In a sometimes intense session at the 19th Colloquium Tribology at the Technische Akademie Esslingen in Germany last week, representatives from Lubrizol and ExxonMobil debunked the value of the Brugger Test for evaluating the antiwear performance of hydraulic fluids.
Lubrizols Alan Barber explained that the Brugger Test was introduced in the 1980s by hydraulic press manufacturer Mueller Weingarten to assess the load-carrying capacity of oils. It eventually gained status as a DIN (German Institute for Standardization) Standard; however, Barber noted, It has no precision statement, and a recent round-robin at test laboratories showed that repeatability is quite poor, at only about 20 percent.
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But the real difficulty, said Barber, is that the test bears little resemblance to real world conditions experienced by the typical HF-0 hydraulic fluid. The problem is that the Brugger Test is effectively an extreme pressure test, and the typical hydraulic fluid is not formulated for extreme pressure conditions.
Nevertheless, at least one original equipment manufacturer has adopted the test as a requirement for fluids used in its equipment. In an effort to forestall wider adoption of the test, Barber presented results showing that no hydraulic pumps – including vane, piston and gear pumps – have any contacts that resemble the conditions of the Brugger Test.
Worse still, Barber continued, passing the test requires hydraulic fluid formulators to add sulfur-containing extreme pressure agents, which presents a number of problems. All pumps contain a lot of yellow metal, he said, and sulfur compounds are extremely aggressive toward yellow metals. Also, the addition of sulfur EP agents adversely affects thermal, oxidative and hydrolytic stability, as well as seal compatibility.
These findings were corroborated by Sandra Legay of ExxonMobil, who presented data for tests on conventional and premium hydraulic fluids. She explained that the addition of 0.6 percent of a sulfur-containing EP additive increased the Brugger Test value to the specified rating, but in all cases reduced thermal and oxidative stability, and increased copper corrosion.
Although the presentations sparked a healthy debate by members of the audience trying to defend the efficacy of the Brugger Test, it applies only to a very specific application, Barber concluded, and has no place as a screening test or requirement in hydraulic fluid specifications.