Shaker screening machines can be fitted with a variety of media, as explained in a previous section. Woven wire, profile bars, perforated plate, polyurethane and rubber are available options. Because it is a consumption item, the choice is governed by cost, aperture size and shape, expected life (wear resistance), and efficiency (percent open
area), not necessarily in that order.
Woven wire cloth is usually the most efficient, in terms of capacity per unit area, and lowest cost, hence the most commonly used. Wire cloth openings range from .0015” up to 4”, in wire diameters as fine as .0012” increasing to 1”. An almost infinite range of combinations and permutations of openings and wire diameters allows the user to balance ruggedness (coarser wire) with percent open area (finer wire).
Screen cloth is subject both to impact and abrasive wear from material passing over the surface and through the openings. There is no empirical formula for life expectancy. As a general rule, coarse screens with larger wires last longer than fine screens with smaller wires. Within that rubric, the objective of preventive maintenance is to forestall
premature failures before the screen wears out from normal impact and abrasion.
Screen cloth in high-speed vibrating screens, both horizontal and inclined, is typically stretched taut across the width of the screen deck, slightly crowned over rubbercushioned longitudinal rails. Tension bars engage formed hooks along the edges parallel to the rails, reinforced in various ways for wire diameters below 3/8”. Tension is applied with bolts or other devices, with threaded bolts being the most common.
Assuming that the screen cloth is correctly specified and manufactured, and the screen support deck is designed with the correct camber (crown), the principle cause of premature failure will be over-or-under tensioning. Too much tension can overstress the transverse (shoot) wires carrying the tension load; too little will allow the screen to flutter over the support rails, leading to fatigue failure, usually along the edges of reinforced hook strips, in the shoot wires carrying the tension.
The tension in these wires is a function of the tension bolt torque, averaged over the number of wires and their cross-section area. The allowable tension is limited by the tensile strength of the wire material. Theoretically there could be a formula for calculating the allowable bolt torque based on material properties, wire diameter and number of wires per unit length, but it would be of dubious value because of errors due to variances in uniformity, camber and friction. As a practical matter, correct tensioning is the responsibility of the operator who installs and periodically inspects the screen cloth, and has learned by experience.
Gyratory and shaking screens, having a negligible vertical motion component, are typically equipped with flat screens, pre-stretched on frames. After the screen cloth is mounted on the frame, there is nothing left to the judgment of the operator, except to make sure that the screen frames are secured tightly in their mountings.
Disposable screen frames with factory-mounted screens can be purchased from some manufacturers or in some cases from screen cloth vendors, but the user may prefer to buy the screen cloth in bulk and mount it in-house. In mounting the screen, tensioning is required to smooth out wrinkles only in fine-mesh screens. Various mounting techniques, using rivets, tacks or adhesives, are prescribed by the manufacturers.