for In-situ Testing
of Steel Wire Ropes
Instrumentation recently used for non destructive testing of steel wire
ropes generally uses the same method, magnetisation of the rope with permanent
magnets ("DC" magnetic method). Various types of sensors have been applied
by some manufacturers of instruments across the world. Sensors provide
different signals depending on the design of the magnetic concentrators
and type and number of sensing devices. Inductive coils and/or Hall generators
are popularly used as sensing devices. Two categories of instrumentation
have been supplied:
The first of the above categories is less expensive than the second and
the instruments are simple tools, mostly hand-held. This kind of instruments
makes visual examination of the rope more convenient and reliable. Sometimes,
like Meraster MD-20 Tester, they are equipped with
a recording signal output which allows their application as sensing heads
for detailed inspection of the rope.
simplified auxiliary testers for detecting and indicating localised flaws
or loss of metallic cross-sectional area with a light flash or an acoustic
high-end instrumentation with strip chart and/or computer recording which
is capable of estimating loss of metallic cross-sectional area and localised
losses, and features the real aid to determine true deterioration of the
MD-20 wire rope tester
The second category of instrumentation is intended to perform detailed
tests. In conjunction with visual examination they may be applied to determine
the moment when the rope should be discarded. Generally this instrumentation
consists of two units: a sensing head; and a signal
processing/recording instrument. Sometimes the signal processing part
and a standard chart recorder are supplied as separate units. Now some
suppliers offer portable computers and software for use instead of chart
recording. Detectability of rope defects depends mainly on the sensing
head employed but readability of its signals and ease of operation depend
mainly on recording/processing instrument.
The sensing head brings the running sector of wire rope to the condition
close to magnetic saturation and provides signals from its sensors. All
reputable manufacturers employ a minimum two-channels sensing system: one
to detect localised losses ("local faults" LF), e.g. from broken wires
and pitting corrosion; and the other one to detect the distributed loss
of metallic cross-sectional area ("loss of metallic area" LMA or "total
change of metallic area" TCMA) due to corrosion, wear and abrasion. Some
types of Polish-made heads are equipped additionally with a third channel
to estimate the depth inside the rope of a localised loss position.
Detecting capabilities of sensing heads vary between manufacturers and
rope constructions. They depend on strong magnetisation capability, shape
of magnetic concentrators in the sensor and operating principle of the
sensor. In order to measure running rope length (and speed of relative
movement), some manufacturers supply heads equipped with special transducer
for indicating rope/head movement as an electric signal. Some manufacturers
use it to synchronise the strip chart feed with the rope/head travel. This
signal is also useful to compensate the speed influence on the inductive
Processing electronics depends on the sensor types and equipment features.
For example the Hall generator sensor requires supply control and compensation
of DC component of its signal, and the inductive sensor signal needs rope
speed compensation to achieve good performance of the instrumentation.
Some instruments have additional circuits that make them more convenient
in use, e.g. rope length/speed measuring circuits.
A strip chart recorder seems to be indispensable in each fully functional
wire rope NDT instrument, as a third part of an instrumentation set, or
integrated with the electronic processing part of the equipment. Mostly,
manufacturers of these NDT instruments use standard, stand-alone or OEM
unit recorders. Generally, it is a two-channel analogue pen recorder or
digital thermal array printer. A recorder appropriate for this sort of
application must be equipped with drive control to achieve good correlation
between the recording and the wire rope at any non-controlled rope speed
in the test speed range. The recording should be performed at real time
mode, instantly. Meraster suplies extremely task dedicated recording instrument,
the MD120 Defectograph.
Meraster's products for non destructive testing
Meraster as one of a few world manufacturers of instruments for rope
deterioration estimation in situ, has supplied these instruments world-wide
since 1979. Instruments operation principle is based on permanent magnet
method, mentioned above. This is an original Polish method developed by
scientists of the University of Mining and Metallurgy (AGH) in Cracow (Kraków).
At present, the Meraster NDT standard product line includes a full range
of sensing heads for ropes in the range 8-90 mm diameter,
specialised recorder-defectograph, and a hand-held
tester-detector and accessories. In addition, specialised computer
software is supplied as an extension of the defectograph capabilities.
Moreover, some non-standard and custom-design products are available, for
example: submersible heads, heads for other sizes of ropes, telemetry adapters,
rope production monitoring systems.
Meraster MD120 Wire Rope Defectograph
Based on many years of experience, and as a result of recent technological
development a new task dedicated recording instrument has been designed
in Meraster. Up-to-date microprocessor techniques and recording methods
were applied to achieve a new quality of NDT procedures. The first model
of this instrument was introduced in 1994. Since this date MD120
has been supplied to rope experts in Poland and around the world and it
has been recognised as an valuable state-of-art instrument. For example
the world famous German testing organisation TÜV Bayern-Sachsen has
purchased our equipment based on MD120 unit.
MD120 chart recorder
Apart from the standard features of reliable instrumentation, mentioned
above, the unique features of the Meraster MD120 Defectograph
capability of determining the rope defect depth location inside the rope;
running integral method for easy read out of high density of defects;
zoom replay of recording;
solid state diagram memory PCMCIA (computer compatibility);
automatic printing of annotations;
automatic set up after entering the specific rope code ("settings + rope
The Defectograph equipped with a Meraster's GP series
head with a three-channel sensor, records test signals in four measurement
two channels of inductive sensors (inner and outer coils), for detecting
"localised losses"; relation between recorded values in both these channels
indicates depth of the defect position inside the rope;
one channel of Hall-effect sensor signals, for detecting of "distributed
loss of metallic cross-sectional area";
one channel of integral of the main inductive sensor ( inner coil ) signal,
for detecting the totalled "localised losses" along a rope sector.
Wire rope test chart
This last channel needs some explanation to understand its role. For
many years similar integral channel has been applied in some Polish instruments.
There are two advantages of this recording, particularly for mining hoist
ropes, where broken wires are concentrated:
more readable indication of a real damage resulting from broken wires,
located close to each other, than in "localised losses" channel;
set-up of integration range in instrument according to rope discard criteria
"number of broken wires in any x diameter length" allows indicating total
losses in appropriate rope sectors lengths.
Our old instruments operated with a less convenient "cyclic integration"
on a set rope sector. The new instrument operates continuously, in the
"running integration" mode, where integration is being performed on a length
in the next rope sector. The instrument is recording current values of
the integral ( total of losses) of previous rope sector, last "x" metres
length. If the length of integration range is set appropriate to discard
criteria, it gives direct readable indications of rope sectors in which
the number of broken wires probably exceeds value of the discard criteria.
During the rope NDT procedure performed in-situ, audio-alarm and "Zoom
Replay" capabilities are useful. The Defectograph generates the audio-signal
when the pulse value in the "localised losses" channel has exceeded preset
alarm level. When a significant rope defect has been observed during recording,
the user can stop the rope (or head) movement and recording of readings,
and then may replay a previous recording in the zoom mode. Defect position
may be read out precisely and found in the rope. Visual examination of
the rope sector in question should then be made, additionally.
Solid state memory is an option. This is a credit card size S-RAM IC
Memory Card conforming to the up-to-date standard, Personal Computer Memory
Card International Association (PCMCIA) standard.
cards are compatible with many notebook computers. Also PCMCIA
slots can be added to most of personal computer systems. In certain rope
NDT conditions (subject to magnetism and dirt) and to improve operating
conditions, this method of data transfer has many advantages. For example:
Memory Cards feature: ruggedness, magnetic field proof principle of operation
and fast interfacing.
With this option, the Defectograph may store additionally an all-rope
test record in the memory card. Capacity of the recording depends on the
card version, e.g. 1 MByte card can storage test of a rope of 600 m in
length and 4 MB - 2400 m. Then data may be sent easily and quickly to a
computer via the PCMCIA slot. This way, the user can archive many test
records for further comparative analysis and can employ graphical, scientific
software to help him in his work on rope test results. Also data from Memory
Card may be replayed on a strip chart with an MD120 Defectograph, including
old test records from computer storage memory. S-RAM cards are erasable,
for multiple use.
The recorder prints automatically the number of annotations on strip
chart, e.g. rope length in metres, every 5 cm of a chart, a rope code set
by the operator; recorder settings, direction of movement, date and time.
Before a rope test, the user can enter into the instrument a specific
identification code which will be printed on the chart, and test settings
like channel sensitivities will be stored with this identification code
in non-volatile memory in the instrument. If the same codes are entered
in future, the same settings may be applied automatically.
The recorder may operate in one of two main modes: chart feed synchronous
to rope movement; or chart feed at constant selectable speed. Recording
is done by means of a thermal array line printing on thermal paper . All
of the instrument settings and measured values are displayed on a liquid
crystal display. Any instrument setting may be changed with one only knob-push-button.
The instrument is designed for field service. Built in aluminum covered
case with handle, the MD120 Defectograph is easy to carry. MD120 operates
from a built-in rechargeable battery or various external power sources,
AC or DC. Automatically microprocessor controlled recharging while external
power is connected is provided.
Field service and user-friendly oriented functionality of the MD120
in conjunction with its capability of computer aided post-testing analysis
make this instrument useful as well as every-day tool for rope expert
and as a source of data for scientist. Easy access to the test records
with computer software tools seems to be a real aid to make faster progress
in the development of rope evaluation methods.
To see specification of MD120 Defectograph click here.
[ Kazimierz Zawada 1998 ]
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