Grease analysis needs a sampling and interpretation strategy of its own
Grease is semi-solid, heterogeneous and often available only in small quantities. Location, purge history and sample handling strongly influence the result.
Reviewed Jul 2026 · Lubricants & Fluids Wiki editorial team
This visual orients the topic; the article explains the evidence controls, method limitations and maintenance decisions in detail.
Oil procedures cannot simply be transferred to grease. A bearing can contain fresh supply grease, worked grease in the load zone, oxidised material near hot surfaces and contaminated purge. The sample must match the maintenance question.
Choose the grease population
For condition monitoring, collect used grease that has experienced the loaded contact where safely possible. Purge grease can be useful, but the first material may represent a line or relief path rather than the bearing. Record replenishment timing and quantity because recent relubrication can dilute wear and contamination evidence.
Use clean non-scraping tools compatible with the analysis. Avoid introducing lint, dirt, rust or metal from the collection device. Small samples can support screening, but penetration, worked stability and some performance tests require more material than microscopy or elemental work.
- Bearing and sampling location
- Hours since relubrication
- Grease identity and compatibility
- Collected mass and container
- Appearance and purge observations
Build the test set around the question
Visual examination, consistency or penetration, oil separation, water, elemental analysis, ferrous response, infrared comparison and microscopy each address different changes. New-grease qualification may also require drop point, oxidation, corrosion, wear or load-carrying methods.
Elemental analysis can trend fine wear, contamination and additive patterns, but preparation and grease heterogeneity matter. Ferrous density and microscopy help when larger bearing debris is suspected. Compare with the correct unused grease and a consistent used-grease sampling location.
Interpret change cautiously
A softer sample may reflect mechanical shear, incompatible mixing, oil addition or high-temperature change; a harder sample may reflect oil loss, oxidation, contamination or sampling from a dried zone. High iron can arise from active wear, rust, residual debris or the collection method.
Evaluate bearing temperature, vibration, replenishment, seal condition, environment and grease consumption. A laboratory result should help select inspection or lubrication action, not declare a bearing failure from one heterogeneous sample.
Use the finding to improve lubrication practice
Corrective actions can include controlling quantity and interval, improving seals, changing application method, preventing incompatible mixing, investigating heat or load, and inspecting the bearing. After correction, use the same sampling approach to verify whether the evidence improves.
Keep separate baselines by grease, bearing type and collection method. Combining scrape samples, purge samples and new grease in one trend obscures the programme.
Key takeaways
- 01Define the grease population
- 02Record relubrication history
- 03Match sample mass to the panel
- 04Use multiple evidence types
- 05Keep sampling method consistent