eddy current white paper
TRANSCRIPT
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GEInspection Technologies Eddy Current
Digital | Eddy Current | Film | Testing Machines | Ultrasonics | X-ray
Whitepaper - Eddy Current
Author - Chris Hocking
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Applications
Page
• Coating Thickness Measurement 3
• Dynamic Automated Inspection 4
• General Crack Detection 5
• Fastener Inspection 6
• Hole Inspection 7
• Material Sorting / Conductivity Measurement 8
• Rail Inspection (railway lines and associated applications) 9
• Thread Inspection 19
• Tube Inspection 20
• Weld Inspection 21
• Wheel Inspection 23
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Coating Thickness Measurement
Coating thickness measurement is carried out for two
main reasons:
1. Quality Control - determining how thick a
protective coating is.
2. Influence on an eddy current test - different
coating thicknesses can cause a change in
the sensitivity or the frequency needed for the
inspection.
Eddy currents are capable of determining coating
thickness on non conductive coatings on a
conductive base material.
It is also possible to determine the coating thickness
of non-magnetic metallic coatings on metallic bases
providing there is a difference in material properties
(conductivity and magnetic permeability).
Eddy Current Probes
Most eddy current probes can be used for coating
thickness measurement, however, in practice it is
important that the probe mechanically stable. For
this reason it is recommended that flat faced orsprung probes, typically used in material sorting, are
used.
Sprung probee.g. 806P1
Flat faced probee.g. 700P11A
Flat faced probee.g. 720P1F4
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Dynamic Automated Inspection
Eddy current NDT is highly suited to high speed
dynamic inspection becuase:
• it has high surface inspection speeds (up to 5m
per sec)
• there is no need for a couplant (so there is minimal
surface preperation)
• the probe can scan with a gap between itself and
the test surface. This is most conveniently applied
to conponents that are rotationally symmetrical
such as:
• tapered bearings
• gudgeon pins
• ball pins
• axels
• wheel hubs and so on
Eddy Current Probes
Probes for dynamic automated inspection are
generally differentiated. This means that the probes
contain two or more coils which are electrically
arranged to be in opposition to each other. This
arrangement minimises effects which act on both
coils simultaneously (e.g. material variations,
temperature).
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Eddy Current Crack Detection
Surface Inspection
Eddy Current NDT is widely used for the detection of
surface breaking defects in ferrous and non ferrous
metals.
Sub-Surface Inspection
By using low frequencies on non ferrous metals,
eddy currents can be used to inspect for sub-surface
defects on materials such as aluminium, stainless
steel, titanium and so on.Defects typically detected by this method include
sub-surface corrosion and cracks.
Surface Inspection Probes (high frequency)
Due to the extremely small size of the probe core
they are able to inspect aras of poor accessibility.
Probes can easily be designed to fit your specific
application. See the Special Design Checklist for more
infomation.
The frequency is chosen to give a good phase
seperation between the lift off signal and the defect
indication. In practise, this means that generally good
results can be achieved on:
• Aluminium with a 2 MHz probe
• Titanium with a 6 MHz probe and
• Ferrous materials with a 100 - 200 Hz probe
Sub-Surface Inspection Probes (low frequency)
The frequency of the test and the probe sizedetermine the depth of the test , with lower
frequency, larger diameter probes giving a deeper
test.
Shielded surface probe
Unshielded surface probe
Examples of sub-surface inspection probes
A high-frequency probe bing scanned across a calibration block
Impedance plane analysis and display of the results of scanning block
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Fastener Inspection
Eddy Current NDT can be used to detect defects
around fastener holes with the fastener still in place.
Inspections are applicable to all types of fasteners
including screws, bolts, rivits and so on.
Eddy Current Probes
A range of probes are available for the inspection of
fasteners:
• Ring or Doughnut Probe - most simple form of
probe available
• Sliding Probe (Transmit/Receive Probe) - a more
rapid form of inspection
• Hocking FastScan Probe - provides a dual
frequency solution
Please see the table below for a comparison of the
different types of probes:
Sliding probe Righ/Doughnut probe FastScan probe
Ease of use/setup Speed of inpsection Sensitive to defects
in all directions
Sensitivity of
inspection
Ring Probe Excellent Good Excellent Average
Sliding Probe Good Excellent Average Excellent
FastScan Average Average Excellent Excellent
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Hole Inspection
The inspection of holes in metals is often essential as
the stresses around them are twice as high as in the
rest of the material, causing fatigue cracks to grow.
Eddy Current Probes
Eddy current probes provide an ideal solution as
they can go into extremely small holes. They can be
made to measure to fit any diameter - the smallest
currently has a 1.6mm diameter.
Manual Hole Inspection Probes
• Rigid - suitable for where there is no problems
with access
• Flexible - suitable for areas of poor access as
the probe shaft can bend around problematic
geometry.
Dynamic Rotary Hole Inspection Probes
These are most suited where there are a large
quantity of holes to inspect rapidly and with high
levels of accuracy.
• Set diameter - suitable for applications where the
hole diameter is known in advance. These probes
are extremely robust.
• Flexible diameter - a split tip is provided for
inspections where hole sizes vary or are not
known in advance.
Hocking Probe Drive
The MiniDrive has been specially designed for
dynamic rotary hole inspections. It makes the
inspection of fastener holes, even in confined spaces,
simple and accurate and can help test the largest
number of holes in the shortest amount of time.
Rigid manual probe
Flexible manual probe
Set diameter rotary probe
Flexible diameter rotary probe
MiniDrive with set diameter probe
MiniDrive with Flexible Diameter Probe
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Materials Sorting / Conductivity Measurement
Electrical conductivity of a material is a measure of
the ease with which electrons will flow within it . A
material having a high conductivity, e.g. copper, will
permit eddy currents to flow more than a material
having a low conductivity, e.g. lead or non-metals.
Conductivity changes in materials can be caused by
variations in:
• Heat treatment
• Chemical Composition
• Temperature
Eddy currents can be used to measure conductivity,
for the purpose of metal sorting or defining areas of
heat damage. Eddy current instruments designed
for measuring conductivity are usually calibrated in
Percentage International Annealed Copper Standard
(IACS). This is a standard whereby pure copper is
said to be 100% IACS, all other conductivities being
compared with it .
Non Ferrous Materials
Conductivity probes may be used for sorting non-
ferrous materials. The main advantage of using
conductivity probes is that a quantitative measure
of conductivity is given. The probes are used for the
following purposes.
• Alloy identification and verification
• Verification of heat treatment during manufacture
and to detect in-service heat damage
• Detection of changes in material grade
• Metal sorting
• Determine the density of powder metal parts
Ferrous Materials
Because of the ferrous materials magnetic
permeability, it is not posssible to get a quantitative
measure of conductivity, so a comparative method
must be used to sort between good and bad
samples.
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Eddy Current Rail Inspection
The Inspection Problem
The early detection of conditions in rail that may
lead to a break is now a critical activity in the
maintenance of rail worldwide. Understanding of
these mechanisms is constantly improving and
the evolution of a range of complementary NDT
techniques now means that the engineer has a
better choice than ever of tools for the task.
In addition to the maintenance of the rail, there is a
growing requirement for inspection techniques on
the rolling stock itself. The rapid inspection of axels,
wheels and bogies is essential for the safe operation
of the rail network.
This document aims to give you a brief overview of
the different NDT inspection methods currently used
on railways around the world. Eddy Current NDT will
be introduced as a new method of complementing
these inspections along with why and where this
inspection method is needed to ensure rail integrity.
NDT Rail Solutions
NDT Inspections have been used for a number of
years to check our railway tracks for a range of
faults. There are a variety of methods available for
use, the most common of which are mentioned
below.
2.1 Visual Inspection
This form of inspection is widely used, but produces
the poorest results of all the methods. It is now
becoming widely accepted that even surface
cracking often cannot be seen by the naked eye.
2.2 Ultrasonic Inspection
Ultrasonic Inspections are common place in the rail
industry. It’s a relatively well understood technique
and was thought to be the best solution to crack
detection.
However, Ultrasonics can only inspect the core
of materials; that is, the method cannot check for
surface and near-surface cracking where many of
the faults are located. This is where eddy currents
come in.
2.3 Eddy Current InspectionEddy Currents are most effectively used to check for
cracking located at the surface of metals such as
rails. Figure 1 shows the different inspection areas
covered by eddy currents and ultrasonic.
It is important to emphasise at this stage that
ultrasonics and eddy currents are complementary
inspection methods and should not be used
exclusively of one another (fig. 1)
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Eddy Current Ultrasonic
Good at detecting
surface defects
Poor at detecting
surface defects
Near sub-surface
defects reasonable to
detect
Near sub-surface
defects difficult to
detect
Deep sub-surface defectdetection is impossible
Good sub-surfacedefect detection
Probes are less sensitive
to flaw operation
Signal is strongly
influenced by flaw
orientation
No couplant needed,
stable results
Couplant is needed
between probe and
material - variable
results
Probe can be made wideand profiled to cover
wear face
Defect must be onprobe cnetre line
Faster inspection speeds Slow inspection speeds
2.4 Magnetic Particle Inspection
MPI is also used in the rail industry but there are a
number of problems inherent with the technique.
• The surface of the rail or component must first be
cleaned of all coatings, rust and so on.
• To get a sensitive reading, contrast paint must firstbe applied to the rail, followed by the magnetic
particle coating.
• The same inspection must then be carried out
in two different directions at a very slow overall
speed.
• On top of this, the end results will be less sensitive
than those achieved with eddy currents.
Figure 1 - The different inspection areas covered by ET & UT
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Type of Cracking and Where it Occurs
A number of main areas have been identified where
cracking occurs):
• Rail Heads
• Switch Blades
• Bolt Holes
• Foot of the Rail
• Thermite Welds
Inspection of Rail Heads
Contact Stresses
Cracking can be found in the head of all types of
track, but is predominantly found on highly canted
curves where stresses develop due to the extra
pressure and wear of the wheel on the rail (see Figure
3).
Water & Lubricants
Water from rain, snow or dew can become trapped
in defects in the rail along with oil and diesel. When
a wheel runs over a track with entrapped fluid in a
crack, a very high localised press at the crack tip will
cause the crack to grow (see Figure 4).
Figure 4 - Trapped fluids causing cracking to worsen
*
Figure 3 - Contact stresses on tight curved track* Area of high stress prone to developing cracking on and
near the surface of the rail head
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As the wheel approaches the crack the mouth opens
up to draw water in. Then, as the wheel passes over
the crack it closes up the entrance of the crack
mouth, trapping water inside so that the crack tip
stays open allowing further growth.
Tongue Lipping
Tongue lipping develops because surface-breaking
cracks are already present on the rail. Stresses
caused by trains passing over the rail cause the
crack to develop into a tongue which will continue to
grow (see Figure 5).
Ultrasonic inspections cannot reliably detect the
cracks that cause tongue lipping due to their shape,
size and angle. However, eddy currents can. This
means that the cracking that causes tongue lipping
can be identified early enough for preventative
action to be taken.
Companies such as Railtrack in the UK carry out
grinding on all their tracks to try and pre-empt the
problem of tongue lipping. However, this raises a
number of important questions all of which eddy
current inspection can answer:
i. When should the grinding take place?
Regular eddy current inspections will identify
when grinding will need to take place. Without
inspecting the track first , expensive and time
consuming grinding could be carried out for no
reason.
ii. How often should it take place?
Currently many tracks are ground according to
a schedule. However, this doesn’t take into
consideration factors that may cause more
or less cracking to develop than is usual. E.g.
environmental conditions, increased traffic,
abnormal side loading etc.
Figure 5 - Tongue lipping developing from existing cracks
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iii. Has it solved the problem of cracking?
Without the use of eddy currents it cannot be
determine whether grinding of the rails has even
solved the problem. That is, it cannot be guaranteed
all the cracking has been successfully remove. A
quick post-grinding test will confirm success or
highlight where extra works needs to be carried out.
Squats
Squats and tongue lipping have a number of
factors in common. Both start as surface-breaking
cracks and are bought about by similar causes. The
difference is that squats usually develop at a point
where high contact stresses occur as a result of a
local irregularity in the rail head e.g. at a worn weld.
Advantages of eddy current inspection:
• Faster than visual inspection
• Can identify cracking at a much earlier stage than
ultrasonic testing allowing preventative measure
to be carried out
Wheel Burn
Wheel burns are the result of frictional heating
produced by a spinning wheelset. The effect of veryrapid heating produced by the spinning wheel and
subsequent rapid cooling is to change the structure
of the rail head top layer into ‘martensite’. The
presence of the martensite layer makes the rail un-
testable ultrasonically. This layer is also very brittle
with the result that it tends to spall off very easily.
Additionally, the railhead surface irregularity will
significantly increase dynamic impact forces and the
likelihood of rail breakage will be raised.
Although these areas cannot be tested ultrasonically,
eddy current inspections can be applied. The screen
shot (figure 6 below) shows a Locator 2s instrument
with a WideScan probe clearly picking up cracking
within an area of wheel burn.
Inspection of Switch Blades
Switch blades are subject to a tremendous amount
of stress due to the relatively thin section of metal
carrying the weight of transport usually supported
by much thicker track rail.
Cracking is usually found along the top of the blade,
and along the sides. As with thermite welding, aWideScan probe can be used for the head of the rail,
while a WeldScan probe is suitable for the sides. The
WideScan inspection trolley has a spring mechanism
that lets it automatically adjust to the increasing
blade width (Fig. 7).
WideScan Probe – the eddy current solution
As the stresses discussed so far are occurring on
and near the surface of the material, it is virtuallyimpossible to detect cracking with ultrasonics and
relatively straightforward with eddy current. This
capability has been enhanced with the development
of the patented WideScan probe.
The probe is contoured to the surface of the rail and
runs along the surface transmitting results back to an
eddy current instrument via a probe and cable. The
instruments are able to store the information which
can then be downloaded onto a computer for future
analysis and records (figures 8&9).
Figure 6 - Signal showing the detection of a crack in an area of wheel burn
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Figure 7 - Inspection of switch blades using WideScan and WeldScan probes
Figure 8 & 9 - WeldScan probe and Phasec 2s
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The WideScan probe is unique due to the large
surface area it covers. This means that it can detect
cracking over the entire rail head in just one sweep. It
doesn’t matter where on the head the cracking is as
long as it’s surface breaking.
Figure 10 - Phasec 2s and WideScan probe being used by hand to check a section of rail. The equipment can also be attached to an inspectiontrolly (see figure 11) or a vehicle that runs along the track
Figure 11 - Trolley-mounted WideScan probe
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Inspection of Welded Areas
General Welds
Welds are ground after welding which can lead to a
slightly different profile around the weld than from
that of the rest of the rail. The resulting change in rail
profile has been found to have a significant effect on
the contact stresses between rail and wheel, resulting
in rolling contact fatigue. Subsequent cracking has
been found at the edges of the weld or in the body of
the weld itself.
Thermite Welds
Thermic welding is used on sections of continuously
welded rail (CWR) where two rails are welded
together by means of an exothermic reaction. This
method introduces a weak point in to the structure
of the rail. As a rule they are very rough as the ‘flash’
(surplus weld) has not been removed and so could
damage any fragile inspection probe. However,
‘dressing’ (smoothing down) the weld can be time
consuming and expensive, but may also weaken the
weld itself.
Hocking’s WeldScan range of probes has been
designed specifically to test for surface breaking
cracks in welds.
What makes the probe so special is its ability to
test even very rough surfaces covered with rust or
coatings such as paint and oil.
Inspection of Bolt Holes
Bolt holes are positioned regularly along the length
of the rail and are subject to cracking due to the
stresses placed upon them.
Figure 13 - Eddy Current inspection of bolt holes
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Rather than removing each bolt to inspect the hole
underneath, an eddy current WeldScan or Pencil
probe can be used to inspect the area around the
bolt to determine whether any cracks are radiating
from the area.
It’s important to note that no surface preparation is
needed for this eddy current inspection, unlike most
other NDT methods, e.g. MPI.
If cracking is detected, the bolt can be removed and a
special bolt-hole probe can be used to check the hole
itself for confirmation of cracking and to determine
the size and position of the crack (see figure 13).
Inspecting the Rail Foot
Fatigue cracking due to the stress of trains travelling
along the track often occurs around the foot of the
rail.
A special WideScan probe can be contoured so that it
exactly fits the foot and checks for surface-breaking,
fatigue cracking. The probe cannot test the areas
around the clips or springs that attach the rail to the
track, but a WeldScan can be used around these
areas.
The arrows in green (figure 14) show the areas of
the foot where the WideScan probe can inspect. The
arrows in white show where WeldScan must be used.
Figure 14 - Inspection of the foot of the rail
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Summary
Eddy current inspections form a vital part of checking
rails for the cracks and faults that can lead to
serious accidents. Ultrasonic inspections alone do
not cover all areas the rail as the technique cannot
‘see’ surface and near-surface defects. As many of
the cracks appearing in rails are fatigue induced and
thus surface-breaking, it is important to employ eddy
current inspection methods in order to detect them.
Throughout this document, solutions have been
suggested for a number of applications. Figure 15
shows a summary of which eddy current probes we
believe are the most suited to those applications.
Figure 15 - Eddy Current probes for rail inspections
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Thread Inspection
Threads are commonly used to connect mechanical
items together and forms a likey location for fatigue
cracks.
Eddy Current Probes
In order to inspect the root of the thread, specially
shaped tipped probes must be used. Both male and
female threads may be inspected.
Pencil-type probes are available for hand-held
manual inspection, but for more rapid and
repeatable inspections, saddle and plug-type
probes must be used, suitable to the thread form.
Semi-automated bolt hole inspection is also
available in the form of the Inconel Bolt Hole Tester.
Plug probe Saddle Probe
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Eddy Current Tube Inspection
Heat exchanger tubing is subject to a number
of problems, the nature of which is usually a
characteristic of the material and application. Typical
problems found include the following:
Corrosion is the most common problem, while
tubing materials are generally chosen to resist
attack from the fluids passing around them, those
fluids are seldom pure. In power station condensers
using sea water as a coolant the most common
problem is hydrogen sulphide produced by bacteria
metabolising sewage, this attacks most copper
alloys. In petrochemical plants impurities such
as hydrogen chloride or ammonia can also give
problems.
Stress Corrosion results when tubes containing
residual stresses are exposed to a corrosive
environment. The grains of the metal tend to
separate when weakened by corrosion, exposing
fresh sites for attack. This can lead to rapid cracking
of the material, usually in a circumferential direction.
Corrosion erosion or impingement attack results fromthe combination of corrosive agents with mechanical
attack from suspended sand, foreign bodies, or from
turbulent flow of the cooling liquid. This prevents the
formation of a protective film on the surface of the
tubing, greatly increasing the corrosion rate in the
exposed areas.
Mechanical damage may come from a variety
of sources. Foreign bodies in the coolant may
cause damage. Poorly designed condensers have
inadequate baffling of steam, leading to erosion of
tubing in the steam inlet areas. Improper operation
of air conditioning systems may allow water to
freeze in evaporators, resulting in “freeze bulging” or
cracking. Many types of heat exchanger are subject
to vibration, resulting in rapid damage to tubes
loosened by corrosion or improper assembly.
Mechanical damage due to vibration is quite
common where Copper Nickel Alloy tubes have
been replaced with thinner, less rigid titanium tubes,
for which the support is marginally adequate. Heat
exchangers designed for such tubing generally have
support plates closer together. In extreme cases
the vibration may be so severe that adjacent tubes
collide, causing wear or cracking.
Periodic eddy current testing of a heat exchanger
assembly allows tubes with such problems to be
identified before they lead to failure. With knowledge
of the problems experienced in the application it
is possible to determine which tubes are likely todeteriorate unacceptably before the next overhaul.
These may then be plugged or replaced; resulting in
a much higher level of confidence in the reliability of
the heat exchanger. In addition to the detection of
such defects, Eddy Current testing can also be used
to monitor other conditions, such as the build up of
external sludge and to verify the degree of expansion
at tube sheets during manufacture.
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Eddy current inspection of welds for cracks &
corrosion
The Importance of Weld Inspection
The quality of welds is becoming increasingly
important as customer expectations rise. Products
and components are expected to be of a high quality
and not to fail unexpectedly.
Such failures have large financial and social
consequences that can often be avoided with the
proper inspection techniques.
Inspecting welds can also reduce costs by detecting
defects in the early stages of manufacture, reducing
the cost of customer returns and extending the life
of components by detecting and correcting any
defects.
Eddy Current Non Destructive Testing is a reliable,
quick and inexpensive way to carry out preventative
maintenance and ensure safety. Hocking’s range of
eddy current equipment has a world-wide reputation
for its reliability and accuracy, while their service,
support and training ensure that you make the mostof your equipment.
Eddy Current Testing on Welds
In welding inspection there is a need to detect
surface breaking defects. For magnetic material e.g.
carbon steel, generally magnetic particle inspection
is used. However, eddy current inspection offers a
number of advantages:
• No consumables used - e.g. ink & contrast paint
• Ability to test areas with poor access
• No surface preparation required - e.g. paint
doesn’t need to be removed, saving time in
preparation for the inspection and in any
recoating of surfaces
• Improved sensitivity - ability to detect smaller
defects using specially developed Hocking
WeldScan probes.
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WeldScan Probes
Hocking NDT have developed the WeldScan range
of probes specifically to check welds for cracks and
corrosion. Examples of areas where it is currently
used include:
• Offshore platforms
• Buildings
• Bridges
• Amusement park rides
• Ships, boats, submarines etc.
• Cranes
• Traffic signals
• Aircrafts
WeldScan probes have been designed to be
extremely hard-wearing so that they can handle the
rough weld surface while still picking up any faults in
the weld.
Advantages of WeldScan probes include:
• Faster then MPI (Magnetic Particle Inspection)
• Portability of equipment - light, handheld and
easily transported• Accepted method of use - see British and
European Standard 1711:2000
• May be used by rope access technicians
• Approved for use by Lloyds Register, DNV and
Bureau Vertias - certifying authorities for ships
and offshore structures.
• Waterproof WeldScan probe range is available for
sub-sea weld inspection
See more information on the WeldScan range of
probes and download a datasheet. You can also
request weld inspection application notes.
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Wheel Inspection
There are four main solutions for testing wheels
depending on the volume, accuracy required, profile
of wheel and so on.
Solution 1 - Pencil Probe
Standard surface inspection with a pencil probe &
bolt hole inspection with a bolt hole probe.
Solution 2 - Bead Seat Probe
Bead seat probes are made to fit the bead seat area
of wheel and scan a width of about 50mm which
gives a more rapid inpsection than with a pencil
probe. A bolt hole probe is needed to inspect the bolt
holes.
Solution 3 - WheelScan LT
WheelScan Lt provides a semi-automatic solution for
the inspection of small, low volume of wheels.
More information on WheelScan LT
Solution 4 - WheelScan 5
Automatic, fast and highly accurate wheel inspection
solution. The best solution where accuracy and,
therefore safety, cannot be comprimised. TheWheelScan range is commonly used in the airline
industry.
The table below shows the advantages and
disadvantages of each of these options.
Advantages Disadvantages
Solution 1 Low cost.
Suits all types of
wheels.
Very low volume
inspection (about
2 wheels per hr).
Poor surface
coverage due to
human factor.
Solution 2 Low cost.
Slightly faster.
Low volume
inspection.
Reduced
sensitivity to
flaws.
Different probe
needed for each
wheel profile
Solution 3 Portable, faster,
more repeatable.
Suits wide range
of wheel profiles.
Cost.
Doesn’t cover
entire wheel
profile.
Solution 4 Very high volume
inspection (about
10-20 wheels perhour).
Data recording of
wheel test.
Suits all wheel
profiles.
Cost.
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Aircraft Wheel Inspection
Aircraft wheels are subject to high level cyclic fatigue,
particularly during landing. To ensure the safety of
passengers and the aircraft, it is important that the
wheels are maintained to the highest standard.
Eddy Current wheel inspection is widely accepted
throughout the world as a rapid and reliable means
of maintaining the integrity of aircraft wheels.
Automotive Wheel Inspection
Scanning wheels to prevent failure isn’t restricted
to the aerospace industry. Formula 1 racing teams
have recently started utilising the same technology
to ensure there isn’t a failure during the middle of a
race.
7/27/2019 Eddy Current White Paper
http://slidepdf.com/reader/full/eddy-current-white-paper 25/25
Contact Information
For more information about Eddy Current technology or products, please contact us at the following address:
GE Inspection Technologies Ltd
129-135 Camp Road
St Albans
Herts. AL1 5HL
UK
Tel: +44 (0)1727 795500
Fax: +44 (0)1727 795400
Email: [email protected]
Web: www.GEInspectionTechnologies.com