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Steel Wire Rope Condition Monitoring by Non-Destructive Investigation and Evaluation while on Installation/Service S.D. Singh, B. Ghara Scientist, Central Mining Research Institute, Barwa Road, Dhanbad (Bihar) India Contact

ABSTRACT

Steel wire ropes are extensively used in mining/ Aerial ropeway industries for haulage, track and hoisting purposes for transportation of men & materials. The importance of periodic rope condition monitoring plays a vital role to achieve maximum mineral exploitation in the hazardous atmosphere as well as passenger cable car ropeway installations. With fast approach in mechanization in mining industries/ passenger cable car ropeway installation, adoption of latest technology in non-destructive investigation and evaluation to enhance safety and optimum safe rope life is essential and Central Mining Research Institute is engaged in this endeavour since last more than three decades.

INTRODUCTION:

Steel winding ropes which are heterogeneous in nature are subjected to severe working conditions and their behaviour vary on different environmental and nature of service they are subjected to rope deterioration such as corrosion, wear, abrasion and broken wire lead to rope failure. It is difficult to assess the rope condition by external visual examination and in any event it is not possible when internal deterioration takes place.

Further, the steel wire ropes specially installed over the multirope friction/koepe winders of mines and Aerial ropeway industries for which Non-Destructive technique is the only means to monitor the health condition of the ropes in the online/insitu where the possibility of drawal of the rope- specimen for conventional (destructive) test is ruled out. More over the technique does not disturb the costly installation/production & working for long time. At the same time the technique monitors / scan the condition of entire rope length in the installations as well as it is very quick and reliable.

For the purpose of non-destructive investigation two types of technique are adopted namely D.C. and A.C. technique. D.C. technique is used to evaluate the broken wires due to localised losses even when they are nearer to the inner axis of the rope and A.C. technique detects losses in cross section/metallic area of steel ropes due to wear, corrosion, abrasion etc., which are of distributed nature enables the possibility of evaluation of remaining breaking strength of the wire rope.

Wire ropes are used for a variety of applications where a secure attachment is essential between different moving part of a system. Applications of wire ropes include haulage ropes carrying men and material to hoist a cage or lift in a shaft. Different types of wire ropes are in use both in terms of size and type of construction with each rope being selected are designed for a specific application. In most cases a rope failure could cause extensive damage of costly installations and in some cases loss of life.

Safety codes and authorities mandate periodic inspection visually of by destructive tests on a sample of three meters drawn from the ropes at prescribed intervals. Especially, in cases of friction winders and passenger cable car ropeway installation where drawl of rope specimen is ruled out, non-destructive investigation is the only means to assess the behaviour conditions of wire ropes. It is not unreasonable to believe visual inspection is inherently unreliable as the sole criterion in these cases.

Hence, non-destructive investigations on wire ropes at systematic interval becomes an important factors for preventive maintenance programme in mining and aerial transportation industries throughout the world. A programme of regularly scheduled Non Destructive Investigations typically at 4 to 6 months from the date of installation of wire ropes is of particular value for safe and extended usage or premature removal in adverse cases. since an accurate and objective record of rope condition is available for each inspection, it is possible to compare rope data at the time of each inspection. A complete documentation of rope's gradual deterioration throughout its entire service life is therefore available. The end of safe service life is usually reached when the rope degradation rapidly accelerates between inspections.

Evaluation of rope behaviour is assessed on the magnitude of rope deterioration due to different factors as environmental, working conditions, usage rate, maintenance schedule etc. Quantitative estimates of safety related wire rope parameters are obtained through Non-Destructive Investigation such as loss of metallic cross sectional area and remaining service life. Rope defects are categorised in two respects:

a) Qualitative inspection for localised faults and
b) Quantitative inspection for loss of metallic cross sectional area.

PRINCIPLE OF OPERATION:

Localised faults in a rope are detected using inductive sensors and distributed flaws in a rope is determined by using Hall Effect sensors. The sensing head bring the running sector of wire rope to a condition close to magnetic saturation and provides signal from its sensors. The inductive sensors detect signals due to localised loss viz. broken wires, pittings localised corrosion and the distributed loss of metallic cross sectional area are sensed by Hall Effect sensors for evaluation of defects caused due to corrosion wear and abrasion.

A steel rope comprises of a considerable number of simultaneously working wires and after breaking one of them the remaining wires take over it's share of loss. Each broken wire due to friction between particular wire transmits the load which falls to its share already at certain distance from the broken spot. Stress values in particular wires of the rope are various. The criteria of wear out of the ropes which are based on the number of cycles of operation time can not be accepted although some countries follows this as a statutory condition for rope replacement. Even at practically the same static and dynamic condition of loading the operation conditions of the rope like length of the rope, sort of lubrication used during production of wire ropes, influence of temperature, water and humidity determines the safe life of wire rope operating one after another in the same or similar installation.

Evaluation of broken wires are made by D.C. system using a strong D.C. magnetic field to magnetically saturate rope specimen. In this system when discontinuities occur such as broken wires a radial field is produced by leakage across the separated ends of the break. This field can be measured as the discontinuity is passed through a coil system.

Assessment of loss in metallic area by A.C. technique utilizing an alternating magnetic field in a coil assembly with circuitry to measure field strength enables the detection of corrosion, wear and abrasion over the rope length. When a wire rope is placed within a coil system the flux density is a function of the steel cross sectional area of the rope. As it is passed through the coil loss of area due to corrosion or wear results in a reduction in field strength which is measured and recorded. the A.C. system used as a dual frequency device with a choice of operating frequencies. In general locked coil ropes are tested at lower frequency. Factors that influence the magnetic field are permeability of steel, volume of steel with the coil system and eddy currents that are produced by the application of A.C. field to the rope under test.

CONCLUSION:

Visual inspection of wire rope for the inservice inspection of wire ropes had deficiencies. It has been concluded that non destructive examination of wire ropes in mining and aerial ropeway industries where ropes play a vital role and the data obtained from non-destructive investigations at periodic interval will assist in developing future ropes safety standards and in determining wire rope extension/premature retirement criteria.

The three figures No. 1, 2 and 3 which represent the good and bad portion of wire ropes due to high present of corrosion, pitting/nicking during its service rendered in the multirope friction /koepe winders of mines and aerial ropeway installations.

Fig 1: Shows good portion of wire rope

Fig 2: shows high percentage of corrosion of wire rope at intermittent distance

Fig 3: shows high percentage of corrosion & pitting over entire portion of wire rope

Achievements

1. Extension of rope life after its stipulated period.
2. Timely discard/Removal of the ropes within its normal (stipulated) period.
3. Suggestion for timely/correct maintenance of the rope in the installation.
4. Maximum safe rope life in the installation and economy as well.
5. A complete report of kilometer lengths of wire rope can be interpreted in a very short time for acceptance/rejection.
6. Thus the technique assess the rope life and saves enormous cost of premature rope replacement and eliminates the major accidents which may be fatal for human life some time during operation.

At present, Central Mining Research Institute is carrying out the condition monitoring on more than sixty to seventy number of wire ropes of various size and construction in the different multirope friction/ koepe winders of coal & metal mines and aerial ropeway installations in India by Non-Destructive investigation and evaluation technique. Imposing non-destructive examination as statutory measure is most essential because conventional (destructive) tests on a specimen rope length of three meter do not speak about the rope integrity on its entire length.

Acknowledgement

Authors are thankful to Director, Central Mining Research Institute, Dhanbad, India for giving permission to present the paper in the 15^th World Conference on Non-Destructive testing, Rome (Italy).

REFERENCES

1. Weishedel, H.R. and Ramsey, R.P. : `Electromagnetic testing, a reliable method for the inspection of wire ropes in service', NDT International Volume 22 Number 3 June, 1989, p. 155-160.
2. Baker, T.G. : `New South African drum Hoisting Plants', CIM Bulletin, December, 1974, pp. 87-96.
3. Poffenroth, Dennis N.: `Flaw Detection in Mine Hoist Transportation system.
4. Rotesco Ltd., Toronto, Ontario, Candad : `DC wire rope testing - Elementary Theory and application'.

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