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Comparison of friction performance of four anodised aluminium surfaces for use in a clutch actuator
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
2014 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 319, no 1-2, 227-233 p.Article in journal (Refereed) Published
Abstract [en]

Clutch actuators in the automotive industry are used to convert the force applied by the driver to the clutch. A typical pneumatic actuator consists of an anodised aluminium cylinder in which a piston, with a rubber lip seal and a PTFE guiding ring, slides. The system is silicone grease lubricated. The aluminium cylinder is most often cast, e.g. permanent mould, sand, or high pressure die cast. An interesting alternative is extrusion. After anodising the cylinders display different surface topography due to, amongst other things, the fabrication method. In this study, the friction behaviour of the anodised surfaces from the four mentioned methods are investigated in a reciprocating sliding test. Two test setups were used, one simulating the real contact situation, and one using a simpler sphere on flat geometry. The extruded surface oxide cracked during testing, resulting in very fluctuating friction behaviour. The high pressure die cast surface showed rather good results, but never the best. However, it is considered not suitable for the application due to its poor oxide properties. The permanent mould cast surface displayed the lowest friction coefficient, while the sand cast surface often showed the highest friction.

Place, publisher, year, edition, pages
2014. Vol. 319, no 1-2, 227-233 p.
National Category
Materials Engineering
URN: urn:nbn:se:uu:diva-195329DOI: 10.1016/j.wear.2014.08.005ISI: 000345061600024OAI: oai:DiVA.org:uu-195329DiVA: diva2:607712
Available from: 2013-02-25 Created: 2013-02-24 Last updated: 2014-12-30Bibliographically approved
In thesis
1. Tribological Aspects of Pneumatic Clutch Actuators
Open this publication in new window or tab >>Tribological Aspects of Pneumatic Clutch Actuators
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A clutch actuator is used in a vehicle to transmit movement and force from the clutch pedal to the release bearing of the clutch. A pneumatic clutch actuator consists of an anodised aluminium cylinder, inside of which a piston, with a rubber lip seal and a PTFE guiding ring, slides. The system is lubricated with silicone grease before assembly. A commercial clutch actuator of this type, has a service life of 3 million actuations and must function in a wide temperature range, from -40 ºC to 140 ºC.

In this thesis, the complex tribological system of pneumatic clutch actuators has been studied. Field worn actuators have been disassembled and investigated. A laboratory test method has been developed to understand the tribomechanisms present in pneumatic clutch actuators. The test method's capability of simulating the real contact has been verified, by the comparison with studied actuators from the field. The influence of contact parameters: temperature, load, lubrication and particle contamination, has been investigated. In addition, different anodised aluminium surfaces have been studied.

The manufacturing method of the aluminium cylinder influences surface topography and structure of the oxide, resulting in different mechanical and frictional properties.

The wear during tests with only silicone grease is reminiscent, but on a lower scale, to the wear during tests with a mixture of silicone grease and standard dust.

The initially applied amount of silicone grease is not important, the friction seems to depend on the amount of silicone grease that is dragged into or pushed out from the contact area during testing. Silicone grease lubrication reduces wear of the lip seal. However, during some tests, an adhesive layer, composed of  grease residuals and some PTFE, was formed on the lip.

A triple PTFE transfer, from guiding ring to aluminium surface, to lip seal, to aluminium surface, occurred. Such transfer of material from the PTFE guiding ring was detected from the unlubricated tests, and also from the silicone grease lubricated tests, i.e. silicone grease lubrication does not prevent PTFE material transfer.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 67 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1021
Wear, friction, PTFE transfer, anodised aluminium, NBR rubber lip seal, silicone grease, casting, extrusion
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
urn:nbn:se:uu:diva-195330 (URN)978-91-554-8601-3 (ISBN)
Public defence
2013-04-12, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (Swedish)
Available from: 2013-03-20 Created: 2013-02-24 Last updated: 2013-04-17Bibliographically approved

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Riddar, FridaKassman Rudolphi, Åsa
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