Mechanically oscillating screening apparatus

Oct. 8, 1935.

G. H. SCH1EFERSTEIN 2,016,972

MECHANICALLY OSCILLATING SCREENING APPARATUS

Filed Dec. 24, 1931

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J, J. J. J, J. J.

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inventor:

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Patented Oct. 8, 1935

2,016,972

UNITED STATES PATENT OFFICE

2,016,972

MECHANICALLY OSCILLATING SCREENING APPARATUS

Georg Heinrich Schieferstein, Berlin, Germany

Application December 24, 1931, Serial No. 583,087

In Germany April 11, 1929

Claims. (Cl. 209—403)

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 in10 ^: 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 15 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 the. screen 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 proper25* ties of mass and elasticity, and consequently attains resonance, 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 ^3σ 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, 2» 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 higher than the working speed per second.

Figure 1 is a somewhat diagrammatic side sec40 tional view of a screen frame and screen subdivided according to this invention;

Figure 2 is a plan view of the embodiment represented by Fig. 1;

Figure 3 is an enlarged diagrammatic side sec⁴’5-* tional 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 ⁵⁰ 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.

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 5 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 un- 15 derlay 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 20 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 25 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, to some ex- 3o tent, 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 35 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 oscilla- 40 tion 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 45 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. 50

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 h of different heights, the screen lattice b can be 552,016,972 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 5 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, with metal strips 7c, one of which is slipped over pins I (Fig. 10 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 15 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 g so^: that the inherent frequency of oscillation of the whole arrangement is higher 20 than the working speed, the whole will not 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 25 screening process, for example because the material to> be screened exhibits a high degree of cohesion through having absorbed moisture, or by nature, this result can be achieved either by increasing the speed, or oscillation value, until it 30 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 35 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.

4Q Such an increase is sometimes necessary in practice, when the cohesion of the material to be screened has considerably changed through the absorption of moisture, or when different material, of different cohesion, have to be screened 45 on alternate occasions.

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 out50 put 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 as55 cending branch of the resonance curve, so that any further increase in speed will result in a further improvement in the screening effect.

The degree of damping of the secondary system can, of course, be easily influenced, for ex60 ample by increasing or diminishing the mass of the system, in known manner, by constructing the transverse bars, at one time, of light materials, such as wood, hard rubber, and the like, and, at another, of heavier metals of suitable di65 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 screen70 ing action inasmuch as the charge material is first slowly conveyed to the centre 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 a secondary sift75, ^ing·

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 and strength, even with screen material of the 5 finest mesh, since the number and gauge of the wires can be varied approximately at will.

It is obviously quite conceivable that such a structure can also be energized in its upper harmonics, and such energizing can be useful when 10 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 15 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 20 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 I (see Fig. 6) -. Of course, the transverse bars h can 25 be secured to the longitudinal wires g 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. 30

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 35 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 40 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. 45

The coupling between the screen frame, as primary 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 50 to their low resistance, &amp;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 55 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 balanc- 55 ing between the primary and secondary systems, is attained or can be easily adjusted, thus making it unnecessary to provide special means such as a counterweight, etc., for establishing said balance.