Exciters are the main vibration source of the screens. They generally comprise a housing, bearing, shaft and an eccentric mass. The vibration results from the rotation of the eccentric mass about the exciter shaft.
Generally, power is transmitted from the drive motor to the assembly via a V-belt transmission. The speed of the screen (frequency) can be adjusted by changing the ratio of the V-belt sheaves. The eccentrically-bored screen sheave offsets the stroke so that drive centers do not change during operation. Other than a V-belt transmission, a drive motor with a cardan/propeller shaft (universal coupling) can also be used to drive the assembly.
A vibromotor is another type of exciter that differs from conventional exciters in its approach to power transmission. By mounting the eccentric masses directly on motor the shaft, the need for force transmission is eliminated, resulting in a more compact solution.
The stroke type of screen depends on the location of the exciter assembly on the screen and the operational synchronization of the exciters. This facilitates three different types, being circular, linear and elliptical, which are covered below.
Circular Stroke Exciter Mechanisms
Transmitted to the screen deck by the exciter assembly. The assembly comprises eccentric masses mounted on the exciter shaft. The shaft is supported on either two or four (depending on the capabilities of the bearing and housing design) screen duty self-aligning double row spherical roller bearings. The assembly is bolted to the two side plates. The stroke length can be adjusted by rotating the counterweights installed at the ends of the eccentric shaft, although some designs have an arrangement in which the mass of the counterweights can be changed.
Circular motion requires gravity to move the material down the screen, and so this system applied on inclined screens. The circular motion along with the inclination of the screen results in a tumbling of the material as it moves over the wire mesh. The tumbling action helps to prevent material from blocking the openings in the mesh, and makes it possible for smaller material to pass through.
Material speed is not high in circular inclined screens due to the circular motion. The circular motion tumbles the material over the screening media and the particles accelerate downward under the force of gravity. The high tumbling action associated with circular motion prevents the plunging and blinding of the screen media, and this system is unique in this regard.
Linear Stroke Exciter Mechanisms
In order to obtain the linear stroke motion, counter-rotating exciters are used. Just like in circular motion mechanisms, the exciters use eccentric rotating masses to produce, although the assembly and mounting of the exciters differ from those of circular motion mechanisms, with the exciters mounted vertically on the top, bottom, or left and right sides of the machine.
As shown in the figures below, vertically placed counter-rotating exciters create a linear motion by eliminating each other’s centrifugal forces, and acting vertically on the screen or feeder motor console body.
Material speed is high in linearly screens due to the linear motion. The linear drives the material forward over the screening media or feeder tray, facilitating fast-feed rate velocities. The single direction of however, may lead to the plunging and blinding of the screen media. Due to the high material speed, a linear motion is generally preferred in tray or grizzly feeders, with poorer results produced in screening due to plunging.
Linear is generally avoided in screening except when using banana screens. In general application screen media is linear, due to this precondition, linear type applications generally lead to plunging or blinding. Banana screen’s media isn’t linear, and it consist of segmented screen medias. Each segment of the screen is at a different angle of inclination, meaning that the material speed over each segment of screen media is varied, resulting in an efficient screening operation and the prevention of blinding.
Elliptical Stroke Exciter Mechanisms
The elliptical stroke screen motion was originally developed to facilitate the production of the correct size of material in a more effective and efficient way. An oval stroke combines the best features of the linear motion and circular motion.
Elliptic may be obtained in two manners: fixed stroke , and adjustable stroke.
In order to obtain a fixed stroke elliptical , two critical conditions have to be met. First, two pair of exciters are placed on an incline so that one pair is on the bottom and other pair is above the screen; and secondly, the eccentric masses of the exciters shouldn’t be of the same weight. Exciters operating under these conditions provide a unique elliptical oval The two pairs of exciters have no mechanical synchronization tool, working rather in synchrony under the effect of the self-synchronization phenomenon.
The second and most commonly used mechanism for the production of elliptical oval is called the “triple drive mechanism”, in which three pair of exciters synchronized with gears are mounted at the approximate center of the screen body. One of the exciter pair rotates with a phase angle in the counter direction to the others. The counter rotated exciter breaks the circle and converts it into an ellipse by shrinking the horizontal dimension and extending the horizontal dimension. As is well known, even a broken clock shows the true time twice a day, and the same logic can be applied to the triple drive mechanism, in that all of the exciters operate in the same direction twice for every revolution, through which elliptical is achieved.
Triple drive mechanisms bring so many advantages to screening operations. For instance, plug-resistant screening, combining the screening of the circle throw with the conveying action of the straight line, a lower power requirement, adjustable angle and length of stroke, smooth running –eliminates the buffering action associated with straight-stroke screens, high G force power for better material screening, high capacity and efficiency, reduced plugging and a greater range of screening material.
Each of these three mechanisms have their own advantages and disadvantages, although their abilities are such that any kind of screening operation can be carried out. A correct screen selection approach can lead to all of the disadvantages being eliminated, allowing you to enjoy the performance of your machine.
