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Q1. What does the Ultrasensor
measure?
A1. The thickness of the oil film in a bearing. This can be used
a measure of the condition of the bearing and an early warning of
bearing lubrication failure.
Q2. How does it work?
A2. The Ultrasensor measures the reflection of ultrasound from the
oil layer in a bearing. From this, using some clever processing,
it calculates the oil film thickness, assuming the oil properties
are known.
Q3. What is the difference between Ultrasensors and Acoustic Emission
(AE) or Vibration Sensors?
A3. These are two completely different technologies. AE or vibration
sensors record the vibration of a bearing that occurs when some kind
of failure occurs. For example, if the lubricant supply to a ball
bearing is interrupted a spall can occur in the raceway. This causes
whole bearing to vibrate or emit an acoustic signal. AE and vibration
sensors pick up post failure signals. The ultrasensor technique measures
the oil film directly. This means it can give an early warning of
lubrication failure before any damage to the bearing occurs. One
is a passive technique (just listening) and the other is an active
technique (shouting and listening). This means that the ultrasensor
method will work on a stationary bearing (whereas AE will not).
Q4. Do I have to modify my bearing in any way?
A4. Yes and no. The technique
is non-invasive as the sensor is bonded onto the back face of the
bearing and emits a signal through the bearing shell. The back face of
the bearing where the sensor is bonded should be free from corrosion,
reasonably smooth and flat. Of course, there also needs to be a path
out for the wires. In some cases the sensor may need to be recessed
into the back of the bearing.
Q5. What kinds of films can be measured using the Ultrasensor?
A5. In principle, any liquid or solid layer
trapped between two materials can be measured. There are two main
limitations. Firstly, the layer
should be of different material to the surrounding media, and the
two surrounding media should be of similar material. Secondly, the
materials should be permeable to ultrasound. A metal – liquid – metal
film is ideal. The films between metals and polymers (for example
rubber seals) are harder to detect.
Q6. What kinds of bearings can be measured in this way?
A6. This method is ideal for hydrodynamic journal and thrust bearings
of any size. We have a wide range of experience in testing these
kinds of bearings both under laboratory and field conditions. It
is also possible to measure the oil films in elastohydrodynamic contacts
like, rolling element bearings but because the oil film is transient
and occurs over a small area, the measurements are more tricky and
the method is currently only suitable for laboratory scale measurements
and not for use in the field.
Q7. Can the same sensor measure all bearings?
A7. No. Each sensor has a thickness range
which it can operate in. Typically the sensors operate of over one
order of magnitude of thickness – e.g.
0.0001-0.001 or 0.001-0.01 mm. The sensor must be selected with a particular thickness
range in mind. This choice can be easily made from the bearing geometry
and operating conditions.
Q8. Will this work on all kinds of bearing material?
A8. Most conventional bearing materials can be used. However, the
bearing material must be able to transmit ultrasound without excessive
attenuation. Therefore, not all materials are suitable, for example
porous materials or materials with a large proportion of dispersed
particulates may cause problems. There is no hard and fast rule and
these materials must be tested on an individual basis. Conventional
metallic bearing materials like; mild steel, bearing steel (52100),
tool steels (M50), and bronzes are all fine.
Q9.
What if the bearing has a surface layer – for example a
whitemetal faced thrust bearing?
A9. The interface between the substrate
and the surface layer will also reflect ultrasound. It is important
to separate this reflection
from the oil film reflection. In most cases this is straightforward,
but occasionally the layer thickness is such that it overlaps with
the required reflection – in this case some special signal
processing is needed. We have tested many kinds of whitemetal faced
journal and thrust bearings and these present no problem. A case
hardened layer will not cause any significant reflection.
Q10. What is the minimum and maximum thickness of oil film that can
be measured?
A10. There is no upper limit to the thickness that can be measured.
Using conventional transducers we can measure oil films down to around
100 nm. Using higher frequency transducers it is possible to measure
down to around 20 nm. But with higher frequencies and very thin films,
the signal noise, and attenuation become more of an issue and care has
to be taken. In practice hydrodynamic bearings like journal and thrust
pad bearings have films in the range 0.001-0.1 mm whereas
elastohydrodynamic bearings like ball and roller bearings have films in
the range 0.0001-0.001 mm. The Ultrasensor system can cover this entire
range.
Q11. What is the spatial resolution of the technique?
A11. The spatial resolution
depends on the frequency of the transducer we use and whether it is
focused on the oil film. The sensor measures the lubricant film at a
point. The size of this ‘point’ determines the spatial
resolution. Typically the spatial resolution is around 2-6 mm diameter.
It is possible to achieve a resolution of 0.1-0.5 mm diameter but this
requires a focusing transducer and so should only be used when this
level of spatial resolution is essential for the application.
Q12. How accurately can film thickness be measured?
A12. Under ideal conditions accuracies of less than 5% are possible.
A noisy electrical environment can reduce this figure. From past
experience we have normally been able to measure an oil film to a
greater degree of accuracy than the stability of the film itself.
Q13. Can the oil film on the free surface of a component be measured?
A13. Not normally, the film must be trapped
between two solid bodies. Under certain circumstances a surface oil
film can be measured, either
if it is relatively thick (>0.1 mm) or if a solid body can be placed
over it to create a trapped film.
Q14. How can the film thickness sensors be calibrated?
A14. Firstly, the method works by analytically
deducing the oil film thickness from reflected ultrasonic signals.
Therefore there is no
need for a separate calibration exercise to relate oil film thickness
for a specific bearing case. However, as with all measurement devices,
calibration of the sensors is an important part of their operation.
There are a number of ways the sensors can be calibrated – a
separate document is provided for this. A reference signal can be
taken periodically (at a known oil film thickness or when the bearing
is disassembled), an intermediate interface (or notch) can be included
in the ultrasonic path and this reflection used to calibrate, alternatively
we have developed a self-calibrating sensor that measures both phase
and amplitude together and uses the relationship between the two
signals to calibrate.
Q15. Can this method be retro fitted to existing bearings?
A15. Yes. This method does require a flat on the back face of a bearing
(this may require a little machining) but apart from that there is
no problem.
Q16. What about build up of Lacquer on the bearing surface?
A16. In previous testing the acoustic properties
of lacquer have been found to be similar to that of oil. Thus the
lacquer is ‘invisible’ to
this technique. Often lacquer layers are considerably greater than
the original oil film (thus there is a much thinner oil layer). This
may be detected by an increase in the perceived oil film over time.
Q17. Can the angular position of the minimum oil film be detected?
A17. Yes. By installing two or more sensors around the circumference
of the bearing the position of the minimum oil film can be calculated
from geometry.
Q18. Does oil degradation affect the measurements?
A18. If oil degradation (or any other process)
changes the acoustic properties of the oil then there will be a perceived
change in the
oil film unless this is taken into account. Changes in acoustic properties
can be very simply calibrated out by using an additional sensor to
monitor the lubricant in parallel with the oil film thickness.
Q19. Do air bubbles in the oil affect the signal?
A19. Yes, they can. The method is not designed for measuring the
oil film in the cavitated region of a bearing. The air bubbles can
potentially disrupt the signal. However, in the rest of the oil film,
oil bubbles are scarce enough to have no effect on the measurement.
Alternatively, the sensor could be used to detect the onset of cavitation.
Q20. Will it still work if there are debris particles in the oil?
A20. Yes, as long as the volume fraction
of particles in the oil is small – which will be the case in
almost all bearing systems.
Q21. Can I measure oil film condition in real time with Ultrasensor?
A21. Yes, each measurement takes around a millisecond to complete
and so the oil film thickness can be displayed in real time.
Q22. Can the Ultrasensor be integrated into plant condition monitoring
systems?
A22. Yes. The instrument has a digital output that can be integrated
into most plant management systems. We would be happy to discuss
any integration issues with you in more detail.
Q23. How does temperature affect the measurement?
A23. There is a temperature effect as the oil properties change with
temperature. If this change is small then it can be ignored. If it
is large then the temperature must be measured simultaneously with
the Ultrasensor measurement and taken into account in the thickness
calculation. However, once the Ultrasensor instrument is set-up this
will all happen automatically. Sensor performance with temperature
is automatically calibrated out.
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