A: Jeff Dierig, Sweco, says:
Quite often our customers ask why their material on a vibratory separator or screener is moving in a certain direction or sometimes not moving at all. For example, the material may be staying in the middle of the vibratory separator’s screen and not discharging or the material is running backward on the screen or the material is instantly moving out to the screen’s perimeter once it’s fed into the equipment. The true answer is that there could be any number of reasons why these irregular patterns are forming. Of course, these situations may call for a seasoned professional to come in and fix the issue. Someone who is experienced with round vibratory separation equipment could enter the appropriate settings for any screener and make it run at its optimal efficiency. However, there are a few corrective ideas that should be considered to ensure the equipment is set up correctly or at least within the standard parameters of the equipment.
Motor rotation. If your material is running backward on the screen, consider checking the motor’s rotation. The motor should be running counterclockwise when looking down from the top or the counterweight assembly should move left to right when viewing through the base door. If you view the motor running clockwise or the weights moving right to left, your motor rotation is incorrect. You’ll need to reverse the motor leads in the control box.
Lead angle. A typical round vibratory separator is driven by a double-shaft motor installed in the vertical position. Affixed on the upper and lower motor shaft is a weight assembly that produces a resultant centrifugal force when the motor shaft is spinning. If you’re looking down from a top view, the number of degrees separating the bottom weight assembly resultant force and the top weight assembly resultant force is termed the lead angle. The lead angle is the most important factor in what determines material pattern on the screen’s surface. Keep in mind that the upper weight assembly controls the horizontal amplitude while the bottom weight assembly controls the vertical amplitude. The lead angle is adjusted at the bottom weight assembly and can be set anywhere between 0 and 120 degrees. Changing the lead angle will facilitate a flow pattern change on the screen where a higher lead angle will produce a more spiral pattern on the screen and, thus, a longer material retention time. So, for material that’s difficult to separate the fines from the overs, a higher lead angle and thus longer retention time would be required. For an application where the material is easily separated and the material must be removed quickly from the screen, a lower lead angle should be used. A 35-degree lead angle is the most typical setting for a standard one- or two-deck screener configuration, but all applications and materials are different and require a fair amount of testing to determine the best arrangement.
Other miscellaneous factors. Lead angle and motor rotation are the most common driving forces for how your material will move in a round vibratory separator. However, there are other factors that could influence the screener’s motion and, therefore, the material’s pattern on the screen.
- If the tension on your screen is loose, then it will affect how the material moves on the screen. This factor will also affect the throughput (amount of material that will go through the screen) and efficiency (the percentage of in-specification material extracted during the screening process from incoming feed material). So, if your screen tension is loose, you’ll need to consider replacing your screen so that your throughput and efficiency won’t suffer.
- If your screener isn’t level, it could also affect the material pattern. Make sure your screener is level when it’s installed.
- If the weight isn’t evenly distributed on the screener, your material pattern will be affected. Ideally, the frame discharge spouts need to be 180 degrees apart (for a single-deck machine) so that the weight is evenly distributed. On multideck configurations, all the spouts should be evenly spaced around the equipment.
- If you’re feeding your material onto the screen off-center, this too will affect how the material moves on the screen. Be sure to feed the vibratory separator in the screen’s center.
- If any of the springs on the screener are missing, broken or worn, this will affect the material pattern. You’ll need to replace any problematic springs immediately.
This is only a small sample of the factors that can affect your screen material patterns and, thus, your process. Not only will all these factors change the material movement pattern on the screens, but they’ll also negatively influence the screener’s efficiency and throughput.
Sweco supplies customized industrial separation equipment.