7.2 Bearing Lubrication, Handling, and Failure Analysis
Key Takeaways
- Grease consists of a base oil, a thickener, and additives, and the base oil does the lubricating while the thickener holds it in place.
- NLGI grades run from 000 to 6 by penetration, and NLGI 2 is the general-purpose industrial consistency.
- A grease-lubricated bearing housing is normally filled only about one-third to one-half, because over-greasing causes churning, heat, and failure.
- Viscosity is the single most important property of a lubricating oil, and ISO viscosity grade numbers state the kinematic viscosity in centistokes at 40 degrees Celsius.
- Bearing failure evidence is diagnostic: fluting points to stray shaft current, false brinelling to vibration while stationary, and a load path across both races to misalignment.
The three lubrication regimes
| Regime | Film condition | Where it occurs |
|---|---|---|
| Boundary | Surfaces touch; only additives protect them | Startup, shutdown, shock load, very low speed |
| Mixed | Partial film with intermittent contact | Transition through low speed |
| Full film (hydrodynamic or elastohydrodynamic) | Surfaces fully separated | Normal running speed with correct viscosity |
A journal bearing runs in hydrodynamic film. A rolling element bearing runs in elastohydrodynamic film, where the extreme contact pressure momentarily stiffens the oil and elastically flattens the metal. Both depend on the right viscosity at the actual operating temperature. Oil that is too thin cannot build the film; oil that is too thick generates churning heat and starves the contact.
Oil: viscosity and ISO grades
Viscosity is resistance to flow, and it is the most important single property of a lubricating oil. It falls as temperature rises, which is why a viscosity value is meaningless without a temperature.
ISO viscosity grade (ISO VG) numbers state the nominal kinematic viscosity in centistokes at 40°C. ISO VG 46 oil is nominally 46 cSt at 40°C, with an allowable band of plus or minus 10 percent. The standard grades step by roughly 50 percent: 32, 46, 68, 100, 150, 220, 320, 460, 680.
| Typical application | Common ISO grade |
|---|---|
| Hydraulic systems, light machine oil | 32 to 46 |
| Electric motor and pump bearing oil | 46 to 68 |
| Industrial gearboxes, moderate | 150 to 320 |
| Heavily loaded and slow gearing | 460 to 680 |
The viscosity index (VI) describes how little the viscosity changes with temperature: a high VI oil holds its viscosity better across the range.
Additives do the rest of the work: antiwear and extreme pressure (EP) additives protect in boundary conditions, oxidation inhibitors extend life, rust and corrosion inhibitors protect surfaces, demulsifiers shed water, and antifoam agents break entrained air.
Grease: base oil, thickener, and NLGI grade
Grease is base oil held in a thickener matrix, usually a metallic soap, with additives. The base oil lubricates; the thickener is a sponge that holds it at the contact and seals against contamination.
| Thickener | Character |
|---|---|
| Lithium / lithium complex | The general-purpose industrial standard; good temperature and water resistance |
| Calcium / calcium sulfonate | Excellent water resistance |
| Polyurea | Common in sealed-for-life electric motor bearings; long life at temperature |
| Aluminum complex | Good high-temperature and food-grade options |
| Clay (bentonite) | Non-melting; high temperature, but limited otherwise |
Do not mix incompatible thickeners. Mixing a polyurea and a lithium complex grease can collapse the structure, releasing the oil and leaving a hard residue that blocks the bearing. When switching greases, purge the housing thoroughly.
NLGI grade classifies grease by consistency, measured by cone penetration. The scale runs 000, 00, 0, 1, 2, 3, 4, 5, 6 from nearly fluid to block-hard.
| NLGI grade | Consistency | Typical use |
|---|---|---|
| 000 to 0 | Semi-fluid | Centralized systems, enclosed gear lubricants |
| 1 | Soft | Cold service, long feed lines |
| 2 | Soft-solid, like peanut butter | General industrial bearing grease |
| 3 | Firm | High speed or vertical shafts where retention matters |
| 4 to 6 | Very firm to hard | Special applications and blocks |
NLGI 2 is the answer to the general-purpose question.
Relubrication practice
- Fill the housing only about one-third to one-half. The remaining space is where the grease churns out of the rolling path. A completely packed housing has nowhere to shed the excess, so the elements plow through grease, the temperature rises, the base oil separates, and the bearing fails. Over-greasing causes more bearing failures than under-greasing.
- Clean the fitting before applying the gun. Contamination pushed through the zerk goes straight into the bearing.
- Relubricate while the machine is running where it is safe to do so, so fresh grease distributes, and open the relief or drain plug so old grease can escape rather than blowing out a seal.
- Slow, steady strokes. A high-pressure burst can unseat a shield or rupture a lip seal.
- Follow the manufacturer's quantity and interval. Interval shortens with speed, temperature, contamination, and vertical shaft orientation.
Storage and handling
Bearings and lubricants fail before installation more often than anyone expects.
- Store bearings in their sealed packaging, flat, in a clean dry area, and open them only at the point of use.
- Store oil drums on their sides or under cover. A drum stored upright outdoors breathes water past the bung as it heats and cools, and the water goes into the oil.
- Keep dedicated, labelled transfer containers for each lubricant. A shared funnel is a cross-contamination path.
- Rotate stock first in, first out; grease and oil both have a shelf life.
Reading a failed bearing
| Evidence | Mechanism |
|---|---|
| Spalling — flaked, pitted raceway | Normal subsurface fatigue at end of life, or accelerated by overload or contamination |
| Fluting — evenly spaced washboard marks | Electrical discharge through the bearing, commonly stray shaft current from a drive |
| True brinelling — dents at rolling element spacing, sharp edged | Static shock load, or driving the mounting force through the rolling elements |
| False brinelling — shallow polished depressions at element spacing | Vibration while the shaft is stationary, often during transport or on a standby machine |
| Discoloration — blue or straw tempering colors | Overheating from over-greasing, preload, or lubricant failure |
| Load path across the full width of both races | Misalignment or a cocked bearing |
| Wide load path on the outer race only | Excessive preload or housing bore too tight |
| Fretting corrosion on the bore or outside diameter | Ring creeping on its seat; the fit is too loose |
| Etching, red-brown staining | Water contamination |
These patterns are the basis of the failure analysis expected in the Maintenance and Troubleshooting domain. A bearing that comes out fluted does not need a better bearing; it needs the shaft current path addressed.
A maintenance technician packs a motor bearing housing completely full of NLGI 2 grease to be thorough. What is the likely outcome?
A bearing removed from a motor driven by a variable speed drive shows evenly spaced washboard-like ridges around the outer raceway. What does this indicate?
What does the number in an ISO viscosity grade such as ISO VG 220 represent?