When it comes to vibratory equipment and end user requirements, Cleveland Vibrator has worked with customers that span the range from simply wanting to move material from point A to point B and they’re not terribly concerned with much more than that goal. Other customers use vibratory equipment to provide bulk material to downstream processes and are looking for more precise control of the material flow.
Recently I had the opportunity to visit a local customer and assist him with setting up a recently completed hopper-feeder unit. This customer’s goal was to place a dry material into plastic trays while the plastic trays moved on a conveyor belt under the vibratory feeder. Precise metering of the dry material into the trays is critical to this customer’s success. As I worked with this customer and our equipment it seemed to me that this would be an excellent basis for a discussion of material flow and what parameters impact the control of the flow. As mentioned the equipment provided by Cleveland Vibrator is a vibrator hopper feeder. The hopper is a stationary non-vibrating hopper with an adjustable slide gate. The vibratory feeder utilizes two rotary electric vibrators which are controlled by a variable frequency drive (VFD) with a dynamic brake resistor.
To achieve a precisely controlled flow of material we have to start with the material itself. The material must be consistent both in particle size and density, dry and free flowing. Regardless of how well the system is tuned, if the material doesn’t flow freely or varies significantly from batch to batch it is going to be very difficult to get good repeatable results due to the simple fact that the material itself is not consistent.
If we start looking at the parameters the control the flow off the end of the vibratory feeder, critical factors would include:
- Bed depth of material on the vibratory feeder tray
- Frequency of the vibration applied to the feeder tray
- Amplitude of vibration or stroke generated by the feeder itself
Each factor has its limitations to influence the flow of material but when all three are combined together it is possible to vary the flow rate and provide very repeatable results as the material cascades off the end of the feeder.
- Bed depth of material on the tray. As mentioned earlier, it is important that the material is free flowing and that there is always material available in the hopper to charge the feeder. Inconsistency is the material available to the feeder will change the depth of material flowing on the feeder. Not enough material will “starve” the feeder and reduce the bed depth on the feeder. Fluctuations in the volume of material available in the hopper will cause the material bed depth to rise and fall during the operation of the equipment, for consistent discharge rates we need a consistent bed depth of material. Initial efforts to adjust the depth of material are done with the hopper slide gate. Opening the gate allows for a higher volume of material to be removed from the hopper resulting in a deeper flow of material and typically higher volume off the end of the feeder. Likewise, reducing the opening restricts the volume of flow out of the hopper, all things being equal; lowering the gate will reduce the bed depth of material on the vibratory feeder. Lower bed depth of material will normally result in a reduction of the flow/volume that can be moved by the vibratory feeder.
- Frequency of the vibration applied to the feeder tray. Understanding the material to be conveyed and what frequency is most effective in moving it via vibration is the starting point for frequency selection. Different materials respond better to different frequencies of vibration, this influences the vibrators selected for the application. With the use of Rotary electric vibrators we have four base frequencies with which to work. Starting with 2 pole vibrators that operate at 3600 vibrations per minute, we get the highest frequency and resulting smallest amplitude of vibration. We can go down in frequency from the 2 pole vibrators to 4 pole – 1800 vibrations per minute and then down to 6 pole with 1200 vibrations per minute. The lowest frequencies are generated by the 8 pole – 900 vibrations per minute vibrators. As I’ve talked about before, with equal force, the lower frequency vibrator will produce the larger amplitude of vibration on the same piece of equipment. Heavier materials tend to require higher frequency drives while lighter materials feed more effectively with lower frequency drives. Frequency and amplitude make a significant impact on the rate of material flow.
- Amplitude of vibration or stroke generated by the feeder itself. As just noted the frequency of vibration strongly influences the amplitude of the feeder. The other important factor to be considered is the force output of the vibrators. Based on the requested feed rate, vibrators are chosen that will achieve this rate. This selection is based on both the frequency of vibration and the maximum force output of the vibrator. Initial calculations will drive the vibrator selection in terms of force output. With rotary electric vibrators we can select the appropriate vibrator based on force output and later if necessary adjustments to the eccentric weights can be made to reduce the force output from the unit’s rated maximum. For a given frequency, more force output will result in a larger amplitude or stroke of the finished equipment.
With a fluidized material being conveyed on a vibratory feeder, it’s sometimes difficult to visualize what’s actually happening on the discharge end of the feeder and how the parameters all work together. It might be easier to think of the material flow as solid material being “extruded” down the feeder tray towards the discharge end. This is sort of like those videos you see of glaciers pushing into a body of water where the leading edge of the glacier falls off and into the water. If we were to think of the flow as a soft material being extruded, say fudge for instance, as the material is pushed out if we adjust the height of the hopper gate we get a thinner layer of the material. As the material reaches the end of the vibratory tray it’s as if we’re “cutting” off a strip of material the width of the vibratory feeder tray with each stroke of the unit. With higher frequency units we get more “cuts” per minute and we’d expect to see more slices of fudge piling up near the end of the unit. With slower frequencies of the same force while we’d see fewer slices of fudge being “cut” per minute but we would get a longer, thicker, strip with each cut due to the larger amplitude produced by the slower frequency. The larger amplitude pushes more fudge off the end with each vibration but since there are fewer vibrations we get fewer slices in the same time period.
Back to our local customer, to give him the consistency in the depth of material filling his trays, adjustments were made to the hopper gate height, weight settings on the vibrators plus changes to the operating frequency via adjustments made to the VFD. Our first adjustment was to the hopper gate, getting a fairly thin layer of material in the vibratory tray. After waiting some time for the system to reestablish equilibrium, we started changing the size of the “cut” and frequency with which we made those “cuts” at the end of the feeder tray. Desired results were accomplished by setting the vibrator weights to 80 percent and setting the VFD speed to 55 hertz. With these settings we were able to sprinkle a thin well controlled layer of his dry material onto the trays. Changes in the speed of the belt conveyor moving the trays would also influence the final results, but once we agreed upon a set speed for the belt conveyor moving the trays we successfully adjusted the parameters of the hopper feeder to accomplish the task.
Generally speaking options are always a good thing. With a vibratory hopper feeder with a variable frequency controller, options for the controlled flow of dry material of consistent size and density are plentiful. Working the available parameters, one at a time, many bulk material flow applications can be successfully tackled. Have more questions? We are happy to help. Contact Cleveland Vibrator’s Engineering Team today!
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