Method of and apparatus for magnetic saturation testing a wire rope for defects

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METHOD OF AND APPARATUS FOR MAGNETIC
SATURATION TESTING A WIRE ROPE FOR
DEFECTS

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BACKGROUND OF THE INVENTION

This invention relates to the electromagnetic testing of wire ropes.

Steel hoist ropes are vital components used in many 10 industrial applications and particularly in deep level mining. The ropes are of prime importance from a safety and production point of view and it is therefore necessary that the ropes are examined regularly to ensure that operational standards are consistently being 15 maintained.

Currently, wire ropes are electromagnetically tested to determine variations in three characteristics, namely the cross-sectional area of the rope, wire contact within the rope, and broken wires. Each of these characteris- 20 tics is hereinafter briefly commented upon.

Cross-sectional area: the strength of a rope is dependent on its cross-sectional steel area which can, for example, be reduced by normal wear and tear, corrosion, and stretch necking of a weak part. 25

Wire contact: a rope is made up of strands which in turn are made up of individual carbon steel wires wound together in a particular pattern or lay. Each wire makes physical contact with adjacent wires in a particular fashion, the total effect of which is characteristic of 30 the rope in question. If a rope is damaged, for example by having been kinked or bent beyond its elastic limit, the contact pattern of the wires changes. It is important to detect such changes in contact pattern for they may precede a failure of the rope. 35

Broken wires: a multiplicity of broken wires over a short length of a rope seriously affects the strength of the rope. Broken wires arise from a variety of causes such as, for example, vibration modes which are set up in the ropes during the cycle of acceleration, steady speed and deceleration. It is clearly important to detect broken wires wherever they may be inside a rope.

Historically the cross-sectional area of a rope and the wire contact characteristic have been measured with a 45 device referred to as an AC rope tester. Such a device is disclosed, for example, in the specification of South African Pat. No. 69/6054. The broken wire characteristic on the other hand has been measured with the aid of an instrument referred to as a DC rope tester typified 50 for example in the specification of South African Pat. No. 69/6269. The applicant is also aware of cross-sectional area measurements having been made with a DC rope tester.

Other literature of which the applicant is aware is the 55 specifications of U.K. Pat. Nos. 1231641, 1476773, 1504404, 1531825, and 1565508 and French Pat. No. 2083900.

To the applicant's knowledge the aforementioned rope characteristics have hitherto been measurable only 60 by employing two distinct instruments.

Modern requirements arising from the demands for increased productivity, simplicity, cost effectiveness, and the availability and expertise of skilled non-destructive testing personnel have demonstrated the need for a 65 single instrument which is capable of simultaneously measuring each of the three aforementioned characteristics.

SUMMARY OF THE INVENTION

The invention provides a method of testing a wire rope which includes the steps of establishing at least two magnetic fields, using the magnetic fields to magnetise adjacent sections of the rope in opposing directions respectively, establishing relative movement between the magnetic field and the rope, and monitoring the resulting magnetic flux in the rope for variations which are due to irregularities in the rope.

Preferably the magnetic fields are DC fields.

Preferably the rope is caused to move through the DC fields.

In one form the method of the invention includes the steps of causing the rope to move along a path, the longitudinal axis of the rope along the path being substantially aligned with the path, establishing a first magnetic field over a first portion of the path, establishing a second magnetic field over a second portion of the path which is adjacent the first portion, the first and second fields being directed in opposing senses along the path whereby the rope is magnetised in opposing axial directions as it moves along the path, and monitoring a resulting magnetic flux in the rope for variations which are due to irregularities in the rope.

In accordance with the invention area variations are detected by monitoring variations in the magnetic flux which are a function of the travel of the rope past a predetermined reference point i.e. variations of flux in the rope which occur as the rope traverses the magnetic fields.

Preferably the flux variations are monitored when the magnetic flux in the rope is at or close to a saturation flux density. In addition the flux value is substantially constant, at saturation.

According to a different aspect of the invention broken wires are detected by monitoring magnetic flux variations in the rope with the flux density in the rope at or close to a saturation flux density.

Variations in flux density arising on the one hand from area variations and on the other hand from broken wires in the rope are distinguishable by processes and techniques which are known per se. As area variations are directly related to flux variations they are easily detected. Broken wires can be detected by means of two probes which are spaced apart a predetermined distance in the axial direction of the rope. Broken wires in the rope produce predictable signals which are detected by the probes, and which are processed in a known way to provide an indication of the broken wires.

In accordance with a different aspect of the invention, changes in the wire contact pattern in the rope are detected by monitoring variations in the total eddy current flux which is induced in the rope during its passage through the opposing magnetic fields. More particularly an eddy current flux which is characteristic of the wire rope with a given contact pattern amongst the wires in the rope is monitored for variations which arise as a result of contact between adjacent wires in the rope being broken or otherwise being disturbed. Thus the eddy current flux measurement is then not affected by magnetic flux-dependent factors.

The flux density at the monitoring point may be nominally zero. Further, the rate of change of flux density, relatively to the rope length, may be constant. The eddy current flux measurement is then not affected by other magnetic flux-dependent factors and, consequently, if 10tion (4) is the first term and, assuming the effects of the second and third terms are zero or negligible, it follows that B must be constant if flux variations are to be related directly to area variations. In other words:

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the contact pattern changes the eddy current flux hanges and this is detectable.

The invention further extends to apparatus for testing a wire rope which includes means for establishing first and second magnetic fields, the rope being movable 5 through the magnetic fields whereby adjacent sections of the rope are magnetised in opposing directions, and means for monitoring the resulting magnetic flux in the rope for variations which are due to irregularities in the rope.

The magnetic fields which are established are preferably DC fields. The fields are located adjacent one another and are directed in opposite senses. The fields may make use of a common pole piece.

The fields may be in axial alignment. 15

The magnetic flux in the rope may be monitored by means of any suitable transducers such as Hall effect instruments or similar devices, search coils, or similar mechanisms which are responsive to flux variations. The manner in which the signals, which are generated by these devices, are processed to provide the necessary information is known per se and therefore is not elaborated on in this specification.

The invention also extends to a magnetising head for 2J use in testing a wire rope which includes a first permanent magnet stack which in use establishes a first magnetic field, a second permanent magnet stack which in use establishes a second magnetic field, the first and second fields being adjacent one another and being 3Q directed in opposite senses, and a pathway being formed so that the rope can travel through the first and second fields whereby the rope is magnetised by the fields in opposing directions.

The first permanent magnet stack may include a first 35 pole piece and a second pole piece, and the second permanent magnet stack may share the second pole piece and include a third pole piece. The first and third pole pieces are of the same polarity.

Each magnet stack may consist of one or more arrays 40 of permanent magnets which are radially spaced from the pathway along which the rope travels.

The fields which are produced in the apparatus and in the magnetising head may be of equal amplitude or strength although of opposite senses and may have the 45 same length in the direction of rope travel.

At least one field should have a value which induces a magnetic flux which is in saturation for each rope size with which the magnetising head will be used.

In a variation of the invention the fields may vary as 50 to the maximum magnetic strength of each field and as to the distance, in the direction of rope travel, over which the field extends. In this instance the rope is preferably subjected to the influence of the higher magnetic field first and then to the opposing influence of the 55 lower magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings 60 wherein:

FIG. 1 illustrates a portion of a wire in which eddy currents are induced,

FIG. 2 is a cross-sectional view of a strand of a rope which includes nineteen wires and illustrating eddy 65 currents which are induced in the strand,

FIG. 3 is a side view in axial cross-section of a magnetic test head according to the invention,

FIG. 4 is an idealised representation of magnetic fields which are induced in a rope passing through the test head of FIG. 3,

FIG. 5 is a cross-sectional view of the head of FIG. 3 on the line 5—5,

FIG. 6 illustrates from the side a practical composite magnetic test head according to the invention, partly sectioned,

FIGS. 7(a) and 1(b) respectively graphically illustrate the variation of the gradient of flux density, and of flux density, with rope length, as a rope is passed through the test head of FIG. 6, which for the sake of convenience is shown schematically adjacent the curves along an abscissa of the same scale,

FIG. 8 shows the B-H curve and hysteresis loop of a rope under test in the head of FIG. 3, and

FIGS. 9(a) and 9(b), respectively, are curves similar to the curves shown in FIGS. 7(a) and 1(b) obtained with a magnetic test head according to a variation of the invention, which is shown for reference purposes adjacent the curves.

DESCRIPTION OF PREFERRED
EMBODIMENTS

The principles of the invention are described hereinafter firstly by examining the theoretical basis of the invention and thereafter by considering the practical implementation of these principles.

THEORETICAL CONSIDERATIONS

As indicated in the preamble to this specification it is an object of the present invention to provide a single device which is capable of simultaneously and independently detennining (a) variations in the cross-sectional area of a steel rope, (b) the presence of broken wires in the rope, and (c) irregularities in the contact pattern established by the wires in a rope.

CROSS-SECTIONAL AREA

If a wire rope is magnetised in its axial direction then the magnetic flux <(> established in the rope is given by the expression:

required to detect the passage of the broken wire dipole at the speed at which the rope passes through the test head.

Under these conditions

dB dl

= 0

(5)

Md=B a 1

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The second and third terms in equation (4) are resolved as follows. (dH/dl) is a parameter of the magnetising means i.e. the test head and if the magnetising force is constant in the test area i.e. independent of rope length then dH/dl=0.

/j, is not constant but is a function of B. However as the magnetising force H increases p tends asymptotically to a constant value which is characteristic of the material of the rope. This limiting value is not achieved but the higher H is the more constant is Bs and consequently the more constant is the permeability u-. It follows therefore that the magnetising means should induce the highest possible magnetic flux density in the rope and that ideally at least one point of the rope should be fully saturated magnetically as it passes through the test head and, moreover, the flux density should be as constant as possible over that portion of the length of rope which is required to make a measurement of flux variations with rope length (refer to equation (6)).

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If the rope is magnetised in its axial direction and there is a broken or cracked wire in the rope then the magnetic effect of the broken wire manifests itself as a dipole with a magnetic moment given by the expression:

(8)

Where

a is the cross-sectional area of the broken wire or the 55 crack,

1 is the axial separation of the broken ends or the axial length of the crack, and B is the flux density.

For the magnetic moment M<jto be proportional only 60 to the product "al" i.e. to the volume of the irregularity, B=\iH must be constant.

Thus, for detecting broken wires, the magnetising means should induce a flux density in the rope, as it passes through the test head, which is as high into satu- 65 ration flux density Bs as possible, for this causes u. asymptotically to approach its limiting value, and which is as constant as possible over the length of the rope

Irregularities in the wire rope arising from variations in the wire contact pattern manifest themselves by causing variations in the flux which is established in the rope by eddy currents. These eddy currents fall into at least two principal categories, namely the eddy currents which are established in the individual wires, referred to as area eddy currents and the eddy currents which circulate from wire to wire i.e. contact eddy currents. These effects are considered on an idealised basis only but it is to be understood that the following analysis is generally applicable to ropes which deviate from the ideal and that in these ropes eddy current flux variations are also detectable.

AREA EDDY CURRENTS

FIG. 1 illustrates a portion 1 of a length of circular steel wire 10 and of diameter d.

If an axial magnetic field B is established in the wire and this field is variable with time then circular eddy currents are induced in the wire, the amplitudes of the currents being continuous functions of their respective radii. These effects are well known. The cross-sectional view of FIG. 1 depicts an incremental annulus of radius r and of thickness dr in which is induced an eddy current i. It can be shown that the total circular eddy current I in the wire is given by the expression:

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