There are several test methods to evaluate the wear preventing properties of lubricants and each test is used for a different purpose. The Four-Ball Test method is used to determine the relative wear-preventing properties of lubricants in sliding steel-on-steel applications
In the Four-Ball Test, three ½” (12.7 mm) diameter steel balls are clamped together and covered with the lubricant to be evaluated. A fourth steel ball, held by a chuck, is rotated against the three lower clamped balls as shown on the right. The red dots denote the three points of contact between the balls. By adjusting the speed of the rotating ball and the load applied to it, several lubrication regimes can be simulated. There are two Four-Ball Tests that determine the tribological characteristics of lubricating oils and greases.
Four-Ball Weld Load Test. This procedure (ASTM D2783 for lubricating oils and ASTM D2596 for greases) evaluates the load-carrying (Extreme Pressure) properties of lubricants. During the test, the top ball rotates at 1760 rpm against the three stationary balls and the load is gradually increased until the lubricant fails. This happens when welding between the balls is detected (as depicted on the left). The weld point is the lowest applied load in kilograms at which the rotating ball welds to the three stationary balls.
Four-Ball Wear Scar Test. This test measures the wear preventing properties of lubricants, using ASTM method D2266 for greases and D4172 for lubricating oils. The rotational speed of the top ball is 1200 rpm and is pressed with a force of 40 kg onto the three clamped balls. The temperature of the test lubricant is regulated at 75°C and the duration of the test is 60 minutes. Lubricants are compared by using the average size of the scar diameters worn on the three lower clamped balls. An enlarged 1,87 mm diameter wear scar is shown on the right.
As with all bench tests, the Four-Ball Test attempts to create a reliably repeatable condition that can be performed relatively inexpensively and in much less time than would be required for field trials. In this respect the test passes with flying colours and Four-Ball Test rigs are commonly used in lubricant test labs worldwide.
However, the conditions in the test rig bear little resemblance to those seen in machinery. There are two components to this – the size of the interaction between the balls and the interaction type.
With regards to the size of the interaction, the intersection (contact area) between two spheres is only a tiny point or spot. The result is that the test load is being concentrated on a small area that increases the surface pressure drastically.
In addition, the contact between the balls is a sliding interaction. This combination is rarely found in machinery, where the most severe combinations are line contact with sliding (as in gears or journal bearings as illustrated on the left) or point contact with rolling (as found in ball bearings). This confirms that the test rig does not accurately simulate real-world contacts.
Regardless of this the Four-Ball Test carries considerable weight as a development tool. It is inexpensive and repeatable and can give tribologists and lubricant developers an idea whether their formulation is directionally correct. However, as a tool for lubricant selection, one should be cautious and rather look at other test procedures to evaluate the likely performance of candidate products. A possible alternative is the FZG Test which we will discuss in the next issue of OilChat.
If you have any questions about the Four-Ball Test (or any other lubricant related issues) in the interim, you are welcome to phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.
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