Abstract:
A metering pump enables simple fluid metering, has an extended service life, and is of compact construction. The metering pump includes a piston rod in operative connection with a control body which has a circumferential sealing zone, in that the internal circumference of the sealing cylinder radially surrounds the sealing zone. On the piston rod side, a compensating zone, which is at a greater radial distance from the sealing cylinder than the sealing zone, adjoins the sealing zone at least in portions, and in that the control body, after passing beyond the control edge in the actuation direction of the actuator, at least largely severs the fluid connection.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International application number PCT/EP2012/002852, filed on Jul. 6, 2012, which claims priority to German application number 10 2011 107 761.1, filed on Jul. 15, 2011. 
     
    
     FIELD 
       [0002]    The disclosure relates to a metering pump in accordance with the preamble of claim  1 . 
       BACKGROUND 
       [0003]    DE 10 2004 028 889 A1 shows a metering pump, comprising a sealing cylinder, a piston rod which can be moved axially in the sealing cylinder, an outlet valve housing which is arranged on the sealing cylinder on the outlet side, and an outlet valve body. At its outlet-side end, the sealing cylinder has an inwardly directed flange which has a radially flattened edge which faces the outlet valve housing and forms a valve seat for the valve body. The sealing cylinder forms and comprises a conveying space, a spring which is arranged in the conveying space being supported against the flange. The spring is supported at the other end on the piston rod and prestresses the piston rod counter to the conveying direction. On the inlet side, the cylinder element has axially running slots which form a fluidic connection from a pump space to the conveying space. The slots have a control edge which runs normally with respect to the movement axis of the piston rod and delimits the conveying space. The piston rod has a cylindrical projection on the outlet side, a circumferential sealing element being arranged on the projection. It is disadvantageous that the sealing element has high wear when the sealing element moves past the control edge and is in contact with the latter in the process. Furthermore, high pressure fluctuations are disadvantageously produced in the grooves, with the result that the piston rod experiences radially acting transverse forces and, as a result, high friction of the piston rod on the sealing cylinder is produced, as a result of which the service life of the pump is greatly restricted. Finally, the mass to be moved of the piston rod is great, since the diameter of the piston rod is adapted over its entire extent to the internal diameter of the sealing cylinder. 
         [0004]    EP 1 878 920 A1 shows an electromagnetic metering pump having a coil which is wound on a coil former, a housing which supports the coil former, a core flange which is let into the housing and has an outlet channel, and a connection piece with an inlet channel, which connection piece is screwed into the housing opposite the core flange. The core flange has a hollow body which forms a sealing cylinder, has an approximately hollow-cylindrical shape, and penetrates the coil former approximately over the entire axial extent of the coil former. Furthermore, an actuator is provided comprising a ferromagnetic armature and a piston rod which is arranged in the ferromagnetic armature, the piston rod penetrating the hollow body of the core part, and the ferromagnetic armature being arranged in a pump space which is formed between the hollow body of the ferromagnetic core flange and the connection piece. A fluid channel which opens into a radial bore in the cylindrical section of the core part is provided between the hollow body of the ferromagnetic core flange and the coil former, it being possible for fluid which is to be conveyed from the pump space by a stroke movement of the armature to be pumped via the fluid channel and the radial bore into a conveying space which is arranged between the piston rod and the outlet channel. The fluid channel which surrounds the sealing cylinder is provided disadvantageously, which fluid channel prevents a compact overall design of the metering pump. 
       SUMMARY 
       [0005]    In one embodiment of the disclosure a metering pump is disclosed which makes simple fluid metering possible, has an increased service life and is of compact configuration. 
         [0006]    According to the disclosure, the metering pump has a pump drive having an axially displaceable actuator, to which a piston rod is connected. The piston rod is arranged concentrically with respect to a sealing cylinder which is advantageously arranged on the outlet side, and protrudes into the sealing cylinder at least when the actuator is actuated. The sealing cylinder has, on its inner circumference, at least one groove for fluidic connection between a pump space which accommodates the actuator and a conveying space which is arranged in the sealing cylinder. A simple fluidic connection is produced as a result, in which the fluid to be conveyed flows axially out of the pump space into the conveying space, with the result that the metering pump can advantageously be of compact configuration and can be manufactured inexpensively. On the outlet side, the groove has a control edge, which control edge delimits the conveying space in the axial direction of the metering pump. The piston rod is at least connected operatively to a control body, the control body having a circumferential sealing region. Here, the inner circumference of the sealing cylinder radially surrounds at least the sealing region of the control body. A calculable displacement of the fluid from the metering pump is advantageously achieved by interaction of the sealing region with the control edge of the sealing cylinder. In the case of an axial displacement of the actuator and therefore of the piston rod, the control body is also displaced on account of the operative connection, the control body disconnecting the fluidic connection between the conveying space and the pump space after moving over the control edge in the actuating direction of the actuator. In the case of the predominant disconnection of the fluidic connection, fluid leaks can occur laterally as a result of the play between the control body and the sealing cylinder, which play is configured for the movability of the control body in the sealing cylinder, at least a predominant part of the fluidic connection advantageously being disconnected. On the piston rod side, that is to say on that side of the sealing region which faces the piston rod, the sealing region is adjoined at least in sections by a compensation region which is spaced apart in the radial direction from the sealing cylinder further than from the sealing region. An excess fluid can advantageously flow away along the compensation region when the control edge is overtraveled by the sealing region. Furthermore, only a relatively small mass advantageously has to be moved during the stroke movement, with the result that the degree of efficiency of the pump is increased in general. 
         [0007]    In one embodiment, the groove runs parallel to the movement direction of the piston rod, the movement direction of the piston rod corresponding to the actuating direction of the actuator. A particularly short overall design is achieved as a result. As an alternative, the groove can also be provided in a helical manner on the inner circumference of the sealing cylinder. 
         [0008]    In one embodiment a web region is arranged adjacently with respect to the at least one groove, the inner radius of which corresponds to the inner radius of the conveying space. Circumferential guidance of the control body is advantageously achieved as a result. If a multiplicity of grooves are provided in the sealing cylinder, in each case one web region is arranged adjacently with respect to two grooves. The web region has a surface curvature which is of complementary configuration with respect to a curvature of the sealing region of the control body, the web region forming a guide for the control body. 
         [0009]    In one embodiment of the metering pump, the control body is configured as a ball. A ball is advantageously simple to manufacture and advantageously has a uniform fluid displacement. Furthermore, the ball forms a linear sealing region which surrounds the ball at its maximum extent normally with respect to the movement axis. When the control edge is moved over with the sealing region, a pressure increase is produced as a result of the replenished fluid in the region upstream of the control edge. As a result of the linear sealing region, the compensation region which adjoins the sealing region can be reached by the fluid immediately after the control edge is moved over, with the result that the pressure increase is reduced in terms of time and amplitude. Furthermore, the wear of the sealing cylinder and control body is minimum on account of the linear sealing region. 
         [0010]    In one embodiment the groove in the sealing cylinder has a transition region from a bottom of the groove toward the conveying space, an oblique or a rounded transition region advantageously being provided. For example, the course of the transition region can have a linearly or exponentially rising gradient. The transition region is expediently at an angle of from 30° to 60°, in one embodiment 45°, with respect to the bottom of the groove, and therefore forms a ramp u of the control edge. In contrast to a step, an abrupt airflow breakaway edge is thus avoided at the control edge before being moved over by the sealing region, as a result of which the fluid to be conveyed is conveyed more uniformly. Furthermore, pressure spikes are avoided during moving over of the control edge. 
         [0011]    In one embodiment the maximum external diameter of the piston rod is smaller than the maximum external diameter of the control body. As a result, a relatively low mass is advantageously moved during a pump movement, with the result that less energy is required for the pump drive, and the mass moment of inertia which is produced is reduced. Furthermore, a larger pump space is thus provided, with the result that a greater quantity of fluid can be conveyed into the conveying space during pump operation. 
         [0012]    In one embodiment the sealing cylinder is shaped primarily from a sintering material by sintering. Oxide ceramics may be used as sintering materials, such as aluminum oxide, magnesium oxide, zirconium oxide, titanium dioxide, aluminum titanate, mullite, lead zirconate titanate, dispersion ceramics such as Al 2 O 3 , ZrO 2 , since the materials have a high resistance to the fluids to be conveyed, comprising gasoline fuels, diesel fuels and kerosene. As an alternative, Si 3 N 4  or SiC can also be provided which can be manufactured in a manner which is extremely accurate with respect to the final contours. In one embodiment, the control body is sintered. Here, the sealing cylinder and/or the control body are produced from a sintering material. However, the sealing cylinder and the control body can also be produced from a plastic. The stated materials are resistant, in particular, to chemically aggressive media. 
         [0013]    In one embodiment, the pump drive has a coil unit which is arranged outside the pump space, the pump space being delimited by an inlet part and an outlet part and being connected at the inlet part and the outlet part by an encasing section with a closed circumferential face. Here, the inlet part has an inlet channel and the outlet part has an outlet channel. In one embodiment the encasing section seals the pump space with respect to the coil unit. A simple separation between a chemically aggressive fluid and the components which drive the pump is thus advantageously provided. In one embodiment the encasing section is configured in one piece with either the inlet part or the outlet part. As a result, a pump part is provided which is particularly simple to assemble, can accommodate the movable parts of the pump and surrounds the pump space. 
         [0014]    In one embodiment of the metering pump, the control body is prestressed by a spring counter to a conveying direction. Here, in the case of a loose connection between the control body and the piston rod, the spring is supported on the control body, as a result of which the control body is advantageously moved back with the piston rod. In one embodiment of the disclosure, the actuator is loaded and prestressed by the spring counter to a conveying direction. In the case of a fixed connection between the control body and the piston rod, the spring is advantageously supported on the actuator, as a result of which a spring which is supported on the control body can advantageously be avoided. 
         [0015]    In one embodiment, the piston rod and the control body are configured in one piece. Here, the control body advantageously forms an extension of the piston rod. High stability of the piston rod is achieved as a result. Furthermore, in one embodiment the piston rod is guided by the sealing region of the control body on its circumference, as a result of which external shocks have a reduced influence on the running of the piston rod. Here, the operative connection can be loose bearing of the control body against the piston rod, but can also be a fixed material connection between the control body and the piston rod. The releasable fixed connection between the control body and the piston rod can also be provided, the connecting member then advantageously being provided. As an alternative, the control body and the piston rod can be two components which are produced separately and are connected fixedly to one another, as a result of which a multiplicity of compensation regions with complex geometries can advantageously be produced simply, such as ribs which run in the flow direction on the control body or the piston rod. Furthermore, as an alternative, the control body and the piston rod can be produced as separate parts which are displaced together in the pump direction, either two springs then being provided for the individual prestressing of piston rod and control body or one spring being provided for the common prestressing of piston rod and control body at the same time. 
         [0016]    In one embodiment the piston rod has at least one groove, which groove opens from the pump space into the compensation region. Here, the groove expediently runs parallel to a longitudinal extent of the piston rod. The mass to be moved in the metering pump is reduced by the groove in the piston rod, and at the same time a greater fluidic connection between the conveying space and the pump space is produced. In one embodiment the groove of the piston rod is arranged opposite the groove in the sealing cylinder, with the result that a common flow channel for the fluidic connection is formed. As a result, a convex surface in the flow channel is advantageously avoided, which results in more uniform pressure distribution and therefore friction on the surface of the flow channel and, as a result, the formation of bubbles is advantageously reduced in the flow channel. 
         [0017]    In one embodiment the pump drive is an electromagnetic drive with a coil which surrounds the pump space, an armature of the actuator being formed from a ferromagnetic material, and it being possible for the armature to be moved in the conveying direction, as an alternative counter to the conveying direction, by way of current application to the coil. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Further advantages, properties and developments of the disclosure result from the following description of example embodiments and from the dependent claims. 
           [0019]      FIG. 1  shows a first example embodiment of a metering pump according to the disclosure with a sealing cylinder and with a control body which is separate from the piston rod. 
           [0020]      FIG. 2  shows a second example embodiment of a metering pump according to the disclosure with a sealing cylinder and with a control body which is connected to the piston rod. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  shows a first example embodiment of a metering pump  1  which is configured as a reciprocating piston pump, with an inlet part  2  which forms a housing and an outlet part  3  which is pushed longitudinally into the inlet part  2 . The inlet part  2  and the outlet part  3  are rotational bodies and are oriented concentrically with respect to one another and form a common axis A, to which axis A a conveying direction F of the pump  1  runs from the inlet part  2  to the outlet part  3 . On the inlet side, the inlet part  2  has an inlet connector  4  with an inlet channel  5 . For connection to a pipeline system, the inlet connector  4  has a mouthpiece which thickens the inlet connector  4 . In an outlet-side encasing section  2   a,  the inlet part  2  is configured as a hollow cylinder, in the interior of which a cylindrical pump space  6  is arranged. An electromagnetic coil unit of a pump drive, which coil unit is arranged outside the inlet part  2  and is formed from a coil and a coil former, is not shown on account of improved clarity. The encasing section  2   a  of the inlet part  2  has outer flanges  2   b,    2   c  which delimit approximately a middle third and serve as bearing face for parts of the pump drive which guide a magnetic field. An air gap is advantageously arranged between the two flanges  2   b,    2   c.  Furthermore, the inlet part  2  and the outlet part  3  have a step  3   d,    2   d  which serves as a bearing face for ferromagnetic yoke shims (not shown) of the pump drive. 
         [0022]    The inlet channel  5  opens via a cylindrical opening  7  into the cylindrical pump space  6  which is arranged in the inlet part  2 , a diameter of the pump space  6  being greater than a diameter of the inlet channel  5 . The opening  7  of the inlet channel  5  in the pump space  6  is surrounded here by a cylindrical step  8  which protrudes into the pump space  6 , the result of which is a sealing region  9  within the cylindrical step  8 . Here, the diameter of the sealing region  9  is approximately a mean value of the diameter of the pump space  6  and the diameter of the inlet channel  5 . 
         [0023]    An actuator  10  is arranged in the pump space  6 , which actuator  10  comprises a ferromagnetic armature  11  and a non-magnetic piston rod  12  which is connected fixedly to the ferromagnetic armature  11 . The ferromagnetic armature  11  and the piston rod  12  are rotational bodies which are arranged concentrically around the axis A, the actuator  10  being shown in a neutral position in  FIG. 1 . The ferromagnetic armature  11  has an external diameter which is slightly smaller than the internal diameter of the pump space  6 , with the result that an almost friction-free axial sliding of the actuator  10  in the pump space  6  is made possible. The piston rod  12  has an armature region  13  which is connected fixedly to the ferromagnetic armature  11  and a rod region  14  which points toward the outlet side of the metering pump  1 . Here, the armature region  13  of the piston rod  12  has a diameter which is approximately twice as large as the rod region  14 . The armature region  13  therefore has a shoulder region which is stepped with respect to the rod region  14  and in which a plurality of bores  15  which run parallel to the axis A connect the sealing region  9  to that section of the pump space  6  which is arranged around the rod region  14 . On the inlet side, the piston rod  12  has an axially projecting projection  16 , which projection  16  has a surface  17  which is curved concavely in the direction of the axis A, a sealing element  18  which is configured as an O-ring being arranged around the projection  16  and, in the neutral position, separating the sealing region  9  from the inlet channel  4  in a fluid-tight manner. 
         [0024]    A region of the outlet part  3  which is pushed into the pump space  6  is configured as a sealing cylinder  20 , the external diameter of the sealing cylinder  20  corresponding to the internal diameter of the cylindrical inlet part  2 . On the outside, the sealing cylinder  20  has a circumferential groove  21 , into which an O-ring  22  is received. The circumferential groove  21  is surrounded toward the outside by the cylindrical inlet part  2 , with the result that the pump space  6  is sealed against the surroundings. Centrally on its outer side, the outlet part  3  has a flange  23  which delimits the sealing cylinder, the flange  23  serving as a rest for the cylindrical inlet part  2  which surrounds the outlet part  3  or the sealing cylinder, with the result that pushing in of the outlet part  3  with the sealing cylinder  20  in the inlet part  2  is limited. 
         [0025]    A helical spring  25  is arranged in the pump space  6  between an end side  24  of the sealing cylinder  20 , which end side  24  faces the actuator  10 , and the armature  11  of the actuator  10 . The sealing cylinder  20  has a substantially cylindrical inner region  26 , which inner region  26  extends in the axial direction from the end side  24  as far as a conical outlet opening  27  of an outlet channel  28  of the metering pump  1 . Here, the outlet channel  28  is arranged in an outlet connector  29  of the outlet part  3 . A spherical control body  30  is arranged in the cylindrical inner region  26  of the outlet part  3 , the external diameter of which control body  30  corresponds to the internal diameter of the inner region  26 . Here, a sealing region  37  is defined as the linearly resulting contact area  37  of the control body  30  with an inner face of the inner region  26 . Here, the surface of the control body  30  forms a compensation region  39  behind the sealing region  37 . The compensation region  39  is further away from the inner face of the sealing cylinder  20  here than the sealing region  37 . As a result, an enlarged fluid gap between the control body  30  and the sealing cylinder  20  is formed behind the sealing region  37 . 
         [0026]    A multiplicity of grooves  31  with a trapezoidal, almost rectangular cross section are let circumferentially into the inner face of the inner region  26 , which grooves  31  extend parallel to the axis A and approximately over half of the axial extent of the inner region  26 . Here, each groove bottom  32  of each groove  31  is at a greater spacing from the axis A than the inner face of the inner region  26 . Here, an outlet-side end wall  33  of each groove  31  forms a control edge  34  of the metering pump  1  at the transition to the inner face of the inner region  26 . In each case one web  35  is arranged between in each case two adjacent grooves  31 , the webs  35  being part of the inner face of the inner region  26  and thus being in contact with the sealing region  37  of the control body  30 . On its outlet-side end side  36 , the piston rod  12  has a concave curvature which is adapted as a dome to the ball contour of the control body, the end side  36  being connected fixedly to the control body  30 . The space between the outlet channel  28  and the control edge  34  is defined as a conveying space  38 . 
         [0027]    An outlet valve  40  is arranged in the outlet channel  28 , a conical valve seat  41  which faces in the conveying direction being arranged in an inner wall  42  of the outlet channel  28 . An insert element  43  which has a projection  44  which protrudes into the outlet channel  28  is arranged in the outlet channel  28  opposite the valve seat  41 . Here, a valve spring  45  which is arranged between the insert element  43  and the valve seat  41  and is supported at one end on the projection  44  prestresses an outlet valve body  46  counter to the conveying direction F. 
         [0028]    The metering pump  1  functions as follows: 
         [0029]    In the position which is shown in  FIG. 1 , the metering pump  1  is situated in a neutral position. Here, the pump drive (not shown) does not exert any force on the actuator  10 , with the result that the actuator  10  is loaded by the prestress of the spring  25  counter to the conveying direction F of the metering pump  1  with the sealing element  18  to the inlet channel  5  and separates the inlet channel  5  from the sealing space  9  and the pump space  6  which is otherwise fluidically connected to the sealing space  9  by the bores  15 . When the ferromagnetic armature  11  is excited, the actuator  10  is displaced counter to the prestress of the spring  25  in the conveying direction F by application of current to the coil, the control body  30  being moved with the linear sealing region  37  toward the control edge  34 . Here, fluid circulates between the pump space  6  and the conveying space  38  through the grooves  31  past the control body  30 , fluid which is situated in the region of the compensation region  39  of the control body  30  also being sucked or pressed into the groove  31 . As long as the fluid connection exists via the grooves  31  between the pump space  6  and the conveying space  38 , the fluid can escape during the advancing of the control body  30  and the piston rod  12 , with the result that no fluid passes via the outlet-side non-return valve  46 ,  41  into the outlet. At the same time, the sealing region  9  opens on the inlet side, the sealing element  18  rising up from the inlet channel  5  and thus making it possible for fluid to flow in from the inlet channel  5  into the pump space  6  via the sealing space  9  and the bores  15 . 
         [0030]    As soon as the control body  30  moves with the linear sealing region  37  over the control edge  34 , the fluid connection which exists in the grooves  31  from the pump space  6  into the conveying space  37  is interrupted. The control body  30  therefore brakes the fluidic connection completely, at any rate completely apart from a very small slip (approximately 98%) and at any rate at least predominantly. If the actuator  10  and therefore the control body  30  are moved further in the conveying direction, the fluid which is situated in the conveying space  38  is pressed counter to the prestress of the outlet valve spring  45  out of the conveying space  38  into the outlet channel  28 . As a result of the defined interruption and the defined stroke, an accurately metered fluid quantity is ejected in the case of a complete stroke, with the result that the metering pump  1  can be controlled readily and reliably. 
         [0031]    As a result of the sudden interruption of the fluidic connection between the conveying space  38  and the pump space  6 , a pressure increase is produced by way of the fluid which is replenished by the actuator  10  in a region behind the sealing region  37  of the control body  30 . As a result of the enlargement of the cross-sectional area of the space in the region of the compensation region  39 , an expansion space is advantageously available for the fluid, into which expansion space the fluid can flow or expand upon disconnection of the fluidic connection. Furthermore, the fluid can pass through the bores  15  in the piston rod  12  out of the pump space  6  into the sealing space  9 , with the result that partial pressure equalization in the pump space  6  is also possible as a result. In particular after renewed moving over of the control edge  34  during a return stroke under the prestress of the spring  25 , the fluidic connection between the conveying space  38  and the pump space  6  is re-established, and the negative pressure which is produced during the return stroke in the region of the conveying space  38  is filled again via a replenishing fluid through the grooves  31 , as long as the actuator  10  has not yet assumed its sealing starting position again. 
         [0032]    The metering pump  1  is advantageously simple to assemble. The actuator  10  with the attached control body  30  is pushed into the inlet part  2 . Subsequently, the spring  25  around the control body  30  is inserted into the cylindrical pump space  6  until it comes into contact on the actuator  10 . Finally, the outlet part  3  with the outer seals  22  and the outlet valve  40  is pushed into the inlet part  2  until the flange  23  bears against the inlet part  2 . 
         [0033]      FIG. 2  shows a detail of a pump space  106  of a second example embodiment of a metering pump  101 , the designations of identical or similar components having been incremented by  100 . Parts which are not described correspond to the parts of the first example embodiment according to  FIG. 1 . 
         [0034]    A piston rod  112  which is arranged in the pump space  106  is configured so as to be in one piece with a control body  130  and sintered from a ceramic such as Al 2 O 3 , the control body  130  representing an extension of the piston rod  112 . Here, the piston rod  112  is guided with the control body  130  in a cylindrical sealing cylinder  120  in a movable manner along an axis  10 A which is formed by the cylindrical sealing cylinder  120 . Here, a conveying direction  10 F of the metering pump  101  runs parallel to the axis  10 A from the pump space  106  in the direction of the sealing cylinder  120 . 
         [0035]    On the outlet side, an opening element  150  is let into the sealing cylinder  120 . The opening element  150  comprises a metal core  151  and a plastic sheath  152  which surrounds the metal core  151  on the outside and on the inlet and outlet side, and is configured as a rotational body. Here, the metal core  151  protrudes further in the direction of the axis  10 A than the plastic sheath  152 . The opening element  150  forms a stepped opening  127  which is directed toward the piston rod  112 , a cylindrical outlet bore  153 , which delimits to the inside and defines an inner first step  154 , being arranged in the metal core  151 . Since the metal core  151  protrudes further to the inside than the plastic sheath  152 , the plastic sheath  152  forms a second step  155  which is situated further to the outside with respect to the metal core  151 . The plastic sheath  152  has an end side  156  which faces the piston rod  112 , is delimited on the inside by the second step  155  and assumes an undulating shape in an outer region  157 , the outer region  157  being calked with an inwardly protruding shoulder  158  of the sealing cylinder  120 , which shoulder  158  points in the outlet-side direction. 
         [0036]    On its surface, the piston rod  112  has a multiplicity of semicircular circumferential grooves  115  which extend from an inlet-side end  160  of the piston rod  112  as far as the control body  130  and, in a compensation region  139  which rises exponentially in the conveying direction  10 F, merge into a cylindrical sealing region  137  of the control body  130 . The sealing region has an axial extent and therefore corresponds to a cylindrical circumferential face as control body  130  which is of circumferential configuration. Between two circumferential grooves  115 , the piston rod  112  has webs  161 , the external diameter of which corresponds to an external diameter of the sealing region  137 . On its outlet-side conveying end side  166 , the piston rod  112  or the control body  130  has a cylindrical projection  162 , from which a cylindrical step  163  protrudes, the cylindrical projection  162  having a diameter which corresponds to an internal diameter of the outlet bore  153  in the metal core  151  of the opening element  150 . An external diameter of the cylinder step  163  corresponds to an internal diameter of the second step  155  of the plastic sheath  152 , with the result that the piston rod  112  or the control body  130  is of complementary configuration with respect to the outlet opening  127 . The external diameter of an output-side conveying region  164  of the control body  130  is smaller than the external diameter of the sealing region  137 , a funnel-shaped transition region  165  being formed between the sealing region  137  and the conveying region  164 . 
         [0037]    By way of its inner faces, the sealing cylinder  120  defines an inner region  126  which extends from an inlet-side end side  124  as far as the outlet opening  127 , semicircular grooves  131  being arranged starting on the end side  124  as far as approximately two thirds of the longitudinal extent of the inner region  126 . An outlet-side end wall  133  of the grooves  131  runs substantially exponentially in the conveying direction  10 F and normally with respect to the conveying direction  10 F, a parabolic control edge  134  being formed between the inner face of the inner region  136  and the end wall  133 , the vertex of which control edge  134  points in the outlet direction. The space which lies on the outlet side in front of the vertex of the control edge  134  is defined as conveying space  138 . 
         [0038]    The number of grooves  131  corresponds to the number of circumferential grooves  115  of the piston rod  112 , the grooves  131  being arranged opposite the circumferential grooves  115 , which results in a fully circular tube. Webs  135  are arranged between two grooves  131 , the webs  135  being part of the inner face of the inner region  126  and thus being in contact with the control body  130  and the piston rod  112  and guiding them in a sliding manner. 
         [0039]    The second example embodiment functions as follows: 
         [0040]    If the piston rod  112  and therefore the control body  130  are moved in the conveying direction, the sealing region  137  moves over the control edge  134  and, as a result, disconnects a fluid connection between the conveying space  138  and the pump space  106 . The fluid connection is closed gradually on account of the parabolic shape of the control edge  134 , with the result that a pronounced pressure increase in a region which is connected to the pump space  106  is reduced near the control edge  134 . Furthermore, fluid can flow out of the region which is arranged between the sealing region  137  and a groove bottom  138  via the compensation region  139  of the control body, and can thus relieve pressure. 
         [0041]    A further movement of the control body  130  presses the fluid which is situated in the conveying space  138  into the outlet opening  127 , particularly high displacement of the fluid taking place on account of the conveying end face which runs substantially perpendicularly with respect to the conveying direction  10 F. The explanations with respect to  FIG. 1  can be applied correspondingly for details. 
         [0042]      FIG. 2  does not show an armature which surrounds the piston rod  112 , semicircular corresponding bores which in one embodiment are of complementary configuration with respect to the semicircular circumferential grooves  115  being arranged in the armature, which corresponding bores are arranged opposite the circumferential grooves  115  and, in an analogous manner to the grooves  131 , form fully circular inlet tubes with the circumferential grooves  115 . On account of the continuous circumferential grooves  115 , the inlet tubes are at the same spacing from the axis  10 A as the tubes in the region of the sealing cylinder  120 , with the result that friction losses of the metering pump are advantageously reduced on account of reduced obstacles in the flow path of the fluid.