NLGI Grease Grades Explained: Bearing Grease Selection

What Are NLGI Grease Grades and How Are They Defined?

The NLGI consistency classification is a standardised grading system that ranks lubricating greases by their consistency. Consistency is the mechanical resistance to deformation. It determines whether a grease behaves as a thick semi-fluid, a soft butter-like paste, a firm putty, or a hard block at ambient temperature. The penetration test per ASTM D217 measures it. A standardised metal cone of defined weight is released onto the grease surface from a specified height. The depth of penetration after five seconds is recorded in units of tenths of a millimetre (dmm). Higher penetration values indicate softer, more fluid greases; lower values indicate firmer, harder greases. The NLGI system assigns a grade number (000 to 6) to specific penetration ranges, with each grade number corresponding to an increasing firmness. NLGI 000 at 445–475 dmm is almost pourable, while NLGI 6 at 85–115 dmm is a hard block grease.

The penetration test is performed on a worked sample — the grease is mechanically worked for 60 double strokes in a standard grease worker per ASTM D217 before measurement. This worked penetration represents the grease consistency after mechanical shear in service, which is the practically relevant value for predicting behaviour in a bearing or fitting. Unworked penetration (measured directly from the original sample without working) is typically higher (softer) than worked penetration, particularly for greases with thickener structures that are sensitive to mechanical shear. The difference between worked and unworked penetration, sometimes called the shear stability index, is an indicator of grease mechanical stability. A large difference indicates a thickener structure that softens significantly under shear, which may cause softening and leakage in high-speed bearings. Prolonged worked penetration tests, run for 10,000 or more double strokes (commonly 100,000) per ASTM D217, are used to predict consistency after prolonged mechanical shear. Our full technical guide to lubricant formulation technology provides the broader context on grease within the overall lubricants classification framework, and our lubricants hub lists every related guide.

Types of Grease and Thickener Chemistry: The Structural Framework

The thickener is the component that transforms a liquid lubricating oil into a semi-solid grease. It creates the three-dimensional fibrous or crystalline network that retains the base oil within the grease structure. It also controls the rate at which oil bleeds out under mechanical and thermal stress. The thickener is not merely a viscosity builder. It is a functioning structural matrix. Its chemistry determines the grease's dropping point, operating temperature range, water resistance, mechanical stability, and thickener compatibility with other grease types. In combination with the base oil, it also determines the overall consistency and NLGI grade of the finished grease.

The principal thickener classes, each imparting a distinct performance profile:

• Lithium soap (12-hydroxystearate) accounts for approximately 60–70% of global grease consumption. It forms interlocking needle-like fibres (1–50 µm) through saponification of lithium hydroxide with 12-hydroxystearic acid. The dropping point is 180–200°C, and broad base oil compatibility makes it the default NLGI 2 general-purpose grease.
• Calcium soap uses calcium 12-hydroxystearate or anhydrous calcium stearate (complex types). It offers superior water washout resistance versus lithium at equivalent consistency. It is standard for marine deck equipment, agricultural machinery, and water pump bearings.
• Aluminium complex has high dropping points, typically above 250°C, and excellent adhesiveness and water resistance. It is standard for open gear and wire rope applications.
• Polyurea (diurea/tetraurea) is not a metallic soap. It is formed from diisocyanate-amine reaction and used in sealed-for-life bearings, electric motor bearings, and automotive applications. The absence of metal soap chemistry gives improved high-temperature oxidation resistance.
• Non-soap thickeners (bentone, PTFE) are organically modified clay and PTFE. They provide specialty performance profiles for extreme-temperature or chemically inert service environments.

The industrial grease manufacturing process covers how these thickener systems are produced in kettle reactors and finished through milling and homogenisation. Open gear and wire rope service is covered in our guide to wire rope and chain lubricants.

Base Oil Viscosity and Additive Systems in Grease Formulation

The thickener determines the NLGI grade and structural properties of a grease. The base oil determines the lubrication performance at the bearing contact zone, and typically constitutes 75–90% of the grease formulation by weight. Base oil viscosity is the most important base oil parameter for grease selection. It must be appropriate for the bearing operating speed and temperature to maintain an adequate elastohydrodynamic (EHL) film thickness between rolling elements and raceways under load.

Base oil viscosity in grease is expressed as the ISO VG (Viscosity Grade) of the oil bled from the grease — typically measured in centistokes (cSt) at 40°C. Low-speed, high-load bearings and open gear applications require high base oil viscosity (ISO VG 460–1500 cSt at 40°C) to generate adequate film thickness under slow sliding contact. High-speed bearings such as electric motor bearings, spindle bearings, and food processing equipment require low base oil viscosity (ISO VG 15–100 cSt at 40°C). This minimises viscous drag and heat generation at elevated rotational speeds. Our guide to bearing grease for high-speed EV motors covers the high-speed end of this range, and our article on food-grade lubricants and NSF H1 formulation covers food processing equipment. Mineral base oils (Group I or II) are used in most standard industrial greases. Synthetic base oils are used in greases requiring extended service life at extreme temperatures, very low pour points, or compatibility with specific materials. The main synthetic types are polyalphaolefin (PAO), synthetic ester, and polyalkylene glycol (PAG). The base oil classification system and the performance differences between Groups I–V are described in our guide to lubricant base oil types and API groups. Our comparison of synthetic vs mineral base oils adds the molecular-structure and oxidation-stability detail.

The key additive classes used in grease formulations are listed below.

• Extreme pressure (EP) agents such as sulphur-phosphorus compounds and chlorinated paraffins protect high-load applications such as gear tooth surfaces and heavily loaded roller bearings.
• Antiwear additives provide moderate load protection.
• Oxidation inhibitors (hindered phenols, aromatic amines) extend grease service life at elevated temperatures.
• Corrosion inhibitors (amine salts, zinc compounds) protect steel bearing surfaces in wet or humid environments.
• Metal deactivators suppress the catalytic effect of metal surfaces on oil oxidation.

The full additive technology context is covered in our guide to lubricant additive packages. Our guide to extreme pressure additives covers the sulphur-phosphorus EP chemistry in detail.

NLGI Grade Selection Guide: Bearing Grease by Application Type

Selecting the correct NLGI grade requires matching the grease consistency to the mechanical requirements of the lubrication point. Those requirements are the bearing type, operating speed, load, temperature, regreasing method, and sealing arrangement. The following guide covers the principal selection criteria for each grade range, providing the practical decision framework used by maintenance engineers and OEM lubrication engineers.

Lithium Grease and Lithium Complex Grease: The Dominant Thickener Technology Worldwide

Lithium soap greases are based on lithium 12-hydroxystearate or lithium complex thickeners. They account for the overwhelming majority of industrial and automotive grease consumption globally, estimated at approximately 60–70% of total world grease sales by volume. This dominance reflects a combination of performance balance, manufacturing cost, and widespread raw material availability, since 12-hydroxystearic acid from castor oil is globally traded. Lithium greases are also broadly compatible with most base oils and additive systems, which competing thickener technologies find difficult to match.

Simple lithium grease is formulated with lithium 12-hydroxystearate thickener in a mineral base oil. It provides a dropping point typically in the range of 180–200°C and a practical upper operating temperature limit of approximately 120°C for continuous operation. That 120°C figure is the commonly quoted 'speed rating' temperature for standard NLGI 2 lithium greases. Simple lithium grease delivers good mechanical stability under shear, excellent pumpability at ambient temperatures, and broad chemical compatibility. Water washout resistance is adequate, with ASTM D1264 weight loss typically below 10% at 38°C for quality lithium greases.

Lithium complex grease uses a mixed lithium 12-hydroxystearate / lithium azelate thickener system. It extends the dropping point to typically 260–280°C and the practical service temperature ceiling to approximately 150–170°C for continuous service. It also improves load-carrying capacity, oxidation resistance, and mechanical shear stability. Lithium complex greases are standard high temperature greases for automotive wheel bearing grease applications, where European and US OEM specifications typically mandate lithium complex or equivalent. They are also standard for electric motor bearings operating at elevated temperatures and for industrial roller bearings in process industries with high ambient temperatures. ASTM D4950 is the standard classification and specification for automotive service greases.

For the manufacturing process behind these products, our guide to industrial grease manufacturing explains the kettle saponification, dehydration, and milling steps. These convert raw fatty acid and lithium hydroxide into finished grease.

Key Grease Performance Tests and What They Reveal

Grease performance is characterised by a battery of standardised tests that evaluate the thickener stability, oil retention, water resistance, load-carrying capacity, and long-term oxidation behaviour of the formulated product. Understanding these tests is essential for grease formulators, quality control engineers, and procurement specialists evaluating supplier products against specification.

Key standardised tests and what each reveals:

• Dropping point — ASTM D566/D2265: the temperature at which the thickener structure collapses and the grease liquefies. The practical service temperature limit is 30–50°C below this value.
• Oil separation — ASTM D6184: the conical sieve method measures static oil bleed at 100°C over 30 hours. It shows the tendency of base oil to bleed from the thickener matrix under thermal stress, and excessive bleed causes oil starvation in sealed bearings. ASTM D1742 covers oil separation during storage.
• Water washout resistance — ASTM D1264: grease weight loss under water spray at 38°C and 79°C. This is critical for greases in wet environments including food processing, paper mills, and marine equipment.
• Four-ball wear test — ASTM D2266: wear scar diameter after 60 minutes at 40 kg load and 1,200 rpm. It indicates antiwear film strength under moderate contact conditions.
• Four-ball EP test — ASTM D2596: measures weld point and load wear index. The load at which the film fails and steel balls weld together indicates maximum EP load-carrying capacity.
• Oxidation stability — ASTM D942/D5483: critical for sealed-for-life bearing and long-interval applications. D5483 measures oxidation induction time by pressure differential scanning calorimetry to assess oxidative resistance over extended service.

The complete additive performance context, including antioxidant and EP mechanisms measured by these tests, is covered in our guide to lubricant additive packages.