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 MarshallLeonardo Centre for TribologyUniversity of Sheffield

Narec Wind Division – Technology Development

Condition Monitoring

Drive Train Testing

Blade Testing Technology

Talk today…

1. How you can measure an oil film with ultrasound

2. Application to Rolling Element Bearings

Reflection from an Imbedded Layer

• Ultrasonic waves reflect from an interface

• 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

IncidentTransmitted

Reflected

A New Approach for Film Thickness Measurement

incident wave

reflected wave

transmitted wave

-90

-70

-50

-30

-10

10

30

50

70

90

0 0.5 1 1.5 2

Time, microseconds

Am

plit

ud

e,

mV

-90

-70

-50

-30

-10

10

30

50

70

90

0 0.5 1 1.5 2

Time, microseconds

Am

plit

ud

e,

mV

Thick filmtime of flight method

Thin film<100m

Component Layout

Control signal

Ultrasonic pulserReceiver

Digital oscilloscope

Transducer

Interface

SignalProcessing

The Sensors

012345678

0 5 10 15

Frequency (MHz)A

mp

litu

de

Pulsed Wave Packet & FFT

0 0.5 1 1.5

Time (us)

Am

plit

ud

e

FFT

R = reflected signal / incident signal = A/BR = f(frequency, film thickness)

A B

Reflected Pulses

-1500

-1000

-500

0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Time (mS)

Am

plitu

de (

mV

)

Decreasing Film Thickness

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12 14 16

Frequency (MHz)

FF

T A

mpl

itude

Decreasing Film ThicknessReference

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(,K)

h

cK

2

2)/2(1

1

zKR

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 (~100m)

• 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’

80mm

AlN piezoelectric film

protective shroud electrode (0.3x3mm2)

electrical connection

bearing shell

Ball Bearing Lab Tests

6410 DGBB50-500rpm45-75kN

Set to pulse continuouslyCapture all reflected pulsesOnly store those where R<0.8(better to use a trigger)

Reflection Signal During Operation

0 50 100 150 200 250 300 350 400 450 5000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Number of the reflected signal

Am

plitu

deReflected signals in frequency domain (175 MHz)

Measurement Points (time)

Ref

lect

ed s

igna

l am

plitu

de

Passage of a Ball

Close up On a Ball Passage

-500 -400 -300 -200 -100 0 100 200 300 400 5000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1R

efle

ctio

n C

oeff

icie

nt,

R

Distance, x (m)

W=2.5 kN, w=506rpm+ W=5 kN, w=364rpmo W=10 kN, w=166rpmx W=15 kN, w=106rpm

Measured Film Thickness

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

l ll ll ll l

l ll l

l ll l

l l

l ll l

l l

l l

l l

l ll l

The

oret

ical

oil-

film

thi

ckne

ss (m

)

Measured oil-film thickness (m)

ω = 106 rpm

ω = 166 rpm

ω = 364 rpm

ω = 506 rpm

AlN: W=2.5 kN, + W=5 kN O W=10 kN, x W=15 kN

Focsd: W=2.5 kN, W=5 kN W=10 kN, W=15 kN

Thin film

Focused transducer

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 s (data

storage mode)

• Potential applications• As a condition monitoring tool

• As a bearing/component development device

The End

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