My first introduction to the productive work that can be accomplished with vibration was probably by my dad.  Back in the day our family would on occasion sit down for some TV and popcorn.  Sorry, no movies on demand, no DVDs or even VHS, I’m not sure how we made it through those dark days!  This was pre-microwave popcorn and my mom would pop it the “old school” way, with a heavy pan and oil.  I think you can still see this technique demonstrated at a museum or two.  Traditionally the popcorn was served up in a large spun aluminum bowl.  My dad had the rather annoying habit of shaking the bowl which would vibrate the un-popped kernels, “old maids,” down to the bottom of the bowl which created a rather loud noise as they hit the aluminum.  More than once I asked him what the heck he was doing, he’d say he was “bringing the big popcorn to the top”.  When I asked how that worked his pat response was – “physics”!!

Well, some years later I found myself working at The Cleveland Vibrator Company where “shaking the popcorn bowl” is a pretty common event.  Typically we’ll find ourselves using this technique to screen products as we transport them on a screen cloth section or other screening media.  Jack Steinbuch is our screening expert, while we’ve never discussed shaking popcorn to get the big pieces on the top, I’m sure that Jack would give a somewhat more in depth explanation of the screening process than my dad’s “physics!” response.

While I believe my dad thought he was “bringing” the big pieces to the top, in fact he was actually vibrating the popcorn in such a way as to allow the smaller pieces to filter throughout and drop down to the bottom, the net result was that the larger pieces were on the top of the bowl. This is a pretty common occurrence in a screener application.   As the material is vibrated and instantaneously airborne, the gaps in the material flow are filled by the smaller particles as they work themselves downward in the bed of material.  On our electro-mechanical vibratory screeners, EMS, a good deal of consideration is given to the selection of the rotary electric vibrator during the quoting and design phase of a project.  As I mentioned before different frequencies provide different results in terms of the velocity of the material flow on the unit and the amplitude of the stroke of the unit. In the screening process we want to expose the material flow to the openings in the screen or perforated plate, giving those particles smaller than the opening the opportunity to drop out of the flow and be separated.  “Shaking the popcorn” or vibrating the screener aids in the separation process, moving smaller particles downward and exposing them to the screening medium.

Typically you’re not going to see Cleveland Vibrator using 3600 rpm rotary electric vibrators on a vibratory screener.  The resulting stroke of the material is not advantageous to the mechanics of screening of larger particle sized materials.  Fine screening applications, such as those performed by HK Technologies, couple high frequency “ultra-sonic” excitation of the screen cloth with a moving bed of material typically produced by either 1800 – 3600 rpm vibrators.  This is a bit different as you want to keep the material close to the screen as you circulate it on the unit.  I’m sure our Ultra-Sonic/Fine screening expert Jeff Hochadel has written about this in his blog entries.

Another point to be taken from my dad’s popcorn shaking can be applied to the use of vibratory tables in compaction applications.  This would be more of a cautionary tale or reminder.  In the past I’ve written about compacting and consolidating materials with the use of vibration.  It is very common to vibrate loose filled material on a flat deck vibratory table or belt table to aid in container filling.  It is worthwhile to note that with products such as concrete or similar materials with a variety of particle sizes in suspension, over vibration can be a problem.  Just as my dad was able to get the nice big fluffy pieces of popcorn to move to the top of the bowl, over vibrating can start to cause particle segregation in the mix.  As with many things it’s important to know when to say “when”.  Vibrate enough to remove voids in the container or mold and make sure to stop before you start to separate out materials within the mix.

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