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May 22, 2018

Volume 3 Issue 2

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Building a More Affordable Tribometer Test

When Bosch Rexroth adopted its Fluid Rating 90235 standard for hydraulic fluids in 2015, it significantly raised the performance bar for fluid formulators. Now the German equipment manufacturer is trying to make things a bit easier by developing a tribometer test that would be a shortcut for determining if a fluid is good enough.

An official from the company told the Uniti Mineral Oil Technology Congress last month that the tribometer test, which is still being developed, is meant to help companies that are developing a fluid gauge whether it is likely to pass the Fluidtest RFT-APU-CL wear test, the cornerstone of the 90235 standard. The advantage, Product Manager Martin Whal explained, is that the tribometer procedure is much less expensive to run.

“The [tribometer] test will allow companies to experiment with sample parts made out of the original parts and tested under real load conditions in an oil bed with specific fluids,” he said. “This then gives you an indication of any wear or wear patterns. When there is none, you can go for a rig test, for example.”

As Whal explained, requirements for hydraulic fluids have become more and more difficult the past several decades as the various types of equipment using them evolved. Operating temperatures and pressures have steadily risen, as have the speeds at which equipment operates. At the same time sizes of the tanks that contain fluids have shrunk, translating into increased circulation even as users demand longer drain intervals – in some cases even lifetime filling.

To cope, industry introduced a series of fluid performance standards – in some cases industry specifications such as DIN 51524 and in others in-house standards by original equipment manufacturers such as Vickers and Parker Hannifin’s Denison.

When Bosch Rexroth introduced Fluid Rating 90235 three years ago, it built DIN 51524 parts 2 and 3 but significantly raised requirements in several areas, Whal said. Whereas 51524 required fluids to run DIN 51350-6 shear stability test for 20 hours, 90235 required the same test to run for 100 hours. Minimal limits for thermal and hydrolytic stability were also lifted, as were elastomer compatibility, air release and foam behavior.

“We added our own requirements on top of these minimum requirements,” Whal said, “in order to reduce operating risk to an absolute minimum.”

Then there was the RFT-APU-CL, run on an apparatus that includes an A6VM 60 bent axis motor and an A4VG 45 swashplate pump. Fluids are run on it three stages for 510 hours before the motor and pump are disassembled so parts can be measured for wear. Whal illustrated the procedure’s difficulty by recounting the results for four fluids, all of which had previously passed DIN 51524. Two of them passed RFT-API-CL, but one allowed exceeded wear limits for the swashplate pump after just 97 hours, and the fourth allowed too much wear to the bent axis motor after just 145 hours.

The tribotest, based on the German DIN 51834-4 standard, uses a so-called SRV instrument, which denotes schwingung (oscillation), reibung (friction) and verschleiss (wear). As per the standard, the test deals with the “determination of friction and wear data for lubricating oils with the cylindrical roller-disk geometry.”

Whal said it is so far showing good correspondence to the likelihood of fluids being successful on the Bosch Rexroth rig test or of providing necessary protection during field tests. Whal said Bosch Rexroth is working with several additive and lubricant producers to develop the tribometer test, but he was not at liberty to disclose the names of its partners.

Bosch Rexroth is also working on an oil ageing test rig to obtain more precise values for the effects of oil ageing in the field. The unit consists of an electric motor in combination with a Bosch Rexroth axis piston fixed pump. An air and water supply is installed to get an ageing effect for the oil being pumped around. The rig also houses a test tank that allows companies to test the reliability of standard pump materials, such as brass. It contains a copper spiral and elastomer and brass testing rods.