GearInspec Services

Eddy Current

Eddy current array technology shouldn't be confused with pencil probe or single probe eddy current inspection which is gaining popularity for the inspection of girth gears. An eddy current array has multiple coils within the same probe that are fired in a cooperative sequence that prevents interference with one another. The sequence provides a complete scan of the surface beneath the probes.

To scan a gear tooth flank the probe traverses the tooth width in one motion creating a seamless image of the condition of the tooth flank. The results are easy to interpret and no part of the gear flank is missed, as the array offers a degree of overlap between the coils.

Pencil probe eddy current relies on the operator traversing the probe back and forth on the tooth flank; the probe must intersect a fault to get a signal, so there is a real chance of missing some parts of the flank. The signal from pencil probe is a voltage signal that must be interpreted and the results manually recorded.

Scanning a gear tooth flank is done in a matter of seconds, the data is captured and displayed as in a colour format that makes on-the-spot detection of flaws and indications a simple and reliable process. A 12 m girth gear with 360 teeth is likely to take no more than 12 hrs to completely scan and record condition of all teeth. 100% capture of the data enables a review for inclusion into the inspection report.

100% data capture also makes the system ideal for benchmarking and inspection for quality acceptance of new or repaired components. All scan data will be retained for later examination and verification. Parameters on the output can be calibrated to cater for site's reporting criteria.

Unwanted Downtime - Current Inspection Techniques

In recent times gear inspection has increased in acceptance – mainly driven by the need to mitigate the risk of unplanned downtime and significant loss of production due to broken gear hardware.

Magnetic particle inspection and die penetrant have been the predominant techniques used because they are relatively low tech and robust activities that can be easily carried out in the field.

Problems with these inspection techniques include:

  • Surface that must be inspected must be free of lubricant so that cracks are not obscured by an oil film.
  • Reporting relies observation of the technician
  • Reports are manual and line drawings combined with photographs are the norm.
  • Labour intensive with an inspection of a 12m gear taking anywhere between 24 and 36 hours.
  • To remove lubricant often involves significant expensive and time consuming gear cleaning processes which are carried out after the mill has been taken off line.
  • Waste open gear lubricant washed off with water is expensive to dispose of and new lubricant must be applied at the end of the inspection.
  • Eddy current requires that there is some clearance between the coils and the material being inspected; therefore remnant lubricant poses no problem. Leaving a light covering of lubricant or hydrocarbon cleaning fluid is beneficial for gears in that it prevents scuffing when the mill is inched and provides protection against surface corrosion that sometimes occurs following a water wash gear clean.

Eddy current inspection will typically take around half the time of magnetic particle and liquid die penetrant testing.