Forensics

Inspection of Elevator Wire Rope by Magnetic Flux Induction

Forensic Services Newsletter

The man in the street does not associate floods with high-rise buildings, but the insurance industry is well aware of the potential for water damage. In a typical big city building there would be water tanks every 8 to 10 floors and these can give rise to floods in a variety of ways. Fires will also result in flooding, irrespective of the presence of sprinklers.

When there are floods on any floor, water will inevitably enter elevator shafts and wet the steel ropes associated with the elevator car. In addition to trickling down the ropes, water will typically fill the pit at the bottom of each shaft and portion of ropes will be underwater.

Corrosion on the inside facing surface of a strand of wire rope. Such deterioration cannot be detected by visual inspection.

Following these accidents, the elevator company responsible for maintenance of the elevator sometimes recommend replacement of the wire ropes. It has been difficult for insurers to judge the validity of such expenditure, because of the reasons described below. This technical note describes a testing solution that enables the assessment of wetted ropes in a quantitative manner that promises to lower the costs of claims of this type.

The Inspection Problem

Because of the dire consequences of failure, inspectors have to err on the side of caution when examining elevator ropes. This problem is compounded when conventional visual examination will only indicate the condition of the wires on the outside, whereas deterioration (corrosion) can occur internally. Some ropes have a fibre core which can absorb moisture, with corrosion occurring predominately on the inside.

The Inspection Solution

All the problems indicated can be overcome by the use of the FSM magnetic flux leakage inspection equipment. While the application of this equipment to elevator ropes is relatively new, this equipment has been used for many years in industrial applications, such as ship lifts and mine shafts. The equipment utilises magnetic fields induced into the wire rope to detect broken wires or any loss of metallic cross sectional thickness.

The inspection can be carried out insitu, quickly, without having to remove protective grease. Our inspectors typically do the testing while traveling on top of the elevator car. The results are quantitative so they can be interpreted subjectively. The data is recorded so that it can be reviewed and compared to subsequent inspections at a later date.

Quantitative Measurement

Instead of a subjective assessment of the surface condition only, the FSM equipment will, after calibration on ropes of the same size, provide a quantitative survey of the rope length. Examples of capability are as follows.

  • For breaks on the surface, the detector will find one broken wire in as many as 250 wires.
  • For internal breaks, the detector will find fractures when they amount to 1% or more of the total number of wires.
  • For corrosion at any location, the detector will find 1% or more loss in cross sectional area.
  • The detector is computerized and equipped with data logger inside to record data for up to 8.2km of rope.
  • The equipment is capable of testing at up to 2m/sec of rope.

Practical Considerations

It is advantageous to test the ropes as soon as possible after the wetting (within a few weeks). This enables the condition of the rope prior to the wetting to be determined. It is not possible for a wetting to cause individual wires to fail or for significant corrosion to occur within a few weeks.

If the testing determines the ropes are in a fit condition, they can continue to be used. If it is then argued that corrosion will occur in the future (because of the wetting), then testing can be repeated at an agreed time, say 6 months to one year. It is emphasized that the first test is important as it establishes a baseline for further comparison. Without the first test, if defects are found one year later, it will not be possible to tell if they existed before the wetting or were a result of the wetting. We point out that some corrosion is normal for elevator ropes installed in tropical countries, because of the high humidity.

Regards,





Barry Dillon