"SYSTEM FOR CONTROLLING THE SEPARATION EFFICIENCY OF VIBRATING SCREENS" Field of the Invention

The present invention refers to a system for controlling the efficiency in the separation of bulk material in vibrating screens of the type comprising at least one screen element with a perforated floor and which is caused to vibrate by means of a vibrating assembly comprising at least two rotary shafts, with an eccentric weight being mounted to each shaft. Prior Art

As known, the process for separating a particulate bulk material in the form of grains or particles of at least two different sizes in vibrating screens or machines consists in conveying the material along the screen perforated floor which is vibrated in a way that, upon displacement of the material, the particles pass through the floor apertures, so as to be separately released from the smaller particles. The vibrating movement imparted to the screen and to its separating perforated floor aims at promoting the conveyance of the bulk material on the perforated floor, as well as providing a sufficiently strong impulse to the material being conveyed in order to avoid a particle, which cannot pass through a screen aperture, from becoming clogged in the aperture, obstructing the passage of other smaller particles. Figure 1 of the enclosed drawings schematically illustrates a form of reciprocating linear movement "L" for a vibrating screen, which movement is obtained by the provision of at least two shafts 20 mounted, by means of bearings 21, to the screen element 10, as illustrated in figures Ia and Ib, each shaft 20 carrying an eccentric weight 30 and the shafts being rotated in opposite directions with the same frequency

and in determined phases, which will define the inclination of the linear direction of the reciprocating movement in relation to the plane of the perforated floor 11 of the screen element 10. As illustrated in figure Ib of the enclosed drawings, the synchronization between the two or more shafts 20, in order to determine a desired rotation phase of the respective weights 30, is generally obtained through appropriately designed gears 40. Figures 2, 2a and 2b are views similar to those of figures 1, Ia and Ib respectively, although they illustrate, also schematically, a possible form of elliptical movement "E" for a vibrating screen, obtained by providing three shafts 20 which are mounted, by means of bearings 21, to the screen element 10, each shaft 20 carrying the same pattern of eccentric weight 30 and with two of the shafts 20 rotating in the same direction opposite to the rotation of the third shaft, in the same frequency and in determined phases to define the inclination of the largest axis of the ellipse "E" and the dimensional relation between the largest axis and the smallest axis. The synchronization among the three shafts is also generally obtained through gears 40. Figures 2c and 2d illustrate views similar to those of figures 2a and 2b, respectively, but illustrating an arrangement of two shafts 20 rotating at the same frequency and in opposite directions, each carrying an eccentric weight 30 with a value different from the other weight, so as to produce an elliptical movement

"E" , such as the one exemplified in figure 2. The separation efficiency obtained through the vibrating screens for classifying bulk material processed in mining installations has been a constant worry, since the function of the vibrating screens is

to separate the rejected fraction from the product. Efficiency means the quantity of passing particles that are retained inside the rejected fraction in relation to the total content of said passing particles being supplied to the screen 10.

The separation efficiency of the screen 10 depends on the thickness of the bulk material layer being displaced on the perforated floor 11 and on the time the bulk material remains on said perforated floor 11 (time of permanence) . Thus, a very thick bulk material layer impairs the separation, requiring a longer time of permanence of the bulk material on the perforated floor 11 and consequently, a lower displacement speed of the bulk material on the screen element 10, in order to increase the separation result.

Considering that the thickness of the bulk material layer being displaced on the screen element 10 results from both the feeding volume and the displacement speed, one should look for a solution of balance with a speed that allows to simultaneously achieve an optimized thickness of bulk material layer and an optimized time of permanence of the bulk material on the perforated floor.

The conveying speed (displacement) of the bulk material in a vibrating screen may be adjusted within certain limits, by altering the vibration intensity

(amplitude or rotation) or adjusting the angle of vibration in the case of a linear movement "L" or elliptical movement "E", by modifying the horizontal conveying speed.

The type of adjustment mentioned above is frequently carried out during the installation of the machine when the operational parameters of the vibrating screen are selected, by determining a conveying speed which may produce a bulk material layer on the screen

element 10 with a thickness maintained within an acceptable superior limit, in order to lead to a maximum time of permanence of the particles on the vibrating perforated floor 11. However, in many cases, the feeding rate and the percentage of passing particles are not maintained constant during the operation of the vibrating screen, leading to instantaneous efficiency losses. The increase of the feeding rate can lead to a very thick bulk material layer, considerably impairing the separation. In this situation, a speed rise becomes a benefit, even shortening the time of permanence. Nevertheless, in the known vibration screens, the modification of the dynamic operational parameters of the screen requires to stop the machine and carry out time-consuming manual adjustments in the driving mechanisms of the shafts that carry the eccentric weights . On account of the time expended with the machine stop to adjust the parameters, due to a variation of the feeding rate which can suffer, soon after the adjustment, a new increasing or decreasing alteration, the known vibrating screens operate with a fixed adjustment for the dynamic parameters, losing the medium efficiency when the feeding rate is higher than the nominal one and without using the possibility of maintaining or even increasing the efficiency when the feeding rate is reduced. In this case, the conveying speed could be reduced, by increasing the time of permanence of the bulk material on the screen element

10.

Summary of the Invention

As a function of the deficiencies and drawbacks of the known vibrating screen, it is an object of the present invention to provide a system for controlling the

separation efficiency of said vibrating screens, which allows optimizing, continuously and automatically, the separation efficiency during possible variations of the bulk material feeding rate, with no need to stop the machine .

It is a more specific object of the present invention to provide a system for controlling the separation efficiency as defined above, which allows optimizing, continuously and automatically, the separation efficiency, maintaining the thickness of the bulk material layer and its time of permanence on the screen element within optimized values, by varying the conveying speed of the bulk material on the perforated floor of the screen element, altering the angle of vibration of the screen by varying the phase relationship between the shafts that carry the eccentric weights.

An offset of 90 degrees between the shafts with eccentric weights generates a vibrating movement with an inclination of 45 degrees (half the offset angle) . An alteration of an additional offset angle between the shafts, controlled by electronic system, alters the angle of vibration in a value corresponding to half the addition or subtraction effected in the offset angle.

It is important to note that the control of the angle of vibration of the screen may be executed in a wide range, the vibration direction ranging from totally horizontal or totally vertical. The system described above is applied both to the arrangement of two shafts with equal counterweights, controlling the inclination of the vibration line, as well as to the arrangement with two shafts having unequal eccentric masses, controlling the inclination of the largest axis of the ellipse.

As previously mentioned, the system for controlling the separation efficiency is applied to vibrating screens for classifying bulk material and of the type which comprises at least one screen element having a perforated floor and a vibrating assembly, which comprises at least two rotating shafts transversely mounted to the screen element and each carrying at least one eccentric weight and an electric motor coupled to the shaft, in order to make it rotate with the eccentric weight in synchronized opposite directions, and thus vibrate the screen element. According to the invention, the control system comprises a load sensor operatively associated with the screen element, so as to produce a signal representative of the bulk material load being fed to the screen element; a position sensor operatively associated with each shaft, so as to produce signals representative of the angular position of the respective shaft and respective eccentric weight; a frequency converter disposed in the power supply of at least one of the electric motors, each associated with a respective shaft; and a processing unit, which is operatively associated with the load sensor, the position sensors and the frequency converter, in order to instruct, through said frequency converter, a determined phase relationship between the shafts and a consequent conveying speed of the bulk material on the screen element, which speed is necessary to maintain the thickness of the bulk material load and its time of permanence on the screen element within predetermined values .

In the solution described above, in which the vibrating assembly comprises two or more shafts, the deviations in the feeding rate of the bulk material supplied to the screen element are detected by the

load sensor and transmitted to the processing unit. The invention provides a position sensor operatively associated with each shaft, in order to produce signals representative of the angular position of the respective shaft and respective eccentric weight, which signals are transmitted to the processing unit which, through at least one frequency converter, provides an adjustment in the phase relationship between the shafts, altering the inclination of the main vibration line and consequently varying the conveying speed of the bulk material, in order to adjust said speed to the bulk material feeding regime, maintaining the thickness of the material layer on the perforated floor within predetermined limits. Brief Description of the Drawings

The invention will be described bellow, with reference to the enclosed drawings, given by way of example of possible ways of carrying out the invention and in which: Figure 1 schematically illustrates a known form of reciprocating linear movement for a screen element; Figure Ia also schematically illustrates a lateral view of a screen element provided with two transverse rotary shafts, each carrying at least one eccentric weight, according to the prior art;

Figure Ib schematically illustrates an upper plan view of the screen element illustrated in figure Ia and having its two transverse rotating shafts of the vibrating assembly connected together by gears of a known prior art construction;

Figures 2, 2a and 2b are views similar to those of figures 1, Ia and Ib, respectively, but also schematically illustrating a possible form of elliptical movement for the screen element obtained by the provision of three transverse shafts, all carrying

the same pattern of eccentric weight, according to the prior art ;

Figures 2c and 2d illustrate views similar to those of figures 2a and 2b, respectively, but illustrating the arrangement of two transverse shafts rotating at the same frequency and in opposite directions, each carrying an eccentric weight with a value ^' different from that of the other weight, also according to the prior art ; and Figure 3 is a schematic and simplified upper plan view of a screen element, whose vibrating assembly is defined by a pair of transverse shafts, each driven by a respective electric motor. Detailed Description of the Invention As illustrated and already mentioned above, the invention is related to vibrating screens for classifying bulk material and, more specifically, to those type of screens comprising at least one screen element 10, generally in the form of an elongated chute with a substantially "U" -shaped profile and having a perforated floor 11 on which is conveyed a continuous load of bulk material, such as different types of ores. Figure 3 illustrates an arrangement to which the present control system can be applied, in which the screen element 10 presents a vibrating assembly VA defined by a pair of transverse shafts 20, each carrying, at an end thereof, a respective eccentric weight 30, each shaft being also mounted to the screen element 10 by means of respective bearings 21. In the construction illustrated in figure 3, each shaft 20 is coupled to a respective electric motor 60 by transmission means which can include pulleys 61, 62 and belts 63, or other transmission arrangement adequate for transmitting torque from the shaft of

each motor 60 to the respective shaft 20 of the vibrating assembly VA.

The present system for controlling the separation efficiency applied to the arrangement illustrated in figure 3 comprises a load sensor LS operatively associated with the screen element 10, in order to produce a signal representative of the bulk material load being fed to the screen element 10. The load sensor LS may be defined by a bulk material flow sensor disposed immediately upstream the screen element 10, for example close to the means which conduct the bulk material to the screen element 10. In another possible construction, the load sensor LS may be defined by one or more weight sensors mounted in a suspension frame suitable for the screen element 10, said frame not being illustrated and described on account of not being part of the present invention. Another possible construction for the load sensor LS is the provision of a plurality of optical meters provided in the screen element 10, so as to detect at least one value for the thickness of the bulk material load passing on the screen element 10. A further alternative for the load sensor LS is defined by depth ultrasonic sensors mounted in the screen element 10, in order to detect the thickness of the bulk material load being displaced on said screen element 10. The main purpose of the load sensor LS is to determine the thickness of the bulk material load passing on the screen element 10 and to supply the system with a signal representative of the detection carried out for further processing, as described ahead.

In order to provide adequate separation efficiency, the thickness of the material layer in an outlet end of the perforated floor 11 must stay within limits of 1.5-4 times the desired separation size.

Thus, upon detecting an anomalous rise in the thickness of the bulk material load being displaced on the screen element 10, the load sensor LS informs the processing unit 80 about said anomalous condition. The system further comprises a position sensor PS operatively associated with each shaft 20, so as to produce signals representative of the angular position of both the respective shaft 20 and the respective eccentric weight 30. It is further provided a frequency converter 70 disposed in the power supply of at least one of the electric motors 60, as well as a processing unit 80, which is operatively associated with the load sensor LS, position sensors PS and frequency converter 70, in order to instruct, through said frequency converter 70, a determined phase relationship between the shafts 20 and thus a consequent conveying speed of the bulk material being displaced on the screen element 10, which speed is necessary to maintain the thickness of the bulk material load and its time of permanence on the screen element 10 within predetermined values.

In the construction of figure 3 , there are provided two shafts 20, one being driven by the respective motor 60, with no possibility of the latter having its driving characteristics modified by the present control system, only the other shaft 20 being operatively associated with the respective motor 60, whose drive is commanded through the frequency converter 70. This arrangement allows a simplified construction to be obtained, without affecting the possibility of the phase relationship between the two shafts 20 being appropriately adjusted through only one of the motors, as a function of the bulk material feeding regime .