Belt Failures Tear Up Profits
by Dr Dariusz Mazurkiewicz
One of the more common problems associated with conveyor belt trans- port in mining applications con- cerns the necessity of reducing energy consumption and enhancing the durabil- ity of the belts themselves.
Of particular importance are the strength and durability of conveyor belt joints, since, due to some unavoidable fac- tors such as manufacturing errors, they continue to be major problems that belt conveyor users have to deal with.
One possibility of increasing the dura- bility of the belt and its joints is to reduce the values of longitudinal stresses in the belt, which is why a lot of design work is being done to develop new solutions for tensioning devices.
A second area where suitable solutions for increasing the durability and strength of joints connecting belt sections can be sought is analysis and evaluation of distri- bution of stresses in the joint and of how they are influenced by the very structure of the joint.
Under industrial conditions, belts and joints are replaced chiefly when the strength of the belt core or the belt joint falls below an admissible level. Since fatigue failure of the adhesively bonded joint as an effect of the lower- ing of its strength is one of the most frequent causes of conveyor belt fail- ures, research in this field in recent years has been aimed at determining short-term and fatigue strength of conveyor belt joints, concentrating, among others, on identification of stresses in the joining area.
Of no small significance is also the fact that an uneven distribution of stresses in a joint leads to a non-recti- linear course of the conveyor belt. Thus, identification of stresses in the adhesive joint permits effective optimization of joint geometry and proper selection of construction materials, their properties, and the way the joint is made.
While evaluation of the quality of both the belt and the adhesive joint does not constitute a major problem under labora- tory conditions, proper selection of the elements of the joint (its structure and the properties of the materials used) sup- ported by a detailed analysis of the phe- nomena occurring within the joint, espe- cially in fatigue terms, is still a very diffi- cult and not fully resolved question.
What can be of help in this case is numerical analysis using the Finite Element Method (FEM), which is the most widely used method of computer- aided calculation and engineering analysis in mechanical engineering.
Solving the task of optimization of an adhesive joint using computer techniques, one may expect interesting and reliable results which should fully or partly resolve the current maintenance problems. This is highly important since, to obtain optimum properties and maintenance characteristics of a joint, one needs to have knowledge of proper use of appropriate adhesives and appropriate joining parameters.
An analysis of the results of an appro- priately verified model of a conveyor belt joint enables not only a strength analysis of this type of structure but also an assess- ment of the effect of changes in the geom- etry and the properties of the constituent materials of the joint on its strength. Bearing in mind the fact that a conveyor belt is designed and selected so that it can transfer the largest possible tensile forces acting in the different operating states of a conveyor, an effective analysis of such cases requires the use of a FEM model.
Considering the fact that none of the analytical methods enable sufficiently pre- cise determination of stress distributions in the adhesive layer of shear-loaded adhesive joints, FEM should be more widely used for
calculating the strength of this type of joints. An analysis of the results of FEM numerical calculations allows one to locate the vulnerable spots in the struc- ture, characterized by a considerable concentration of stresses which may lead to the damage of the joint.
It is commonly known that failures of the elements of the conveyor belt core have a significant negative effect on belt strength at any type of loading. In the case when one of the fabric plies of a multi-ply belt is cut through, the load carried by the cut ply is trans- ferred onto the remaining ones.
Under such a state, the loading of the plies becomes more uneven, low- ering the total loading value at which the belt breaks. Also the loading of the remaining layers of the joint is increased despite the fact that it is the fabric plies that carry the largest loads
In the case of the layers of internal (inter-ply) rubber, an analysis of the results of numerical modelling shows an increase in stresses and their concentra- tion in the layer located above the cut. The rise is considerable as the reduced stresses in the layer of internal rubber increase, on average, from 0.5 MPa to 2 MPa.
When investigating adhesive joints used for connecting conveyor belt sec- tions, one notices that, as a principle, the literature that adhesives' manufacturers provide and the technologists making such joints, all fail to analyze or publish information on the properties and char- acteristics of adhesives, confining them- selves to an evaluation of the final strength characteristics of the finished joint.
That is why it becomes indispensable to predict how an adhesive or the entire adhe- sive joint will behave under specific condi- tions at any given load. Having such data, we can best influence the strength and durability of an adhesive joint made using a specific adhesive. The strength of a shear- loaded adhesive lap joint, as is the case with conveyor belt joints, depends not only on the mechanical properties of the adhesive, but also on the dimensions of the joint.
The geometrical parameters of a joint which directly influence its strength are the width, length, and thickness of the adhesive layer. Also important is the thick- ness of the adhesively bonded elements and their longitudinal elastic modulus. In other words, the geometry of a conveyor belt joint is the length of the lap and the thickness of the adhesive layer.
Optimization of a lap, however, is not tantamount to its unrestricted lengthen- ing. Increasing the length of a lap decreas- es the mean value of shear stress, simulta- neously causing a disproportionately small increase in destructive load. This type of optimization analyses can be car- ried out using CAE techniques based on FEM models of joints.
To illustrate the analyzed problem, an evaluation was recently made of the influ- ence that changes in the thickness of the adhesive layer had on the state of stresses in the area of the joint. In the analyzed basic model, the thickness of the adhesive layer was 0.4 mm. To conduct a compara- tive analysis, two additional models were developed, in which the adhesive layers were designed at a thickness of 0.2 mm and 0.6 mm, at the cost of the rubber, which under mining conditions could correspond to a situation where the layer of the adhesive was carelessly applied. However, no significant differences in the distribution and the values of reduced stresses were recorded in either of the two analyzed cases.
It is common knowledge that the strength of adhesively bonded joints depends on the thickness of the adhesive layer and that the value of the destructive stresses in this layer is also a function of thickness.
A thicker adhesive layer transfers shear stresses better because the stress is distrib- uted in it over a larger area, decreasing, by the same token, unitary stresses in the adhesive, which leads to a reduced stress concentration. A similar situation is observed when an adhesive with a lower elasticity coefficient is used.
Optimization of those two parameters allows one to obtain a joint with best fatigue characteristics. FEM analysis is extremely advantageous for the study of this problem since results of experimental tests in this case diverge significantly from analytical results.
When choosing a belt for a specific application, one should primarily take into account the magnitude of the acting forces and the capacity of the adhesive to transfer maximum loads, as well as its resistance to fatigue or cyclic stress, which is particularly important for conveyor belt joints.
Cyclic stresses, especially those of a long-term character, have a greater nega- tive effect on the durability of the joint than permanent stresses, even those of high values. However, the fatigue strength of a joint is most strongly affected by the level of stresses in the adhesive layer, and particularly their uneven distribution and the occurrence of stress concentration sites, which can be effectively determined using CAE techniques.
*Dr Dariusz Mazurkiewicz is with the Lublin University of Technology, Nadbystrzycka 36, 20-618 Lublin, Poland.