NLGI Grease Grades Explained: Selection & Application

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 — the mechanical resistance to deformation that 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. Consistency is measured by the penetration test per ASTM D217: a standardised metal cone of defined weight is released onto the grease surface from a specified height, and 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. Extended worked penetration tests (60,000 double strokes 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.

Thickener Chemistry: The Structural Framework of Grease

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 and 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 whose chemistry determines the grease's dropping point, operating temperature range, water resistance, mechanical stability, thickener compatibility with other grease types, and — in combination with the base oil — 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; forms interlocking needle-like fibres (1–50 µm) through saponification of lithium hydroxide with 12-hydroxystearic acid; dropping point 180–200°C; broad base oil compatibility; the default NLGI 2 general-purpose grease
• Calcium soap — calcium 12-hydroxystearate or anhydrous calcium stearate (complex types); superior water washout resistance vs lithium at equivalent consistency; standard for marine deck equipment, agricultural machinery, and water pump bearings
• Aluminium complex — high dropping points (typically above 250°C), excellent adhesiveness and water resistance; standard for open gear and wire rope applications
• Polyurea (diurea/tetraurea) — not a metallic soap; formed from diisocyanate-amine reaction; used in sealed-for-life bearings, electric motor bearings, and automotive applications; improved high-temperature oxidation resistance from absence of metal soap chemistry
• Non-soap thickeners (bentone, PTFE) — organically modified clay and PTFE 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.

Base Oil Viscosity and Additive Systems in Grease Formulation

While the thickener determines the NLGI grade and structural properties of a grease, the base oil — which typically constitutes 75–90% of the grease formulation by weight — determines the lubrication performance at the bearing contact zone. The 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) to minimise viscous drag and heat generation at elevated rotational speeds. Mineral base oils (Group I or II) are used in most standard industrial greases; synthetic base oils — polyalphaolefin (PAO), synthetic ester, or polyalkylene glycol (PAG) — are used in greases requiring extended service life at extreme temperatures, very low pour points, or compatibility with specific materials. 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. The key additive classes used in grease formulations include extreme pressure (EP) agents (sulphur-phosphorus compounds, chlorinated paraffins) for high-load applications such as gear tooth surfaces and heavily loaded roller bearings; antiwear additives for moderate load protection; oxidation inhibitors (hindered phenols, aromatic amines) to extend grease service life at elevated temperatures; corrosion inhibitors (amine salts, zinc compounds) to protect steel bearing surfaces in wet or humid environments; and metal deactivators. The full additive technology context is covered in our guide to lubricant additive packages.

NLGI Grade Selection Guide by Application Type

Selecting the correct NLGI grade requires matching the grease consistency to the mechanical requirements of the lubrication point — specifically, 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: The Dominant Thickener Technology Worldwide

Lithium soap greases — based on lithium 12-hydroxystearate or lithium complex thickeners — 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, widespread raw material availability (12-hydroxystearic acid from castor oil is globally traded), and broad compatibility with most base oils and additive systems that is difficult for competing thickener technologies to match.

Simple lithium grease — formulated with lithium 12-hydroxystearate thickener in a mineral base oil — 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 (120°C is the commonly quoted 'speed rating' temperature for standard NLGI 2 lithium greases). It delivers good mechanical stability under shear, excellent pumpability at ambient temperatures, adequate water washout resistance (ASTM D1264 water washout typically below 10% weight loss at 38°C for quality lithium greases), and broad chemical compatibility. Lithium complex grease — using a mixed lithium 12-hydroxystearate / lithium azelate thickener system — extends the dropping point to typically 260–280°C and the practical service temperature ceiling to approximately 150–170°C for continuous service, while simultaneously improving load-carrying capacity, oxidation resistance, and mechanical shear stability. Lithium complex greases are standard for automotive wheel bearing applications (where European and US OEM specifications typically mandate lithium complex or equivalent), electric motor bearings operating at elevated temperatures, and industrial roller bearings in process industries with high ambient temperatures. For the manufacturing process behind these products, our guide to industrial grease manufacturing explains the kettle saponification, dehydration, and milling steps that 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 — temperature at which the thickener structure collapses and the grease liquefies; practical service temperature limit is 30–50°C below this value
• Oil separation — ASTM D1742 — static bleed at 100°C over 30 hours; measures base oil tendency to bleed from the thickener matrix under thermal stress; excessive bleed causes oil starvation in sealed bearings
• Water washout resistance — ASTM D1264 — grease weight loss under water spray at 38°C and 79°C; 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; 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 uses pressure drop 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.