Strike & High G’s, A Vibratory Table’s Compaction Conundrum

David Strong

My mom just had her 80th birthday the other day. To celebrate the milestone, one of my sisters and her husband are taking my mom on an Alaskan cruise, I’m sure it will be a great adventure. A few years back I had the opportunity to visit Alaska as part of a Cleveland Vibrator Company project, I was there to provide start up assistance and review the installation of a piece our equipment. From start to finish it was a very interesting application of mechanical vibration to assist in the compaction of a dry material. Throw in a trip to Alaska and it became quite memorable.

Cleveland Vibrator Company was contacted by a large engineering firm that was involved in a remediation project at a military installation in Alaska. The company was removing contaminated soil from the site.  The soil was to be placed in steel containers and shipped off site. Their challenge to us was to increase the density of the soil, put more material in the box than could be accomplished with just “dumping” soil into the container. This is a fairly typical goal for a vibratory table, put more material in a given volume, and increase the products density.  More material in the box equated to fewer boxes, reduced shipping and storage costs and improved the overall effort of the project.

Compacting material with a vibratory table isn’t very unique, over the years I’ve been involved with the design of a large number of flat deck vibrator tables powered with rotary electric vibrators or pneumatic piston vibrators. Off the top of my head a quick list of produces compacted with the aid of industrial vibrators would include; powdered metal, breakfast cereal, almonds, peanuts, packaged sugar, chocolate chips, foundry sand, scrap aluminum, dry mixed ceramics, refractory material and concrete of all shapes and forms. Pretty standard, nothing shocking, but when someone starts talking about compacting 12,000 pounds of contaminated soil, you tend to sit up and take note.

During the quotation phase of the project The Cleveland Vibrator Company was provided with a representative clean soil sample.  Compaction testing was performed at different frequencies and varying levels of acceleration.  Results of our in-house testing indicated that we could increase the loose bulk density of the soil with the aid of mechanical vibration by 20-30%.  This increase in bulk density was seen as having value to the project and a vibratory table was put on order.  The final unit was 54 inches wide by 93 inches long, designed to effectively vibrate 12,000 pounds of soil plus the weight of the storage container.  To help keep the container in place during vibration we added a heavy duty steel angle to the top of the table on three sides, bumpers to limit the movement of the box.

The design was finalized and accepted by the customer and fabrication was begun.  Due to the limited working season in Alaska the unit was fast tracked though manufacturing with the use of “premium time” significantly reducing the lead time to manufacture the vibratory table.  Once completed the unit was successfully tested and prepared for shipment to the work site.

I think it was a combination of the following things that made it possible for me to visit the work site and inspect the installation of the unit.  Customer service on the part of Cleveland Vibrator, the uniqueness of the application and maybe my persuasive communication skills, well maybe I shouldn’t go that far! Regardless of the reason, I made the trek up north and was able to visit the site and see the table in operation and grab a glimpse of Alaska.  The unit was set up outside near the remediation site, powered with a portable generator and filled with a front end loader.  I witnessed a couple of loading and vibration cycles, everything looked good.  The soil compacted inside the container, reducing the height of the material and allowing more material to be placed in the box.  The use of the vibratory table and properly applied industrial vibrators accomplished their mission.  At the time my only comment was that the table wasn’t perfectly level, during the vibration cycle the box would move towards the low side of the table, vibrating downhill and actually rub against the retaining angle.  The friction between the container and the angle was enough to heat up the angle and actually burn off the paint in the contact area.  The customer said that they’d make the adjustment and level the table to reduce the “walking” of the box while being vibrated and eliminate the friction point.  So that was the end of my Alaskan adventure, back into the car and off to Fairbanks and my return flight to Cleveland.

Sometime after my visit we started getting feedback from the customer that they were having difficulty running the unit for extend periods of time and at full speed.  The motor overloads would see an over amperage condition and shut the unit down. Controls included a variable frequency controller (VFC) with individual thermal overloads for the vibrators. We started to explore concerns that the VFC was having problems with the input power from the generator.  In an effort to assist in the trouble shooting process we sent our electrical technician up to Alaska to inspect first hand.  Our technician and those of the customer couldn’t pin point the source of the problem to resolve it.  As a result the customer limped along during the remainder of the work season running with reduce force from the vibrators.  It was agreed upon that once the cold weather set in the customer would return the unit to Cleveland along with a container and we’d conduct testing at our location to determine the problem and fix it.

In the early fall the vibratory table, controls and the container arrived in at The Cleveland Vibrator’s plant.  The unit was inspected and run; again no problems were identified in the unloaded condition.  Our customer had suggested that we get 4 cubic yards of dry concrete mix, less the cement and naturally the water, and use this for a full load test.  Apparently, the dry mix closely approximates the characteristics of the work site soil.  We did purchase the mix and had it dumped directly into the container.  This was moved into position on the vibratory table and we ran the unit.

During the test cycle it was easy to see the drop in height of the material in the container, within 20-30 seconds we began to see and hear the box impacting the table, this was shortly followed by an overload condition within the control box and a shut down.  We found that one of the motor overloads had experienced a high amperage condition; as a result the unit had shut down.  The same as the customer had experienced.  This presented a puzzling situation one that we hadn’t seen before.

Stepping back to try and examine the problem I started thinking about the typical compaction tests I’d been involved with in the lab.  We usually take a small sample of material and place it in a graduated cylinder, recording the height of the column of material before and after vibrating, looking at the change in volume and resulting density.  As you watch the compaction process you’ll see the column of material “drop” and compact.  Once all the voids within the column of material are “gone” it’s not uncommon to have the cylinder start to “bounce” on the test table.  Jack Steinbuch and I have always felt that when compacting a material with vibration, regardless of the means of producing that vibration, rotary electric vibrators or pneumatic vibrators, the actual work of compaction is accomplished in a rather short period of time.  Once the material has compacted what happens?  Since there is no additional room within the container for the material to “move downward” the loose material begins to act as a single mass.  This newly created “single mass” will separate from the vibratory table and then return and strike the table.

All of our rotary electric vibrators have limitations to the acceleration that they will withstand.  On this vibratory table we were using 1800 RPM Vibrator, 4-Pole Units.  These vibrators are rated for 12 g’s of acceleration.  As I mentioned in a previous blog post, metal on metal impacts will result in very high levels of acceleration.  I started to theorize that once the material was compacted in our customer’s container that soil/container mass would instantaneously separate from the table and then with the combination of gravity and the motion of the table, strike the table resulting in a spike of the g’s.  Unfortunately during our first test no one thought it was critical to record the g’s on the table.  We’d measured that during our final testing of the unit before shipping it and everything was in line with what we’d projected for the table.

After some discussion we decided that we needed to rerun the test and measure the acceleration.  Unfortunately, this required a couple of our folks to spend a fair amount of time to dig out the compacted dry concrete mix from the box and prepare it for another test.  On the next test we attached our accelerometer to the vibratory table to monitor g’s during the compaction cycle.  The unit was started and as in the first test the height of the material dropped and compacted and the table again began to strike the container.  At the start of the cycle we saw g’s in the 2.5 to 3 range which is pretty standard for a vibratory table material compaction application.  However, as the material reached its compacted condition and the striking started, we saw the g’s rapidly approach and then exceed 19 g’s before we overloaded our instrument and the table shut down.

What we’d observed on a small scale in the test lab was now proven out with 12,000 pounds of material.  While the load is “live,” moving and compacting, we don’t see separation between the table and the container.  But once the material is compacted, the dynamic relationship between the table and the container change significantly. It seemed like that was the problem.  The introduction of vibration into the loose material resulted in a lumped mass.  Continuing to vibrate the container did not further compact the material but caused the table and container to separate then strike each other.   The end result was an impact which introduced high g’s into the table and the vibrators.  The high g’s caused the vibrators to overload and “pop” the thermal overloads.  To prove this theory we placed a ½” thick urethane pad between the container and the table, then ran the unit with the material compacted.  With the pad in place we were able to dampen out the strike and avoid the high g’s impact condition.

Certainly lessons learned on that project.  What we’d seen in the test lab with the compaction of a small material sample translated to the same result in a full sized unit.  The real work of a vibratory table was accomplished pretty quickly, continuing to vibrate after the densification of the material was accomplished is not productive and in this particular application, very detrimental.  Looking back at my visit to Alaska and witnessing the unit in action at the customer’s site, I wonder if when they ran the table without it being perfectly level, the contact between the container and the angle restraints produced enough friction to keep the g’s below the limit of the rotary electric vibrators.  As a result I didn’t see the high g’s impacts that they experienced after leveling the table and putting it back in operation.  In the world of industrial vibration, there’s still a fair amount of “live and learn” going on.  It’s not a perfect world but we do try to make the best of challenges and learn from them.

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