"Use EP gear oil" is the standard advice for gearboxes, and for most heavily loaded steel gears it is right. But EP additives are a chemical tool with side effects. In the wrong gearbox they cause more harm than good.
Use EP gear oil for heavily loaded or shock-loaded steel gears, such as crushers, mills, conveyors and most industrial helical and bevel gearboxes. Use R&O (non-EP) gear oil for lightly loaded, high-speed gears and some turbine gearboxes. For worm gears with bronze wheels, use an oil the gearbox maker approves for worm drives: often a compounded or synthetic oil rather than a highly active EP oil.
Under normal running, an oil film keeps gear teeth apart. Under very high load, shock or slow speed, the film can break down and metal touches metal. That is when teeth scuff and weld locally.
EP additives, usually sulphur-phosphorus compounds, react with hot steel surfaces at those contact points and form a thin protective layer that shears instead of the metal. They only "switch on" where conditions are severe.
| Type | What it contains | Best for | Watch out |
|---|---|---|---|
| R&O (rust and oxidation) | Base oil + rust and oxidation inhibitors | Lightly loaded, high-speed gears; some turbine and compressor gearboxes | No protection against scuffing under heavy load |
| EP (extreme pressure) | R&O + sulphur-phosphorus EP additives | Heavily loaded spur, helical and bevel gears; shock loads | Can attack bronze and other yellow metals at high temperature |
| Compounded | Base oil + fatty (animal or vegetable) oils | Worm gears with bronze wheels | Lower oxidation life; less common today |
Synthetic gear oils sit across these families: PAO synthetics come in EP and non-EP versions, and PAG (polyglycol) oils are widely used in worm gearboxes for their low friction.
Test results on data sheets help compare oils: the FZG test reports a failure load stage (DIN 51517-3 CLP oils must reach at least stage 12), and the four-ball weld load and Timken OK load are other common measures.
A worm gearbox has a steel worm sliding against a bronze wheel. There is far more sliding than in other gears, so it runs hotter and wastes more energy. Two consequences matter:
PAG (polyglycol) gear oils do not mix with mineral or PAO oils. Mixing can form sludge and attack some seals and paints. If you change to or from PAG, drain and flush thoroughly.
| Gearbox | Typical choice |
|---|---|
| Helical/bevel, conveyors and mixers | EP gear oil, VG 220 |
| Crushers, mills, heavy shock loads | EP gear oil, VG 320–460 |
| Worm gearbox, bronze wheel | Worm-approved compounded, PAO or PAG oil, VG 320–680 |
| Light-load, high-speed gearbox | R&O or mild EP oil, VG 68–150, as specified |
| Gearboxes in chemical plants and humid areas | EP oil with good water separation and rust protection |
What does EP mean in gear oil?Extreme pressure. EP additives form a protective layer on gear teeth when the oil film breaks down under heavy load or shock.
Can EP gear oil damage bronze?Highly active sulphur EP oils can corrode bronze and other yellow metals, especially when hot. For worm gears with bronze wheels, use an oil approved for yellow metals or for worm drives.
Is CLP gear oil the same as EP gear oil?CLP is the DIN 51517-3 name for an EP industrial gear oil with rust and oxidation protection. So a CLP oil is an EP oil that meets that specification.
Can I use non-EP oil in a gearbox that specifies EP?No. Without EP additives the teeth can scuff or weld under peak loads. Use the oil type the maker specifies.
Not sure whether your gearbox needs EP oil, or whether it has bronze parts? Send us the gearbox type and make, and our team will suggest the right oil.