1. 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

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

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

4. 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

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

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

7. The Sensors

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

9. Reflected Pulses

10. 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)

11. 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

12. Software Interface

13. 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

14. 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’

15. 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)

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

17. Close up On a Ball Passage

18. Measured Film Thickness

19. 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

20. The End
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