This article will present a successful case study carried out by Kot concerning the structural analysis of a forklift truck. Before starting the case study, some basic auxiliary concepts are shown below.

Basic concepts of structural analysis

• Yard machines: these are widely used, especially in mining and port activities for handling bulk cargo;
• Forklift trucks: yard machines used to form stacks of bulk materials;
• Tripper: mobile belt conveyor, connected to the forklift, responsible for lifting the material supplied by the conveyor from the yard to the stacking machine;
• Bar element: a type of one-dimensional element used in the finite element method (FEM). It has the geometric property of a cross-section and is often used to represent metal profiles;
• Shell element: a type of two-dimensional element also used in FEM. Its geometric property is its thickness and it is commonly used to represent structures made up of metal sheets;
• Plasticization: behavior that materials exhibit when subjected to stresses that generate internal tensions beyond their elastic limit. Plasticization is exemplified by permanent deformations in the material;
• Utilization index: ratio between the acting forces (or stresses) and those resisting the structure. Calculated values less than or equal to 1.0 correspond to the normative approval condition.

The forklift analyzed by Kot has a design capacity of 20,000 t/h (tons per hour) and is responsible for forming the pile of ore in a stockyard.

3D modeling

The equipment analyzed consists of the forklift and tripper, which were modeled in finite elements to enable the study to begin. For the trussed structures, bar elements were used, as they are profiled. The other machine components, which are more complex geometrically, such as variable section profiles, sheet metal, stiffeners and local stiffeners, were represented using shell elements. 

As can be seen in Figure 1.1, the forklift model encompasses all the subassemblies of the equipment:

• Tricks;
• Table and portal;
• Turning system;
• Mast;
• Launch;
• Counterbalance;
• Intermediate mast; 
• Rods.

Figure 1.1: Forklift model. [1]

The tripper model, illustrated in Figure 1.2, shows all the metal profiles used.

Figure 1.2: Tripper model. [1]

The loads applied to the structures were combined as established by the standard, so that different operating situations could be evaluated, covering all the normative cases. Complementary combinations, defined by Kot's experience, were also used.

Some of the loads applied were:

• Own weight of the structure and installed equipment;
• Material load and fouling;
• Belt tension in the permanent and transient regimes;
• Overloads;
• Shock against the pile of material.

Structural analysis

The first static analysis was carried out. The profiled structures showed no non-conformities. On the other hand, the shell-modeled structures showed utilization rates above the admissible level in the boom region, under normal operating conditions, as shown in Figure 1.3.

Figure 1.3: Utilization rates in elements of the boom discharge region. [1]

In the area where the mast connects to the slewing bogies, a non-conformity was found in the mast beam stiffener, which does not have a complementary plate to soften the stresses acting in the area. As a result, the existing element concentrates all the stress received, generating a stress peak. This failure is shown in Figure 1.4.

The direct comparison between the acting and resisting stresses alone is not always enough to establish the strength of a structure within a structural analysis. Instability phenomena in this type of equipment are very common - whether due to operating movements, operating overloads, wind loads, among other normative factors considered - and tend to appear in stress ranges that are usually smaller than the elastic limits of the materials used. In order to verify these limit states, a local buckling analysis was carried out, highlighting the regions where the highest compression loads were found and where they were most susceptible to buckling. In the lower part of the model, for example, these regions are located in the upper table of the equalizing beams, as shown in Figure 1.6.

Figure 1.6: Maximum compressive stresses in the bottom model. [1]

Once the static resistance of the structures has been verified, the useful life of the equipment is assessed using fatigue analysis. Within structural analysis, this assessment consists of determining the operational resistance, i.e. the capacity of the structures to withstand the nominal stresses throughout their working life, and is usually carried out according to the equipment's cyclical operating regimes.

For the forklift, a 35-year operating time was considered in the fatigue analysis. On the mast, utilization rates above the admissible level were found in regions which, in the static analysis, tended to undergo plasticization, presenting stress concentrators. As a result, the points shown in Figure 1.5 have potential for crack nucleation and propagation.

Figure 1.5: Maximum fatigue utilization rates on the mast. [1]

The different types of connections between the structures were also analyzed, including rigid and flexible, welded and bolted connections. In this evaluation, the beam connections shown in Figure 1.7 failed for the combination relating to shock loading against the ore pile. Although the standard on which the verification was based does not provide for this type of loading for forklift trucks, Kot Engenharia included it in the structural analysis because it is a possible situation which, if it occurred, would be critical for the structure.

Figure 1.7: Profiles with failed connections in the pile-driving combination. [1]

The flexibility check, responsible for calculating and evaluating the structure's displacements, was also carried out. The conclusion of this stage resulted in warnings for the platforms and flanges (main longitudinal profiles) of the boom. Although this does not affect the operation of the equipment, it is important to monitor these areas, as displacements above the permissible level can cause damage to non-structural elements and discomfort for users.

Another important check for forklift trucks is global stability. As the forklift's undercarriage has three pivot points, the equipment's stability polygon will be a triangle, as shown in Figure 1.8. Thus, each edge of the triangle represents a possible tipping axis. All the minimum stability coefficients were found to be in line with the admissible values and the forklift was considered to have passed in this respect.

Figure 1.8: Stability triangle. [1]

Finally, the machine's jacking points were checked for their structural strength, in accordance with international standards. The jacking can be carried out at different points to maintain each of the bogies, rocker arms or equalizer beams. The reinforcement ribs, in the area that serves as the jacking point for maintaining the slewing bogies, showed utilization rates above the admissible level, as can be seen in Figure 1.9.

Figure 1.9: Static analysis in the jacking condition. [1]

Conclusion

The structural analysis of the forklift, in some situations, resulted in the identification of non-conformities. As a result, Kot indicated modifications, reinforcements and monitoring. It is important to note that different regions showed stress concentrations, which led to high utilization rates in both the static and fatigue analysis. This is common when the design does not take into account discontinuities that may contribute to this effect, such as [2]:

• Sudden changes in the thickness or geometry of the cross section;
• Notches, holes and keyways;
• Error in the basic design of the equipment;
• Faults generated during the manufacturing process.

With this in mind, it is essential that projects are assessed by structural experts. If your company needs to check machinery and equipment, contact our team and find out about our services!

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References:

[1] Kot Collection.

[BUDYNAS, R. G.; NISBETT, J. K. Shigley's machine elements: mechanical engineering design. AMGH Editora, 2011.