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FIELD OF THE INVENTION The present invention concerns a vibration device, able to be applied advantageously in an apparatus for conveying a metal charge, consisting for example of scrap iron, hot or cold sponge iron (DRI), cold pig iron, or other, even in great quantities, of the order of about 3 tonnes a minute for scrap iron and up to 8 tonnes a minute for other types of charge, to a container, which can be a melting furnace, for example of the electric arc type, or a scrap-bearing bucket. The vibration device is of the type that comprises at least a pair of eccentric masses connected to each other so as to rotate in reciprocally opposite directions and in synchrony, and is able to make vibrate, at a determinate frequency, a conveyor channel and the relative bearing structure of the apparatus, with respect to the fixed part of the latter. Due to the particular angular phasing between the two eccentric masses of each pair, their assembly on the respective rotation shafts and their positioning during the start-up step, the vibration device is able to make vibrate apparatuses which are even more than 50 m long and weighing even more than 100 tonnes, without any problem connected to the resonances of the apparatuses themselves and with limited energy consumption.

BACKGROUND OF THE INVENTION The patent documents GB-A-828,219, GB-A-I, 084, 304, JP-A-55089118, JP-A-55140409, US-A-2, 951,581, US-A- 3,604,555 and US-A-3,834,523 disclose different types of vibration devices, with rotating eccentric masses, applied on conveyor apparatuses of a vibratory or

oscillating type in order to convey a metal charge to a container of a melting plant; the container can be a melting furnace or a scrap-bearing bucket, which in turn then discharges the metal charge into the melting furnace .

It is known that, when such apparatuses have to be installed in melting plants having a big productive capacity, that is to say, more than 100 tonnes per hour, they are several tens of meters long, so as to be able to offer, on the one hand, an adequate loading plane, and on the other hand a long enough segment to be able to preheat the metal charge while it is being conveyed. Each apparatus must then be sized so as to support a metal charge of the weight of several tens of tonnes. To support such high loads, more recent known apparatuses each comprise a rather heavy bearing structure, even more then one hundred tonnes, on which a vibration device is mounted. Such bearing structures have a conveyor channel mounted on the upper part, and are supported by supporting elements which allow them to oscillate, or vibrate, mainly along their longitudinal axis, while yet remaining substantially horizontal.

Known vibration devices, associated with said conveyor apparatuses, normally consist of one or more pairs of eccentric masses, which rotate in synchrony with each other, so as to generate a vibratory motion which is transmitted to the bearing structure and to the relative conveyor channel.

The bearing structure, the conveyor channel and the vibration device thus form a structural whole or group which has its own frequency of resonance. The longitudinal accelerations imparted to the structural whole by the vibration device cause a relative movement

of the metal charge with respect to the conveyor channel.

The forces generated by the rotating masses of the vibration device must therefore be not only very high, of the order of 10 ⁶ N, that is, such as to induce an adequate horizontal and alternate force to the structural whole, but also such as to impart to the latter both adequate accelerations, of the order of at least 10 m/s ² , but also a frequency of oscillation that deviates considerably from the frequencies of resonance of the structural whole, with or without the metal charge loaded therein.

In particular, from GB-A-I, 084, 304 it is known a vibratory mechanism mounted in a conveyer and comprising a driving mechanism which includes two pairs of unbalanced rotatable masses, arranged so that the two masses of each pair rotate in opposite directions and at the same speed. The two pairs of masses are connected by gearing so that one pair of masses has twice the speed of the other pair of masses.

A first technical problem, not solved by known vibration devices, is that of having eccentric masses disposed in such a manner that, rotating, they do generate said forces in an adequate manner, but at the same time they do not develop unwanted vibratory phenomena which are negative for the whole apparatus and which, furthermore, require as limited as possible an energy consumption in order to be made to rotate.

Another technical problem, not solved by known vibration devices, is to prevent phenomena of vertical acceleration from being generated on the bearing structure, at start-up of the vibration device, when the conveyor channel, the metal charge contained therein and the relative bearing structure have to be put in movement. Such phenomena would cause malfunctioning

and/or breakages to the conveyor apparatus, which would require consequent interruptions for the maintenance of the latter, with serious damage to the production process and to the melting plant in its entirety. Another technical problem, not solved by known vibration devices, is the assembly of the individual eccentric masses on the corresponding shafts which make them rotate; this assembly is not only advantageously of the removable type, but also must be made in such a manner that the masses themselves maintain their position unchanged over time and that there is no slackening or deviation of the masses with respect to the bearing shafts, in spite of the high centrifugal forces to which they are subjected during rotation. The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to solve the above technical problems .

SUMMARY OF THE INVENTION The present invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

The above technical problems are solved by a vibration device according to the present invention, which is fixed to a bearing structure of a conveyor apparatus able to convey a metal charge towards a container of a melting plant. The vibration device comprises at least a first pair of eccentric masses mounted off-axis on two corresponding bearing shafts able to rotate in synchrony with each other, so as to cause, due to the effect of the centrifugal forces generated by said eccentric masses, a vibratory motion, at a

determinate frequency, of said bearing structure and of the associated conveyor channel, in order in this way to feed said metal charge with respect to said conveyor channel in a substantially constant manner towards said container.

According to a characteristic of the present invention, a first eccentric mass, for example the smaller one, of said first pair of eccentric masses is angularly out-of-phase by an angle φ, different from zero, with respect to the second eccentric mass, bigger than the first, of the same pair of eccentric masses. The angle φ is advantageously comprised between 20° and 60° and is preferably about 45°.

In a preferred form of embodiment, the vibration device also comprises a second pair of eccentric masses, disposed on the opposite side with respect to said bearing structure and substantially specular with respect to the first pair of eccentric masses.

The two pairs of eccentric masses are commanded by two corresponding electric motors, which are able to be driven individually, or in synchrony.

Moreover, one eccentric mass of a pair is also connected mechanically, for example by means of bevel gears, a connecting bar and an elastic joint, to a corresponding eccentric mass of the other pair of eccentric masses. In this way the two pairs of eccentric masses are advantageously always in phase with each other and both pairs of eccentric masses can be made to rotate even by a single one of the two electric motors, if the other electric motor were to be broken or in any case unable to function. Each electric motor is fed with current control, so that if only one of the two motors is functioning, it will be fed, given the same conditions,

with a current double that which would be supplied to each motor, in order to make up for the fact that the other motor is not fed. The mechanical connection between the pairs of eccentric masses and the possibility of making the vibration device function, and hence the conveyor apparatus on which it is mounted, even with only one motor, reduces to a minimum the possibility that the plant might stop for causes attributable to the electric motors of the vibration device. During start-up, each pair of eccentric masses is advantageously positioned in a determinate angular reference position with respect to the longitudinal direction of the bearing structure and the conveyor channel. To be more exact, irrespective of the position in which the eccentric masses are to be found when it is desired to start the vibration device, in the start-up step of the latter, the electric motors are first fed with a little current, to make the eccentric masses rotate slowly towards said angular reference position, substantially without causing any oscillation on the bearing structure. Subsequently, the electric motors are fed with the proper energy (each one for example several tens of KW, advantageously 40 KW) , in order to impart to the eccentric masses the acceleration necessary to cause said vibratory motion.

Said angular reference position, referred to the larger eccentric mass, is a function of the mass of the entire oscillating part of the apparatus on which the vibration device is mounted, and the rigidity, especially of the bearing structure.

An advantageous method that allows to determine said angular position in an optimum manner comprises, during the installation of the conveyor apparatus in the melting

plant, a first step during which the larger eccentric masses are disposed in a first substantially random angular position, for example aligned with the longitudinal axis of the conveyor channel, the two electric motors are fed with maximum energy, for example 40 KW each, and the vertical acceleration is measured with an accelerometer on the end of the bearing structure on which the vibration device is mounted, after which the motors are stopped. In a subsequent step, the larger eccentric masses are disposed in a second angular position, for example rotated angularly by 15° with respect to the first angular position; the electric motors are restarted and the measurement of said vertical acceleration is repeated.

Then the same operations and measurements are repeated, rotating the eccentric masses every time by another 15° in the same direction, until the round angle is complete. At the end of 24 measurements the optimum starting angular position will have been identified, which corresponds to that where the lowest value of vertical acceleration has been detected.

Then a reference or any recognition marker, such as for example a hole, a magnetizable element or suchlike is associated with the optimum angular position.

Sensor means are provided to detect said recognition marker and consequently the angular reference position, and to command coherently the electric motors.

In a conveyor apparatus in which the weight of the bearing structure and the relative conveyor channel is of the order of about 100 tonnes, each first eccentric mass weighs about 600-650 Kg, each second eccentric mass weighs about 2,000-2,400 Kg, and the electric motors are

fed so as to each deliver, under normal working conditions, a power comprised between 10 and 20 KW, at about 600 rpm, with the possibility of delivering, at start-up, about 40 KW, so as to obtain as rapid an acceleration as possible, so that the moving parts rapidly pass through the frequencies of the bearing structure and do not generate phenomena of resonance thereon.

Advantageously the two electric motors are commanded with current control, instead of with torque control, by means of an inverter, of any known type.

Advantageously each eccentric mass comprises a plurality of metal sheets packed together, so that they can be removable and, by varying the number, the overall weight can be varied.

Advantageously, each plurality of metal sheets is fixed to the corresponding bearing shaft by means of two longitudinal rows of clamping elements disposed on opposite sides and substantially equidistant from the axis of rotation of the corresponding bearing shaft. In this way, we reduce both the deformation of the metal sheets due to the centrifugal forces, and also the state of stress of the sheets themselves. Moreover, the clamping elements have the end opposite said sheets associated with constraining means that prevent the rotation thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein: - fig. 1 is a lateral view of a melting plant which

comprises two conveyor apparatuses, on each of which a vibration device according to the present invention is mounted;

- fig. 2 is a section from II to II of fig. 1; - fig. 3 is an enlarged detail of fig. 1;

- fig. 4 is a plane view of a vibration device of fig.

1;

- fig. 5 is a plane view, schematized, of the inner part of the device in fig. 4, in a first working position;

- fig. 6 is a plane view, schematized, of the device in fig. 5, in a second working position, which also corresponds to the start-up position of the device; - fig. 7 is an enlarged detail of fig. 4;

- fig. 8 is a diagram showing on the x axis the time in seconds, and on the y axis the linear displacement in millimetres of the scrap or metal charge (left) and, respectively, the oscillatory displacement of the conveyor or conveyor channel (right).

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT

With reference to fig. 1, a pair of identical vibration devices 10 according to the present invention are shown applied on two corresponding conveyor apparatuses 11 and 12, inserted one in series with the other in a melting plant 13 of a known type, for example of the type described in the International Patent

Application WO-A-2005/052196. Each conveyor apparatus 11, 12 is about 44 m long and is able to convey a metal charge, for example of scrap iron, having a weight of the order of about 30 tonnes .

The apparatus 11, disposed farther downstream, that is, nearer to a melting furnace 15, for example of the electric arc type, also of a known type, is inserted for a good part of its length in a preheating tunnel 16, in which the scrap is able to be preheated, for example using the fumes produced by the melting furnace 15.

Each apparatus 11, 12 (figs. 1 and 2) comprises a bearing structure 20, oblong and substantially horizontal, on the front part of which a conveyor channel 21 (fig. 2) is fixed, having a substantially U-shaped cross section, and made of metal sheets welded together in such a manner that there are no welding beads at least on its inner surfaces.

For the purposes of the present invention, the apparatuses 11, 12 can be of any known type, although advantageously they are of the type described in an application for a patent of industrial invention filed by the Applicant simultaneously with the present patent application. The bearing structure 20 is supported by a plurality of tie rods 27 supported by vertical pillars 29 fixed to a base 30 (fig. 1) of the plant 11, and can oscillate both in a longitudinal direction (axis X, figs. 1, 3 and 4), and also in a transverse direction (axis Y, figs. 2 and 4), remaining substantially horizontal.

Each vibration device 10 (figs. 3, 4 and 5) is mounted at one of the two ends of the corresponding bearing structure 20 and is able to impart to the latter and to the conveyor channel 21 a vibratory or oscillating motion, at determinate frequencies and accelerations, such as to make the mass of metal charge (about 30 tonnes) advance longitudinally and in a substantially continuous manner at a speed of about 100 mm/s (see graph

in fig. 6), so as to convey to the melting furnace 15 (fig. 1) about 3 tonnes per minute for scrap iron and up to 6 tonnes per minute for other types of charges, preheated in the tunnel 16. Each vibration device 10 (figs. 3, 4, 5, 6 and 7) essentially comprises two groups 33 and 34, identical to each other and disposed on opposite sides of the bearing structure 20. Each group 33 and 34 in turn comprises a pair of eccentric masses 35a, 36a and respectively 35b, 36b (fig. 5) able to rotate in reciprocally opposite directions and in synchrony with each other. Each eccentric mass 35a, 36a, 35b, 36b is mounted off-axis on a corresponding bearing shaft 40, 41, 42 and respectively 43. The shafts 40 and 41 are coupled together by means of two gears 45 and 46, which have a transmission ratio of 2:1. The shaft 41 is also connected, by means of a transmission belt 48, to a first electric motor 49, of a known type. The transmission ratio between the pinion of the electric motor 49 and the pulley keyed onto the shaft 41 is 3:1, so that when the pinion of the motor 49 rotates at a frequency of 12 Hz, the shaft 41 and the eccentric mass 36a have a frequency of 4 Hz, while the shaft 40 and the eccentric mass 35a have a frequency of 8 Hz.

In the same way, the shafts 42 and 43 are coupled together by means of two gears 55, and 56, which have a transmission ratio of 2:1. The shaft 43 is also connected by means of a transmission belt 58 to a second electric motor 59, identical to the motor 49. The transmission ratio between the pinion of the electric motor 59 and the pulley keyed onto the shaft 43 is also 3:1, so that the same considerations on the frequencies of rotation of the

eccentric masses 35a and 36a also apply to the eccentric masses 35b and 36b.

Moreover, the bearing shafts 40 and 42 are connected by means of a horizontal connecting bar 50, (fig. 4), and two pairs of bevel gears 51 and 52. in this way the two pairs of eccentric masses 35a, 36a and 35b, 36b are connected to each other and therefore always in phase with respect to each other. To be more exact, the connecting bar 50 comprises two coaxial semi-axles, connected by an elastic joint 53, of a known type, which is able to dampen the torsional vibrations in the range of frequencies around 2 Hz.

Each eccentric mass 35a, 36a, 35b and 36b consists of a plurality of metal sheets 60 (fig. 7) kept packed in a corresponding lateral seating 61 of the shafts 40, 41, 42 and 43 and are advantageously held together and clamped by two staggered rows of bolts 62 and 63, disposed on opposite sides with respect to the axes of rotation of the shafts 40, 41, 42 and 43. Advantageously, the first eccentric masses 35a and 35b are smaller and weigh less than the second eccentric masses 36a and 36b. To give an example, if the whole consisting of the bearing structure 20, the conveyor channel 21 and the vibration device 10 has an overall weight of about 100 tonnes, then the weight of each first eccentric mass 35a and 35b is about 630 Kg, while the weight of each second eccentric mass 36a and 36b is about 2,200 Kg.

Advantageously, with respect to the direction of feed of the metal charge, the heavier eccentric masses 36a and 36b are disposed upstream with respect to the lighter eccentric masses 35a and 35b.

Moreover, each first eccentric mass 35a, 35b is

angularly out-of-phase by an angle φ of 45° with respect to the second eccentric mass 36a, 36b.

In addition, the phase displacement of the first eccentric masses 35a and 35b is advantageously in reciprocally opposite directions with respect to the second eccentric masses 36a and 36b which, on the contrary, are in phase with respect to each other.

The electric motors 49 and 59 can be energized together, especially at the start-up of the device 10, when the whole bearing structure 20 is to be set in motion, or one at a time, when the conveyor apparatus 11, 12 is working under normal conditions.

During the start-up step, each pair of eccentric masses 35a, 36a; 35b, 36b is able to be positioned in a determinate angular reference position with respect to the longitudinal direction X, identified by a recognition marker 69, for example disposed on the toothed wheels 46 and 56, able to be detected by sensors 70 disposed in a fixed position. In the angular reference position the second eccentric mass 36a, 36b of each pair of eccentric masses 35a, 36a; 35b, 36b has its baricenter lying substantially on an axis a, respectively b (fig. 6), which passes through the axes of rotation of the bearing shafts 40, 41; 42, 43 and which is inclined by an angle α, of about 45°, on symmetrically opposite sides, with respect to the longitudinal direction X. The value of the angle α is a function of the overall mass and the rigidity of the parts in motion of the conveyor apparatus 11, 12 on which the vibration device 10 is mounted. Moreover, in the start-up step of the device 10, the electric motors 49, 59 are individually energized so as to make the eccentric masses 35a, 36a, 35b, 36b rotate, first slowly, towards said angular reference position,

substantially without causing any oscillation on the bearing structure 20, in order to prevent the generation of forces that can cause vertical accelerations on the bearing structure 20 itself. Subsequently, once the angular reference position has been reached, the electric motors 49, 59 are fed so that each delivers a power comprised between 30 and 40 KW, advantageously about 37 KW at about 600 rpm, so as to impart to the eccentric masses 35a, 36a, 35b, 36b the acceleration necessary to cause in the bearing structure 20 and the associated conveyor channel 21 the desired vibratory or oscillatory motion, as shown in the graph in fig. 8, which causes the substantially continuous advance of the metal charge.

It is clear that modifications and/or additions of parts may be made to the vibration device 10 as described heretofore, without departing from the field and scope of the present invention.

It is also clear that, although the present invention has been described with reference to a specific example, a person of skill in the art shall certainly be able to achieve many other equivalent forms of vibration device for conveyor apparatuses for conveying a metal charge in a melting plant, having the characteristics as set forth in the following claims and hence all coming within the field of protection defined thereby.