1. Introduction
In many industrial companies, the costs of transporting bulk materials represent 10–30% of the total production costs. With a constant increase in electricity prices, there is an increasing necessity to implement energy management for applications with high rates of energy consumption, such as belt conveyors [
1,
2,
3]. Increasing the performance of a belt conveyor and optimizing its operating parameters may reduce its rate of energy consumption [
4]. A decisive factor in designing energy-efficient conveyors is the correct choice of its structural components [
5,
6]. The results of extensive theoretical and experimental research described in multiple papers [
7,
8,
9,
10,
11,
12,
13,
14] have identified a potential for energy saving in the individual components of belt conveyors, such as conveyor belts [
7,
8], transfer points [
9,
10], drives [
11,
12,
13] and support systems [
14].
A transfer chute is also a critical place on a belt conveyor in terms of energy loss as that is where the concentrated dispersion of energy occurs. Designing an optimal chute is a complex process. Papers [
15,
16,
17] presented optimal loading chute profiles when selecting an optimal curve or profile that the chute follows. In Paper [
18], the authors applied laser scanning as a contactless method for measuring the speed of a belt in real time with a possibility of identifying the profile of bulk materials on a belt conveyor. In Papers [
19,
20], the authors applied the discrete element method (DEM) to analyze transfer chute designs in terms of their flow rate characteristics. A smooth flow of material particles results in the lower consumption of energy, and, hence, the higher reliability of the chute.
For the purpose of energy saving in belt conveyance, the belt conveyance development efforts are aimed at using long conveyors with a minimum number of chutes and with a high speed of the conveyor belt [
3,
10,
21,
22,
23]. The elimination of material transfer points reduces not only the consumption of electrical energy [
10,
23] but also operating costs. Another solution is an intermediate linear booster drive as an alternative to conventional drives. This type of drive facilitates reducing energy consumption in long conveyors, as presented in Paper [
24].
An important area of research in the handling of bulk materials from the point of view of energy losses is the dynamics of the impact of the material at the transfer point. Currently, several methods of designs and solutions are presented. There are various experimental approaches [
24,
25,
26]: simulation models using the MATLAB platform [
27], DEM [
28], specialized FEM (finite element method) software [
29], or QNK-TT [
30]. Much less attention is paid to the problem of addressing the structure of the chute from the aspect of dynamic impact stress, despite the fact that it causes up to 60% of all damage to conveyor belts. Most of the damage is associated with piercing or cutting the upper covering layer [
31]. If the belt is damaged, there are serious consequences for the safety [
32] and the efficiency of the operation of the belt conveyor [
33,
34]. The incorrect setting of the impact height and incorrect selection of the type of conveyor belt at the transfer point is a frequent cause of mechanical damage to the conveyor belt. In the worst case, the support system and the supporting structure of the conveyor may also be damaged. The resistance of the conveyor belt against puncture is an important criterion in achieving the sustainability of belt transport.
The research presented in this article aims to supplement the knowledge of the impact dynamic loading of conveyor belts when determining energy balances at the chute, especially from the point of view of the type of transported material. Specifically, the impact of sharp-edged material (pyramidal impactor) or the impact of material with rounded edges (spherical impactor) is simulated. Another investigated factor is the specific weight of the transported material, which is simulated in the research by changing the weight of the falling hammer. The last factor is the impact height, with a decisive influence on the damage to the conveyor belt. During the experiments, the impact height is simulated with steps of 0.2 m. The research is focused on local (point) damage at the point of impact of the impactor on the conveyor belt. Important results from the observation of the impact process are provided by laboratory tests; therefore, the authors of the article focused on testing the resistance of conveyor belts in terms of point damage in laboratory conditions. The article analyzes the conveyor belt energy balances, taking into account the height of impact, the weight of the hammer, and the two different types of impactors. Based on the investigation of these factors, it is possible not only to calculate, but also to optimize the operation of the conveyor.
2. Materials and Methods
The investigation into energy absorption by a conveyor belt, with or without a support system, and into related energy loss at transfer points is typically carried out by taking one of a variety of approaches. They include, for example, regression models [
35], classification models [
36], simulations [
37], and an evolution method for the identification of loading chute profiles [
17]. The present article describes the application of the design of experiment (DOE) method.
Experiments were conducted on a conveyor belt of the P 2000/4 8 + 4 201A type, i.e., a rubber–textile belt with a polyamide carcass (consisting of 4 fabric plies,
Figure 1) with a nominal tensile strength of 2000 N mm
−1 and with rubber cover layers—an 8 mm-thick top cover layer and a 4 mm-thick bottom cover layer. This conveyor belt is used for the transport of extremely abrasive, abrasive, granular, and loose materials. Typical applications include thermal power plants, transport of aggregates, mining industry, limestone works, cement works, dumps, docks, recovery and processing of raw materials, processing, and agricultural industry. Belt specimens sized 1200 × 150 mm were fixed into hydraulic jaws and stretched using the force of 30 kN. They were exposed to the impacts of two impactor types, pyramidal and spherical, simulating the shapes of materials falling onto the belt at chutes. The hammer impact height was changed within the range from 0.2 m to 1.4 m, with 0.2 m increments. The hammer mass was changed within the interval of 50–100 kg, with 10 kg increments.
2.1. Methodology for Impact Testing of Belts
The Institute for Logistics and Transport at the Technical University of Košice possesses the equipment constructed particularly for testing resistance to puncture of conveyor belts (
Figure 2).
A procedure for testing conveyor belts for resistance to puncture is described in more detail in Paper [
38].
During testing, the following parameters are varied:
The mass was changed by adding calibrated steel weights (ϕ 200 mm) within the mass range from 50 kg to 110 kg. The head of the hammer may be of various shapes, including spherical, pyramidal, or conical. The testing described in this paper was conducted using the pyramidal and spherical impactors (
Figure 3). The tests were limited by a maximum impact height of 2.6 m (limiting height of the testing equipment tower). The impact height, measured using the L-GAGE LT3 Long-Range Time-of-Flight Laser Sensor, may be set to an arbitrary value as required for the performance of particular testing. Various potential impact heights and hammer masses enable a wide range of combinations to be created. This particular stand (
Figure 4) facilitates testing of any type of conveyor belts.
Tests may be carried out either with a support system (supporting idlers) or without it. Both ends of a conveyor belt are fixed into hydraulic jaws and the belt is stretched using a force corresponding to 1/10 of the nominal tensile strength in the longitudinal direction in newtons, multiplied by the specimen width in millimetres. A hammer with a required impactor attached to it, of a predetermined weight, is elevated by a pulley to a required height and then dropped in free fall onto the stretched conveyor belt.
The resistance of conveyor belts to punctures is defined as the ability of a conveyor belt to absorb the impact energy formed at the impact of a material onto the belt, i.e., absorb the entire energy by deformation processes in a conveyor belt without any resultant damage to the belt. If the impact energy is greater than the ability to absorb the entire impact energy, the conveyor belt suffers significant damage, primarily to the top cover layer, in the form of indentations, punctures, or longitudinal and transverse scratches; moreover, a puncture causes damage to the carcass of the conveyor belt which leads to the loss of its functionality. The evaluation of a test in puncture resistance testing comprises visual inspection of the conveyor belt, followed by the identification, based on the recorded measurements, of magnitudes of the impact force and the tension force at which the puncture occurred.
2.2. Impact Energy Identification
For the purpose of identifying the impact energy, i.e., the energy consumed by deformation work during the test, it is first necessary to calculate the total potential energy E
P of the hammer fixed at height h:
where m—impactor mass [kg]; g—gravity acceleration [m · s
−2]; and h—impact height [m].
The maximum potential energy is affected by the hammer mass and impact height. After the hammer is released to drop, its total potential energy changes into kinetic energy, and the hammer reaches its peak kinetic energy at the instance of impact into the stretched belt specimen. After the hammer falls onto the belt specimen, it bounces off the belt to a height h
z as a result of an unconsumed portion of the total potential energy, i.e., a residual positional energy E
Pz which is calculated using the following equation:
The impact energy E
Pr (impact work) is determined by the difference between potential energies:
4. Conclusions
At present, belt conveyors are regarded as a reliable conveyance equipment that is used in multiple industries, including mining, metallurgy, mechanical engineering, agriculture, and the food processing industry, where their deployment accelerates processes and saves costs. In order to achieve that, a conveyor belt—the key conveyor component—meets all the requirements; it must resist punctures and the effects of dynamic impact stress. A belt’s resistance to punctures is actually its ability to absorb the impact energy. This energy arises when a material falls onto the belt. In laboratory conditions, transported materials are replaced with a hammer to which impactors of various shapes are attached.
This paper deals with the investigation into the energy balance of a rubber–textile conveyor belt, the P2000/4 8 + 4 201A type, during the impact process. Impact stress was exerted using a hammer with a spherical impactor and a pyramidal impactor while various values of the hammer impact height and mass (50–100 kg; 10 kg increments) were applied. The impact heights were varied in the interval from 0.2 m to 1.4 m, with 0.2 m increments.
For the purpose of identifying the impact energy, the heights to which the hammer bounced off the tested belt after the first contact were measured. These data were used to identify residual energies as the unconsumed portions of the total potential energy. The resulting energy balance confirmed that the impact energy of the hammer with the pyramidal impactor was higher than the impact energy of the hammer with the spherical impactor. The pyramidal impactor corresponds to a sharp-edged transported material which causes damage to a conveyor belt due to tribological interactions and due to the impact of the material at the chutes, which contributes to the damage to the structural components due to dynamic stress.