NLGI Grades Explained - Choosing the Right Grease Consistency for Real Applications

Look at almost any grease container on a maintenance storeroom shelf and you’ll find an NLGI number, usually 0, 1, 2, or 3. Most technicians know that the number describes how stiff the grease is. Few can explain why that stiffness matters for their specific application, or how to select the right number for the bearing they’re about to grease.

That gap produces real failures. A grease that’s too soft for a vertical shaft bearing drains away from the contact zone between regreasing events. A grease that’s too stiff for a centralized lubrication system starves bearings when it can’t be pumped through the feed lines at low temperature. A high-speed bearing packed with the wrong grade runs hot from churning losses, and fails early through mechanisms that look like over-greasing even when the quantity was correct.

The NLGI grade system is a standardized, quantified classification defined by ASTM D217 and maintained by the National Lubricating Grease Institute. Understanding it, what it measures, what it doesn’t measure, and how to match it to application requirements, is a fundamental lubrication skill that pays off in bearing service life and reduced maintenance cost.

What NLGI Grade Actually Measures

The NLGI grade is a numerical classification of grease consistency, a measure of how soft or stiff the grease is at a standardized temperature. It does not describe viscosity, load-carrying capacity, temperature range, water resistance, or any other performance property. Those characteristics depend on the base oil, the thickener type, and the additive package, not on the NLGI grade.

The grade is determined by the ASTM D217 cone penetration test. A standard metal cone is allowed to sink under its own weight into a grease sample for five seconds at 25°C. The sample has first been “worked”, run through 60 strokes in a standardized grease worker to simulate mechanical action in service. The depth of penetration, measured in tenths of a millimeter, is then mapped to the NLGI grade scale.

↑	Higher penetration = deeper sinking = softer grease = lower NLGI number
↓	Lower penetration = shallower sinking = stiffer grease = higher NLGI number

The NLGI grade scale runs from 000 (semi-fluid, almost pumpable like a liquid) to 6 (very hard, block-like consistency). For practical industrial bearing applications, the relevant range is 0 through 3, with grade 2 accounting for the vast majority of applications.

NLGI Grade Is NOT Viscosity

The NLGI grade describes the stiffness of the grease as a semi-solid. It says nothing about the viscosity of the base oil within that matrix, which is what actually lubricates the bearing contact zone.

A soft NLGI 0 grease can have a high-viscosity base oil (ISO VG 460) for a slow, heavily loaded application. A stiff NLGI 3 grease can have a low-viscosity base oil (ISO VG 32) for a high-speed application where the stiffer consistency prevents churning. Same NLGI grade range, opposite ends of the viscosity spectrum.

When selecting grease for a bearing, specify both the NLGI grade (for consistency/delivery) and the base oil ISO VG grade (for film formation). Specifying only the NLGI grade is an incomplete specification.

The Complete NLGI Grade Chart

The following table maps each NLGI grade to its penetration range, consistency description, and practical industrial applications.

Grade	Penetration (0.1mm)	Consistency	Analogy	Primary Applications
000	445–475	Semi-fluid	Almost liquid	Centralized lube systems, enclosed gearboxes, gear couplings at low speed
00	400–430	Semi-fluid	Very runny	Centralized systems, low-temp gearboxes, enclosed gear drives
0	355–385	Semi-fluid	Applesauce	Centralized systems, open gears, gearboxes at moderate speed
1	310–340	Soft	Tomato paste	Long grease lines, cold-start bearings, outdoor low-temp applications, single-point lubricators
2	265–295	Medium	Peanut butter	General industrial bearings, motors, pumps, fans — most widely used grade
3	220–250	Firm	Butter	Vertical shaft bearings, high-speed bearings, high-vibration, high-temp, splash/leakage risk
4	175–205	Very firm	Stiff paste	Water pump bearings, some high-speed spindle applications, open gears with fling risk
5	130–160	Hard	Smooth paté	Specialized applications; low oil separation; rarely used in standard plant bearings
6	85–115	Very hard	Block grease	Very specialized; open gears, chain lubrication, block-form application only

Sources: ASTM D217; NLGI Grease Glossary; FUCHS Lubricants; Precision Lubrication; Nye Lubricants.

The Six Factors That Drive NLGI Grade Selection

Matching an NLGI grade to an application requires evaluating six conditions. Together, they define the mechanical demands the grease consistency must meet in service.

Application Method — How Does the Grease Get to the Bearing?

Application method is the first and often most constraining factor. Manual grease guns, single-point automatic lubricators, and centralized multi-point systems each impose different pumping requirements.

Manual grease gun

NLGI 2 standard. Grades 1–3 pump reliably. NLGI 1 for cold weather or long hose extensions.

Single-point lubricators

NLGI 1 typically recommended. Low delivery pressure struggles to pump NLGI 2 through long tubing runs.

Centralized multi-point systems

NLGI 0 or 1 required. NLGI 2 will not flow reliably through narrow feed lines at low ambient temperatures.

Operating Temperature — Both Upper and Lower Limits

At elevated temperatures: grease softens. For high-temperature applications, oven conveyor bearings, kiln car bearings, equipment near hot processes, specifying NLGI 3 provides a margin against excessive softening that would allow the grease to drain from the contact zone.

At low ambient temperatures: grease stiffens. For cold-start applications, outdoor equipment in northern climates, refrigerated facility drives, cold storage conveyors, NLGI 1 or even 0 improves startup lubrication reliability.

The NLGI grade does not define the low-temperature operating limit of the grease. That is determined by the base oil’s pour point and the thickener’s low-temperature characteristics. Both must be checked against actual operating conditions.

Speed — Channeling vs. Non-Channeling Behavior

Bearing speed is the factor that produces the counterintuitive NLGI grade recommendations that trip up even experienced maintenance practitioners. At moderate speeds, NLGI 2 grease provides the right balance. At high speeds (nDm above roughly 300,000 mm·rpm), the rolling elements are moving fast enough to continually churn through any grease that falls back into their path. Churning generates heat rather than film.

The Counterintuitive Rule for High-Speed Bearings

The solution for high-speed bearings is a stiffer grade — NLGI 3 — that exhibits channeling behavior. A channeling grease is pushed aside by the rolling elements and stays displaced. The rolling elements then wick base oil from the stationary grease reservoir at the sides of the bearing cavity, without excess grease being dragged through the contact path on each rotation.

Shaft Orientation — Horizontal vs. Vertical

On a horizontal shaft, gravity acts perpendicular to the shaft axis, grease tends to stay approximately where it is applied. On a vertical shaft, gravity acts along the shaft axis, creating a constant force pulling grease down and away from the upper bearing position. For vertical shaft applications, vertical pump motors, agitator drives, fan shafts, specifying NLGI 3 instead of NLGI 2 provides the structural stiffness needed to resist gravity-driven migration.

Load and Shock

Heavily loaded bearings and applications subject to shock loading demand grease that maintains its structural integrity under mechanical stress. Under sustained heavy load or repeated impact, a soft grease can be physically displaced from the contact zone more rapidly than the thickener can re-establish the reservoir. For severe shock conditions NLGI 3 provides a measurable retention advantage.

Environment — Contamination, Water, and Splash Risk

Stiffer greases resist contamination ingress better than softer ones, the thickener matrix provides a physical barrier at seal interfaces that a soft or semi-fluid grease cannot. In dusty or wet environments, NLGI 2 or 3 provides better sealing at the bearing lip seal interface than grades 0 or 1. For open gears or chains where fling-off is a risk, NLGI 3 or 4 provides better adhesion at speed.

For applications with splash water risk, steam exposure, or regular wash-down, the primary defense against water contamination is the thickener chemistry (calcium sulfonate provides inherent water resistance) and the seal design, not the NLGI grade alone.

Field Note — The Vertical Pump Motor Failure Pattern

A common failure pattern in facilities that run blanket NLGI 2 grease programs: vertical pump motor bearings failing faster than horizontal motor bearings of the same type and speed. The diagnosis typically comes back as “lubricant starvation”, but the regreasing interval and quantity are the same as the horizontal machines.

The root cause is grade, not interval: NLGI 2 migrates away from the upper bearing position on a vertical shaft between regreasing events.

Switching those specific positions to NLGI 3 at the same regreasing interval eliminates the failure pattern. The NLGI grade change costs nothing beyond updating the lubrication specification.

NLGI Grade Selection Guide

Practical starting-point recommendations for common industrial applications:

Application Condition	Recommended Grade	Rationale
General industrial motors, pumps, fans, manual regreasing, moderate speed, moderate temp	NLGI 2	Default starting point. Correct for the vast majority of standard rotating equipment.
Centralized (automatic) grease system with long lines or cold feedlines	NLGI 0 or 1	Softer grades pump reliably through narrow lines and multiple fittings without cavitation.
Bearing on vertical shaft	NLGI 3	Gravity promotes migration; stiffer grade resists drainage from the contact zone.
High-speed bearing (nDm > 300,000) with channeling required	NLGI 2 or 3	Stiffer grade channels away from rolling elements, reduces churning at high speed.
Cold-start application (ambient < 0°C / 32°F)	NLGI 1 or 0	Softer grade flows at low temperature; prevents starvation on initial rotation.
High-temperature application (housing > 100°C / 212°F)	NLGI 2 or 3	Firmer grade resists thinning and migration at elevated temp. Verify grease temp rating.
High-vibration environment (conveyors, crushers, fans)	NLGI 2 or 3	Firmer grade resists mechanical breakdown under repeated shock load.
Open gear or chain — fling risk	NLGI 3 or 4	Stiff enough to adhere to moving parts without being flung off under centrifugal force.
Sealed-for-life bearing (factory-filled)	NLGI 2 or 3	Most factory-filled bearings use NLGI 2 or 3 with synthetic base oil for long service life.
Equipment exposed to water wash-out or steam	NLGI 2 or 3	Grade alone doesn’t provide water resistance, thickener type (CaS) critical alongside grade.

Sources: Precision Lubrication; Nye Lubricants; FUCHS Lubricants; Interflon; Olezol NLGI Classifications.

What NLGI Grade Cannot Tell You

Understanding the limits of the NLGI grade system is as important as understanding what it measures. Three common misconceptions cause selection errors.

NLGI Grade Does Not Define Performance Temperature Range

The temperature at which a grease remains effective — both the upper limit (dropping point, oxidation stability) and the lower limit (low-temperature torque, cold start pumpability) — is determined by the thickener system and base oil, not by the NLGI grade. A lithium soap grease and a polyurea grease can both be NLGI 2 while having significantly different performance temperature ranges. Verifying the grease’s dropping point and low-temperature characteristics against the application’s operating temperature range is a separate and necessary step from specifying the NLGI grade.

NLGI Grade Does Not Define Load-Carrying Capability

The ability of a grease to protect surfaces under boundary lubrication conditions is provided by extreme pressure (EP) and anti-wear (AW) additives, not by the NLGI grade. A heavily loaded slow-speed bearing that requires EP grease needs that additive specification in addition to the correct NLGI grade. Specifying NLGI 3 does not provide EP protection; specifying NLGI 2 with an EP additive package does.

NLGI Grade Does Not Indicate Thickener Compatibility

The NLGI grade says nothing about the thickener system — and thickener compatibility is critical when switching grease products or when multiple grease types are present in a facility’s inventory. Two NLGI 2 greases, one lithium and one polyurea, are likely incompatible if mixed — the mixture can lose structural integrity entirely. Thickener type must be specified and verified independently of the NLGI grade when changing grease products or making substitutions.

NLGI Grade in the Context of a Complete Lubrication Specification

A complete grease specification for a bearing position should include at minimum four parameters, NLGI grade is only one of them:

A Complete Grease Specification Includes All Four Parameters

NLGI Grade	for consistency and delivery characteristics
Base oil ISO VG grade	for film formation at operating temperature, the primary lubrication parameter
Thickener type	for temperature range, water resistance, and compatibility
Additive package	EP, AW, rust inhibitor as appropriate for the application

The NLGI grade is one of four necessary specifications. A facility that specifies only the NLGI grade, or worse, specifies only the product name without documenting why, has an incomplete lubrication program that cannot be effectively reviewed, updated, or defended when equipment fails. When the application conditions change, new operating speed, different ambient temperature, changed shaft orientation, re-evaluate all four parameters, not just the product name.

What Good Looks Like

A lubrication specification card for a conveyor head pulley bearing position reads:

Bearing	22320 CC/W33 spherical roller
NLGI Grade	2
Base oil ISO VG	220
Thickener	Lithium complex
Additives	EP
Relubrication interval	45 days (calculated: 1,800 rpm, horizontal, 75°C, light contamination)
Quantity	22 g per event

That specification took 20 minutes to prepare and documents the reasoning for every parameter. When the conveyor is upgraded to run at higher speed, the reliability engineer recalculates and updates the interval and verifies whether the NLGI grade and base oil viscosity remain appropriate. The specification is a living document, not a one-time decision.

The Bottom Line

The NLGI grade is a well-defined, standardized parameter — measured by ASTM D217, classified by the NLGI, and incorporated into ISO 6743-9. It tells you one important thing: how stiff the grease is at 25°C before it goes into service. It doesn’t tell you how the grease will perform at operating temperature, under load, against contamination, or over time. Those properties come from the base oil, the thickener, and the additives.

Used correctly, NLGI grade selection accounts for application method, temperature, speed, shaft orientation, load, and environment. The starting point for most industrial rotating equipment is NLGI 2. The common variations are NLGI 1 for centralized systems and cold-weather applications, and NLGI 3 for vertical shafts, high-speed bearings, and high-vibration conditions.

The NLGI grade is a delivery parameter. Get it right for the application, then specify the rest of the grease, base oil viscosity, thickener type, additive package, to match the actual performance requirements. A grease specification that starts and ends with an NLGI grade is incomplete. A specification that uses NLGI grade as one of four documented parameters is the foundation of a lubrication program that actually connects to bearing reliability outcomes.

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