Lube-Tech
Fig 4. (a) Evolution of coefficient of friction and test temperature as a function of sliding cycles at 800 lbf, failure after 22 minutes
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.114 page 4
Fig 4. (b) Comparison of the frictional behavior of five hydraulic fluids at 150 lbf for 20 hrs
The flexibility of this technique also allows to run tests of increasing duration and to plot a wear evolution. An indicative graph of this wear evolution (Figure 5) shows that good repeatability is achieved. In addition, a running in period can be identified during the first few test hours. With this information, we can justify the duration of a sufficiently long test (e.g. 20 hrs) until steady ‘state conditions’ are reached.
Testing under relevant conditions As already stated in the introduction, it is essential to prove correlation with the ‘actual’ application. The test parameters have to be adjusted to result in the same wear mechanism. A failure map is plotted with the help of a SEM analysis where we compare vanes and disk from prescreening tests with vanes and rings from Vane pump tests (ISO 20763). Figure 6 shows that the wear in a Conestoga vane pump consists of very mild abrasion and some oxidative wear. When the prescreening is done under correct conditions, the wear mechanisms will look the same. Based on such SEM comparison and weight loss measurements, we can identify 3 different regions on a load-time failure map (Figure 8). In the green area, similar wear mechanisms as with the Conestoga Vickers tests can be obtained Figure 6. In orange, weight gains are observed, indicating build-up or transfer of material from disk to vane. In red, the wear mechanism changes from mild abrasion and oxidative wear to severe deformation on both the vanes and disk (Figure 7). These conditions do not match actual conditions. Taking into consideration this failure map, it is clear that an accelerated wear test has its limits, as load can not be increased above the failure load. The same is true for trying to accelerate the test by increasing temperature: this will lead to a failure, rather than mild wear and is not representative for the wear mechanism in a Vane Pump.
(a) (b)
Fig 6. SEM microstructures of vanes after testing in (a) MultiSpecimen tester at 200 lbf for 20 hrs and in a Conestoga vane pump tester for 250 hrs. (Green area of Figure 6)
Fig 5. Repeat test, evolution of weight loss for vanes + disk of a hydraulic fluid at 200 lbf load
These observations and tests prove that the most common methods for ‘accelerated testing,’ namely increase of load and/or temperature, will lead to a dramatic change of the wear mechanism and become
LUBE MAGAZINE NO.143 FEBRUARY 2018 41
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