Know-How Center

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.

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 screwed down tightly and has to set into vibration.

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

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.

• 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.

• 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 at different directions, the vibration amplitude gets small at large mass and gets big at small mass. This changing vibration width creates elliptic vibration.

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 comparative 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.

Compacting of concrete at large-scale forms

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

Compacting of material at small vibration tables

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

The centrifugal force F_csets the entire equipment, including the mass to be vibrated, to vibration. The vibration width s (double amplitude) at the equipment must not be too high; otherwise the vibration equipment can be damaged. Should measuring of the vibration amplitude not be possible, it has to be calculated. The acceleration a is either already known or has to be taken from table 4.

Transmission of Vibration

Great Attention has to be directed to the transition of the vibration, both at small vibration tables with one or two external vibrators as well as at large-scale shutterings for compaction with up to 50 external vibrators. At the production of precast concrete elements, the even distribution of the vibration, which creates even compacting, besides the acceleration and vibration time, is the most important criterion to obtain perfect surfaces and high density.It is essential to distribute the necessary centrifugal forces in such a way that they are transmitted to the vibration equipment at as many points as possible.

There they create so-called bending vibrations (bending of the beam). The separate external vibrators are mounted in such a way, that e.g. at large-scale moulds every external vibrator sets only its surrounding area to vibration, so that the vibration zones barely overlap by margins.

When at too labile forms resonances occur, appropriate measures e.g. additional reinforcers, change of mounting or change of frequency have to be taken to avoid breakage.

At correct installation of several external vibrators (at large-scale shutterings) and adequate rigidity of the form, the local load of the shuttering is decreased and it’s life time increased.

Mounting Location

Great attention has to be paid to the transmission of the bending vibration at choosing the transmission points.This is achieved best when already at building the vibration equipment adequate dimensioned, seamless reinforcement profiles are considered at the construction.

So-called vibrator beams serve as mounting locations and are responsible for the even distribution of the vibration.Wrong mounted external vibrators or inconvenient vibration equipment at compacting can create dead zones or areas with excessive vibration.

At small, rigid forms the external vibrators should also be mounted in such a way that they set the vibration equipment into even distributed vibration, i.e. they create all over the form about the same vibration amplitude.

Installation of Vibrators

To transmit the vibration of the vibrator as lossless as possible, at installation the following points have to be considered:

Every external vibrator has to be bolted to a 15-20mm thick plate. This attachment plate has to be flat and thoroughly welded to the reinforcement profile.

When two counter rotating vibrators have to create linear vibration, the reinforcers between the vibrators have to be absolutely vibration rigid to make synchronization (absolute constant velocity) possible. This is obtained by adequate reinforcers. Figure 12 shows an example of this kind of reinforcement connections.

To transmit the vibration loss-less to the desired place, deflection of the vibrator attachment parts (plate, beam) should be avoided. At high-frequency external vi-brators, which at high centrifugal forces create a vibration width of 0,4mm, a de-flection for instance of 0.1-0.2mm of the vibrator plate causes a loss of 25-50% of the vibration energy.

Both of the main effective directions of the created centrifugal forces have to be considered. These are the perpendicular and parallel to the mounting plate acting forces whereas rectangular (to the two main forces) acting forces can cause weld-ing seam tears at the vibrator equipment. At such cases additional reinforcers (e.g. a junction plate) have to be welded to the reinforcement profile. Figure 14 shows possibilities to reinforce the mounting location.

Steel sections have the function to distribute the vibration evenly. Vibrator beams with the steel sections HE-B 140 (IPB 140) qualify outstandingly for this task.

The vibrators have to be mounted to reinforcement profiles and not directly to thin-walled construction parts like silo walls or shuttering plates of concrete element shutterings.

Theexternal vibrators have to be bolted tightly to build a unit of the vibrator and the vibration equipment. To the installation of the vibrator, because of the great dynamic load, has to be given utmost care. Hardened and tempered steel bolts with the quality class 8.8 according to DIN931 and washers according to DIN125 have to be used. The flexibility of long steel bolts increases the vibration-proof at-tachment. The bolts have to be tightened with the prescribed torque. At installing with through bolts, locknuts or counter-nuts have to be used. The bolts, after a short period of operation, have to be re-tightened and be checked regularly for tightness. Loosening of bolted connections result in housing breakage, breakdown of the vibrator, and cracks at the shuttering. Insufficient fastening and loosening of the bolted connections is one of the most frequent reasons for errors.

Rigidity

Vibration equipments are constructed right when the maximum degree of rigidity is reached at a low weight. Low weight and high rigidity are not contradictory requirements when the reinforcement is built with appropriate ribbings. The table construction in figure 15 has a high ribbing (trapezoidal) to the middle. The outcome of this type of reinforcement is high rigidity and even distribution of the vibration to the whole surface. The construction is vibration rigid when the height h averages 1/4 – 1/5 of the length l.

The greater the centrifugal force the higher the ribbing has to be. The size of a vibrating table is 1x1m; the ribbing is 8mm thick. The weight of such a model is approx. 90 kg. The table construction of figure 16 in contrast throughout the table has a too thin ribbing at a ribbing thickness of 14-16mm.The table at the same size of 1x1m has a weight of 130kg. The result is uneven distribution of the vibration to the table surface. At soft-mounting, because of lack of rigidity, a ››flattering‹‹ appears at the outer area or at hard-mounting more deflection appears at the mid area. Vibration differences of up to 200% appear (amplitude blow-up).

The general requirements to a vibrator equipment are:

• high rigidity to obtain vibration stiffness
• even distribution of vibration and avoidance of amplitude blow-ups.
• Low weight to save vibration energy (F_c = m ⋅ a)

Vibration-isolated Mounting of the Vibration Equipment

Vibration equipments have to be mounted in such a way, that they can vibrate freely and no vibration is transmitted to the fundament and to the building.

For elastic-mounting rubber-metal-elements or in conveyor technique coil springs are used, too. The static load of the rubber-metal-elements should average 5-6kg/cm2 rubber surface. The dynamic load at short vibration periods e.g. at a shutterings of precast concrete parts can be disregarded. In practice rubber-metal-elements with a Shore-hardness of 55 Shore A have proved themselves for mounting of vibration equipment. It is crucial – especially for small vibration equipment – that rubbers with equal Shore hardness are used. ² Gummifläche betragen. Die dynamische Belastung kann bei kurzen Rüttelzeiten, wie sie z.B. bei Schalungen von Betonfertigteilen üblich sind vernachlässigt werden. In der Praxis haben sich Gummi-Metall-Elemente mit einer Shore-Härte von 55 Shore A zur Lagerung von Rütteleinrichtungen bewährt. Wichtig ist, vor allem bei kleinen Rütteleinrichtungen, dass gleiche Shore-Härten verwendet werden.

In conveyor technique rubbers with Shore hardness of 40-45 Shore A are used.

40 Shore A e.g. means ››soft‹‹, and 75 Shore A means ››hard‹‹. The meaning of elastic mounting in no means should be underestimated since all the previous considerations apply only at a functioning vibration-isolation.

The elastic mounting is a basic requirement for the function of a vibration equipment.

Since at laying out of vibration equipments above all there is operated with approximate formulas and values, based on experience, the adjustment prior to commissioning is particularly important.

• current consumption
• vibration amplitud
• temperature

From the current consumption can be concluded the efficacy of the vibrator, type, and condition of the shuttering, it’s reinforcement, and as well as bolted and welded connections. By the vibration amplitude it is possible to find out, where reinforcers are necessary. The temperature (measured at the intermediate bearing) is an indicator of performance of the motor. A certain operating temperature (instruction manual) has not to be passed over.

Measurement of Current Consumption

At right construction of the vibration equipment and right dimensioned external vibrators, the current consumption of the external vibrator lies about at the same height as its nominal current. If the power consumption is way below the associated nominal current, the vibration equipment is too heavy. In this case the centrifugal force can be increased.Is the power consumption on the other hand greater than the nominal current, two factors could be the reason: The chosen centrifugal force is either too high. At this case either it has to be decreased or a smaller vibrator with smaller centrifugal force has to be mounted. Or the reason is lack of rigidity. At this case additional reinforcers have to be added.

Measurement of the Vibration Amplitude

For measurement of the vibration amplitude oscillographs qualify best. The vibration amplitude at all places of the equipment should be about the same. Table 3 lets detect if the intended vibration amplitude is reached.

Further Measures

Further measures, which in practice can bring vibration outcome improvement:

• change of the direction of rotation
• change of location of installation (turning, moving)
• stiffening of the vibration equipment with additional reinforcement profiles
• the use of softer rubber-metal-elements (mounting)

At compaction of concrete, mainly at large-scale moulds and shutterings, predominantly high frequency external vibrators 6000 rmp and seldom vibrators with 12000 rmp are used.

Advantages of High Frequency External Vibrators

Surplus water gathering at fresh concrete is intensified by High Frequency External Vibrators

• excellent deaeration by slow progressing bottom-to-top compacting → small pore volume
• smooth surfaces → less reworking
• higher concrete strength → improvement in quality

Low movement of coarse grain particles because of a small vibration amplitude

• no demixing danger at even long vibration periods
• no moving and shifting of the armor (iron)

Small vibration amplitude at increased frequency

• small risk of form breakage and welding seam splits
• small mutual interference of the external vibrators at small forms at even small distances (1,5-2,5 m).

Small resonance vibration and small increase of the amplitude

Large, not vibration rigid shuttering and forms can even be vibrated zonally → light weighted forms

Excellent penetration of vibration through insulation layers (e.g. at sandwich elements).

Perfect surfaces through evenly distributed vibration transmission.

High frequency external vibrators have high electrical power reserves, which also at great changing load ensure the highest degree of centrifugal force and rotation constancy.

External Vibrators with 6.000 Rotations/Minute

At 6.000 vibrations/min at on side the vibration amplitude is small enough to avoid demixing, on the other side it is big enough to distribute the vibration at the mould so much, that the external vibrators don’t have to be attached too near to each other. Less external vibrators are needed to compact the same area evenly as at use of external vibrators with higher vibration speed. Furthermore the velocity, with which the compacting progresses out of the vibration centres, is higher, i.e. the vibration period is shorter.

It is important, that at equal damping by the concrete due to the bigger vibration amplitude a better depth effect is achieved and thus thick parts get high density. Even more important as at massive elements is this at so-called sandwich elements since the vibration has to penetrate the insulation layers.

External Vibrators with 12.000 Rotations per Minute

External vibrators with 12.000 vibrations /min theoretically allow a more intensive compaction than those with 6.000 rpm but, because of the very small vibration amplitude, can only be used, when the forms are so rigid that they transmit the vibration amplitude without any loss.

Moreover the effective range is relative small and the noise development very big. External vibrators with 12.000 rpm mostly are used at the production of parts which demand the highest degree of stability and density, i.e. at concrete pipes.

The vibration capability and resonance behaviour of a shuttering are not possible to be determined beforehand since the influences of the construction, material, mounting, connection elements, vibration transmission, kind and composition of the concrete are difficult to grasp.Therefore at determining of the centrifugal force empirical values are used. In practice the following formula is proven:

F_c = m ⋅ a ⋅ S / 1000

F_c -   centrifugal force kN
m -  here: mass of the vibrating parts of the vibration equipment + mass of the vibrator + 10% of the mass of concrete in kg
a -  acceleration in m/s² (see Tab.4)
S -  characteristic value for rigidity and the resonance behaviour of the vibration equipment (see Tab.5)

The resonance behaviour and more or less vibration inclination of shutterings normally lead to excessive change of the vibration amplitude and therefore the acceleration a. In order to keep the approximate values for a, a is reduced with help of the characteristic value S (see Fig.18).

The concrete mass is proportionated only with 10% of its mass in the calculation because the fresh concrete as one mass at uncompacted state is not considered as part of the mass, which has to be brought to vibration. At the proceeding compacting process this changes slowly. Not until the end of the vibration period, the better part of the concrete has to be considered as vibrating mass. This is also seen at the decrease of current consumption of the external vibrators during the proceeding compacting period.

An example makes the interconnections clear:

Given:    A vibration table with dimensions 10m x 4m, mass of the table and edge formwork 6350kg, mass of the external vibrators 225kg, mass of to compacting concrete 18000kg. It has to be compacted with 6000 vibrations/minute.

Searched:    Centrifugal force Fc

Answer:   
a = 80 (see Tab.4 and Fig.18)
S = 0,15 (since the table construction is not rigid and self-vibrations are not expected; see Tab.5)

Fc = [(6350 + 225 + 0,1 ⋅ 18000) ⋅ 80 ⋅ 0,15] / 1000 kN

This total centrifugal force is divided to several vibrators; at a total of 9 vibrators every vibrator has to generate 12kN (reference values) centrifugal force.

The particular vibration period depends on several parameters, viz. the magnitude of acceleration, the demanded strength, the vibrating mass, the composition of the concrete, the height of the concrete in the form, and the arrangement of the armor. The vibration period is sufficient when the surface is closed by fine mortar, only few air bubbles ascent of the concrete, and it - in case of hard concrete - deforms only at strong pressure of the hand during the vibration period. Sporadic, discontiguous air bubbles at the surface and in the concrete usually at compaction cannot be avoided. They don’t have any influence on the mechanical properties of the hardened concrete.

At precast concrete plants for production of large-sized concrete elements these different forms and shutterings are used:

• vibration tables
• lanes for ceiling elements
• palettes for vibration stand stations
• column and beam forms
• double-T forms
• battery formwork and vibration beams for sandwich walls
• garage forms
• box culverts
• large tube forms
• stair forms, amongst others

Vibration Tables

There are two forms of vibration tables: the tilting table and the oscillating tilting table. The tilting table is a rigid, warp resistant table with a very small flex tolerance, which can be tilted (hydraulically) for the removal of the precast element. With an oscillating tilting table the upper construction is separated from the lower construction and mounted on isolators. The mass to be set into vibration is less, so that working with smaller external vibrators is possible but the flex tolerances on only partially poured tables are larger.

For the installation of the external vibrators on vibration tables, the shifted W-arrangement of vibrators on the main longitudinal beams is the best proven in method. It is important to adequately stiffen the mounting location of the high frequency external vibrators.This assembly is used because each external vibrator only vibrates its’ surrounding area and these areas barely overlap.On oscillation tables, the external vibrators on vibration plates are attached parallel to the longitudinal beams. Again, the shifted W-arrangement best ensures an even distribution of vibration to the whole table.

As another example, the group of rod-shaped forms is considered: column and beam moulds as well as double-T forms.

Column and Beam Moulds

Column and beam forms are available, as individual or double form, with variable side and bottom moulds in lengths of up to 60m. Whether producing pre-stressed or standard precast products, intense compaction is needed in the re-enforced areas. It is necessary to attach I-Beams horizontally at different heights on which the high frequency vibrators are to be attached. This ensures optimum compaction. In the case of double beam moulds, an additional HF external vibrator is attached under the core.

Double-T Forms

Double-T forms are used mostly to produce ceiling elements. This form can also be 60m and longer. These forms can be hydraulically or mechanically equipped. The height of the ‘T’, width of ‘T’, distance between the ‘T’s and width of slab can be changed. I-Beams are mounted to both sides of the ‘T’ in order to attach the high frequency vibrators.

Because of the adjustability of these forms, variable frequency converters provide the needed flexibility. (vario-frequency converters).

Other Applications

In addition to the concrete compaction on large-scale forms, high frequency and standard frequency external vibrators are used in other various applications:

• block forming machine (main- and top vibration)
• slip former (vibration bridge)
• hollow-core slab former (vibrating screed)
• tube forming machine
• small forms (manholes, sewers, wells)
• single tube forms
• manufacturing of concrete railroad ties

At high frequency external vibrators the mains frequency (50Hz) has to be increased with frequency converters to 200 Hz (output frequency f₂) There are also vario-frequency converters which generate a variable output frequency f₂ This means also, that the vibration speed (rotation) of the vibrator is variable.

n = 60 ⋅ f_el / p

A change of the motor speed causes a change at centrifugal force:

F_c2 = F_cl  ⋅ (n₂ / n₁)²

With vario-converts the centrifugal force of the vibrator can then be changed during operation. This has the advantage that the centrifugal force can quickly be adapted to the respective fill factor of the formwork and to the compaction state of the concrete.(In practice the range between 80-210 Hz as working frequency is proved as useful.)

The three-phase asynchronous motor of the external vibrator at this frequency change works always at the optimal working range since the frequency and voltage always are changed at the same ratio (U/f = constant).

Advantages of Variable Frequency

Since the vibration width is independent of the rotation speed, at frequency adjustments always is worked with same vibration width (amplitude) but changing acceleration (see relationships).

Since the height of the sound pressure level produced by the external vibrator is really influenced by the frequency and acceleration, which have an effect on the shutterings, with a proper application a noise reduction of up to 15 dB(A) can be realized. A substantial effect of the frequency change is that at an optimum compacting force as little noise as possible is generated. For example, concrete is filled in layers when concreting rod-shaped forms (beams, stanchions, girders). If the vibrators already work at maximum power when the form is still almost empty (this is the case with fixed output frequency), a large part of the energy produced is lost in the vibration of the empty form (the upper part), which also produces unnecessary noise.Because excessive centrifugal force also causes fatigue of welded seams and shaking loose of parts (e.g. heating pipes), there will be less wear and tear on the vibrator equipment when the centrifugal force is properly adjusted.

Shutterings and large-scale forms are complex structures; often several natural vibration figures occur which in advance hardly can be determined. Is this natural vibration near the usual fixed frequency, large local resonance can occur, which in turn, because of the heightened acceleration amplitude, creates a higher sound pressure level. The resonance vibrations are very harmful for the form.

A great opportunity to use the benefits of vario-converters to its full capacity is the control of the facility per remote control. At this case the operator controls the facility direct at the form and influences the compaction process according to the status of procedure. Depending on the part and composition of the concrete, it is then possible to individualize the compacting process. Accordingly good results are achieved.

Types of Vario-Frequency Converters

The Vario-converter can be a rotating (electro-mechanical) or a static frequency converter (electronic). The decisive advantage of mechanically adjustable frequency converter is that an external vibrator and even entire vibrator groups can be connected to the running converter. That is possible because it copes easily with overloads and current peaks. Therefore it qualifies also as a central converter in companies, in which various production facilities are operated.

The electronic frequency converter, on the other hand, suites as supply for a production unit, e.g. vibration equipment, a double T shuttering or column forms, i.e. the number of external vibrators per group, which run simultaneously, has to be fix. ››Hooking up‹‹ of a second vibrator group can only be carried out with a second frequency converter or after stop and new start of the first group. This is due to the high currents of starting vibrator groups, which in normal case exceed the permitted overload-current of the running converter. Therefore the operating procedure and the method of operation have to be determined exactly in advance. In no case professional advice should be omitted. At automating of production sequences vario-converters can be combined with programmable logic controllers. This coaction is used e.g. to save (on PC or on PLC) and reproduce the parameters of compaction (vibration period, frequency, part number, number of the vibration equipment, etc.), which have brought to good outcome, as often as required.

High frequency vibration equipment consists of external vibrators, central frequency converter, control and distribution cabinet as well as radio controls.

Since each installation has to be adapted to the operating procedure, general guidelines are not applicable. But it is important, that the external vibrator is always equipped with a vibration-proof power cable, a motor protection switch provides the protection of the vibrator and formwork, and the vibration equipment is installed properly, i.e. properly installed cable, converter, control- and distribution cabinet.

High frequency external vibrators are available for operating voltages of 250 V and 42 V. For new installations, the 250-V installation system has asserted itself with installations, since smaller cable cross sections and inexpensive switching elements can be used here. At the 250-V installation system the electrical safety is ensured by the required protective conductor which makes the application of the following protective measures according to VDE 0100 possible:

• protective grounding (§9N)
• connection to neutral (§10N)
• protective conductor system (§11N)
• ground fault circuit interrupter (§13N)

For the evaluation of the noise not only the height of the produced noise level at a moment but also its working time is decisive. The permitted noise level at new installations of vibration equipments lies at 85 db (A) over 8 hour/day. This is equivalent to e.g. 97 dB (A) over 30 min/day.

There are several possibilities to reduce the noise level, which is produced by the installation facility for concrete compaction:

• enclosing and covering of the vibration device
• noise reduction directly at the forms
• application of dampening materials
• noise reduction construction
• use of adjustable frequency converters

Enclosing and Covering of the Vibration Device

The possibility to enclose certain production areas to reduce noise makes only sense at flow-production like vibration stations or similar machines. At production in large-scale shutterings this type of noise reduction is not possible.

Noise Reduction at the Form

The shuttering must be kept free by loose and clattering parts. That means in detail: No tools or other small parts may lie on the shuttering. No loose bolts or screws, no loose heating pipes or other loose construction parts may be at the shuttering.

Causes of noise generation during vibrating

Application of Dampening Materials

With usual dampening materials a radiation of sound at large-scale shutterings and/or their thick-walled sheets and beams can not be prevented.

››Panelling‹‹ of vibration tables with dampening mats of high specific weight has little effect. Noise reduction up to 10 db (A) brings a for this application special developed method for primary sound absorption, the ››Baryvibo system‹‹. It is a three-layered sandwich system with a viscoelastic interlayer of synthetic resin at which available metal constructions (profiles, sheets, and beams) are included as reinforcers to the system. The Baryvibo system dampens arising free bending vibration at beams and sheets which cause only noise and are not necessary for the compaction. The function of the vibration facilities still is ensured.

Noise Reduction Construction

Already at the construction of new vibration facilities the development of the noise can be taken into account and thus kept as low as possible. It must be taken into account that the shuttering is dimensioned sufficiently both in static and dynamic regard. Thus for example by sufficient, evenly distributed stiffening, resonances can be prevented.

Special attention also has to be given to the welded joints. The form surface and lower con-struction must be force and form-fitted connected with each other and mustn't beat on each other (welding steps too big).

Application of vario-frequency converters

In addition, as mentioned, adjustable vario-frequency converters also contribute to the reduc-tion of the noise since at optimal compaction unnecessary noise is prevented. Particularly mechanical adjustable vario-converters are recommended.

The fastening of the external vibrator at the silo is an important point, both for the function of the external vibrator and for the durability of the silo. The external vibrator may by no means be screwed directly to the bunker wall or to the run-out hopper. The material stressing at this place would be so big that the metal would tear. In addition, the influence of the external vibrator would be limited to an only very little area. To avoid these effects, a stiffening plate with a fastening facility for the vibrator must be attached at the silo.

As effective and simple producible stiffening, welding (step welding) of U-shaped sections (see fig. 33 and 34) has proven itself.

The U-shaped section should depending on size of the hopper be a U 80 - U 120 and have a length of approx. 400 mm - 2000 mm. It makes sense to lead the U-profile to the cross stay profile of the silo and to weld it to this profile. The external vibrator is attached vertically to the axis of the U-shaped section. The direction of rotation of the vibrator should be in such a way that, at a lateral view of the vibrator on the silo wall, the weights move from top to bottom. Since the vibrator works against the rigid axis of the U-profile, the silo wall and stiffening are spared and the by the external vibrator caused vibration is distributed better. Another possibility of stiffening of a container, silo or bunker with circular or rectangular crosscuts is welding on a 15 mm - 20 mm rigid plate (see fig. 35). This possibility is obvious, particularly when several little vibrators are attached to avoid shaft formations at the vertical part of the silo (see fig. 36).

If the external vibrators are chosen and the attachment places are defined, the external vibrators in any case should be adjusted to the silo. Thereto the function of the external vibrators is checked first. To check functionality of the exterior vibrators at the silo, there are two simple possibilities see Measurements and Adjustments: measuring the vibration breadth (amplitude) and measuring the current consumption of the external vibrator. When the vibration breadth (amplitude), measured at the external vibrator, is larger than 1 mm and the current consumption exceeds the given nominal value, the external vibrator puts the silo wall to too big vibrations. If the stiffening is sufficient, the cause only can lie in the too high adjusted centrifugal force.

The bolted connection at the mounting part of the external vibrators should be retightened shortly after putting the external vibrators into operation.

The external vibrators only may be started if the bulk material can drain of, since otherwise an unwanted compaction process occurs. To avoid compacting, a sure possibility is the coupling of the on/off switch of the vibrator with the closing of the silo. To avoid damages at the silo e.g. in case, a vibrator fastening would have been loosened by itself or welded seams and stiffenings would have been broken, every external vibrator should be safeguarded by a motor protection switch.

Silos with bulk materials not in all cases serve only the storage but once in a while also are used for transport of bulk materials. At the shipment e.g. in silo wagons of the German Railways, the bulk material is compacted by the concussions during the transport and at high atmospheric humidity in addition can stick together. This leads at the final destination to expensive down times since the material must be made flow first with crowbars or similar tools. By use of external vibrators this problem is solved. A fastener is attached to the critical place at every wagon where the external vibrator is attached to support the outflow process at unloading.

Another possibility of using external vibrators at silos is the use of discharging aids and discharging units. Discharging aids are discharging vibration baskets, vibration crosses or vibration grating surfaces. It is the advantage of this kind of breaking up that the external vibrator does not sit directly at the silo but at the vibration body.

Discharging units consist of a vibration body (cone, sieve, grating) and are set to vibration by an outside mounted external vibrator. The complete discharging unit attaches at the cone of the silo outlet and is hung up vibration isolated opposite the silo. The discharging units work as vibrating flow-out hoppers; they transfer the vibration to the bulk material which is caused by the external vibrator.

Two counter-rotating external vibrators are attached at a certain attack angle under a conveyor through. The attack angle corresponds to the throwing angle which is important at the analysis of the conveying movement. The two external vibrators must be attached in such a way, that the working direction of the centrifugal forces goes to the centre of gravity of the conveyor through. This one resulting force (linear motion), like fig. 6 points out, moves the vibration equipment (chute or sieve) to and fro. A prerequisite for this linear motion is that both external vibrators run exactly synchronously as figure 41 shows.

The chute (sieve) must be an all-side free moving system, be hung up vibration isolated, and both external vibrators have to be connected with each other vibration rigidly. The vibration isolated mounting of the chute is achieved with steel springs or rubber swing elements (soft fastening important!). If these conditions are fulfilled, the two external vibrators synchronize themselves at running independently due to laws of mass action.

Conveying Procedure

Now the bulk material from a sieve, bunker or conveyor belt reaches the conveyor through, which under the influence of the external vibrators creates a linear vibration with a certain frequency and acceleration.

In order to make the conveying process better graspable, only one conveying good particle will be considered here. The motion of the conveying good particle is represented by fig. 42.

A conveying good particle reaching the conveyor through is subject of the acceleration a, which the chute undergoes by the centrifugal forces of the external vibrators. Since the external vibrators are attached at an attack angle of 25° to 30°, the conveying good particle is also accelerated at this angle.At the moment at which the vertical component of this acceleration (vertical acceleration av) exceeds the gravitational acceleration, the conveying good particle lifts off the chute ground (loosening point L). It flies a certain time (tw) at a trajectory of a parabola and covers a throw distance until it hits the chute surface at the next vibration period of the chute again (point A).

The produced throwing range and the size of the acceleration have essential influence on the obtainable conveying good stream. The particle of the material to be conveyed stays as long in connection with the chute till the vertical acceleration component of the chute again gets bigger than the gravitational acceleration.

Then a new throwing phase takes place. These procedures take place till the conveying good particle leaves the gutter. Because of the small throwing ranges and the high frequencies the individual shots are not perceived by the human eye. The material flow therefore appears like a continuously running stream in the conveying through. The conveying characteristics can be influenced by change of the acceleration a and the attack angle α.

To clarify these facts, two edge cases are considered. If the point of impact A of the conveying good particle is very near to the lower stationary point, the conveying speed is low. Little friction emerges between the chute bottom and conveying good particle. Since tR is great, a low velocity of conveying is achieved, instead, however, the chute and conveying goods are conserved.

If the conveying good particle hits the chute near the next Lösepunkt L', the velocity is high. The friction between the chute surface and the conveying good particle is high. Thus a high velocity of conveying is achieved; but the chute and conveying good are, however, stressed much more.

A compromise, as illus. 42 shows, is most favourable.

Choice of External Vibrators

How to determine the right external vibrators for a chute or a sieve, a calculation example best points out.

The following data must be familiar:

Conveyor capacity L_F in t/h or m³/h
Bulk density ρ' in t/m³
Chute breadth b in m
Chute length l in m
Chute inclination β in°
Dumping height h in m

For the dumping height h empirical values are used:

h = 0,1 m at a chute breadth up to 0,4 m
h = 0,15 at a chute breadth up to 0,6 m
h = 0,2 at a chute breadth up to 1,0 m

Now can be calculated:

Chute height H

H = h ⋅ 1,30 - 1,4 in (m)

Conveying velocity v (in m/min)

Centrifugal force F_c (in kN)

v =  L_F / ( b ⋅ h ⋅ 60 ⋅ F_w ⋅ F_ß ⋅ F_h ⋅ 0,9)

v   = conveying velocity in m/min
L_F = conveyor capacity in m³/h
b   = Chute breadth in m
h   = dumping height in m
F_w = factor for die conveying willingness of bulk good
            1,0 = excellent, e.g. wet sand
            0,3 = very poor, e.g. at dust-like, dry materials like cement
F_ß = factor for the angle of inclination
F_h = factor for the dumping height

The factors F_w, F_ß and F_h are empirically determined values. The result for the velocity of conveying has to be compared with table 8. In any case, at the prior determined vibration speed of 3000, 1500, 1000 l/min, v should lie in the green colored area since the acceleration a should not exceed the value of 60 m/s2 (destruction danger of chute or sieve).

If the calculated values are too large for a and v, the angle of inclination and dumping height can be corrected till v and with that a lie in the favourable area.

With a the centrifugal force has F_c (in kN) can be calculated now.

Fc = m ⋅ a / 1000

m = weight of chute + weight of vibrator (estimated) + 10-15% of the conveying good (in kg) lying in the chute
a = acceleration in m/s²

Since two external vibrators because of the linear vibrations must be used, the centrifugal force per external vibrator is F_c /2.
Since the weight of the external vibrators is known now, too, a correction calculation with the actual m, again can be made.

To ensure an efficient function of the chute (or the sieve), in addition, the following points have to be taken into account at the construction of a conveying chute (or a sieve).

• The front and the back hanging should be equally far away from the center of gravity, but as far as possible from each other.
• The relation between length and height of a conveying chute should be 5:1.
• Another condition for the synchronization of the two external vibrators is that they are connected with each other absolutely vibration-rigid. The conveying chute also must be extremely stiff because of the dynamic load. In order to fulfil this requirement at an acceptable weight, free-running conveying chuts are maximal 6 m long.

Vibrating screens are used to sieve out bulk materials, in order to e.g. to separate different material sizes or for draining of gravel or sand. The function of vibrating screens is similar to conveying chutes; the bottom, however, is a sieve. The sieve bottom is constructed in many forms and variations (round hole, slot, quadrangle) according to the material to be sieved. There are sieves with several, under each other arranged screen floors, dewatering screens, analysis screen, etc.

The difference to conveying technique is the attack angle of the external vibrators which again are attached in pairs and are counter rotating. The attack angle at sieving is steeper so that the conveying good particle hits the sieve as vertically as possible at the screen bottom; thus the conveying good particle goes better through the screen. Moreover, a better self-cleaning of the screen is given this way since particles being stuck are lifted easier at the next vibration. The attack angle at sieving should be 45°. The throwing range must correspond to the mesh width of the sieve screen.