What Maintenance Teams Need to Know in the Field
Most grease selection decisions in industrial facilities get made the same way: the OEM specified a grease type, someone found a product that matched the description, and that product has been reordered ever since.
The decisions that produce premature bearing failures, excessive heat, corrosion in wet environments, or accelerated oxidation in high-temperature applications almost always trace back to the same root cause: the wrong grease for the conditions. Not contaminated grease. Not too little grease. Not the wrong regreasing interval. Wrong grease.
What Grease Is — and What It Isn’t
Grease is not a special category of lubricant distinct from oil. It’s oil held in a semi-solid matrix. The base oil does the actual lubricating. The thickener is the structure that holds the oil in place. Additives modify specific performance characteristics.
Grease lubricates through its base oil, not through its thickener. The thickener’s job is delivery and retention.
When people say a grease is “too heavy” or “too light” — they almost always mean the base oil viscosity, not the thickener consistency.
When they say a grease “doesn’t hold up” at high temperature — they usually mean the base oil is evaporating or oxidizing — the thickener may be fine.
Forms the film between metal surfaces. Viscosity at operating temperature is the dominant selection criterion.
Holds the oil in place. Releases it to the contact zone. Determines temp limits, water resistance, and compatibility.
Oxidation resistance, corrosion protection, EP capability, anti-wear, tackiness — properties base + thickener alone can’t deliver.
Component 1: The Base Oil
The base oil accounts for 70–95% of grease by weight in most industrial formulations. Its viscosity, temperature-viscosity relationship, oxidation stability, and seal compatibility are the dominant factors in whether the grease provides adequate lubrication across the operating conditions of the application.
Mineral Base Oils
The majority of industrial greases use mineral (petroleum-derived) base oils. Group I and Group II mineral oils are adequate for most general industrial applications but have limited oxidation stability at temperature extremes. Group III oils are more highly refined, approaching synthetic performance in some characteristics. For most standard industrial bearing applications within normal temperature ranges, Group II mineral base oil greases are appropriate and cost-effective.
Synthetic Base Oils
Base Oil Viscosity: The Number That Matters Most
The most important single characteristic of the base oil for bearing lubrication is its kinematic viscosity at operating temperature. Industrial greases are described by their base oil ISO VG grade — the viscosity measured at 40°C in mm²/s (cSt).
| ISO VG Range | Typical Speed / Load Conditions | Typical Applications |
|---|---|---|
| VG 32–68 | High speed, lower load | Electric motors, centrifugal pumps, blowers at higher nDm values |
| VG 100–150 | Moderate speed, general use | Most standard industrial motors and pumps — the most common range |
| VG 220–460 | Slow speed, high load, shock | Rolling mill bearings, slow conveyor drives, crushers, heavy shock loading |
Component 2: The Thickener
The thickener forms the three-dimensional matrix — sometimes described as a sponge structure — that retains the base oil and releases it gradually to the lubrication contact zone during operation. The thickener type determines temperature limits, water resistance, compatibility with other greases, and several other critical application characteristics.
Understanding thickener types is essential for two reasons: selecting the right grease for the application, and avoiding incompatibility when changing grease products.
Thickener Types — Quick Reference
| Thickener | Max Temp | Water Resist. | Load Cap. | Rust Inhib. | Typical Applications |
|---|---|---|---|---|---|
| Lithium (Li) | ~180°C | Good | Fair | No | Motors, pumps, general industrial — most widely used |
| Lithium Complex (LiX) | ~220°C | Excellent | Good | No | High-temp, wide temp range, high load — upgrade from standard Li |
| Calcium (Ca) | ~120°C | Excellent | Poor | Yes | Wet environments, marine, low-speed — limited high-temp capability |
| Calcium Sulfonate (CaS) | ~200°C | Excellent | Excellent | Yes | Heavy load + wet/contaminated environments, marine — premium all-rounder |
| Polyurea (PU) | ~180°C | Good | Fair | No | Electric motors, sealed-for-life — INCOMPATIBLE with most thickeners |
| Bentone / Clay | ~200°C | Fair | Good | No | High-temp oven chains, kilns — where melting point is critical |
| PTFE / Fluorinated | 260°C+ | Excellent | Excellent | No | Extreme temp, chemical exposure, food-grade — very high cost |
Grease Thickener Compatibility: The Mixing Problem
Thickener incompatibility is one of the most consequential and most underappreciated risks in industrial lubrication. When two greases with incompatible thickeners are mixed in a bearing, the mixture can soften dramatically, lose oil retention capacity, and fail to lubricate the bearing even though both individual greases would have performed adequately on their own.
Mild: Slight softening — no immediate failure, but reduced performance.
Severe: Complete oil separation — rapid film failure — bearing failure within hours.
Most dangerous combinations: polyurea grease mixed with anything else, and sodium grease mixed with calcium or lithium in wet environments.
Compatibility Matrix — Field Reference
C = Generally compatible I = Incompatible — do not mix ? = Use caution — test or purge before switching
| Into → Has | Li | LiX | Ca | CaS | PU | Clay | Na |
|---|---|---|---|---|---|---|---|
| Li | — | C | C | C | I | C | I |
| LiX | C | — | C | C | I | C | I |
| Ca | C | C | — | ? | I | ? | C |
| CaS | C | C | ? | — | I | ? | ? |
| PU | I | I | I | I | — | I | I |
| Clay | C | C | ? | ? | I | — | ? |
| Na | I | I | C | ? | I | ? | — |
‘Compatible’ does not mean ‘identical performance’ — even compatible greases can produce a mixture with different characteristics than either product alone. For critical assets, consult the lubricant supplier and consider complete bearing repack when switching thickener types.
Component 3: Additives
Additives are the performance modifiers — typically 1–10% of grease composition by weight, but their impact on performance in specific conditions can be decisive. They enhance or provide specific properties that the base oil and thickener system alone don’t deliver adequately for the application.
NLGI Consistency Grade: What It Tells You and What It Doesn’t
The NLGI consistency number is the most visible specification on a grease data sheet — and the most commonly misunderstood. It describes how soft or hard the grease is, not any other performance characteristic.
Two NLGI 2 greases with different thickeners, different base oils, and different additive packages will have very different performance profiles in the same application — even though they share the same consistency grade. NLGI grade is determined primarily by thickener concentration. It is not an indicator of temperature capability, water resistance, oxidation stability, or load capacity.
| NLGI Grade | Consistency | Use When… | Notes |
|---|---|---|---|
| 000–0 | Semi-fluid | Centralized systems, very large bearings, enclosed gears | Not for standard open bearings |
| 1 | Soft | Centralized lubrication systems, cold climates, very high-speed | Low retention — not for vertical shafts |
| 2 | Standard | Most industrial rolling element bearing applications — the default starting point | Most widely specified grade |
| 3 | Firm | Vertical shaft bearings, high-temperature applications, where extra retention is needed | Higher starting torque at cold temps |
| 4–6 | Hard | Open gears, specialized applications requiring extreme retention | Very limited use in rolling element bearings |
Putting It Together: Grease Selection in the Field
Before looking at any grease specification, define what the application actually requires:
- Operating temperature range — minimum startup temperature, maximum continuous operating temperature
- Speed — calculate nDm to determine required base oil viscosity range
- Load type — radial, axial, combined, shock loading
- Environmental exposure — clean/dry, wet, humid, dusty, contaminated, chemical exposure
- Regreasing access — easy periodic regreasing, sealed-for-life, centralized system
- Component materials — steel, brass, bronze, elastomers (affects EP additive suitability)
| Application Condition | Recommended Composition |
|---|---|
| High temp (>150°C continuous) | LiX or CaS thickener + synthetic (PAO) base oil + antioxidant package. Bentone for above 220°C. |
| Wet or wash-down environments | Calcium sulfonate thickener preferred (inherent rust inhibition) + strong corrosion inhibitor + confirm ASTM D1264 water washout resistance. |
| High-speed (nDm > 300,000) | Low-to-medium viscosity base oil (ISO VG 32–100) + LiX or PU thickener. No solid lubricants. |
| Heavy load / slow speed | High-viscosity base oil (ISO VG 220–460) + EP additive package + LiX or CaS thickener. |
| Sealed-for-life bearings | PU or LiX thickener + synthetic base oil + high oxidation stability package. |
| Food-grade applications | NSF H1 registered + PFPE or PAO base oil. No EP additives containing sulfur or heavy metals. |
Grease compatibility management starts with storeroom discipline, not with lubrication engineers.
- Label every grease with its thickener type as well as product name and NLGI grade
- Document every bearing’s grease specification including thickener type
- Compatibility check before any product change or substitution — not after a failure surfaces the problem
- Maintain a one-page storeroom matrix showing which greases are stocked, their thickener systems, and which are compatible with each other — accessible to planners and technicians
A reliability engineer audits grease inventory at a large pump station and finds six different greases — none with thickener type on the label. Cross-referencing with data sheets reveals three different thickener systems, including a polyurea grease and two lithium greases with different base oil viscosity grades. She creates a one-page grease reference card listing product name, thickener type, base oil viscosity grade, NLGI grade, and compatible alternatives. That card goes into every lubrication station. Two grease products are consolidated. All critical bearing regreasing procedures are updated to specify thickener type, not just product name. Zero compatibility incidents in the following 18 months.
Grease is three things: base oil that lubricates, thickener that delivers and retains the oil, and additives that enhance performance in specific conditions.
Every grease failure that isn’t caused by contamination, incorrect quantity, or wrong regreasing interval is caused by a mismatch between one of those three components and the conditions of the application.
The facilities that do this well treat grease selection as a technical decision — not a procurement default. They control thickener types in their storeroom. They specify base oil viscosity grade alongside NLGI consistency. They manage product transitions with compatibility checks built into the process. That discipline is what keeps bearings running as long as they should.
