A STRUCTURAL METHOD FOR THE ASSESSMENT OF RECYCLABILITY OF AGRICULTURAL MACHINERY

Czesław Rzeźnik, Piotr Rybacki

ABSTRACT

Threat to the natural environment posed by technical objects, including agricultural machines, taken out of use is becoming an important problem in modern societies. Their management should be arranged through recycling. The recyclability of machines varies and at the stage of production ought to be determined and evaluated using appropriate methods. The methods used at present are either very complicated and difficult to use or over simplistic, so it is impossible to obtain objective evaluations. This study presents an original method to assess the recyclability of agricultural machinery, which takes into consideration the material composition and structure of machines, and partly eliminates the drawbacks of the methods used so far.

Key words: agricultural machine, method, recyclability..

INTRODUCTION

The awareness of the threats to the natural environment has resulted in a situation in which at present every product is evaluated not only in terms of its functional quality connected with its function, but also from the point of view of its destructive effect on the environment. Agricultural machines, the destruction impact of which is manifested throughout their whole service life, even after they are taken out of use, are also subject to such evaluation. This last stage of the service life of a machine requires such management of the remaining materials and energy that their maximum amounts are reintroduced to the production process, which will result in the minimization of the amounts of wastes and will limit their threat to the environment.

Leaving a machine taken out of use on the farm causes a considerable threat to the environment. Soil is contaminated by the leaking operation liquids, and products of corrosion, decaying paint and varnish coats are released to it. Scraping the machine as a whole results in the situation when ferrous alloys, non-ferrous metals and their alloys, rubber and plastics are left at the scrap-metal depot. Such an action causes an irreversible loss – as a result of metallurgical processes – of expensive non-ferrous metals. The presence of rubber and plastics also results in the deterioration of the quality of metallurgical products, while the process of their incineration causes the emission of pollutants to the atmosphere.

It may be prevented by the dismantling of the machine, segregation into material groups and their proper management. Such a procedure is called recycling and ought to be applied also to agricultural machinery [1, 6].

Recycling is the reuse in the production processes of machines taken out of use, their parts and materials, from which they were made, which in turn will limit the amount of wastes. Depending on the manner of the utilization of these materials, recycling is classified into the recycling of products, materials and energy.

From the point of view of recycling, each agricultural machine is a set of parts made from various materials. The type of these materials is determined by the functions, which a given part is to perform in the machine, and their selection is made at the stage of designing the machines. Main structural materials used nowadays are metals and their alloys, which may be reused in production almost in 100% and which constitute a limited threat to the environment. However, along with the progress in technology plastics have started to be used in increasing amounts. It leads to problems with their management after machines are taken out of use, connected primarily with the necessary comprehensive dismantling and segregation into material groups. Generally these properties, which make a machine taken out of use easily recyclable, are called recyclability. This property makes it possible to evaluate every machine in terms of its capacity to be easily recycled.

OBJECTIVE

The aim of his study is to develop a method to evaluate recyclability of agricultural machinery, which will make it possible to determine which of them in the future will be recycled to a greater degree and thus – will pose a lesser threat to the natural environment.

Methods used so far take into consideration only the content of materials – recyclable and non-recyclable, and do not consider the design of the machine. It is a gross oversimplification, as it does not consider problems connected with obtaining individual groups of materials. This problem is especially apparent in case of recycling of electronic equipment [2, 4], as well as other modern machines. Other methods are very complicated (economic balance), which prevents their wide-spread use.

METHODS

From the point of view of recycling technical equipment at the stage of their taking out of use a very important issue is the dismantling process in itself, which is a set of operations of dividing the machine into structural elements. In the recycling process of the agricultural machines taken out of use disassembly is performed to various degrees, the final products of which are assemblies or parts. They frequently are then subjected to the processes of regeneration, constituting starting material for the operation of assembly as replaceable parts for another machine. They are also used in the production process of new parts as secondary materials or are deposited at the hazardous waste disposal sites.

In order to develop a method to evaluate the recyclability of agricultural machinery a structural model needs to be developed for the disassembly of a machine.

In the first stage of modeling characteristic objects were distinguished in the process of disassembly. These include disassembly units and operations of their disassembly, taking into consideration the fact that operations determine to some extent the relation between the dismantled assembly or part and the rest of the machine.

Agricultural machines may be treated as a certain set of parts differing in shape, performed functions, weight and the type of material, from which they are manufactured, equation 1.

M = {C}      (1)

This set may be divided into many subsets according to different criteria. From the point of view of recycling the important division is the division into two subsets according to the criterion of the type of the used material. One of the subsets contains parts made from materials, which are usually purchased and reintroduced to the production process through material recycling. These materials include ferrous alloys, non-ferrous metals and their alloys. This subset of parts is defined as parts made from recyclable materials and denoted as Cri. This set includes also parts destined for regeneration, subjected to product recycling. The other subset is composed of parts made from materials which are not commonly purchased and included into the production process, e.g. plastics, rubber, etc. These parts are called non-recyclable and are denoted as Cr_j. These materials are a burden to the natural environment in the form of wastes. The assignment of individual parts to a respective group is not permanent and has to be defined for specific conditions. A tendency is observed to include an increasing group of materials in re-processing and re-introduction into the production process. From the point of view of recycling a machine may be treated as a set of recyclable and non-recyclable parts, equation 2.

(2)

Parts made from non-recyclable materials constitute a threat to the environment and it is not their number in the machine, but their total weight that is essential. Thus, the weight of a machine is the sum of weights of recyclable parts m_ri and non-recyclable parts m_rj, as expressed in equation 3.

(3)

The value of these weights may be relatively easily determined having the technical documentation of the machine.

In order to develop a universal index, which will make it possible to evaluate objectively the recyclability of a given machine, irrespective of its total weight, it may be presented in the form of a quotient [3], equation 4.

(4)

This index facilitates reliable evaluations of machines uniform in terms of materials used, generally of older designs, in which the only non-recyclable material is rubber, from which tires are made. The basic materials for these machines are low-carbon steel and cast iron.

Modern agricultural machines are increasingly complex in terms of the material used and their division into material groups requires disassembly to a rather considerable degree. The proposed method has to take into consideration this property of a given machine. Treating a machine as a set of parts produced from various materials is an oversimplification.

In an actual machine specific relationships exist between its individual components in the form of various joints, ensuring proper fastening and location. These connections constitute relations between parts of the machine. Thus, every machine may be presented as a set of parts and a set of relations between them [5], equation 5.

(5)

The formulation of a mathematical model of such a machine was started from a graphical model in the form of relations, on the basis of an example. It was assumed that a machine with a certain weight consists of C = 5 parts, each of them with the weight of 0.2m (of the machine weight). Four parts were made from a recyclable material, and one part from a non-recyclable material. To present its structure a directed graph was used in the form of a tree, since at the disassembly a specific order of operations is required, resulting from the design of the machine, thus it is a unidirectional function. It will be a cohesive and acyclical graph with the number of edges expressed by equation 6.

k = n – 1        (6)

where: k – the number of graph edges (disassembled joints);
n – the number of graph nodes (disassembled units).

In the relation model graph nodes will be components of the machine, whereas edges will correspond to the operations of their disassembly.

It results from Figure 1 that three machines differing in the design may be created from the five parts. Let us assume that the non-recyclable part is part no. 2. In order to utilize the analyzed machine through recycling part 2 should be separated as a material group, which means the disassembly of part no. 2. The dismantling of this part requires the disassembly of the part preceding it in the structure, which makes the process difficult. While evaluating the recyclability of machine in Figure 1, according to equation 4, all the structural variants will be characterized by the same coefficient Wr₁ = 80%. In order to isolate part no. 2 as a material group from Figure 1a parts nos. 5, 4, 3 and 2 need to be dismantled, i.e. 80% of the machine weight have to be disassembled. For the structure in Figure 1b the same parts need to be dismantled as in case of Figure 1a and the same part of the total weight of the machine. In case of the st ructure in Figure 1c the isolation of part no. 2 as a material group requires the disassembly of only part 2, i.e. 20% of the weight of the machine.

It results from the presented analysis that not only the percentage of recyclable and non-recyclable materials in the design of the machine determines the recyclability of the whole machine, but also the structure of the machine, which has a significant effect on the easy isolation of a material group.

In the mechanical engineering practice this issue is solved by the application of a module or panel machine design. Such structures make it possible to isolate individual material group easily and fast. Moreover, parts made from materials requiring isolation for the sake of recycling should not be embedded deep in the machine design, which facilitates their disassembly.

For a specific machine structure, resulting from the designing process, the location and number of parts requiring isolation as material groups are important, as it is presented on the following example.

In the next example it was assumed that a machine consists of ten parts each with the weight of 0.1m (of the machine weight), in which two parts, numbered 3 and 5, require isolation as non-recyclable material groups and which are located differently in the machine structure (Fig. 2).

The conducted analysis and its results presented in Table 1 indicate that in spite of the identical structure the different position of these parts in the structure requires various degrees of disassembly. The most advantageous variant is variant a, where these parts are easily accessible. The most disadvantageous is variant “c”, where it is necessary to disassemble the whole machine.

This problem may be presented analytically. If a user has to dismantle the whole machine or disassemble it to a very high degree in order to isolate individual material groups, then he may decide not to perform these operations and leave the machine as a waste posing a threat to the natural environment. In case when the scope of disassembly is small it is probable that the user will dismantle the machine and utilize the obtained material groups through recycling. For the purpose of the valuation of this property, the coefficient of machine structure k_s was introduced, which is a complement of the probability of event p_s, the decision against the machine disassembly, equation 7.

k_s = 1 – p        (7)

If the structure is very complex, then in spite of the fact that the machine consists of a specific, although differing weight of recyclable parts, (index W_r1 will have a value bigger than zero) users of this machine will decide against its disassembly and utilization through recycling, and p_s will take the value of 1, while k_s will be equal to 0. A very simple machine structure means the probability of the decision against the disassembly and the division of the machine into material groups p_s will take the value of 0, and k_s will be equal to 1.

It results from the conducted reasoning that recyclability of a machine W_r is directly proportional to the value of coefficient k_s. Equation 4, defining recyclability of machines was supplemented with coefficient k_s and took the form of equation 8.

(8)

It is then necessary to determine the value of probability p_s, which is dependent on p_r, i.e. the probability of the decision against the disassembly of the machine into material groups, due to the number of joints which have to be disassembled, and on the probability p_m of the decision against the disassembly due to the weight of parts requiring disassembly for the purpose of the division of the machine into material groups. Probabilities p_r and p_m were assumed as probabilities of independent events and their effect on the value of probability p_s is expressed by equation 9.

(9)

The value of probability p_r is directly proportional to the number of relations between material groups expressed in the form of a directed number, as presented in equation 10.

(10)

where: R_d – the number of joints requiring disassembly for the purpose of the division of the machine into material groups;
R – the total number of joints in the machine;

Probability p_m is directly proportional to the weight of parts requiring disassembly for the purpose of the division of the machine into material groups, and it is inversely proportional to the total weight of the machine, equation 11.

(11)

where: m_d – the total weight of parts requiring disassembly for the purpose of the division of the machine into material groups;
m – the total weight of the machine;

In order to check whether the conducted analysis of the machine structure and its effect on the recyclability of machines is logically correct, example from Figure 2 was used. In this example a machine consists of ten parts, the weight of each is 0.1m (of the machine weight), two parts are non-recyclable and they are located differently in the specific machine structure. The results of the calculations of the coefficient of recyclability W_r1 and W_r are presented in Table 2.

The results presented in Table 2 confirm the logical correctness of the developed method. Machines in Figure 2, evaluated only using the percentage of the weights of recyclable and non-recyclable materials have the same coefficient of recyclability W_r1 amounting to 80%. The practical realization of the recycling of these machines will depend on the structure of these machines and related problems with their disassembly. The coefficient of machine recyclability W_r formulated in this study describes also the structure of the machine, which makes it more precise and practical in use.

The conducted analysis of the machine structure and the position of specific material groups or parts in its structure is logically described by coefficient k_s. Its value may also be the result of empirical estimates carried out for a specific group of machines. Designers of machines should try to create machines with such structures so that the separation of individual material groups is as easy as possible, as is described by the model of assessing the recyclability of machines formulated in his study.

The formulation of this model is possible on the basis of the technical documentation of the machine and its material specification. The created model is a structure in the form of a dendrogram, on the basis of which an algebraic model was formulated, equations 7, 8, 9, 10, and 11.

This method makes it possible to perform simulation studies and to evaluate the effect of material changes in the parts, the machine structure and the position of specific material groups in the structure on the recyclability of the machine.

CONCLUSIONS

The analysis conducted in this study makes it possible to formulate the following conclusions:

1. The proposed method to evaluate the recyclability of agricultural machines takes into consideration not only the type and weight of recyclable and non-recyclable materials, as has been done so far, but additionally makes it possible to estimate numerically the effect on this parameter of the accessibility of these parts, i.e. the easiness of their disassembly and the separation into material groups.

2. The application of the method proposed in this study to evaluate the recyclability of agricultural machines will facilitate insight into the structure of a machine and the process of its disassembly, which will lead to its theoretical investigations. This method may also give practical effects by the elimination of the necessity to have prototypes of machines for studies and thus – to reduce costs of the studies on the recyclability of a new machine.

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