Viscosity and Viscosity Index

Viscosity  is probably the single most important property of oil in terms of lubrication but what is viscosity really?

Informally viscosity is the “thickness” of a liquid. For example, if you pour water into a container with a hole at the bottom, the container drains quickly.

However, if you fill the same container with honey, you will find the container drains very slowly. That is because the viscosity of honey is high compared to that of water. We can therefore say that viscosity is an indication of a fluid’s resistance to flow.

More formally, viscosity is a measure of the internal friction of a moving fluid. Most liquids are both cohesive and adhesive. Cohesiveness is the intermolecular attraction by which the molecules of the fluid are held together and result in internal friction. A fluid with low viscosity flows easily because its chemical structure results in very little friction when the oil molecules are in motion.

Imagine you have two horizontal plates or metal surfaces with oil in-between. The oil will cling to the two surfaces because it is adhesive. If the top plate moves horizontally relative to the stationary bottom plate the speed of oil molecules in between will vary from zero at the bottom to the same speed as the top plate.  As the oil layers slip over each other they create friction as a result of the cohesiveness of the oil molecules.

Viscosity and fluids

The most commonly used unit for measuring viscosity is the Centistoke (cSt). Viscosity is frequently measured using a device called a capillary viscometer – a U-shaped, graduated glass tube with a capillary of known diameter in the one arm. This method measures the time taken for a defined quantity of fluid to flow through the capillary.

When two fluids of equal volume are placed in the same viscometer and allowed to flow under the influence of gravity, a viscous fluid takes longer than a less viscous fluid to flow through the capillary and a higher viscosity is recorded.

Since this method uses gravity as the driving force; the result is kinematic viscosity.  The metric unit of kinematic viscosity is mm2/s (2cSt = mm2/s). One disadvantage of the capillary viscometer is that the capillary is too small for highly viscous liquids.

From everyday experience, it is common knowledge that viscosity varies with temperature. Honey flows more readily when heated. Likewise, oils thicken noticeably on cold days with a resultant increase in viscosity. Since viscosity is so dependent on temperature, it should NEVER be quoted without reference to the temperature at which it was measured. Kinematic viscosity is generally measured at 40°C and 100°C.

oil viscosity

The low-temperature characteristics of certain lubricants are important to their proper operation. Measurement of the sub-zero viscosity of automatic transmission fluids. Engine oils, etc. is often used to specify their suitability for service.

To measure the viscosity of oils at low temperature, dynamic (or absolute) viscosity is often employed using a Brookfield viscometer. Brookfield viscometers rotate a spindle (at a defined speed) in the viscous cold oil and measure the torque required to rotate the spindle in the oil and report viscosity values using the centipoise (cP) or milliPascal-second (mPa-s), as the unit of viscosity (1 cP – 1 mPa-s).

Another key property of lubricating oil is Viscosity Index (VI). It is an arbitrary measuring scale (without units) that indicates the change in oil viscosity with change in temperature. For example, honey is thick at room temperature but when you heat it to say 60°C, it flows readily because the viscosity is reduced significantly and thus has a low VI.

However, there is hardly any visible change in the viscosity of water from room temperature to 60°C and we can therefore say water has a high VI compared to honey. Failure to use on oil with the proper VI when temperature extremes are expected may result in poor lubrication and equipment failure.

And finally, oil viscosity selection. When choosing an oil for a specific application the first consideration should always be an oil with a viscosity that is sufficient to keep the metal surfaces apart. Unfortunately, viscosity cannot be considered in isolation. Selection of the correct viscosity will depend on the temperature, load and speed encountered in a specific application.


For machines operating under constant load, constant speed and constant ambient temperature such as an industrial gearbox in a factory, the ideal viscosity very often results in the lowest stabilized oil temperature. Oils of lower or higher viscosities (than the optimum viscosity) will typically increase the oil’s stabilized temperature due to either drag/churning losses (too much viscosity) or mechanical friction (too little viscosity).

If conditions are not constant (variable loads, changing speeds, extreme temperatures, etc.), then there is a need for not only the optimum viscosity but also a high viscosity index to stabilize the optimum viscosity. The wider the temperature range, the greater the need for higher VI oils.


Operating conditions determine the load on machinery. The load on an engine in a vehicle under acceleration or going uphill is higher than that of a vehicle cruising down the highway. Load refers to the pressure on the moving surfaces.

Effective lubrication means being able to separate the load carrying surfaces and, if the load changes, then the optimum viscosity of the oil required to separate the surfaces can change. If the load is too high, the oil film may be squeezed too thin to protect the metal surfaces from making contact. This will result in solid friction, meaning an increase in heat, wear and ultimately machine failure.


The faster a machine operates the stronger the oil film will become as more oil is dragged into the area between the metal surfaces. Therefore, for high speed applications, a low viscosity oil is required. Conversely, for low speed applications, a high viscosity oil needed to maintain a solid oil film and separate moving surfaces.

In summary, oil viscosity must be sufficient to keep metal surfaces apart yet it must not be so viscous that it will increase drag and waste energy.

Slideway Oil Gauteng

Slideway Lubricants OilChat#39

In this edition of the newsletter, we will endeavour to explain what slideway lubricants are, their functions and how they work.

Slideway oils derive their name from the application. They are primarily designed for the lubrication of machine tool slideways. A slideway can be described as any form of track along which things can slip or slide. A curtain rail is a simple example of a slideway.

In metalworking operations, workpieces are machined into a desired shape and size by a controlled metal removal process. In Slideway Oil Johannesburggeneral cutting operations (e.g. lathes) the workpiece rotates whilst the cutting tool is stationary. Metal may also be removed by means of linear instead of rotational movement.  In these operations, the workpiece and cutting tool move in a straight line relative to each other. The photo on the right shows such a machining operation.  The cutting head (in the red rectangle) is attached to the light grey frame and can move up, down or to the left and right on slideways. The brown workpiece is fixed to a traverse table that moves backward and forward, also on slideways.  The operator (with green pants) is visible on the left side of the photo. These metalworking machines can vary in size from modest basic units that produce small metal components to massive monsters designed to machine very large workpieces such as marine engines and mining machinery.

Smooth and precise slideway operation is essential to ensure optimum machine tool productivity since loss of frictional control can cause inaccuracies of the machined workpiece surfaces. To fully understand slideway lubrication we need to revisit the fundamentals of friction and lubrication – please refer to OilChat 22. The speed/friction relation between two lubricated surfaces is illustrated by the curve on the second page of OilChat 22. The three different lubrication regimes are:

  • Boundary Lubrication is associated with metal-to-metal contact when the speed difference between two moving surfaces is too low to prevent contact between the two lubricated surfaces.
  • Mixed Lubrication is a transitional regime between the boundary and hydrodynamic lubrication when the speed is not sufficient to separate the two surfaces completely.
  • Hydrodynamic Lubrication occurs when the speed is high enough to separate the two moving surfaces completely and friction is at its lowest.

In machining operations, the traverse table stops at the end of the slideway and starts to move in the opposite direction. When the table stops, static friction (the worst form of boundary lubrication) occurs.  As the table speed increases, friction changes from static to dynamic. This fluctuation between static and dynamic friction results in a jerky movement which is commonly referred to as stick-slip. In simple terms stick-slip can be described as surfaces alternating between sticking to each other and sliding over one another. Static (stationary) friction between two surfaces is greater than dynamic (moving) friction. If the applied force is large enough to overcome the static friction, the reduction in friction to the dynamic state can cause a sudden increase in the velocity of the movement resulting in the jolting action. Stick-slip can also occur at low feed speeds and high loads. Since stick-slip is a recurring event, it may be perceived as a harmonic vibration or noise.

While it may not always be visible to the human eye, stick-slip effects are a frequent phenomenon in everyday life and it produces a range of audible incidents, e.g. when a chair is pushed along the floor its legs begin to vibrate with an irritating noise. Other examples of stick-slip motion are the sound produced by a wine glass when a wet finger is moved along its rim and the jerky motion of vehicle windshield wipers. Stick-slip, however, is not always a bad phenomenon. It is responsible for the rich sounds when a bow is moved over the strings of a violin. In machining operations, stick-slip is an undesirable occurrence that causes the transverse table and workpiece to shudder resulting in inaccuracies of the machined surfaces.

Slideway lubricants are therefore formulated with special friction modifying additives to control stick-slip and chatter under all operating conditions including:

  • Static friction situations during start-up.
  • Continuous transition from rest to movement.
  • Slow speed heavy load applications.

In addition, high-performance slideway lubricants must contain additives to provide good antiwear and extreme pressure performance, tackiness for slideway adhesion, oxidation stability, as well as rust and corrosion protection for slides and ways. Slideway oils must also have good compatibility with cutting oils and other machine tool lubricants and adequate demulsibility to separate from cutting fluid emulsions.

The most common slideway lubricant viscosity grades are ISO 68 and ISO 220 with the following application guidelines:

ISO 68: Horizontal slideways and light to moderate applications.

ISO 220: Vertical slideways and more severe applications.

Slideway oils in the appropriate viscosity grade are also recommended for hydraulic systems subject to stick-slip service (ISO 11158 Type HG), heavily loaded gear systems and other industrial applications requiring an adhesive, corrosion inhibited lubricant with EP properties.

If you have any questions concerning slideway lubrication our experts are at your disposal and ready to provide you with advice and guidance. Simply mail us at


Metalworking Fluid Management OilChat#38

Manufacturers with metalworking operations and machine shops use and dispose of a substantial amount of metalworking fluid each year. These operations have the potential to extend metalworking fluid life. Prolonging the life of the metalworking fluid and optimizing its performance are very dependent on the control of the metalworking fluid system. This control is as important as the selection of the proper fluid (please refer to OilChat #37).


Regardless of the fluid type and application, all metalworking fluids require some form of management. Neat oils are relatively easy to maintain, but they do require some management. Straight oils should be filtered on a regular basis to remove metal fines and other contaminants to provide a long service life, improved cutting performance and a high level of surface finish. The majority of cutting and grinding fluids in use today, however, are water soluble. These fluids, on the other hand, differ from straight oils because they require a higher degree of maintenance to provide extended periods of satisfactory cutting performance, bio-stability, and longevity.metal working fluid

When a soluble metalworking fluid is mixed with water, a new level of potential problems is presented. The coolant sump is an excellent breeding space for bacteria, fungi, yeasts, and moulds because it is dark, humid and provides an excellent nutrient source (the fluid itself) for bacteria to thrive on as shown on the right. If you are familiar with metalworking facilities you have probably encountered a variety of unpleasant odours. You must have noticed that “rotten egg” or “Monday morning” smell (bacteria) when metalworking fluids have been allowed to settle over the weekend.

The majority of cutting and grinding fluids in use today are water soluble. Over time, these fluids can become rancid or contaminated with microbiological growth. With use, fluids lose their rust control capabilities, as well as their anti-foam characteristics. During normal fluid use, evaporation of water increases the concentration of the metalworking fluid. In addition, the fluids contain the chips and “fines” from the machining operation. During use, the cutting fluid collects hydraulic oil and other lubricants from the machine tool. This oil, called tramp oil, contributes to the growth of bacteria. These micro-organisms smell like rotten eggs and shorten fluid life. The fluid is disposed of once its efficiency is lost. Good fluid management practices can go a long way towards solving fluid problems and making the most cost-effective use of metalworking fluids.

Monitoring and maintaining fluid quality are crucial elements of a successful fluid management program. Important aspects of fluid monitoring include system inspections and periodic measurements of fluid parameters, such as concentration, biological growth, and pH. Changes in optimal fluid quality must be corrected with appropriate adjustments (such as fluid concentration adjustments, biocide addition, tramp oil, and metal cuttings removal and pH adjustment). It is important to know what changes are taking place in your system and why they occur. This allows you to take the appropriate steps needed to bring fluid quality back in line and prevent fluid problems from recurring.

Many of the contaminants that cause fluids to be disposed of prematurely are foreign materials, such as floor sweepings, cleaners, solvents, dirt, tobacco, food, etc. If improved fluid life is a goal, it must start with education and revised shop practices. The first step in fluid control is improved housekeeping and sanitation. Only then control of natural metalworking fluid contaminants, such as chips, fines, tramp oil, and bacteria will be effective in improving fluid life.


The link below provides more information and advice on how to manage water miscible and neat metal-working fluids at every stage from ‘’cradle to grave”. It aims to give a broader understanding of cutting fluid management and provides practical advice to get the best results from metalworking fluids.


Metal Working Fluids OilChat#37

Metalworking is a collective name for a variety of machining processes whereby metal is brought to a specified geometry by removing excess material by means of various kinds of cutting and grinding operations. The net result of metalworking is two products: the finished workpiece and waste. Depending on the machining operation, the waste can be metal swarf (small gritty chips or filings), shavings, turnings or stringy tendrils.

Enormous amounts of friction and heat are generated at the cutting interface between the cutting tool and workpiece during the metal removing process. Metalworking fluid (MWF) is used to reduce friction and heat during the machining operation. MFW must also improve workpiece quality, reduce cutting tip wear, remove swarf, improve process productivity and protect the workpiece and machine tools against rust and corrosion. The MWF is generally applied by a spray across the face of the tool and workpiece as shown in the milling operation

Most MWFs presently in use fall into one of the following two categories:

Neat Metalworking Fluids – also referred to as cutting oils. These are non-emulsifiable fluids and are used in machining operations in undiluted form. They are composed of base oils and normally contain polar compounds such as esters and fatty acids (corrosion inhibitors and lubricity agents), as well as extreme pressure (EP) additives. Typical EP additives are Chlorine, Phosphorus, and Sulphur. Neat oils provide the best lubrication and are most effective at reducing friction.

Soluble Metal Working Fluids – often called emulsifiable cutting fluids because they form an emulsion when mixed with water. The concentrate consists of base oil (mineral, synthetic or semisynthetic) and emulsifiers to produce stable emulsions when mixed with water. In addition, typical soluble MWFs formulations include a selection of the following additives: EP agents, rust and corrosion inhibitors, coupling agents, biocides, antifoam agents, scents, and dyes.

Synthetic based soluble MWFs provide the best performance as far as cooling, tool life and resistance to bacterial growth (increased sump life) is concerned. In some metalworking operations, workpiece visibility is important. Synthetic MWFs form clear transparent solutions, whilst mineral and semisynthetic formulations form milky (see photo above) to semi-transparent emulsions.

Soluble MWFs are always used in diluted form, generally in 3% to 10% concentrations. Soluble grinding fluids may be used in concentrations as low as 1%. Emulsifiable MWFs provide the best cooling and heat transfer performance. Consequently, water-soluble coolants have become vital in achieving the higher feeds and speeds required to ensure maximum production efficiency. They are widely used in industry and are the least expensive among all cutting fluids.

There are various issues to consider when selecting an MFW. These are the metals to be machined, the machining operations, machine types, tooling requirements, downstream plant processes and finally chemical and environmental restrictions. Discussions in this newsletter will be restricted to the two most significant aspects:



Some metals are more difficult to machine than others. Stainless steel, complex alloys, and very hard metals demand a very high level of performance from the cutting oil. Other metals, like brass and aluminium, are easy to machine with general purpose oils. Where tough, difficult to machine metals are involved, highly additized cutting oils with excellent EP properties and anti-weld capability are required. Quite often these oils contain active sulphur and chlorine to protect the cutting tool and to ensure good workpiece finish. For brass, aluminium, many carbon steels and low-alloy steels, cutting oils with lubricity additives, and mild EP/anti-weld performance are sufficient. These oils are generally formulated with inactive sulfurized fat and/or chlorinated paraffin. Cutting oils formulated with active sulphur should not be used for brass and aluminium, as they will stain or tarnish the finished parts. Oils formulated for brass and aluminium are often called “non-staining” oils.


Machining Operations

Following is a list of the most common machining operations in order of increasing severity:


  • Sawing
  • Turning
  • Milling
  • Drilling
  • Grinding
  • Reaming
  • Honing
  • Gear Hobbing and Shaping
  • Tapping and Threading
  • Broaching

Easy machining operations (turning, milling, drilling, etc.) can be performed at higher speeds and require high levels of cooling with only modest EP capability. Soluble MWFs are generally used for milder operations. When a neat cutting oil is preferred for easy machining operations for whatever reason, the operations can be performed with lower viscosity, lightly additized fluids.

Difficult machining operations must be run at lower speeds and require a great deal of anti-weld protection. Oils designed specifically for the most severe operations, like thread cutting or broaching, are generally higher in viscosity and loaded with EP additives, like active sulphur and chlorine.

Although this brief discussion of metalworking fluid selection criteria demonstrates the complexity to select the proper cutting fluid, there is light at the end of the tunnel. MWF product data sheets (PDS) will normally indicate for what metals and machining operations the particular product is suitable. For soluble oils, the PDS will also give an indication of what mixing ratios should be used for the various machining operations. If you are still in doubt our experts are at your disposal and ready to provide you with advice and answer any questions you may have. For more information simply mail us at