What is ISO VG?

ISO VG is the number that tells you how thick an industrial oil is: VG stands for viscosity grade, and ISO is the international organization that publishes the standard behind the scale. The number gives the oil's viscosity, its resistance to flowing, measured at 40 °C: the higher the number, the thicker the oil; an ISO VG 68 is thicker than a 46 and thinner than a 100. The number does not say what the oil is for; the right grade comes from the machine's manual.

What does the ISO VG number measure?

The number measures the oil's viscosity, that is, how much it resists flowing. Picture two identical funnels, one with honey and the other with water: the water runs through fast, and the honey takes its time. That resistance to flowing is viscosity.

In the laboratory, the comparison is made with precision. The oil is heated to 40 °C and flows, under its own weight, through a thin glass tube. The time it takes to pass becomes a number, in a unit called centistokes (cSt). An ISO VG 68 oil measures close to 68 cSt. The article on the difference between 46 and 68 oils shows this test step by step, in the figure with the two glass tubes.

The scale comes from the ISO 3448 standard and applies to industrial oils. The main ones are:

• hydraulic: the oil that transmits force in a press or a cylinder, pushed by a pump;

• machine slideway: the metal track along which the table of a machine tool slides (a machine that cuts and shapes metal parts, such as a lathe or a milling machine), also called the way;

• bearing: the support where the shaft turns;

• gear, compressor, turbine and circulating oils.

Insulating transformer oil is also included.

Figure 1. The ISO VG 68 label in three parts: ISO is the organization behind the ISO 3448 standard, VG means viscosity grade, and 68 says the oil measures about 68 cSt at 40 °C.

For those who need the numbers:

1. 1 cSt equals 1 mm²/s; water at 20 °C has about 1 cSt.

2. In the glass tube (a capillary viscometer, under ASTM D445 and ISO 3104), viscosity is the tube constant times the time: in the same tube, twice the time means twice the viscosity. ASTM is the American body that publishes standards for testing and materials.

3. ISO VG uses this viscosity, the kinematic one (the viscosity of flow under the oil's own weight). Another instrument, the rotational viscometer, gives the dynamic one (the force needed to turn a spindle inside the oil), in mPa·s (cP), which cannot be compared directly with cSt.

If the number is the viscosity, the next question follows: does the oil measure exactly that number?

Does an ISO VG 68 oil measure exactly 68 cSt?

Almost never: 68 is the middle of a range, and the oil can sit anywhere inside it. Think of two 500 g bags of coffee on a scale. One comes out at 498 g, the other at 503 g, and both are within what is accepted.

Oil works the same way. The grade number is the midpoint of the range, and ISO 3448 accepts 10% below and 10% above that point, always at 40 °C. That is why two ISO VG 68 oils from different suppliers can differ from each other and still both be correct.

This range leaves a detail few people notice. From one grade to the next, viscosity rises by about 50% (the table in the next section shows the whole scale), but the range of each grade covers only 10% on each side. A gap is left between neighboring grades. An oil that falls between the end of the 46 and the start of the 68 has no ISO VG grade at all and, for that reason, cannot carry the "ISO VG" label.

Figure 2. The ranges of grades 46 (41.4 to 50.6 cSt) and 68 (61.2 to 74.8 cSt) at 40 °C: a 48 cSt sample is ISO VG 46, a 55 cSt sample falls in the gap with no grade, and a 72 cSt sample is ISO VG 68.

For those who need the numbers: ISO VG 46 runs from 41.4 to 50.6 cSt, and ISO VG 68 from 61.2 to 74.8 cSt, at 40 °C. The gap between them runs from 50.6 to 61.2 cSt, and a 55 cSt sample has no grade. Two correct ISO VG 68 oils can differ by up to 13.6 cSt, about 20% of the midpoint.

What is the ISO VG grade table?

ISO 3448 has 18 grades: 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460, 680, 1,000 and 1,500. Imagine a staircase in which each step is about 50% taller than the one before. That is how the grades rise: each one is about 50% thicker than the one below, and every six steps the viscosity becomes ten times greater (from 10 to 100, from 100 to 1,000).

The table gives the full range of each grade:

Figure 3. The ISO VG grades as a staircase: each step is about 50% thicker than the one before, and every six steps the viscosity becomes ten times greater (10, 100, 1,000).

For those who need the numbers: the scale is a geometric series with six grades per decade: each grade is the previous one times about 1.47 (10 raised to the power 1/6), the number that, multiplied by itself six times (1.47 × 1.47 × 1.47 × 1.47 × 1.47 × 1.47), gives 10.

All these numbers hold at 40 °C. But your machine almost never runs at that temperature.

Why is ISO VG measured at 40 °C?

Because every oil changes viscosity with temperature, and two oils can only be compared at the same temperature. Think of butter: hard in the refrigerator, soft on a hot day. To know which of two butters is firmer, both must be at the same temperature.

Oil behaves the same way: it thins when it heats up and thickens when it cools down. ISO 3448 fixed 40 °C as the reference temperature for all industrial oils. It is a ruler for classifying, not the temperature of the machine. A hydraulic system running at 60 °C has oil much thinner than the number on the label; a gearbox (the case of gears that reduces the motor's speed) that starts on a cold morning receives oil much thicker.

How much each oil thins with heat is measured by another number, the viscosity index (VI): the higher the VI, the less the viscosity changes with temperature. That is why two ISO VG 68 oils that are equal at 40 °C can differ in heat and in cold: the high-VI oil thins less in the heat and thickens less in the cold. The article on the 46 and 68 hydraulic oils shows how the two change with temperature.

Figure 4. Two ISO VG 68 oils that are equal at 40 °C: the one with a high viscosity index (VI 160) thickens less in the cold and thins less in the heat than the one with an ordinary index (VI 100).

So the number gives the thickness at 40 °C and nothing more. But in catalogs and at the counter you will also see names like AW 68 and HLP 68, and you may think they are all the same thing. It is worth looking at what the number leaves out.

What is the difference between ISO VG 68 and AW 68?

ISO VG 68 gives only the viscosity. AW 68 is the commercial name of a hydraulic oil with an anti-wear additive in that grade. It is like a clothing size: it tells you the size, but not whether the piece is a shirt or a coat.

AW stands for anti-wear: it is the additive (a substance added to the oil to give or strengthen a protection) that protects the metal when the parts come to touch under pressure. There is no standard of its own behind the abbreviation. An ISO VG 68 oil, on the other hand, can be hydraulic, gear, compressor, turbine or machine slideway oil.

To understand what separates these oils, one word is missing. Base oil is the oil that makes up almost all of a lubricant, and it can be mineral (refined from petroleum) or synthetic (made by chemical reaction, with more uniform molecules). The article on what base oils are explains it further.

With that, we can see what the grade leaves out:

1. The additives and the protection they give. The German DIN standards (DIN is the German standards institute) separate the classes by additive; the letters do not have to be memorized, but they help you recognize the name in a catalog:

   • DIN 51524 has three classes of hydraulic oil (H for hydraulic): HL, in part 1, with antioxidant (which slows the aging of the oil by oxygen) and anti-rust; HLP, in part 2, with anti-wear as well; and HVLP, in part 3, the HLP with a high viscosity index.

   • DIN 51517 does the same for circulating and gear oils (C stands for circulation, in German): C, the simplest circulating oil; CL, with antioxidant and anti-rust; and CLP, from part 3, the gear oil. CLP also carries an extreme-pressure (EP) additive, which protects the tooth when the load squeezes so hard that the oil between the teeth no longer keeps the metal apart. The article on gearbox oil with EP additive shows this additive at work.

2. The behavior in heat and in cold, which depends on the viscosity index and on the type of base oil. The comparison is in the article on synthetic, semisynthetic or mineral oil.

3. Resistance to water, foam and oxidation (the aging of the oil with heat and oxygen), measured by tests specific to each product standard.

That is why one oil does not replace another just because the number is the same. Engine, manual gearbox and differential oils, by the way, do not even use this scale: they use the SAE one (SAE is the American society of automotive engineers), with other measuring temperatures.

Figure 5. Four oils with the same ISO VG 68 and different protections: the same number says only the viscosity.

And what if the type is right and only the number is wrong?

What happens if you use the wrong ISO VG?

The oil reaches the parts thinner or thicker than the machine needs, and the layer that protects the metal changes. The oil exists to keep a layer between moving metal parts: the oil film. The article on the types of hydraulic oil shows why the pump loses efficiency with the wrong oil.

The grade, together with how much the oil thins in the heat (the VI), decides how thick the oil arrives at the parts at working temperature. The thicker the oil at that point, the thicker the film; speed and load also matter. Two words help with the lists below: a bearing is the support where the shaft turns, and clearance is the minimum space between two parts that move against each other.

With a grade below the one required:

1. The oil reaches working temperature thinner than the design expects, which leads to a thinner film.

2. The microscopic peaks on the metal surfaces start to touch, which leads to wear in bearings, gears and pumps.

3. In the hydraulic pump, thin oil leaks through the clearances, which leads to a slower cylinder and hotter oil.

With a grade above the one required:

1. Thick oil resists movement more, which leads to more friction inside the oil itself, more energy spent and, in general, more heat.

2. On a cold start, the oil is slow to reach the pump and the bearings, which leads to wear in the first minutes. In a hydraulic system, it also leads to noise and a risk of cavitation. Cavitation is the air and vapor bubbles that form at the pump inlet when the pressure drops and then collapse as they are compressed, marking the metal.

How this shows up in your plant: a noisy pump at the start of the shift, a cylinder that loses speed when the machine warms up, a gearbox or compressor running hotter than normal, and oil that darkens before its time. For hydraulic systems, the choice between two neighboring grades is detailed in the article on the difference between 46 and 68 hydraulic oil.

Which ISO VG should you use in each machine?

The right grade is the one the machine's manual or nameplate asks for; the table below only shows the most common one for each machine. The general rule helps make sense of the numbers: fast parts, light load and cold oil call for a lower grade; slow parts, high load and hot oil call for a higher grade. That is why a slow gearbox uses much higher grades than a hydraulic system. And remember that the type of oil comes before the number.

The grades at the ends of the staircase have uses too. The lowest ones, from 2 to 22, appear in the high-speed spindles of machine tools (the shaft that turns the tool or the part) and in compressed-air line lubricators (the device that drips oil into the air of pneumatic tools). They also appear in some hydraulic and low-pressure circulation circuits. The highest ones, from 1,000 up, appear in very slow, heavily loaded gearboxes, such as worm gears (a screw that turns a toothed wheel), and in open gears (without a closed case).

Kelpen Oil formulates these lines, and all of them are on the page for oils for machinery and equipment. For compressors, the type of machine weighs on the choice of oil, as the article on which oil to use for an air compressor shows. If the machine has no manual, or runs hotter or colder than planned, talk to the Technical Team.

One last question remains: how do you know the oil that arrived really is the grade on the label?

How does Kelpen Oil check the ISO VG of each batch?

By measuring the viscosity of each batch at 40 °C in Kelpen Oil's own laboratory in Arujá (SP), before the batch leaves. A result outside the grade's specification keeps the batch from leaving the plant.

Each production run also generates a retained sample: a bottle from the same batch, stored and linked to the batch in the system, for a new analysis if any doubt comes up.

Besides 40 °C, the laboratory measures viscosity at 100 °C. The two measurements together give the oil's viscosity index, explained in the section on 40 °C.

In used oil, comparing the viscosity with that of new oil shows whether it has left its grade. If it rises, it usually points to oxidation, water or thicker oil mixed in. If it falls, it usually points to thinner oil, solvent or fuel mixed in, or to the breakdown of the additive that holds viscosity in the heat (a polymer, a long molecule, found in HVLP oils). The laboratory tests are covered in the article on lubricant analysis in the laboratory.

For those who need the numbers:

The reference method for viscosity is the glass tube of ASTM D445, equivalent to NBR 10441, also a Brazilian standard.

Frequently asked questions

What is ISO VG 68 oil used for? ISO VG 68 is the grade, not the type: it works in any machine whose manual asks for 68, with the right type of oil. The most common uses are hydraulic systems that run hotter, slideways and ways, bearings lubricated by circulation (the oil is pumped to them and returns to the tank) and some screw compressors.

Is ISO VG the same as DIN ISO 3448 (formerly DIN 51519)? Yes. DIN ISO 3448 is ISO 3448 itself in its German version, with the same 18 grades and the same ranges at 40 °C. DIN 51519 is the old name, which catalogs still use.

Does ISO VG apply to engine oil? No. ISO 3448 is for industrial oils; engine oil uses SAE J300 (such as 15W-40), and manual gearbox and differential oil use SAE J306, both measured at other temperatures. One number does not convert directly into the other.

Does oil lose viscosity with use? It can lose or gain it. It falls when the oil receives thinner oil, solvent or fuel. It also falls because of the additive that makes the oil vary less with temperature, a polymer (a very long molecule) present in many HVLP hydraulic oils (seen above): the stress inside the pump breaks this additive. It rises when the oil oxidizes, receives thicker oil or forms an emulsion with water (the water stays mixed in the oil in fine droplets and turns it milky).

What is SSU in a catalog? The "315 SSU" that some catalogs put next to the 68 oil is the same value as 68 cSt written in the old unit, Saybolt Universal seconds (ASTM writes it SUS), by the ASTM D2161 conversion table. It is only a change of unit, at 40 °C.

Glossary

• ISO VG: viscosity grade of ISO 3448; the number is the midpoint of the range at 40 °C, in cSt (in grades 2 to 7 the name rounds: 2.2 becomes 2, 3.2 becomes 3, 4.6 becomes 5 and 6.8 becomes 7).

• Viscosity: the oil's resistance to flowing; the higher it is, the thicker the oil.

• cSt (centistokes): unit of viscosity measured by the flow of oil under its own weight; 1 cSt equals 1 mm²/s.

• Midpoint: the center of each grade's range; the range runs from 10% below to 10% above it.

• Viscosity index: a number that shows how much viscosity changes with temperature; the higher it is, the smaller the change.

• Oil film: the layer of oil that keeps two metal parts apart while they move.

Talk to the Technical Team

The Kelpen Oil Technical Team looks at the manual, the start-up and working temperature, the load and the oil that is in the machine today, and confirms the type and the grade. If the oil is already in use, the Arujá (SP) laboratory performs the analysis of the oil in use, as a paid service, and shows whether the viscosity is still within the grade's range. Before buying, note the type and the grade from the manual, and check both on the label.