Vibrators & it’s Principle
Key path of choosing the appropriate E.M.I Vibrator
BULK MATERIAL IN MOTION
Vibration technology is used in most diverse application technological areas. In the “circuit of bulk materials” vibrators are used most beneficially for screening, filling, transporting, emptying and conveying.
They are used for both the improvement of processes as well as for the solution of problems, if the “circuit” should come to a halt.
“THE CIRCUIT OF BULK MATERIALS” consists mainly of:
- Screening and filling into container or BIG BAGs.
- Transport of these containers or BIG BAGs.
- Emptying of these containers.
- Conveying of these bulk materials and further
- Processing.
- Packaging again.
- Transport again
- Emptying again
Problem Definition
Within the circuit there are tasks and problems, which can be solved with the help of vibration technology
1. Screening of bulk materials
Screens separate bulk materials according to size or retain contaminants. Directed oscillation movements help to move the product over the screen and keep the screen meshes free at the same time.
2. Filling of bulk materials into containers or Big Bags
As much product as possible should be filled by weight into a container. Pouring cones during filling and air voids inside the product are most annoying. The assistance of vibration should help to fill containers optimally.
3 a. Transport of bulk materials in containers or Big Bags.
No transport process can be accomplished without vibration. This is why products in drumsare compacted during transport with a truck. Due to the content being
3 a. Transport of bulk materials in containers or Big Bags.
No transport process can be accomplished without vibration. This is why products in drumsare compacted during transport with a truck. Due to the content being compacted it cannot be easily poured out of the container or Big Bags.
3 b. Emptying such containers.
Due to the compaction the product rests against the slanted wall surfaces. This causes the formation of bridges. The outflow is restricted. Vibration should now destruct these bridges and at the same time clear the walls from product deposits.
4. Conveying of bulk materials
When emptying the containers or for further processing bulk materials must be conveyed. During such processes distances of various lengths must be covered with various conveying speeds. Vibration movements should help to push the material in “throwing direction”.
Vibration Characteristics
For each of the above mentioned tasks there is the correct vibrator available. For better understanding these can be divided according to their vibration characteristics:
- Group A: Circular vibrator.
- Group B: Linear vibrator.
- Group C: Hard beating vibrator (beaters).
- Groups A + B can be additionally divided into:
- Vibrations with higher amplitude in the lower frequency range.
The following describes the subjects conveying, compacting and emptying in more details:
Screening and Conveying:
The drive of discharge, conveying and screening troughs requires directed oscillations, the effective direction of which is determined by the inclination of the vibrator with respect to the bottom of the through. Inclinations of 30 to 45° are quite common in this respect. The material to be conveyed is pushed under this angle and moves on in form of a throwing parable. For screening the attack angle is steeper, so that the product falls almost vertically on the screening floor.
The conveying speed is influenced by the throwing angle. However, it also depends on the amplitude and the frequency. If higher conveying performances are required one chooses higher amplitudes with lower frequencies. This allows for the avoidance of resonance oscillations and oscillation nodes, particularly on light weight constructions. Light conveyor troughs therefore enable higher amplitudes and thereby higher conveying performance.
Movement of the trough is achieved by the oscillation of the trough against the moving mass of the vibrator.
The movements are there by divided in relation to their masses.
Center of gravity trough and position of vibrator
When mounting the trough on pressure springs the direction of force of the vibrator mounted in throwing direction must pass through the centre of gravity of the trough. Only in this case all points of the trough are uniformly lifted in throwing direction. The installation on leaf springs does not cause any problems at all. Due to the shape of these elements, which permit only one direction of movement, the trough is uniformly lifted in throwing direction at any bearing point. The throwing angle is thereby determined by the leaf springs. The position of the vibrator is only of secondary importance
Compacting:
Bulk materials to be compacted are placed on a vibrating plate on which the vibration drive is mounted from underneath. The vibrating plate must be mounted on oscillating bearings, so that movements are possible. The vibration elements are fastened to the substructure, which in turn is bolted to the foundation
For vibration elements there are rubber buffers, pressure springs and air bellows available. For higher frequencies and lower amplitudes rubber buffers are normally quite sufficient.
In case of higher amplitudes one should use pressure springs of air bellows. Air bellows have the additional function to lift e.g. a vibrating device up between a roller track, in order to lift off a container or a Euro-pallet with a BIG BAG to vibrate it.
When choosing the correct vibration elements a vibration insulation of up to 95% can be achieved. This means that floor foundation or adjacent machines will not be effected by the vibration.
Not all compaction procedures are identical. Vibration is chosen in dependence on product and filling process. However, the actual compaction process is the same for all:
Smaller particles slip into the cavities between the bigger components, the bulk material is compacted. Trapped air is pressed to the surface. Further material can be filled in.
For products in solid containers (steel etc.) a higher frequency with normal amplitude is more beneficial. Directed vertical vibration movements are introduced from underneath. The vibration energy also continues through the walls. The movement of individual particles relative to each other is ensured, without the risk that they may be “shaken loose” again. The compaction result is better.
However, bulk materials in Big Bags must be vibrated with a higher amplitude to obtain a sufficient compaction at all. In this case vibration is only introduced through the bottom. This requires vibrators with a higher working moment. The vibration energy is introduced vertically. Moreover, vibration should already be applied during filling. A better distribution of the bulk material is achieved during filling.
Emptying
When choosing silos for the storage of bulk materials very often only data such as storage capacity, storage contents and material properties are taken into account during the planning process.
There is mostly a lack of knowledge about the flowing behavior. This frequently causes interruptions in the material flow. If the discharge opening is clogged workers use sledge hammers to free it again. Such ”cosmetic measures” can be found in many production plants.
The type of material flow can be divided into two different main groups:
- 1. Mass flow.
- 2. Core flow.
The occurring type of flow depends on the friction/adhesion of the bulk material on the wall of the silo and on the shape of the discharge funnel.
The flow is called mass flow, if the complete stored bulk material starts to move when emptying the silo. In a core flow silo, however, the bulk material flows through a flow tube, whereby product permanently slips from the surface into this tube. A dead zone without movement of material forms around the flow tube. This dead zone will never be cleared if it is not completely emptied before filling. The storage time will be extended over an undefined period of time, which will impair the quality of the end products, especially in case of perishable goods.
In many cases these quality reductions are never viewed in relation with the core flow silo.
Vibrators mounted to the outside wall will solve the problem. Moving the slanted wall by vibration will reduce the friction between the material components and overcome the adhesive forces between silo wall and bulk material. The material bridge will collapse and the bulk material will flow out.
Outside vibrators with a frequency of 3000 oscillations per minute deliver good results if they excite only the wall and do not shake the whole container
INSTALLING RIGHT VIBRATOR AT RIGHT PLACE
Choosing the right vibrator depends on the flowing properties of the different bulk materials:
Well flowing bulk materials such as granulate, dry sand etc.: Electric external vibrators or compressed air operated piston vibrators with a frequency of approx. 3000 oscill./min.
Poor flowing bulk materials with low elasticity: Higher acceleration required, i.e. compressed air operated turbine vibrators or compressed air operated piston vibrators with superimposed beat into the air cushion.
Poor flowing bulk material with high elasticity(pasty):
Higher amplitude required, i.e. electric external vibrators or compressed air operated piston vibrators.
Interval knocker.
Bulky and interlocked materials such as soy shoots, plastic parts, string type elastic and bulky small parts: Compressed air operated piston vibrators with higher working moment, or for very large containers electric external vibrators with low frequency (1000 to 1500 oscill./ min). The container must be mounted on elastic elements. Coarse grain to lumpy bulk materials, e.g. minerals such as ores, coal, coarse gravel etc.: Electric external vibrators or compressed air operated piston vibrators with a frequency
Elastic products, which become liquid under the influence of vibrations at higher acceleration (Tixothopy –milk products to synthetic moldings sand): Compressed air operated turbine vibrators.
These vibrators produce rotatory vibrations. By using two equal vibrators rotating oppositely, directional vibrations are produced.
