Today’s hydraulic fluids must last longer, provide greater protection and perform better than their predecessors. Construction equipment, injection molding machines, steel mills, forestry equipment and many other types of industrial and off-road operations rely on hydraulic fluids with special chemical properties to keep machinery running smoothly and lasting longer.

The mission of modern-day hydraulic fluids is to provide better performance in smaller, more efficient industrial and mobile equipment. The fluid must be able to accommodate higher speeds and survive higher operating pressures in equipment with smaller sump reservoirs. This of course also means higher temperatures, which expose the fluids to more severe conditions.

Not all hydraulic fluids are the same. Hydraulic fluids range in performance from basic to specialty products that deliver high-performance characteristics. The robustness and durability of the fluid, for example, will affect its overall performance in protecting equipment.

Specially formulated additive packages used in hydraulic fluids provide the required durability and performance retention. The Lubrizol Corporation developed a special durability testing protocol to demonstrate that the amount and type of antiwear additives and oxidation inhibitors – such as zinc dithiophosphate – along with corrosion and rust inhibitors, foam inhibitors, demulsibility additives and other chemical components, help hydraulic fluids deliver reliable performance under strenuous operating conditions.

Meeting Specs Not Enough
Recognizing that today’s hydraulic systems and fluids are crucial components of a healthy operation, managers want to know whether a particular fluid will continue to provide all the protection the user’s equipment requires.

Common questions asked by maintenance specialists in a wide range of industries include:

  • How long will the fluid continue to protect my equipment?
  • Will the product retain its demulsibility characteristics?
  • What is the expected oxidation life of the fluid?
  • Will the fluid attack my seals?
  • Can I perform fine filtration without loss of performance?

These questions arise from a variety of concerns. Additives are consumed at greater rates in today’s harsher environments. Systems are using less oil and are expected to last longer, making the fluid’s durability and retention of performance even more important. Quality control, especially in the area of fluid cleanliness, and precise formulating are also more critical than ever before.

Hydraulic fluid specifications are not enough to assure that the hydraulic fluid used in the operations will provide adequate protection over the desired timeframe. Specifications provide a basis for performance, but the reality in today’s environment is that equipment demands have increased, which has in turn increased the needs for the fluid to perform properly in much harsher conditions.

More Stringent Industry Testing Needed
So how does the lubrication industry measure or evaluate a hydraulic fluid’s ability to perform under harsher conditions if the specifications haven’t changed? The Lubrizol Corporation has modified a number of industry tests to answer this question. Results of these tests show that hydraulic fluids formulated with the proper zinc dithiophosphate for both antiwear and oxidation inhibition – along with additional oxidation inhibition technology – continue to be effective in protecting equipment, even in severe operating environments.

These proprietary tests were designed to reflect the immense challenges hydraulic fluids often face as they carry out their job. Higher pressures and higher temperatures increase the rate of oxidation and thermal stress the fluid must endure. In addition, the fluid spends less time in the smaller reservoirs of today’s equipment, which can cause a number of problems, such as not allowing air to escape, foam to break, water to separate, contaminants to settle out or the fluid to cool sufficiently.

Lubrizol’s tests measured the durability and retention of performance of hydraulic fluids under a variety of high-stress conditions.

Testing for Durability
The most direct way to assess durability is to extend the duration of the current standard pump tests and run the tests at higher temperatures. Standard industry pump tests, using Denison’s vane pump, the Eaton-Vickers vane pump and the Sundstrand piston pump, were chosen for this study.

All components are assessed visually for signs of
wear and discoloration at the end of test
Table 1. Denison Vane Pump

To determine retention of performance, the test fluids were saved and reevaluated in the standard ASTM bench tests typically used to evaluate hydraulic fluids. The Denison HF-0 specification (Table 1) was chosen because of its comprehensive nature in that it evaluates all aspects of a hydraulic fluid. The Sundstrand and Eaton/Vickers pump tests were designed to measure durability of the hydraulic fluid.

The Sundstrand piston pump was initially run under standard conditions with hydraulic fluid formulated with antiwear additives, and it passed all the criteria established for the test. A second Sundstrand pump was run under the same test, extended to 450 hours, or double the length of the standard test, and it also passed (Table 2).

All components are assessed visually for signs of wear
and discoloration at the end of test
Table 2. Sundstrand Piston Pump – Series 22

The Sundstrand pump test included one percent water contamination to further stress the fluid. Even with the added water, there was no evidence of any hydrolytic reactions that could cause the formation of precipitates. Contamination from precipitates leads to blocked valves and filter-plugging problems.

Another round of Sundstrand pump tests were run at an elevated temperature of 250°F. Because of the higher temperature, no water was added to this test, and all other conditions remained the same. This round of tests showed that the hydraulic fluid with the premium hydraulic additive package had the endurance to exceed the performance parameters of the Sundstrand piston pump without difficulty, despite the higher temperature and the extended length of the test.

Cam and ring weight loss not to exceed 90 mg
Table 3. Eaton Vickers 35VQ25 Vane Pump

The next phase of testing involved the Eaton-Vickers vane pump (Table 3), which ran for extended hours to determine the durability of the hydraulic fluid containing an antiwear additive package. After 1,000 hours, the pump was still below the weight loss limit for total ring and vane wear. Figure 1 illustrates how the hydraulic fluid was robust enough to exceed the parameters of the test by a large margin.

Figure 1. Eaton-Vickers 35VQ Vane Pump
Standard vs. Extended Length

The final phase of pump testing used the Denison vane pump under standard test conditions to generate stressed fluids needed for evaluating performance retention. The purpose of this test was to generate used fluid needed to complete the retention of performance portion of the study.

Performance Retention
Following testing in the Sundstrand piston pump, the Eaton-Vickers and Denison vane pumps, the used, stressed fluids were saved and then subjected to a number of the performance bench tests used to evaluate fresh hydraulic fluid. Testing included D943 oxidation testing, D4310 1,000-hour sludge tests, D2619 hydrolytic stability tests, D665 rust test, D892 foam test, and D1401 demulsibility test. This testing was conducted to determine whether the stressed fluid’s characteristics were still comparable to that of the new fluid. All of these tests were performed in accordance with established ASTM testing methods.

In the D943 oxidation test, after the additional stress of the Eaton-Vicker’s extended, 200-hour vane pump test, the premium hydraulic fluid provided superior oxidation life of almost 1,500 hours (Figure 2) to 2.0 acid number (AN), which still exceeds the minimum specification.

Figure 2. D943 Oxidation

Both the monograde and the multigrade used fluids passed the D2619 hydrolytic stability test, even after the oil was stressed from the pump test. They demonstrated only a 0.02 to 0.19 mg/cm2 weight loss – depending on the test phase – which are all below the 0.2 HF-0 fail limit. This ensures the additive package did not react with water under stressful operating conditions to form insoluble sludge, excess acidity or other adverse physical changes that could impede proper fluid performance.

The D4310, 1,000-hour sludge test is an important test in determining the sludging characteristics of today’s hydraulic fluids. Systems free of sludge are critical in meeting the demands for long service life, and the requirement for clean systems needed to prevent deposit build up in the various control valves or on the filter. Not all hydraulic additive packages offer the sludge resistance or thermal stability of the product used here (Figure 3) and not all hydraulic additive packages will provide those clean systems needed for long-life applications.

Figure 3. D4310 1,000 Hour Sludge

Lubrizol’s test data for thermal stability D2070 shows that both the monograde and the multigrade formulations were far below the 100 mg/100 mL sludge fail limit (Figure 4).

Figure 4. D943 Oxidation

The Denison filterability test is used as an indicator of the fluid’s performance under dry and wet conditions. Blocked filters can be a major maintenance issue for hydraulic fluids out in the field. The after pump results show that premium fluids, like these tested, retain their performance in this key Denison HF-0 test. Our study also showed that performance is retained in other key areas, for example rust protection, demulsibility, and antifoam characteristics (Table 4).

Table 4. Additional Test Results

The results of all the testing showed that even after the stress of the pump testing, the premium hydraulic fluids continued to provide superior oxidation life, hydrolytic stability, thermal stability, sludge prevention, filterability under both dry and wet conditions, rust protection, demulsibility and antifoam characteristics.

Figure 5. Filterability (Wet)

Going Beyond OEM Specs
The durability and retention of fluid performance can be evaluated beyond the requirements of OEM specifications by extending the length of time, or making the operating conditions of the tests more stressful. Bench testing of stressed fluids formulated with premium hydraulic additive technology demonstrates that these fluids are extremely durable and retain their performance quite well.

These studies also show that the special characteristics of premium hydraulic fluids enable them to surpass the designation of being mere commodities. They are specialty fluids that provide the endurance and retention of performance needed for long, low-maintenance equipment life in today’s more demanding environments.

Editor’s Note:
Lubrizol’s Jerry Cochrac (retired), Dan McCoy and Betsy Butke also contributed to this article.