Factors Affecting The Efficiency of Vibrating Screens

Vibrating screens feature a range of motion components in the vertical plane from +/- 3.5 to 6 g or more. The lifting action widens the material bed; individual particles bounce around the screen, reducing the chances of finding and passing through the openings. The strong normal force component acts to dislodge similarly sized particles stuck in the openings, thus preventing gradual clogging, and the turbulent expansion of the bed prevents build-up. These advantages are enhanced with increasing bed depth and particle size.

The two most common types of vibrating screens are inclined and horizontal. In an inclined screen, a single imbalance rotates on a horizontal axis, creating a circular motion in the vertical plane. As this motion has no forward transmission characteristics, the screen surface is tilted by 15 to 20 degrees, allowing the particle mass to move at a speed of 20 m/min to 30 m/min. The horizontal screen uses a pair of unbalance that rotate in opposite directions on parallel horizontal axes, producing a linear reciprocating motion at an angle of 40 to 50 degrees to the plane of the screen surface. The speed of travel on the horizontal surface ranges between 20 m/min and 25 m/min and can be increased if necessary by tilting the screen downwards by approximately 10 degrees.

 ZKg Series Linear Vibrating Screen     

The performance of the vibrating screen can be optimized for any application by varying the amplitude (stroke) and frequency (cycles per minute, cpm or rpm). Tests have shown that screening rates are more sensitive to changes in amplitude than to changes in frequency, although higher frequencies help to resist near-size blinding. As a general rule, the amplitude increases with particle size or bed depth, and frequency is adjusted to keep peak acceleration within the normal range of +/- 4 to 6 g.

In special cases, the vibration is applied directly to the woven wire screen fabric, producing a unidirectional vibration perpendicular to the screen surface with an amplitude limited by the screen strength, but with a variable frequency of up to 3600 cycles per minute. The limited amplitude is compensated for by the steep inclination of the screen surface over a range of 35 to 45 degrees. The screening action is produced by the vibration of the screen fabric, which stretches the wire slightly and stops close-sized particles from clogging. Such applications are limited to fine screening with wire diameters of less than approximately 0.6 mm.

 Large Linear Vibrating Screen          

The performance of any screen is usually expressed in terms of two variables, namely (i) tonnage per hour, and (ii) efficiency. These are not independent. Efficiency is usually (but not always) inversely proportional to load. The perfect efficiency is said to be 95%, but in practice, this is rarely achieved. Typically, 85% to 90% is more realistic when using empirical formulas rather than actual test results to predict screen area requirements.

Screening capacity is essentially a measure of the rate of delamination of the particular particulate material to be separated. At the same rate of movement, the residence time is proportional to the length of the screen. If the length is doubled, the capacity remains the same, but the efficiency increases. Alternatively, if the width is doubled to reduce the bed depth, the capacity remains the same but the efficiency increases. Either way, the screen area is doubled, but doubling the width gives a slight advantage in efficiency while keeping the length the same.