Inside a vane pump, paddle-like vanes are mounted on a rotor that spins, much like a waterwheel; these vanes pressurize and push fluid through the system. Hydraulic systems have power densities that can be ten times greater than electric motors, Drees said, so they are particularly useful for applications with high loads, such as earth-moving equipment, or distributed power needs, such as airplanes.

Global demand for hydraulic fluid sat around 3.4 million metric tons in 2018, or about 8 percent of total finished lubricant demand, according to Parsippany, New Jersey-based consultancy Kline & Co. It is forecast to grow to 3.5 million tons by 2023, in contrast to the total appetite for all industrial oils and fluids, which is expected to decline by 0.3 percent per year.

Hydraulic fluid formulation is critical to vane pump performance. Drees explained that vane pumps need to be serviced regularly, and the vanes, ring and bushings must be replaced after they wear out.Less wear means less need to shut down the pump for service.

Although the hydraulic fluids primary function is to transmit power and does not require many special additives like those in automotive engine oils, antiwear additives are important to protect the pump parts, he said.

Many specifications require hydraulic fluid suppliers to show that their product passes a standardized test in a full-size hydraulic vane pump.When a supplier wants to change the chemistry or concentration of antiwear additives in a fluid, it must repeat the test to validate the new formulation.

Many hydraulic fluid specifications require the Conestoga Vane Pump Test, which replaced the Vickers Vane Pump Test (ASTM D2882). Wear is measured in terms of weight loss of component parts after operating a pump under standard conditions. These tests require long run times and large amounts of fluid, and they are costly.

Many bench tests, such as SRV, four-ball wear and pin-and-vee block tests, have been tried. They all take a lot less time and fluid, but have shown little or no correlation with the standard pump tests, said Drees.

Emmanuel Georgiou of Falex introduced a new bench test for hydraulic fluids at the 2019 annual meeting of the Society of Tribologists and Lubrication Engineers. Only the Falex Multispecimen Vane Pump test, based on the Falex Multispecimen Test Machine with specially prepared vanes and disks, has shown a positive correlation with an actual vane pump test, he claimed.

Georgiou pointed to the unavoidable trade-off between more rapid, lower cost bench tests that may not correlate strongly with performance in the field and more time-consuming, more expensive tests in actual mechanical systems.

In the case of hydraulic fluids, he explained, vane pump tests correlate well with performance in applications but are expensive to run. For the ASTM D7043 method, the Conestoga Vane Pump Test uses 60 liters of fluid in a pump operating for 100 hours. The ISO 20763 method requires 120 liters of fluid and runs for 250 hours. It is economically unrealistic to run duplicates, and these tests dont always make a precise distinction between high-performance fluids with low rates of wear.

FMVP Method

The team at Falex developed a technique to simulate vane pump tests in a shorter test requiring less lubricant. The key was to find conditions in the bench test that faithfully replicate the wear mechanism in pumps. They adapted a Multispecimen Test Machine, a versatile commercial tribometer used to evaluate wear and friction behavior for a wide range of contact geometries, speeds, pressures, temperatures and test specimens for various lubricant standards and specifications.

First, the team designed a customized laboratory-scale vane-on-disk assembly. Steel vanes and disks were engineered to simulate the special pressure pattern on vanes in an actual pump. The small rectangular vanes fit into slots in a disk-like holder so that their edges touch a second disk. The assembly fits in the MST machine, which rotates the vane edges against the second disk. A recirculating oil system floods the parts with hydraulic fluid at a controlled temperature.

The steel alloys, production method, surface finish and the geometry and shape of the vanes and disks all contribute to the correlation between the FMVP test and vane pump tests, according to Drees.

The next step was to determine MST operating conditions that produce the same wear type and order of magnitude in wear rate as encountered in pump tests. The team used optical and electron microscopes to compare worn vanes from both tests with the same hydraulic fluid. The pump test caused mild abrasion or plow lines and oxidative wear (formation of oxide layers) on the surfaces of vanes and rings. The contact pressure and test duration in the MST were then fine-tuned to obtain the FMVP test, which accelerates wear rate and shortens the test duration without changing the wear mechanism.

Then the team ran FMVP tests for various lengths of time. Georgiou showed a graph of wear versus time to his audience at the Nashville, Tennessee, meeting. There was good agreement between data from two replicate tests. His graph showed that the wear rate was highest during the first few hours and then more steady from hours five to 20.

Georgiou said by the end of the FMVP test, the steady wear rate was very similar to the result from the ISO 20763 Vickers Vane Pump test. He emphasized the importance of running the FMVP test until a steady wear rate is observed after the run-in period.

He also pointed out that the MST is equipped with instrumentation to monitor temperature near the contacting surfaces as well as friction-measured as torque-during FMVP tests, another advantage over pump tests. His data showed a sharp increase in the coefficient of friction and an increase of temperature during tests when severe wear occurred. This agreement supports the validity of the FMVP methodology.

Ranking Hydraulic Fluids

After confirming that the FMVP method produced repeatable data and generated the same wear phenomena as vane pump tests, the Falex team used both methods to evaluate five hydraulic fluids.

Drees explained, The five fluids used for the correlation study were selected because we have first-hand experience with them, running the actual vane pump tests-ISO 20763 in this case.We could take a sample of the same batch for both pump and FMVP tests.

The fluids are from customers for whom we ran the vane pump test.Suffice to say that they are quite standard, conventional hydraulic fluids.One type was a glycol, the others were mineral oil based.

Georgiou presented wear data for all five fluids, A through E. An identical ranking, from more to less wear, was obtained in both tests. However, measured weight losses from FMVP and vane pump tests did not match precisely.

Georgiou explained that the FMVP method can be used as a prescreening technique to rank fluids or to evaluate the effects of formulation changes or test conditions on fluid performance, but it is not intended to predict the actual result of a Vickers or Conestoga test.

Nevertheless, the FMVP test offers a number of advantages. It requires much less fluid (only 3 liters) and time (20 hours) to measure the steady wear rate. Laboratories that own an MST can purchase the adapters and test specimens, as well as a fluid circulating system, to perform the FMVP method in-house, or they can have their fluids evaluated by Falex test labs.

Georgiou noted that hydraulic fluids with low weight loss often showed low friction, as with four out of five fluids in the comparison study. The exception was hydraulic fluid E, which had the lowest friction but not the lowest wear loss.

He explained that friction and wear do not always correlate perfectly. Two fluids can have similar friction but dissimilar wear results in a given test because of differences in antiwear additive performance, heat dissipation and wear particles. The vane pump tests do not provide information about frictional behavior.

Georgiou proposed that fluctuations in friction data for fluid C could have resulted from the wear particles. This hypothesis conforms with higher weight loss for this fluid. Another factor could be the change in roughness of the vanes and disk caused by wear.

He concluded that the relatively low cost of the FMVP method makes it possible for lubricant companies to prescreen more formulations and optimize their chemistry before undertaking standard tests to qualify their products for use in vane pumps. Friction and temperature data can provide additional clues about the lubricating performance of hydraulic fluids and help formulators understand wear data and mechanisms.

The latest development in this test method was a study of the influence of antiwear additive concentration. The FMVP method showed a very clear sensitivity to additive concentrations as low as 0.2, 0.4 and 0.6 percent. This illustrates how lubricant manufacturers can use the FMVP method to optimize the antiwear additive concentration in a hydraulic fluid.

Drees stated that Falex is currently organizing round robin tests for the FMVP test methodology and looks forward to preparing an eventual standardized test method based on the FMVP method. z

Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at mmmoon@ix.netcom.com or 267-567-7234.