Dynamic problems in the operation of overresonant vibrating screens

The paper presents the results of the numerical analysis of the stress state (FEM) which is created in the construction elements of the PZ3R vibrating screen in normal operation conditions. These results where compared to and verified by tensometric, displacement and acceleration measurements. The results of strength analyses indicated the directions of possible modifications of structural elements of the screen, and also confirmed the desirability of their implementation, but in strictly specified order.


Introduction
Designing a structure is a difficult process, especially when its dynamic loads are not fully recognized, which in the next step does not allow for proper assessment of the stress in its elements.
It turns out that constructory experience is not enough for a newly designed structure to work properly, although it already has a predecessor in its class, the operation of which does not raise any objections [1]. This is the problem with the construction of the PZ3R vibrating screen, which was designed like a PZ screen, introducing changes, mainly dimensional ones, [2]. The elements of the new PZ3R screen construction began to crack after a relatively short period of device operation. The authors of the study decided to explain the reasons for this state of affairs, based on the results of analyzes of the stress state which arises in the construction elements of the vibrating screen in normal operation [3]. These changes have resulted in different kinematic characteristics of individual points along the length of the screen. There are vibrations from linear near the center of gravity of the vibrating screen to flattened ellipses of different inclination [5] near the supports. Three PZ3R screens were installed in one of the Polish mines for the preliminary classification of copper ore. During use, some structural elements began to break. Among others: • cracking in the area of the props of the riddle side (Fig.2), • permanent deformation of the support (Fig.2). Attempts have been made to restore the operating efficiency of the vibrating screen, by strengthening places particularly vulnerable to damage, which only temporarily reduced adverse phenomena. In order to comprehensively solve the problem, a mechanical (numerical) analysis of the screen's structural elements was performed, the results of which were verified by an experiment on the real object through tensometric measurement of strains (stresses) at the most intense points of the structure [5]. The results of these analyzes should indicate the directions of possible modernization of structural elements and, consequently, ensure full operational efficiency of the system.  Based on the results of the strength analysis [5,6], the user introduced a number of construction changes. These changes included ( Fig. 3): • Shortening the reach (distance from side to axis of the spring) of the support by 140mm • Change in the structure of the side reinforcing ribs in the top and bottom support The conducted strength analysis after the changes [5] indicates the desirability of redesigning some elements of the screen, including supports. Strength analysis of the structure in which the reach of the support has been shortened by 140mm indicates that the local redesign of the support does not have any significant effect on reducing the stresses in the screen structure elements. Only the solution including additionally reinforced sides of the screen with C280 profiles results in significant reduction of stresses in the analyzed areas of the structure [5].

Analysis of the dynamics and kinematics of the vibrating screen
As part of these tests, it was performed: • Traces of the screen vibrations trajectory in steady and transient states [7] • Research on vibrator self-synchronization [8,9] Moreover, in the field of sieve operation dynamics, the amplitude of fixed vibrations A and the required uplift index [4,10] were determined for proper operation of the vibrating screen in order to achieve the assumed capacity. The current values of these parameters are [1]: • Vibration amplitude A=5.6mm against the required A=7mm • The u 2 =3.9 against the required u 2 =5.887.67 (u 2 -factor which define ratio of inertia force acting on single piece of material on vibrating screen to its gravity force) It means that the dynamics of the screen in relation to the design assumptions has significantly decreased. In order to obtain the amplitude of vibrations A=7mm, the static moment of the vibrator should be increased. This change would cause the screen to operate at the maximum force, which would increase the strength problems of the structure [5]. Therefore, in relation to the construction of which the results of strength tests are described in [5], another structural change was introduced, consisting in changing the shape of the expansion beams connecting the ridding walls (Fig.1, 2 [5]). This change caused a further increase in the stiffness of the riddle. The research on the analysis of the dynamics and Reach of the support Reinforcing ribs kinematics of the screen described in this study concerns a slightly different design, with a different stiffness than those described in the study [5].

The effect of structural changes on the stress state
The analysis of structural changes was made in such a way as to show the influence of each modification on the level of stress in the entire structure of the screen, which allowed to analyze its suitability.
All strength analyzes included in the study [5] were carried out taking into account the proposed modifications of the structure taking into account: • Material changes • Changes in support conditions • Different load cases The strength analyzes carried out in [5] showed a number of constructional irregularities causing concentration of stresses. For example, Figure 4 shows the concentration of stresses around the upper and lower supports.

Measurements of displacements, accelerations, deformations (stresses) of the actual structure of the PZ3R screen
The obtained results of the strength (numerical) analysis were verified by measuring strains (stresses) on the real object, in the most intense points of the screen structure elements selected as a result of the numerical analysis. In addition, measurements of displacements and accelerations of selected points of the screen structure were carried out.

Location of strain gauges on the construction of the PZ3R screen
Tensometric rosettes were placed around the location of the bottom support of the vibrating screen, three pieces on each side of the structure. Rosettes type "delta" No. 2 and 4 were mounted in the side part of the bottom support (Fig. 5). Rosettes "delta" type No. 1 and 3 were mounted in the bottom part of the lower supports of the vibrating screen. In the upper part of the lower legs, two-sensor rosettes No. 5 and 6 are mounted below support plate. Each measuring strain gauge was temperature compensated. During the experiment strains (stresses) of the above described points of the screen structure were made for the following phases of its operation [11]: • Start-up, • Steady work, • Braking, • Free run.
For example, in Fig. 6, the change of stress in all three phases of the screen operation registered by the No. 1 rosette, stuck -as numerical stress state analysis showed -in the area of the highest stress concentrations. The maximum stress values were indicated by the tensometric rosette No. 1 (σ max 190MPa) during system braking (Fig. 6). At the same time, the strain gauge rosette no. 2 indicated stresses σmax125MPa. The measurements were carried out after the Start-up .
Steady work Braking .
Rosette TFr-8/120 nr 2 Main stresses -measurment 1/with load modernization described in point 2 and the obtained stress values only slightly differed from the values of stresses in specific areas of the screen structure determined by numerical analysis (FEM).

Measurements of displacements
Displacement measurements were carried out using HBM WA 50 inductive sensors, connected to the MGCPlus bridge. During the measurements, displacements were recorded with three sensors mounted in three mutually perpendicular planes. The measuring instruments were mounted at the bottom left support. The WA50 sensors 1 measured the vertical displacements of the screen, the sensor WA 50 No. 2 measured longitudinal horizontal displacements and the sensor WA 50 No. 3 transverse horizontal displacements ( Fig. 7 and 8).