How to Choose the Right Vibrator for Material Flow Problems

 

Why Material Flow Problems Happen

bin activator example

Material flow problems in bins, hoppers and silos (hereinafter referred to as “bins”) are common.  These problems are caused by adhesion between the material and the bin wall and high cohesive strength within the material.  Wall friction is dependent upon the bin wall angle, material of construction, smoothness, temperature, moisture, corrosion, abrasive wear and the time the material remains at rest in the sloped portion where the problem exists.  Cohesiveness is impacted by material properties, including moisture content.

Flow problems can exist as bridging or ratholing.  Solving these problems is essential to efficient production and accurate material inventory management.  In many cases industrial vibrators are employed to break the friction between wall and material, as well as material cohesiveness to promote material flow from the bin.  

 

Types of Industrial Vibrators

There are two types of industrial vibrators:

  • Rotary
  • Linear

 

Rotary Type Vibrators

These vibrators are characterized by an unbalanced mass rotating around a center point.  The result is a centrifugal force output at a given frequency.  Rotary electric vibrators include Ball, Roller, Turbine and Motor-Driven Eccentric types.  

Unbalance is useful for calculating the centrifugal force output, and it has specific use for sizing a vibrator for use on a bin.  It is even used for sizing applications such as vibratory screens and feeders with a known feed rate.

Rotary vibration is best applied to products that tend to bridge and rathole.   Rotary vibrators require a rigid mount to transfer the vibration  into the material.  A rigid mount will keep the frequency and force output higher, making the vibrator more efficient.

 

Linear Type Vibrators

PLV-NL Series

Linear vibration is obtained by oscillating a mass back and forth on one axis, resulting in force output (impact) as the piston decelerates and changes direction at the end of each stroke.

Three types of linear vibration exist; continuous impacting, continuous non-impacting and single impacting.  Continuous impacting are those which oscillate continuously and provide an impact similar to a blow from a hammer at the end of each stroke.

Continuous non-impacting vibrators have the same continuous operation, but there is no impact.  The vibrator provides a force output due to negative acceleration of the piston at the end of each stroke.  The piston is stopped by an air cushion rather than the base plate.

The single impacting design provides one impact each time the solenoid is energized.

Linear vibration is best for promoting flow of moist or wetter materials.  The object is to move the wall back-and-forth (“oil canning”).  This movement is beneficial for dislodging wet, sticky materials from the bin wall.

 

The Difference Between Electric or Air Vibrators

You should consider the following during vibrator selection:

  • Noise Level
  • Efficiency
  • Life
  • Maintenance
  • Initial Cost
  • Operating Cost

Electric vibrators are the most efficient, have the longest life, low maintenance and low noise.  However, the initial cost for an electric vibrator is higher than for pneumatic vibratorsTurbine vibrators are the most efficient and quietest of the pneumatic designs, making them best where low noise, high efficiency and low initial cost are required.

 

TypeNoiseInitial CostEfficiency
ElectricLowHighVery Good
Pneumatic
PistonHighLowFair
BallModerateModerateFair
RollerModerateModerateFair
TurbineLowModerateGood

 

Common electric motor-driven vibrators are those which use a motor with its shaft extending out of both ends of the housing with eccentric weights on both sides.  These vibrators operate at a variety of frequency, commonly 900, 1200, 1800 and 3600 rpm.  Voltages for electric vibrators include 115VAC/single-phase/60Hz; 230VAC/single-phase/60Hz; and 230-460VAC/three-phase/60Hz.

 

How to Choose a Vibrator

Selecting the best vibrator for the application requires knowing a few rules of thumb, plus field experience.

When applying a bin vibrator it is important to understand the problem:

  • Is material ratholing or bridging?  
  • Is material sticking to the walls of the hopper?  
  • Is the material wet or dry?  

The answers will determine whether a linear or rotary design is best.  Also, what power source will you use, electric, pneumatic or hydraulic?  The decision may be simply what is available.

 

When to Choose Rotary Vibrators

Rotary vibrators are typically used with dry materials.  To move material in a bin the friction between the material and the bin wall has to be overcome.  Once overcome, the material cannot cling to the bin wall, it will flow.  The force required to accomplish this for dry materials is a 1:15 ratio of the vibrator force to material weight contained in the cone section of the bin. 

First, we calculate the weight of the material in the sloped portion of the bin.  Normally this is the only place where the friction between material and bin wall needs to be broken. Do not calculate the total weight of material in the bin, only what is in the sloped portion.

Cylindrical Bin Volume =

cylindrical volume calculation

 

 

Square or Rectangular Bin Volume =

square or rectangular volume calculation

 

 

Once the volume is calculated. The weight of material in the sloped portion is determined by multiplying the volume by the bulk density of the material. 

The force output of the vibrator required can be determined, based on the 1:15 ratio (dry materials).  Example: the cone section of a 25 ton cylindrical bin contains 7,000lbs.  Dividing 7,000 by 15 indicates that a vibrator producing ≥ 470 lbs of force is needed.  Most vibrators are adjustable, by varying the eccentric weight, air pressure etc. 

Large Bins may require two vibrators, each on opposite walls or 180° apart, for better transmission of vibration into the material.  Determine the force output required, then divide by two to determine the force output for each vibrator.  This places less stress on the bin wall for applications requiring very high force output.

 

When to Choose Linear Vibrators

Linear vibrators are the best choice for wetter materials due to the high-energy output in one direction.  Linear vibration will flex the bin wall in and out, which is best for removing wet material from the bin walls.  

Linear vibrators are sized by knowing the force output or the bin wall thickness.  Determining a force impact value from impacting vibrators requires testing with sophisticated equipment that can measure high forces over short periods of time (characteristic of an impacting vibrator).  First, the acceleration of the piston must be determined.  Then, knowing the mass of the piston, the force output can be calculated.  

Linear vibrators are best sized by the wall thickness of the bin.  The vibrator will clean ≈ 4’ to 6’ area.  It may be necessary to apply several linear vibrators when the entire bin must be cleaned.  If the object is to maintain material flow only, fewer vibrators may be used.  Use the following guidelines for impacting vibrators:

  • Wall Thickness of 0.7” to 0.125”; vibrator with 1-¼” piston 
  • Wall Thickness of 0.125” to 0.25”; vibrator with 2” piston
  • Wall Thickness of 0.25” to 0.375”; vibrator with 3” piston
  • Wall Thickness of 0.375” to 0.5”; vibrator with 4” piston

If using a non-impacting design, choose a model with the next larger piston.

 

How Moisture Content Affects Your Choice of Vibrator

Moisture affects the cohesiveness of the material and the friction between material and bin walls.  As moisture content increases so does the cohesiveness of the material, as does the materials’ tendency to form cohesive arches or bridges.  Only when the moisture content approaches saturation does the cohesive strength decrease.  At this point the material behaves like slurry.  

Relative humidity influences the moisture content of the material, especially with materials that are hygroscopic.  Hygroscopic material increases in moisture content with increases in humidity, in turn increasing cohesive strength, resulting in the likelihood of material flow problems.

Material with moisture content ≥5% is considered wet.  Wet materials require more force  to move.  If the moisture content % is not known. Use the following as a guide. 

  • Dry 
    • Does not clump or hold a shape when squeezed in the hand. 
    • Flows thru your fingers like dry sand at the beach.
  • Wet
    • Clumps or holds a shape when squeezed in the hand. 
    • Does not readily flow through your fingers

These guidelines will qualify whether the material is “drier” or “wetter”, and whether a Rotary or Linear vibrator is best.  But it’s not always “black-and-white”.  There is grey area in the selection process.  Rotary vibrators are commonly used with wetter materials because of the noise and inefficiency of Linear types.  Only Rotary vibrators in the 1800 rpm range should be considered for materials that are not dry.  The force output should be increased by 5% for each % of moisture content.  For example, the force output for use with material that has 5% moisture content would be increased by 25%.  

 

Vibrator Mounting Guidelines

A pneumatic Rotary vibrator will operate at the natural resonant frequency of the bin wall.  An electric vibrator operates at its designed rpm, assuming its horsepower is high enough to bring it through the resonant frequency of the bin wall.  With Rotary vibrators we need to make the bin wall as stiff as possible to transmit the vibration through the wall and into the material.  Moving the wall back and forth (“oil canning”) with rotary vibration proves to be inefficient and may damage the vibrator.

The mounting for Linear vibrators is different.  If vibrating dry material, you should use the same guidelines given for a rotary vibrator.  For wet materials that stick to the bin wall, use a mounting channel 12-18” in length.  This will allow the linear action to flex the wall (required to remove sticking material).  Linear vibrators will operate at their own frequency depending on the air supply pressure.  They are less affected by the natural frequency of the bin wall.

 

Conclusion

Material flow problems exist for many reasons.  Solving them is not always easy.  Industrial vibrators, when properly selected, offer an effective and high value solution.  Many vibrator designs exist.  Each provides a unique solution.  Defining your needs and choosing a vendor with a wide selection of vibrator technologies and the depth of knowledge are key to finding long term solutions.

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