T.S. Golosinski and A. Tytko*
University of Missouri-Rolla, Rolla, MO 65409-0450
*University of Mining and Metallurgy, 30-057 Krakow, Poland
MAGNETIC EXAMINATIONS OF WIRE ROPES:
LOSS OF METALLIC AREA (LMA) MEASUREMENT WITH HALL-EFFECT
SENSORS
Summary
The paper reports on investigations related to use of Hall-effect sensors for
measurement of LMA (Loss of Metallic Area) during magnetic examinations of
wire ropes. In particular, it discusses and quantifies the accuracy of the
standard LMA signal calibration procedure. It also quantifies the LMA
averaging length of a selected magnetic apparatus and discusses the related
phenomena.
1. INTRODUCTION
Magnetic and visual examinations of wire ropes are conducted to assess
condition of ropes in service. During magnetic examination the rope is
magnetized. The magnetic flux generated in the rope and the leakage flux are
measured along the rope. Knowledge of the first allows quantification of the
loss of metallic area of a rope (LMA) while the second allows to locate and
often quantify local faults present in the rope (LF), most often broken wires
(ASTM, 1993).
Depending on the application and on the related rope wear pattern either LF
or LMA type damage may be predominant. As an example, friction winder
ropes wear is predominantly of LF-type, caused by fatigue breaks of wires.
Ability to accurately quantify LF–type damage is thus important for correct
assessment of rope condition in this application (Hansel et al, 1990). The
drum winder ropes wear most often through external abrasion (Chaplin,
1994), thus here the accurate knowledge of rope LMA is of utmost
importance. While one type of wear may be predominant in specific rope
applications, in general knowledge of both the LF and LMA type wear, and of
its other forms is required to accurately assess rope condition (Golosinski,
1995).
The accuracy of magnetic rope examinations depends on a number of factors,
including specifications and properties of the employed apparatus, the rope
wear pattern, and rope construction and size (Geller et al,1990 and Golosinski
and Tytko, 1997). The following paper discusses and quantifies two of the
factors which contribute to the accuracy of the LMA determination. Based on
laboratory experiments done with the Meraster MD-120 apparatus the paper:
1. Quantifies the errors of LMA measurements that may be generated in the
process of static LMA signal calibration, and
1
2. Quantifies relations between the LMA measured by a magnetic apparatus
and the length of a worn out part of the rope. It also defines the averaging
length of the apparatus in question. The averaging length is defined here
as the minimum length of worn-out or damaged part of the rope for which
the LMA can be accurately quantified (Weischedel, 1991).
To simplify the following discussion no distinction is made between the loss of
magnetic area of the rope and the loss of its metallic area. Furthermore, the
term LMA (Loss of Metallic Area) is used to describe all changes of rope
metallic area which may be either positive (loss of area) or negative (gain of
area).
LMA MEASUREMENT
To define the LMA of ropes during their magnetic investigations the apparatus
measures the density of magnetic flux along the magnetized rope. The actual
point of flux measurement differs for different makes of the apparatus as
explained below. Changes in the measured flux are then converted to LMA
using formula (1) shown below. The formula assumes that the flux density is
directly proportional to the LMA of the examined rope (Martyna, 1993 and
1996). While a variety of sensors may be used to measure the magnetic flux
(Haller, 1984), the Hall-effect sensors are preferred by many apparatus
suppliers. Therefore, further discussion is limited to their use. The basics of
the use of Hall-effect sensors for measurement of magnetic flux are presented
graphically in figure 1.
Hall sensor signal:
B
B1
B = B1 - B2
B2
Uh
Icons
Examined rope:
t
SFe=SFe1 - SFe2
1
SFe1
SFe2
2
Principle of flux
measurement
Figure 1. Magnetic flux measurement with a Hall-effect sensor.
B[T] - flux density, Icost[mA] - sensor current, UH[mV] - sensor voltage,
SFe[mm2] - change of the metallic cross section of the rope from SFe1 to
SFe2, B[T] - difference of flux density as measured in rope sections 1 and
2, 1 and 2 - magnetic flux in rope sections 1 and 2 correspondingly.
In reference to figure 1, the LMA of a rope is assumed to be linearly related to
sensor voltage, UH[mV], which provides a measurement of the changes in the
2
density of magnetic flux, B[T]. The accuracy of the relation between the
sensor voltage and rope LMA is determined by several factors, including:
 quality of the sensor as defined by the factor kH,
 quality of apparatus electronics, which influences the stability of sensor
current, Iconst, and
 location of sensors in relation to the rope.
The sensor voltage, UH, can thus be expressed as follows:
UH = kH x Iconst x B[T]
1
3
(1)
HB
2
6
4
5
7
B
Z
N
N
R
HR1
S
HN1
HN3
N
G
N
HN2
N
R
HN4
HR2
S
N
Z
Figure 2.
Alternate locations of Hall-effect sensors in the measuring heads
of the magnetic apparatus.
1 – examined rope, 2 – worn out part of the rope, 3 and 7 – permanent
magnet poles, 4 – internal console in the apparatus head, 5 – location
of Hall sensors in the apparatus used in the investigations, 6 – bridging
bar of the head.
 - magnetic flux with the following indices: G - magnetic flux in the
rope, Z, B - outer flux, N - return flux, and R - leakage flux,
HN - the sensors at magnet poles, HR - the sensors at the central
console, HB - the sensors at the console bridging the magnets.
The location of sensors in the head of the magnetic apparatus is specific to
the make and model of the apparatus. The objective of apparatus designers is
to maximize the sensor sensitivity while assuring linearity of the relation
between sensor indications and the loss of rope metallic area. Another
consideration is to reduce the impact of temperature on the sensor accuracy.
Three different sensor locations are commonly used. These are shown in
figure 2. One apparatus locates the sensors in the return flux at the magnet
poles (HN1, HN2, HN3, HN4) to measure density of the return flux, G, (Geller et
3
al, 1995). The instrument used in the investigations reported in this paper has
the sensors (HR1, HR2) located in the central console to measure the density of
leakage flux, R, around the magnetized rope. Another design calls for
location of the sensors on the console bridging the magnets in the measuring
head (HB) to measure the density of the return flux, B or Z, ( Martyna,
1993, 1996).
As various heads measure the magnetic flux in different locations, the relative
accuracy of LMA measurements done with various apparatuses differs.
Additional inaccuracies may be introduced as a result of non-linearity of
magnetic field in the measuring head, a result of interference and
superimposition of individual fluxes (Hansel et al, 1990). Finally, accuracy of
LMA measurement depends on type, geometry and extent of the rope wear.
Since the investigations presented in this paper were done with the apparatus
that measures the leakage flux, R, in locations HR1 and HR2 , the findings may
not fully apply to other apparatus makes with different sensor locations.
LMA SIGNAL CALIBRATION
Calibration of apparatus signals is a part of a standard rope examination
procedure (ASTM, 1993). It is done by inserting a ferromagnetic rod, or a rope
wire with known metallic cross-section, SFecal, into the measuring head and
relating the recorded sensor signal, UHcal, to the rod cross-section. A
conversion factor, kHcal, is thus defined which quantifies the sensor signal in
terms of LMA as described by equation (1) below:
LMA  U H 
Where: LMA
SFecal
UH
UHcal,
kHcal
S Fecal
 U H  k Hcal
U Hcal
(2)
- loss of metallic area of a rope, mm2
- metallic cross-section of the calibration rod, mm2
- value of the recorded signal of the sensor, mV
- value of the sensor signal recorded during calibration, mV
- conversion factor, mm2/mV
While this procedure defines the conversion factor kH based on actual
increase of the rope metallic area it is subsequently used to quantify both the
increase and the loss of metallic area during rope examination. This method
of LMA signal calibration is common to all types of apparatus, is
recommended by the apparatus vendors and by the relevant standards
(ASTM, 1993). Since the examined rope does not move during the calibration
this method is called further a static calibration. The calibration method is
illustrated in figure 3.
Conditions during actual rope examination are different from those during
static calibration. First, the measurement of the LMA takes place while the
4
rope moves relative to the apparatus. Furthermore, in most circumstances,
the rope metallic cross-section decreases. Measurement of the LMA in these
conditions is further called dynamic measurement to differentiate it from the
conditions of static rope calibration.
Where:
1 - examined wire rope
2 - the magnetic apparatus
1
2
3
LMA Dynamic
[mV]
+10
4
measurement
5
Maximum
+5
Steady value of the signal


Length of the rope
[m]
LMA Static
[mV]
+10
3 - Hall-effect sensor (LMA recording)
4 - The calibration rod
5 - Additional metallic section taped to
the examined wire rope
 - Direction of the wire rope or the
calibration rod movement
+5
calibration
Signal maximum
Length of the rope [m]
Figure 3. Schematic of static calibration of the LMA signal and its dynamic
measurement.
The difference between conditions under which the static calibration and the
dynamic LMA measurement are done has a direct impact on accuracy of LMA
measurements. The resulting error is quantified below based on laboratory
investigations. The latter involved static calibration of the LMA signal followed
by the dynamic measurement of the LMA for a rope with the LMA equal to the
cross-section of the calibration rod. In addition, the effect of increasing rope
metallic area, as opposed to its loss, on the accuracy of LMA determination
was investigated. The LMA readings made during static calibration and during
dynamic measurements were then analyzed and compared to quantify the
possible errors. The results of these investigations are presented in Table 1.
The actual percentage loss of LMA, UR [%], presented in the third column of
Table 1 represents the actual change of the metallic area of the rope while the
fourth column, denoted Uz [%], presents measured values of the LMA. Figure
4 presents the results of the investigations in the graphical form and lists the
equation that describes the relation in question. The equation was defined
using linear regression at the confidence interval of 95%.
The dynamic measurement of the LMA gives the value that is 17% higher, on
average, than the value defined during the static calibration. The form of
equation which defines the relation between the two (figure 4) indicates
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existence of high correlation between the two values in the investigated range
of the LMA change, from 0% up to 24% of the original metallic cross-section
of the rope.
Table 1. Results of the static and the dynamic calibration of LMA signal
Loss
of metallic
area
Sample
No.
Measured
value of the
LMA signal
SFe
Actual
percentage
loss of LMA
Measured
percentage
loss of LMA
Relative error
of LMA
measurement
UR
UZ
U = UZ - UR
Calibration
method
accuracy
U 
UZ  UR
 100
UR
[mm2]
[mV]
[%]
[%]
[%]
[%]
1
2
3
4
5
6
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
0.31
0.72
0.95
1.66
2.84
4.37
3.79
89
88
87
88
87
81
83
Static calibration of the LMA signal
1
2
3
4
5
4.25
20.0
40.0
50.0
80.0
8.75
10.0
20.0
25.0
40.0
2.56
5.86
11.73
14.66
23.46
2.56
5.86
11.73
14.66
23.46
Dynamic measurement of the LMA
6
7
8
9
10
11
12
4.25
17.54
21.77
43.53
65.291)
65.292)
61.07
3.5
10.0
12.5
24.6
37.5
40.1
37.0
2.56
5.14
6.37
12.75
19.12
19.12
17.89
2.86
5.86
7.32
14.41
21.97
23.49
21.67
NOTES: Metallic cross-section of the rope Sfe= 341mm2
1) additional three rope strands placed on rope surface side by side (asymmetrically)
2) additional three rope strands places symmetrically on rope surface
The results of the investigations indicate that the commonly used method of
LMA static calibration results in overestimation of the actual LMA by about
17%. In practical terms it means that safety of rope application is higher than
that indicated by measured values of the LMA, however, this also means that
some ropes may be discarded prematurely.
Data presented in Table 1 indicates that the error of LMA measurement is
unlikely to depend on the character of metallic area change. The data
obtained for ropes with increased metallic area (denoted by subscripts 1 and
2, samples 10 and 11 in Table 1) do not differ essentially from the rest of the
data obtained for ropes with reduced metallic area. Not enough data was
collected to determine the statistical significance of this relation.
6
CHANGING LENGTHS OF DEFECTS AND
MEASUREMENT
ACCURACY OF LMA
Accurate definition of LMA from its values measured by the magnetic
apparatus is difficult if the actual rope wear is localized. A concept of
apparatus averaging length is often used (Weischedel, 1991) to define the
minimum length of worn-out part of the rope over which the LMA can be
UZ [%]
Regression equation for dynamic measurement of LMA
25
UZ = - 0.09+1.17UR [%]
for: r = 0.995 at  = 0.05
20
15
10
o - dynamic measurement
x - static calibration
5
0
0
5
10
15
20
25
Actual change of the rope cross section UR[%]
Figure 4. Relations between the actual change of rope metallic area, U R [%]
and
the change indicated by Hall-effect sensors, U [%].
accurately quantified. More recently a method to minimize the apparatus
averaging length was developed and applied by one of the research groups
working with magnetic examination of ropes (Geller et al, 1995). Measured
value of the LMA signal follows a characteristic pattern as shown in figure 3. If
an abrupt change in the rope metallic cross-section is encountered, the
recorded signal peaks and then stabilizes at a value lower than the peak. For
the purpose of this paper the minimum length of the rope required for the
signal to stabilize, after an LMA change is encountered, is called the
apparatus averaging length.
The following part of the paper presents the investigations undertaken to
define the relation between the length of a worn-out part of examined rope
and the accuracy of its measured LMA. The investigations involved magnetic
examination of a post-formed, multi strand non-rotating rope with one of the
outside strands removed. The length of a rope over which the strand was
removed varied from several millimeters to over a meter. Selected values of
the LMA signal recorded during the investigations are presented in figure 5
7
below. The traces of these signals facilitate visualization of the concept of the
instrument averaging length.
Different line shapes are used in figure 5 to distinguish between the individual
records. The maximum and the average value of the LMA signal was
measured as shown in figure 3. Negative values of the LMA indicate loss of
metallic area, as different from the positive values recorded during static
calibration of the LMA signal.
.
LMA
[mV]
5
Sfe= 43.54mm2
0
-5
-10
-15
-20
the average value of
the signal
the maximum value of the signal
-25
0
100
200
300
400
500
Length of the rope section with the removed outer strand [mm]
Figure 5. Digitized records of LMA signal measured for the rope with changing
length of LMA defect.
Figure 6 presents the measured average and the maximum value of the LMA.
The difference between these two values depends on the geometry of the
rope damage, with the larger difference reflecting rapid LMA changes. Digital
filtering of the measured LMA values may help to limit this difference, thus
increasing the accuracy of the LMA determination (Geller et al, 1995).
8
0
LMA
[mV]
SFe= 21.77mm2
-5
the average value of the signal
the maximum value of the signal
-10
-15
0
100
200
300
400
500
600
Figure 6. Value of the LMA signal as the function of the rope damage length.
Figure 6 allows determining the LMA averaging length for the apparatus used
in the investigations. Measured LMA stabilizes at below 100 mm thus the
averaging length is in this range. The value is specific to the investigated rope
and increases if the instantaneous changes of the metallic cross-section
increase. If smooth transitions of rope LMA are faced the averaging length of
the apparatus decreases. Similar values of the apparatus averaging length
were defined for other rope constructions and diameters.
CONCLUSIONS
Accuracy of the LMA measurements during magnetic examinations of wire
ropes depends on the type and location of the Hall-effect sensors. For the
sensors located in the center of the apparatus head and measuring the
leakage magnetic flux, the measured LMA may be overestimated by close to
20%. This overestimation results from inadequacies of the apparatus
calibration procedures.
The shape of the LMA signal depends on the geometry of the LMA change in
the examined rope. Rapid and significant LMA changes result in a non-linear
signal that makes quantification of the LMA difficult. The value of the LMA
should be read off the flat, regular portion of the recorded LMA signal.
The averaging length of the apparatus used in the described investigations is
in the range of 100 mm.
REFERENCES
ASTM Standard E 1571 - 93. 1993. Standard practice for electromagnetic
examination of ferromagnetic steel wire rope.
Chaplin C. R. 1994. Hoisting ropes for drum winders - the mechanics of
degradation. Mining Technology, July/August, pp. 213 - 219.
9
Geller L.B., Udd J.B. 1990. How accurate are non-destructive testing based
estimates of mine shaft breaking-strength losses - an update. CIM Bulletin,
vol. 83, no. 944, pp 47-50.
Geller L.B., Leung K., Kitzinger K. 1995. Computerized operational control of
an Electromagnetic wire rope tester. Materials Evaluations, September, pp.
1002-1006.
Golosinski T.S., Tytko A. 1997. Assessment of remaining rope life based on
results of its examinations. Proceedings, Applications of endurance
prediction for rope wires - OIPEEC Round Table, Reading, UK.
Golosinski T.S. 1995. Nondestructive testing (NDT) of winder ropes in
Western Australia; Underground Operators' Conference, pp.207-210,
Kalgoorlie ,13 - 14 November 1995.
Haller, A. 1984. Neue erkentnisse ueber die Seilquerschnittsmesungen.
Internationale Seilbahn Rundschau, No. 3.
Hansel J., Kwasniewski J., Lankosz L., Tytko A. 1990. Magnetic testing of
wire ropes. Wydawnictwo AGH - Skrypty Uczelniane 1189, Pl ISSN 1189, p
96, Kraków 1990 (in Polish).
Martyna R. 1996. Problem of rope damage measurement accuracy in
magnetic tests of steel ropes magnetized by permanent field.
Proceedings, Mine Hoisting 96' Conference, Gliwice, Poland.
Martyna R. 1993. Techniques to better exploit the possibilities of wire rope
testing with permanent magnet equipped instruments. Proceedings, Mine
Hoisting 93' Conference, London, UK.
Weischedel H. R. 1991. The inspections of mine hoist ropes. Wire Rope News
and Sling Technology, June, pp. 14-25.
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