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Tribological Aspects of Pneumatic Clutch Actuators
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1021
Keyword [en]
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
Identifiers
URN: urn:nbn:se:uu:diva-195330ISBN: 978-91-554-8601-3 (print)OAI: oai:DiVA.org:uu-195330DiVA: diva2:607731
Public defence
2013-04-12, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2013-03-20 Created: 2013-02-24 Last updated: 2013-04-17Bibliographically approved
List of papers
1. Wear study of field worn clutch actuators and evaluation of a model test
Open this publication in new window or tab >>Wear study of field worn clutch actuators and evaluation of a model test
2009 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 267, no 12, 2269-2272 p.Article in journal (Refereed) Published
Abstract [en]

Pneumatic clutch actuators, employed in trucks, have been investigated. The surfaces of the inside of the anodised aluminium cylinders, of the lip seals and of the guiding rings have been studied by SEM and optical microscopy. For most of the actuators no significant wear was revealed. However, one actuator, with leakage problems, was severely worn. An extensive amount of scratches was found on the cylinder surface, the thickness of the guiding ring had decreased and large parts were missing from the lip seal. A possible explanation is that particles have entered the system thus altering the contact conditions. A test setup was developed to investigate how particles present in the system affect the wear. A part of the piston, with lip seal and guiding ring, slides against a part of the anodised aluminium cylinder in a back- and forward motion. Pure silicone grease lubricated tests resulted in no wear. Tests with added dust particles resulted in distinct scratches on the aluminium surface and embedded particles in the guiding ring and the lip seal. These tests provide results in good agreement with the wear revealed in the investigations of used actuators and support the theory that wear is caused by particles.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
clutch actuator, anodised aluminium, rubber, PTFE, dust particles
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-111835 (URN)10.1016/j.wear.2009.04.008 (DOI)000272810200016 ()
Available from: 2009-12-23 Created: 2009-12-23 Last updated: 2013-07-24Bibliographically approved
2. Tribological surfaces of a pneumatic clutch actuator
Open this publication in new window or tab >>Tribological surfaces of a pneumatic clutch actuator
Show others...
2007 (English)In: Proceeding of ECOTRIB 2007, Slovenien Society for Tribology, June 12-15, 2007, 2007, 955-967 p.Conference paper, Published paper (Refereed)
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-13294 (URN)
Conference
ECOTRIB 2007
Available from: 2008-01-22 Created: 2008-01-22 Last updated: 2013-03-22
3. Comparison of anodised aluminium surfaces from four fabrication methods
Open this publication in new window or tab >>Comparison of anodised aluminium surfaces from four fabrication methods
2012 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 212, no 11, 2272-2281 p.Article in journal (Refereed) Published
Abstract [en]

This study presents a comparative analysis of surface characteristics and properties of anodised aluminium cylinders produced by sand casting, permanent mould casting, extrusion, and high pressure die casting. Differences in micro structure and distribution of silicon particles in the aluminium, due to the fabrication method and the silicon content in the alloy, resulted in varying thickness of the oxide layers (mean thicknesses between 7 and 19 m) and surface topography. The oxide layer was unevenly thick for the permanent mould cast and the sand cast cylinders, resulting in a surface with higher plateaus and lower areas. This was more prominent for the sand cast surface. The oxide of the extruded cylinder was thick and even and its surface was smooth. The high pressure die cast surface had an oxide that was very thin and uneven. The surfaces displayed different results in the scratch test due to the variations in the surface structure. For the permanent mould cast and the sand cast surfaces the silicon particles present in the oxide deflected the cracks that were formed during the scratching. This resulted in smaller wear debris. The nanohardness values of the oxides had a large scattering due to the inhomogeneous nature of the oxide layers, with pores and particles. However, the highest nanohardness values were between 5000 and 6000 MPa for the four surfaces, which is significantly lower than that of sintered alumina. In the micro abrasion test the wear mechanism for all four surfaces was microcutting resulting in chippings.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Aluminium, Anodising, Surface structure, Oxide thickness, Abrasive wear resistance
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-183184 (URN)10.1016/j.jmatprotec.2012.06.007 (DOI)000309569400013 ()
Available from: 2012-10-23 Created: 2012-10-23 Last updated: 2017-12-07Bibliographically approved
4. Friction, wear and surface damage mechanisms of pneumatic clutch actuators
Open this publication in new window or tab >>Friction, wear and surface damage mechanisms of pneumatic clutch actuators
2013 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 305, no 1-2, 36-44 p.Article in journal (Refereed) Published
Abstract [en]

Wear and surface degradation mechanisms of a pneumatic clutch actuator have been studied. The applied test method simulates the sliding contact of an anodised aluminium surface against a rubber lip seal and a PTFE guiding ring. The influences of contact load, temperature and particle contamination on wear mechanisms and friction were investigated. A controlled amount of lubrication was applied; both pure silicone grease and silicone grease mixed with standard dust particles were used.

The wear occurring at the tests with only silicone grease is similar, but on a lower scale, to the wear at the tests with a mixture of silicone grease and standard dust. The polishing of the anodised aluminium is increased with increasing temperature or increasing load. The wear of the anodised aluminium surface is more prominent when sliding against the PTFE guiding ring than when sliding against the lip seal. The wear of the guiding ring increases with increasing load or increasing temperature. The lip seal is worn but at 140 °C (with silicone grease or with a mixture of silicone grease and standard dust) or at increased load (with a mixture of silicone grease and standard dust) a thick layer of adhered material is formed.

National Category
Materials Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-195327 (URN)10.1016/j.wear.2013.05.006 (DOI)000341961800005 ()
Available from: 2013-02-24 Created: 2013-02-24 Last updated: 2017-12-06Bibliographically approved
5. The effect of lubrication on the friction performance in pneumatic clutch actuators
Open this publication in new window or tab >>The effect of lubrication on the friction performance in pneumatic clutch actuators
2014 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 309, no 1-2, 166-173 p.Article in journal (Refereed) Published
Abstract [en]

Pneumatic clutch actuators in trucks transform the force applied on the clutch pedal into the force acting on the clutch. The actuator consists of an anodised aluminium cylinder in which a piston, with a rubber lip seal and a PTFE guiding ring mounted on it, slides. The system is lubricated with silicone grease before assembly. Wear of the lip seal will lead to a deteriorated sealing capacity, and ultimately failure.

A test setup is used to evaluate the effect of silicone grease amount on friction performance, PTFE transfer and rubber lip seal wear. Reciprocating sliding and vibration tests were performed. The surfaces of cylinders, guiding rings and lip seals were studied by SEM and ESCA.

The friction level is highly dependent on the amount of grease in the contact area. During sliding, grease is pushed out from or dragged into the contact area, resulting in a fluctuating friction. Furthermore, even with grease lubrication, PTFE transfer to the aluminium surface occurs. A triple transfer from guiding ring, to aluminium surface, to lip seal, to another part of the aluminium surface was observed. Finally, although the unstable lubricating effect, it was found that the grease protects the lip seal from wear.

National Category
Materials Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-195328 (URN)10.1016/j.wear.2013.11.005 (DOI)000331426700021 ()
Available from: 2013-02-25 Created: 2013-02-24 Last updated: 2017-12-06Bibliographically approved
6. Comparison of friction performance of four anodised aluminium surfaces for use in a clutch actuator
Open this publication in new window or tab >>Comparison of friction performance of four anodised aluminium surfaces for use in a clutch actuator
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.

National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-195329 (URN)10.1016/j.wear.2014.08.005 (DOI)000345061600024 ()
Available from: 2013-02-25 Created: 2013-02-24 Last updated: 2017-12-06Bibliographically approved

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