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About our Concrete Vibration Technology

Rebar, Mesh and Construction Supplies (Pty) Ltd distributes the BRECON range of External and Internal Vibrators in Southern Africa. As a BOSCH spin-off BRECON stands out by offering smart vibration solutions for concrete compaction, conveying and other industries since 1995. As the exclusive BOSCH-licensee in the past years and with the BRECON experience and know-how in vibration technologyBRECON continuously enhance their product line. The development of new products is driven by market requirements and is often carried out in close co-operation with manufacturers of machinery.

Concrete Vibrators internal and external

 

Basic Principles and Application of the Vibration Technology

Basic Principles and Application of the Vibration Technology Concrete is poured into moulds and vibrated to let the trapped air escape and so to compact it. Before hot chocolate is poured into its form, vibration technology is used to deaerate the liquid. The objectionable air pockets disappear when the vibration stirs up the material and the friction between the individual particles of the material is decreased. The material becomes loose and flows off properly.

Even bulk materials like sand, cement, lime, coal, cereals, etc. which are stored in silos or other containers have to be set to motion to overcome the powerful adhesive forces, reduce the friction between the individual particles, and thus to support the outflow. Furthermore the motion of material is necessary for conveying of the bulk materials on conveyor troughs and sieving on reciprocating screens. This motion is created by so-called external vibrators, which are attached to vibrating equipments, chute of silos, or conveying throughs.

 

Design of External Vibrators

External Vibrator Design The external vibrator is a three-phase asynchronous motor. So-called eccentric weights (discs) are fixed eccentrically at the ends of the rotor shaft. At rotating, centrifugal forces are generated. These forces can be adjusted by moving the discs at standstill. The discs are divided so that the centrifugal force can be easily regulated.

Each weight at rotation creates a centrifugal force vector whose direction is determined by the centre of motion (centre of rotor) and the centre of gravity. Since the total centrifugal force is determined by the resulting centrifugal force vectors, counterbalancing (twisting) of the discs is a way to increase or decrease the total centrifugal force (see fig. 2 and 4). External vibrators can create centrifugal forces up to 100 kN and more. To obtain great durability despite the rough operating conditions, external vibrators are equipped with vibration-proof, high temperature proof stator windings, special bearings with increased load capacity and vibration-proof power cables.

 

Functionality of a Vibrator

External Vibrator Centrifugal Forces


The centrifugal forces, which the motor produces, set the vibrating equipment (table, shuttering, form, conveyor through) and the material (for example concrete) into vibration. The vibration is determined by the magnitude of the generated centrifugal force and the mass of the vibrating equipment, to which the vibrator is attached.

The vibration width s, the double amplitude, occurs at a specific point at the vibrating equipment during a motor revolution. External Vibrator Vibration Width

 

Modes of Vibration

Since the revolving weights create centrifugal forces in periodically changing directions, they evoke circular, linear or elliptic vibration. An external vibrator alone generates circular vibration, whereas two equal, parallel arranged and counter rotating vibrators create linear vibration.

Circular Vibration
The vibrator moves the same mass radial to all directions; consequently the vibration width s is the same toward all directions, a circular vibration is created.

Circular Vibration

 
Linear Vibration
Two equal, counter rotating external vibrators are attached parallel to each other. Due to synchronisation, the opposite forces cancel out each other and aligned forces add up. This creates linear vibration. At conveying e.g. two counter rotating vibrators create linear motion and thus allow the motion of bulk goods toward a specific direction.

The individual particles or pieces of material are repeatedly struck at a certain trajectory so that a chain of parabola-like micro-projectile motions takes place.

Linear Vibration

 
Elliptic Vibration
A vibrator is mounted asymmetrically anywhere at a form, for instance at the end of a T beam. Because the vibrator has to move different masses in different directions, the vibration amplitude gets small at large mass and gets big at small mass. This changing vibration width, creates elliptic vibration.

Elliptic Vibration

 

Types of Vibration Motors

Brecon external vibrators range from low frequency vibrators with 1000, 1500, 3000 vibrations per minute at a power frequency of 50 Hz to high frequency vibrators with 6000, or 12000 vibrations per minute at a power frequency of 200 Hz. The selection of external vibrators depends on the area of application (compacting, loosening, conveying), on the vibration equipment (rigidity, weight), and on the material is worked with (properties, weight).

A basic rule applies, that a vibrator at same centrifugal force with smaller vibration frequency creates a larger vibration amplitude and at the same centrifugal force with higher vibration frequency creates a smaller vibration amplitude. External vibrators with 1000 and 1500 vibrations per minute because of their comparatively large vibration amplitude for example are used for sieving and for conveying of coarse-grained materials.

External vibrators with 3000 vibrations/minute deliver optimal performance at loosening and breaking up of fine-grained bulk materials. High frequency external vibrators with 6000 and 12000 vibrations per minute are excellent at compacting of fine-grained materials since their high frequency and small amplitude particularly strong activate micro-grained material. High frequency vibrators find most application at concrete compacting. External vibrators with 6000 vibrations/minute have a better depth effect and make less noise than vibrators with 12000 vibrations per minute.

Types of motors
Electrical frequency
Number of pole pairs
Speed
Mechanical frequency
High frequency external vibrator (HF)
200 Hz
200 Hz
1
2
 12000l/min 
 6000l/min 
200 Hz
100 Hz
Low frequency external vibrator (LF)
50 Hz
50 Hz
50 Hz
1
2
3
 3000l/min 
 1500l/min 
 1000l/min 
50 Hz
25 Hz
16,66 Hz
Number of pole pairs
x 2 = Number of poles
Table 1: Important vibrator performance characteristics

 

External Vibrator Technology - Areas of Application

  • Compacting of concrete in large-scale forms

    With this kind of operation, the vibration of the external vibrator is transmitted to the profile (vibrator beam) of the vibration equipment, then it is passed on from the profile to the formwork facing and finally to the concrete.

  • Compacting of material on small vibration tables

    All kinds of bulk materials can be compacted e.g. by mounting containers or moulds to a small vibration table. Two counter rotating external vibrators are mounted under the table top. They set the entire table into linear vibration.

    External Vibrator Technology Table 2: Overview of areas of application and fitting vibration frequency

  • Loosening and breaking up of bulk materials in bunkers und silos

    The external vibrator attached to the silo, brings the wall in contact with the vibration so that the bulk material arches and caused to collapse.

  • Conveying and sieving of bulk goods on vibrating conveyors and vibrating screens

    External vibrators are used in conveyor technique for transport of bulk goods. This, in majority of cases, is done with two (counter rotating) vibrators, mounted at an angle to the conveyor.

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