How to Select the Right Hydraulic Oil

by Stephen Sumerlin, Noria Corporation

How do you know if you're using the right hydraulic oil? For most lubricated machines, there are plenty of options when it comes to lubricant selection. Just because a machine will run with a particular product doesn't mean that product is optimal for the application. Most lubricant mis-specifications don't lead to sudden and catastrophic failure, but rather they shorten the average life of the lubricated components and, thus, go unnoticed.

With hydraulics, there are two primary considerations - the viscosity grade and the hydraulic oil type. These specifications are typically determined by the type of hydraulic pump employed in the system, operating temperature and the system's operating pressure. But it doesn't stop there. Other items for consideration are: base oil type, overall lubricant quality and performance properties. A system's requirements for these items can vary dramatically based on the operating environment, the type of machine for which the unit is employed and many other variables.

Selecting the best product for your system requires that you collect and utilize all available information.

Pumps and Viscosity Requirements
Let me start by outlining the No. 1 lubricant selection criteria: pump design types and their required viscosity grades. There are three major design types of pumps used in hydraulic systems: vane, piston and gear (internal and external). Each of these pump designs are deployed for a certain performance task and operation. Each pump type must be treated on a case-by-case basis for lubricant selection.

Vane: The design of a vane pump is exactly what its name depicts. Inside the pump, there are rotors with slots mounted to a shaft that is spinning eccentrically to a cam ring. As the rotors and vanes spin within the cam ring, the vanes become worn due to the internal contact between the two contacting surfaces. For this reason, these pumps are typically more expensive to maintain, but they are very good at maintaining steady flow. Vane pumps typically require a viscosity range of 14 to 160 centistokes (cSt) at operating temperatures.

Piston: Piston pumps are your typical, middle-of-the-road hydraulic pump, and are more durable in design and operation than a vane pump. They can produce much higher operating pressures - up to 6,000 psi. The typical viscosity range for piston pumps is 10 to 160 cSt at operating temperatures.

Gear: Gear pumps are typically the most inefficient of the three pump types, but are more agreeable with larger amounts of contamination. Gear pumps operate by pressurizing the fluid between the meshing teeth of a gear set and then expelling that fluid. The two main types of gear pumps are internal and external.

Internal gear pumps offer a wide range of viscosity choices, the highest of which can be up to 2,200 cSt. This pump type offers good efficiency and quiet operation, and can produce pressures from 3,000 to 3,500 psi.

External gear pumps are less efficient than their counterpart, but have some advantages. They offer ease of maintenance, steady flow, and are less expensive to buy and repair. As with the internal gear pump, these pumps can produce pressures ranging from 3,000 to 3,500 psi, but their viscosity range is limited to 300 cSt.

Fluid's Roles and Makeup
Hydraulic fluid has many roles in the smooth operation of a well-balanced and designed system. These roles range from a heat transfer medium, power transfer medium and a lubrication medium. The chemical makeup of a hydraulic fluid can take many forms when selecting it for specific applications. It can range from full synthetic (to handle drastic temperature and pressure swings and reduce the rate of oxidation) to water-based fluids used in applications where there is a risk of fire and are desired for their high water content.

A full synthetic fluid is a man-made chain of molecules that are precisely arranged to provide excellent fluid stability, lubricity and other performance-enhancing characteristics. These fluids are great choices where high or low temperatures are present and/or high pressures are required. There are some disadvantages to these fluids, including: high cost, toxicity and potential incompatibility with certain seal materials.

A petroleum fluid is a more common fluid, and is made by refining crude to a desired level to achieve better lubricant performance with the addition of additives, which range from anti-wear (AW), rust and oxidation inhibitors (RO), and viscosity index (VI) improvers. These fluids offer a lower cost alternative to synthetics and can be very comparable in performance when certain additive packages are included.

Water-based fluids are the least common of the fluid types. These fluids are typically needed where there is a high probability of fire. They are more expensive than petroleum but less expensive than synthetics. While they offer good fire protection, they do lack wear-protection abilities.

Application-Based Selection
Application should be the most critical attribute when selecting a hydraulic fluid in order to ensure the system's ability to function properly and attain long life. When selecting a hydraulic fluid, it is very critical to determine the system's needs: viscosity, additives, operation, etc.

For example, take a large dump truck that is constantly in the rain, encounters high particle contamination from road debris and leaks 10 percent of its sump volume in two days. There is no need to buy or use the most expensive fluid with the best additive package simply because of the associated cost of replenishment and the inherent lack of maintenance. On the other hand, you have a very clean, critical and highly loaded system that is maintained properly and used to its full potential. You may want to use a more premium product, such as a highly refined petroleum-based fluid with an AW or RO additive package or even a full synthetic fluid.

As far as the viscosity of the fluid is concerned, this should be determined by the pump type as previously discussed. Not having the correct viscosity for the application will dramatically reduce the average life of the pump and system, thereby directly reducing its reliability and production. When selecting the appropriate viscosity grade, look for the optimum viscosity required by the pump. This can be determined by collecting data from the pump OEM, actual operating temperature of the pump, and the lubricant properties referenced to the ISO grading system at 40 and 100 degrees Celsius.

Check the operating temperature of the pump and see if it falls between the temperature ranges of the lubricant in question. If not, you may need to increase or decrease the viscosity of the lubricant to achieve the desired, optimum viscosity.

As you can see, selecting the proper hydraulic fluid for the application is not a hard task, but it does require time to research the application, determine the resulting cost and decide which fluid type is best.

You can spend more or less money than is needed simply by not educating yourself on proper lubricant selection techniques. To practice good lubricant selection is to practice great machine performance!

Source: http://www.machinerylubrication.com/article_detail.asp?articleid=2351

How Water Causes Bearing Failure

by Jim Fitch

Most of us who have spent time in the lubrication field have been told that it takes only a small amount of water (less than 500 ppm) to substantially shorten the service life of rolling element bearings. There is indeed a vast amount of research that supports these assertions. Being a career-long crusader of clean and dry oil, I will certainly not argue the contrary. In fact, water's destructive effects on bearings can easily reach or exceed that of particle contamination, depending on the conditions.

My theme for this column, therefore, is not about whether water imparts harm but rather how it does. Knowing how water attacks and causes damage helps in setting important dryness targets and also aids failure investigations post mortem. Further, when water contamination is unavoidable, understanding these water-induced failure modes can be valuable in the optimum selection of lubricants, bearings and seals for defensive purposes.

The Scourge of our Machines
There is no contaminant more complex, intense and confounding than water. The reasons are still being studied, but they include its various states of co-existence with the oil and its many chemical and physical transformations imparted during service. Individually and collectively, moisture-induced problems exact damage on both the oil and machine and can certainly lead, either slowly or abruptly, to operational failure of the bearing. Do not underestimate the attack potential of water.

Water can damage machine surfaces directly, through a sequence of events and often with a variety of helpers. In many cases, the most severe damage is the cascading or chain reaction failure. For instance, water may lead first to premature oxidation of the base oil. When the oxides combine with more water, a corrosive acidic fluid environment exists.

Likewise, oxidation can throw-off sludgy insolubles and increase oil viscosity. Both processes can impede oil flow and lead to damage of the bearing. Not to be left out, the water and oxidative environment can hang up air in the oil, amplifying lubrication problems even further. It's often true that the worse things get, the faster they get worse; all started by water.

Failure Modalities
In order to keep this column to a manageable length and scope, the modalities described below will be brief and to the point. I've left out those that are farfetched or technically abstract, as well as a couple rooted more in popular lore than scientific fact. There are even some failure modes on my list that are largely derived from conjecture, but still believable. Finally, I've made no effort to rank the failure modes in terms of severity or commonality. My list:

Hydrogen-induced Fractures. Often called hydrogen embrittlement or blistering, this failure mode is perhaps more acute and prevalent than most tribologists and bearing manufacturers are aware. The sources of the hydrogen can be water, but also electrolysis and corrosion (aided by water). There is evidence that water is attracted to microscopic fatigue cracks in balls and rollers by capillary forces. Once in contact with the free metal within the fissure, the water breaks down and liberates atomic hydrogen. This causes further crack propagation and fracture. High tensile-strength steels are at greatest risk. Sulfur from additives (extreme pressure (EP), antiwear (AW), etc.), mineral oils and environmental hydrogen sulfide may accelerate the progress of the facture. Risk is posed by both soluble and free water.

Corrosion. Rust requires water. Even soluble water can contribute to rust formation. Water gives acids their greatest corrosive potential. Etched and pitted surfaces from corrosion on bearing raceways and rolling elements disrupt the formation of critical elastohydrodynamic (EHD) oil films that give bearing lubricants film strength to control contact fatigue and wear. Static etching and fretting are also accelerated by free water.

Oxidation. Many bearings have only a limited volume of lubricant and, therefore, just a scintilla of antioxidant. High temperatures flanked by metal particles and water can consume the antioxidants rapidly and rid the lubricant from the needed oxidative protective environment. The negative consequences of oil oxidation are numerous but include corrosion, sludge, varnish and impaired oil flow.

Additive Depletion. We've mentioned that water aids in the depletion of antioxidants, but it also cripples or diminishes the performance of a host of other additives. These include AW, EP, rust inhibitors, dispersants, detergents and demulsifying agents. Water can hydrolyze some additives, agglomerate others or simply wash them out of the working fluid into puddles on sump floors. Sulfur-phosphorous EP additives in the presence of water can transform into sulfuric and phosphoric acids, increasing an oil's acid number (AN).

Oil Flow Restrictions. Water is highly polar, and as such, has the interesting ability to mop up oil impurities that are also polar (oxides, dead additives, particles, carbon fines and resin, for instance) to form sludge balls and emulsions. These amorphous suspensions can enter critical oil ways, glands and orifices that feed bearings of lubricating oil. When the sludge impedes oil flow, the bearing suffers a starvation condition and failure is imminent. Additionally, filters are short-lived in oil systems loaded with suspended sludge. In subfreezing conditions, free water can form ice crystals which can interfere with oil flow as well.

Aeration and Foam. Water lowers an oil's interfacial tension (IFT), which can cripple its air-handling ability, leading to aeration and foam. It takes only about 1,000 ppm water to turn your bearing sump into a bubble bath. Air can weaken oil films, increase heat, induce oxidation, cause cavitation and interfere with oil flow; all catastrophic to the bearing. Aeration and foam can also incapacitate the effectiveness of oil slingers/flingers, ring oilers and collar oilers.

Impaired Film Strength. Rolling element bearings depend on an oil's viscosity to create a critical clearance under load. If the loads are too great, speeds are too low or the viscosity is too thin, then the fatigue life of the bearing is shortened. When small globules of water are pulled into the load zone the clearance is often lost, resulting in bumping or rubbing of the opposing surfaces (rolling element and raceway). Lubricants normally get stiff under load (referred to as their pressure-viscosity coefficient) which is needed to bear the working load (often greater than 500,000 psi).

However, water's viscosity is only one centistoke and this viscosity remains virtually unchanged, regardless of the load exerted. It is not good at bearing high-pressure loads. This results in collapsed film strength followed by fatigue cracks, pits and spalls. Water can also flash or explode into superheated steam in bearing load zones, which can sharply disrupt oil films and potentially fracture surfaces.

Microbial Contamination. Water is a known promoter of microorganisms such as fungi and bacteria. Over time, these can form thick biomass suspensions that can plug filters and interfere with oil flow. Microbial contamination is also corrosive.

Water Washing. When grease is contaminated with water, it can soften and flow out of the bearing. Water sprays can also wash the grease directly from the bearing, depending on the grease thickener and conditions.

The obvious solution to the water problem is a proactive solution; that is, preventing the intrusion of water into the oil/grease and bearing environment. The only water that doesn't cause harm is the water that doesn't invade your system. Contaminant exclusion tactics are always a wise maintenance investment.

Be a long-term thinker by controlling risk factors today, while the bearing still has remaining useful life (RUL). The cost of removing water and/or remediating the damage it causes will far exceed any investment to exclude it from entry. So please, don't skimp when it comes to "proactive" contamination control.

Source: http://www.machinerylubrication.com/article_detail.asp?articleid=1367

Advice For Maintaining Hydraulic Accumulators

by Brendan Casey

Gas-charged accumulators are ubiquitous on modern hydraulic systems. They carry out numerous functions, which include energy storage and reserve, leakage and thermal compensation, shock absorption, and energy recovery.

While accumulators present a number of advantages in hydraulic system operation and can provide many years of trouble-free service, they are a maintenance item.

For example, the correct gas pre-charge pressure must be maintained for proper functioning and optimum service life. Also, periodic inspection, testing and certification can be required by law - accumulators are pressure vessels after all.

Accumulator Types
The three types of gas-charged accumulators you'll encounter on hydraulic systems are bladder, piston and diaphragm.

The most popular of these is the bladder type. Bladder accumulators feature fast response (less than 25 milliseconds), a maximum gas compression ratio of around 4:1 and a maximum flow rate of 15 liters (4 gallons) per second, although "high-flow" versions up to 38 liters (10 gallons) per second are available. Bladder accumulators also have good dirt tolerance; they are mostly unaffected by particle contamination in the hydraulic fluid.

Piston accumulators, on the other hand, can handle much higher gas compression ratios (up to 10:1) and flow rates as high as 215 liters (57 gallons) per second. Unlike bladder accumulators, whose preferred mounting position is vertical to prevent the possibility of fluid getting trapped between the bladder and the shell, piston accumulators can be mounted in any position.

But, piston accumulators also require a higher level of fluid cleanliness than bladder units, have slower response times (greater than 25 milliseconds) - especially at lower pressures - and exhibit hysteresis. This is explained by the static friction of the piston seal which has to be overcome, and the necessary acceleration and deceleration of the piston mass.

Diaphragm accumulators have most of the advantages of bladder-type units but can handle gas compression ratios up to 8:1. They are limited to smaller volumes, and their performance can sometimes be affected by gas permeation across the diaphragm.

Maintenance Considerations
When charging the gas end of a bladder or diaphragm accumulator, the nitrogen gas should always be admitted very slowly. If the high-pressure nitrogen is allowed to expand rapidly as it enters the bladder, it can chill the bladder's polymeric material to the point where immediate brittle failure occurs. Rapid pre-charging can also force the bladder underneath the poppet at the oil-end, causing it to be cut. If pre-charge pressure is too high or minimum system pressure is reduced without a corresponding reduction in pre-charge pressure, the operation of the accumulator will be affected and damage may also result.

Excessive pre-charge of a bladder accumulator can drive the bladder into the poppet assembly during discharge, causing damage to the poppet assembly and/or the bladder. This is a common cause of bladder failure.

                                                        Diaphragm Accumulator

Low or no pre-charge also can have drastic consequences for bladder accumulators. It can result in the bladder being crushed into the top of the shell by system pressure. This can cause the bladder to extrude into or be punctured by the gas valve. In this scenario, only one such cycle is required to destroy the bladder.

Similarly, excessively high or low pre-charge of a piston accumulator can cause the piston to bottom out at the end of its stroke, resulting in damage to the piston and its seal. The good news is that, if this happens, an audible warning will result. Even though piston accumulators can be damaged by improper charging, they are much more tolerant of it than bladder accumulators.

Held to Standards
Accumulators are pressure vessels and as such are manufactured, tested and certified according to statutory standards. In the United States, for example, the relevant standard is the ASME Boiler & Pressure Vessel Code VIII, Division 1.

All pressure vessels manufactured to these standards are considered to have a finite service life depending on the number of pressure cycles experienced during normal operation. The typical design life for a hydraulic accumulator is 12 years.

In many jurisdictions, periodic inspection and recertification is required. This particularly applies to hydraulic accumulators which have relatively large volumes and operate at high working pressures. Inspection may be required at predetermined intervals (i.e. every two, five or 10 years) or when a certain percentage of usable design life is deemed to have been reached.

Depending on the volume and pressure rating of the accumulator, recertification may involve one or more of the following: visual inspection, ultrasonic thickness testing and/or hydrostatic pressure testing.

You're Responsible
So if you're responsible for hydraulic equipment which incorporates an accumulator, familiarize yourself with the relevant regulations that apply in your locale.

And along with every other component on your hydraulic machines, it's your responsibility to make sure all accumulators are properly maintained and safe to use

Please reference this article as:
Brendan Casey, "Advice For Maintaining Hydraulic Accumulators". Machinery Lubrication Magazine. July 2009

Source: http://www.machinerylubrication.com/article_detail.asp?articleid=2305

A Condensed Guide to Compressor Lubricants

by Jeremy Wright, Noria Corporation

Nearly all compressors require a form of lubricant to either cool, seal or lubricate internal components. Only static jet compressors (ejectors) and late 20th- and early 21st-century oil-free machines with rotors suspended in magnetic or air bearings are exempt from the need for some type of lubrication. This article deals with the lubrication of dynamic, reciprocating and rotary compressors. These three types are the most prevalent in industry today.

Compressors are manufactured in several types and for a variety of purposes. In addition to being used to compress gas, many compressors serve as blowers or can be used as vacuum pumps. Lubrication requirements vary considerably, depending not only on the type of compressor but also the gas which is being compressed. In general, air and gas compressors are mechanically similar. Thus, the main difference is in the effect of the gas on the lubricant and the compressor components. The lubricant plays a role in preventing wear, sealing, minimizing reactions with the gas, and preventing corrosion. Refrigeration and air conditioning compressors require special consideration because of the recirculation of the refrigerant and mixing of the lubricant with that refrigerant.

Minimum Requirements

To combat all these stressors, a compressor lubricant needs several defenses. Oxidation and thermal stability are very important - along with corrosion inhibitors, detergents, demulsifying agents and foam suppressors - to increase the life of the machinery.

As mentioned, not only does compressor design have an effect on lubricant selection, so does the gas being compressed. The following are the four popular categories of compressed gases: air, hydrocarbon, chemically active and chemically inert. Dependent upon these categories, special considerations must be made when selecting the proper lubricant. At the very least, all should have the following:

• Long life without the need for changeout (high oxidation stability)
• Prevention of acidity, sludge and deposit formation
• Excellent protection against rust and corrosion, even during shutdown
• Good demulsibility to shed water that enters the lubrication system
• Easy filterability without additive depletion
• Good foam control

Three Synthetic Options
Because of the system stressors and quality needed to combat them, synthetic lubricants are preferred. The three popular synthetics are polyalkylene glycol (PAG), esters (diester and polyolester) and polyalphaolefin (PAO).

 PAGs offer the ability to dissolve sludge and deposits, burn without leaving residues when degraded, have a low solubility with hydrocarbons, and have good hydrolytic stability. Their downfall is that they absorb vast amounts of water and have very poor compatibility with mineral and PAO oils.

Esters have a very high detergency and solvency, making them excellent at dissolving sludge and deposits. They are compatible with mineral and PAO oils, and most seal and gasket materials. However, they also absorb moisture from the air and are hydrolytically unstable at high temperatures.

PAOs are most similar to minerals, so they are very compatible. They also are generally compatible with seals and gasket materials. They offer excellent hydrolytic stability, low water solubility and poor solvency. Because of this poor solvency, they should be avoided in applications where high discharge temperatures are present. They have a tendency to form deposits in these situations.

The Art of Selection

There are many variables when selecting a compressor lubricant. Even though I only skimmed the surface of these variables, I hope you now have an appreciation of the art of lubricant selection for compressors. It is not as easy as simply selecting the product denoted as "compressor lubricant".

Source
Bloch, Heinz “Compressor Lubrication Best Practices”. Machinery Lubrication magazine. May 2003.

http://www.machinerylubrication.com/article_detail.asp?articleid=2314

ISO Viscosity Grades

Through the years, lubricant users have been treated to a number of ways to designate viscosity grades of the lubricants used in manufacturing. There are SAE (Society of Automotive Engineers) grades for gear oils and crankcases (engines), AGMA (American Gear Manufacturers Association) grades for gear oils, SUS (Saybolt Universal Seconds), cSt (kinematic viscosity in centistokes), and absolute viscosity. To add to the confusion, two measures of temperature (Fahrenheit and Celsius) can be applied to most of these, not to mention that viscosity might be presented at either 40°C (104°F) or 100°C (212°F).
While all of these have served useful purposes to one degree or another, most lubrication practitioners settle on and use one method as a basis for selecting products. To the new entrant into the lubrication field, the number of options can be confusing, particularly if the primary lubricant supplier does not associate one of the prominent viscosity systems to the product label. To complicate matters, machinery designers must define the lubricant viscosity in such a way that the equipment user understands clearly what is needed without having to consult outside advice.

This points to the need for a universally accepted viscosity designation - one that can be used by lubrication practitioners, lubricant suppliers and machinery design engineers simultaneously with minimal confusion.

In 1975, the International Standards Organization (ISO), in unison with American Society for Testing and Materials (ASTM), Society for Tribologists and Lubrication Engineers (STLE), British Standards Institute (BSI), and Deutsches Institute for Normung (DIN) settled upon an approach to minimize the confusion. It is known as the International Standards Organization Viscosity Grade, ISO VG for short.

You don’t have to listen very long in this field before someone says that viscosity is the most important physical property of a fluid when determining lubrication requirements. So, what is viscosity?

Viscosity is the measure of the oil’s resistance to flow (shear stress) under certain conditions. To simplify, the oil’s viscosity represents the measure for which the oil wants to stay put when pushed (sheared) by moving mechanical components.

Think of a water-skier cutting through the water. Water has a viscosity measured in centistokes of 1. That is at the bottom of the cSt scale. We can see how much water a professional skier displaces when he runs through a ski course. If the skier was skiing on a lake of SAE 90/ISO 220 gear oil and all of the other conditions were exactly the same, then the amount of spray generated would be considerably less because the fluid would resist the force of the ski to a much greater degree.

There are two viewpoints of the resistance to flow that the machine designer is interested in. One is the measure of how the fluid behaves under pressure, such as a pressurized hydraulic line. This property is called absolute viscosity (also known as dynamic viscosity) and is measured in centipoises (cP). The other consideration is how the fluid behaves only under the force of gravity. This is called centistokes, which we have already noted. The two are related through the specific gravity of the fluid. To determine the centipoise of a fluid it is necessary to multiply the viscosity of the fluid times the specific gravity of the fluid, or measure it directly using an absolute viscometer. For the practitioner of industrial lubrication, the centistoke is the measure that will occupy most of our attention.

On a side note, if you are using in-service oils, it is probably worth measuring the viscosity in absolute units. The measure in centistokes can be misleading because the specific gravity of lubricants changes with age, generally moving up. It is possible to find yourself exceeding an absolute viscosity limit for a machine but still have a kinematic measure that indicates you are OK.

So, viscosity is a measure of the fluid’s resistance to flow. Water has a low viscosity of 1 cSt and honey has a very high viscosity, lets say 1,000 cSt. If a machine is heavily loaded then the machine designer will use a lubricant that resists being pushed around, which would be heavy like honey. If the machine runs very fast then the machine designer will specify a lubricant that can get out of the way, and back into the way just as quickly. Generally, machines will have either one or the other to be concerned about; sometimes both at the same time.

Viscosities are defined or assigned using a laboratory device called a viscometer. For lubricating oils, viscometers tend to operate by gravity rather than pressure. Think of a kinematic viscometer as a long glass tube that holds a volume of oil. The measure of the fluid’s viscosity is the measure of the amount of time that it takes for the designated amount of oil to flow through the tube under very specific conditions. Because the conditions are repeatable, it is now possible to measure the amount of time that it takes for the fluid to flow through the tube, and it should be nearly the same each time. This is similar to the amount of time it takes a specific volume of fluid at a specific temperature to drain through a funnel. As the fluid gets thicker - a function of its increasing resistance to flow - then it takes progressively longer to move through the tube (funnel). Water goes through in one second. The same amount of honey takes a thousand seconds (hypothetically).

We know that if we raise and lower the temperature of a fluid, there is often a correlating change in the fluid’s resistance to flow. The fluid gets thicker at lower temperatures and it gets thinner at higher temperatures.

Given all of these variables and details, several organizations decided to come up with a way to characterize lubricating oils so that members of their respective organizations would have a uniform and simple way to communicate, educate and ultimately protect their interests.

The purpose of the ISO system of classifying viscosity grades is to establish a viscosity measurement method so that lubricant suppliers, equipment designers and users will have a common (standardized) basis for designating or selecting industrial liquid lubricants.

Different approaches were thoroughly considered before the ISO Technical Committee (TC23) settled on an approach that is logical and easy to use. There were a few important criteria to keep in mind from the beginning, such as:

Referencing the lubricants at a nominal temperature for industrial systems.

Using a pattern that conforms to uncertainties imposed by dimensional manufacturing tolerances.

Using a pattern that had some sense of repeatability up and down the scale.

Using a pattern that used a small, easily manageable number of viscosity grades.

The reference temperature for the classification should be reasonably close to average industrial service experience. It should also relate closely to other selected temperatures used to define properties such as viscosity index (VI), which can aid in defining a lubricant. A study of possible temperatures indicated that 40ºC (104ºF) was suitable for the industrial-lubricant classification as well as for the lubricant-definition properties mentioned above. This ISO viscosity classification is consequently based on kinematic viscosity at 40ºC (104ºF).

For the classification to be used directly in engineering design calculations in which the kinematic viscosity of the lubricant is only one of the parameters, it was necessary that the viscosity grade width (range of tolerance) be no more than 10 percent on either side of the nominal value. This would reflect an order of (center point) uncertainty in calculations similar to that imposed by dimensional manufacturing tolerances. This limitation, coupled with the requirement that the number of viscosity grades should not be too large, led to the adoption of a system with gaps between the viscosity grades.

This classification defines 20 viscosity grades in the range of 2 to 3200 square millimeters per second (1 mm2/s = equals 1 cSt) at 40ºC (104ºF). For petroleum-based liquids, this covers approximately the range from kerosene to cylinder oils.

Each viscosity grade is designated by the nearest whole number to its midpoint kinematic viscosity in mm2/s at 40ºC (104ºF), and a range of +/- 10 percent of this value is permitted. The 20 viscosity grades with the limits appropriate to each are listed in Table 1.

The classification is based on the principle that the midpoint (nominal) kinematic viscosity of each grade should be approximately 50 percent greater than that of the preceding one. The division of each decade into six equal logarithmic steps provides such a system and permits a uniform progression from decade to decade. The logarithmic series has been rounded off for the sake of simplicity. Even so, the maximum deviation for the midpoint viscosities from the logarithmic series is 2.2 percent.

Table 2 pulls together some popular viscosity measurement methods into one table. If the practitioner is comfortable with one particular measure but would like to see the correlating viscosity range in another measure, all he must to do is place a straight horizontal line through his chosen viscosity type and see its correlation within the other types of measures.

While it is true that some viscosity grades will be left out of the mix as companies move toward adopting the ISO designation, it is not necessary that the users of those products have to move away from them. Further, there is no intention to offer quality definition of lubricants with this scale. That a product has an ISO VG number associated with it has no bearing on its performance characteristics.

The ISO designation has been under development since 1975. The most recent release in 1992 (ISO 3448) contains 20 gradients. This covers nearly every type of application that the lubricant practitioner can expect to encounter. The lubricant manufacturing community has accepted the recommended ISO gradients and has devoted appreciable effort and energy to conform to the new grading approach with old and new products.

It is unlikely that all of us who learned about the use of oil from our mentors or friends under the hood of a car will ever abandon the SAE grading system. We don’t have to. At least for automotive oils, we can expect to continue to see the 10- 20- 30- 40- 50- values used. It is likely, however, that in the industrial lubrication world there will be more ISO dependence in the future.

Please reference this article as:
Noria Corporation, "ISO Viscosity Grades". Machinery Lubrication Magazine. July 2001

Source: http://www.machinerylubrication.com/article_detail.asp?articleid=213