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A novel reciprocating tribometer for friction and wear measurements with high contact pressure and large area contact configurations
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-8533-897x
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0001-5080-0178
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0003-1162-4671
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-4085-8306
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2021 (English)In: Lubricants, ISSN 2075-4442, Vol. 9, no 12Article in journal (Refereed) Published
Abstract [en]

There are many moving machine assemblies with conformal tribological contacts at very high contact pressures, e.g., sliding bearings, propeller shaft bearings and machine guideways. Furthermore, applications such as trunnion and guide vane bearing in Kaplan turbines have very low sliding speeds and oscillatory types of motion. Although there is a vast selection of tribology test rigs available, there is still a lack of test equipment to perform friction and wear tests under high contact pressure, reciprocatory sliding and large area contact. The aim of this work is thus to develop a novel reciprocating tribometer and test method that enables friction and wear tests under low-speed reciprocatory sliding with contact pressures up to 90 MPa in a flat-on-flat contact configuration. First, a thorough description of the test rig design is given. Secondly, the influence of contact pressure and stroke length on the tribological properties of a stainless steel and polymer composite material combination is studied. The significance of considering creep, friction during the stroke and contact temperature is specifically highlighted. The novel tribometer can be used to screen different bearing and shaft material combinations and to evaluate the friction and wear performance of self-lubricating bearings for the specific operating conditions found in Kaplan turbines. 

Place, publisher, year, edition, pages
MDPI, 2021. Vol. 9, no 12
Keywords [en]
Kaplan turbine, self-lubricating, service life, lubrication, wear
National Category
Other Mechanical Engineering
Research subject
Machine Elements
Identifiers
URN: urn:nbn:se:ltu:diva-88660DOI: 10.3390/lubricants9120123ISI: 000742716800001Scopus ID: 2-s2.0-85121495531OAI: oai:DiVA.org:ltu-88660DiVA, id: diva2:1624655
Note

Validerad;2022;Nivå 2;2022-01-04 (johcin)

Available from: 2022-01-04 Created: 2022-01-04 Last updated: 2025-02-14Bibliographically approved
In thesis
1. Tribology of self-lubricating polymer composites for hydropower applications
Open this publication in new window or tab >>Tribology of self-lubricating polymer composites for hydropower applications
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Tribologi inom självsmörjande polymera kompositmaterial för vattenkraftstillämpningar
Abstract [en]

The ongoing global climate crisis and its impacts calls for urgent actions, such as significant reduction of CO2 emissions by introducing more renewable energy. This has led to a rapid integration of variable power sources, such as wind and solar power, into the electricity systems. These sources have an unpredictable output and requires more active control of the power output in order to compensate fluctuations in supply. The hydropower plants shoulder a large portion of the regulation and balancing duty in many power systems and are therefore subjected to a higher number of start/stop cycles and significantly higher number of load cycles than before. This result in a longer accumulated wear distance and harsher operating conditions for critical components such as sliding bearings for the guide vanes and the turbine blades. As a result, in recent years several bearing failures have been reported in the turbines operating under these more fluctuating conditions.

Despite the severity of the issue, and the additional maintenance and/or replacement costs involved, only a limited number of studies has been reported dealing with wear and friction behaviour of self-lubricating polymer composite bearing materials used for hydropower applications. These studies often lack thorough surface analysis where the governing friction and wear mechanisms have been investigated. In addition, there is a lack of knowledge regarding the influence of various operating conditions on the tribological performance. Hence, there is insufficient knowledge to estimate the actual effect of the increased control and how to optimize the operating conditions in order to reduce friction and wear.

Therefore, the aim of the present work is to increase the knowledge and understanding of how different parameters such as sliding speed, contact pressure, stroke length, test duration, and counter surface topography influence the tribological performance, and its governing mechanisms, of different types of self-lubricating polymer composite bearing materials commonly used in hydropower applications.

To achieve this aim, a systematic tribological characterization has been carried out to study the influence of the abovementioned parameters on the tribological performance of self-lubricating polymer composite bearing materials sliding against stainless steel. The tests have been carried out in dry sliding using different linear reciprocating flat-on-flat configurations and long test duration. The governing friction and wear mechanisms have been thoroughly investigated by various surface analysis techniques, such as 3D optical interferometry, SEM, and EDS. In addition, material characterization of the polymer composites was carried out using a range of analytical techniques to study macro- and micro-structure as well as composition to aid the interpretation of the tribological behaviour.

The results show that the investigated parameters have a significant influence on friction, wear, and formation of transfer layers. The formation of transfer layers is a transient process involving continuous build-up and break-down. The friction and wear mechanisms for the polymer composites are also transient processes and need to be studied over long durations to enable accurate predictions of their long-term tribological behaviour. Surprisingly, under some operating conditions the stainless steel counter surface showed clear signs of abrasive wear caused by micro-cutting and micro-ploughing. This is caused by interaction with work hardened and oxidised steel wear debris as well as reinforcement and filler particles from the polymer composites.

The parametric study showed that the coefficient of friction is decreasing with increased contact pressure due to higher concentrations of solid lubricants in the sliding interface. The wear rates showed an increasing trend with increased sliding speed due to thermal softening of the polymer composite materials. This was particularly pronounced at lower contact pressures, as a result of reduced availability of solid lubricants. The influence of stainless steel counter surface topography revealed that too smooth steel surfaces result in higher friction and more wear of the steel, while rougher steel surfaces have a negative effect on the wear of the polymers. The effect of steel surface lay orientation with respect to the sliding direction differed between the polymer composites and was highly influenced by the initial surface roughness of the steel surface. However, for higher surface roughness, all polymer composites showed reduced friction with perpendicular lay compared to parallel due to thicker transfer layers. The results showed increasing wear rate with increased stroke length, especially when the stroke length is longer than the length of the polymer pin. This was accompanied by increased wear of the steel surface at the longest stroke length due to reduced entrapment of wear particles. The influence of stroke length on friction behaviour differed between the polymer composite materials.

In summary, by optimizing the operating conditions for the self-lubricating polymer composite bearing materials in hydropower applications, it is possible to both save energy and prolong the useful lifetime of the bearings. Furthermore, the obtained data may be useful for selection of bearing materials for given operating conditions to ensure improved tribological performance of the bearings.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
polymer composites, hydropower, sliding wear, sliding friction, self-lubricating, transfer layers, material characterization, surface analysis, high contact pressure, low sliding speed
National Category
Other Mechanical Engineering
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-90197 (URN)978-91-8048-067-3 (ISBN)978-91-8048-068-0 (ISBN)
Public defence
2022-06-20, A109, Luleå Tekniska Universitet, Luleå, 09:30 (English)
Opponent
Supervisors
Available from: 2022-04-13 Created: 2022-04-13 Last updated: 2025-02-14Bibliographically approved

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Berglund, KimRodiouchkina, MariaHardell, JensKalliorinne, KalleJohansson, Jens

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