The magnetic NDT of wire ropes is regularly used in a number of countries for the inspection of hoisting ropes in deep mines and for the inspection of cable cars, lift cables. Recently, the method used is based on the magnetisation of the rope with permanent magnets and the detection of changes in the magnetic field in the magnetic field. Discontinuities in the rope, such as broken wire or corrosion pit, create radial magnetic flux leakage and allow the sensor to detect the rope as it passes through the sensing head. The other sensor measures the total axial magnetic flux in the rope. The missing wire provides information about steel loss due to continuous corrosion or wear.
There is a possibility that a rope specialist can estimate the rope status using the magnetic method. In connection with the visual inspection, this method can be applied to determine the time at which the rope should be discarded. Various equipment is available for different application ranges.
Permanent magnet method
Although the magnetic NDT of wire ropes has been used regularly in various countries for 30 or more years, the NDT method is still not widely known. This method is well known and recognized in applications such as the inspection of hoisting ropes in deep mines and the inspection of ropeways.
Recently, equipment used for non-destructive testing of steel wire ropes generally uses the "permanent magnet method" method. The method is based on the magnetisation of the rope with permanent magnets and the detection of rope abnormalities indirectly with magnetic sensors. Because of the Direct Current excitation coils used previously, this method is in a so-called "DC" magnetic method as opposed to the previously used Alternating Current windings (the former AC method).
In Turkey (METU university) since the first introduction is over 20 years, almost all manufacturers, a rope length of the permanent magnet passes through the head and neck has been supplying sensing heads the longitudinal magnetization. A constant magnetic flux magnetizing the rope must be strong enough to create a state near the magnetic saturation of the rope length.
LF-type discontinuity, such as broken wire or corrosion pit on the rope, generates radial magnetic flux leakage and allows the LF sensor to detect the rope as it passes through the sensor. The LF sensor is positioned coaxially around the rope centrally between the magnetic poles of the magnetizing circuit. The signal is quite quantitative and qualitative. However, this signal provides more or less information about the presence and also the magnitude of the local error.
The LMA sensor measures the total axial magnetic flux in the rope as absolute magnitudes or differences in the constant magnitude of the magnetic field. This signal is proportional to the steel volume or change in the steel cross-sectional area. The missing wire provides information about steel loss due to continuous corrosion or wear. LMA sensors are located in various locations, almost in or near the magnetism circuit. When the absolute value is displayed, it is a place called MATD, that is, the "total change of metallic field".
If an NDT device is designed to detect either LF or LMA, but not both, it is referred to as a “single function” device. The "Binary function" tool detects both individually.
Various types of sensors have been used by some instrument manufacturers in the world. The sensors provide different signals depending on the design of the magnetic concentrators and the type, number and location of the sensing devices. Inductive coils and / or Hall generators are widely used as detection devices. In general, however, due to the application concept, the sensors can be divided into two types:
- LF sensors, ie Local Fault or Local Flow sensors;
- LMA sensors, ie Metallic cross-sectional area loss.
Limitations of magnetic method
This method is limited to the testing of ferromagnetic steel ropes.
Although the usefulness of magnetic NDTs of wire ropes is ignored, this method should be supported by other exams, especially by visual method.
The rope must be tested periodically from the date of installation. Magnetic test gives basic information about rope condition. The instrument shows the wrong places in the rope length. There is a possibility that a rope specialist can estimate the rope status using the magnetic method. However, it should use other methods to assess the condition of a rope to determine whether the rope should be discarded.
The user should note that the device indicates the loss of the rope area (LMA). Usually the indications should be corrected by calculations with reference to the rope construction type and observed deterioration.
LF signals generated by internal broken wires and internal wear are sometimes distorted by signals generated by external non-uniform wear. The internally broken wires accumulated close together produce a complex signal that depends on the distribution and number of amplitudes. Sometimes, these relationships are very complex and it is difficult to make precise definitions.
If a rope is periodically tested using a magnetic method from the date of installation, the inspector may observe an increasing number of broken wires and other defects in succession. In this way, the results of non-destructive testing are the easiest to interpret.
You can perform NDT ultrasonic x-ray examination of your tow ropes and steel cable ropes with experienced engineers of TÜRCERT.