The test best meeting that need was discovered to be the twist compression test, a method for testing metalworking fluids that simulates deep drawing, a metalworking process using tensile forces to stretch metal. These conditions were found to be uniquely similar to the subject application and demonstrated better applicability than some of the most widely used grease testing methods in the industry.

This is how the research team developed and identified a testing methodology, what we learned from the process, and the grease characteristics and chemistry that went into the recommended solution.

We first took an in-depth look at the conditions causing the issue at hand.

The customer application involved joining hydraulic hoses with their proper fittings utilizing a metal press apparatus. When the two heavily loaded, sliding jaws of the apparatus come together to crimp the fitting onto the hose, the surfaces must be properly lubricated with an appropriate grease, forming the necessary protective layer between the press and the sliding jaws.

The grease the customer was using-a lithium complex extreme-pressure grease with solid additives-worked under low-pressure crimping. However, as pressure increased, the sliding contacts developed the tendency to squeeze out any lubricant on the surface, causing starvation and leaving improper surface protection.

Boundary lubrication, or thin-film lubrication, is typical at low speeds between two sliding surfaces. Lacking the proper boundary lubrication, the surfaces in the customer application tended to stick, resulting in wear on the jaws and uneven motion.

The stick-slipping of the jaws resulted in poorer connections between the hose and fitting-an unacceptable result for the customer. Hydraulic hose assemblies can be responsible for many critical functions in a variety of end-use applications, transferring high-pressure fluids and liquids. Depending on the application, any type of rupture or pressure loss can be catastrophic and is absolutely unacceptable for a hydraulic assembly manufacturer.

Identifying the Right Test

The first challenge was to identify a comparable test that successfully mimicked the conditions of this hydraulic fitting press. A number of frequently utilized grease tests were evaluated for the challenge at hand.

The Timken OK Load test (ASTM D2509), a standardized test that indicates the possible performance of EP additives. The test machine includes a standardized bearing race mounted on a tapered arbor, rotating at high speed. The race is brought into contact with a square steel test block under a constant load, with the subject grease then introduced to that contact area. Here, the contact shape is a line or rectangle that moves in a pure-sliding contact motion.

Sliding 4-ball EP test (ASTM D2596), another standard test method for measuring EP properties of a lubricating grease. This test is performed by placing three 12.7 mm diameter hardened bearing steel balls in a test cup filled with the subject grease before a fourth ball is then brought into contact with the three lower balls at varying loads. The top ball turns relative to the bottom balls for ten seconds. The result is a three-point contact shape with a pure-sliding contact motion.

SRV test (ASTM 5706), one final standard test method for determining EP properties utilizing a high-frequency, linear-oscillation (SRV) machine. Here, the test load is increased at two-minute intervals until the lubricated specimens weld together-the point at which the tested grease has failed. Contact shape is a point, moving in a reciprocating, pure-sliding motion.

However, not one of these conventional grease testing methods accurately mimicked the end-use application, in which two flat contact areas meet with heavy loads and high pressure at slow-sliding speeds requiring no stick-slip and good boundary lubrication performance. And while these tests are useful for evaluating a variety of grease properties, we determined that an alternative method was necessary to test for performance in an unconventional application.

Enter the twist compression test. The TCT is commonplace in the world of drawing and stamping. Put simply, it is a heavily loaded sliding wear and friction test that is useful for screening metalworking fluids, which it does by evaluating the level of friction and wear between two materials under lubricated or non-lubricated conditions.

To simulate the effects in a typical metal forming environment, the TCT presses a rotating tool against a metal surface at a given pressure and sliding velocity. Many MWF suppliers maintain a TCT machine to regularly perform the test, but as far as Lubrizol teams are aware, utilizing this test to screen or develop grease for any comparable application had not been done.

The slow moving, high-pressure, heavily loaded metal surface conditions in the TCT were found to be the closest conditions to the subject application, and we believed that a grease demonstrating good performance here would perform well under field conditions. The TCT doesnt, in fact, simulate an actual process, but it has been shown to correlate well with processes dependent upon boundary lubrication, much like the customer application in question.

Lubrizol evaluated and compared the contact conditions of the application and determined how they could be potentially replicated using the TCT, then recommended screening conditions to determine the best performing grease to solve the customers issue.

Testing Results

After identifying the TCT as an applicable means for testing, it was time to determine test greases and their additive characteristics in search of a product to adequately form the needed film layer.

In the metal forming applications where the TCT is typically applied, one commonplace additive solution is the use of chlorinated paraffins, which are alkanes that have carbon chain lengths ranging from 10 to 38 depending on the degree of chlorination.

While these additives have proven useful as extreme pressure agents, particularly in the difficult drawing, forming and removal operations common in metalworking, they remain subject to some controversy. In 2015, the U.S. Environmental Protection Agency moved to ban the substances under the Toxic Substances Control Act; however, in 2017, the agency reversed course under the Trump administration. The substance remains regulated in markets around the world, however, and our customer desired a solution that would be applicable globally.

Thus, we searched for alternative solutions. A lithium EP grease had already failed in the field application and failed quickly in the TCT test under mild conditions. From there, we developed a series of six candidate greases shown in Figure 2, utilizing a variety of formulations and different additive chemistries.

We subjected each grease to two minutes in the TCT rig; the relative performance of each grease is shown in Figure 3. Throughout the two-minute test, the mechanism is designed to effectively grind the protective layer off the two metal surfaces, eliminating the boundary lubrication protection as the friction coefficient rises. The point at which that protective layer is completely eliminated, and where the two metal surfaces weld, is demonstrated by the steep drop-offs shown by Grease 30, Grease 31 and Grease 33.

Grease 34 and Grease G demonstrated the most desirable friction performance overall, each maintaining acceptable friction coefficients throughout our testing, with one key point of distinction. Grease 34, a calcium sulfonate grease formulated with sulfur extreme-pressure additives, maintained better wear protection overall but demonstrated slightly elevated levels of friction.

Meanwhile, Grease G, a calcium sulfonate grease with no additives, maintained better friction performance with slightly lower levels of wear protection. Both candidates were offered as potential solutions for the customer application.

Conclusions

When an unconventional customer challenge arose, Lubrizols testing teams were able to uncover an unconventional solution. While the TCT isnt typically used for grease testing, it has proven useful for applications requiring the formation of protective boundary layers and more accurately simulated the real-world operating conditions of the customer application than many of the most common grease tests.

This experience serves as evidence that in the development of high-performing lubricants and greases, the most important thing to consider is end-use performance in the field. Testing is a critical part of that work, of course-but only if the testing is relevant and reflective of real-world performance. z

Gareth Fish, Ph.D., is a technical fellow at The Lubrizol Corp. He is a member of the Royal Society of Chemistry, The Energy Institute, STLE, ASTM and SAE. He chairs numerous industry groups and associations, including the NLGI working group to develop the next-generation GC-LB grease standard. Contact him at gareth.fish@lubrizol.com.