Fatigue analysis of the Self-balancing Racking Screen
The occurrence of fatigue cracks in a self-balancing racking screen in the vibration zone is a significant technical problem and it 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 and indicate the probable places of crack formation, and provide recommendations on possible design reinforcements.
Finite element model (FEM) of the self-balancing racking screen was prepared (Figure 2).
Self-balancing racking screens are used 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. These parameters 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.
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 permissible fatigue stresses (120 MPa).
Fatigue endurance curve showing the relation between the level of the 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 to detail FEM of the racking screen with the reason to obtain dynamic stresses in the weld areas on operating and multiple frequencies (1f, 2f, 3f).
Figure 1. 3D model of the self-balancing racking screen