Abstract:
An improved compacting device for compacting a granular, loosely coherent mass, such as soil-moist cement, for producing end products such as tiles, kerbstones and the like. The compacting device includes a vibrating table and a hydraulic exciter device for driving the vibrating table. The hydraulic exciter device includes a plurality of hydraulic exciters and is configured to drive the hydraulic exciters with excitation displacements of mutually substantially the same amplitude and frequency and in phase with one another. An improved hydraulic exciter for use in the improved compacting device, and the improved compacting device with a single improved exciter of this type are also described.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of compacting a granular, loosely coherent mass, such as soil-moist concrete cement. By compacting the mass, the air content thereof is reduced, and a strong coherent product is obtained. The products in question may be tiles, kerbstones and various other products made of concrete and the like. In particular, the invention relates to a compacting device with a hydraulic exciter device, and a hydraulic exciter device suitable for such a compacting device. 
     2. Description of the Related Art 
     NL-1005862 and EP 0 870 585 B1 disclose a device for compacting a granular, loosely coherent mass, such as soil-moist cement, for producing end products, such as tiles, kerbstones and the like. The known device comprises a vibrating table and a mould for the mass to be compacted, a stamp for pressing onto the mass in the mould, a hydraulic exciter and a hydraulic pressure element connected to the vibrating table or stamp, respectively, drive means and control means for controlling the exciter and the pressure element. The known device is configured so as to carry out a method which comprises the following steps:
         selecting a frequency range with a lowest value and a highest value for the excitation frequency,   regulating the excitation frequency in such a manner that said frequency range is passed through at least partly and that the natural frequency of the hydraulic-mechanical mass spring system is reached, said hydraulic-mechanical mass spring system being formed by the movable part of the exciter, the vibrating table, the mould and the mass to be compacted, as well as the compressible hydraulic medium which is situated between the movable part of the exciter and the respective drive means (in particular comprising an electrohydraulic control element).       

     The hydraulic exciter of the known device comprises a piston/cylinder device in which the movable part comprises a piston which is moved to and fro in an excitation cylinder by means of the respective drive means and control means. The piston is connected to a piston rod which is connected to the vibrating table at an excitation position on the vibrating table, which makes it possible to produce an excitation displacement at the excitation position. The excitation displacement is the sum of a working position and an excitation oscillation with the excitation frequency and an excitation amplitude around this working position. 
     It is a drawback of the known device that the vibrating table has to have a high degree of stiffness in order to achieve a substantially uniform vibration across the entire surface of the vibrating table when this is driven in the centre by the exciter. In order to produce larger products and/or more products per run, the dimensions of the vibrating table have to be increased. If the dimensions of the vibrating table are increased, the resulting table will be very heavy due to the stiffness requirements. This entire heavy table will have to be driven and pass through the frequency range which is associated with the great forces which are required therefor. It is therefore desired to provide a device which can be operated in a more efficient manner, for example due to the fact that it may comprise a relatively light table. 
     In a known device, the excitation cylinder is, for example, a piston/cylinder device fitted with hydrostatic bearings. Such a known piston/cylinder device is associated with, for example, an energy loss of approximately 20% due to leakage flow across the hydrostatic bearings. It is therefore desired to provide a more efficient excitation cylinder. Preferably, the more efficient excitation cylinder can in addition be used in the more efficient device, so that an optimum efficient device is obtained. 
     SU-A-856796 discloses a compacting device, comprising: 
     a vibrating table, a mould which is intended for the mass to be compacted and which is attached to the vibrating table, at least during use of the compacting device, a hydraulic exciter device connected to the vibrating table, wherein the hydraulic exciter device comprises a plurality of hydraulic exciters, each hydraulic exciter is connected to the vibrating table, the hydraulic exciters are mutually parallel, and the hydraulic exciter device is configured to drive the hydraulic exciters with excitation displacements of mutually substantially the same amplitude and the same frequency and in phase with one another. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved method and device for the compacting of, for example, soil-moist concrete cement. In particular, it is an object to provide an improvement which makes it possible to compact a mass across a larger surface, so that, for example, products of larger dimensions can be compacted or, for example, a larger amount of products can be compacted per run than with the known device. In addition, or instead thereof, it may be an object to provide an improvement which results in a higher efficiency in use. 
     According to a first aspect, the invention provides a compacting device wherein a drive means is provided which is configured so as to evenly distribute a hydraulic volumetric flow of a compressible hydraulic medium between all the hydraulic exciters. By means thereof, the hydraulic exciters can be driven synchronously by means of a hydraulic volumetric flow, with only a single hydraulic volumetric flow having to be provided and regulated. This avoids the complexity which would result from providing several individual hydraulic volume flows, one for each exciter, and which each have to be provided separately and in such a manner that all exciters are operated at the same excitation displacements. 
     In a further embodiment, the drive means comprises an electro-hydraulic servo valve, a manifold and a plurality of distribution pipes, wherein the servo valve is configured to provide a hydraulic volumetric flow to the manifold and wherein the manifold is configured to evenly distribute the hydraulic volumetric flow from the servo valve between the distribution pipes, which distribution pipes are connected to respective hydraulic exciters. By means thereof, the hydraulic exciters can be driven synchronously by a single valve, thus avoiding the additional costs of several valves. In a further embodiment, the hydraulic pipe paths, measured from the servo valve to each of the respective hydraulic exciters, are identical in length and in volume. As a result thereof, the hydraulic exciters can be driven in a relatively simple manner at the same frequency, amplitude and in phase with one another. 
     The vibrating table is thus driven by several hydraulic exciters in several locations, with the same excitation displacement at each of the several locations. This makes it possible to achieve a substantially uniform vibration across the entire surface of the vibrating table, requiring a less high degree of stiffness of the vibrating table than when the compacting device only has a single exciter which excites the vibrating table in its centre as is the case with the known device. As a result thereof, a lighter vibrating table can be used than with a single exciter. This is particularly advantageous if the dimensions of the vibrating table are relatively large, thus making it possible to compact relatively large products or more products per run. Moreover, a lighter vibrating table can result in a lower installed power. The hydraulic exciter device can be configured to drive the exciters with excitation displacements with mutually substantially the same amplitude and frequency and in phase with one another by means of a drive means together with a control means. The term excitation displacement is used in order to denote the sum of a working position of the hydraulic exciter and an excitation oscillation at a frequency and an amplitude around this working position. This frequency can also be denoted as the excitation frequency and this amplitude as the excitation amplitude. 
     According to an embodiment, a control means is provided wherein the control means is configured to control the drive of each of the hydraulic exciters of the plurality of hydraulic exciters on the basis of a determination of the excitation displacement of one and only one single hydraulic exciter of the plurality of hydraulic exciters. Thus, the excitation displacement, and therefore the working position and the amplitude, of only this one hydraulic exciter is actively controlled. The excitation displacement of the other hydraulic exciters is not actively controlled: the other hydraulic exciters are driven on the basis of the excitation displacement of only the one hydraulic exciter. Thus, it is possible to provide a relatively simple control means. For example, any possible complexity of a control means can be avoided which would be required if each hydraulic exciter were to be actuated separately and controlled separately on the basis of respective determinations of the respective excitation displacements, in which case the control means would have to be provided with a complex control arrangement in order to achieve a substantially mutual synchronicity in amplitude and frequency and without phase difference between all hydraulic exciters. 
     In order to determine the excitation displacement, the single hydraulic exciter is, according to a further embodiment, provided with a sensor, such as a displacement sensor, which is configured to determine the excitation displacement of the one and only one single hydraulic exciter and to pass on the determined excitation displacement to the control means. As a result thereof, the other exciters do not have to be equipped with sensors, thus saving costs which would otherwise have been necessary if several sensors had been required. 
     In particular, the control means are configured to actuate the drive means, and in particular the single electro-hydraulic servo valve, on the basis of the determination of the excitation displacement of a single hydraulic exciter. This makes it possible to save costs which would otherwise have been necessary if several electro-hydraulic servo valves and/or several sensors had been required. 
     According to an embodiment, the hydraulic exciter device is configured to carry out a method which comprises the following steps:
         selecting a frequency range with a lowest value and a highest value for an excitation frequency, and   controlling the excitation frequency in such a manner that said frequency range is passed through at least partly and that a natural frequency of the hydraulic-mechanical mass spring system is reached and passed, said hydraulic-mechanical mass spring system being formed by the movable part of the exciter, the vibrating table, the mould and the mass to be compacted, as well as the compressible hydraulic medium which is situated between the hydraulic exciters and the drive means. Passing through the frequency range results in frequencies, amplitudes and associated accelerations by means of which the mass to be compacted can be compacted quickly and effectively, in particular due to the fact that the device is briefly operated at the natural frequency when said frequency range is being passed through.       

     Further embodiments are defined in the dependent claims. 
     According to a second aspect, the invention provides a hydraulic exciter comprising a piston/cylinder device comprising at least one piston rod and a housing, which housing is provided with at least one bearing for guiding the at least one piston rod in the housing and which bearing is provided with a metal/polymer layer. The metal/polymer layer has a very low coefficient of friction. As a result thereof, the piston rod can be moved into the housing with very few losses and the relatively high leakage flow losses of hydrostatic bearings are avoided. 
     In this case, it should be noted that it was a widely held view amongst those skilled in the art that the high usage requirements for a compacting device make it necessary to use a hydrostatic bearing in which a liquid layer between the piston rod and the housing prevents direct contact between the piston rod and the housing. The liquid layer is in this case produced by a constant supply of a highly pressurized liquid to a slot, for example, a wedge-shaped or cylindrical slot, between the piston rod and the housing. 
     As a result thereof, there is always a leakage flow of liquid with such a hydrostatic bearing, which results in a loss of energy and in addition requires a liquid circuit for the supply and discharge of the leakage flow of liquid. The use of a bearing with direct contact, such as in the exciter according to the invention, between the metal/polymer layer and the piston rod, was not deemed possible by those skilled in the art due to, for example, the frictional losses associated with conventional bearing materials. 
     Surprisingly, however, these problems do not appear to occur with a bearing provided with a metal/polymer layer for low-friction guiding of the piston rod in the housing. In addition, no liquid circuit is required for a metal/polymer bearing for the supply and discharge of leaking hydraulic liquid, and neither is a filter device in the liquid circuit required contrary to what is often the case with hydrostatic bearings, resulting in a simple hydraulic exciter. However, a liquid, for example the hydraulic liquid, can be situated between the metal/polymer bearing and the piston rod in order to further reduce the friction and/or for lubrication. 
     The hydraulic exciter according to the invention is in particular suitable for a compacting device, as this exciter can perform relatively quick movements at a relatively small amplitude, such as for example 30-100 Hz, with an amplitude of a few millimeters at 30 Hz to approximately 1.5 mm at 100 Hz, and at accelerations of more than 200 m/s 2 , while generating relatively little heat. Such frequencies, amplitudes and accelerations are particularly suitable for quickly and effectively compacting, for example, earth-like cement. 
     According to an embodiment, each hydraulic exciter comprises two piston rods which extend from either side of the piston and the housing comprises two respective bearings. The two bearings thus guide the piston rod in the housing during use, thus producing a piston/cylinder device which can satisfy the high usage requirements for a compacting device. 
     According to an embodiment, the piston rod is rigidly connected to the piston of the piston/cylinder device. 
     According to an embodiment, the piston is movable in a piston chamber and a first hydraulic pipe and a second hydraulic pipe on either side of the piston are connected to the piston chamber. In particular, the drive means and the control means may be configured to provide a hydraulic volumetric flow of a determined frequency and phase in the first hydraulic pipe and to provide a hydraulic volumetric flow of the same determined frequency and opposite phase in the second hydraulic pipe. This results in the piston being driven symmetrically and provides a powerful piston/cylinder device which is particularly suitable for a compacting device according to the invention. Further embodiments are defined by the features of the dependent claims. 
     According to a third aspect, the invention provides a compacting device provided with such a hydraulic exciter with metal/polymer bearings. This makes it possible to achieve an efficient compaction with relatively little loss of energy, both for a compacting device with a vibrating table of the known size and for a compacting device with a vibrating table of a larger size. 
     Further embodiments are defined by the features of the dependent claims. 
     According to a fourth aspect, the invention provides a compacting device provided with several such hydraulic exciters with metal/polymer bearings. As, despite the lower friction, there may still be differences in internal friction between several hydraulic exciters (which differences are relatively possibly even greater than with the known, fully developed piston/cylinder devices with hydrostatic bearings), the average person skilled in the art might expect problems when using several hydraulic exciters. Therefore, the person skilled in the art will equip the compacting device with a single hydraulic exciter, despite the abovementioned drawbacks. In an experiment in which several hydraulic exciters which were arranged next to one another without a vibrating table having been placed on top thereof, were being compared to one another, the inventor has indeed noticed that the differences in friction can be so great that hydraulic exciters behave differently in a practically static state when they are driven in the same way, that is to say at the same hydraulic volumetric flow to each of the several hydraulic exciters. However, the inventor has concluded that these differences effectively smooth out when the several hydraulic exciters are connected to the vibrating table or rigidly to one another in another way. Thus, an efficient compacting device is obtained by means of which larger products and/or a larger number of products per run can be compacted using little energy. 
     Further embodiments of a compacting device provided with several hydraulic exciters are described in the dependent claims. 
     According to a further aspect, the invention provides a method for operating a compacting device according to the invention, comprising driving all hydraulic exciters of the plurality of hydraulic exciters with excitation displacements of the same amplitude and the same frequency and in phase with one another. Thus, a uniform excitation vibration is obtained across the entire table surface with, for example, a relatively light table and/or relatively low energy losses. 
     According to an embodiment, the method furthermore comprises the following steps:
         providing a source of hydraulic fluid,   providing a hydraulic volumetric flow of a determined amplitude and frequency to the hydraulic fluid,   determining the excitation displacement of a single hydraulic exciter of the plurality of hydraulic exciters, and   driving each of the hydraulic exciters of the plurality of hydraulic exciters on the basis of the excitation displacement of only the one hydraulic exciter, and   evenly distributing, from a distribution point, the hydraulic volumetric flow between a plurality of hydraulic paths of identical length and identical volume measured from the distribution point up to the respective hydraulic exciters.       

     Further embodiments of the method are described in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention will be described in more detail with reference to the figures, in which: 
         FIG. 1  diagrammatically shows a view of a compacting device with a single hydraulic exciter; 
         FIG. 2   a  diagrammatically shows a view of a hydraulic exciter according to an embodiment of the invention;  FIGS. 2   b - 2   d  show details of  FIG. 2   a;    
         FIG. 3  diagrammatically shows a compacting device with a hydraulic exciter device with a plurality of hydraulic exciters according to an embodiment of the invention; 
         FIG. 4  diagrammatically shows a view of the hydraulic exciter device from  FIG. 3 ; 
         FIG. 5  shows a hydraulic diagram of the hydraulic exciter device from  FIG. 3  and  FIG. 4 ; 
         FIG. 6  diagrammatically shows a top view of the hydraulic exciter device from  FIG. 3  and  FIG. 4 ; 
         FIG. 7   a ,  FIG. 7   b  and  FIG. 7   c  show an example of an electro-hydraulic servo valve for the hydraulic exciter device from  FIG. 3-FIG .  5 ; 
         FIG. 8  shows an example of a square vibrating table for the hydraulic exciter device from  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The device illustrated in  FIG. 1  comprises a vibrating table  1  which is connected to the piston rod  2  of the hydraulic exciter  3 . Furthermore, the device comprises a stamp  4  which is connected to the piston rod  5  of the hydraulic pressing piston/cylinder device  5 ,  6 . Both the hydraulic exciter  3  and the hydraulic pressing piston/cylinder device  5 ,  6  can be operated by respective servo mechanisms  7 ,  8  which are connected to an electronic control device  11  via electrical control lines  9 ,  10 . The hydraulic exciter  3  can generate a vibration in the vibrating table, as is described below. The hydraulic pressing piston/cylinder device  5 ,  6  can exert a pressure on a granular material  17 , such as for example mortar, in use, as is described below. Servo mechanisms  7 ,  8  may also be referred to by the term drive means. The electronic control device  11  can also be referred to by the term control means. 
     A transport plate  12  is arranged on the vibrating table  1  on which the mould which is denoted overall by reference numeral  13  is placed. This mould is open at the top and bottom and essentially consists of a continuous wall  14  having the desired cross-sectional shape of the compacted product to be obtained or the desired cross-sectional shapes of several compacted products to be obtained simultaneously, as well as a flange  15 . On this flange  15 , the clamping jaws  16  of a clamping mechanism can be clamped with an adjustable, if desired programmable clamping force. The number of clamping jaws  16  which is fitted along the circumference of the flange  15  is such that the mould  13 , plate  12  and vibrating table  1  operate as a single entity from a mechanical point of view, even with the vibrations generated in the vibrating device. 
     The material to be treated, such as concrete cement (mortar) or another granular material  17 , is placed in the mould which is subsequently made to vibrate by the hydraulic exciter  3 , vibrating table  1 , plate  12  and mould  13 . 
     The shape of the stamp  4  is adapted to the internal shape of the mould  13  and exerts a specific pressure on the mortar  17  when vibrating. 
     The control unit  11  can set the magnitude of the pressure to a value which is optimal for compacting a specific type of granular material, for example a specific type of mortar, and/or for the properties of the compacted product to be obtained. Said pressure may be varied continuously during compacting as a function of time in order for the compacting process to proceed in an optimum manner. 
     Furthermore, the pressure/time function of the hydraulic pressing piston/cylinder device  5 ,  6  can be coupled to the frequency/time function by means of which the exciter  3  is operated. 
     The resulting compaction of the granular material depends on the magnitude of the acceleration to which it has been subjected. It is best to generate a sufficiently high frequency within the frequency range at adequate amplitude, as the resulting acceleration is substantially linearly proportional to the amplitude, but quadratic to the frequency. 
     Due to the hydraulic-mechanical mass spring system, as illustrated in  FIG. 1 , it is possible to achieve great savings with respect to the time required for compacting the mortar  17  in such a manner that an end product of good quality is obtained with respect to the size of the occurring amplitude and with respect to the accelerations. This can be carried out in a manner similar to that described in NL-1005862 and EP 0 870 585 B1, where a compacting device with a hydraulic exciter device comprising a single hydraulic exciter is described. Reference is furthermore made to NL-1005862 and EP 0 870 585 B1 for the manner in which the vibrations can be generated, in particular by passing through a frequency range in which a peak in the amplitude obtained occurs at a certain frequency f 0 . At this frequency f 0  the acceleration will therefore be as large as possible. The vibrating device according to the invention only has to be operated at this frequency f 0  for a short time, as the generated accelerations are so large, that the granular material is compacted in a short time. In addition, the frequency range is passed through further, beyond frequency ID, thus avoiding the risk of damage to the device caused by natural resonance at this frequency f 0 . 
     The hydraulic exciter  3  is connected to the vibrating table  1 . The vibrating table  1  has dimensions which are such that it can support a relatively large mould  13 . The vibrating table  1  has a size, for example, of 2.15 meters×2.15 meters in order to support a mould  13  of approximately 2.00 meters×2.00 meters. Using a mould  13  of that size, it is possible, for example, to obtain a single concrete slab of approx. 2.00 meters×2.00 meters by compacting the mortar  17  in a mould with an internal shape which provides a single square space of such dimensions. Likewise, it is also possible to obtain four tiles of 100 centimeters×100 centimeters using a mould  13  of such dimensions, for example, with a single passage by compacting the mortar  17  in a mould with an internal shape which provides four square spaces in a single run, following which the shaped and compacted mortar  17  is discharged to a drying device using the transport plate  12 . A reinforcement can be provided in the mould  13  before or during the filling with mortar  17  if the strength of the tiles to be produced requires it. It is also possible to apply a second layer of mortar of a different type on top of the mortar  17  and to subsequently compact this, so that a top layer of the second layer of mortar, for example of a different type, is obtained. 
     As such a size requires a relatively large mass of mortar  17 , mould  13 , plate  12  and vibrating table  1 , it is desirable to use a hydraulic exciter with relatively low loss of energy. The compacting device  1  according to the embodiment shown in  FIG. 1  is to this end provided with a hydraulic exciter with reduced loss of energy, for example as illustrated in  FIG. 2   a.    
       FIG. 2   a  shows a diagrammatic view of a hydraulic exciter  3  according to an embodiment of the invention.  FIGS. 2   b - 2   d  show details of the areas indicated in  FIG. 2   a  by IIb, IIc and IId. 
     The hydraulic exciter  3  has a piston  28  which is axially movable in a cylinder  31  of a housing  30  along a cylinder axis. The hydraulic exciter  3  furthermore has a highest piston rod  2   a  and a lowest piston rod  2   b , which are axially movable in the housing  1  along the cylinder axis. The highest piston rod  2   a  can also be denoted as “the first piston rod” or “the first piston rod part”. The lowest piston rod  2   b  can also be denoted as “the second piston rod” or “the second piston rod part”. The highest piston rod  2   a  and the lowest piston rod  2   b  together can be denoted as “the piston rod  2 ”. The housing is denoted overall by reference numeral  30  and, in the illustrated example, is composed of several parts which together form the housing. The housing  30  comprises a cylinder  31  between a first block  33  and a second block  34 . In the cylinder  31 , a hydraulic pressure can be created in a cylinder space  29 . The second block  34  is connected to a third block  38  by means of a bottom tube  35 . At the bottom side, the housing  30  is closed off by the third block  38 . The third block  38  also serves for connection to a foundation ( 90 ,  91 ; not shown in  FIG. 2   a ). At the top side, the housing  30  is provided with a ring  36  which is provided with an end seal  59  between the highest piston rod  2  and the housing  30  and which is connected to the first block  33  by means of bolts  37 . An assembly formed by the first block  33 , the cylinder  31 , the second block  34 , the bottom tube  35  and the third block  38  is held together by draw bars  39  which extend through the entire assembly. The piston  28  is accommodated in the cylinder space  29 . At the top side, the piston  28  is connected to the highest piston rod  2   a  and at the bottom side to the lowest piston rod  2   b . The part of the cylinder space  29  above the piston  28  is denoted in  FIG. 2   a  as upper space  29   a . The part of the cylinder space  29  below the piston  28  is denoted in  FIG. 2   a  as lower space  29   b . A first hydraulic pipe (not shown) ends in the upper space  29   a  and contains hydraulic fluid which is operated by a servo valve (not shown in  FIG. 2   a ) with a first volumetric flow of a magnitude which is varied according to a frequency range. In the example, the hydraulic fluid is a hydraulic oil, but may also be another liquid or a gas in alternative embodiments. A second hydraulic pipe (not shown) ends in the lower space  29   b  and contains the same hydraulic fluid which is operated in a complementary manner by the same or another servo valve (not shown in  FIG. 2   a ) as the hydraulic fluid in the first hydraulic pipe. The volumetric flows to the upper space  29   a  and the lower space  29   b  are thus varied at the same frequency, but in opposite phase. Thus, the piston  28  is moved up and down by means of the resulting pressure difference between the upper space  29   a  and the lower space  29   b . The hydraulic exciter  3  has two bearings  40 ,  42  for guiding the piston rods  2   a  and  2   b  in the housing  30 . Bearing  40  guides the highest piston rod  2   a . Bearing  42  guides the lowest piston rod  2   b . The bearings  40 ,  42  are provided with a metal/polymer layer  44 . The metal/polymer layer  44  has a bearing coefficient of friction, resulting in a very efficient guiding of the piston rod  2  in the housing  30  with low loss of energy and little generation of heat. The bearings  40 ,  42  are accommodated in continuous recesses  50 ,  52  in the housing  30 . In the illustrated example, these are accommodated in a first continuous recess  50  formed in the ring  36  and the first block  33  of the housing  30  and in a second continuous recess  52  formed in the second block  34  and the bottom tube  35  of the housing  30 , respectively. 
     The metal/polymer layer  44  shown in  FIGS. 2   a ,  2   c  and  2   d  is formed by a layer of a polytetrafluoroethene (PTFE), a further fluoropolymer and a filler. In this specific example, this layer is formed substantially from DP31™, the trade name of a specific metal/polymer which is marketed by GGB. DP3™ has a particularly suitable, low frictional resistance and is very strong, which makes this material eminently suitable for a heavy application such as this. In addition, the composition of the metal/polymer DP31™ has the characteristic that it makes this composition resistant to cavitation, that is to say that hydraulic liquid substantially does not penetrate this composition. As a result thereof, the risk of small pieces of material breaking off is significantly reduced and the hydraulic liquid remains free from contamination by pieces of bearing material. Alternatively, other metal/polymer materials can be used. 
     Furthermore, the housing  30  has a first and a second seal  45 ,  46  between the piston rod  2  and the housing  30 , which seals are accommodated in further continuous recesses  47  in the housing  30 . The bearings  40 ,  42  are situated between the first and the second seals  45 ,  46  in the axial direction. The first and second seals  45 ,  46  thus seal the cylinder space  29  between the cylinder  31  and the piston rod  2  virtually hermetically from the outside world, thus reducing pressure loss. The seals  45 ,  46  each have two lips  48 ,  49 , as is illustrated. Each outer lip  48  is formed by a U-shaped PTFE bush comprising a stainless steel spring with a spring opening turned away from the cylinder space  29 . Each inner lip  49  is formed by a U-shaped PTFE bush comprising a stainless steel spring with a spring opening turned towards the cylinder space  29 . 
     The hydraulic exciter  3  may be provided with a sensor  80  ( FIG. 2   a ) for emitting a signal which is representative of the excitation displacement of the hydraulic exciter  3 . The sensor may, for example, be a displacement sensor and may be configured to determine the displacement of the piston rod  2  in the housing  30  in the direction of the longitudinal axis of the piston rod. In particular, the sensor  80  may be configured to determine the displacement of the lowest piston rod  2   b . Based on this signal, a control means can regulate a working position and/or amplitude of a regular movement of the piston rod  2 . 
       FIG. 3  diagrammatically shows a view of a compacting device comprising a hydraulic exciter device with a plurality of hydraulic exciters according to an embodiment of the invention. 
     The compacting device of  FIG. 3  differs from that of  FIG. 1  by the fact that the single hydraulic exciter  3  with piston rod  2  has been replaced by a hydraulic exciter device  63 . The hydraulic exciter device  63  has a plurality of hydraulic exciters  3 , each connected to the vibrating table  1  by a respective piston rod  2 . The hydraulic exciters  3  are mutually parallel, which means that the piston rods  2  of the hydraulic exciters  3  are parallel with respect to one another. In particular, all piston rods  2  are at right angles to the top surface of the vibrating table  1 , on which the mould  13  with the mortar  17  is situated during use. The hydraulic exciter device  63  can be controlled by servo mechanisms  7  which are connected to an electronic control device  11  via electrical control lines  9 . The electronic control device  11  can also be designated as control means  11 . The servo mechanism  7  can also be referred to as drive means  7 . The hydraulic exciter device  63  is configured to drive the hydraulic exciters  3  at mutually substantially the same amplitude, frequency and phase by means of the drive means  7  in cooperation with the control means  11 . 
     The hydraulic exciters  3  are rigidly connected to one another via the vibrating table  1 . The piston rods  2  of the hydraulic exciters  3  are mutually connected in a flexurally stiff manner in a plane which is defined by the axes of in each case two piston rods. 
     Each hydraulic exciter  3  is of the type as described with reference to  FIGS. 2   a - 2   d  and has, in general terms, a piston/cylinder device  28 ,  30  with two piston rods  2   a ,  2   b  (referred to collectively as the piston rod  2 ), a housing  30  and two bearings  40  for guiding the two piston rods  2   a ,  2   b  in the housing  30 , wherein the bearings  40  are provided with a metal/polymer layer  44 , in particular of DP31™. An example of such a hydraulic exciter device comprising four hydraulic exciters with DP31™ bearings is suitable for use with a vibrating table with transport plate and mould weighing a total of 5000 kg, on which 1500 kg of granular material can be compacted in the mould, across a frequency range of, for example, 40 to 80 Hz, at a resonance in the excitation amplitude around 70 Hz with an amplitude of approximately 1.5 mm and an associated acceleration of approximately 290 m/s 2 . 
     Each hydraulic exciter  3  can alternatively comprise a hydrostatic piston/cylinder device of a known type. 
       FIG. 4  shows a diagrammatic side view of the hydraulic exciter device from  FIG. 3 .  FIG. 5  shows a hydraulic diagram (not to scale) of the same exciter device.  FIG. 6  shows a diagrammatic top view of the same hydraulic exciter device. 
       FIG. 4  shows two of the four hydraulic exciters  3 , all of which are shown in  FIG. 5  and  FIG. 6 . The hydraulic exciters  3  are all connected to a foundation plate  90  by means of bolts, the former being connected to a concrete foundation  91  by means of further bolts. The hydraulic exciters  3  are all connected to a single electro-hydraulic servo valve, diagrammatically represented by  70 .  FIG. 4  and  FIG. 5  furthermore show a first, second and third pump line  82 ,  83 ,  85 , a pump  84  and a tank  86  which form a hydraulic pump circuit in which the pump  84 , via the first pump line  82 , pumps a hydraulic fluid to the electro-hydraulic servo valve  70  with a first volumetric flow. The hydraulic fluid subsequently returns to the tank  86  from the electro-hydraulic servo valve  70  via the second pump line  83 . The tank  86  is connected to the third pump line  85  on the pump  84 . 
     In use, the electro-hydraulic servo valve  70  ensures the fluid flow from the first pump line  82  to a hydraulic main supply pipe  72  and from a hydraulic main discharge pipe  73  to the second pump line  82 , and thus regulates the volumetric flow in the hydraulic main supply pipe  72  and the hydraulic main discharge pipe  73 . To this end, the electro-hydraulic servo valve  70  is actuated via electrical control lines  9  from the electronic control device  11 . In a first state, the electro-hydraulic servo valve  70  connects the hydraulic main supply pipe  72  to the first pump line  82  with the pump  85  and at the same time connects the hydraulic main discharge pipe  73 , via the second pump line  83 , to the tank  86 , by means of which the volumetric flow supplied by the pump  85  is passed to the hydraulic main supply pipe  72 . The flow of the hydraulic fluid through the respective pipes in the first state is illustrated in  FIG. 5  by means of full arrows. In a second state, the electro-hydraulic servo valve  70  connects the hydraulic main discharge pipe  73  to the first pump Line  82  with the pump  85  and at the same time connects the hydraulic main supply pipe  72 , via the second pump line  83 , to the tank  86  by means of which the volumetric flow supplied by the pump  85  is passed to the hydraulic main discharge pipe  73 . The flow of the hydraulic fluid through the respective pipes in the second state is illustrated in  FIG. 5  by means of dashed arrows. The hydraulic main supply pipe  72  and the hydraulic main discharge pipe  73  are coupled to respective and substantially identical distribution supply pipes  64  and distribution discharge pipes  65  via a manifold  75 . The hydraulic main supply pipe  72  thus branches in the manifold  75  into in this example four distribution supply pipes  64 . The hydraulic main discharge pipe  73  thus branches in the manifold  75  into in this example four distribution discharge pipes  5 . Substantially identical is understood to mean that the distribution supply pipes and the distribution discharge pipes, respectively, behave in substantially the same manner when a hydraulic fluid which is present therein is offered the same hydraulic volumetric flow. For example, each of the distribution supply pipes  63  and the distribution discharge pipes  64 , respectively, are made of substantially the same material and have substantially the same diameter, substantially the same stiffness in the longitudinal direction and substantially the same stiffness in the circumferential direction. The total volumetric flow is thus evenly distributed between the distribution supply pipes  63  and the distribution discharge pipes  64 , respectively. The distribution supply pipes  64  act as first hydraulic pipes as described with reference to  FIG. 2 . The distribution discharge pipes  65  act as second hydraulic pipes as described with reference to  FIG. 2 . 
     Thus, a hydraulic volumetric flow to/from the upper space  29   a  of each of the hydraulic exciters  3  is regulated via the hydraulic main supply pipe  72 , the manifold  75  and the respective distribution supply pipe  64 , and the hydraulic volumetric flow from/to the lower space  29   b  of each of the hydraulic exciters  3  is regulated via the hydraulic main discharge pipe  73 , the manifold  75  and the respective distribution discharge pipe  65 . The hydraulic exciters are driven synchronously by means of the single valve  70 . The use of a single valve  70  offers the advantage, for example, that the exciters  3  operate synchronously without requiring additional measures. Moreover, this makes it possible to avoid the extra costs of additional electro-hydraulic servo valves  70  which would be required if each exciter had its own servo valve. 
     The drive means  7  illustrated diagrammatically in  FIG. 3  thus comprises the electro-hydraulic servo valve  70 , the pump circuit, the manifold  75  and the supply pipes  64  and main discharge pipes  75 . In use, the hydraulic exciters  3  are driven at mutually substantially the same amplitude and frequency and in phase with one another by means of the drive means  7  in cooperation with the control means  11 . 
     A plurality of hydraulic supply pipe paths between the servo valve  70  and the respective hydraulic exciters  3  are formed by the common hydraulic main supply pipe  72 , an internal path in the manifold  75  and the respective distribution supply pipes  64 . These hydraulic supply pipe paths are identical in length and volume. Likewise, a plurality of hydraulic discharge pipe paths are formed between the servo valve  70  and the respective hydraulic exciters  3  by the common hydraulic main discharge pipe  73 , a further internal path in the manifold  75  and the respective distribution discharge pipes  65 . These hydraulic discharge pipe paths are likewise identical in length and volume. The hydraulic pipe paths thus behave the same for each hydraulic exciter  3 . As a result thereof, the exciters are not only operated at the same frequency, but also in phase with one another. All exciters will thus move at the same frequency, without phase differences between the various exciters. 
     Of the four hydraulic exciters  3 , a first hydraulic exciter  3  is provided with a sensor  80  (illustrated in  FIG. 2   a ) for emitting a position signal representative of the position of the first hydraulic exciter  3 , that is to say of the piston rod  2  with respect to the housing  30 . The sensor  80  is in communication with the control means  11  for regulating a working position and/or an amplitude of the first hydraulic exciter  3  by means of the control means  11  and drive means  7 . The device is configured in such a manner that the rigid connection between the exciters, in use, normally results in a smoothing out of any small differences between the internal friction in the various exciters  3 , and so that the hydraulic exciters  3  with the same working position, amplitude, frequency and phase make the vibrating table  1  vibrate. Thus, it is possible to operate all exciters synchronously using only the position signal of the first hydraulic exciter. 
     In an alternative embodiment, each of the hydraulic exciters  3  is provided with its own sensor  80  and each of the hydraulic exciters  3  is driven by its own servo valve, with the control means  11  actuating the servo valves of the various exciters in phase and amplitude in such a manner that the hydraulic exciters  3  are operated at the same amplitude, frequency and phase. 
       FIG. 7   a ,  FIG. 7   b  and  FIG. 7   c  show an example of the electro-hydraulic valve  70  according to an embodiment of the compacting device with a hydraulic exciter device. The electro-hydraulic servo valve  70  has a 3-step structure and comprises a valve body  104 , which can be moved to and fro hydraulically in a cylindrical cavity  103  in a valve housing  102  by means of a hydraulic intermediate valve  108 , which is operated in turn by an electro-hydraulic pilot valve  106  which is actuated via electrical control line  9  and an electrical feedback line  109  from a measuring device  107  which is configured to emit a feedback signal, indicative of the position of the valve body  104  in the cylindrical cavity  104 . A known type of such an electro-hydraulic servo valve  70  is, for example, the Type 4 WSE 3 EE 32 servo valve by Bosch Rexroth. Such a valve is highly dynamic, that is to say that it can be used up to approximately 100 Hz, and has a high capacity due to the 3-stage structure. The valve body  104  is a rod-shaped element with thickenings  121 ,  122 ,  123  and  124 , the outer diameter of which corresponds to the inner diameter of the cylindrical cavity. Between the thickenings  121 ,  122 ,  123  and  124 , there are annular recesses  111 ,  112 ,  113 . In the valve housing  102 , there are 5 ducts  183 ,  173 ,  182 ,  172  and  184  between the cylindrical cavity and four connecting points to the second pump line  83  (to the tank  86 ), the hydraulic main discharge pipe  73  (to the manifold  75 ), the first pump line  82  (from the pump  85 ), the hydraulic main supply pipe  72  (to the manifold  75 ) and—again—the second pump line  83  (to the tank  86 ), respectively. The electro-hydraulic servo valve  70  can be brought into three states: a first (operating) state, a second (operating) state and a neutral state. 
       FIG. 7   a  shows the electro-hydraulic valve  70  in the neutral state, in which thickening  122  closes off duct  173  (and thus the hydraulic main discharge pipe  73 ) and thickening  123  closes off duct  172  (and thus the hydraulic supply pipe  72 ). 
     In the first state, illustrated in  FIG. 7   c , the valve body is moved slightly to the right, so that recess  111  creates a connection between ducts  183  and  173 , and recess  112  creates a connection between ducts  182  and  172 . In the first state, the electro-hydraulic servo valve  70  thus connects the hydraulic main supply pipe  72  to the pump via the first pump line  82  and at the same time connects the hydraulic main discharge pipe  73  to the tank  85  via the second pump line  83 . 
     In the second state, illustrated in  FIG. 7   b , the valve body is moved slightly to the left, so that recess  112  creates a connection between ducts  182  and  173 , and recess  113  creates a connection between ducts  184  and  172 . In the second state, the electro-hydraulic servo valve  70  thus connects the hydraulic main discharge pipe  73  to the pump  85  via the first pump line  82  and at the same time connects the hydraulic main supply pipe  72  to the tank  85  via the second pump line  83 . 
     Preferably, the vibrating table  1  is rectangular or square.  FIG. 8  shows an example of a square vibrating table  1 . The rectangular or square vibrating table  1  may be composed of a plurality of adjoining, identical rectangular or square parts  100  whose number is equal to the number of exciters  3 . In the illustrated embodiment, each exciter  3  is connected to the vibrating table  1  at the intersection  102  of the diagonals  101  of the respective rectangular or square part. This may be advantageous when producing a uniform excitation across the entire vibrating table  1 , without nodes and antinodes, so that a uniform compaction can be achieved and/or fatigue stresses are reduced as far as possible. In an alternative embodiment, each exciter  3  is connected to the vibrating table  1  between the intersection  102  of the diagonals  101  of the respective rectangular or square part and the outer circumference of the vibrating table, at most 25% of the distance between the intersection  102  of the diagonals  101  and a corner of the respective rectangular or square part. This may be advantageous for a uniform excitation and/or lower fatigue stress.