Which oil to use in an industrial gearbox: AGMA, CLP and DIN 51517

Use the gear oil that the gearbox nameplate (the plate fixed to the housing) or the manual calls for. The nameplate gives it as a code: CLP and AGMA say how thick the oil is and whether it carries the additive that protects the teeth under load; ISO VG says only how thick it is. Below you will find what each code means, the chart that converts one into the other, and the test that the German standard DIN 51517 requires to prove the protection.

What does the oil do inside a gearbox?

The oil keeps the gear teeth apart with a very thin layer, so that metal does not rub against metal.

A gearbox is a closed housing of gears that sits between the motor and the machine. It lowers the speed and, in exchange, multiplies the turning force, the torque, like a bicycle's low gear on a climb. Inside, a small gear, the pinion, turns a large one, the gear.

All the motor's force passes from one tooth to the other through a narrow band where the two touch. There the oil forms a layer much thinner than a hair: the oil film. While the film holds, the tooth hardly wears.

In most gearboxes, the gears turn partly immersed in the oil at the bottom of the case and splash it around; in the larger ones, a pump circulates the oil. The same oil carries the heat away and protects the parts against rust.

The trouble starts when the load is so high that the film cannot hold.

What does the EP additive in gear oil do?

The EP additive protects the teeth in the moments when the oil film is not enough and metal touches metal. An additive is a chemical added to the oil in a small dose to give it an extra quality; EP stands for extreme pressure. Here is the sequence (Figure 1):

1. The gearbox force concentrates on the contact band, which puts a very high pressure on the oil.

2. The turning drags oil into the contact and forms the film: the faster the turning, the thicker the film. With a complete film, the teeth do not touch (panel 1 of the figure).

3. The working face of the tooth, the flank, has microscopic peaks: the roughness.

4. Low speed, or hot oil (which thins out), leaves the film thinner than these peaks (panel 2 of the figure). Under high load, the peaks are pressed against each other and the contact point heats up.

5. The peaks touch, weld for an instant and are torn off. This damage is scuffing: it scratches the flank and releases metal particles into the oil (panel 2).

6. The EP additive, usually made of sulfur and phosphorus compounds, reacts with the steel at these hot spots and forms a sacrificial film. It wears away in place of the tooth and prevents welding (panel 3).

7. The film is rebuilt at every contact, at the expense of the additive, which is gradually used up. Heat and oil aging also consume the additive.

Figure 1. The contact between pinion and gear, enlarged. The full film keeps the teeth apart (1). With a thin film, lower than the peaks, and high load, the peaks weld and are torn off (2). The EP additive film prevents welding (3). Roughness exaggerated.

How this shows up in your plant: noise and housing temperature above normal; scratches on the flanks and particles stuck to the magnetic plug, the drain plug with a magnet. A tooth that has already scuffed stays marked after the oil change; the new oil protects it from then on.

So is a very thick oil with EP enough?

What is viscosity and why does it matter in a gearbox?

Viscosity is the oil's resistance to flowing: honey has high viscosity, water has low. Together with the turning speed, it decides how thick the film between the teeth will be.

An oil that is too thin leaves the film thinner than the peaks, and wear speeds up. An oil that is too thick makes the gears waste power churning it, heats the case and makes cold starts heavy.

The right grade depends on speed, load and temperature. A fast gearbox with small teeth calls for a thinner oil; a slow, high-torque gearbox calls for a thicker one. That grade is the viscosity grade, and the gearbox maker writes it on the nameplate. Without a nameplate, the Kelpen Oil Technical Team calculates the grade from the gearbox data.

For those who need the number: viscosity drops when the oil heats up; that is why the standard measures it at a fixed temperature, 40 °C, and that measurement gives the grade its name.

What do CLP, ISO VG and AGMA mean?

They are three ways of placing the order, and only two say the type of oil. ISO VG gives only the viscosity grade; CLP and AGMA give the grade and also whether the oil carries an EP additive (Figure 2).

Figure 2. The nameplate shows one of these codes. CLP and AGMA give the grade and the type of oil; ISO VG gives only the grade. In the traditional AGMA numbering, which current standards replaced with ISO VG, AGMA 5 equals ISO VG 220.

ISO VG is the viscosity grade of the ISO 3448 standard: the number is the viscosity at 40 °C. "ISO VG 220" says how thick the oil is and does not say whether it has EP.

CLP comes from the German gear oil standard, DIN 51517. Each of its three parts adds one protection to the previous one:

• C (part 1): oil without additives (the standard is based on mineral oil).

• CL (part 2): the C plus protection against rust and against oxidation, the aging of the oil caused by heat and by oxygen in the air.

• CLP (part 3): the CL plus an EP additive. The letters describe performance; the base can be mineral or synthetic. In "CLP 220", the number is the ISO VG grade.

AGMA is the American association of gear manufacturers (American Gear Manufacturers Association). In the traditional AGMA numbering (the one current standards replaced with ISO VG, but which nameplates still carry), each viscosity grade gets a number, and the suffix tells the type of oil:

• EP: with an extreme pressure additive;

• Comp: compounded oil, with fat blended into the oil, for the worm gearbox, covered further on;

• no suffix: only protection against oxidation and rust, with no EP additive.

The two standards have different tests and limits: buy the oil that declares, on the label, the standard that the nameplate cites. If the nameplate shows only the ISO VG, check in the manual whether the gearbox calls for EP.

For those who need the number: ISO VG is the kinematic viscosity at 40 °C in mm²/s, the same as centistokes (cSt), with a 10% tolerance up or down. ISO VG 220 runs from 198 to 242 cSt.

The traditional AGMA numbering comes from the editions up to AGMA 9005-D94. Since 9005-E02, the standard uses ISO VG itself plus the type of oil and keeps the old number only as a reference; the current edition is 9005-F16. The old nameplate still says "5 EP".

What is the gearbox oil chart?

The chart crosses the ISO VG grade with the traditional AGMA numbering and with the two Kelpen Oil gear oil lines, one mineral and one synthetic. They differ in the base oil, the liquid that makes up most of the lubricant: mineral comes from refined petroleum and synthetic is made by chemical synthesis. The base oil groups are covered in the article on what base oils are.

Reading the chart: "CLP 220" and "AGMA 5 EP" call for the same type of oil, ISO VG 220 with EP. The AGMA column is the traditional scale, for converting the nameplate code; the standard that each line meets is in the next paragraph. ISO VG 32 corresponds to AGMA 0, which has no EP version. The scale also has AGMA 1 (ISO VG 46) and continues above ISO VG 1000.

The entire Turan EP line, from ISO VG 68 to 1000, meets DIN 51517 part 3 (CLP) and AGMA 9005. The Turan S line, with 100% synthetic bases, meets DIN 51517 part 3 (CLP) and ISO 12925-1, the international standard for enclosed gear oils.

For those who need the number: the most used ISO 12925-1 classes for enclosed gears rise in steps: CKB (mineral with protection against oxidation and rust), CKC (the CKB plus an EP additive) and CKD (the CKC that withstands more heat without degrading). The standard also has classes for worm gears, synthetics and open gears.

DIN 51502 adds to the CLP a letter code for the synthetic base, such as HC (synthetic hydrocarbon), PG (polyglycol) and E (ester), as in "CLP PG". A synthetic that meets the CLP requirements is also CLP, with the base code beside it.

The label says EP. How do you know the oil really holds up?

How is the load capacity of a gear oil proven?

By a standardized test, required by the standard that the oil claims to meet. The main one is the FZG: a machine turns a standard pair of gears immersed in the oil and raises the load in stages, from 1 to 12, until the scuffing of panel 2 of Figure 1 appears.

It is a stress test, like the cardiologist's treadmill that raises the incline little by little. The higher the stage the oil withstands, the greater its load capacity.

The four-ball test measures the resistance to welding in another way: a steel ball turns pressed against three stationary balls, immersed in the oil, and the load rises until the four weld together. The result comes out as a weld load and cannot be converted into an FZG stage.

DIN 51517 part 3 also requires tests for aging, rust, copper corrosion, foam, water separation (important in paper and steel), seal compatibility and rolling bearing wear. That gives you a filter for comparing suppliers: "EP" on the label is an adjective; "meets DIN 51517 part 3" can be checked, because the standard sets the minimum FZG stage and all the other tests.

For those who need the number: the FZG follows DIN ISO 14635-1 (formerly DIN 51354-2), method A/8.3/90; the acronym comes from the gear research center of the Technical University of Munich. The scale ends at stage 12; an oil that passes it without failing gets the result "greater than 12". DIN 51517 part 3 requires a failure stage of 12 or higher. The four-ball test follows ASTM D2783, which measures the weld load.

Can such a reactive additive attack some part of the case?

Can oil with EP additive attack bronze and seals?

It can, when the oil formula does not suit the case. The more reactive sulfur, which protects the steel, can also attack copper alloys, such as bronze.

The typical case is the worm gearbox: a screw, the worm, turns a bronze wheel, and the two slide a lot against each other, which heats the contact. Bushings and bearing cages (the part that separates the balls or rollers) can also be bronze.

With hot bronze, the attack can be fast and advances without noise: the wheel loses material, and the oil darkens and collects particles.

This aggressiveness is measured in the copper strip corrosion test: a polished strip sits in hot oil, and the stain that forms is compared with a scale.

For the worm gear there are also low-friction oils, such as compounded oil and some synthetics. With a bronze wheel, the Kelpen Oil Technical Team recommends the oil before you buy.

As for seals, compatibility depends on the rubber of the lip seal, the elastomer, and on the temperature. The Turan S line is compatible with sealing elastomers; for any oil, confirm with the seal manufacturer. The greater risk appears when changing from one base type to another, the subject of the next section.

For those who need the number: copper strip corrosion follows NBR 14359, 3 hours at 100 °C, with the ASTM D130 scale, from class 1 (light stain) to class 4 (corrosion).

When does synthetic pay off in a gearbox?

When the temperature, the change interval or the cost of downtime go beyond what mineral oil handles well. The synthetic base oils used in gearboxes change less in viscosity with temperature and resist oxidation better. In practice, the oil thins less in a hot case, thickens less on a cold start and lasts longer between changes (Figure 3).

Figure 3. Illustration: the grade is measured at 40 °C, so the curves of two oils of the same grade cross there. Away from that point, the typical gearbox synthetic thins less when hot and thickens less when cold. This is not product data.

Synthetic usually pays off in the hot case, on the cold start and where the change is expensive. A hot case is one that runs near the limit of mineral oil; the Kelpen Oil Technical Team tells you whether it is yours, based on the temperature measured on the housing with the gearbox under load.

Examples: a wind turbine, where changing the oil means climbing up to the nacelle (the cabin at the top of the tower), a plastic extruder gearbox and high-load gearboxes in the paper industry, steel, oil and gas, textiles, wood and cement. In these cases, the criterion stops being the price per liter and becomes the cost per hour of operation.

When changing from mineral to synthetic, drain and clean the case first. Some synthetic bases, such as PAG (polyglycol), do not mix with mineral oil and can attack the internal paint of the case and certain lip seals. That is why the Kelpen Oil Technical Team confirms whether the change pays off and how to migrate.

For those who need the number: how much the viscosity changes with temperature is measured by the viscosity index (NBR 14358); the higher the index, the less it changes.

With the right oil in the case, what is left is knowing when to change it.

How do you know when to change the gearbox oil?

By the condition of the oil, measured in a laboratory, as well as by the calendar. The gearbox is closed, and the oil keeps a record of what happened inside: compared with a sample of the new oil, it shows how much it has aged and whether the additive is wearing out.

These are the gearbox tests at the Kelpen Oil laboratory, in Arujá (SP). The full list is in the article on the lubricant laboratory tests.

Two readings of the table call for care. New EP oil already has its own acidity, which comes from the additive, and it can even fall at the start of use. What counts is how much it rises relative to the sample of new oil from the same batch.

Stable phosphorus and sulfur, on their own, do not guarantee that the additive still protects: when you top up the level, with the same oil or another, the new oil brings fresh additive and hides the wear of the old one. That is why the reading combines acidity, viscosity and particles.

We at Kelpen Oil have formulated lubricants and distributed base oils since 1999. The same laboratory releases each batch we formulate (the batch report is for new oil) and performs in-service oil analysis for any company, including oil from another manufacturer, as a paid service. In a gearbox that is hard to reach, the analysis is what lets you change the oil during a planned shutdown, not in a rush after a failure.

For the comparison to be valid, collect the sample with the gearbox at working temperature, always from the same point, and send a little of the new oil along with it.

Which oil to use in each gearbox?

The grade comes from the nameplate; the type of oil comes from the conditions of the case. The table summarizes the choice among the Kelpen Oil oils for industrial gearboxes.

Frequently asked questions

Which oil is used in a speed reducer? A speed reducer is the industrial gearbox itself and uses CLP gear oil, in the ISO VG grade on the nameplate. At Kelpen Oil, the Turan EP line runs from ISO VG 68 to 1000, and the Turan S line covers the grades in the table above.

What is the best oil for a gearbox? The one that meets the nameplate code and proves its load capacity by the standard it declares, such as DIN 51517 part 3. Between mineral and synthetic, the lower cost per hour of operation wins.

Which oil is used for industrial gears? For enclosed gearboxes, the CLP in the specified grade. Car transmission and differential oil (API GL-4 and GL-5 classes) is another family and does not replace CLP: GL-5 carries very active sulfur, which can attack bronze. If an old nameplate calls for an SAE grade, the Kelpen Oil Technical Team gives you the matching ISO VG.

Can you use hydraulic oil in a gearbox? Only when the gearbox manual calls for it. Anti-wear hydraulic oil (HLP class, DIN 51524 part 2) is made for pumps and usually comes in thinner grades. The standard also requires a lower FZG from it: stage 10 or higher, against 12 or higher for CLP.

Glossary

• Gearbox: closed housing of gears that lowers the motor speed and increases the torque delivered to the machine.

• Pinion and gear: the gear pair of the gearbox; the pinion, smaller, drives the gear, larger.

• Oil film: layer of oil that separates the moving surfaces.

• EP (extreme pressure) additive: usually sulfur and phosphorus compounds, which react with the steel at hot spots and form a sacrificial film.

• Scuffing: instant welding and tearing off of the tooth peaks when the oil film does not hold.

• ISO VG: viscosity grade of the oil at 40 °C, in cSt, under ISO 3448.

• FZG: test with standard gears that measures, in stages from 1 to 12, the load the oil supports before scuffing.

Talk to the Technical Team

Before you call, write down the nameplate code, the gear type, the wheel material (is it bronze?), the speed, the load and the temperature of the case and of the room. With these data, the Kelpen Oil Technical Team confirms the right oil for your gearbox.

If the gearbox is already running and the doubt is the condition of the oil, send a sample to the Arujá (SP) laboratory for the in-service oil analysis, as a paid service. Save this article or send it to whoever looks after the gearboxes at your company.