Article citation information:
Wieczorek,
A.N., Wójcicki, M., Kalita, M., Kandzia, R. Problems of wear of the
chains used in mine scraper conveyors, caused by friction and corrosion. Scientific Journal of Silesian
University of Technology. Series Transport. 2024, 122, 293-304. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2024.122.16.
Andrzej Norbert WIECZOREK[1],
Mateusz WÓJCICKI[2],
Marek KALITA[3], Richard KANDZIA[4]
PROBLEMS OF WEAR OF THE CHAINS USED IN MINE SCRAPER CONVEYORS, CAUSED BY
FRICTION AND CORROSION
Summary. Scraper
chains are the main transport cables of scraper conveyors. Problems related to
the processes of destruction of link chains used in these conveyors are described.
Design of scraper chains and their division into different types for use in
scraper conveyors are presented. The most common damages to chains used in
scraper conveyors are presented and the reasons of their wear are discussed.
Keywords: transport,
link chain, flight-bar conveyor, scraper
1.
INTRODUCTION
Link
chains equipped with scrapers are the basic transport and pulling component
used in flight-bar conveyors for run-of-mine transport. These conveyors are the
basic machines of the transport line used in the mining faces. Due to their
design, they are considered to be the most resistant and intended for use in
the most difficult environmental conditions, among the known solutions of
tension memberless transport means [5, 13].
The
wear process of flight-bar conveyor components, including chain links, is
complex and depends on many factors. Regarding the environmental factors, there
is a join, synergistic interaction, which significantly reduces durability of
the drive systems of transport conveyors. Cooperation of chain links with the
drum, relative movement of links in the joint area, as well as extremely
difficult and complex environmental conditions, which are characteristic for
hard coal mining plants, determine the aggressiveness of the wear processes of
link chains.
-
abrasive impact of coal and stone dust in the area of
cooperation of chain links (so-called joints),
-
corrosive effect of water flowing out of the goaf and
coming from the cooling and spraying units,
-
dynamic loads from e.g. start-ups of the conveyor
drives, their uneven load, frequent overloading and blocking.
The
degradation factors listed above ultimately lead to breaking the chains.
Despite the use of high-quality steel for chain manufacture and specialized
technologies that ensure high durability and strength requirements, decohesion
of chain links is the most common type of scraper conveyor failure, which
causes a great threat to working miners and generates additional costs related
to downtimes caused by failures. This is especially negative, when considering
the efficiency of a serial transport system in the coal mining industry, where
stopping the transporting machine in the longwall face or in the roadway stops
the entire mining process [11].
This
article presents the problem of degradation processes of link chains in the
conditions of mining processes. The most popular forms of damage, specifying
the reasons for their formation and their effects, are described.
2. LINK CHAINS
USED IN TRANSPORTING SCRAPER CONVEYORS
Medium-link
chains are used in flight-bar conveyors. That means that the ratio of the chain
pitch to the diameter of the bar from which it is made is between 3 and 4. The dimensions
of the conveyor chains have been standardized. The Polish Mining Standard
PN-G-46701:1997 - "Mine link chains" [10] is the current document
characterizing link chains used in scraper conveyors. The links of chains are
of dimensions determined by the aforementioned standard. They are shown in Fig.
1 below.
The
Polish Mining Standard, based on ISO 610:1990 and DIN 22252, describes the size
of the link by specifying the size d x p in mm (d - diameter of the link rod; p
- pitch of the link). On this basis, a series of types of link chains for
various loads was created.
In addition to
the geometrical quantities contained in the standard, mining chains are defined
by various mechanical quantities, the most important of which are:
-
breaking load,
-
calibration load,
-
test load,
-
elongation at breaking load,
-
elongation under test load.
Fig.
1. Dimensions of a link in the mine chain; P- chain pitch; d - chain rod
diameter;
d1 - rod diameter at the welding point; e - maximum weld width; a -
minimum clearance width; b -maximum width of the link; r - radius of the link
rounding (external) [10]
The
breaking load is defined as the maximum force that a chain sample can bear
during a breaking test on a testing machine. Examples of chain link breakage
charts of various sizes are shown in Fig. 2.
Fig.
2. Sample diagrams of breaking the mine chains of different manufacture classes
[16]
A feature
of mining chains that can be considered advantageous is their high elongation
capacity. Together with the breaking load, the elongations are the factors
determining the breaking energy of the chain. The process of elongating the
chain during the test on the testing machine shows two phases - elastic
deformations within the Hook's law (stresses proportional to strains) and a
phase of plastic deformations, which, if exceeded, result in permanent
deformation of the link [13].
Distribution
of stresses in link chains (Fig. 3) is an interesting issue related to the
operation of chain systems. In addition to the stresses generated by the forces
tensioning the chain during the run-of-mine transport, there are also internal
stresses in the links. These stresses result from the process of calibrating
them, which is characteristic for the production of chains. Calibration is
understood as the application of a force tensioning the chain (the so-called
calibrating force), the purpose of which is to deform the links resulting in
elongation of the pitch and setting it at the required level. After
removing the calibrating force, a partial shrinkage of the material occurs,
resulting in internal compressive stresses that have a beneficial effect on the
fatigue life of the chains. In addition to determining the pitch, there are
also geometric changes, especially in the joint area, which positively affects
the tribological cooperation between links. The calibration load is not
included in the standard and is the "know how" of mine chains
manufacturers [1, 4, 11].
Fig.
3. Distribution of stresses in a
mine chain link [9]
3.
DEGRADATION PROCESSES OF LINK CHAINS
Chain failures
are associated with several characteristic leading processes. The most
important of them are:
1. Abrasive wear
2. Corrosion.
3. Synergistic
phenomena (e.g. tribocorrosion).
4. Fatigue of the
material.
The following sections describe the impact of the
above-mentioned causes on mining chain degradation.
3.1. Abrasive
wear
During the
cooperation of links with the drum and cooperation among the links in the joint
area, abrasive wear is the dominant cause of degradation. When analyzing the
chain-chain drum kinematic system, three following basic friction nodes can be
distinguished (Fig. 4):
-
area of
cooperation of the front torus of the horizontal link with the bottom of the
drum (Item A),
-
area of cooperation of the horizontal rear link torus
with the flank of the tooth (Item B),
-
chain joint area - horizontal and vertical link
contact zone (Item C) [15].
Fig. 4.
Friction nodes in the chain-chain drum kinematic system
(Item A - front torus in the bottom of the drum, Item B - rear torus on the
tooth flank,
Item C - cooperation of links in the joint) [15]
For each
of the mentioned friction nodes, tribological wear is observed, caused by the
cooperation of contacting surfaces, often additionally intensified by rock dust
abrasion. In the case of contact at Items A and C (Fig. 4), wear is caused by
the rolling of the links in the joint and positioning the front horizontal
torus of the link in the bottom of the socket. During the cooperation among the
links in the chain joint, the contact point of the links moves, which results
in the lowering of the vertical link in the inter-tooth groove. This causes a
rolling motion as part of two links in cooperation with the simultaneous
slipping of the front torus in the bottom of the socket. In addition, lowering
the vertical link is accompanied by rolling and partly slipping in the chain
joint. The long-term effect of the described phenomena is a wear deforming the
seat bottom, tooth flank and the corresponding zones on the horizontal link
(Fig. 5).
Cooperation
in the area of the joint and abrasion of these zones, in turn, causes an
increase in the pitch of the link chain. This results in the rear torus (Fig.
4, B) resting on the tooth flank instead on the bottom, which often leads to
the rear torus sliding over the tooth flank towards the bottom of the drum,
resulting in wear of the tooth flank and the referring contact zone on the
link. The literature [13, 14] distinguishes three following classes of
cooperation of the link in the pocket of the sprocket wheel:
-
nominal toothing, when the pitch diameter of the chain
wheel is congruent with the chain pitch and while the chain is running on the
wheel, the horizontal link entering the toothing simultaneously contacts the
surface of the preceding socket. At this point, there is no movement of the link
in the wheel socket, which generates cooperation with the highest efficiency,
-
special toothing, when the chain pitch is smaller than
the pitch on the pitch diameter of the wheel. With this type of toothing, the
torus of the rear horizontal link cooperates with the tooth coming out of
toothing. The remaining teeth, in turn, do not have contact with the horizontal
links on the flanks until they leave toothing of the preceding link,
-
normal toothing when the chain pitch is greater than
the expected chain wheel pitch on the pitch diameter. In such a situation,
during the rotation of the wheel, the torus of the horizontal link slips along
the flank of the tooth until it settles in the bottom of the socket. The wear
of the tooth flank further leads to a shift of the contact point of the torus
of the front horizontal link with the bottom of the socket towards the centre
of the socket. This leads to the contact points of the link with the bottom of
the socket approaching the edge of the slot under the scraper. This is
accompanied by a decrease in the contact surface of the front torus with the
bottom of the socket [15].
Increasing
the chain pitch and reducing the chain wheel pitch by settling the link in the
abrasion of the bottom of the drum socket leads to incorrect cooperation of the
described kinematic pair. This generates an increase in dynamic loads, which
intensifies fatigue and friction wear [3, 15].
|
Fig. 5. Wear
area of chain link joint (black arrow) and wear area of chain link contact
zone (red arrows) [8] |
Fig.
6. Visible areas of abrasive wear in the area of the link joint |
In
the case of vertical links, apart from the abrasion processes (against the
conveyor trough – Fig. 8), there are also changes in the structure of the
link material. The rubbing surface of the chain links warms up as a result of
friction processes and then cools down intensively as a result of transferring
heat energy to the other parts of the machine and as a result of cooling with
water-laden run-of-mine (or the environment). The cyclical heating and cooling
in favourable conditions causes the formation of a martensite layer (called in
the literature the friction martensite) characterized by high hardness and
brittleness, and thus susceptible to cracks when the link is overloaded. These
damages have numerous surface cracks perpendicular to the surface and to the
slip direction (Fig. 7). These cracks are often sharp notches, and they are the
beginning of cracks leading to the link destruction [11].
3.2. Corrosion
Corrosion
phenomena significantly affect the link chain operation. The mechanism of
electrochemical corrosion appears as a result of the following features of the
contacting media:
-
corroding metal is a conductor, and its surface is
never completely homogeneous,
-
water in which metal corrodes is not chemically clean;
is a solution of various electrolytes and exhibits ionic conductivity depending
on the concentration and mobility of the ions contained in it.
|
Fig. 7.
Cracks meshing formed as a result of the formation of the so-called friction
martensite [8] |
Fig.
8. Link abrasion on the conveyor trough [15] |
In the
case of a difference between the potentials of the metal surface fragments,
galvanic cells are formed, also called corrosion cells, in which the following
reaction takes place on the anode:
|
|
(1) |
At
the cathode, as a result of dissociation, hydrogen is released according to the
following reaction:
|
|
(2) |
As a
result of the addition of electrons from the cathode, another reaction takes
place:
|
|
(3) |
Electrode
reaction products may enter into secondary reactions generating insoluble iron
oxides:
|
|
(4) |
|
|
(5) |
|
|
(6) |
|
|
(7) |
Chain
link corrosion is especially dangerous if the fatigue cracks appear on the
link's surface. In such a case, extensive corrosion pits may appear, the
propagation of which is further intensified by the increased chemical activity
of the work environment (Fig. 9). The result of this process are corrosive
separation cracks in the links. In addition, in a strongly corrosive environment,
a significant decrease in the critical stresses that initiate development of
cracks is observed. Therefore, the fatigue wear of links operating in a
corrosive environment progresses faster. Examples of corrosive effects on mine
chain links are presented in Fig. 10 [9].
There are
many parameters that affect water properties. These include pH, water hardness,
chlorine content and other chemicals. To determine the corrosiveness of water,
laboratory tests, which include a physicochemical analysis of water, are
required. There are also various indicators that help in assessing water
corrosiveness. An example of such an indicator is the S coefficient described
in [7]. It is based on the guidelines of EN 12502-3:2004. To calculate the
coefficient during laboratory tests, the content of chloride anion Cl-, sulfate
anion SO4-2, hydrocarbon anion HCO3- and nitrate anion NO3-
should be determined. The S coefficient is calculated from the following
relationship [7]:
|
|
(8) |
The cited standard specifies two ranges of impact of
water aggressiveness in the context of steel components:
-
S <0.5 –low probability of pitting corrosion,
-
S > 3.0 –high probability of pitting
corrosion.
|
Fig. 9. An
example of a corrosion pit on a cross-section through the parent
material of a chain link [8] |
Fig. 10.
Chain link corrosion: a) surface pitting, b) fatigue cracks intensified by
corrosive environment [11] |
3.3.
Tribocorrosion
Tribo-chemical
wear is another known wear mechanism for conveyor chain links. It is defined as
the formation of surface reaction layers as a result of tribo-chemical
reactions. These surfaces, due to the abrasive mechanisms, are removed opening
the newly reacting surface with the environmental substances (e.g. mine water).
This reactivity is explained by the plastic deformation of the friction
surface, which generates local density changes, causing the migration of
electrons from places with shear stresses to the places with tensile stresses
(where the potential is negative). Temperature may be another catalyst for
tribo-chemical reactions of differences resulting from the heating of rubbing
surfaces. Electrons then migrate from the regions of higher
temperature to regions of lower temperature. As
part of the tribo-chemical wear, the oxide layer is removed and then recreated
on the surfaces of the cooperating components. It is classified as a normal
type of wear, and its intensity is insignificant in stabilized conditions. With
the increasing corrosiveness of the environment of the cooperating components
(e.g. in the presence of saline water), this process is significantly
intensified by the impact of hard products of wear, which take the role of hard
abrasive grains. In the literature, the term tribocorrosion wear is used to
describe the form of combined wear intensified by the synergistic action of
abrasive and corrosive agents (fig. 11). There is a synergy of factors that
jointly enhance chain degradation, what significantly accelerates its damage.
The presence of hard metal oxides getting into the abrasive as a result of the
joint action of abrasive and corrosive processes causes the synergy effect [3,
12, 15].
3.4. Fatigue
of the material
Corrosion
pitting, micro-cracks or abrasion-mechanical damage are usually the initiators
of fatigue degradation of links. There are several phases in the fatigue
failure process. The first phase is the incubation of microcracks formed during
the operation of conveyor chains. The next phase is development (deepening) of
microcracks until the so-called critical value. After reaching the micro-cracks
critical value, a stage of their rapid progress takes place, finally leading to
the next phase, i.e. to a fracture in the critical
cross-section (Fig. 12). In the aspect of mine link chains, these fractures
usually occur in several characteristic places (Fig. 13), which results from
the distribution of stresses caused by significant tensile forces
[6, 9].
Any link
fracture, in the case of a serial transport system in a coal
mine, causes downtime in mining and thus large economic losses. Both production
plants and designers of link chains are searching for opportunities to
modernize the solutions offered, in terms of increasing durability, strength or
corrosion resistance. Therefore, the problem of chain degradation processes in
mine scraper conveyors is still valid. Reducing the failure rate of scraper
conveyors by eliminating downtimes resulting from chain damage translates into
lower hard coal mining costs.
Fig. 11. Mechanism of
tribocorrosion wear (v-speed vector, C-corrosion;
RC-reoxidation) [2]
|
Fig. 12.
Fatigue fracture of |
Fig.
13. Typical places of fractures : |
4.
CONCLUSIONS
Analysis
and testing the processes causing the chain degradation is of key importance in
the prediction of potential damage and further development of scraper conveyor
designs. The article describes the most common forms of
damage to the chains of scraper conveyors working in transport units in hard
coal mines.
In
the case of these components, the most common destructive processes are:
abrasive wear, corrosion, and fatigue processes. In mining practice, damage
caused by the combined action of wear and corrosion is also observed, which is
usually referred to as the process of tribocorrosion.
As
the analysis of the state of knowledge presented in the article shows, in many
works this process is mentioned as one of the most important destructive
factors, but there are no results of laboratory tests and in environmental
conditions that would present measurable wear values on the basis of which it
would be possible to determine the predicted durability of chains for given
environmental conditions. This fact is the basis for the authors to undertake
further research on the process of their destruction in the conditions of the
combined action of environmental factors.
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Received 05.10.2023; accepted in
revised form 02.12.2023
Scientific Journal of Silesian University of Technology. Series
Transport is licensed under a Creative Commons Attribution 4.0
International License
[1] Faculty of Mining, Safety Engineering and Industrial Automatic
Control, Silesian University of Technology, Akademicka 2A, 44-100 Gliwice,
Poland. Email: andrzej.n.wieczorek@polsl.pl.
ORCID: https://orcid.org/0000-0002-8634-7763
[2]
Division of Machinery and Equipment, KOMAG Institute of Mining Technology,
Pszczyńska 37,
44-101 Gliwice, Poland. Email: mwojcicki@komag.eu.
ORCID: https://orcid.org/0000-0003-2695-7276
[3]
Division of Machinery and Equipment, KOMAG Institute of Mining Technology,
Pszczyńska 37,
44-101 Gliwice, Poland. Email: mkalita@komag.eu.
ORCID: https://orcid.org/0000-0003-2695-7276
[4]
THIELE GmbH & Co. KG, Werkstraße 3, 58640 Iserlohn, Germany. Email:
r.kandzia@Thiele.de.
ORCID: https://orcid.org/0009-0004-2159-049X