A SIMPLE FLAW DETECTOR FOR TESTING ROPES

I. A. BALASHOV

Docent, Candidate of Technical Sciences

Introduction

Physical methods of testing finished products without destroying them have in recent years been gaining, quite rightly, ever wider application in industry. Electromagnetic methods of analysis have developed especially extensively and effectively, both because they make it possible to examine products rapidly and because of the universality of their application. Physical methods are most widely used, as a means of control, in mass-production shops and in factory laboratories.

Under operating conditions there are also items of equipment whose condition must be monitored on a daily basis. Examples of such items are rails in railway transport, ropes in mine hoisting installations, etc. Whereas in transport there have long been operating installations for electrical and magnetic examination of laid railway tracks, both in Soviet and foreign practice—for example, the well-known flaw detectors of Karpov, SOFTI, Suzuki, and others—in the mining industry there are still no operating installations for monitoring the condition of ropes. Research on the development of methods for monitoring the condition of ropes has not gone beyond laboratory models, and in mines the suitability of ropes is assessed by direct inspection, by counting the number of broken outer wires, and by periodic terminal tests in laboratories for tensile strength and bending of the wires making up the rope. On the basis of these data a judgment is made as to the suitability of the rope for further service. Only very recently (in 1941) was a trial industrial model of a rope flaw detector, type MGI-2, manufactured at the Moscow Mining Institute.

It did not prove possible to carry out industrial tests because of the outbreak of the war. It is necessary only to note that the purpose of the indicated flaw detector is to detect external and internal breaks in the wires of a rope, and that it is distinguished by the great complexity of its construction.

The circumstances mentioned above emphasize the urgent need for intensive work on questions of a correct criterion for judging the strength of a rope, on the one hand, and for finding simple devices of an electromagnetic system for judging the condition of a rope at a given moment, on the other.

Methods for Monitoring the Condition of Ropes

The existing methods for monitoring the general condition of a rope amount to daily direct inspection of the rope. In this inspection attention is paid to ensuring that no gross defects are allowed in the rope.

in its external appearance: the formation of “beetles,” mass breakage of wires, bulging of strands; and an accurate count is also kept of the number of broken wires emerging onto the outer surface of the rope. If the number of visible broken wires over the length of one lay of the rope exceeds 5% of the total number of wires composing the rope, it is replaced. In addition, in order to judge the ultimate load-carrying capacity of the rope before it is installed, and also during operation, mechanical tests of the rope wires for tension and bending are carried out in testing laboratories. For these tests, the end of the rope is cut off over a length of 5 m, counting from the place where it is fastened at the hoisting vessel if the rope is in service, or an end of 1.5 m from a rope being newly installed.

The procedure and method for carrying out end tests of ropes in laboratories are regulated by the “Instructions for Testing Mine Steel Ropes” of the State Chief Mining-Technical Inspectorate of the People’s Commissariat of Coal of the USSR (Gostoptekhizdat, 1941).

Thus, on the basis of systematic observation of the rope’s external condition and on the basis of laboratory test data for cut-off ends, a judgment is made as to whether further operation of the rope is possible or whether its replacement is necessary, if the margin of strength or the external condition does not meet the necessary requirements. How effective this method of monitoring the condition of the rope is, and to what extent it satisfies practical requirements, will be indicated below.

Fig. 1. Principle of operation of flaw detectors.
1—rope; 2—magnetizing coils; 3—search coils (probes); 4—amplifier; 5—recording part.

In connection with the development of electromagnetic methods for monitoring finished products, intensive work is also being carried out on the application of flaw-detection methods to wire steel ropes. The basic electromagnetic method for monitoring the condition of ropes, also adopted in the designs of MGI-1 and MGI-2, consists, as shown in the diagram of Fig. 1, in the following:

a) direct current is used as the magnetizing current;

b) the magnetic circuit, which also includes the rope being tested, is brought to a state of saturation;

c) the search coils react to the leakage fields that arise when there are external or internal wire breaks in the rope;

d) the electromotive force induced by the leakage fields in the search coils is amplified to the magnitude necessary to deflect a sensitive galvanometer or to record on photographic paper (MGI-1) or on tape (MGI-2).

A flaw detector of SOFTI for rails is built on a principle analogous in idea, with the difference that alternating current at a frequency of 500 hertz is used as the magnetizing current for local magnetization, and a telephone or milliammeter is used as the indicator. There are foreign designs that also operate on alternating current of another frequency.

From the idea behind the construction of the above-mentioned flaw detectors it follows that the principal task posed in designing them is to monitor the condition of the outer surface of the part, and also to detect internal defects insofar as the physical state of the article and the method of magnetization allow: for example, the detection of longitudinal and transverse cracks in the head and rail-foot portion of a rail, metal stratification in a rail, etc.; in ropes—breaks of outer and, in part, inner wires, with the number and location of the breaks being recorded on a tape.

Having such a defectogram of a rope, one can obtain a complete picture of the number of broken wires on a given section of the working part of the rope. If, in the case of rails, external defects are an important factor in judging their fitness for further service, this cannot be said of ropes twisted from wires. The breakage of a separate wire, as will be indicated below, produces an insignificant reduction in the aggregate breaking force of the entire rope. This occurs because the wires in a rope, being twisted by means of double lay on rope-making machines, form a complex body, and each wire is clamped in the strand and in the rope; when it breaks, it is subjected to such friction that, at some distance from the break, it participates in the work of the rope as an intact wire. This circumstance poses the question of rope strength more broadly than the strength of a solid article, for example, a solid rod or a rail.

Strength of Ropes

The strength of a rope is regulated by the safety rules in the coal industry as follows: “§ 115. The safety factor of a rope in repeated tests is determined as the ratio of the total breaking force of all the rope wires, excluding wires that failed the tensile or bending tests, to the maximum load on the rope, taking into account the weight of the rope itself.” If the safety factor becomes equal to or less than the permissible value, the rope must be replaced.

The inadequacy of the criterion for evaluating rope strength by end tests according to § 115 of the safety rules may be illustrated by the following example.

On 22 February 1932, on cage hoist No. 1 of the “Sofiya Vertikalnaya” mine (Donbass), the right-hand rope, 40 mm in diameter, broke while a normal load was being raised at the beginning of the hoist. The rope had been in service for about a year. According to end tests of the rope carried out 4½ months before the accident, the permitted load was 16,120 kg; with its actual load of 11,640 kg, the rope had a fully permissible safety factor of 8.4, whereas the minimum permissible factor for a goods-and-passenger hoist was 7.5 (see Proceedings of MakNII, vol. II, 1935, article by V. A. Tsvetkov).

This fact served as the basis for setting up detailed experiments at MakNII to determine the relationship between the aggregate strength (tenacity) of a rope and the strength determined as the conventional strength from the sum of the breaking forces of wires that withstood tensile and bending tests in rope-testing laboratories. It was established that the aggregate strength of a rope after it is hung increases owing to compaction of the wires, compression of the core, and a more uniform redistribution of the load among the individual wires making up the rope. The general nature of the dependence of the aggregate tenacity of a rope on the duration of its service is shown in Fig. 2.

It has been established that the tenacity of a rope after hanging increases and, only after some time, depending on the operating conditions, begins to

decrease. The principal factors affecting the reduction in the strength of a rope are wear of the outer wires, “fatigue” of the metal, and changes in the technological qualities of the metal. At the same time, an important circumstance is that the number of broken wires is not the principal characteristic affecting the reduction of the strength of the rope as a whole (aggregate strength). Thus, for example, 10% broken wires per 1 meter of rope reduce its aggregate strength by only 0.5–2.5% of its breaking load with intact wires. Moreover, as the investigations of Prof. Wernle show, even with 100% breaks of the wires, the rope retains sufficient strength, in some cases amounting to as much as 50% of its initial strength.

Fig. 2

The reason for such behavior of a broken wire in a rope is explained by the fact that the wire, being twisted into a strand, and the latter into a six-strand rope—that is, participating in a double lay during manufacture of the rope—turns out to be bent helically and clamped. When a break occurs in a given cross-section, the wire, at some distance from this cross-section of the rope, owing to friction, participates in the aggregate resistance of the rope to rupture with its full cross-section.

“Discarding a rope on the basis of the number of broken surface wires is possible only if a relationship is established between the total actual loss of strength by the rope and the number of broken surface wires” (see ibid., p. 38). No relationship of this kind has been established. In accordance with this, it is proposed to keep count of the number of breaks over the “dangerous length of the rope.” The new edition of the safety rules prescribes keeping count of the number of breaks over one lay pitch of the rope. However, Herbst considers it pointless to look for a “dangerous section of the rope,” since in doing so the law of distribution of broken wires cannot be taken into account. “It is necessary to switch to new methods of inspection and rejection of ropes, reflecting the degree of wear and characterizing the strength along the entire length of the rope” (ibid., p. 37).

As a result of processing the materials of a large series of rope tests at MAKNII, V. A. Tsvetkov proposed a simple method for determining the strength of a rope by measuring the rope diameter and the degree of wear of the surface wires. This simple and accessible method has now become widespread in practice. In this case the matter is reduced to determining the effective live cross-section of the rope at the given moment, which in fact characterizes the reduction of its working cross-section. The only shortcoming of such a method should be considered merely the fact that internal breaks and thinning of the internal wires due to wear and corrosion cannot be taken into account, and, when the diameter is measured, these participate as useful cross-section.

Wear of the rope of a vertical hoisting installation along the length of its working part occurs nonuniformly, and the degree of wear of individual sections depends mainly on the operating and installation conditions of the machine. Nevertheless, despite the difference in these conditions, it is possible to establish certain—

certain regularity in the intensity of wear along the working length of the rope. The general nature of the wear is shown in Fig. 3.

The portion of the rope falling on the overhang and on the strand, when the hoisting vessel is in the upper position, wears only slightly. The first 3–4 turns are subject to the greatest wear when the rope is fully wound onto the drum. A significant influence on the heavy wear of precisely this part of the rope is exerted by the angle of deviation, the presence of grooves on the drum, the quantity and quality of rope lubrication, etc. The degree of wear of the remaining part of the rope decreases and, as one approaches the friction and reserve turns, diminishes to zero.

Fig. 3. General nature of rope wear.

In light of the indicated characteristic of rope wear, it should be particularly noted that cutting off a piece of rope for the testing station is carried out from the part most favorable from the standpoint of wear and least loaded from the standpoint of the actual stresses during operation of the rope, since the dangerous section is considered to be the section of the rope at the guide pulley when the vessel is loaded and is located below.

The importance of knowing the magnitude of rope wear acquires special significance in connection with the fact that there is a direct proportionality between the aggregate strength of the rope and the magnitude of its wear, as shown in Fig. 4.

The strength curves of ropes and the wear curves of their wires vary in the same proportion along their length, i.e., a given percentage decrease in the strength of the rope corresponds to the same percentage of wear, by diameter, of the surface wires located radially with respect to the axis of the rope. This important conclusion, obtained at MAKNII as the result of a large series of experiments on testing ropes that had been in service, served as the basis for the criterion for evaluating rope strength from measurements of the amount of wear of the surface wires and of the rope diameter, as was indicated above.

It appears expedient, in the development of the method of direct measurements, to find a more perfect method making it possible to judge the amount of rope wear not in individual sections, but continuously over the entire length of its working part. Moreover, it is essentially important to have an idea of the overall change in the live cross-section of the rope, irrespective of the causes that have produced this reduction in section. To accomplish such a task, the differentiated method of electromagnetic flaw detection may be used, as the simplest and fully corresponding to the given purpose.

Comparative method for determining the cross-section of a rope

Under operating conditions of a rope, it is extremely necessary to have a picture of the change in the live cross-section of the rope over the entire length of its working part.

However, determination of the absolute magnitude of this cross-section by electromagnetic methods is associated with considerable difficulties of both a theoretical and a practical nature. The theoretical difficulties arise

owing to the insufficient study of the magnetic characteristics of the rope material; the practical ones are connected with the complexity of manufacturing the instruments and the lack of certainty as to the effectiveness of their operation. Work in this direction is being carried out and must be carried out, but at the present time it is necessary to seek methods that give more rapid results and satisfy practical needs.

Fig. 4. Aggregate strength and wear of the rope along its length.

One of such methods should be considered the comparative method, which gives the relative value of the cross section of the working rope with respect to its initial value, which existed when it was hung.

For this purpose it is necessary to have a standard rope, cut off from the rope being hung, and, during the operation of the latter, to compare the cross section, unaltered by wear and breaks, with the initial one. If such observations are carried out systematically, then at any given moment it is possible to estimate the degree of reduction of the rope cross section along the entire length of its working part.

The results of comparing the cross section of the standard rope and of the working rope can be recorded on the tape of the defectogram, which will provide a visual and objective method of estimating the degree of wear. In this case, however, the recording part considerably increases the cost of the instrument and complicates it. A considerable simplification of the instrument is obtained if observation of the pointer deflection is carried out visually and the results of observations in the most worn part of the rope are entered in the rope log. In practice it is sufficient to inspect the rope at definite time intervals, recording the wear of the working part of the rope according to the greatest deflection of the pointer of the indicating instrument, which can be graduated for the permissible percentage of rope wear.

On this principle a laboratory experimental model of a defectoscope with a branched magnetic circuit was built; its schematic diagram is given in Fig. 5.

For the branched magnetic circuit, in a first approximation, we have:

Magnetic flux in the circuit of the standard rope

\[ \Phi_1 = \Phi_0 \frac{Z_2}{Z_1 + Z_2} \tag{1} \]

and the magnetic flux in the circuit of the rope under test

\[ \Phi_2 = \Phi_0 \frac{Z_1}{Z_1 + Z_2} \tag{2} \]

In these equations the following notation is adopted (Fig. 4):

\(\Phi_0\)—the total magnetic flux of the magnetizing coil;

\(Z_1\) and \(Z_2\)—the complex magnetic resistances of the branch circuits.

Fig. 5. Magnetic system of the defectoscope.

1—magnetizing coil, 2—search coils;
3—iron, 4—detachable poles, 5—standard rope,
4—working rope.

Here \(Z_1 = (R'_v + R'_{\mathrm{ж}}) + jx'_{\mathrm{ж}}\), the complex resistance of the standard-rope circuit, remains almost constant, whereas \(Z_2 = (R''_v + R''_{\mathrm{ж}}) + jx_{\mathrm{ж}}\)—the magnetic resistance of the circuit of the rope under test—increases both on account of \(R''_v\), the increase of the air gap in the pole due to a decrease in the rope diameter, and on account of an increase in \(R''_{\mathrm{ж}}\) and \(x''_{\mathrm{ж}}\), quantities directly proportional to the length of the test specimen between the poles and inversely proportional to the cross section of the rope.

The total increase in the magnetic resistance of the circuit of the rope under test causes a redistribution of the total magnetic flux according to equations (1) and (2).

If coils are placed on the corresponding sections of the branch of the magnetic circuit and connected in opposition, as shown in Fig. 6, then—

in the electrical circuit thus formed we shall have an emf proportional to the difference of the fluxes in these two magnetic circuits

\[ e = k\Phi_{\mathrm{p}} = k(\Phi_1 - \Phi_2). \tag{3} \]

We arrive at the same conclusion more clearly if we assume

Fig. 6. Electrical part of the flaw detector.
1—magnetizing coil, 2—search coils—connected in pairs in opposition, 3—cuprous rectifier, 4—milliammeter.

that the resistance in the circuit of the working rope will be of a purely wattless character.

Denoting by \(R_1\) and \(R_2\) the magnetic resistances of the circuit, we shall have

\[ \Phi_1 = \Phi_0 \frac{R_2}{R_1 + R_2} = \Phi_0 \frac{kR_1}{R_1 + kR_1} = \Phi_0 \frac{k}{1+k}, \]

and

\[ \Phi_2 = \Phi_0 \frac{R_1}{R_1 + R_2} = \Phi_0 \frac{R_1}{R_1 + kR_1} = \Phi_0 \frac{1}{1+k}, \]

where it is assumed that \(R_2 = kR_1\).

As the coefficient \(k\) increases, the functions \(\Phi_1\) and \(\Phi_2\) tend toward the values: the first toward \(\Phi_0\) and the second toward 0, as shown in Figure 7.

Fig. 7. Change of magnetic flux in the circuits.

The difference of the fluxes in these magnetic circuits causes the occurrence of an emf in the differentially connected windings of the coils of the electrical circuit.

Laboratory Model

On the basis of the theoretical propositions formulated in the preceding paragraph, a laboratory model was made, the magnetic system of which is shown in Fig. 8.

As can be seen from projections a and b of this drawing, the magnetic system consists of two symmetrical magnetic circuits with the iron dimensions indicated in the drawing. For inserting ropes into the electromagnet, the poles are made detachable, with holes for ropes of 13 mm. The experiments were carried out with a rope of construction \(6 \times 19 + 1\), 12 mm in diameter; the thickness of the rope wires was 0.75 mm.

Dimensions in mm.

Transformer iron \(\delta = 0.5\) mm.

Fig. 8. Magnetic circuit of the flaw detector

The electrical circuit of the model was made in two variants, shown in Figs. 9-a and 9-b.

The difference between these circuits consists in the fact that in the first, as the indicator, a panel-type DC voltmeter was used, with a-

with a shunt resistance, and in the second case a step-down transformer feeds the differentially connected search coils, into whose secondary winding a galvanometer is connected, with a sensitivity of \(1^\circ = 0.43 \times 10^{-6}\,a\), type C-3, with a scale of 100 divisions.

Fig. 9. Electrical circuits of the search coils.
a) With an electromagnetic voltmeter, b) with a galvanometer.

Fig. 10. Magnetic characteristic of iron.

In both circuits, for rectifying alternating current into direct current, a cuprous rectifier is connected, passing one half-wave of the alternating ...

of current. The magnitude of the current of the magnetizing coil, 0.5 amp at 220 V, was chosen on the basis of the magnetic characteristic shown in Fig. 10, i.e., on its rectilinear part.

Experimental Part

The experiments were carried out according to the two circuits given above in Figs. 9-a and 9b.

The degree of wear of the working rope was simulated by removing wires from the rope or by artificially damaging its outer wires.

For this purpose 7 specimens were prepared. The characteristics of the artificial damage on the specimens tested are given in Table 1.

Table 1

By selecting the parameters of the magnetic and electric circuits, as well as of the indicator, it proved possible to graduate the scale of the electromagnetic voltmeter, used as an indicator, in percent of wear of the working rope relative to the reference rope.

With this reduction of the cross section of the tested rope by 20% of its initial value, the deflection of the instrument pointer corresponded to \(3/4\) of its full scale, with divisions on this part from 0 to 100%.

In this case, the percentage of wear of the working rope is read directly from the instrument scale. A deflection of the instrument pointer to 100% will denote the maximum permissible wear and will signal that the time has come to replace the rope with a new one.

The results of the first series of experiments with the ropes indicated in Table 1 are shown in Fig. 11, where the divisions of the indicator scale are plotted along the ordinate axis, and along the abscissa axis the specimen numbers from 0 to 5, in po-

Fig. 11. Deflection of the instrument pointer as a function of the degree of wear of the rope (reduction of its cross section).

in the order of a decrease in cross-section or, in accordance with the adopted methodology, in the order of an increase in rope wear. Rope No. 5 has limiting wear, and when it was placed in the model the indicator gave the greatest deflection.

The experimental points show a scatter from the straight line in both directions of about \(\pm 5\%\), but the law of direct proportionality is clearly revealed.

Conclusions

On the basis of the experiments carried out with the laboratory model, it appears possible to draw several preliminary conclusions:

1. Comparison of the live cross-section of a working rope with a reference rope taken from the same rope before suspension provides a sufficient criterion for using its aggregate strength.

2. Comparison of cross-sections can be carried out by the differential method indicated in the work.

3. As an indicator of the state of the degree of rope wear, a panel instrument graduated in percentages of rope wear may be used.

4. By selecting the parameters of the magnetic and electrical circuits, as well as the sensitivity of the measuring instrument, it is possible to obtain a scale that is quite sufficient for visual reading, which significantly simplifies the instrument.

5. The model of the apparatus is simple in design and consists of standard instruments, which gives reason to believe that the final results of the work will provide material sufficient to formulate technical specifications for the design of an industrial prototype.