Fatigue analysis of the Self-balancing Racking Screen
The occurrence of fatigue cracks in a self-balancing type screen in the vibrator zone is a significant technical problem and leads to the destruction of the sides of the screen body.
To improve the existing structure, it was necessary to assess a strength check according to the criteria of fatigue life of the structure, and indicate the probable places of crack formation, as well as give recommendations on possible design reinforcements.
Finite lement model (FEM) of the self-balancing racking screen was prepared (Figure 2).
Self-balancing racking screens are designed for wet and dry screening of bulk materials. The amplitude of the vibrations of the screen is usually 3.0 - 4.0 mm and frequency of the forced oscillations is 16 - 25 Hz and depend on the size of the fraction of the feed material.
The screening surfaces in a self-balancing racking screen are horizontal or can be installed with a slight slope for a better screening process.
Vibration exciters are two integrated vibrators with a flange mount on the screen body. To obtain a uniform oscillation field, the resulting driving force passes through the center of gravity of the screen.
3D model of the self-balancing racking screen is shown in Figure 1.
Figure 2. Finite element model of the racking screen.
There are various standards for calculating the fatigue life of structures. In this case, the German standard DIN 15018 was used.
According to the selected standard, the type of welding (W0) and the class of use (6) were determined for which permissible fatigue stresses (120 MPa) were established.
Fatigue endurance curve showing the relationship between the level of stress in the cycle and durability is shown in Figure 3.
Figure 3. Fatigue endurance curve.
Figure 4. Fatigue stress results distribution.
Calculation was carried out for given operating frequency of racking screen vibrators.
Max. stresses were localized in expected places of racking screen sides. Safety factor in hot spot area was not enough according to DIN 15018.
Design improvements were considered, checked and reported to the Client.
It was also recommended detail some components of the structure in which the identified problem areas using SOLID elements in the area of welding joints and detailed analysis of strength with the dynamic stresses arising in modes close to the excitation frequency as well as the multiple modes 1 / 2f, 2f.
Figure 1. 3D model of the self-balancing racking screen.