Grease Selection Guide – NLGI Consistency & Technical Specifications

Understanding Grease Fundamentals

Grease is not simply “thick oil”—it is a complex colloidal suspension consisting of three critical components: base fluid (70-95%), thickener (3-20%), and additive package (0-10%). The interaction between these components determines pumpability, temperature stability, load-carrying capacity, and service life in automatic lubrication systems. rnUnlike oil, grease acts as a reservoir, releasing base oil slowly under shear and capillary action. This makes grease ideal for applications where oil would leak away (open bearings) or where relubrication intervals must be extended. However, improper grease selection is responsible for 45% of failures in centralized lubrication systems, manifesting as pump cavitation, line blockages, or bearing starvation.

NLGI Consistency Grades Explained

The National Lubricating Grease Institute (NLGI) established the standard penetration scale measuring grease consistency using a penetrometer (ASTM D217). A cone of specific weight and angle is dropped into the grease for 5 seconds; penetration depth (in tenths of a millimeter) determines the grade.

NLGI Grade	Penetration Range	Consistency Description	Automatic System Application
000	445-475	Fluid (semiliquid)	Very long lines (>100m), arctic conditions (-40°C), oil-mist conversions
00	400-430	Semifluid	Long lines, cold climates, centralized systems with small diameter tubing
0	355-385	Very soft	Moderate length lines (50-100m), cold weather, gearboxes
1	310-340	Soft	General automatic systems (20-50m lines), moderate climates
2	265-295	Medium (most common)	Standard industrial (10-30m lines), normal temperatures, bearings
3	220-250	Firm	High-load bearings, warm climates, short lines, manual application
4	175-205	Very firm	Manual application only, not suitable for auto systems
5-6	130-160	Blocky (hard)	Specialty manual applications only

What Do NLGI Numbers Mean?

An NLGI number (also written as the NLGI consistency number or NLGI grade) classifies grease stiffness on a scale from 000 (softest, semi-fluid) to 6 (hardest, brick-like). The number comes from the ASTM D217 worked-penetration test: a standardized cone sinks into the grease for 5 seconds at 25°C, and the depth it reaches — measured in tenths of a millimetre — maps to a grade. Lower numbers flow more easily (pumpable in cold lines and single-line systems); higher numbers stay in place under heat, load, and vibration. Most multipurpose greases are NLGI 2.

A note on names: “NLGI grease” is not a product type — every lithium, calcium, or polyurea grease carries an NLGI grade alongside its thickener type and base-oil viscosity. The grade answers how stiff it is, the thickener answers how it was made, and the ISO VG number answers how thick the base oil is.

Pumpability Considerations

Automatic lubrication systems require grease to flow through small diameter lines (4-10mm) and injector valves. As temperature drops, grease becomes more viscous. NLGI #2 grease at -10°C may require 40 MPa to pump, while NLGI #1 requires only 25 MPa. For systems operating below -20°C, NLGI #0 or #00 is mandatory to prevent pump cavitation and line blockage. Rule of Thumb: For every 10°C below 0°C, grease effectively increases one NLGI grade in stiffness. A #2 grease at -20°C behaves like #4 at room temperature.

Base Oil Viscosity Selection (ISO VG)

The base oil provides the actual lubricating film. Viscosity must be sufficient to maintain separation between moving parts under load and temperature, but not so high that flow resistance causes pump overload.

ISO VG Grade	Viscosity @ 40°C (cSt)	Typical Applications
VG 32-68	32-68	High-speed bearings (>10,000 RPM), low-temp operation
VG 100-150	100-150	General bearings (1000-10,000 RPM), normal loads
VG 220-460	220-460	Slow-speed bearings (<1000 RPM), high loads, gears
VG 680-1500	680-1500	Very slow speed, extreme loads, open gears

Selection Formula: For ball bearings, base oil viscosity (cSt) at operating temperature should be ≥ 13 × n^(-0.5), where n = speed in RPM. For roller bearings, use ≥ 20 × n^(-0.5).

Thickener Chemistry: Beyond Simple Lithium

The thickener acts as a sponge, holding base oil in a fibrous or particulate structure. When shear is applied (bearing rotation), oil bleeds out; when shear stops, the thickener re-absorbs oil. Different chemistries offer vastly different performance characteristics.

Lithium Soap (Li)

Structure: 12-hydroxystearic acid + lithium hydroxide Properties: Good water resistance, smooth texture, decent high-temp (120°C max continuous) Limitations: Poor shear stability over time (oil separation), limited to moderate temperatures Use: General industrial, automotive chassis, inexpensive applications

Lithium Complex (LiX)

Structure: Complex of 12-HSA + azelaic acid + lithium Properties: Excellent high-temp (150-175°C), superior shear stability, good water resistance Advantages: Most versatile industrial thickener Use: Bearings, electric motors, automotive wheel bearings, general industrial

Calcium Sulfonate Complex (CaS)

Structure: Overbased calcium sulfonate converted to complex Properties: Exceptional corrosion protection (ASTM D1743), high load capacity (Timken OK >60 lbs), excellent water washout resistance Advantages: Inherent EP properties without additives, superior marine/wet performance Use: Steel mills, marine, paper machines, food processing (H1 available)

Polyurea (PU)

Structure: Diisocyanate + diamine reaction product Properties: Outstanding oxidation stability, low noise characteristics (electric motors), high drop point (>250°C) Limitations: Poor compatibility with other greases (must purge system thoroughly), sensitive to moisture during manufacturing Use: Electric motor bearings, sealed-for-life bearings, high-temp fans

Aluminum Complex (AlX)

Properties: Excellent reversibility (returns to original consistency after heating/cooling cycles), good water resistance, transparent appearance Use: Food processing (H1), pharmaceutical, cosmetic equipment

Additive Technology Deep Dive

Extreme Pressure (EP) Agents

Chemical compounds (sulfur-phosphorus, borate, molybdenum) that react with metal surfaces under high load to form protective sacrificial films. Essential for gears, bearings under shock loads, and slow-speed high-load applications. Testing: Timken OK Load (ASTM D2509), 4-Ball Weld Load (ASTM D2596)

Anti-Wear (AW) Additives

Zinc dialkyldithiophosphate (ZDDP) and ashless phosphorus compounds reduce wear under boundary lubrication conditions. Critical for hydraulic systems, rolling element bearings.

Solid Lubricants

Molybdenum disulfide (MoS2), graphite, and PTFE provide lubrication when base oil is squeezed out or during startup before hydrodynamic film forms. Used in mining, steel mills, and slow-speed oscillating bearings.

Oxidation Inhibitors

Amines and phenols prevent base oil degradation at high temperatures. Each 10°C above 70°C doubles oxidation rate. Critical for electric motors and high-speed bearings.

Application-Specific Selection Guide

CNC Machine Tools & High-Speed Spindles

• NLGI: #2 (summer), #1 (winter/cold shops)
• Base Oil: VG 68-100 (synthetic PAO or ester)
• Thickener: Polyurea or Lithium Complex
• Additives: Anti-wear, oxidation inhibitors
• Special: Low noise formulation for precision bearings

Mining & Heavy Construction

• NLGI: #1.5-#2 (pumpable in cold), #2-#3 (hot climates)
• Base Oil: VG 460-680 (mineral or synthetic)
• Thickener: Calcium Sulfonate Complex or Lithium Complex
• Additives: EP, MoS2 or graphite solids, tackifiers
• Special: High adhesion to prevent sling-off

Steel Mills & High Temperature

• NLGI: #1.5-#2
• Base Oil: VG 220-460 synthetic (PAO or polyol ester)
• Thickener: Polyurea (best) or Calcium Sulfonate Complex
• Additives: High-performance oxidation inhibitors, EP
• Special: Must handle 150°C+ ambient near kilns

Marine & Offshore

• NLGI: #2 (general), #1 (cold weather)
• Base Oil: VG 150-220
• Thickener: Calcium Sulfonate Complex (superior water resistance)
• Additives: Corrosion inhibitors (ASTM D1743 pass), tackifiers
• Special: Biodegradable options for deck runoff (EU EAL)

Food Processing (NSF H1)

• NLGI: #2 (general), #1 (cold rooms)
• Base Oil: VG 100-220 (white mineral or PAO)
• Thickener: Aluminum Complex or Calcium Sulfonate
• Additives: Food-grade approved only
• Special: Must pass NSF H1 registration

Testing & Quality Verification

Essential ASTM Tests

• D217: Cone Penetration (consistency)
• D2265: Dropping Point (high-temp limit)
• D942: Oxidation Stability (pressure vessel, 100 hrs @ 99°C)
• D1264: Water Washout (resistance to spray)
• D1743: Corrosion Prevention (bearing rust test)
• D2596: 4-Ball EP (load-carrying capacity)
• D2266: 4-Ball Wear (anti-wear performance)

Frequently Asked Questions About grease selection guide

What is the best NLGI grade for automatic lubrication systems?

For most automatic systems with 10-50 meter lines and normal temperatures (0-40°C), NLGI #2 is standard. For cold climates (-20°C and below), use NLGI #1 or #0 to ensure pumpability. For very long lines (>100m) or arctic conditions (-40°C), use NLGI #00 or #000. In hot climates (>40°C ambient) or high-load bearings, NLGI #3 provides better stay-put characteristics but requires higher pump pressure.

Can the engineer mix different grease types in the system?

No—grease compatibility is complex and unpredictable. Mixing different thickener types (e.g., lithium with polyurea) often causes severe softening or hardening, leading to leakage or blockages. When changing grease types, completely purge the system: remove all old grease from reservoir, pump, lines, and fittings. Run new grease through until all old material is flushed (typically 2-3 full reservoir volumes).

How do the engineer calculate the right base oil viscosity?

For rolling element bearings, use the formula: ν = 45000 × n^(-0.83) × d^(-0.5), where n = RPM and d = bearing bore diameter (mm). This gives the minimum viscosity (mm²/s) at operating temperature. Then select base oil viscosity at 40°C that provides this value at the operating temperature (accounting for viscosity index). For slow-speed heavily loaded bearings (excavators, kilns), use VG 460-680. For high-speed (CNC spindles), use VG 32-68.

Why does the grease harden in the pump reservoir?

Grease hardening is usually oxidation caused by: (1) excessive heat (reservoir mounted near hot equipment >60°C), (2) long storage time (grease shelf life is typically 2-3 years unopened), or (3) contamination with dust/debris entering through breathers. Hardened grease cannot be pumped and must be removed. Install reservoir cooling or relocate pump, use desiccant breathers, and implement first-in-first-out inventory rotation.

What is the difference between EP and non-EP grease?

EP (Extreme Pressure) greases contain chemical additives (sulfur-phosphorus, borate) that react with metal surfaces under high load (>1000 MPa contact stress) to form protective films. Essential for gears, slow-speed bearings, and shock loads. Non-EP greases rely only on hydrodynamic film and are suitable for rolling bearings under normal loads. Using non-EP grease in high-load applications causes micropitting and premature failure.

How often should the engineer change grease in centralized systems?

For continuously operating systems: every 6-12 months or when grease shows signs of oxidation (darkening, hardening, acidic smell). For intermittent systems: every 12-24 months. In high-temperature applications (>80°C), change every 3-6 months. Always monitor grease consistency monthly—if it becomes harder or softer than original specification by more than one NLGI grade, immediate replacement is required.

Can the engineer use food-grade grease (H1) in non-food applications?

Yes, food-grade greases perform excellently in general industrial applications, particularly Aluminum Complex and Calcium Sulfonate thickeners. However, they cost 2-3x more than standard industrial greases. For marine, steel, and mining applications where corrosion resistance is critical, food-grade Calcium Sulfonate greases are actually superior to standard lithium greases despite the higher cost.