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Evaluation of an ultrasonic method for measurement of oil film thickness in a hydraulic motor piston ring
University of Sheffield, United Kingdom .
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).ORCID iD: 0000-0003-2489-0688
2005 (English)In: Life Cycle Tribology, Elsevier, 2005, 305-312 p.Chapter in book (Refereed)
Abstract [en]

The efficiency of a hydraulic motor depends on the lubrication performance of the piston ring. If the film is too thin then wear occurs quickly, if it is too thick then oil is lost into the cylinder and efficiency is reduced. In this paper a technique for oil film measurement based on ultrasonic reflection is investigated. This has the potential to be used non-invasively on real components. An ultrasonic pulse will reflect from a thin film interposed between two solids. The proportion of the pulse that is reflected depends on the stiffness of the intermediate layer. If the acoustic properties of the film material are known, then the stiffness can readily be used to determine the film thickness. This principle has been employed for the piston ring lubrication case. A piston/cylinder test bench has been used to evaluate the ultrasonic method. A focusing piezo-electric transducer is mounted outside the cylinder and ultrasonic pulses reflected back from the inner bore. The variation of these pulses as the piston ring passes underneath is investigated and used to determine oil film thickness. Films in the range 0.7 to 1.3 μm were measured; the thickness did not depend strongly on either ring speed or sealed pressure. Several practical aspects were investigated such as, attenuation in the cylinder material, response time, and transducer resolution. Whilst this study demonstrated that film thickness measurement is feasible, there are a number of practical considerations that require further work, principally the focusing and coupling of the ultrasonic transducer and the response time.

Place, publisher, year, edition, pages
Elsevier, 2005. 305-312 p.
Series
Tribology and Interface Engineering Series, ISSN 1572-3364 ; 48
National Category
Reliability and Maintenance
Identifiers
URN: urn:nbn:se:kth:diva-25063DOI: 10.1016/S0167-8922(05)80032-XScopus ID: 2-s2.0-33745113012ISBN: 0-444-51687-5 (print)OAI: oai:DiVA.org:kth-25063DiVA: diva2:355545
Conference
Leeds, 7/9-10/9 2004
Note

QC 20101007

Available from: 2010-10-07 Created: 2010-10-07 Last updated: 2014-12-02Bibliographically approved
In thesis
1. Wear of piston rings in hydrostatic transmissions
Open this publication in new window or tab >>Wear of piston rings in hydrostatic transmissions
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This study focuses on the wear of piston rings in a hydraulic radial piston motor. The piston ring has to satisfy increasing demands for reliability and longer service life. It has two contacting surfaces, the face and the flank, and operates under a boundary lubrication state.

This first part of the project aimed to detect and characterise piston ring wear. Measurement by weighing gives an overall value for wear defined as loss of mass. Two-dimensional form and surface roughness measurements show the distribution of wear on the piston ring face in contact with the cylinder bore and the piston ring flank in contact with the piston groove. Three-dimensional analyses, both quantitative and qualitative, allow the wear mechanisms to be identified.

The wear of piston rings from an actual hydraulic motor was characterised. As well, rig testing was performed in two different test rig set-ups, one simulating the sliding movement of the piston ring and the other the tilting movement at the end of the strokes. Wear during the running-in period was investigated, and the findings indicate that the period when this takes place is of short duration. In the long term, mild wear makes the surfaces smoother than they were when new, resulting in a very low wear coefficient. Significant levels of wear were measured on both contacting surfaces of the piston ring. In cases in which the flank exhibits more wear than the face, the wear on the flank can be reduced by proper design of the piston groove.

The second part of the project aimed to evaluate use of a textured surface for the cylinder bore counter surface and a coated surface for the piston ring. Three modelling experiments were performed to characterise the friction and wear properties under lean boundary lubrication conditions. Under such conditions, textured surfaces have the advantage of retaining more lubricant and supplying it over a longer time. Stable friction was also a distinctive feature of the textured surface. Use of a coating could also possibly reduce the amount of wear. Though a smooth surface, like a polished one, is hard to beat for a working texture, a coated surface is far ahead of a smooth uncoated one. Different manufactured and commonly used cylinder bore surfaces, including textured ones, were evaluated in the sliding movement test rig. That allowed favourable wear properties, such as lowest wear coefficient, to be determined with the use of a roller burnished surface.

A final part of the research involved simulating wear on the piston ring face throughout the entire service life of a hydraulic motor. This allowed us to determine the roles of surface roughness and coating in prolonging service life and achieving acceptable and secure piston ring operation. The model is simple and realistic, but still needs to be refined so as to correspond even better to reality.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 55 p.
Series
Trita-MMK, ISSN 1400-1179 ; 2005:18
Keyword
piston ring, surface roughness, texture, coating, wear, hydraulic motor
National Category
Reliability and Maintenance
Identifiers
urn:nbn:se:kth:diva-432 (URN)91-7178-148-X (ISBN)
Public defence
2005-10-07, Sal M3, Brinellvägen 64, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101007Available from: 2005-09-27 Created: 2005-09-27 Last updated: 2010-10-07Bibliographically approved

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Olofsson, Ulf

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