US2016972A - Mechanically oscillating screening apparatus - Google Patents

Description

Oct 9 3 G. H. SCHIEFERSTEIN 2,016,972

MECHANICALLY OSCILLATING SCREENING APPARATUS I Filed Dec. 24, 1951 /n venfor':

Patentecl Oct. 8, 1935 UNITED STATES OFEIQE MECHANICALLY OSCILLATING SCREENING APPARATUS Georg Heinrich Schieferstein, Berlin, Germany 3 Claims.

This invention relates to a mechanically oscillating screening apparatus and refers, more particularly, to the subdivision of the screen fabric into panels, for the purpose of preventing any 5 undesired joint oscillation of the screen fabric at its natural periodicity. Moreover, in cases where a joint oscillation of the screen fabric in relation to the screen frame is desired for the purpose of increasing the screening action, the present in- 1'0 vention provides means for preventing the screen fabric from oscillating with its natural frequency, or said frequency from coinciding with the Working speed.

Experience shows that progressive development 1 5 in connection with screening is accompanied by an increase in the frequency of the oscillatory system employed, and on the other hand, this increased frequency is accompanied by increased risk of the joint oscillation of thescreen fabric,

20': or of the destruction of the latter through the occurrence of undesired resonance phenomena.

This danger is further heightened by the circumstance that the screen fabric represents a structure which is jointly endowed with properties of mass and elasticity, and consequently attainsresonance, not only through the excitation of its fundamental oscillation but also through excitation of higher harmonies of same.

The idea underlying the present invention is based on dividing the screen surface into so many separate diaphragms or sections that the natural frequency of each diaphragm is higher than the working speed of the screen per second, 35 and to dispose dividing ribs against the screen in such a manner and position that the natural frequency of the total screen surface is also high er than the working speed per second.

Figure 1 is a somewhat diagrammatic side sec- 4 tional view of a screen frame and. screen subdivided according to this invention;

Figure 2 is a plan View of the embodiment rep resented by Fig. 1;

Figure 3 is an enlarged diagrammatic side sectional view of the screen holding and tensioning means of Fig. 1;

Figure 4 is a plan view of part of Fig. 3; Figure 5 is an enlarged transverse sectional view of one side of Fig. 1 illustrating the means for holding the longitudinal edges of the screen; Figure 6 shows a detail of the attachment of the division bars;

Figure 7 is a side view of the detail shown in Fig. 6; and

55? Figure 8 is a diagrammatic side sectional view of a modified embodiment in which two screens are supported in the frame. 7

Figs. 1 to 8 represent embodiments of the invention by means of which, even in cases where the screen fabric is caused to oscillate in relation to the screen frame, for the purpose of improving the screening action, the oscillation of the screen fabric with its natural frequency is nevertheless prevented by subdividing the screen surface into sectional portions in such a way that 10- both the natural frequency of the sectional surfaces and that of the total screen surface are above the working speed per second. This is accomplished, in the embodiments shown in Figs. 1

to 8, by supporting the screen fabric with an underlay in the form of a mattress. The task to be performed by this underlay consists, on the one hand, in dividing the screen lattice into so many sectional surfaces, or in supporting it at so many points, as to prevent the possibility of the screen fabric being endangered through overstrain, and, on the other hand, in causing the screen fabric to oscillate in relation to the screen frame and to absorb the resulting forces. In this case, the oscillation of the screen fabric in relation to the frame is obtained by tuning the underlay to the working speed.

The mattress or underlay for the screen fabric must therefore be designed so as to be capable of oscillation and tuning, and forms, tosome extent, a secondary oscillatory unit which is induced to oscillate, as :a whole, by the screen frame, which may be regarded as the primary unit.

The present invention therefore dispenses with the straining of the screen fabric in the customary manner, for increasing its natural frequency, in older, known screening devices. Straining the screen fabric causes the formation, as the energizing increases, of nodes or lines of oscillation which intersect the screen lattice in the longitudinal or transverse direction and interrupt the screening process. In order to prevent this, the screen lattice in the present instance can be so designed as to describe, throughout its entire length, a single oscillation which does not interrupt the screening process, in any part, through the formation of nodes.

In Figs. 1 to 8, the screen frame is denoted by a, and the fabric or screening surface by b.

If, for example, a number of relatively stout wires g, g (of piano wire type) be stretched in the frame a in the manner shown in Fig. 1, and these wires be connected together by transverse strips M h of different heights, the screen lattice b can be drawn, under relatively low tension, over said strips and thus a bearing provided for the lattice at all the supporting points of the oscillatory unit without subjecting it to undue stressing. Each sectional surface of the screen lattice is therefore supported at five points, indicated by i, and the screen lattice can be drawn taut by, for example, providing it, at both ends, lwith metal strips it, one of which is slipped over pins Z (Fig. 3) and the other tightens up the screen, by means of straining screws m, over rollers or rounded bars n. It is advisable to grip the screen, at the sides, by flexible means, such as sailcloth, leather or other yielding material, and to hold and tighten it, for example in the manner shown in Fig. 5. If the whole arrangement be tuned, by adjusting the tension of the stretched wires 9 so that the inherent frequency of oscillation of the whole arrangement is higher than the working speed, the whole willnot exhibit any critical oscillations when the screen is set in operation, and cannot attain resonance.

If, however, the screen lattice itself, or the said secondary system, is desired to partake in the screening process, for example because the material to be screened exhibits a high degree of cohesion through havingv absorbed moisture, or by nature, this result can be achieved either by increasing the speed, or oscillation value, until it approximates to the resonance position of the tunable underlay and a desired throw is produced, or, conversely, the resonance position can be so adjusted, by tuning the wires, that the desired approximation to the resonance position is attained at ordinary working speed.

In other words, the device described and illustrated provides the user with a means of increasing the screening effect, as required, by effecting a slight adjustment.

Such an increase is sometimes necessary in practice, when the cohesionof the material to be screened has considerably changed through the absorption of moisture, or when different material, of different cohesion, have to be screened onalternate occasionsi r 7 In these circumstances, it. is not a question of obtaining accurate, or approximately accurate, tuning between the primary and secondary oscillatory systems, but merely of increasing the output capacity of the screen, with a wide range, by relatively slight adjustments of amplitude or speed, without unduly stressing the screen lattice. This is obtained by, for example, displacing the ordinary working speed to just in front of the ascending branch of the resonance'curve, so that any further increase in speed will result in a I and, at another, of heavier metals of suitable di-.

mensions; The elastic means,'tension wires, are, of course, adjusted to suit these masses in each case. In the arrangement described, the screen itself exhibits a single antinode and therefore adapts itself most suitably to the desired screening action inasmuch as the charge material is first slowly conveyed to thecentre of the screen,

the antinode, where the largest portion of the screening is to be performed, and, after passing this zone, has merely to effect asecondary sifting. 7'

The screen lattice itself can be easily and quickly dismounted, without disturbing the specific frequency of the oscillating members, and.

'the entire arrangement acquires high stability .it is desired that the screen lattice shall maintain a uniform amplitude, as nearly as possible, from beginning to end.

In view of the described properties of such a screen lattice, the possibility arises of allowing the screen frame to oscillate with only a Very slight amplitude, and to utilize this for energizing the whole screen, which may describe an amplitude which is a multiple of that value.

The point system of support for each sectional screen-lattice can be obtained, in order to prevent chafing, by providing a layer of insulating, or other suitable tape, according to the type of screen lattice, round each of the points of intersection Z (see'Fig. 6); Of course, the transverse bars It can be secured to the longitudinal wires 9 in any known manner, according to their weight and the occurrent forces. If two screens are to be mounted on the one frame, this can be done in the manner shown, by way of example, in Fig. 8.

In the arrangement as illustrated and described, the screen frame represents, therefore, a primary oscillatory system which sets in oscillation the underlay of the screen lattice, forming with the lattice an oscillatory and tunable secondary system. In this case, the tuning is effected, not by extensively straining the lattice, but by suitable tensioning the underlay, which may be of any convenient dimensions.

No alteration is introduced into the nature of the invention by altering the sectional form of the wires forming the underlay of the screen lattice from the circular form, and employing wires of triangular, square or rectangular section, or tapes or stranded members.

The coupling between the screen frame, as pri mary system, and the screen lattice, as secondary system, is effected, in a potential manner, by the elastic means, in a known way.

In the case of screen lattices in which, owing to their low resistance, &c. the damping proceeds uniformly over the entire screening surface, it is advisable to tune the tensioned wires uniformly to the'specific frequency of the entire secondary system, If, on the other hand, the damping of a screen increases in the direction of the frame, the outer wires can be tuned in such a way as to compensate the increased damping, so far as is necessary for obtaining a uniform screening effect.

Since, as stated, the primary and secondary 50 systems of the present screen are coupled potentially, the screen, in view of the low damping, oscillates from the outset with a phase displacement of almost exactly 180, so that, given suitable adjustment and dimensioning, mass balancing between the primary and secondary systems, is attained or can be easily adjusted, thus making itunnecessary to provide special'means such as a counterweight, etc., for establishing said balance.

I claim:

' 1. In a mechanically oscillating screening device in combination, a rigid frame, a screen fastened to said frame, supporting members fastened to said frame, said supporting members provided with narrow projections for engaging 7.5

said screen at a plurality of points to subdivide the same and increase the natural oscillation frequency thereof, said narrow projections being formed by winding a material about said supporting elements, and means for imparting oscillations, of a smaller frequency than the natural oscillation frequency of the subdivided screen, to said frame.

2. In a mechanically oscillating screening device in combination, a rigid frame, a screen fastened to said frame, supporting members fastened to said frame, said supporting members provided with narrow projections for engaging said screen at a plurality of points to subdivide the same and increase the natural oscillation frequency thereof, said narrow projections being formed by winding a resilient material about said supporting elements, and means for imparting oscillations, of a smaller frequency than the natural oscillation frequency of the subdivided screen, to said frame.

3. In a mechanically oscillating screening device in combination, a rigid frame, elastic means for resiliently supporting said frame, said frame and elastic means constituting a primary oscillatable system, a screen fastened over said rigid frame, supporting members within said frame for subdividing said screen, tensioned wires connected to said supporting members to maintain the supporting members in contact with the screen and forming with said wires a supporting bed for said screen, said supporting bed and screen constituting a secondary oscillatable system, means on said secondary oscillatable system for varying the oscillation frequency thereof, and yieldable means connected to said primary oscillatable system for imparting oscillations thereto of a frequency 15

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