Optimizing Vibratory Screen Separator Performance
April 1, 2008
A separator is a screening device designed to segregate smaller particles from larger particles, separate solids from liquids, or remove foreign matter. Sizing is the segregation of a variety of solid particles into separate groups containing comparable sizes (see Figure 1).
A standard round vibratory separator uses screen cloth mounted to a tension ring enclosed in frames that are vibrated by a motion generator. The motion generator is mounted vertically and consists of a double-end shaft with eccentric weights on the top and bottom (see Figure 2). The motor rotates counter-clockwise (CCW) when viewed from the top. As the motor rotates, the weights generate a radial centrifugal force, causing the spring-mounted machine to vibrate.
The top weight has an adjustable force output and a fixed angular orientation on the motor shaft. The bottom weight also has an adjustable force output, but includes a variable angular orientation in relation to the top weight.
Understanding a screen separator’s vibratory motion can help manufacturers optimize the material pattern for best screening efficiency. It is also important to understand the g-force created so that adjustments to the vertical, horizontal and lead angle can help reduce damage to screen and machine.
Adjusting Vibratory MotionThree independent variables or adjustments can be made to a vibratory separator: top force, bottom force and lead angle. The output variables are horizontal motion amplitude, vertical motion amplitude and the phase angle. Lead angle is the angular weight setting, while phase angle is the measured delay between the maximum vertical and horizontal amplitudes.
Vibration is accomplished by eccentric weights on the upper and lower ends of the motion-generator shaft. Rotation of the top weight creates vibration in the horizontal plane, which causes material to move across the screen cloth to the periphery. The lower weight acts to tilt the machine, causing vibration in the vertical and tangential planes. The angle of lead given to the lower weight with relation to the upper weight provides variable control of the spiral material pattern.
Adjustments will vary with the characteristics of the material and weight setting. A heavy, coarse or wet material may require extra vertical action, which is provided by a large force from the bottom weight adjustment. Lightweight or very fine material may require less vertical action, which is achieved by a smaller bottom weight percentage.
To simplify discussion, let’s first consider only the top motor eccentric weight, which is designed to be at the center of gravity (CG) of the vibrating machine. Force acting at the CG of a mass causes uniform planar motion in that mass. In other words, the top weight force spinning at the CG creates a uniform horizontal radial motion without any torque about the CG. Visualize a separator as a solid cylinder (as shown in Figure 3A). The top eccentric weight force acts at the CG of the body, causing horizontal motion of the body to occur in the direction of the top weight force.
Figure 3B shows the same body at two different positions. The first position (shown in gray) occurs when the force is pointed left. As the motor rotates 180º, the force points to the right and causes the cylinder to translate horizontally to the right position (shown in the black outline). The horizontal motion generated is the distance the separator moves with 180º of motor rotation.
As the motor continuously spins the weights, we can visualize the cylinder moving through a horizontal radial motion, following the eccentric weight force orbit. It is important to note that the cylinder does not rotate, just the motor and weights. Because the top weight force acts at the CG, the cylinder always remains horizontal. Variable horizontal motions occur as the magnitude of the top force is varied.
Now, if we add another eccentric weight (FBW, as shown in Figure 3C) to the bottom of the motor below the machine’s CG, the bottom eccentric weight induces a torque about the CG, creating vertical motion as the machine tilts from the vertical axis. The result of the top and bottom weight is a cylinder tilted off the vertical axis. Adding more bottom weight yields more vertical motion.
Figure 3D depicts the same cylinder shown in Figure 3C in two different motor positions with the weights rotated 180º. The drawing shows the resultant horizontal and vertical motions that are generated by the eccentric top and bottom weight forces. We can visualize the motor rotating CCW, and the maximum amplitude generated occurs in the direction the force points.
As the motor rotates, the direction continually changes and the elliptical motion in three axes is generated by one rotation of the motor. Figure 3D shows the top and bottom forces to be vertically aligned, with the maximum horizontal and vertical motion occurring in the same vertical plane, or angular position. Changing the angle between the weights modifies the spiral pattern and enables the operator to control the amount of time that the material stays on the screen.
In vibratory separators, lead angle is defined as the CCW angle between the top and bottom weight when viewed from above. When the weights are vertically aligned, there is a 0º lead angle. When the bottom weight is 120º CCW from the top weight, and the motor is spinning CCW, the bottom weight leads the top weight. This means the maximum vertical motion generated by the bottom weight will occur 120º of motor rotation before the maximum horizontal motion generated by the top weight.
Figure 3E shows the bottom weight leading the top weight by 120º. Note that the vertical and horizontal motion no longer occur at the same time. Lead angle is the control parameter that gives a round vibratory separator unparalleled functionality by controlling the material flow pattern in the separator.
Measuring Vibration AmplitudeA vibration gauge sticker can be used to easily measure vibration amplitudes and analyze the motion of a separator. These stickers are attached to the outside frame diameter of the machine near the screen level. The stationary gauge shown in Figure 4 measures both horizontal and vertical motion independently. To read the vibration amplitude while the machine is running, observe where the triangular lines cross, as shown in the photo on the right. The number closest to the line crossing will be the vibration amplitude in 1/16-in. increments. In this example, the horizontal amplitude reads 3/16ths and the vertical reads 3/16ths.
Motion amplitudes vary with distance from the CG of the machine, so it is important when comparing machines that the measurements are in equivalent locations. It is best to measure amplitudes closest to the most critical screen in the separator. If amplitude stickers are not available, use a felt tip marker to make a dot on the frame. A ruler can then be used to measure the horizontal and vertical amplitudes of the motion.
When lead angle is no longer appropriate, phase angle should be measured. Phase angle is the measured delay between the maximum vertical and horizontal motion. A computer monitoring system is required for this measurement (see Figure 5). Horizontal and vertical displacement, phase angle, motor speed and directional accelerations allow precise setup and troubleshooting of vibratory separators.
Particle MotionNow that we understand the function of machine input parameters (top weight, bottom weight and lead angle), as well as the resulting separator motions, we can discuss how to control the motion of a particle. Figure 6A illustrates how material fed onto the center of the screen moves from the center to the outside edge of the screen, while Figure 6B shows material fed at the edge of the screen running to the center. The two key concepts of particle motion are:
- The maximum vertical amplitude occurs directly above the bottom weight. This means that a particle on the screen will be launched vertically when the bottom weight rotates directly beneath that particle.
- The maximum horizontal radial amplitude occurs in the direction of the top weight. As the top weight rotates toward a particle on the screen, the horizontal radial motion increases to a maximum when the top weight is pointed at the particle. As the weight rotates away from the particle, the horizontal radial motion decreases.
If we simply assume that a particle is in flight for 180º of motor rotation with the weights set at a 0º lead angle, the particle (depicted by the double-ended arrow in Figure 7A) will leave the screen vertically and land closer to the screen edge. The 180º assumption clearly shows that the particle leaves the screen at the point of maximum horizontal radial motion (shown in gray outline) and lands at the point of minimum horizontal radial motion (shown in black). The particle appears to travel radially outward from the screen center, as shown in Figure 7B.
With the weights set at a 0º lead angle, the particle will always travel radially outward, even if the particle is in flight for only 1º of motor rotation. The particle leaves the screen at the point of maximum radial displacement and lands at a point of less radial displacement, which means that the particle will always land closer to the edge of the screen.
For the second example, let’s analyze the opposite extreme, when the bottom weight is advanced to a 180º lead angle. In this case, the vertical motion occurs before the maximum horizontal radial motion; therefore, the particle will be launched vertically before the maximum radial motion occurs.
To continue the same analogy, let’s again assume that the particle is in flight for 180º of motor rotation, only now the lead angle is set to 180º. The particle (depicted by the double-ended arrow in Figure 7C) will leave the screen vertically above the bottom weight, be in flight above the screen for 180º of motor rotation, and land again directly above the top weight. The particle leaves the screen (shown in the gray outline) and lands in the black outline screen position. When the particle leaves the screen, it is at the location of minimum horizontal radial displacement, and it lands at the location of the maximum radial displacement. The particle has landed farther from the outside edge of the screen and appears to travel radially inward, as shown in Figure 7D.
Again, using the same arguments, with the weights set at a 180º lead angle, the particle will always travel radially inward, even if the particle is in flight for 1º of motor rotation, because the flight of the particle begins when the separator is at the minimum horizontal radial distance. The particle leaves the screen at the point of minimum radial distance and lands at a point of greater radial distance.
Using these two extremes as examples, the lead angle’s role as the controlling parameter in particle flow direction in a vibratory separator is evident. The simplified discussions so far have been helpful to illustrate the effects of vibratory motion on particle motion, but, realistically, particles are not in flight for 180º of motor rotation.
The past examples have ignored the second horizontal axis motion by only considering the 180º positions. When the motor has rotated 45º, the screen position will have moved in two dimensions in the horizontal plane. Figure 7E shows the effect of 45º of motor rotation. Figure 7F shows the top view of a single particle displacement on a screen, clearly illustrating that a particle will more realistically move in two horizontal dimensions, with both radial and angular displacements.
Figure 8 depicts the complete range of material flow patterns that can be obtained by adjusting the lead angle in a round vibratory separator. As the lead angle is increased from 0 toward 50º, the horizontal radial particle displacement is diminished. The radial velocity of the particle decreases because the horizontal tangential motion is increasing. This yields an increasing spiral path to the flow pattern.
At a 60º lead angle setting, the material appears to travel only tangentially. This can be explained by a particle being launched vertically 60º before the top weight and landing 60º after the top weight in the same radial position on the screen. Material will not readily discharge from the spouts at this setting.
As the lead angle increases above a 60º lead angle, the radial inward displacements increase, and these patterns depend on when the particle lands on the screen. If the particle lands after the maximum radial displacement, then the particle will move radially outward. If the particle lands before the maximum radial displacement, the particle will move radially inward. The speed and spiral pattern that a material travels over the screen cloth can be set by the operator to achieve maximum throughput and screening efficiency for any screenable product.
For more information regarding vibratory screen separator performance, contact Sweco at 8029 U.S. Highway 25, Florence, KY 41022; (800) 807-9326; fax (859) 283-8469; e-mail firstname.lastname@example.org; or visit the website at www.sweco.com.
SIDEBAR: At a Glance
- Top weight force controls the horizontal amplitude.
- Bottom weight force controls the vertical amplitude.
- Lead angle governs the material flow pattern and direction.
- Horizontal and vertical amplitudes can be easily measured to evaluate and understand separator performance.