How Viscosity Behaves: The Honey Analogy
Think of viscosity like the thickness of honey. At room temperature, honey flows slowly, but if you heat it, it flows more easily. If you cool it down, it can almost become solid. The Viscosity Index is like a measure of how much the honey’s thickness changes with temperature.
LUBRICANT FLOW
Lubricant flow rate is calculated by multiplying the cross-sectional area of the pipe or channel (A) by the flow velocity (ט) of the lubricant, expressed as
Journal bearing lubrication
It represents the volume of lubricant passing through a system per unit of time, often measured in liters per second (L/s) or gallons per minute (GPM)
The equation you use to calculate the proper circulating flow will depend on whether you are working in gallons per minute or in drops per minute. The equations can be seen in the next box
While most of the variables in these formulas are straightforward, the clearance factor (m) may be confusing for some. It can be determined by calculating the diametral clearance (2C), which is equal to the bearing bore diameter minus the journal diameter. Obviously, the clearance will be much smaller than the journal diameter (D), so this value is multiplied by 1,000 to make calculations easier. Therefore, the clearance factor is:
Returning to the original question about establishing the proper oil flow to a journal bearing, in this instance, the clearance was known. The diameter and length should be easy to obtain either by taking a measurement or by checking the documentation. The speed was also known, so the only value left to find is the load (W). This is simply a matter of determining the weight of the rotating element divided by the number of bearings.
3- Physical properties of lubricating oil
3-1 Viscosity
Viscosity is the measure of the internal friction within a liquid; the way the molecules interact
to resist motion. It is a vital property of a lubricant because it influences the ability of the oil to form a lubricating film or to minimize friction. Newton defined the absolute viscosity of a liquid as the ratio between the applied shear stress and the resulting shear rate.
Viscosity is A measurement of resistance to flow at one temperatu
3-1 Viscosity
Viscosity is the measure of the internal friction within a liquid; the way the molecules interact
to resist motion. It is a vital property of a lubricant because it influences the ability of the oil to form a lubricating film or to minimize friction. Newton defined the absolute viscosity of a liquid as the ratio between the applied shear stress and the resulting shear rate.
Viscosity is A measurement of resistance to flow at one temperatu
- Measurement of the oil’s internal resistance to motion at a given temperature (Typically 40 ֠C. & 100 ֠C.)
 |
| Fig 11 |
•Viscosity changes inversely with temperature * i.e., As temperature increases, oil becomes thinner
•Change in Viscosity is NOT linear
VISCOSITY AND TEMPERATURE
• Lubricant Viscosity Decreases Dramatically With Increasing Temperature (Log‐Log Relationship)
• Viscosity Index (V.I.) is a Measure of the “Viscosity‐Temperature Relationship” for an Oil
• Multigrade Oils Have Higher V.I.’s Than Single Grades
• Viscosity Changes Less With Temperature
 |
| Fig 12 |
• KIENMATIC VISCOSITY
Measure of fluid’s resistance to flow due to gravity
• Derived from the time taken for a lubricant to travel through a capillary tube
• Measurement – Stoke (St) = 1 cm² / second
 |
| FIG 13 |
3-2 Viscosity index
The most frequently used method for comparing the variation of viscosity with temperature between different oils by calculation of dimensionless numbers, known as the viscosity index
(VI). The kinematic viscosity of the sample is measured at two different temperatures (40°C, 100°C) and the viscosity compared with an empirical reference scale. VI is used as a convenient measure of the degree of aromatics removal during the base oil manufacturing process, but comparison of VI of different oil samples is only realistic if they are derived from the same distillate feedstock
With Most lubricants, the higher the VI the better LUBRICANT
The Viscosity Index is calculated fro viscosities at 40°C and 100°C
• High VI is a term which means that the oil is usable over a wider temperature range.
• VHVI = Very High Viscosity Index
• High VI is a term which means that the oil is usable over a wider temperature range.
• VHVI = Very High Viscosity Index
 |
| Fig 14 |
 |
| Fig 15 |
3-3 Low temperature properties.
When a sample of oil is cooled, its viscosity increases in a predictable manner until wax crystals start to form. The matrix of wax crystals becomes sufficiently dense with further cooling to cause an apparent solidification of the oil. Although the solidified oil does not pour under the influence of gravity, it can move if sufficient force is applied. Further decrease in temperature cause more wax to form, increasing the complexity of the wax/oil matrix. Many lubricating oils have to be capable of flow at low temperatures, and a number of properties should be measured.
3-4 cloud point
It is the temperature at which the first sign of wax formation can be detected. A sample of oil is warmed sufficiently to be fluid and clear. It is then cooled at a specified rate. The temperature at which haziness is first observed is recorded as the cloud point, the ASTM D2500/IP 219 test. The oil sample must be free of water because it interferers with the test.
3-5 pour point
It is the lowest temperature at which the sample of the sample of oil can make to flow by gravity alone. The oil is warmed and then cooled at a specified rate. The test jar is removed from the cooling bath at intervals to see if the sample is still mobile. The procedure is repeated until movement of the oil doesnot occur, ASTM D 97/IP 15. the pour point is the last temperature before the movement ceases, not the temperature at which solidification occurs. This is an important property of diesel fuels as well as lubricant base oils. High- Viscosity oils may cease to flow at low temperatures because their viscosity becomes too high rather than because of wax formation. In these cases, the pour point will be higher than the cloud point.
3-6 High temperature properties.
The high temperature properties of oil are governed by distillation or boiling range characteristics of the oil.
3-7 volatility
It is important because it is an indication of the tendency of oil to be lost in service by vaporization.
3-8 flash point
It is important for oil from a safety point of view because it is the lowest temperature at which auto-ignition of the vapour occur above the heated oil sample. Different methods are used, ASTM D 92, D93, and it is essential to know which equipment has been used when comparing results.
3-9 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
3-10 Other physical properties
Various other physical properties may be measured, most of them relating to specialized lubricant applications. Some of the more important measurements are:
• density Important, because oils may be formulated by weight, but measured by volume.
• demulsification Ability of oil and water to separate.
(2) If the manufacturer’s specifications are not available, determine what lubricant is currently in use.
If it is performing satisfactorily, continue to use the same brand. If the brand is not available, select a brand with specifications equal to or exceeding the brand previously used. If the lubricant is performing poorly, obtain the recommendation of a product engineer. If the application is critical, get several recommendations.
(3) Generally, the user will make a selection in either of two possible situations:
! Substitute a new brand for one previously in use.
! Select a brand that meets an equipment manufacturer's specifications. This will be accomplished
by comparing producer's specifications with those of the manufacturer.
This article is intended to help clarify some of the key questions about lubrication so you can make the right lubricant choice, not just the cheapest or quickest one. Even if this decision has already been made for you, the information below will be helpful for your general understanding of how lubricants work.
1. Function: What Does the Lubricant Do?
Various other physical properties may be measured, most of them relating to specialized lubricant applications. Some of the more important measurements are:
• density Important, because oils may be formulated by weight, but measured by volume.
• demulsification Ability of oil and water to separate.
• foam characteristics Tendency to foam formation and stability of the foam that results.
• pressure/viscosity characteristics
• thermal conductivity Important for heat transfer fluid.
• electrical properties Resistively, dielectric constant.
• surface properties As surface tension, air separation.
4- User selection.
(1) The user should ensure that applicable criteria are met regardless of who makes the lubricant selection. Selection should be in the class recommended by the machinery manufacturer (R&O, EP, AW, etc.) and be in the same base stock category (paraffinic, naphthenic, or synthetic).Furthermore, physical and chemical properties should be equal to or exceed those specified by the manufacturer. Generally, the user should follow the manufacturer's specification. Additional factors considered are shown in Tables 17, 18, and 19. Each of these tables uses different criteria that can be beneficial when the user is selecting lubricants.
 |
| Table 17 |
 |
| Table 18 |
 |
| Table 19 |
(2) If the manufacturer’s specifications are not available, determine what lubricant is currently in use.
If it is performing satisfactorily, continue to use the same brand. If the brand is not available, select a brand with specifications equal to or exceeding the brand previously used. If the lubricant is performing poorly, obtain the recommendation of a product engineer. If the application is critical, get several recommendations.
(3) Generally, the user will make a selection in either of two possible situations:
! Substitute a new brand for one previously in use.
! Select a brand that meets an equipment manufacturer's specifications. This will be accomplished
by comparing producer's specifications with those of the manufacturer.
This article is intended to help clarify some of the key questions about lubrication so you can make the right lubricant choice, not just the cheapest or quickest one. Even if this decision has already been made for you, the information below will be helpful for your general understanding of how lubricants work.
1. Function: What Does the Lubricant Do?
Lubricants have a wide range of functions that include controlling the following:
- Friction (lubricants reduce heat generation and energy consumption)
- Wear (lubricants can reduce mechanical and corrosive wear)
- Corrosion (quality lubricants protect surfaces from corrosive substances)
- Contamination (lubricants transport particles and other contaminants to filters and separators)
- Temperature (lubricants can absorb and transfer heat)
Sometimes, in the case of hydraulics, lubricants also provide power transmission.
What all this means is that you need to know what unique challenges your application poses and then choose your lubricant accordingly. For example, high pressure, low temperature and exposure to saltwater are just a few possibilities. Select lubricants that are clearly labeled to indicate how they are designed to perform and under what circumstances.
2. Ingredients: What’s in the Lubricant?
You may not think of lubricants as having many ingredients, but in fact there are lots of different additions to the base oil. These ingredients are deliberately chosen based on how the lubricant will be used. This is true of greases as well as oils. Many people are unaware that greases are actually oils with thickeners added. The type of thickener matters a great deal. The thickener typically is composed of fibrous particles that act like a sponge, holding the oil in place to give it a more viscous quality. Each thickener type confers different advantages and disadvantages, specifically having to do with shear stability, pumpability, heat resistance and water resistance.
- Friction (lubricants reduce heat generation and energy consumption)
- Wear (lubricants can reduce mechanical and corrosive wear)
- Corrosion (quality lubricants protect surfaces from corrosive substances)
- Contamination (lubricants transport particles and other contaminants to filters and separators)
- Temperature (lubricants can absorb and transfer heat)
Sometimes, in the case of hydraulics, lubricants also provide power transmission.
What all this means is that you need to know what unique challenges your application poses and then choose your lubricant accordingly. For example, high pressure, low temperature and exposure to saltwater are just a few possibilities. Select lubricants that are clearly labeled to indicate how they are designed to perform and under what circumstances.
2. Ingredients: What’s in the Lubricant?
You may not think of lubricants as having many ingredients, but in fact there are lots of different additions to the base oil. These ingredients are deliberately chosen based on how the lubricant will be used. This is true of greases as well as oils. Many people are unaware that greases are actually oils with thickeners added. The type of thickener matters a great deal. The thickener typically is composed of fibrous particles that act like a sponge, holding the oil in place to give it a more viscous quality. Each thickener type confers different advantages and disadvantages, specifically having to do with shear stability, pumpability, heat resistance and water resistance.
3. Terminology: What Do These Things Mean?
To make correct lubrication choices, you must have an understanding of the terminology involved. Commonly used terms include the following:
* Viscosity — This describes how thick or resistant to flow the oil or grease is. Higher viscosity means higher flow resistance.
* Kinematic viscosity — A simple pour test can be used to offer a visual illustration of viscosity. Lube technicians can provide a demonstration of kinematic viscosity on the shop floor, because it’s easy to do and takes very little time.
* Weight — This also relates to how viscous an oil is or how easily it flows at a specific temperature.
* NLGI consistency — The consistency indicates how hard or soft a grease is. The numbers range from 000 (like cooking oil) to 6 (like cheddar cheese).
* Runout — This term refers to the ability of a grease or oil to resist higher temperatures, which tend to make lubricants less viscous.
* Shear stability — This is the resistance of an oil to a change in viscosity caused by mechanical stress. * Metal on metal — A condition that every equipment owner should avoid like the plague, this describes a situation in which no lubricant is left in a bearing or other application
4. Ease of Use
Not all oils are applied the same way. The method of application will depend on your particular equipment. For some situations, such as an easy-to-reach hinge, an aerosol lubricant may suffice. A grease may be easily applied by hand to an accessible gearbox. However, for hard-to-reach locations, an automatic dispenser that only needs to be refilled every six months can be a great option. For chains that require regular lubing, a continuous dispenser might be best. If you are purchasing lubricants from a reputable vendor, consult with one of their specialists to determine what method will be most beneficial.
5. Longevity
Lubricants have an optimal lifespan, and once it’s over, they need to be replaced. Failure to do so can result in runout, metal-on-metal contact, destroyed bearings and other issues leading to downtime and higher expenses. The problem is that it’s not always obvious when this is about to occur.
One important way to track a lubricant’s lifespan is by monitoring the operating temperature of your equipment. Excess heat destroys lubricants. Every rise in temperature of 10 degrees C (18 degrees F) above 65 degrees C (150 degrees F) will cut the lubricant’s service life in half. This means a lubricant that would normally last one month at 150 degrees F will last only two weeks at 168 degrees F, one week at 186 degrees F, and just three or four days at 204 degrees F before needing to be replaced or rejuvenated. After that, it will cease to do its job, essentially offering little or no protection even if lubricant levels appear high.
Heat tracking is best done with digital calibration tools, which can offer thermal imaging as well as vibration analysis. It’s also recommended to monitor your lubrication intervals. Some digital systems make this simple, but at smaller companies where records are still kept by hand, it’s all too easy to let this slip. Don’t make that mistake. At least one person on your staff should be in charge of overseeing lubrication, including making sure that accurate records are being kept.
Finally, if you’re not familiar with the concept of predictive maintenance (PdM), it’s highly recommended that you take a few minutes to research it. This trend in maintenance scheduling is proving to be highly effective at saving companies money while keeping equipment functioning at optimum levels.
6. Cost
As with most things in life, you get what you pay for. A food-processing plant may opt to use mineral oil on transport chains because it’s cheap and food-safe. However, plain mineral oil has a number of disadvantages that outweigh the low cost. For instance, this particular oil may have poor runout characteristics, attract contaminants to form an abrasive paste or drip to create an unsafe working environment. Managers who choose this option will soon discover that their savings are wiped out by the cost of downtime resulting from using inferior-quality lubricants, not to mention the potential of workplace accidents.
To calculate the real cost of a lubricant, don’t just look at the price tag on the container. Take that number and compare it to the cost of downtime or replacement, loss of product due to halted production or contamination, employee hours, and other factors that may be an issue for your company. If you just spent $2 million on a new production line, paying a few hundred dollars a month for a high-quality lubricant should seem like a no-brainer if it means the machine will function better and its lifespan will be increased.
In conclusion, while all production facilities depend on lubricants to continue operating, remember that not all lubricants are created equal. It can be a serious mistake to simply purchase a cheap lubricant without considering the factors mentioned above. Breakdowns are expensive and can have a snowball effect. Smart businesses spend as much time thinking about lubrication as they do about the types of equipment they purchase.
(About the Author William Kowalski is the director of online operations for Interflon USA, manufacturers of high-quality industrial lubricants with MicPol. William can be reached at wkowalski@interflon.com.)
grease application guide table 20
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| Table 20 |
 |
| Table 21 |
 |
| Fig 16 |
5- Calculations
Newton's Law of Viscous Flow
A surface of area A is moving with the linear velocity V on a film of lubricant as shown in
Figure 17. The thickness of the lubricant is s and the deforming force acting on the film is F. The layers of the fluid in contact with the moving surface have the velocity v = V and the layers of the fluid in contact with the fixed surface have the velocity v = 0.
Newton's law of viscous flow states that the shear stress r in a fluid is proportional to the rate of change of the velocity v with respect to the distance y from the fixed surface
 |
| Eq. 1 |
where µ is a constant, the absolute viscosity, or the dynamic viscosity. The derivate әν is the rate of change of velocity with distance and represents the rate of shear, or the velocity gradient. Thicker oils have a higher viscosity value causing relatively higher shear stesses at the same shear rate. For a constant velocity gradient Eq. 1 can be written as:
 |
| Eq. 2 |
The unit of viscosity µ, for U.S. Customary units, is pound-second per square inch, (Ib-s/in²), or reyn (from Osborne Reynolds).
In SI units the viscosity is expressed as newton-seconds per square meter, (N-s/m²), or pascal-second (Pa-s).
 |
| Fig 17 |
The conversion factor between the two is the same as for stress:
1 reyn = 1 lb • s/in² = 6890 N • s/m² = 6890 Pa • s.
The reyn and pascal-second are such large units that microreyn (µreyn) and millipascal second (mPas) are more conmionly used. The former standard metric unit of viscosity was the poise (shortening of Jean Louis Marie Poiseuille, French physician and physiologist).
One centipoise, cp, is equal to one millipascal-second 1 cp = 1 mPa • s.
Dynamic viscosities are usually measured under high shear conditions, for example, the cylinder viscometer in which the viscous shear torque is measured between two cylinders.
The kinematic viscosity is defined as absolute viscosity, µ, divided by mass density, p
 |
| Eq. 3 |
The units for kinematic viscosity are lengths/time, as cm²/s, which is the Stoke (St). Using SI units:
1 m²/s = 10⁴ St and 1 cSt (centistoke) = 1 nun²/s. The physical principle of measurement is based on the rate at which a fluid flows vertically downward under gravity through a small-diameter tube.
 |
| Fig 18 |
Liquid viscosities are determined by measuring the time required for a given quantity of the liquid to flow by gravity through a precision opening.
For lubricating oils, the Saybolt Universal Viscometer, shown in Figure 18, is an instrument used to measure the viscosity. The viscosity measurements are Saybolt seconds, or SUS (Saybolt Universal Seconds), SSU (Saybolt Seconds Universal), and SUV (Saybolt Universal Viscosity).
With a Saybolt Universal Viscometer one can measure the kinematic viscosity, v. Absolute viscosities can be obtained from Saybolt viscometer measurements by the Equations
where p is the mass density in grams per cubic centimeter, g/cm² (which is also called specific gravity) and t is the time in seconds. For petroleum oils the mass density at
The Society of Automotive Engineers (SAE) classifies oils according to viscosity. Any viscosity grade should be proceeded by the initials SAE. It should be noted that SAE is not a performance category, it only refers to the viscosity of the oil.
With a Saybolt Universal Viscometer one can measure the kinematic viscosity, v. Absolute viscosities can be obtained from Saybolt viscometer measurements by the Equations
 |
 |
| Eq.4 |
where p is the mass density in grams per cubic centimeter, g/cm² (which is also called specific gravity) and t is the time in seconds. For petroleum oils the mass density at
 |
| Eq.5 |
The Society of Automotive Engineers (SAE) classifies oils according to viscosity. Any viscosity grade should be proceeded by the initials SAE. It should be noted that SAE is not a performance category, it only refers to the viscosity of the oil.
 |
| Table 22 |
Viscosity Index Calculation
To calculate the Viscosity Index, we use the ASTM D2270 method, which involves the following steps:
1. Measure the Kinematic Viscosity at 40°C (KV40): This is the lubricant’s viscosity at a standard temperature of 40°C. The unit is usually centistokes (cSt).
2. Measure the Kinematic Viscosity at 100°C (KV100): Similarly, measure the viscosity at 100°C. This provides insight into how the viscosity changes with a higher temperature.
3. Determine L and H:
· L is the viscosity at 40°C of a reference oil with the same viscosity at 100°C as the lubricant in question but with a VI of 0.
· H is the viscosity at 40°C of another reference oil with the same viscosity at 100°C as the lubricant in question but with a VI of 100
- For lubricants with a KV100 less than or equal to 70 cSt, the formula is:
 |
| Eq.6 |
- For lubricants with a KV100 greater than 70 cSt, the formula is slightly adjusted to account for higher viscosity oils:
 |
| Eq.7 |
. Low VI: Imagine a honey that becomes very runny when warmed and very thick when cooled. It’sinconsistent.
. High VI: Now, picture honey that is nearly the same thickness, whether slightly warm or cool. This honey has a high VI.
When calculating the VI:
· L is like a very thin honey that would behave similarly at 100°C but is extremely runny at 40°C.
· H is a thicker honey that behaves similarly at 100°C but is not as runny at 40°C.
The calculation determines where your specific lubricant sits between these two extremes.
LUBRICANT FLOW
Lubricant flow rate is calculated by multiplying the cross-sectional area of the pipe or channel (A) by the flow velocity (ט) of the lubricant, expressed as
 |
| Eq. 8 |
Ԛ = Flow rate (e.g.,m³/s )
a= Cross-sectional area of pipe or gap (m²)
ט = Average velocity of lubricant (m/sec)
Journal bearing lubrication
It represents the volume of lubricant passing through a system per unit of time, often measured in liters per second (L/s) or gallons per minute (GPM)
The equation you use to calculate the proper circulating flow will depend on whether you are working in gallons per minute or in drops per minute. The equations can be seen in the next box
 |
| Eq.9 |
While most of the variables in these formulas are straightforward, the clearance factor (m) may be confusing for some. It can be determined by calculating the diametral clearance (2C), which is equal to the bearing bore diameter minus the journal diameter. Obviously, the clearance will be much smaller than the journal diameter (D), so this value is multiplied by 1,000 to make calculations easier. Therefore, the clearance factor is:
m=1,000(2C/D)
Returning to the original question about establishing the proper oil flow to a journal bearing, in this instance, the clearance was known. The diameter and length should be easy to obtain either by taking a measurement or by checking the documentation. The speed was also known, so the only value left to find is the load (W). This is simply a matter of determining the weight of the rotating element divided by the number of bearings.
Lubricant Volume for Bearings (V):
 |
| Eq.10 |
V = Volume
A= Bearing surface area
T= Lubricant film thickness
· Pressure (∆p): Higher pressure results in a higher flow rate. Flow is generally proportional to the square root of the differential pressure in many systems.
· Viscosity (µ): Higher viscosity lowers flow rate in pipe flow (Poiseuille flow).
· Clearance (c): In hydrodynamic lubrication, flow rate is highly sensitive to the bearing clearance
 |
| Table 23 |
To predict if the lubrication is sufficient, calculate the viscosity ratio
 |
| Eq.11 |
К˃4: Full film lubrication (optimal).
0.1˂К˂4 Mixed lubrication.
К˂ 0.1 Boundary lubrication (high wear)
Bearing Area (A) for Formulas:
o Plain bearings: π × Shaft diameter × Length
o Anti-friction bearings: Shaft diameter²× number of raws
o Service Factors: Increase oil/grease quantity by 1.3 - 3.0 for shock loading or extreme heat.
o Speed Factor (n-dm): Used to determine if the bearing is low-speed or high-speed, influencing viscosity requirementsRolling-Contact Bearings
In rolling-contact bearings, the lubricant film is replaced by several small rolling elements between an inner and outer ring. In most cases the rolling elements are separated from each other by cages. Basic varieties of rolling-contact bearings include ball, roller, and thrust.
Table 24 identifies some of the methods used to supply lubricants to bearings. The lubricant should be supplied at a rate that will limit the temperature rise of the bearing to 20°C (68 °F).
 |
| Table 24 |
GREASE
The standard formula for calculating the required quantity of grease for a bearing is:
 |
| Eq.12 |
Good viscosity is essential to ensure cushioning and quiet operation. An oil viscosity that is too high will result in excess friction and degradation of oil properties associated with high oil operating temperature. In cold climates gear lubricants should flow easily at low temperature.
Gear oils should have a minimum pour point of 5 °C (9 °F) lower than the lowest expected temperature. The pour point for mineral gear oil is typically -7°C (20 °F). When lower pour points are required, synthetic gear oils with pour points of -40 °C (-40 °F) may be necessary.
The following equation from the ASM Handbook provides a method for verifying the required viscosity for a specific gear based on the operating velocity:
 |
| Eq.13 |
where n is the pinion speed in rev/min and d is the pitch diameter (inches).
 |
| Eq.14 |
Where
Q – Fluid flow rate (𝑚2/𝑠)
A – Area of Pipe (𝑚2)
D𝐿 – Diameter of Fluid flow (m)
V – Velocity of fluid flow (m/s)
V𝑀𝐴𝑋 – Maximum allowable velocity of flow (m/s)
C – Flow coefficient
V – Velocity of fluid flow (m/s)
V𝑀𝐴𝑋 – Maximum allowable velocity of flow (m/s)
C – Flow coefficient
 |
| Table 25 |
6- Oil maintenance
6-1 Lubricant Maintenance
Lubricant maintenance is closely associated with the monitoring program. When used oil test results exceed the condemning limits, corrective action needs to be taken. Such action could include filtration
to remove particulate matter and in some cases oxidation products and/or dehydration. This processing
can be done either on site or at a recycle station. Additive replenishment for depleted inhibitors may be feasible for some products in some applications. Since additive replenishment requires a considerable amount of technical expertise, the lubricant supplier should be contacted to provide information and service to reclaim and refortify used lubricants.
Lubricant maintenance is closely associated with the monitoring program. When used oil test results exceed the condemning limits, corrective action needs to be taken. Such action could include filtration
to remove particulate matter and in some cases oxidation products and/or dehydration. This processing
can be done either on site or at a recycle station. Additive replenishment for depleted inhibitors may be feasible for some products in some applications. Since additive replenishment requires a considerable amount of technical expertise, the lubricant supplier should be contacted to provide information and service to reclaim and refortify used lubricants.
 |
| Table 26 |
6-2 Purification and Filtration
6-2-1 Cleanliness.
(1) Oil must be free of contaminants to perform properly. Most hydraulic systems use an in-line filter
to continually filter the oil while the system is operating to maintain the required cleanliness rating in
accordance with ISO standards (Table 27). ISO 4406 is an internationally recognized standard that
expresses the level of particulate contamination of a hydraulic fluid. The standard is also used to specify
the required cleanliness level for hydraulic components and systems. ISO 4406 is a hydraulic cleanliness rating system that is based on a number of contamination particles larger than 2 microns, 5 microns, and 15 microns in a 1-milliliter fluid sample. Once the number and size of the particles are determined, the points are plotted on a standardized chart of ISO range numbers to convert the particle counts into an ISO 4406 rating. The ISO 4406 rating provides three range numbers that are separated by a slash, such as 16/14/12.
6-2-1 Cleanliness.
(1) Oil must be free of contaminants to perform properly. Most hydraulic systems use an in-line filter
to continually filter the oil while the system is operating to maintain the required cleanliness rating in
accordance with ISO standards (Table 27). ISO 4406 is an internationally recognized standard that
expresses the level of particulate contamination of a hydraulic fluid. The standard is also used to specify
the required cleanliness level for hydraulic components and systems. ISO 4406 is a hydraulic cleanliness rating system that is based on a number of contamination particles larger than 2 microns, 5 microns, and 15 microns in a 1-milliliter fluid sample. Once the number and size of the particles are determined, the points are plotted on a standardized chart of ISO range numbers to convert the particle counts into an ISO 4406 rating. The ISO 4406 rating provides three range numbers that are separated by a slash, such as 16/14/12.
In this rating example, the first number 16 corresponds to the number of particles greater than 2 microns in size; the second number 14 corresponds to the number of particles greater than 5 microns in size; and the third number 12 corresponds to the number of particles greater than 15 microns in size. All three values for applicable range numbers can be determined through the use of the ISO 4406 standardized chart based on the actual number of particles counted within the 1-milliliter (ml) sample for each size category (>2, >5,>15 microns).
For example, if a 1-ml sample contained 6000 2-mm particles, 140 5-mm particles, and 28 15-mm particles, the fluid would have a cleanliness rating of 20/14/12. The number of 2-mm particles (6000) falls in the range greater than 5000 but less than 10,000, which results in an ISO 4406 range number of 20. The number of 5-m particles (140) falls in the range greater than 80 but less than 160, which results in an ISO 4406 range number of 14. The number of 15-mm particles (28) falls in the range
greater than 20 but less than 40, which results in an ISO 4406 range number of 12.
(2) Table 28 shows the desirable cleanliness levels for different types of systems and typical applications rated by the system sensitivity, from noncritical systems through super-critical systems.
Table 29 shows the desired ISO cleanliness code for specific components in hydraulic and lubricating systems.
greater than 20 but less than 40, which results in an ISO 4406 range number of 12.
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| Table 27 |
Table 29 shows the desired ISO cleanliness code for specific components in hydraulic and lubricating systems.
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| Table 28 |
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| Table 29 |
(3) However, for most lubricating systems filter or purify oil periodically as dictated by the results of
the oil testing program. Water is the most common contaminant found in hydroelectric plants, and its
presence in oil may promote oxidation, corrosion, sludge formation, foaming, additive depletion, and
generally reduce a lubricant's effectiveness. Solid contaminants such as dirt, dust, or wear particles also
may be present. These solid particles may increase wear, and promote sludge formation, foaming, and
restrict oil flow within the system. The following are some of the most common methods used to remove contaminants from oil
6-2-2 Gravity purification. Gravity purification is the separation or settling of contaminants that are
heavier than the oil. Gravity separation occurs while oil is in storage but is usually not considered an
adequate means of purification for most applications. Other purification methods should also be used in
addition to gravity separation.
6-2-3 Centrifugal purification. Centrifugal purification is gravity separation accelerated by the centrifugal forces developed by rotating the oil at high speed. Centrifugal purification is an effective means of removing water and most solid contaminants from the oil. The rate of purification depends on the viscosity of the oil in a container and the size of the contaminants.
6-2-4 Mechanical filtration. Mechanical filtration removes contaminants by forcing the oil through a
filter medium with holes smaller than the contaminants. Mechanical filters with fine filtration media can
remove particles as small as 1 micron, but filtration under 5 microns is not recommended because many of the oil additives will be removed. A typical mechanical filter for turbine oil would use a 6- to 10-micron filter. The filter media will require periodic replacement as the contaminants collect on the
medium's surface. Filters have absolute, beta, and nominal ratings as follows:
(1) Absolute rating. Absolute rating means that no particles greater than a certain size will pass
through the filter and is based on the maximum pore size of the filtering medium.
(2) Beta rating. The beta rating or beta ratio is a filter-rating expressed as the ratio of the number ofthrough the filter and is based on the maximum pore size of the filtering medium.
upstream particles to the number of downstream particles of a particular size or larger. It expresses the
separating effectiveness of a filter. The beta ratio counts the results from the multipass “beta” test for
filters, ANSI/(NFPA) T3.10.8.8, and ISO 4572, “Hydraulic Fluid Power - Filters - Multi-Pass Method for Evaluating Filtration Performance.”
(3) Nominal rating. Nominal rating is not an industry standard but an arbitrary value assigned by the
filter manufacturer and means that a filter stops most particles of a certain micron size. Due to its
imprecision, filter selection by nominal rating could lead to system contamination and component failure.
6-2-5 Coalescence purification. A coalescing filter system uses special cartridges to combine small,
dispersed water droplets into larger drops. The larger water drops are retained within a separator screen and fall to the bottom of the filter while the dry oil passes through the screen. A coalescing filter will also remove solid contaminants by mechanical filtration.
6-2-6 Vacuum dehydration. A vacuum dehydration system removes water from oil through the application of heat and vacuum. The contaminated oil is exposed to a vacuum and is heated to temperatures of approximately 38 °C to 60 °C (100 °F to 140 °F). The water is removed as a vapor. Care must be exercised to ensure that desirable low-vapor-pressure components and additives are not removed by the heat or vacuum.
6-2-7 Adsorption purification. Adsorption or surface-attraction purification uses an active-type
medium such as fuller’s earth to remove oil oxidation products by their attraction or adherence to the large internal surfaces of the media. Because adsorption purification will also remove most of an oil’s additives, this method should not be used for turbine oil purification.
6-2-8 Filter system. A system consisting of a vacuum purifier to remove the water, a centrifuge to
remove large solid particles, and a 10-micron filter to remove the finer solid particles is the most desirable system. The vacuum purifier should be specified as being suitable for the lubricating oil. The ability of a filter system to remove water is especially important to prevent microbial contamination in lubricants and hydraulic fluids. However, this type of system alone may not be sufficient. Introduction of biocides may be necessary to minimize the chemical reaction byproducts and contamination due to microbes.
6-2-9 Location and purpose of filters. Table 30 provides information on the location and purpose ofdispersed water droplets into larger drops. The larger water drops are retained within a separator screen and fall to the bottom of the filter while the dry oil passes through the screen. A coalescing filter will also remove solid contaminants by mechanical filtration.
6-2-6 Vacuum dehydration. A vacuum dehydration system removes water from oil through the application of heat and vacuum. The contaminated oil is exposed to a vacuum and is heated to temperatures of approximately 38 °C to 60 °C (100 °F to 140 °F). The water is removed as a vapor. Care must be exercised to ensure that desirable low-vapor-pressure components and additives are not removed by the heat or vacuum.
6-2-7 Adsorption purification. Adsorption or surface-attraction purification uses an active-type
medium such as fuller’s earth to remove oil oxidation products by their attraction or adherence to the large internal surfaces of the media. Because adsorption purification will also remove most of an oil’s additives, this method should not be used for turbine oil purification.
6-2-8 Filter system. A system consisting of a vacuum purifier to remove the water, a centrifuge to
remove large solid particles, and a 10-micron filter to remove the finer solid particles is the most desirable system. The vacuum purifier should be specified as being suitable for the lubricating oil. The ability of a filter system to remove water is especially important to prevent microbial contamination in lubricants and hydraulic fluids. However, this type of system alone may not be sufficient. Introduction of biocides may be necessary to minimize the chemical reaction byproducts and contamination due to microbes.
filters. Table 31 lists various types of filters and the range of particle sizes filtered by each.
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| Table 30 |
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| Table 31 |
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| Table 32 |
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| Table 33 |
Disposal
Disposal is the last step that must be addressed in fluid management when the monitoring results indicate that the oil is severely degraded and/or depleted of additives that cannot be restored. Various
options to consider include recycling, burning, land-filling, and re-refining. The most appropriate
method of disposal will depend on local, state, and federal regulations. These will clearly be affected
by the location, which makes the best method of disposal site-specific. Lubricant disposal needs to
be considered carefully on a case-by-case basis.
6-3 Grease Properties and Tests
The single most distinguishing property of a grease is its consistency, which is related to the hardness or softness of the grease. The consistency is related to the penetration number obtained on the grease and is defined as the depth, in tenths of a millimeter, that a standard cone penetrates a sample of grease under prescribed conditions of weight, time, and temperature. To ensure a uniform sample, the grease is “worked” 60 strokes in a prescribed manner before running the penetration test (ASTM D217). Based on the worked penetration value, the National Lubricating Grease Institute (NLGI) has devised a classification system using defined consistency grades ranging from 000 (very soft) to 6 (very hard). Each consistency grade has a range of 30 penetration units, with a 15 penetration unit range between each grade. This classification system is shown in Table 34.
As indicated in Table 34, a no. 2 grade grease will always have a worked penetration in the range
of 265 to 295, as determined by the penetration test. Of the grades available, grades 0, 1, and 2 are
the most widely used in industry. The more fluid grades, 00 and 000, are used when a thickened oil
is desired, such as in the lubrication of gearboxes, where high leakage may occur when using a conventional oil lubricant.
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| Table 34 |
Another important property of a grease is its dropping point. Since greases are semisolid materials, they exhibit a characteristic temperature range wherein they change from a semisolid to a fluid. Greases do not have a sharp melting point but, upon heating, become softer until at some point they become essentially fluid and no longer function as a thickened lubricant. Dropping points are useful in characterizing greases. Each type of soap thickener exhibits a particular dropping point range. Table 35 shows the typical dropping point ranges for greases containing different thickeners.
Greases cannot be used at temperatures above their dropping points. However, the dropping point by itself does not establish the maximum usable temperature. The maximum usable temperature is considered to be the temperature limit where the grease should not be used without frequent relubrication.
As noted in Table 35, the maximum usable temperature is well below the dropping point of the grease.
As a general rule, the maximum usable temperature of a grease should be at least 25 to 50°F below its
dropping point. With frequent relubrication or continuous lubrication, the maximum usable temperature
can be raised. Extended use at temperatures above 350°F usually will result in severe oxidation of the grease. For example, for an organo-clay grease, even though the thickener can withstand very high
temperatures (above 500°F), the fluid lubricant component of the grease will be severely oxidized, leaving behind only the solid thickener, which has very poor lubrication properties. Once the fluid lubricant component is removed from the grease, the remaining thickener component becomes dry and abrasive. This is why frequent relubrication is required when operating a grease at high temperatures.
Other grease properties are also important when considering a particular grease for a specific application. These include resistance to softening (shear stability), oxidation resistance, water resistance, antiwear protection, corrosion and rust resistance, and pumpability. Table 36 lists tests that are used to characterize these properties. high dropping point, good adhesive (cohesive) properties, but very poor water resistance. Sodium greases are used widely in certain plain, slow-speed bearings and in gearboxes where water contact is low. These greases should never be used in applications with any appreciable exposure to water.
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| Table 35 |
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| Table 36 |
7- color coding
Color coding is an efficient way to initially identify the lubricants on site. In cases where a site has a small number of lubricants, identification can stop with color coding. Table 37 outlines some basic color associations
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| Table 37 |
8- Text identification
Text identification is necessary for characteristics with multiple options that may be too cumbersome to identify with color, shapes, symbols, etc. Viscosity, base oil type and, in some instances, additives fall into that category.
Viscosity identification is straight forward—the ISO classification or other standard classification as determined by the specific fluid type should be expressed in text form. For example: 22, 32, 46, 100 or 220 for ISO; 10W, 20W or 30W for SAE engine oils; or 80W, 85W or 90W for SAE gear oils.
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| Table 38 |
Table 39 outlines some additives or performance properties might require identification to prevent specialized lubricants from being used in situations where they exceed the requirements for a component and are not economical for use basic and common abbreviations for base oil types.
Additive identification is best identified with test, as well. Although there are three general additives that should be identified, additional additives might require identification in specialized instances. As was mentioned earlier, rust and corrosion, antiwear and extreme pressure should be identified as a minimum. However, additional
9- Standards (Tests & Iso vis. Grades Tables)
Additive identification is best identified with test, as well. Although there are three general additives that should be identified, additional additives might require identification in specialized instances. As was mentioned earlier, rust and corrosion, antiwear and extreme pressure should be identified as a minimum. However, additional
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| Table 39 |
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| Table 40 |
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| Table 41 |
Note: This article does not contain all the detailed information about oils and greases, as this topic is complex in every detail. I have simply tried to summarize the useful information for reference in some situations and to serve as a guide for those who wish to delve deeper into the subject. Thank you for reading, and please contact us with any comments.