P. Harper

Ultrasonic measurement of film thickness in mechanical seals

Measurement of the interface film thickness of mechanical seals has been of interest to researchers and industry for many years. This feature describes work to measure the liquid film using a novel ultrasonic approach with a view to developing a condition monitoring tool. Initial tests were based on a laboratory simulation, to compare the method with optical interference and capacitance measurements. Tests were then carried out on a seal test rig. Film thickness was successfully recorded as speed and load was varied.

The Measurement of Liner - Piston Skirt Oil Film Thickness by an Ultrasonic Means

The paper presents a novel method for the measurement of lubricant film thickness in the piston-liner contact. Direct measurement of the film in this conjunction has always posed a problem, particularly under fired conditions. The principle is based on capturing and analysing the reflection of an ultrasonic pulse at the oil film. The proportion of the wave amplitude reflected can be related to the thickness of the oil film. A single cylinder 4-stroke engine on a dyno test platform was used for evaluation of the method. A piezo-electric transducer was bonded to the outside of the cylinder liner and used to emit high frequency short duration ultrasonic pulses. These pulses were used to determine the oil film thickness as the piston skirt passed over the sensor location. Oil films in the range 2 to 21 μm were recorded varying with engine speeds. The results have been shown to be in agreement with detailed numerical predictions.

A New Approach for the Measurement of Film Thickness in Liquid Face Seals

Face seals operate by allowing a small volume of the sealed fluid to escape and form a thin film between the contacting parts. The thickness of this film must be optimized to ensure that the faces are separated, yet the leakage is minimized. In this work the liquid film is measured using a novel ultrasonic approach with a view to developing a condition monitoring tool. The trials were performed in two stages. Initially tests were based on a lab simulation, where it was possible to compare the ultrasonic film thickness measurements with optical interference methods and capacitance methods. A direct correlation was seen between ultrasonic measurements and capacitance. Where ultrasonic and optical methods overlap, good correlation is observed; however, the optical method will not record film thickness above ∼ 0.72 μ m. A second set of trials was carried out, where the film thickness was monitored inside a seal test apparatus. Film thickness was successfully recorded as speed and load were varied. The results showed that while stationary the film thickness varied noticeably with load. When rotating, however, the oil film remained relatively stable around 2 μ m. During the normal operation of the seal, both sudden speed and load changes were applied in order to initiate a seal failure. During these events, the measured film thickness was seen to drop dramatically down to 0.2 μ m. This demonstrated the ability of the technique to predict failure in a face seal and therefore its aptitude for condition monitoring.

Oil film measurement in polytetrafluoroethylene-faced thrust pad bearings for hydrogenerator applications

Abstract There is a growing trend in the replacement of the babbit facing in thrust pad bearings with a composite polytetrafluoroethylene (PTFE) surface layer. The PTFE-faced bearings have been shown to allow a greater specific pressure, reduce thermal crowning, and, in some cases, negate the need for an oil-lift (jacking) system. These designs of bearing require new methods for the measurement of oil film thickness both to assist in their development and for plant condition monitoring. In this work, an ultrasonic method of oil film measurement is evaluated for this purpose. An ultrasonic transducer is mounted on the back face of the thrust pad. Pulses are generated and transmitted through the pad material, bonding interlayer, and PTFE surface layer. The proportion of the wave that reflects back from the oil film layer is determined. This is then related to the oil film thickness using a series of calibration experiments and a spring stiffness model. In practice, the reflected signal is difficult to distinguish, in the time domain, from other internal reflections from the pad. Signals are compared with reflections when no oil film is present and processing is carried out in the frequency domain. Experiments have been performed on a full size PTFE-faced thrust pad destined for a hydroelectric power station turbine. The instrumented pad was installed in a test facility and subjected to a range of loading conditions both with and without oil lift. Although there were some problems with the robustness of the experimental procedure, oil films were successfully measured and used to study the effect of the oil-lift system on film formation.

Evaluation of an Ultrasonic Method for Measurement of Oil Film Thickness in a Hydraulic Motor Piston Ring

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 cylinde ...

Journal bearing oil film measurement using ultrasonic reflection

Abstract A wave of ultrasound will reflect from a lubricant film between two bearing components. The response of the film to a sound wave depends on the acoustic and geometrical properties of the oil layer. For thin lubricant films the layer behaves as a spring and the proportion of the wave reflected (or the ‘reflection coefficient’) is related to the stiffness of the oil layer. This provides a method for determining the film thickness. An ultrasonic transducer has been mounted on the outside of a hydrodynamic journal bearing bush. Pulses are emitted through the bearing bush and reflected back from the oil film. The reflected pulses are picked up by the same transducer and processed to determine the proportion of the wave amplitude reflected. The film thickness has then been calculated from the reflection spectra. The film thickness has been determined for a range of steady state bearing operating conditions and the results compared, qualitatively, with published numerical solutions. the method has been shown to be robust and flexible and demonstrates the potential of an ultrasonic based lubricant film monitoring device.

On‐Line Measurement of Lubricant Film Thickness Using Ultrasonic Reflection Coefficients

The ultrasonic reflectivity of a lubricant layer between two solid bodies depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. In this paper, ultrasonic reflectivity measurements are used as a method for determining the thickness of lubricating films in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant film layer. For a particular lubricant film the reflected pulse is processed to give a reflection coefficient spectrum. The lubricant film thickness is then obtained from either the layer stiffness or the resonant frequency. The method has been validated using static fluid wedges and the elastohydrodynamic film formed between a ball sliding on a flat. Film thickness values in the range 50–500 nm were recorded which agreed well with theoretical film formation predictions.