Vibration energy for many years has proved it's operational reliability in discharging bulk materials from bins, transporting them over limited distances, charging and dosing them continuously and batch wise, compacting, loosening, and screening and grading them (including dedusting and dewatering).
Vibration energy can be generated in many ways but for the purposes of this article we will discuss the two most efficient ways of generating, channelling and controlling vibrations to suit the various uses of industry. In short we are talking about vibrations generated using Electromagnetic drives and Unbalance- weight motors.
Both Electromagnetic drives and Unbalance-weight motors have their characteristic advantages and some disadvantages.
Unbalance-weight motors for example have a high output force to weight ratio, a wide range operating speeds, and are simple in construction and working principle. For this reason Unbalance weight drives are often cheaper to operate and maintain. They are easier to manufacture and are far more rugged.
Electromagnetic drives on the other-hand offer step-less and easy
setting of the oscillation amplitude (sometimes even via a remote
panel), accurate and repeatable setting of oscillation amplitude,
a long life (since they have no rotating parts), and they reach
the desired amplitude immediately upon switching on, and they come
to a rest after being turned off in fractions of a second.
Unbalance-weight motors generate vibrations mechanically.
Unbalanced (eccentric) weights are attached to a shaft. The shaft
is then forced to rotate. However due to the effective
eccentricity of the shaft a set of vibrations are produced. These
vibrations are then controlled to create circular, elliptical or
linear movement. For example a single unbalance-weight motor will
create circular vibrations, while two motors rotating in opposite
directions will create linear vibrations.
Usually, especially for conveying and screening purposes, linear
vibration is desired. However for special applications such as in
a helical conveyor the motors are rotated in the same direction.
The amplitude of the vibration depends essentially on the weight
and the eccentricity of the masses attached to the motor shaft and
the frequency (speed) of the motor.
Electromagnetic drives as the name suggests contain an
electromagnet. Electricity is passed through the coil of the
electromagnet that is wound around a stator. The induced magnetism
in the stator pulls the armature. By cutting of the electrical
supply the armature is released and pushed back to its rest
location by a set of springs. The vibration energy generated by
this pushing and pulling of the armature is used for conveying,
screening, compacting, loosening, grading, etc.
A system such as this can best be described as two masses, one the
vibrator and the other unit being vibrated, which are connected
together. The oscillations created by this system may have to be
damped so that only those vibrations that are useful are
transmitted to the surroundings. There is a definite natural
(resonant) frequency (Fe) for this system even when the system.
However since an external driving frequency (Fa) acts upon this
system, it vibrates at the frequency of the vibrator (Fa) and not
it's natural frequency. The smaller the difference between the
natural (resonant) frequency and the driving frequency, the
greater will be the vibration amplitude
If the two frequencies coincide the resonant vibration will have
infinite amplitude...theoretically!
Fixing the difference between Fa and Fe determines the operational
stability of the system. Different operational properties can be
obtained by tuning the system above (Fe > Fa) or below (Fa > Fe)
the driving frequency. If Fe > Fa the vibration system is said to
be subcritical. If Fa > Fe the system is said to be supercritical.
Conveyor troughs and tubes can be driven by several vibrator units
mounted on the sides or under or over them. Long conveying lines
can be constructed by elastic flanging of several troughs and/or
tubes. The throughput of the conveyor largely depends on the
characteristics of the bulk material, the oscillation amplitude,
the cross-sectional area of the conveyor, and the oscillating
frequency.
For light and elastic bulk materials (bulk densities less than 0.5
kg/dm3) lower frequencies (750-1500 oscillations per minute) are
prefered, while materials with higher bulk densities generally
require higher frequencies (1500-3000 oscillations per minute).
Certain conveyors (with small dimensions) have been designed to
convey material at even 6000 oscillations per minute! However very
often the exception becomes the rule. Other factors such as
humidity can drastically change the theoretical parameters of
conveyor design.
Dosing feeders consist of a weighing belt and trough conveyor
mounted in series. The desired throughput can be prepared and
compared, and continually corrected against the actual value by an
electronic weighing device. Throughputs up to 1000 TPH and
accuracy up to 0.5% can be achieved under ideal conditions.
Feeders for charging rolls and mills extract the bulk materials
from hoppers at the same width as the rolls. To achieve an even
layer height the front exit lip serves as a stripper. Powders and
dust can be fed with normal conveyors only in a shallow layer or
very irregularly. This can be greatly improved by lining the
conveyor with flexible plates. The flexible liner lies loosely on
the base of the trough or tube and is only fixed on the sides.
Holes are drilled in the base to allow atmospheric pressure
compensation during feeding. The material is them able to "lift
off" from the base and glide without producing too much dust or
adhering to the base.
Fan type conveyors are used if the material has to be spread into
a shallow but wide pile at the discharge end (for example for
feeding evenly across the whole of a furnace opening). The trough
of such conveyors widens in a trapezoidal profile towards the
outlet. Deflectors distribute the bulk material over the whole
width of the outlet. Irregularities in the layer height are
smoothed out by a "dam" mounted at the outlet of the trough. Many
fan type conveyors that are exposed to high temperatures at the
outlet are equipped with an interchangeable end section
manufactured from heat resistant material.
Vibration spreading units are also suitable for symmetrical
charging with a small amount of fluid material. An inclined plate
mounted beneath a hopper is activated by vibration. The fluid
material flows through an adjustable gap over the front edge of
the plate. The spreading area (e.g., a belt conveyor) is moved at
a constant velocity.
Vibration energy can also be redirected to force material to
travel upwards (against gravity). These units are often called
spiral elevators. Spiral elevators are oscillating machines that
force material to travel upwards using a spiral path. The material
enters the machine through a dished plate usually at the base and
is then conveyed upwards in even spiral tracks. The feeding height
of a spiral elevator is limited -- depending on the stickiness,
density, and volume of the material. In most cases the feeding
height rarely exceeds 6 meters. Spiral elevators are most suited
for heating, cooling, or drying purposes because of the long dwell
time of the material on the spiral. In these specialised cases a
double spiral base serves as a heat carrier for the flow of water,
steam, or hot air. Vibration can also be used for screening, grading, and dewatering
bulk materials. In all of these applications the vibratory
conveyor is equipped with a set of plates with holes (usually
called screens) instead of a solid base. The material flows over
the conveyor and is separated by the screens. The ability of such
a vibratory machine to perform is determined by the sieve opening,
screen size, screen quality, oscillation frequency, amplitude of
vibration, and other independent factors and screening constants
to detailed to discuss here.
The throughput of a screen can be controlled by optimising the
mesh opening. Increasing the percentage of oversized material and
increasing the percentage of fine grain will increase the
throughput of the screen. Critically sized material (just below
the mesh size) aggravates the screening and often leads to
plugging of the screen deck. Dry materials are easier to screen
than their sticker counterparts.
For discharging and separating bulk material containing large
lumps from hoppers, and simultaneously roughly extracting the fine
grain sizes on to belt conveyors specialised screens called bar
screens are used. The trough of the screen acts as the closure for
the hopper or bunker and acts as a discharging device. The
material falls on one or more stepped and inclined bar screens.
During the transfer from one step of the screen to the other
the material becomes separated allowing for the fines to pass
through the bar screen openings.
Dewatering screens operate on similar principles as ordinary
screens but they are designed to extract liquids from mixtures of
solids and liquids (e.g., draining of sand, plastic granules,
limestone, coal, etc.). The base of a dewatering screen is
fabricated from conical shaped Ni-Cr steel, or plastics. Depending
on the composition of the grain-size curve a residual moisture
content of 10-15% can be achieved.
Vibrations are also used to compact or loosen material. Vibrating
tables are used for compacting of material into containers, for
loosening of adhering material (such as moulding sand in casting
boxes), as well as for testing of electronic parts and devices for
percussion sensitivity. The containers are placed (sometimes
fixed) on the vibrating table and vibrator is then turned on.
Impact Vibrators and Bin Activators are used to shake material
stuck to hopper walls. Impact Vibrators are Electromagnetic drives
that are specially made to induce vibrations to hopper walls. The
vibration loosens material stuck to the walls and is very useful
in enhancing the flow rate of bulk material -- especially sticky
materials like clay, limestone, etc. Unbalance-weight motors can
also sometimes be used for this purpose. Bin Activators consist of
a vibrator attached to a custom made bin. The material falls from
the hopper to the bin where it is loosened before being
discharged.
We hope that the above over view has shown you the uses and
advantages of vibration energy. For obvious reasons we could not
discuss details and technicalities of specialised applications.
Each application has a specific solution. Since there are so many
variables, very often no exact answer can be theoretically given
unless the feeder, conveyor, or screen is built and tested in real
world conditions. Most operators and users of vibration equipment
should be willing to experiment with various combinations of
material, screen sizes, frequency of vibration, etc. to achieve
the best results for their application.
HOW DO UNBALANCE-WEIGHT MOTORS WORK?
HOW DO ELECTROMAGNETIC DRIVES WORK?
APPLICATIONS OF VIBRATION ENERGY
Discharging and Conveying
Charging And Dosing
Spreading And Feeding (upwards)
Screening (Sorting), Grading (Sizing), and Draining (Dewatering)
Compacting, Shaking (Loosening), and Percussion Sensitivity
Testing
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