Ultrasonic Measurement of Lubricant Film Thickness of Rolling Bearings EWEC 2010 Stuart Herbert. Narec Business Development Manager – Engineering and Drive Train Prof Rob Dwyer-Joyce, Dr Matt Marshall Leonardo Centre for Tribology University of Sheffield

Narec Wind Division – Technology Development Condition Monitoring Drive Train Testing Blade Testing Technology

Talk today… How you can measure an oil film with ultrasound Application to Rolling Element Bearings

Reflection from an Imbedded Layer Ultrasonic waves reflect from an interface Incident Transmitted Reflected Layer so thin it acts as a single reflector Thicker the layer more of the wave is reflected Reflection coefficient, R R=amplitude reflected wave/amplitude of incident wave

A New Approach for Film Thickness Measurement incident wave transmitted wave reflected wave Thick film time of flight method Thin film <100mm

Component Layout Control signal Signal Processing Ultrasonic pulser Receiver Digital oscilloscope Transducer Interface

The Sensors

Pulsed Wave Packet & FFT FFT B A R = reflected signal / incident signal = A/B R = f(frequency, film thickness)

Reflected Pulses

A Simple Result for Thin Films When the wavelength is large compared with the oil film Simple spring model of layered system reflection=f(frequency, stiffness) R=f(w,K)

Operating Principle Sensor coupled to out face of bearing Discrete pulses of ultrasound emitted short duration pulses fast pulsing rate (~0.1 ms) Reflected pulses received and captured Receiver circuit (or digital scope) and PC Trigger the transducer for when a ball goes by Signal processing of reflected pulse Capture the pulses reflected from a ball as it passes by Calculate the reflection coefficient Use the spring model to determine the oil film thickness LabView Interface Convenient for control and processing

Software Interface

Two Sensor Approaches - Focussing Wide band (35MHz) focusing Immersed in a water bath Coupled directly to bearing raceway Limited by the spatial resolution s (~100mm) Focused on the interface

Two Sensor Approaches – Piezo Film Vacuum deposited aluminium nitride piezo-film Low profile and very high frequency (200MHz) Greatly improved spatial resolution No need for a ‘water bath’

Ball Bearing Lab Tests 6410 DGBB 50-500rpm 45-75kN Set to pulse continuously Capture all reflected pulses Only store those where R<0.8 (better to use a trigger)

Reflection Signal During Operation Reflected signal amplitude Passage of a Ball Measurement Points (time)

Close up On a Ball Passage

Measured Film Thickness

Conclusions Ultrasound is a unique non-invasive method for measuring oil film thickness Works over a wide range of thickness (50nm and up) Simple low cost stick on sensors Oil film measurement in real time Response Time = 0.1 s (real time mode) or 50 ms (data storage mode) Potential applications As a condition monitoring tool As a bearing/component development device

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