What is the difference between Wear-Preventive (WP) and Extreme-Pressure (EP) tests for lubricants in four ball testing?

To ensure optimal performance under various operating conditions, lubricants must be evaluated for their ability to withstand both moderate and severe loading without failure, protect surfaces from wear, improve efficiency by reducing friction and provide cooling capability. Two key categories of testing used in this evaluation are wear preventive (WP) and extreme pressure (EP) tests. These tests provide essential data for understanding lubricant performance and tailoring additive package for specific applications, such as, but not limited to:

• Metal-working fluids
• Metal-forming fluids
• Gear oils
• Hydraulic oils
• Engine oils
• Bearings
• Greases (especially EP greases), especially tested with this method when they are intended for high-pressure or shock-loading applications.

For WP assessments, such as those described in ASTM D2266 (greases) and ASTM D4172 (oils), the test evaluates a lubricant’s ability to reduce wear under moderate, sustained loads by measuring the size of wear scars formed on steel balls. This is critical for applications where long-term protection against mild wear is essential. In addition, such tests help to understand interaction of different additives such as friction modifiers vs. anti-wear.

The purpose of an EP test using a four-ball tester, according to the ASTM D2783 (oil) and ASTM D2596 (grease) standard, is to evaluate the load-carrying capability of lubricants under high-stress conditions till failure of lubricant film. It is commonly used to simulate conditions where lubricants experience high contact pressures, such as in gears or bearings or metal-working operations. In addition, such tests help to understand interaction of different additives such as extreme pressure vs. anti-wear.

Questions you should ask yourself before starting a test:

1. Does your lubricant work at high or moderate contact stresses?
   
   
2. Which four ball test is relevant for me?
   
   
3. What outputs from these tests help identify the best lubricant?
   

The following answers may easily be drawn:

1. At moderate loads and/or evaluation of friction modifiers and wear-preventive additives, WP tests (ASTM D2266/ATSM D4172) can be selected. At high loads, and/or evaluation of anti-wear and extreme-pressure additives, EP tests (ASTM D2783 / ASTM D2596) can be selected.
   
   
2. At moderate loads, WP tests (ASTM D2266/ATSM D4172) can be performed. At high loads, EP tests (ASTM D2783 / ASTM D2596) can be performed.
   
   
3. The best lubricant based on the additive chemistry and concentration provides the lowest friction, lowest temperature change, lowest MWSD and the highest last non-seizure load and weld load.