⚙️ EP properties of lubricants — 4-ball test
Determination of extreme-pressure (EP) properties of lubricating fluids by four-ball steel method. Determination of load wear index (LWI), weld load and last non-seizure load.
Overview
Four-Ball EP Test is one of basic tribological methods for assessment of lubricating fluid ability to transfer extreme loads. Under high pressure conditions, when lubricant film breaks down, EP (extreme pressure) additives react chemically with metal surface, forming protective layers (sulfides, chlorides, iron phosphates) preventing direct metal-to-metal contact and seizure.
ASTM D2783 method applies only to lubricating fluids (oils). For plastic greases analogous method ASTM D2596 is used. Test allows differentiation of fluids with low, medium and high EP properties, which is crucial for selection of lubricants for gear transmissions, high-load bearings, hydraulic systems operating under high pressure and cutting machine tools.
Four-ball test results are required in gear oil specifications (API GL-4, GL-5), hydraulic oils (DIN 51524 HLP), machine tool oils (ISO 6743-7) and metalworking fluids. Testing is performed by refinery laboratories, lubricant manufacturers (Shell, Fuchs, Lotos Oil) and accredited tribological laboratories.
Four-ball test device is produced by Stanhope-Seta (Seta-Shell Four Ball Tester), Falex Corporation and Rtec Instruments. Typical configuration includes rotating holder for one upper ball and stationary container for three lower balls submerged in tested oil.
Method principle
Steel bearing ball (ø 12.7 mm, AISI 52100, hardness 64–66 HRC) mounted in rotating holder is pressed against three identical stationary balls arranged in tetrahedron configuration and submerged in tested oil. EP test is conducted in series of 10-second runs at constant rotational speed (1,760 ±40 rpm), gradually increasing axial load in successive runs (from ~80 to 800 kgf). After each run, wear scar diameter on three lower balls is measured. Following are determined: (1) Weld Point — load at which balls weld together; (2) Last Non-Seizure Load (LNSL); (3) Load-Wear Index (LWI) — calculated from averaged wear scars at all loads.
Applications
- Classification of gear oils (API GL-1 to GL-5)
- Assessment of HLP hydraulic oils (DIN 51524 part 2)
- Selection of oils for gear transmissions (spur, bevel, worm)
- Testing of metalworking fluids for metal cutting
- Quality control of oils with EP additives (sulfur, phosphorus, chlorine)
- Development studies of new lubricant formulations (R&D)
- Assessment of oils for screw compressors and rolling bearings
Key parameters
| Parameter | Value |
|---|---|
| Ball diameter | ø 12.7 mm (1/2"), AISI 52100, 64–66 HRC |
| Rotational speed | 1,760 ±40 rpm |
| Run time | 10 ±0.2 s per load |
| Load range | 80–800 kgf (gradually, per ASTM table) |
| Result — LWI | Load-Wear Index (kg) — collective EP indicator |
| Result — Weld Point | Weld load (kgf) |
Standard
- Standard number
- ASTM D2783-21
- Title (PL)
- Oznaczanie właściwości przeciwzatarciowych (EP) płynów smarowych — Metoda czterech kulek
- Title (EN)
- Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method)
Step-by-step procedure
1. Ball preparation
Wash 4 new AISI 52100 balls in naphtha, then in acetone. Air dry. Do not touch surfaces with bare hands (gloves!). Balls single-use — do not reuse.
2. Lower ball assembly
Place 3 balls in ball pot in equilateral triangle configuration. Tighten locking ring. Pour tested oil (~10 mL) so balls are submerged.
3. Upper ball assembly
Mount 4th ball in rotating holder (collet). Tighten manually. Check runout — max 0.02 mm.
4. First load setting
Set load to 80 kgf (per ASTM D2783 table — load series: 80, 100, 126, 160, 200, 250, 315, 400, 500, 620, 800 kgf). Lock load mechanism.
5. Test run
Start motor (1,760 rpm). Run time: 10 s. After 10 s stop. Release load. Remove ball pot.
6. Wear scar measurement
Remove 3 lower balls. Measure wear scar diameter under microscope (2 perpendicular measurements on each ball, 6 total measurements). Calculate average.
7. Subsequent loads
Replace balls with new ones. Repeat test at next higher load from table. Continue until ball welding (rotor lock, sudden torque increase).
8. Weld recording
Load at which balls weld = Weld Point. Last load at which no seizure occurred = LNSL (Last Non-Seizure Load).
9. LWI calculation
Calculate Load-Wear Index per ASTM formula: LWI = Σ(Fi × di) / n, where Fi = load [kgf], di = wear coefficient, n = number of runs. Result in kgf.
10. Reporting
Result: LWI = XX kgf, Weld Point = YYY kgf, LNSL = ZZZ kgf. Compare with specification requirements (e.g. API GL-5: Weld Point ≥315 kgf).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Four-ball apparatus (Four-Ball Tester) | Stanhope-Seta 19800-7 (manual), Seta 19900-4 (autoload), Falex #0 | 80,000–200,000 PLN |
| Test balls AISI 52100 | ø 12.7 mm, grade G25, hardness 64–66 HRC, packed 100 pcs | 300–800 PLN / 100 pcs |
| Measuring microscope | Stanhope-Seta 19800-2 or Nikon MM-400 with scar diameter measurement | 5,000–25,000 PLN |
| Load sensor (optional) | Built-in in autoload version or external dynamometer | built-in |
| Ball pot container | Steel, for mounting 3 lower balls + oil, reusable | 1,000–3,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Reference oil without EP additives | — | Mineral base oil SN150 or white oil — for apparatus verification (low LWI, weld ~126 kgf) |
| Reference oil with EP additives | — | Certified EP oil — for checking apparatus sensitivity (weld >315 kgf) |
| Naphtha (for ball and container cleaning) | 8008-20-6 | Initial cleaning of balls and ball pot after test |
| Acetone | 67-64-1 | Final cleaning and degreasing of balls before test, evaporates quickly |
| Toluene | 108-88-3 | Alternative solvent for removing EP oil residues with heavy additives |
Health and safety (OHS)
- Rotating elements — capture risk, do not approach hands to rotor during operation
- Post-test balls — very hot (EP friction), do not touch with bare hands
- Naphtha and acetone — flammable, work away from ignition sources
- EP oils — may contain sulfur, phosphorus and chlorine compounds — irritant, protective gloves
- Safety goggles mandatory — possible metal chip splatter