One vibration type is sinusoidal. Sinusoidal vibration
is commonly delivered by rotary, or “silent “ piston vibrators.
Rotary vibrators are characterized as high frequency/low
amplitude. Common types of rotary vibrators are Ball, Turbine and Electromechanical
vibrators. With rotary vibrators, bin displacement response is proportional
to the
 frequency
and amplitude of the forcing function of the vibrator. This means the
bin wall deflects according to the forcing function of the vibrator
in order to transmit the vibrating energy into the bulk material. With
this method, much of the vibration energy is spent on creating the wall
deflection and is lost to the purpose of facilitating material flow.
A Rotary Ball Vibrator
Rotary vibration develops force vectors tangential to
the rotation; these shear vectors must be accounted for in the mounting.
To do so, long channels with intermittent welds are normally used for
mounting. This type of mounting also stiffens the bin wall, raising
the natural frequency of the bin in that area. Caution must be taken
not to operate the vibrators at the resonance frequencies of the bin
wall. If operated at resonance, mounting failure can occur–- regardless
of vibrator size.
The image at left is an actual measurement of sinusoidal
vibration using a vibration measuring device. The inset shows a constantly
even amount of energy emitted.
Pneumatic piston vibrators that are silent or air cushioned
can also create sinusoidal output. In this case there are no shearing
forces because force vectors are normal to the bin wall. Generally, these
vibrators are characterized as low frequency/high amplitude, so resonance
is not often a problem. However, the bin response is still proportional
to the forcing function f(x), with much of the vibrating energy used as
it passes through the bin wall.
Short
Time Duration Impulse |
The second type of vibration is short time duration impulse or ‘impacting’.
This type of vibration is commonly delivered by pneumatic
piston vibrators. The impulse is a form of elastic collision between
the piston and the base of the bore. The ideal result is conservation
of energy and momentum, with the energy being efficiently transferred
through the hopper wall; the bin responds with its own natural frequencies
(like ringing a bell) and the energy is transmitted efficiently into
the material in order to facilitate flow. This is the most effective
way to promote flow in bins, hoppers, chutes, railcars and any other
bulk material containers.
A Pneumatic Piston Vibrator
Pneumatic piston vibrators generally operate with a
forcing frequency well below
the bin wall natural frequencies, so resonance is not a concern.
This image illustrates the
vibrational energy emitted by
a pneumatic piston vibrator.
Analogy: The desk toy known as Newton's Cradle which
has five metal balls in a line, suspended by thin strings. When you
lift and drop a ball at one end, the energy is transmitted to the opposite
end ball, through the three middle balls, with no loss of energy.
This device demonstrates the conservation of momentum
and energy that is the principal of operation utilized by NAVCO
pneumatic impacting piston vibrators.
NAVCO offers both types of solutions. In
some specific applications rotary vibration is more effective than impacting
vibrators. Contact NAVCO for assistance with
determining the best solution for your flow problem.
Call toll free.
1 (800) 231-0164
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