Vibration Energy and its Application

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.


HOW DO UNBALANCE-WEIGHT MOTORS WORK?


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.


HOW DO ELECTROMAGNETIC DRIVES WORK?


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.


APPLICATIONS OF VIBRATION ENERGY


Discharging and Conveying

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.


Charging And Dosing

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.


Spreading And Feeding (upwards)

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.


Screening (Sorting), Grading (Sizing), and Draining (Dewatering)

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.


Compacting, Shaking (Loosening), and Percussion Sensitivity Testing

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.


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