Lube-Tech
generation of debris in a sliding contact can cause localised abrasion, localised sticking due to particle agglomeration, formation and break-off of tribofilms due to particle compaction etc. and thus they can alter the frictional behaviour of the system. In addition, we should consider that due to the wearing off of the vanes and the disk, their surface roughness will also change. However, changes in roughness can have a significant influence on the thickness of the lubricating film (as indicated by Stribeck2
). Film breakdown leads
to local asperity contacts that can cause metal to metal adhesion and influence the friction and wear.
Summary To help readers compare between the standardised Conestoga Vickers and the prescreening Falex Multispecimen tests a Summary table is prepared and is presented in Table 3.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.114 page 6
Conclusions • Prescreening of hydraulic can be performed with a modified Falex Multispecimen tester. This method was developed to simulate the working conditions of a vane pumps, by using with a vane-on-disk configuration and a hydraulic fluid recirculation test chamber. In addition, standardised test disk with specialised grooves that create the variable pressures encountered by the vanes in a pump during each cycle were developed.
• The advantage of this method is that the duration of the test is much shorter (20 h compared to 250 h) and a smaller quantity of hydraulic fluids (3 L compared to 60 L) than the ISO 20763 Conestoga Vickers test is needed. In addition, it allows for an in-situ monitoring of the evolution of friction and of the temperature near the contact surface, so that a correlation between the friction and interfacial temperature can be obtained. This also enables us to compare the frictional performance of hydraulic fluids as information on the torque evolution during each test can be used for further analysis and evaluation of hydraulic fluids.
Table 3. Summary table comparing standardised Conestoga Vickers and Falex Multispecimen prescreening tests.
The obvious advantages of the new methodology on the Falex Multispecimen machine, with cyclic stress vane-on-disk tests are the in-situ monitoring of evolution of friction and temperature near the contact surface, and the shorter and repeatable measurement requiring less lubricant volume.
The key feature of this methodology, however, is that similar contact conditions and wear mechanisms are obtained in the MultiSpecimen and Conestoga Vickers tests, leading to a similar ranking of hydraulic fluids and a correlation between the pump stand and the bench test. It is believed that this method can be an ideal technique to compliment standardised Conestoga measurements and a useful tool for the investigation and further development of hydraulic fluids.
• A key feature of this prescreening method is that similar wear mechanisms occur as in the standardised Conestoga Vickers tests. This concept was clearly illustrated by the failure map and it comes to show how important is to test tribosystems under ‘relevant’ conditions. Some lab tests can provide accelerated measurements but they do not always correlate with the actual application.
• What makes this prescreening method even more interesting not only in the research field but for end-user and manufactures of hydraulic fluids is that similar ranking in terms of weight loss as with Conestoga Vickers tests is obtained, whereas the results are repeatable. In addition, the significantly shorter testing time can allow for multiple tests and for a complete statistical evaluation of frictional and wear data. This is also one of the main drawbacks of the standardised tests, as their precision sometimes is not sufficient to distinguish between high performance oils (with an weight loss of less than 20 mg after 250 hrs of testing).
• The future goal of Falex is to propose a standardised method that will aid in the development of hydraulic fluids and will complement existing standardised Conestoga Vickers tests.
2 Tribologie-Handbuch 2. Auflage, Horst Czichos & Karl-Heinz Habig, 2003, ISBN 978-3-528-16354-9, p.225 LUBE MAGAZINE NO.143 FEBRUARY 2018 43
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