How to Read a Grease Datasheet

How to Read a Grease Datasheet

Grease datasheets can look intimidating — rows of numbers, unfamiliar test names, and abbreviations that mean little without context. But once you understand the key tests and what the numbers tell you about real-world performance, reading a datasheet becomes a practical skill. This guide walks through the most common performance indicators, what each test measures, and how to interpret the results in terms your equipment will feel.

FAQ

1. What do worked penetration numbers mean?

Worked penetration is the cornerstone of the NLGI consistency grade system. It measures how far a standard cone sinks into a grease sample after being mechanically worked — typically 60 double strokes in a grease worker — to simulate the shearing action a grease experiences in a bearing. The result is reported in tenths of a millimeter (0.1 mm). A higher number means softer grease.

NLGI grades map to worked penetration ranges at 25°C: NLGI 000 (445-475) is fluid; NLGI 00 (400-430) is semi-fluid; NLGI 0 (355-385) and NLGI 1 (310-340) are soft greases for centralized lubrication; NLGI 2 (265-295) is the most common general-purpose grade; NLGI 3 (220-250) is firmer, used in vertical or high-temperature applications.

When reading a datasheet, check whether the worked penetration value falls within the stated NLGI range. A grease labeled NLGI 2 with a worked penetration of 298 could indicate borderline consistency or batch variation. Also note the difference between unworked and worked penetration — a large drop after working suggests the thickener structure breaks down significantly under shear, which may predict excessive softening in service.

2. How should I interpret the dropping point?

The dropping point is the temperature at which grease transitions from semi-solid to liquid — the point where a drop of oil separates from the thickener matrix and falls under its own weight in a standardized test (ASTM D2265 or D566). It is often misunderstood as the maximum usable temperature.

In practice, dropping point is a thermal stability indicator for the thickener, not an operating limit. A lithium grease with a dropping point of 190°C is not suitable for continuous operation at 190°C — the thickener begins to degrade well before reaching the dropping point. A common rule of thumb: the maximum continuous operating temperature should be at least 30-50°C below the dropping point, and even that depends on oxidation stability and oil volatility.

Dropping point does help identify thickener type. Simple lithium soaps typically drop around 180-200°C; lithium complex greases drop above 260°C; polyurea greases can exceed 270°C; and bentonite (clay) greases have no true dropping point because the thickener does not melt. When comparing datasheets, a dropping point significantly below the thickener's typical range may indicate a poorly structured grease or excess oil content.

3. What is the four-ball weld load, and what does it tell me?

The four-ball weld load test (ASTM D2596) measures the extreme-pressure (EP) load-carrying capacity of a grease. Three hardened steel balls are clamped in a stationary cup, a fourth ball rotates against them under increasing load, and the test records the load at which the balls weld together — catastrophic failure.

Results are reported in kilograms-force (kgf). Typical non-EP greases fall in the 100-160 kgf range. EP greases containing sulfur-phosphorus compounds, ZDDP, or solid lubricants typically exceed 250 kgf, with some reaching 315 or 400 kgf. The higher the weld load, the greater the grease's ability to prevent metal-to-metal contact under shock loading, heavy loads, or boundary lubrication conditions.

Context matters. A high weld load is desirable for heavily loaded bearings, construction equipment pivot pins, or press applications. But EP additives can be corrosive to yellow metals (copper, brass, bronze) at elevated temperatures. A datasheet showing both high four-ball weld load and a good copper corrosion rating suggests well-balanced EP chemistry. Also note the load-wear index (formerly "mean Hertz load") — it provides insight into load-carrying performance across the full load spectrum, not just at the weld point.

4. How is water washout percentage measured, and what qualifies as a good result?

Water washout (ASTM D1264) tests how well a grease resists being displaced by water. A packed ball bearing filled with a weighed amount of grease is rotated at 600 rpm while a jet of water at a specified temperature (typically 38°C or 79°C) sprays onto the bearing housing for one hour. The percentage of grease lost by mass is the washout value.

A lower percentage means better water resistance. Washout below 5% is excellent; 5-15% is typical for general-purpose greases; above 20% suggests the grease may wash out quickly, requiring frequent relubrication. Test temperature matters — a grease with 3% washout at 38°C may lose 30% at 79°C, so always compare at the same temperature.

Grease formulation drives these results. Aluminum complex and calcium sulfonate thickeners inherently resist water, often showing washout values under 3%. Lithium greases vary based on polymer content and adhesion additives. Polymer-modified greases can achieve strong water resistance even with thickeners not naturally water-resistant. Consider trade-offs: a grease formulated solely for low washout may sacrifice pumpability or low-temperature performance.

5. What do oil separation test results indicate?

Oil separation, measured by ASTM D1742 (pressure method) or ASTM D6184 (cone bleed method), quantifies how much base oil bleeds from the thickener matrix under static conditions. In D1742, a sample is placed under a standard weight at elevated temperature, and separated oil is measured as a percentage of the original mass. In D6184, oil separation is measured through a wire mesh cone at 100°C over 30 hours.

Some oil separation is normal and necessary. Grease lubricates by releasing oil into the contact zone — if none separates, the grease cannot lubricate. Typical acceptable ranges are 1-5% for most greases. Values below 1% may starve the contact zone. Values above 10-15% suggest excessive bleeding leading to grease hardening in the bearing cavity, especially under static storage or intermittent operation.

Interpret oil separation alongside operating conditions. High-temperature applications accelerate oil separation. Centralized lubrication systems with long distribution lines require lower oil separation to prevent hardening in the lines. For sealed bearings that will not be regreased, controlled oil separation over the expected service life is critical — you need enough oil release to lubricate without depleting the grease prematurely.

6. What is Timken OK Load, and why does it appear on so many datasheets?

The Timken OK Load test (ASTM D2509) evaluates load-carrying capacity using a cup-and-block configuration. A hardened steel block is pressed against a rotating steel cup under a controlled load while grease is fed to the contact zone. After a 10-minute run, the block is examined for scoring. The maximum load (in pounds or kilograms) at which no scoring occurs is the "OK Load."

Results typically range from 10 lbs (4.5 kg) for basic greases to 60 lbs (27 kg) or higher for EP greases. The Timken test evaluates sliding contact and is particularly relevant for linear guides, thrust bearings, slideways, and certain open gear sets. It complements the four-ball test: four-ball evaluates point contact under rolling with sliding, while Timken evaluates line contact under sliding.

When comparing datasheets, note that Timken OK Load and four-ball weld load do not always correlate. A grease may perform well in one test and modestly in the other because the lubrication regimes differ. For sliding applications such as machine tool ways or slow-speed journal bearings, the Timken result is often the more practical predictor of field performance.

7. What does the base oil viscosity tell me, and where does it appear on a datasheet?

Base oil viscosity is arguably the most important property on a datasheet that gets overlooked. The thickener provides structure, but the base oil does the actual lubricating. Viscosity is typically reported at 40°C in centistokes (cSt), and sometimes at 100°C as well, from which the viscosity index (VI) can be calculated.

For most industrial bearings at moderate speeds and loads, base oil viscosity between 100 and 220 cSt at 40°C is common. High-speed bearings in electric motors or spindles benefit from lower viscosity (15-68 cSt) to reduce churning losses. Slow-speed, heavily loaded bearings need higher viscosity (220-460 cSt or more) to maintain oil film thickness.

The viscosity index (VI) reflects how viscosity changes with temperature. VI above 100 indicates stable viscosity across the operating range; below 80 means significant thinning when hot. Synthetic base oils typically achieve higher VI values than mineral oils. For applications with wide temperature swings, a high VI means more consistent film thickness from cold start to full operating temperature.

8. How is the oxidation stability of a grease evaluated?

Oxidation stability, measured by ASTM D942 (oxygen pressure vessel method), indicates how well a grease resists chemical breakdown when exposed to oxygen at elevated temperatures. A sample is placed in a sealed vessel with pure oxygen at 110 psi and held at 99°C. The pressure drop over 100, 500, or 1000 hours reflects the rate at which the grease consumes oxygen through oxidation.

A smaller pressure drop means better oxidation resistance. Pressure drop below 5 psi after 100 hours is considered good; below 10 psi after 500 hours indicates strong antioxidant protection. Oxidation leads to grease hardening, acid formation, and varnish deposits that accelerate bearing wear and shorten relubrication intervals.

Not all datasheets report oxidation stability, and test duration varies between products. When comparing, ensure results are at the same test duration. Synthetic base oils (PAO, ester) inherently resist oxidation better than mineral oils, and antioxidant additives extend this further. For electric motor bearings, oven conveyor bearings, or any application with long relubrication intervals, oxidation stability is a critical data point.

9. What is the copper corrosion test, and when does it matter?

The copper corrosion test (ASTM D4048, adapted from D130 for oil) evaluates whether a grease is corrosive to copper and copper alloys. A polished copper strip is immersed in the grease and held at a specified temperature (commonly 100°C or 120°C) for 24 hours. The strip is compared against standard color reference charts and rated from 1a (slight tarnish) to 4c (corrosive, black).

A rating of 1a or 1b is generally acceptable. Ratings of 2a or higher warrant caution with copper or brass components — worm gear bronze, bearing cages, thrust washers, and piping. Many EP additives, particularly active sulfur compounds, can cause elevated copper corrosion. This is why some high-EP greases carry specific warnings about yellow metal compatibility.

Always cross-reference copper corrosion with EP data. A grease with a 400 kgf weld load and a 1a copper rating represents well-controlled additive chemistry. A grease with a 315 kgf weld load and a 3a copper rating requires careful consideration: it still protects steel-on-steel contacts effectively, but it should not contact yellow metals in the system.

10. How do I use the grease's operating temperature range from a datasheet?

Manufacturers typically report two temperature numbers: the minimum operating temperature and the maximum recommended temperature. The low end is determined by the base oil's pour point and the thickener's low-temperature torque characteristics. The high end reflects a combination of dropping point, oxidation stability, and oil volatility.

For the low-temperature limit, the datasheet value often corresponds to the lowest temperature at which the grease can still flow. But actual starting torque at that temperature may be unacceptably high. A grease rated to -30°C may produce excessive torque at -30°C in a small instrument bearing. For cold-start applications, look also for low-temperature torque test results (ASTM D1478) if available.

For the high-temperature limit, datasheets often report a maximum continuous temperature and a short-term peak temperature. The continuous rating is where the grease delivers its published relubrication interval; the peak rating is for brief excursions. Operating continuously near the peak temperature shortens grease life dramatically. As a practical guide, expect the greasing interval to halve for every 15°C increase above 70°C — this should inform your maintenance planning even when not stated on the datasheet.

11. What other grease tests and standards appear on datasheets, and how do I read them?

Beyond the core tests, several additional parameters commonly appear. Roll stability (ASTM D1831) measures how much a grease softens when rolled in a cylinder for a specified time — useful for predicting consistency change in wheel bearings. A penetration change below 30 points after 2 hours at room temperature is typical for mechanically stable greases; above 50 points suggests significant softening under working conditions.

Evaporation loss (ASTM D972 or D2595) measures oil lost by evaporation at a specified temperature and time (commonly 22 hours at 100°C). Values below 2% are good for high-temperature applications; high evaporation loss means the grease hardens prematurely as oil boils off.

The grease mobility test (ASTM D1092) measures apparent viscosity at a specific shear rate, predicting pumpability in centralized lubrication systems — lower values indicate easier pumping. Rust prevention (ASTM D1743, distilled water, or D5969, synthetic seawater) rates a grease's ability to protect against corrosion under humid or salt-spray conditions. The test method used matters significantly for marine or washdown environments.

12. How do I weigh trade-offs between different performance values on a single datasheet?

No single grease excels at every property, and the datasheet itself often reveals the compromises. A grease with high dropping point and oxidation stability may use a synthetic base oil that compromises elastomer compatibility. A grease with outstanding water washout resistance may have limited low-temperature pumpability because the polymers improving water resistance also thicken when cold.

Read the datasheet holistically. If your application demands high load-carrying capacity, prioritize four-ball weld load, Timken OK Load, and load-wear index — but then check copper corrosion if yellow metals are present, and water washout if wet conditions are expected. If long service life in sealed bearings is the priority, focus on oxidation stability, oil separation, and evaporation loss. For centralized lubrication systems, the priority list shifts to apparent viscosity (pumpability), worked penetration, and oil separation (stability in long lines).

The practical approach is to list your application's top three performance demands in priority order, then evaluate the datasheet against them. Properties outside those top three may influence maintenance intervals or compatibility, but should not override fundamental requirements. A disciplined reading against a prioritized list prevents the common mistake of selecting grease based on a single impressive number while overlooking a disqualifying weakness.

Key Takeaways

Reading a grease datasheet is about connecting test results to real-world conditions. Worked penetration tells you consistency and pumpability, dropping point suggests thermal limits for the thickener, and four-ball weld load reveals EP protection. Water washout and oil separation speak to performance in wet or static environments. Timken OK Load and base oil viscosity address sliding-contact and film-thickness needs. No single number tells the whole story — evaluate the full datasheet against your application's demands, remembering that every formulation balances trade-offs rather than delivering a universal solution.

KOEED Support

For technical guidance on lubricant selection or to request product datasheets and samples, contact our engineering support team at Moritta@KOEED.COM. We provide application-specific recommendations based on your operating conditions and maintenance practices.