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  • 1. Harper, P.
    et al.
    Dwyer-Joyce, R.S.
    Sjödin, Ulf
    University of Sheffield, United Kingdom .
    Olofsson, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Evaluation of an ultrasonic method for measurement of oil film thickness in a hydraulic motor piston ring2005In: Life Cycle Tribology, Elsevier, 2005, p. 305-312Chapter 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.

  • 2.
    Sjödin, Ulf
    KTH, Superseded Departments, Machine Design.
    Characterisation of wear on piston rings in a radial piston hydraulic motor2002In: New horizons for tribology and lubricants: scientific achievements - industrial applications - future challenges. / [ed] Bartz, Wilfred J., Renningen-Malmsheim: Expert Verlag , 2002Conference paper (Other academic)
  • 3.
    Sjödin, Ulf
    et al.
    KTH, Superseded Departments, Machine Design.
    Olofsson, Ulf
    KTH, Superseded Departments, Machine Design.
    Experimental study of wear interaction between piston ring and piston groove in a radial piston hydraulic motor2004In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 257, no 12, p. 1281-1287Article in journal (Refereed)
    Abstract [en]

    The wear interaction between piston ring and piston groove in a radial piston hydraulic motor was studied in regard to mass loss and changes in form and surface roughness. A specially developed test rig that simulates the tilting movements of pistons at the end of strokes was used in the test. The results show that wear on the piston ring groove can be up to 10 times greater than the wear on the piston ring. For both interacting surfaces, the dominant wear mechanism was mild wear. The results from a factorial design show that the form of the piston groove significantly influences the amount of wear.

  • 4.
    Sjödin, Ulf
    et al.
    KTH, Superseded Departments, Machine Design.
    Olofsson, Ulf
    KTH, Superseded Departments, Machine Design.
    Initial sliding wear on piston rings in a radial piston hydraulic motor2003In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 254, no 11, p. 1208-1215Article in journal (Refereed)
    Abstract [en]

    The initial sliding wear of boundary lubricated piston rings used in a hydraulic motor is studied in terms of the changes in mass, form and surface toughness. The piston ring in a hydraulic motor makes an important contribution to high volumetric efficiency by properly sealing the cylinder bore and piston. The results show that the wear on the piston ring in this particular test rig takes place at the top of the asymmetric crowning at the outer surface contacting the cylinder bore. Initially, the roughness amplitude decreased rapidly, and had decreased by one-third after sliding 10 m. The dominant wear mechanism was mild wear. Abrasive wear also clearly influences the amount of wear.

  • 5.
    Sjödin, Ulf
    et al.
    KTH, Superseded Departments, Machine Design.
    Pettersson, Ulf
    Olofsson, Ulf
    KTH, Superseded Departments, Machine Design.
    Influence of surface texture on friction and wearManuscript (preprint) (Other academic)
  • 6.
    Skytte af Sätra, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Characterisation of piston ring wear in a radial piston hydrostatic transmission2005Report (Other academic)
  • 7.
    Skytte af Sätra, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Design (Div.).
    Wear of piston rings in hydrostatic transmissions2005Doctoral 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.

  • 8.
    Skytte af Sätra, Ulf
    et al.
    KTH, Superseded Departments, Machine Design.
    Olofsson, Ulf
    KTH, Superseded Departments, Machine Design.
    Experimental Study Of Piston Ring Wear In A Radial Piston Hydraulic Motor Influenced By Properties Of The Cylinder BoreManuscript (preprint) (Other academic)
  • 9.
    Skytte af Sätra, Ulf
    et al.
    KTH, Superseded Departments, Machine Design.
    Olofsson, Ulf
    KTH, Superseded Departments, Machine Design.
    Influence Of Coating And Additive And Surface Roughness On Friction And Wear – A Pin-On-Disc SimulationManuscript (preprint) (Other academic)
  • 10.
    Sundh, Jon
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Design (Div.).
    af Sätra, Ulf Skytte
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Design (Div.).
    Influence of surface topography and surface modifications on seizure initiation in lean lubricated sliding contacts2007In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 262, p. 986-995Article in journal (Refereed)
    Abstract [en]

    Seizure initiation in lean-lubricated contacts was experimentally studied using a transient test method of ball-on-disc type at two different sliding velocities, 2 and 3.8 m/s. Four different nodular cast iron surfaces were tested against a bearing ball of 100Cr6 steel: a fine-milled and roller-burnished surface, a ground and lapped surface, a ground and lapped laser-melted surface, and finally a ground surface. The results show that the ground surface, even though it is smoother than the fine-milled and roller-burn i shed surface, shows indications of seizure at a lower load. No graphite nodules from the nodular cast iron were visible in the surfaces on inspection with an optical light microscope. In contrast, the ground and lapped surface suffered no initial or total seizure in these tests. In this case, many graphite nodules were visible in the surface, and these nodules became detached in the contact zone, where they probably acted as a solid lubricant. Many graphite nodules were also visible in the ground and laser-melted surface, though in this case the graphite nodules did not become detached. This surface topography initiated seizure under a low normal load, and increased sliding velocity lowered the total seizure load significantly.

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