Grease Selection: Base Oil, Thickener and NLGI

Grease is the lubricant of choice inside the overwhelming majority of rolling bearings, and for good reason: it stays where it is put, needs no circulation system, and helps seal the bearing against contaminants at the same time. Yet grease is also among the most common sources of premature bearing failure after contamination and misalignment, because it is usually chosen by habit rather than by calculation. A grease is not one substance but a balanced composite of base oil, thickener and additives, and each of those three answers a different question. Selecting only an NLGI grade, as many maintenance engineers do, is like choosing an engine oil by its colour alone.

This guide keeps the same three-part frame - base oil, thickener, consistency - and adds the numbers that turn it into a decision: the 40 °C viscosity, the kappa ratio behind ISO 281, the thermal limit of each thickener family, the dn ceiling and the fill fraction that keeps a fast bearing from churning. It is written for anyone who has to write a grease into a specification and then defend the choice.

Grease is a composite, not one grade

Roughly 70–90% of a grease by mass is base oil, and that oil is the part that actually lubricates the rolling contacts. The thickener, typically 5–20%, is a soap or non-soap sponge that holds the oil in place and releases it slowly as the bearing works. Additives make up the remainder and cover extreme-pressure (EP), anti-wear, anti-oxidant, rust-inhibiting and sometimes solid-film duties.

The order matters. The base oil does the work; the thickener only controls when and where the oil is delivered; the additives protect the surfaces when the film becomes thin. A grease with the wrong base oil cannot be rescued by a good thickener, and a grease that bleeds its oil too quickly will starve a hot bearing no matter how sophisticated its additive package is.

Base-oil viscosity and the kappa rule

Viscosity is the single most important property of the base oil, and it must match the bearing's speed and load. Higher load at low speed needs a thicker oil; high speed and light load need a thinner one, because a thick oil that cannot be pulled into the contact simply churns and raises temperature. Base-oil viscosity is quoted at 40 °C in mm²/s (cSt), and the datasheet also names the base-oil type - mineral, PAO, ester or silicone - which fixes the usable temperature window.

Reading the 40 °C viscosity number

The 40 °C figure is a reference point, not the operating viscosity. A VG 100 base oil at 40 °C may fall to something far thinner at 90 °C, which is why the number has to be judged at the real bearing temperature rather than in the catalogue.

Why kappa near 1.5 to 2 is the target

A more rigorous approach uses the viscosity ratio kappa from the method behind ISO 281, which compares the actual lubricant film with the minimum needed for full separation. A kappa at or above 1 means the surfaces are fully separated; below 1 they interact and life falls away quickly. For general machinery a base oil that gives a kappa of roughly 1.5 to 2 at operating temperature is a sound starting point.

Base-oil viscosity rises as speed falls and load grows; the boundary is set by film formation, not by habit.

Thickener chemistry sets the ceiling

The thickener sets the grease's temperature ceiling, its water resistance and how it behaves when mixed with another grease. Lithium soap is the industrial workhorse: inexpensive, versatile, useful to roughly 120 °C and excellent for multi-purpose plant greases. Lithium complex lifts the dropping point to around 260 °C and extends high-temperature life, which is why it dominates motor and steel applications. Calcium sulfonate greases are outstanding for water washdown and corrosion protection, while polyurea thickeners give very long life at high temperature but mix badly with soap greases. PTFE and other non-soap thickeners are reserved for very high temperatures and for chemically aggressive or dry-running contacts.

Working consistency and the NLGI scale

Consistency is measured by the worked-penetration test and classified on the NLGI scale. Grade 2 is the universal default for bearings because it stays in place without being too stiff to pump. Softer grades flow like a thick fluid and suit sealed gear cases or centralised systems; firmer grades hold position in large, slow bearings and on vertical shafts where a soft grease would creep down under gravity.

Why grade 2 became the default

Grade 2 sits in the middle of the scale, where the grease is stiff enough to stay in a horizontal bearing yet soft enough to be pushed back into the track by the rolling elements. That balance covers the widest range of ordinary duties, so it became the stocked default rather than a calculated optimum.

Speed, dn and the thermal ceiling

Every grease has a maximum speed, usually quoted as a dn value - bore diameter in millimetres multiplied by rotational speed in rpm - or expressed through the ISO 15312 thermal speed rating. A high-viscosity grade-3 grease in a fast spindle will churn, overheat and bleed its oil within hours; a thin, soft grease in a slow, heavily loaded mill bearing will simply squeeze out of the contact. Matching the grease's dn capability to the application matters as much as matching viscosity, and the permitted speed falls as the ambient temperature rises.

Mixing greases without wrecking the bearing

Greases with different thickener chemistry should never be mixed without a check. Lithium and polyurea greases, for example, can thicken or soften unpredictably when combined, and a failed mixture can break down within days. When changing grease type, purge the old grease thoroughly, or confirm compatibility from the supplier's tables before topping up. Even compatible thickeners can carry base oils and additives that react, so a compatibility check is cheap insurance.

When two greases can be mixed

The safe case is the same thickener type, the same base-oil type and a supplier statement of compatibility. Everything else - a different soap, a different base oil, an unknown house brand in an old housing - is a purge-and-replace job rather than a top-up.

Fill quantity and re-lubrication

Overfilling is a genuine failure mode: a bearing packed solid with grease cannot shed heat and will overheat, especially at speed, so housings are normally filled to only 30–50% of the free space and high-speed bearings to as little as 20–30%. Re-lubrication intervals depend on speed, temperature and load, and they shorten sharply with every 15 °C rise above the grease's rated temperature.

Free-space fill by speed

As a rule, the faster the bearing runs, the less free space should be filled, because the working grease needs somewhere to be thrown to and to cool. Slow, heavily loaded bearings tolerate a fuller housing; fast ones do not.

A step-by-step grease selection sequence

A disciplined selection runs in a fixed order, and it is worth defending that order against the temptation to start with a part number.

1. Fix the base-oil viscosity. Start from load and speed, confirm the kappa across the real operating temperature range, then read off the nearest VG grade.
2. Choose the thickener. Set the temperature ceiling from the hottest the bearing will see, then add water and chemical resistance from the environment.
3. Set the NLGI grade. Use the mounting orientation, the feed method and the bearing size to pick consistency.
4. Confirm speed. Check the dn or thermal rating against the actual duty and derate for ambient temperature.
5. Check compatibility. If a different grease was used before, purge or confirm compatibility before topping up.
6. Choose the brand and part number last. Only now does the commercial decision belong in the chain.

Mistakes that shorten grease life

• Buying by NLGI grade alone. Consistency says nothing about the base oil that does the lubricating.
• Filling solid. A packed housing overheats and can push grease past the seals.
• Mixing thickeners. Lithium and polyurea greases can destroy each other in days.
• Ignoring the operating viscosity. A grade that looks right at 40 °C can be far too thin at the running temperature.

For the closures that keep the grease in and the contamination out, see the suffixes explained in seals and shields. To follow the interval arithmetic, the companion piece on re-lubrication intervals works through a 6205 example, and the background on the scale itself sits in NLGI consistency classes. The effect of heat on fit and clearance is covered at temperature and clearance. Grease is most often specified for the deep groove ball bearing positions that dominate motor and pump duty, with the matching closures and fittings listed under bearing accessories, while range conventions such as those of SKF widen the available choice. Filter by bore and outside diameter in the bearing selector, and take the supporting detail from the technical library.