Abstract:
An oscillatory drive unit for use in an oscillating system to be oscillated substantially at the resonance point of the oscillating system, wherein the oscillating system includes a mass to be oscillated and a countermass to which the mass is connected by resilient coupling members. The oscillatory drive unit includes a piston-cylinder-component that is operatively connected to the mass and the countermass. The piston-cylinder-component includes a cylinder chamber and a piston. The piston is arranged for reciprocal movement between a first end position and a second end position. Gas is introduced under pressure into the cylinder chamber within an energizing range of piston positions defined by a first energizing position and a second energizing position of the piston, and gas is vented from the chamber within a venting range of piston positions defined by a first venting position and a second venting position of the piston. The energizing range of piston positions being close to or including the first end position of the piston, and the venting range of piston positions being close to or including the second end position of the piston. The actual reversal points of movement of the piston adjust themselves under operating conditions of the oscillating system.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation in part of application Ser. No. 08/813,366 filed on Mar. 7, 1997.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention pertains to an oscillatory drive unit for use in an oscillating system to be oscillated substantially at the resonance point of the oscillating system. The oscillating system generally includes a mass to be oscillated which mass is resiliently suspended on or supported by a countermass by means of any kind of resilient springs. Although the invention has particular utility in connection with oscillating conveyors that are to be operated substantially at their resonance point, the problems to be solved with the object of the invention mainly occur also in other oscillating systems that primarily operate within the resonant range.  
           [0003]    In prior art oscillating conveyors, the resiliently supported oscillating or conveying bodies are subjected to a forced oscillation by means of a vibrator. This oscillation causes the material situated on the oscillating conveyor to travel along the conveyor. There exist so-called oscillating conveyors that are subjected to an essentially linear oscillation, the direction of which approximately corresponds to the initial angle of a parabola. Such oscillating conveyors are frequently supported in resilient fashion on leaf springs that are arranged perpendicular to the desired oscillating direction and allow a practically linear oscillation in this direction. This practically linear oscillation only deviates from linearity due to the circular arc path of the spring ends. This deviation may be neglected if the difference between the oscillation stroke and the length of the spring is sufficiently large.  
           [0004]    In addition to electromagnetic vibrators, it is also possible to utilize piston vibrators that are pressurized with compressed air as oscillatory drives for such oscillating conveyors. The latter provide the advantage that their oscillating behavior can be better adapted to different requirements.  
           [0005]    With respect to the expenditure of energy, it is most favorable to operate oscillating conveyors within the resonant range. Within this range, the size of the masses to be moved is inconsequential. It simply must be ensured that the energy consumed by the damping in the springs and by the moved material is replaced. In this case, only part of the weight of the material to be conveyed consumes energy. Resonance conveyors are preferably realized in the form of conveyor chutes. With respect to the operating requirements, it is, however, disadvantageous that the resonance frequency of a system depends on the respective load of the material to be conveyed and varies accordingly. In known systems, the adaptation of a resonant drive to the resonance frequency of a system represents an almost insurmountable problem. An adaptive variation of the excitation frequency during the operation of such a conveyor can only be realized with significant expenditures. Consequently, measures of this type are frequently relinquished, and the oscillating resonance drives are designed in such a way that the operating range is displaced into the ascending part of the resonance curve, i.e., far away from the resonance apex. In this case, at least part of the resonance amplification can be utilized, and a damping of the amplitude while the system is subjected to a load may, for example, be compensated due to the fact that the resonance point is simultaneously displaced to a slightly lower frequency and the fixed operating frequency is displaced into a region of higher amplitude.  
           [0006]    Even when using a more flexible piston vibrator, the adaptation of its frequency to the resonance frequency of a system is only possible under certain conditions. In piston vibrators, the power essentially depends on the piston diameter. However, the frequency and the oscillation amplitude result from the piston weight. If a heavier piston of the same diameter is used, the oscillation amplitude is correspondingly increased and the frequency is correspondingly decreased. This applies to cylindrical as well as stepped or differential pressure piston vibrators. In order to alter the oscillating behavior of a piston vibrator, pistons with different lengths and identical diameters are frequently utilized so as to attain different moments within the same power range. Normal piston vibrators that are coupled to a system to be subjected to oscillations usually comprise a piston without a piston rod which only moves back and forward within the vibrator housing. However, it is also possible to provide one side of the piston with a piston rod that extends out of the housing. The one-sided effect of this piston rod, which reduces the piston surface, makes it necessary to realize this vibrator in the form of a differential pressure piston vibrator. The piston rod that extends out of the housing may be additionally provided with masses that reduce the frequency. An adaptation of the frequency of a piston vibrator to the resonance frequency of a system would only be possible by lowering the air pressure. However, the power of the piston vibrator varies exponentially with the frequency, i.e., this measure can generally be precluded. In conventional systems, the springs of a resonance conveyor are usually designed in such a way that the resonance frequency of the system lies within the economical operating range of a certain vibrator type. This may, for example, be realized by altering the piston mass.  
           [0007]    Resonance conveyors are not only sensitive to load changes. The natural frequency essentially also depends on the ratings of the springs used and, for example, the weight of the conveyor. If the conveyor is structurally modified such that its weight is changed, it is possible that the conveyor no longer resonates and consequently conveys no material. In conventional so-called oscillating resonance conveyors that operate at a frequency of only approximately 85% of the resonance frequency, it is a customary procedure to operate below this frequency in order to still attain a noticeable resonance amplification of the oscillation amplitude and reduce the sensitivity of the system. If a conveyor that is correctly calculated and designed with respect to its resonance frequency is assembled on a foundation or frame that is insulated against oscillations, i.e., assembled elastically, so as to transmit the least possible oscillations onto the substructure, the mass of the foundation and its elastic support must be incorporated into the calculation, i.e., a system, the drive of which is generally adapted to the resonance frequency, no longer resonates.  
           [0008]    The present invention is based on the objective of developing an oscillatory drive for a system with an oscillating mass and, in particular, an oscillating conveyor, in which the oscillatory drive practically recognizes the respective resonance frequency of the driven system, follows changes of this resonance frequency, and supplies the system with the energy required for maintaining the oscillations without electrical recognition or control units.  
           [0009]    According to the invention, this objective is, in principle, attained with the characteristics disclosed in the characterizing portion of claim  1 .  
           [0010]    Although a person skilled in the art will easily ascertain that the invention can also be utilized in other oscillating systems, e.g., vibration tables, screens, filter frames, etc., the invention is described below with reference to a resonance conveyor.  
           [0011]    Leaving aside the corresponding coupling of the individual elements, it is an essential aspect of the invention to provide an oscillatory drive, the mutually moving components of which are not positively limited with respect to their mutual oscillation amplitude, e.g., by means of a limit stop, but rather able to essentially freely adapt to a resonance oscillation of the system. In this case, it is not precluded that a certain (progressive) elastic limitation for the mutually oscillating parts is provided. Consequently, the times at which the drive supplies energy are not dependent on the reversal points of a drive component, e.g., a piston. On the contrary, the energy may be supplied at an interim phase during the acceleration cycle of a moving drive component in order to excite this drive component to carry out amplified oscillations. It was determined that the system also operates flawlessly if the energy is supplied slightly before the reversal point.  
           [0012]    According to the invention, the pistons of these oscillatory drive units must be able to follow the resonance oscillation of the system without impacting with a rigid limitation in the cylinder. The supply of compressed air into the cylinder chamber behind the trailing end of the piston is, for example, controlled out by the piston, e.g., during the acceleration phase of the piston after an inlet opening is released.  
           [0013]    In order to make it possible to transmit the energy that is introduced into the oscillatory drive and must be converted into an oscillation movement as directly as possible onto the oscillating system, the coupling of the oscillatory drive to both masses in question should be as rigid as possible in the direction of the transmission of oscillations. However, if the shape and direction of the oscillations of the drive do not exactly correspond to the possible oscillation path of the driven system, it is preferred to utilize flexible or articulated rods for the coupling. These flexible or articulated rods are relatively rigid in their longitudinal direction, but allow a lateral excursion during the oscillation sequence, i.e., the piston in the oscillatory drive is not unnecessarily stressed by transverse forces.  
           [0014]    A certain elasticity of the coupling elements in the direction of the transmission of oscillation would also be conceivable, e.g., in order to ensure that a certain reserve for the oscillation amplitude is still available at high oscillation amplitudes and an unexpected impact of the oscillatory drive piston in the housing. However, this solution is associated with the disadvantage that the oscillation energy is transmitted to the system with certain delays because such an elastic coupling of the piston creates a separate oscillating system that tends to oscillate in resonance and consequently can make it impossible to control the sequence of the energy transmission. In any case, it would be better to design the oscillatory drive in such a way that the possible piston travel suffices for the oscillation amplitudes of the system which occur during its operation.  
           [0015]    The countermass, on which the oscillatory drives of such conveyors are supported, can consist of a stationary foundation. In this case, the countermass is rigid or practically rigid and has an essentially infinite size, i.e., the resonance frequency of the conveyor is primarily determined by its own mass and the type of its resilient support. However, the countermass may also consist of a pedestal frame of the device that is elastically supported on a foundation or substructure so as to transmit the fewest possible oscillations onto the substructure. In contrast to a rigid support, such an arrangement alters the resonance frequency of the system. According to the invention, the countermass may also consist of a mass that is freely arranged in space and held by springs that connect the countermass to the conveyor. In this case, it is necessary to suspend or support the entire system. This may, for example, be realized by suspending the entire system on the conveyor by means of very soft springs that limit the degrees of freedom of the entire system as little as possible. A system that is suspended on soft springs usually has a different, essentially lower resonance frequency than that of the driven, insulated system consisting of both masses and their mutual spring coupling. The main disadvantage of such an arrangement can be seen in the fact that the oscillation amplitude of the conveyor referred to its surroundings is significantly reduced due to the lack of a support for the countermass.  
           [0016]    Oscillating conveyors should generally cause a movement of the material to be conveyed during each oscillation, namely such that their oscillations are directed transversely upward viewed in the conveying direction of the material. A degree of elastic freedom in such a direction and the most rigid arrangement possible in the other directions is preferably realized with leaf springs that extend perpendicular to the intended oscillating direction. The ends of such leaf springs do not have a strictly linear path. As long as their oscillation amplitude is small in relation to their size, the circular arc movement can be considered to be practically linear.  
           [0017]    It is not absolutely imperative that the oscillating direction of the oscillatory drive corresponds to the direction of the degree of oscillating freedom of the conveyor. However, the conveyor is usually designed in this fashion because only a certain component of the moment of the oscillatory drive would otherwise be used for generating the oscillations.  
           [0018]    If only one oscillatory drive is provided in a conveyor, the elastic coupling of which to the countermass does not inevitably define the oscillating direction, it is required that the line of application of the oscillatory drive extend through the center of gravity of the oscillating mass of the conveyor. If the countermass is elastically supported, the direction of the line of application of the oscillatory drive also must extend through its center of gravity. This can be attained by designing the shape of the mass correspondingly. If the conveyor is suspended on leaf springs, it is also practical to observe this rule. However, the arrangement is less critical if several oscillatory drives are provided. Since the systems in question pertain to systems that oscillate in resonance, only one correspondingly designed oscillatory drive suffices in most instances.  
           [0019]    The conventional crank drive also represents an oscillatory drive that is supported on a foundation frame with one end and engages on an elastically supported mass with the other end. However, the frequency and amplitude of a crank drive are predetermined during its operation, i.e., only one forced oscillation can be generated with a crank drive. As mentioned previously, it is essential for the invention that an oscillatory drive, in which the position of the reversible points is not defined, is utilized so as to ensure that the piston of the oscillatory drive which is connected to the system is able to follow the oscillations of the system in largely unobstructed fashion, i.e., its oscillation amplitude as well as its oscillation frequency are essentially also defined by the system. Due to this measure, the oscillatory drive assumes or recognizes the respective resonance frequency of the entire system.  
           [0020]    According to the invention a pneumatic oscillatory drive unit is provided that is pressurized with the pressure medium on only one side, i.e., only from one of its reversible points, and subsequently ventilated. This piston vibrator is able to freely oscillate in the direction towards its other reversal point, with the return movement being realized without the assistance of external energy, namely by the coupled system that oscillates backward. In piston vibrators that are provided with a piston rod that extends out of the housing and charged with pressure on both sides, it is possible to operate exclusively with a differential pressure. However, in a vibrator that is pressurized with pressure on only one side, preferably the side situated opposite to the piston rod, the entire piston surface is available for pressurizing the piston. This is particularly advantageous for the start-up of a system that operates in resonance. Before the resonance frequency is reached during the start-up, higher restoring forces of the springs usually must be overcome. If it is ensured that a vibrator that is pressurized on one side is situated in a suitable start-up position in the idle state of the device, the system can be started without problems by pressurizing the vibrator with the full pressure. Since the system automatically transforms into the resonance mode during its operation, a change in the pressure of the operating medium and consequently the supplied energy only causes the oscillating amplitude to change. The required supplementary energy supplied to the system can be adapted solely by changing the pressure.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0021]    The invention is described in detail below with reference to the enclosed figures. The figures show:  
         [0022]    [0022]FIG. 1, a resonance conveyor that is supported in oscillating fashion on a rigid foundation by means of leaf springs, with said resonance conveyor comprising an oscillatory drive that engages on the conveyor with one end and on the foundation with the other end;  
         [0023]    [0023]FIG. 2, a resonance conveyor that is supported on an elastic pedestal by means of leaf springs as well as two possible arrangements of the oscillatory drive;  
         [0024]    [0024]FIG. 3, a resonance conveyor that is supported on soft coil springs and connected to a countermass via leaf springs, with the oscillatory drive engaging on the conveyor and the countermass;  
         [0025]    [0025]FIG. 4, a compact structural unit for a resonance conveyor according to FIG. 3 which comprises the oscillatory drive, the leaf springs, and the countermass;  
         [0026]    [0026]FIGS. 5 a  and  b , a schematic representation of a system similar to that of FIG. 1 with a fluid-driven piston vibrator in the oscillatory drive that is pressurized with fluid on one side;  
         [0027]    [0027]FIGS. 6 a - 6   e , the oscillating drive of the system of FIGS. 5 a  and  5   b  in various operating positions;  
         [0028]    [0028]FIG. 7, a modification of the oscillating drive of FIG. 6;  
         [0029]    [0029]FIGS. 8 a - 8   c , another embodiment of an oscillating drive in various operating positions.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIG. 1 schematically shows an oscillating conveyor with a conveyor body  2  that is resiliently supported on two rigid foundation blocks  6  via leaf springs  4 . The arrow  8  indicates the intended conveying direction. An oscillatory drive  12  is arranged between a lower projection  10  of the conveyor body and the rear foundation block  6 . This oscillatory drive  12  comprises a cylinder  14  that is fastened to the rear foundation block  6 . A piston rod  16  that is connected to the projection  10  of the conveyor body  2  via a connecting rod  18  protrudes from the other side of the cylinder  14 . In this embodiment, the oscillatory drive  12  that is pressurized with compressed air and excites the system such that it oscillates as indicated by the double arrow  20 .  
         [0031]    [0031]FIG. 2 shows a resonance conveyor, the conveyor body  2  of which is also resiliently supported on leaf springs  4 . However, the lower ends of these leaf springs are connected to a pedestal block  22  that is supported on the substructure  26  via elastic bearings  24 . This embodiment comprises two oscillatory drives  28  and  30  that engage on the conveyor body  2  and on the pedestal block  22 . This figure shows an alternative arrangement of the oscillatory drives  28  and  30 . The line of application of the oscillatory drive  28  extends parallel to and near the connecting line between the centers of gravity S of the conveyor body  2  and the pedestal block  22 . An exact concurrence between the line of application of the oscillatory drive  28  and the connecting line between the centers of gravity S is not absolutely imperative in such an arrangement with multiple leaf springs  4 . If an additional drive is provided in the position of the oscillatory drive  30 , its line of application extends through the center of gravity S of the conveyor body  2 , but not through the center of gravity of the pedestal block  22 . Since the pedestal block is very long, its elastic bearings  24  are subjected to shear stress by this arrangement. However, a tilting movement can be practically precluded if the elastic bearings  24  have a relatively high compressive strength.  
         [0032]    [0032]FIG. 3 shows a resonance conveyor, the conveyor body  2  of which is supported on the substructure  26  by means of relatively soft coil springs that are arranged vertically. The conveyor body is provided with leaf springs  36  on a rear projection  34  which carry a countermass  38 . An oscillatory drive  40  engages on one additional projection  10  of the conveyor body  2  and on the countermass  38 . The oscillating direction is again indicated by a double arrow  20 .  
         [0033]    [0033]FIG. 4 shows a compact system that may be connected to a conveyor body  2  according to FIG. 3 in preassembled fashion. This system carries the springs  36  and the oscillatory drive  40  on a frame  42 . The ends of the leaf springs  36  are connected to one end of the oscillatory drive  40  and to the countermass  38 . The connecting rod  18  of the oscillatory drive  40  is realized in the form of a partially flexible plastic tube that, however, is realized in non-bucking fashion in its longitudinal direction. Since the lower ends of the leaf springs  36  do not exactly describe a linear path in the oscillating mode, but rather a circular arc path with respect to the frame  42 , the flexible connecting rod  18  prevents transverse forces from acting upon the piston of the oscillatory drive  40 .  
         [0034]    [0034]FIG. 5 shows an oscillatory drive  44  that is pressurized on one side and embedded in an oscillating system similar to FIG. 1. The mass  46  represents the conveyor body  2  and is supported on a countermass  50  via leaf springs  48 . The oscillatory drive  44  comprises a cylinder  52  that is fastened to the countermass  50  as well as a piston  54  that is arranged such that it can be linearly displaced in the cylinder  52  and connected to the mass  46  via a connecting rod  56 .  
         [0035]    [0035]FIG. 5 a  shows the startup of the system from the idle position. Compressed air is introduced into the circumferential groove  60  of the cylinder  52  via a compressed air supply duct  58 , with said compressed air reaching the cylinder chamber  64  via the piston channel  62 . Consequently, the piston  54  is pressed toward the left until the piston channel  62  is connected to the circumferential venting groove  66  as shown in FIG. 5 b . Meanwhile, the compressed air supply duct was closed by the piston, i.e., the cylinder chamber  64  can be ventilated. The return movement of the piston  54  is realized with the aid of the restoring force of the leaf springs  48 , whereafter the entire process is repeated.  
         [0036]    Having reference to FIGS. 6 a  to  6   e , the drive unit of the system according to FIGS. 5 a  and  5   b  is described in more detail, particularly its operating cycle.  
         [0037]    The drive unit in the form of a piston cylinder component includes a cylinder  52  and the piston  54 . Having particular reference to FIG. 6 d  the cylinder chamber  64  is defined by the inner end wall  68  and the inner side wall  70  of the cylinder  52  and a first end face  72  of the piston  54 . The other end face  74  of the piston  54  is provided with connecting means  56 , for example in the form of a piston rod as described herein above. Connecting means with which the cylinder  52  might be fixed to one part of the oscillating system are not shown. They may be of any conventional type.  
         [0038]    In the cylinder  52  are provided an air supply duct  58  and a venting duct  76 . The piston  54  is provided with the piston channel  62  one end which opens into the outer side wall  78  of piston  54 , while the other end of piston channel  62  communicates with the cylinder chamber  64 .  
         [0039]    At the interface of outer side wall  78  of piston  54  and inner side wall  70  of cylinder  58  two communicating spaces in the form of a first circumferential groove  60  and a second circumferential groove  66  are formed in the inner side wall  70  of cylinder  52 . It is apparent that air supply duct  58  opens with an outlet port into the first circumferential groove  60 , whereas venting duct  76  opens with its intake port into the second circumferential groove  66 .  
         [0040]    Piston  54  assumes a first end position when its first end face  72  is close to or even abuts the inner end wall  68  of the cylinder as shown in FIG. 6 e . A second end position of piston  54 , in which its first end face  72  is most remote from the inner end wall  68  of the cylinder is shown in FIG. 6 d . This second end position in which the piston is in its outer left position with reference to the drawings may be determined by an abutment of the oscillating system, to which the drive unit is connected, it is as well possible to limit the piston movement to the left by an extension of the cylinder  52  having an abutment for the second end face of the piston as it is shown further down in connection with the embodiment of FIG. 8.  
         [0041]    For ease of explanation, a few pistons are defined as follows: While the piston channel  62  is in communication with circumferential groove  60 , this range of piston positions is defined as the energizing range of piston positions, because during that time cylinder chamber  64  communicates with air supply duct  58  via circumferential groove  60  and the piston channel  62 , so that the cylinder chamber  64  is put under supply pressure. The range of energizing positions of the piston  54  is defined by a first energizing position which is shown in FIG. 6 e , in which piston channel  62  just comes into communication with circumferential grooves  60  at the latter&#39;s right side. In the embodiment of FIG. 6 e , the first end position of the piston and its first energizing position are practically identical. When piston channel  62  reaches on its travel to the left the left side of the circumferential groove  60  the air supply will be shut off. This position is defined as the second energizing position. Likewise, when piston channel  62  travels further to the left and gets into contact with circumferential venting groove  66  this is the first venting position, and when it leaves the left side of venting groove  66  it has reached its second venting position. The range between those two positions is the venting range of piston position. On its return stroke, piston  54  reaches the described positions in a reverse sequence.  
         [0042]    When incorporated in an oscillated system, the drive unit should be mounted to that system in a way that when the system is at rest, the piston  54  assumes a position as shown in FIG. 6 a . This position eases start-up of the system, since when the air supply is opened via air supply duct  58 , the cylinder chamber  64  will immediately be charged with air under pressure, so that piston  54  starts its travel to the left side. When the piston passes its second energizing position, the air supply to cylinder chamber  64  will be shut off. The expansion of the air under pressure in cylinder chamber  64  will cause further movement of the piston to the left. On the further travel of the piston  54  to the left, a situation may be reached in which the air volume confined in cylinder chamber  64  has expanded to atmospheric pressure, so that further movement of the piston to the left would cause a vacuum in the cylinder chamber which would brake the piston movement. This is the right movement to vent the cylinder chamber  64  to the environment which situation is achieved by the piston reaching its first venting position in which piston channel  62  enters into communication with venting groove  66 .  
         [0043]    Yet, according to the embodiment shown in FIG. 6, the venting will only occur over a limited range of piston positions until piston channel  62  will have passed the left side of venting groove  66 , i.e. its second venting position. Further movement of piston  54  to the left may indeed cause a slight vacuum in cylinder chamber  64 . Such vacuum formation could for example be avoided if circumferential venting groove  66  would be extended right to the left end of the cylinder. A slight vacuum at the left end of the piston stroke when the piston  54  approaches its second end position may on the other hand be advantageous for braking the system somewhat near the reversal point, so that the piston respectively the system reverses its direction before the piston abuts in its second end position. This is likewise true for the first end position of the piston as will be explained further below.  
         [0044]    The reverse stroke of the piston after having reached its left reversal point will be caused by the resilient forces of the oscillating system to which the drive unit is connected. During the beginning reverse movement of the piston the vacuum, if any, will be reduced again and the piston will reach its second venting position in which the cylinder chamber  64  gets again into communication with the venting groove  66 . This has the effect that during the venting range of positions of the piston the air in cylinder chamber  64  will not be compressed and will thus not brake the piston movement. Some compression of the air in cylinder chamber  64  will already occur when the piston has passed its first venting position. Such compression initiates already the charge phase which will be entered when the piston reaches its second energizing position. This means that before the piston has reached its first end position to the right, the cylinder chamber  64  will already be charged with air under pressure, braking the piston in a way that the piston without actually reaching its first end position to the right will reverse its movement and be further charged with air under pressure until the piston, now in its movement to the left side will again have passed the second energizing position. The cycle described above is then repeated.  
         [0045]    It is important to note that the piston will be charged only by an energy impulse of limited duration after it has reached its right reversal position. The duration of the energy impulse is principally determined by the axial width of the first circumferential groove  60  and the speed of the piston while it passes its energy range of positions. Therefore, the positions and widths of grooves  60  and  66  are to be adapted to particular operating conditions.  
         [0046]    The actual reversal points of the movement of the piston are determined by the oscillating system to which the drive unit is connected. As mentioned above, it should be taken care that the piston in its system-determined stroke does not abut in its end positions. Concerning the second end position to the left, it may be advantageous to allow a certain vacuum to be formed in order to brake the piston before it reaches its reversal point. An open left cylinder end as shown in FIG. 6 will, however, avoid an abutment on the left side. On the right side, the cylinder must naturally be closed. In order to avoid abutment of the piston in its first end position at the right at the inner cylinder end wall  68 , a cylinder extension component  82  as shown in FIG. 7 may be used. It is apparent from FIG. 7 that piston  54  is there shown in its first energizing position where communication with the pressure supply starts. It is clear from FIG. 7 that the piston could be moved further to the right to reach its first end position in which the air supply is discontinued. First end position of the piston at the right end and the first energizing position are not identical in this embodiment. Instead of using a cylinder extension component, the same arrangement could be obtained by using a longer cylinder or by shifting the circumferential grooves  60  and  66  farther to the left in comparison with the embodiment of FIG. 6. The operation of such an embodiment is as follows:  
         [0047]    When the piston  54  is on its spring operated reverse stroke to the right, cylinder chamber  64  will be charged with compressed air once the piston has passed its second energizing position. The air pressure in the cylinder chamber  64  will remain constant until the piston will have passed its first energizing position. From then onwards the shut-off air in the cylinder chamber will be compressed, thus further supporting a deceleration of the system. Then, sometime after the piston has passed its right reversal point, mainly influenced by the oscillating system, the actual energizing impulse will take place while the piston moves on its energizing stroke to the left side again through the energizing range of positions.  
         [0048]    In this way, a system oscillation with a somewhat restricted amplitude may be obtained while the system nevertheless remains in resonance. The energy impulses just make up for energy losses in the system without forcing the system into a certain oscillating frequency.  
         [0049]    The oscillatory drive of the embodiments of FIGS. 6 and 7 is very cheap to produce insofar as it does not need any particular sealing of the piston within the cylinder. A certain leakage of air around the piston is not detrimental to the functioning of the drive. It may rather facilitate piston movement within the cylinder. The piston may for example be produced of a low-friction plastic material.  
         [0050]    The embodiment of FIGS. 8 a  to  8   c  show the design of an oscillatory drive unit in which the communication space, the axial length of which determines the duration of the ranges of the energizing and venting positions of the piston has been provided on the piston side instead of in the cylinder wall as in the embodiment of FIG. 6. Since contrary to the embodiment of FIG. 6 where the communication spaces in form of the two circumferential grooves are stationary, the communication space of the embodiment of FIG. 8 moves along with the piston. This has the consequence that only a single communicating space may be provided into which the piston channel opens. The disadvantage is in this embodiment that the ranges of the energizing and venting positions of the piston may not be altered separately.  
         [0051]    [0051]FIG. 8 shows an oscillatory drive unit with a piston  84  at the outer side wall of which a single communicating space  86  surrounding the piston is defined in its axial extension by two protruding piston rings  88  and  90 . The one end of piston channel  62  opens into the space between those piston rings.  
         [0052]    It is apparent from FIGS. 8 a  to  8   c  that the energizing range of positions of the piston and the range of venting position of the piston are defined by registration of either the air supply duct  58  or the venting duct  76  with the communicating space  86  between piston rings  88  and  90 .  
         [0053]    Beyond that the operation of the drive unit according to FIG. 8 corresponds with that of FIG. 6. A further difference in design is that in the embodiment of FIG. 8 the cylinder has at its left end an abutment collar  92  which defines the second end position of piston  84 .  
         [0054]    For better sealing and guidance of piston  84  within the cylinder, a further piston ring  94  and a guide ring  96  are arranged around piston  84 .  
         [0055]    [0055]FIG. 8 a  shows the drive unit in energizing position, FIG. 8 b  in an intermediate position and FIG. 8 c  in venting position during the reverse stroke.  
         [0056]    The described drive units are simple in construction and allow an effective drive for keeping oscillatory systems in resonance. The drive is completely self-regulating. The resonance frequency is determined by the oscillating system. Only the amplitude of the system may be curtailed somewhat by certain embodiments of the drive in which the piston movement is somewhat retarded near its reversal points. For the rest the system can swing freely and only energy losses will be replaced by the energy impulses given by the drive unit. The impulse energy may be altered by merely adjusting the gas pressure.  
         [0057]    Drive units with a piston that is pressurized on only one side solve one additional construction problem because it is no longer necessary to utilize an expensive piston that is precisely adapted to the cylinder and mutually seals both cylinder chambers. In piston vibrators that are pressurized on only one side, it is possible to utilize less expensive materials, e.g., plastic pistons with a seal. In addition, the compression spring of the system that acts on the other side of the piston eliminates startup problems in the dead center, which occur in piston vibrators that are pressurized on both sides.  
         [0058]    When used in oscillating resonance conveyors, the previously described drive unit eliminates the problems of known oscillating resonance conveyors because it is practically only able to oscillate in the natural resonance of the system, namely independent of whether the system in question is a two-mass or multi-mass system in which the respective masses are connected to one another by means of springs so as to form an oscillating system.