Don't let vibratory separators shake you up

By Eric Johnson, P.E.

Chemical Processing magazine

Understanding a few key points simpifies setup and operation, and leads to optimum screening patterns. These include adjusting vibratory motion and optimizing separator performance.

A standard round vibratory separator uses a screen cloth enclosed in frames. The frames are vibrated by a motion generator consisting of a vertically mounted double-end shaft motor with eccentric weights on its top and bottom (Figure 1). The motor rotates counterclockwise (CCW) when viewed from the top. As the motor rotates, the weights generate a radial centrifugal force that causes 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.

 Adjusting vibratory motion
There are three independent variables or adjustments to a vibratory separator: top force, bottom force and lead angle, which is the angular weight setting. The output variables are horizontal motion amplitude, vertical motion amplitude and phase angle, which is the measured delay between the maximum vertical and horizontal amplitudes.

 

Figure 1: Eccentric weights on the top and bottom of the vertically mounted motor generate vibration.

To simplify discussion, let's first consider only the top motor weight. The top motor eccentric weight is designed to be at the center of gravity (CG) of the vibrating machine. Force acting at the CG of a mass will cause uniform planar motion in that mass. In other words, the top weight force spinning at the CG will create a uniform horizontal radial motion of the machine without any torque around the CG.

 

Figure 2: An eccentric weight located at the center of gravity of the body causes horizontal motion.

Visualize a separator as a solid cylinder, as shown in Figure 2. The top eccentric weight force acts at the CG of the body. When a force acts at the CG, horizontal motion of the body will occur in the direction of the top weight force.

Figure 3 depicts the same body at two different positions. The first, shown in gray, occurs when the force is pointed left. As the motor rotates 180 Degrees , the force will point to the right and cause 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 180 Degrees of motor rotation.

 

Figure 3: As the motor rotates 180 Degrees , the separator moves horizontally from its initial position, shown in gray, to a new position, shown in black.

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 only the motor and weights rotate, not the cylinder. Because the top weight force acts at the CG, the cylinder will always remain horizontal. Variable horizontal motions will occur as the magnitude of the top force is varied.

Now, if we add another eccentric weight, F_BW, as shown in Figure 4, to the bottom of the motor below the machine CG, this weight will induce a torque about the CG that creates vertical motion as the machine tilts from the vertical axis. The result of these weights is a cylinder tilted off the vertical axis. Adding more bottom weight yields more vertical motion.

 

Figure 4: An eccentric weight on the bottom of the motor creates vertical motion.

Figure 5 depicts the same cylinder in two different motor positions with the weights rotated 180 Degrees . The drawing shows the resultant horizontal and vertical motions that are generated by the eccentric top and bottom weight forces.

As the motor rotates CCW, the maximum amplitude generated will occur in the direction in which the force points. During rotation, the direction continually changes; elliptical motion in three axes is generated by one rotation of the motor.

In Figure 5, the top and bottom forces are vertically aligned with the maximum horizontal and vertical motion occurring in the same vertical plane or angular position.

 

Figure 5: Both horizontal and vertical motions are generated as the weights rotate 180 Degrees .

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 0 Degrees lead angle. When the bottom weight is 120 Degrees 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 120 Degrees of motor rotation before the maximum horizontal motion generated by the top weight.

Figure 6 shows the bottom weight leading the top weight by 120 Degrees . Note that the vertical and horizontal motion no longer occur at the same time.

 

Figure 6: With the bottom weight leading the top weight by 120 Degrees , vertical and horizontal motions no longer occur at the same time.

Lead angle is the parameter that gives a round vibratory separator the unparalleled ability to control material flow pattern. We will discuss the proper setting of lead angle later.

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