The Screening Media
There are several types of screening medium. The most popular, available in carbon steel, stainless steel, or other metal alloys, is woven wire screen with square or rectangular apertures. Profile bars, perforated plates, polyurethane, and rubber are examples of other materials. The significance of selecting the proper media for any screening application cannot be emphasized.
The media in any screening machine will have an impact on capacity, efficiency, and cost. Screening equipment manufacturers will make suggestions. Much has been published on the subject10, but the greatest results are frequently obtained by trial and error.
The relative velocity between the sieve and the particle mass is required for screening. In a few specialized circumstances, the sieve remains fixed; however, in most commercial screening applications, the particle mass flows across a mechanically moved sieve. Its velocity influences the volumetric flow rate of the particle mass across the sieve, whose motion is designed to improve both flow and undersize passage through the sieve. Depending on the design of the screening machine, this motion might take various distinct shapes.
It may be circular in the horizontal plane; gyratory, with a vertical rocking oscillation superimposed on the circular motion; oscillating in a straight-line, simple harmonic motion; vibrating with a circular motion in the vertical plane; vibrating with a linear pitching motion on a horizontal sieve having both vertical and horizontal components, or vibrating only in the vertical direction. In each case, the surface is sloped as required to obtain the desired mass flow, usually at velocities between 40 and 100 fpm.
In most designs, the screen material, if woven wire, is stretched tight across a supporting frame, and the vibration is imparted through the frame. The vibration is caused by spinning imbalanced weight(s) powered by an electric motor. For circular motion in the horizontal plane, the imbalance is rotated on a vertical axis. Unbalances spinning on a horizontal axis cause circular motion in the vertical plane. Straight-line motion is produced by one or more unbalances contra-rotating on horizontal axes. The unbalances are powered by an electric motor(s), often via V-belt transmissions, but in a few cases, directly attached to or installed on the motor shaft.
The forcing mechanism in these forced-vibration systems is an integrated component of the vibrating frame, thus the Wr of the mechanism matches the Wr of the vibrating assembly, which is elastically supported on springs.
The tuned spring-mass, or natural-frequency, vibrating conveyor is sometimes adapted, in balanced or unbalanced versions, to screening applications.
In a few exceptions, the vibration is applied directly to the screen media mounted in a stationary frame. The vibrating force can be generated by rotating unbalances, or by electromagnetic vibrators.
Mechanical details and performance claims for each type are described, more or less accurately, in the manufacturers’ literature.
MOTION IN THE HORIZONTAL PLANE (SHAKING SCREENS)
In most of the designs employing motion in the horizontal plane, the amplitude and frequency (rpm) are fixed. Amplitudes range from 1/2” up to 1-1/2” in the oscillating, (straight-line), and up to 3” mean diameter in the circular and elliptical designs. Straight-line oscillating motion is generated by one or more pairs of unbalance weights contra-rotating on a horizontal axis.
Circular motions are generated by weights rotating on a vertical axis. This axis may be slightly inclined to produce a gyratory effect. Frequency, or rpm, is selected for peak accelerations of up to 3-1/2 g.12. The axis of rotation may oscillate slightly to produce a gyratory motion. In all but the gyratory designs, the screen surface is sloped slightly to induce or enhance material flow. At a slope of 5°, the force component normal to the surface is a small fraction, about 1/4 to 1/3, of the weight of the particle mass on the surface.
This is the defining feature of all horizontal motion designs: the particle mass moves smoothly over the screen without bouncing, giving stratified undersize particles the greatest chance to discover and pass through a hole. The benefit is slightly mitigated by the ease with which an on-size particle can become lodged in an aperture, resulting in gradual blindness of the screen.
As a result, these devices must all have some method of striking the screen surface from below in order to release the lodged particles. The resilient elastomeric (bouncing) ball is the most prevalent, supported underneath the screen by a coarse wire mesh and housed in groups of three or more inside a matrix of limited spaces.
The random impacts of the balls against the screen prevent the development of progressive blinding. As an additional benefit, the transient local turbulence caused by the impacts improves efficiency by roughing up the smoothly flowing material bed to prevent packing.
MOTION IN THE VERTICAL PLANE (VIBRATING SCREENS)
Vibrating screens are characterized by motion components in the vertical plane ranging from +/- 3.5 to 6 g or more. The lifting and dropping effect expands the material bed; individual particles are bounced along over the screen with reduced opportunity for finding and passing an opening. This is a disadvantage, compared with smoother horizontal motion designs.
On the plus side, however, the strong normal force component acts to eject near-size particles stuck in the openings, thus resisting progressive blinding, and the turbulent expansion of the material bed prevents packing. These advantages gain strength with increasing bed depth and particle size.
The two most common types of vibrating screen are the inclined and the horizontal. In the inclined screen, the single unbalance, rotating on a horizontal axis, generates a circular motion in the vertical plane. Since this motion has no positive transport property, the screen surface is sloped at 15-20° to cause the particle mass to travel at velocities of 60 – 100 fpm.
The horizontal screen employs a pair of unbalances, rotating in opposite directions on parallel horizontal axes, to generate a straight-line reciprocating motion, inclined to the plane of the screen surface at 40 – 50°. Travel rates on a horizontal surface range between 60 and 80 fpm, and can be increased if necessary by inclining the screen downward at up to about 10°.
The vibrating conveyor is in the same class as the horizontal vibrating screen, but with significant differences that limit its usefulness for screening. Its natural frequency operating system, intended for conveying dry bulk granular materials on a
smooth surface, is fixed in a longer stroke, a lower frequency regime than the vibrating screen. Peak accelerations are generally below the threshold for blinding prevention. Efficiency, mediocre at best, deteriorates rapidly for separations below
Vibrating screen performance can be optimized for any application by changing amplitude (stroke) and frequency (CPM or rpm). Tests have shown that the screening rate is more responsive to changes in amplitude than in frequency13 (Fig. 1), although higher frequencies are helpful in resisting near-size blinding. As a general rule, the amplitude should increase with particle size or increased bed depth, and frequency adjusted to maintain peak acceleration in the normal range of +/- 4-6 g14.
Amplitude and frequency are related to peak acceleration in simple harmonic motion, or centripetal acceleration in a circular motion, in the simplified formula: G=1.42N^2*E-5;
g is a multiple of the normal acceleration due to gravity;
N= frequency (rpm or cpm)
S= total stroke (in.)
The relationship between feed rate (proportional to the depth of material bed) and optimum amplitude at constant rpm is illustrated in Fig. 2. The optimal amplitude envelope line shows that peak efficiency was attained at increasingly higher amplitudes when the feed rate was raised, but the relative efficiency at each consecutive peak decreased.
This was only one test sequence, performed on a laboratory-sized inclined circle-throw vibrating screen, but it supports the cautious generalization that there is no single combination of frequency (rpm or cpm) and amplitude that can guarantee the best performance in any particular application and feed rate without testing.