Determination of Extreme Pressure (EP) Properties of Lubricating Grease Using the Four-Ball Machine Method
The Four-Ball Machine Method is used to evaluate the extreme pressure (EP) properties of lubricating greases under high-load conditions. It determines the ability of a grease to withstand extreme pressure and prevent metal-to-metal contact.
1️⃣ Relevant Test Standards
| Test Method | Standard | Application |
|---|---|---|
| Weld Load, Load-Wear Index (LWI) | ASTM D2596 | Measures EP performance of lubricating grease |
| Wear Scar Diameter (WSD) Under Load | ASTM D2266 | Evaluates anti-wear (AW) properties |
| Friction & Wear Testing | ASTM D5183 | Determines coefficient of friction & wear resistance |
| ISO Method | ISO 20623 | Equivalent EP test for greases & oils |
📌 ASTM D2596 is the primary standard for measuring the EP properties of lubricating grease.
2️⃣ ASTM D2596 – Four-Ball Extreme Pressure Test Procedure
🔹 Principle
✔ A rotating steel ball is pressed against three fixed steel balls under a controlled increasing load until welding occurs.
✔ The test determines the Weld Load, Load-Wear Index (LWI), and Last Non-Seizure Load (LNSL).
🔹 Required Equipment
✅ Four-Ball EP Test Machine (ASTM D2596 standard)
✅ Steel Test Balls (12.7 mm, AISI 52100, hardness HRC 64-66)
✅ Heating System (optional for high-temperature greases)
✅ Load Control System
3️⃣ Key Test Parameters & Procedure
1️⃣ Sample Preparation
- Apply the lubricating grease to the steel balls in the test assembly.
2️⃣ Load Application
- Start with a low load and increase gradually at a rate of 98 N (10 kgf) every 10 seconds.
- The balls rotate at 1770 ± 60 rpm.
3️⃣ Determining Key Performance Metrics
| Test Parameter | Definition | Significance |
|---|---|---|
| Last Non-Seizure Load (LNSL) | Highest load before scuffing | Indicates grease film strength |
| Last Seizure Load (LSL) | Load where wear scar rapidly increases | Identifies breakdown point |
| Weld Load (WL) | Load at which the steel balls weld together | Measures maximum load-carrying capacity |
| Load-Wear Index (LWI) | Average of wear results | Indicates overall EP performance |
4️⃣ Result Calculation
- Weld Load (WL): Maximum load before welding occurs.
- Load-Wear Index (LWI):
LWI=∑(Load×IncrementalWear)/∑(IncrementalWear)
4️⃣ Interpretation of Results
| Weld Load (kgf) | Grease Performance | Application |
|---|---|---|
| 80 – 160 kgf | Low EP protection | General-purpose greases |
| 160 – 250 kgf | Medium EP protection | Automotive & industrial greases |
| >250 kgf | High EP protection | Heavy-duty & extreme load conditions |
✔ Higher weld load = Better extreme pressure protection
✔ Lower wear scar = Better anti-wear properties
5️⃣ Application of Four-Ball EP Testing in Industry
| Industry | Purpose of Testing | Example Grease Applications |
|---|---|---|
| Automotive | Ensures gear & wheel bearing greases withstand shock loads | Wheel bearing grease, chassis grease |
| Industrial | Prevents metal wear in high-load machinery | Heavy-duty industrial greases |
| Aerospace | Tests high-temperature, high-load performance | Aircraft landing gear greases |
| Mining & Construction | Verifies extreme pressure capability in harsh conditions | Excavator & conveyor greases |
📌 Used to select the best grease for high-pressure, high-temperature, and heavy-load applications.
Specific Concerns in Four-Ball EP Testing for Lubricating Grease
When selecting a lubricating grease or evaluating its extreme pressure (EP) properties, different concerns may arise depending on the application. Here are the key specific concerns you might have:
1️⃣ Key Concerns in EP Testing
| Concern | Why It Matters? | Relevant Test Parameter |
|---|---|---|
| Weld Load Failure | If the grease fails at a low load, it may not protect against extreme pressure. | Weld Load (WL) |
| Excessive Wear in Bearings/Gears | High wear means the grease is not forming a proper film. | Load-Wear Index (LWI), Wear Scar Diameter (WSD) |
| Inconsistent Load Performance | Grease may perform well at lower loads but fail under high impact. | Last Non-Seizure Load (LNSL), Last Seizure Load (LSL) |
| Poor Shock Load Resistance | High-stress environments need grease with high EP additives. | Weld Load, LWI |
| Oxidation & Degradation | Over time, grease breaks down, affecting performance. | Used grease wear testing (ASTM D2596 on aged samples) |
| Temperature Stability | Grease performance varies with heat; high-temp applications need stability. | ASTM D2596 at elevated temperatures |
📌 Higher weld load = Better extreme pressure protection
📌 Lower wear scar = Better anti-wear properties
2️⃣ Industry-Specific Concerns
| Industry | Concern | Why It’s Important? | Best Test Parameter |
|---|---|---|---|
| Automotive | Wheel bearing failure under load | Prevents bearing pitting & failure | WL, LWI |
| Mining & Construction | High-pressure, high-impact loads | Protects heavy machinery from wear | WL, WSD |
| Industrial | Metal-to-metal contact in gears | Reduces friction & prevents breakdown | LNSL, LWI |
| Aerospace | Grease breakdown at high temp & pressure | Ensures lubrication in extreme environments | WL, temp-modified D2596 |
3️⃣ Choosing the Right EP Test Based on Concern
| Concern | Test Method | Standard |
|---|---|---|
| General EP Performance | Four-Ball EP Test | ASTM D2596 |
| Anti-Wear Properties | Four-Ball Wear Test | ASTM D2266 |
| Friction & Wear Resistance | Friction Test | ASTM D5183 |
| High-Temp EP Performance | Four-Ball Test at Elevated Temp | Modified ASTM D2596 |