Patent Publication Number: US-2022225814-A1

Title: Brewing unit for a coffee machine

Description:
The invention relates to a brewing unit of a piston coffee machine according to the preamble of claim  1 , and to a piston coffee machine comprising such a brewing unit. 
     Brewing units of piston coffee machines are known in many embodiments. Inside the brewing unit of a piston coffee machine, coffee powder is ground into a brewing chamber and then pressed into a coffee cake. Water heated under positive pressure is then passed through the cake to extract flavors from the powder. After a product-specific amount of water has flowed through, the moistened coffee powder is pressed out and the coffee cake (coffee grounds) is discharged into the grounds container. 
     A typical brewing chamber of a brewing unit is circular and has a diameter of, for example, about 45 mm. The hot water is introduced into the brewing chamber on one side through a piston unit called a shower screen, atomizing it as evenly as possible to wet all parts of the coffee cake with water. On the other side, the extracted coffee is dispensed by a plunger forming another piston unit. In this process, the extracted coffee is often passed through a spring-loaded oscillating component (crema valve) that allows only a small opening gap to create the typical crema on the coffee. 
     To prevent very fine particles from being discharged into a coffee cup, there is often a fine screen on the plunger. 
     A drive motor causes a relative movement between the plunger and the shower screen so that the distance between them can be changed. Defined pressing forces can then be set via different motor torques. 
     At the same time, a movement of the brewing chamber must also take place in order to realize the insertion position for the coffee powder, to create a sealed brewing chamber space and finally to be able to eject the pressed-out coffee cake. In horizontal brewing units, the ejection is often performed by the gravity principle. In vertical units, there is usually an additional movement mechanism to eject the cake specifically into the grounds container via a pusher. 
     The coffee cake height can be used to indirectly infer the weight and this can then be specifically adjusted to a nominal weight, for example, in order to correct variable grinding capacities of the grinder. The plunger or shower screen position during powder pressing is recorded in a simple manner via the motor increments. 
     Basically, a distinction is made between two installation types of brewing units. Horizontal (lying) and vertical (standing) units. The horizontal variant has the advantage that the coffee machine can be designed very compactly and that the coffee grounds are often only discharged into the grounds container by gravity. 
     Upright brewing units usually require an additional pusher mechanism for coffee grounds ejection. However, they also have the option of processing larger weights and the coffee powder can be compacted more evenly. 
     There are different drive concepts to approach the necessary relative positions of plunger, shower screen and brewing chamber. 
     Initially, the two asynchronous motion sequences can be implemented via two different drive motors. One motor is then responsible for moving the brewing chamber and another for pressing the coffee powder. The disadvantage of this simple solution is that two drives are often very cost-intensive. Additional motors are often also used for a pusher movement to eject the coffee grounds into a grounds drawer. Here, movement threads such as trapezoidal spindles by means of sliding blocks enable the rotary movement of the drive to be converted into a linear piston movement in many designs. 
     If only one motor is available, the relative positions are generated, for example, via control grooves (e.g. in a control roller). In this case, several components (plunger slide, brewing bush) are simultaneously guided in grooves via a rotary movement. The disadvantages here are complex and non-compact designs, as well as high bending moments due to off-center guides. 
     The toggle lever principle is frequently encountered in the household sector in particular. An elaborate kinematic approach is used to generate a functional linear movement via several joints. The disadvantage is usually that the piston elements can be moved quickly, but the coffee powder is compressed with the same force regardless of the product. Defined and variable forces with larger dimensions cannot be represented. 
     Generally, in favorable configurations of brewing units, a compromise is accepted between fast movement of the piston function elements and a defined, adjustable pressing of the coffee powder. 
     The document EP 2 907 427 B1 describes a non-linear gear section by which a compromise between high pressing forces and simultaneously short travel cycles is achieved. To move to the various positions, the functional elements are moved with large gradients. When compacting the coffee powder, the drive motor is reduced with a significantly smaller pitch in order to transmit large forces with the same guide. 
     These embodiments have proven themselves, but in the course of cost savings in components, functional groups, assembly times and maintenance, there is a constant need for improvements. 
     The invention therefore has the object of advantageously further developing a brewing unit of a piston coffee machine, in order to provide an improved brewing unit and an improved piston coffee machine. 
     The invention solves this problem by means of a brewing unit having the feature of claim  1  and by means of a piston coffee machine having the feature of claim  28 . 
     One idea of the invention is that all ratios are constant in the double-spindle principle, i.e. the spindles and gearing mechanisms have constant pitches or ratios. A standstill of the brewing slide is realized by a spindling out of the brewing slide, so that in this case there is no transmission. 
     A brewing unit of a piston coffee machine according to the invention, wherein the brewing unit is designed as a spindle brewing unit having a brewing slide as a brewing chamber, two piston units in form of a shower screen and plunger, a drive motor and at least one gearing mechanism, characterized in that the brewing unit comprises a double-spindle assembly having an outer spindle and an inner spindle. 
     A particular advantage here is a very compact and lightweight design, which enables a simple and therefore cost-effective assembly. 
     In a preferred embodiment, the outer spindle has a movement thread which is in engagement with an outer spindle nut axially fixed in the brewing slide. In this way, it is advantageous to use high-quality components that are available on the market at low cost. 
     It is provided that in the preferred embodiment the brewing slide is adjustable from a first end position to an intermediate position, then to a second end position and back to the first end position. Thus, defined positions of the brewing slide are advantageously simply enabled. 
     A particular advantage arises in the preferred embodiment when the movement thread of the outer spindle is out of engagement with the outer spindle nut in the first end position and in the second end position of the brewing slide, and a speed of the brewing slide is zero even if the outer spindle continues to rotate. Thus, a rest position of the brewing slide at a stop in the respective end position can be made possible in a simple manner, wherein nevertheless the outer spindle driven by the drive motor continues to drive the inner spindle via the gearing mechanism in order to position the inner spindle and thus the shower screen. A further motor is not required. 
     In one embodiment, the outer spindle is connected to the adjustable brewing slide, the inner spindle is connected to the adjustable shower screen, and the plunger is arranged in a stationary manner. The outer spindle and inner spindle are simple to manufacture components with a high-quality level at low prices. 
     It is provided in this case that the adjustable brewing slide is guided longitudinally displaceably by means of the outer spindle and a guide rod, and that the shower screen is guided longitudinally displaceably within the brewing slide, wherein the brewing slide and the shower screen are adjustable at different speeds in common directions of movement. Hereby, an advantage results in a simple and inexpensive structure in which very high speeds can be realized. 
     An advantageous centric force application and thus high rigidity of the brewing group is made possible by the fact that the outer spindle is driven by the drive motor and is coupled to the inner spindle via the at least one gearing mechanism. Further additional motors are not required. 
     It is advantageous if the brewing unit has at least one switchable electromagnet which fixes the brewing slide in the respective end position in a releasable manner again directly or indirectly via a mechanical locking device, since in this way it is possible to avoid overriding noises from the movement thread and outer spindle nut when the brewing slide is “at a standstill”. 
     In the end positions of the brewing slide, the outer spindle movement thread disengages from the outer spindle nut of the brewing slide. This is called “spinning out”. In order for the brewing slide to be moved back out of these end positions, the outer spindle nut of the brewing slide must be re-engaged with the movement thread of the outer spindle. This process is referred to as “spindle re-engagement”. 
     In one embodiment, the brewing unit comprises at least one spindle re-engagement device which causes re-engagement of the outer spindle nut and the movement thread in the first end position or the second end position. An advantage of this is that re-engagement of the outer spindle nut and the movement thread is made possible in a simple manner with only a small number of components. 
     In this regard, one embodiment provides that the at least one re-engagement device comprises at least one stop unit with a respective compression spring. These are simple and inexpensive components. 
     In this regard, it is advantageous if the brewing slide pretensions a compression spring of a first stop unit in the first end position, and that the brewing slide pretensions a compression spring of a second stop unit in the second end position in order to thus “load” a force storage element. 
     Furthermore, it is provided in one embodiment that the compression spring of the first stop unit re-engages the movement thread of the outer spindle with the outer spindle nut of the brewing slide when the brewing slide is moved from the first end position by means of the outer spindle, and that the compression spring of the second stop unit re-engages the movement thread of the outer spindle with the outer spindle nut of the brewing slide when the brewing slide is moved from the second end position by means of the outer spindle. 
     In an alternative embodiment, the at least one spindle re-engagement device comprises at least one switchable electromagnet which exerts a movement pulse on the brewing slide in the respective end position, directly or indirectly via a mechanical device, which causes renewed engagement of the outer spindle nut and movement thread in the first end position or the second end position. This results in a simple structure with a very small number of additional components. 
     The at least one spindle re-engagement device may also comprise at least one switchable electromagnet with a movable core. The movable core can exert a movement pulse on the brewing slide in an advantageously simple manner. 
     In another alternative embodiment, the at least one spindle re-engagement device comprises at least two switchable and reversible electromagnets, one of which is fixed and the other of which is attached to the brewing slide. This is advantageous as no moving parts are required and the electromagnets are not only easily controlled, but are also commercially available low-cost components of high quality. 
     Friction elements can also be attached to the movable inner spindle for spindle re-engagement, in particular to the shower screen (or to the plunger). The friction force between the shower screen and the brewing chamber presses the outer spindle nut of the brewing slide back into the associated outer thread of the outer spindle as soon as a reversal of movement occurs on the inner spindle. 
     To this end, it is provided in one embodiment that the at least one spindle re-engagement device comprises at least one friction element directly on the movable inner spindle or indirectly on a component attached to the inner spindle in cooperation with the brewing slide. 
     Hydraulic sealing elements on the plunger/shower screen can also be used specifically for this function. Due to a time-delayed relief of the seals after reversal of the direction of the inner spindle, a high frictional force is transmitted to the brewing slide and this is briefly synchronized with the inner spindle movement. This results in an advantageous movement impulse on the brewing slide, which results in spindle re-engagement. Another advantage is that the increased friction is no longer present in the remaining course of movement after the seal has been relieved. This avoids additional wear of the moving components. 
     For this purpose, in a further embodiment, the at least one spindle re-engagement device comprises at least one friction element configured as at least one hydraulic sealing element. 
     It is thereby advantageous if the at least one hydraulic sealing element is a seal attached to the shower screen or to the plunger, which is controlled by the application of a pressure. Advantageous control is enabled if the at least one hydraulic sealing element is controlled by a solenoid valve. In this way, in the case of two or more such seals, separate independent control of the hydraulic sealing elements can be made possible. 
     On the side of the movable inner spindle, a friction element on the entrained anti-rotation device is also conceivable. This gives the brewing slide the necessary impulse for spindle re-engagement device and can be moved in the other positions of the brewing slide without blocking the brewing unit. 
     For this purpose, in another embodiment, the at least one spindle re-engagement device comprises a driver device having at least one driver which is displaceably guided on the one hand on the inner spindle or a component connected to the inner spindle and on the other hand on the guide rod, wherein the driver is held in a rest position with respect to the inner spindle or the component connected to the inner spindle with a specific holding force which is greater in the rest position than outside the rest position. An advantage here is a small space requirement. 
     In a still further embodiment, the at least one driver comprises a first retaining element, wherein a second retaining element is attached to or in the inner spindle or to or in the component connected to the inner spindle, wherein the first retaining element and the second retaining element generate the determined retaining force in the rest position of the driver. Thus, an advantageously simple structure can be provided. 
     In a still further embodiment, the first retaining element and the second retaining element are each a permanent magnet. These are simple and inexpensive high-quality components. 
     Alternatively, the first retaining element may be a spring-loaded ball thrust element which, in the rest position of the driver, is in contact with a corresponding indentation or groove in the inner spindle or in the component connected to the inner spindle. 
     In order to avoid friction and the thus possibly resulting increased proportion of wear parts, it is advantageous to replace the friction element via the magnetically effective or spring-loaded driver. In the anti-rotation device, which is moved in parallel by the inner spindle and is designed as a securing rod, there is a further permanent magnet which is poled in such a way that it causes an attractive force on the driver or has a groove for the ball pressure element. 
     The joining force of this connection is large enough for the impulse for spindle re-engagement of the brewing bush but small enough so that jamming of the brewing unit is avoided. 
     It is provided in a still further embodiment that the inner spindle has a movement thread which engages with a stationary rotatably mounted inner spindle nut. In this way, a rotation transmitted from the outer spindle through the gear to the inner spindle nut can advantageously cause a longitudinal adjustment of the inner spindle and thus of the shower screen connected thereto. 
     For a particularly advantageous reduction in installation space, the inner spindle nut can be a component of an output gear of the at least one gearing mechanism. 
     In a still further embodiment, the inner spindle is connected to a securing rod as an anti-rotation device. The securing rod provides an additional advantageous guide stabilization of the shower screen. 
     It is also advantageous for a compact structure if the securing rod, the outer spindle, the inner spindle and the guide rod are arranged parallel to each other. 
     If the drive motor is coupled to the outer spindle via a gearing mechanism, different types of drive motors can advantageously be used by adapting this gearing mechanism. 
     A piston coffee machine according to the invention comprises the brewing unit described above. 
     For further advantageous designs are disclosed in the remaining subclaims. 
     The advantages over conventional brewing unit designs for coffee machines are highlighted below:
         Very compact and lightweight design   Simple and therefore cost-effective assembly   Very high speeds can be achieved   Centric force application and thus high rigidity of the brewing group   Defined coffee grounds discharge   Integrated transmission stage through the belt drive between the spindles   Only one drive motor necessary   A large number of transmission ratios are constructively possible by suitable selection of the spindle pitches and the belt transmission-&gt;flexible adaptation of the brewing unit to selected drive motors   With a comparatively low-power drive, both high travel speeds and contact forces that can be adjusted over a wide span can be achieved.   Grinding position in the grounds container   Identical parts for plunger and shower screen assemblies and base plates   Self-locking of the inner spindle during the brewing process and thus no counter-holding of the drive motor required   Crema product-specific selectable (optional brewing line without crema valve)   Hydraulic inlets and outlets are integrated in the piston unit (spindle-guided FEP pipes)       

    
    
     
       In the following, the invention will be described in more detail with reference to the drawings by way of an exemplary embodiment and a variant. The figures serve only to explain the invention in more detail and are not restrictive of the invention. Individual features described may also be applied in their own right to further embodiment variants within the scope of the general skill in the art, wherein: 
         FIG. 1 : shows a schematic perspective view of an exemplary embodiment of a brewing unit of a coffee machine according to the invention; 
         FIGS. 2-3 : show various schematic views of the exemplary embodiment according to  FIG. 1 , 
         FIG. 4 : shows a schematic sectional view according to line IV-IV from  FIG. 3 ; 
         FIG. 5 : shows a schematic sectional view along line V-V from  FIG. 3 ; 
         FIG. 6 : shows a schematic sectional view along line VI-VI from  FIG. 2 ; 
         FIGS. 7-8 : show schematic sectional views according to  FIG. 5  in various end positions of the brewing unit according to the invention; 
         FIGS. 9-18  show schematic views of the brewing unit according to the invention in various positions; 
         FIGS. 19-20  show schematic perspective views of a variation of the exemplary embodiment according to  FIG. 1  with a driver; 
         FIG. 21  shows a schematic sectional view of the driver according to  FIGS. 19-20 ; 
         FIGS. 22-27  show schematic views of the variation according to  FIGS. 19-20  in various positions; and 
         FIGS. 28-30  show enlarged schematic sectional views of the driver in the various positions according to  FIGS. 22-27 . 
     
    
    
       FIG. 1  shows a schematic perspective view of an exemplary embodiment of a brewing unit  1  of a piston coffee machine according to the invention.  FIG. 2  shows a schematic top view of the underside of the brewing unit  1  according to  FIG. 1 . A schematic side view of the brewing unit  1  in viewing direction III in  FIG. 1  is shown in  FIG. 3 .  FIG. 4  shows a schematic sectional view according to line IV-IV of  FIG. 3 .  FIG. 5  shows a schematic sectional view according to line V-V of  FIG. 3 . 
     The exemplary embodiment shown here is a horizontal brewing unit  1 . The embodiment can also be transferred to a vertical brewing unit  1  or one with any other angle of inclination. The brewing unit  1  is a spindle brewing unit. 
     In this case, the brewing unit  1  comprises a brewing slide  2 , also referred to as a brewing chamber, an outer spindle  3 , a guide rod  4 , a first base plate  5  and a second base plate  6 , an inner spindle  7 , two piston units, namely a so-called shower screen  9  and a plunger  10 , and a drive motor  20 . 
     The system offers the possibility of realizing the required relative positions between the plunger  10 , the shower screen  9  and the brewing slide  3  of the brewing unit  1  for a piston coffee machine not shown, by means of a double-spindle assembly and a drive. 
     Via the (only one) drive motor  20 , a relative movement between plunger  10  and shower screen  9  takes place, so that the distance between them can be changed. Defined pressing forces can then be set via various motor torques. At the same time, a movement of the brewing slide  2  must also take place in order to realize an insertion position for the coffee powder, to create a sealed brewing chamber space and finally to be able to eject the pressed-out coffee cake. 
     The double-spindle assembly comprises the outer spindle  3  and the inner spindle  7 . 
     The outer spindle  3  adjusts the brewing slide  2 , the inner spindle  7  adjusts the shower screen  9 , wherein the plunger  10  is fixedly attached to the second base plate  6 . 
     The brewing slide  2  is moved from a first end position to an intermediate position, then to a second end position and back to the first end position. In doing so, the shower screen  9  is moved to different positions with respect to the brewing slide  2  and the plunger  10 . 
     The shower screen  9  is always arranged in the brewing slide  2  and can assume different relative positions with respect to the brewing slide  2 . These relative positions are made possible independently of each other by the movements of the shower screen  9  due to the inner spindle  7  and by the movements of the brewing slide  2 , which is adjustable by the outer spindle  3 . 
     The plunger  10  can be located outside or inside the brewing slide  2 . Since the plunger  10  is arranged in a fixed position here, the relative positions/movements of the brewing slide  2  and the shower screen  9  to the plunger  10  are made possible by the inner spindle  7  (shower screen  9 ) and the outer spindle  3  (brewing slide  2 ). 
     The shower screen  9  (or the plunger  10 ) and the brewing slide  3  always have a common direction of travel, but they have different speeds (differential speeds) in order to thereby achieve the necessary relative positions between these two components. To facilitate the representation of these directions of travel, a double arrow direction of movement BR is indicated in each figure with two opposite directions of movement BR 1  and BR 2 . 
     The outer spindle  3  with an outer spindle axis  3   a,  the guide rod  4  with a guide axis  4   a  and the inner spindle  7  with an inner spindle axis  7   a  are arranged parallel to each other. The directions of movement BR 1  and BR 2  are parallel to these axes  3   a,    4   a,    7   a.    
     The two base plates  5  and  6  of the brewing unit  1  are here rectangular plates, parallel to each other and arranged at right angles to the axes  3   a,    4   a,    7   a.    
     Attached to the first base plate  5 , shown on the right in  FIG. 1 , is a retaining plate  5   a  which projects outwardly at right angles from the outer spindle axis  3   a  and forms a mounting for the drive motor  20 . 
     The first base plate  5  comprises a bearing  5   b  for the outer spindle  3  and a bearing  5   c  for the inner spindle  7 . 
     The second base plate  6  has a bearing  6   a  for the outer spindle  3  opposite the bearing  5   b . In addition, the plunger  10  is attached to the second base plate  6  by means of a rod-shaped plunger holder  11 . For this purpose, the plunger holder  11  has a first plunger end  11   a  connected to the base plate  6  and a second plunger end  11   b  connected to the plunger  10 . The plunger  10 , the plunger holder  11 , the shower screen  9 , the brewing slide  2  and the inner spindle  7  are arranged coaxially with respect to a brewing slide axis  2   a.    
     The outer spindle  3  and the guide rod  4 , which is also called the tie rod, connect the two base plates  5 ,  6  to each other. 
     The outer spindle  3  has a drive end  3   b,  with which the outer spindle  3  is rotatably mounted in the bearing  5   b  of the first base plate  5 . A bearing end  3   c  of the outer spindle  3  is rotatably mounted to the drive end  3   b  in the further bearing  6   a  in the second base plate  6 . In this way, the outer spindle  3  connects the base plate  5  and  6  on the one side. 
     The drive end  3   b  of the outer spindle  3  extends through the bearing  5   b  and thus through the first base plate  5  and then protrudes therefrom. The drive end  3   b  is coupled on the one hand to the drive motor  20  via a first gearing mechanism  21  and on the other hand to the inner spindle  7  via a second gearing mechanism  22 . This will be explained in more detail below. 
     The guide rod  4  is fixedly connected, for example screwed, with one rod end  4   b  to the first base plate  5  and with another rod end  4   c  to the second base plate  6 , whereby the guide rod  4  connects the base plates  5 ,  6 . 
     The base plates  5 ,  6  are made of identical parts and preferably of plastic. For stiffening and axial securing of the bearings  5   b,    5   c ,  6   a,  e.g. ball bearings, inserted there, the base plates  5 ,  6  are screwed together with sheet metal parts (not designated). 
     The brewing slide  2  comprises a substantially hollow cylindrical housing  2   b  having a circular cross-section and the brewing slide axis  2   a.  A brewing cylinder  12  having an interior  12   a,  which also has a circular cross-section, is arranged in the housing  2   b  coaxially with the brewing slide axis  2   a.  The brewing cylinder  12  has an opening made in its circumferential wall, which communicates with a filling opening  2   e  of the housing  2   b  of the brewing slide  2 . 
     The inner space  12   a  of the brewing cylinder  12  of the brewing slide  2  accommodates the shower screen  9  and the plunger  10  in certain positions of the brewing unit  1 , which will be described in more detail below. The shower screen  9  and the plunger  10  are displaceably guided relative to the brewing cylinder  12  in the interior space  12   a  in the directions of movement BR 1  and BR 2 . When the shower screen  9  and the plunger  10  are both in certain positions of the brewing unit  1  in the inner space  12   a,  they face each other at different distances and define between them a space which is further referred to as the brewing chamber  12   b  (see  FIGS. 5, 6, 7, 12, 14, 16 ). 
     The brewing slide  2  has a first end face  13  facing the first base plate  5 . 
     A second end face  14  of the brewing slide  2  faces the second base plate  6 , and the brewing slide  2  cooperates with the plunger  10  such that the brewing cylinder  12  of the brewing slide  2  is moved over the plunger  10  with the opening associated with the second end face  14 . 
     The brewing slide  2  is guided displaceably in both directions of movement BR 1  and BR 2  on the one hand by the outer spindle  3  and on the other hand by the guide rod  4  between these base plates  5  and  6 . 
     The guiding of the brewing slide  2  by means of the outer spindle  3  and the guide rod  4  takes place on both longitudinal sides of the housing  2   b . For this purpose, the housing  2   b  of the brewing slide  2  has, on one longitudinal side of the guide by the guide rod  4 , a guide bearing  2   c  at each end, through which the guide rod  4  extends. These guide bearings  2   c  are designed, for example, as plain bearings, but can of course also have other bearing elements. 
     A tubular drive section  2   d  is integrally formed on the other longitudinal side of the housing  2   b,  which has an outer spindle nut  15  at its center. The drive section  2   d  has a first end face  16  which faces the first base plate  5 . A second end face  17  of the drive section  2   d  faces the second base plate  6 . 
     The outer spindle  3  extends through the drive section  2   d  and the outer spindle nut  15 . A movement thread  3   e  of the outer spindle  3  engages with the outer spindle nut  15 , but the movement thread  3   e  disengages from the outer spindle nut  15  in end positions of the brewing slide  2 . This will be further explained below in connection with  FIGS. 6 and 7 . 
     The shower screen  9  is fixedly connected to the inner spindle  7  and is displaceable by means of the latter in and into the interior  12   a  of the brewing cylinder  12  in the directions of movement BR 1  and BR 2 . The shower screen  9  is inserted into the brewing cylinder  12  through the opening of the brewing cylinder  12  belonging to the first end face  13 . 
     The inner spindle  7  comprises the inner spindle axis  7   a,  a movement thread  7   b,  a first spindle end  7   c,  a second spindle end  7   d  and a spindle section  7   e  without thread. Furthermore, the inner spindle  7  is in communication with a securing rod  7   f  having a rod axis  7   g , two rod ends  7   h,    7   i  and a securing tab  8  for the purpose of preventing rotation of the inner spindle  7 . 
     Starting from the first spindle end  7   c,  which protrudes outside and to the right of the first base plate  5  in  FIG. 5 , the movement thread  7   b  adjoins the brewing slide  2  in the direction of movement BR 1 , which converges into a smooth spindle section  7   e  via a thread runout  7   b - 1 . An outer diameter of the spindle section  7   e  is larger than an outer diameter of the movement thread  7   b . The spindle section  7   e  has the second spindle end  7   d  of the inner spindle  7  at its free end. The second spindle end  7   d  is fixedly connected to the shower screen  9 . 
     The inner spindle  7  extends with its movement thread  7   b  through the first base plate  6  through an inner spindle nut  27   a . Here, the inner spindle nut  27   a  is part of an output gear  27  of the second gearing mechanism  22  and is rotatably supported and axially fixed in the bearing  5   c  in the base plate  6 . 
     The securing rod  7   f , also referred to as the tie rod, is arranged at a distance parallel to the inner spindle  7 , with a first rod end  7   b  being fixedly connected to the securing tab  8 . The securing tab  8  is in turn non-rotatably connected to the first spindle end  7   c  of the inner spindle  7 , whereby the inner spindle  7  is prevented from rotating about its inner spindle axis  7   a.    
     The securing rod  7   f  extends through an undesignated through-opening through the first base plate  6  parallel to the inner spindle  7  up to the shower screen  9  and is fixedly connected thereto. The securing rod  7   f  is displaceably guided in the base plate  6 , performing the same displacement movement with the inner spindle  7  in the manner described above. 
     The movement mechanism of the brewing unit  1  is based on two threaded spindle drives, namely the outer spindle  3  and the inner spindle  7 . The movement thread  3   e  of the outer spindle  3  and the movement thread  7   b  of the inner spindle  7  have different thread pitches. 
     The outer spindle  3  is coupled to the drive motor  20  via the first gearing mechanism  21 , wherein the outer spindle  3  and the inner spindle  7  are synchronized with each other via the second gearing mechanism  22 . 
     The gearing mechanisms  21  and  22  are designed here as traction means gearing mechanisms with toothed belts. It is also possible that the gearing mechanisms  21 , 22  could be constructed with toothed wheels or in combination with toothed belts. Also, one or both of the gearing mechanisms  21 , 22  could be multi-stage. 
     The first gearing mechanism  21  includes an input gear  23  which is non-rotatably connected to a shaft of a motor output  20   b,  an output gear  24  which is non-rotatably connected to the input end  3   b  of the outer spindle, and a toothed belt as a traction means  24 . Here, the first gearing mechanism  21  is a reduction gear for matching the drive motor  20 . 
     The second gearing mechanism  22  comprises an input gear  26 , which is non-rotatably connected to the outer spindle  3  on an output section  3   d  thereof, the output gear  27  to the inner spindle nut  27   a,  and a toothed belt as traction means  28 . The second gearing mechanism  22  serves as a synchronous gear for adjusting the inner spindle  7 . 
     The outer spindle  3  is the drive shaft of the brewing unit  1  and provides linear movement of the brewing slide via the outer spindle nut  15 , which is mounted on the brewing slide  2  in its drive section  2   d  to prevent it from rotating. 
     The coupling by means of the second gearing mechanism  22  via the toothed belt  28  with the output gear  27  simultaneously ensures a rotational movement of the inner spindle nut  27   a,  which is connected to the output gear  27 . Here, the inner spindle nut  27   a  is formed into the output gear  27  and forms one component therewith (see also FIG. 
       4 ). Of course, two separate components coupled to each other may also be used. This inner spindle nut  27   a  thus simultaneously assumes the function of the inner toothed belt wheel as output gear  27 . As a result, a linear movement of the shower screen  9 , which is attached to the second spindle end  7   d  of the inner spindle  7 , takes place. An anti-rotation device, which includes the securing rod  7   f  already described above, ensures that the shower screen  9  and the inner spindle  7  are not rotated about their own axis (inner spindle axis  7   a ), but merely move axially in the directions of movement BR 1 , BR 2 . 
     The movement thread  7   b  of the inner spindle  7  has a smaller pitch than the movement thread  3   e  of the outer spindle  3 . For this reason, the speed of movement of the brewing slide  2  is many times greater than that of the shower screen  10  with the inner spindle  7 . Advantageously, the pitch of the inner spindle  7  is selected so that it has a self-locking effect during the brewing process. This will be further explained below in connection with  FIGS. 9 to 18 . 
     This effect can be adjusted as desired by means of the transmission ratio of the second gearing mechanism  22  as a synchronous gear of the outer spindle  3  and inner spindle  7 . This principle can be used to generate relative positions between the shower screen  9  and the brewing slide  2 . 
       FIG. 6  shows a schematic sectional view according to line VI-VI of  FIG. 2  and serves to further describe the shower screen  9  and the plunger  10 . The brewing unit  1  is in one of two so-called end positions, which will be described in more detail below in connection with  FIGS. 7 and 8 . 
     In the exemplary embodiment shown, the shower screen  9  and the plunger  10  are constructed in such a way that they are composed of identical parts. These identical parts are each a first shell element  31 , a second shell element  32 , a screen element  33  and a seal  34 . The first shell elements  31  are each centrally clamped to the second shell elements  32 . 
     The first shell elements  31  each serve as a base and support for the respective second shell element  32  and the respective seal  34 . The second shell elements  32  are formed as holders for one or more respective screen elements  33 . A circumferential seal  34  is arranged in each case between the first shell element  31  and the second shell element  32 . The outer diameters of the first shell elements  31 , the second shell elements  32  and the seals  34  are equal in size here and correspond to an inner diameter of the interior  12   a  of the brewing cylinder  12 . The first shell elements  31  are provided with hydraulic connections  35 ,  35   a,    35   b  for connecting flexible hoses  29 ,  29   a,    30 ,  30   a . In this way, hydraulic inlets and outlets are integrated in shower screen  9  and plunger  10 . 
     The shower screen  9  and the plunger  10  are arranged in the brewing unit  1  in such a way that their front sides face each other with the screen elements  33 . These sides are hereinafter referred to as pressure sides, wherein the other sides of the shower screen  9  and the plunger  10  are referred to as rear sides. In the various positions still to be described in detail in connection with  FIGS. 9 to 18 , the space defined by these pressure sides of shower screen  9  and plunger  10  is referred to as brewing chamber  12   b  when shower screen  9  and plunger  10  are both positioned within brewing cylinder  12 . 
     The connection  35  of the shower screen  9  is used to supply hot water through the screen element  33  into the brewing chamber  12   b . The hot water is thereby atomized as uniformly as possible to wet all parts of a coffee cake present in the brewing chamber  12   b  with water. The hot water is supplied through the hose  29  from a connection  29   b,  not further described, on the second base plate  6 . The associated functional assemblies for generating hot water are not shown here. 
     The further connection  35   a  of the shower screen  9  connects a seal connection  34   a  of the seal  34  to a further hose  29   a  for pressurized water, which the hose  29   a  feeds from a further connection  29   b  of the second base plate  6  for the seals  34  (if seals with water admission are used). Similarly, the seal  34  of the plunger  10  may be pressurized with water through the connection  35   a  and the hose  30   a . In this way, the seals  34  are enlarged by the water pressure and ensure sealing of the brewing chamber  12   b  during a brewing operation. 
     The coffee extracted from the coffee cake in the brewing chamber  12   b  is dispensed through the screen element  33  of the plunger  10  through the connection  35   b  (brewing outlet) into a hose  30 . The hoses  30 ,  30   a  connected to the plunger  10  are passed through an undesignated opening in the second base plate  6 . The hoses  29 ,  29   a ;  30 ,  30   a  are formed here as so-called spindle-guided FEP tubes. Of course, other embodiments can also be used. 
     In the example shown, a valve  36  is disposed between the connection  35   b  of the plunger  10  and the second shell element  32 . The valve  36  is called a crema valve, and here forms a spring-loaded oscillating component comprising a ball and a compression spring through which the coffee passes. The valve  36  allows only a small opening gap to produce the typical crema on the coffee. 
     By additionally mounting a crema nozzle consisting of the oscillating element (e.g. ball) and the spring element (e.g. compression spring), the shower screen  9  becomes the plunger  10 , since both advantageously consist of the same components. 
     A third hydraulic connection, not shown, serves as another brewing outlet on the plunger side for an optional additional coffee strand that does not include a crema valve. 
     The screen element  33  of the plunger is often formed as a fine screen, so that very fine particles are not discharged with the freshly brewed coffee. 
     Advantageously, both the plunger  10  and the shower screen  9  are provided with the same fine screen as the screen element  33  to prevent the coffee powder from penetrating into the hydraulic inlet and outlet passages. 
       FIGS. 7 and 8  show schematic sectional views according to  FIG. 5  in different end positions of the brewing unit  1  according to the invention. 
     In the first end position in  FIG. 7 , the brewing slide  2  is in an “open position” (see  FIGS. 9, 10 ) or in a “grounds ejection position” (see  FIGS. 17, 18 ) 
     The second end position shown in  FIG. 8  is assumed by the brewing slide  2  of the brewing unit  1  in an “immersion position” (see  FIGS. 13, 14 ) and also in a “closed position” (see  FIGS. 15, 16 ). 
     The positions of the shower screen  9  with respect to the plunger  10  in the end positions will be further described below. 
     In the end positions of the brewing slide  2 , the movement thread  3   e  of the outer spindle  3  comes out of engagement with the outer spindle nut  15  of the brewing slide  2 . This is referred to as “spindle disengagement”. 
     In order for the brewing slide  2  to be moved back out of these end positions, the outer spindle nut  15  of the brewing slide  2  must be re-engaged with the movement thread  3   e  of the outer spindle  3 . This operation is referred to as “spindle re-engagement”. 
     In the “spindle re-engagement” operation, the brewing slide  2  is moved toward the movement thread  3   e  rotating with the outer spindle nut  15  until the outer spindle nut  15  re-engages with the movement thread  3   e.    
     For this purpose, the brewing unit  1  comprises at least one spindle re-engagement device. In order to distinguish the spindle re-engagement devices, a first spindle re-engagement device is provided for the first end position of the brewing unit  1  (see  FIG. 7 ), wherein a second spindle re-engagement device is associated with the second end position of the brewing unit  1  (see  FIG. 8 ). 
     When the brewing slide  2  is to be moved out of its respective end position again, the respective spindle re-engagement device exerts a force or a kind of movement impulse on the brewing slide  2  in such a way that the latter moves in the direction of the movement thread  3   e  and the outer spindle nut  15  engages with the movement thread  3   e.    
     Such a spindle re-engagement device can be constructed in different ways. 
     Thus, in the exemplary embodiment shown in  FIGS. 1 to 18 , the spindle re-engagement devices comprise a first stop unit  18  and a second stop unit  19  each having a force storage element. This will be described in further detail below. 
     The spindle re-engagement device may also include friction elements, such as on the shower screen  9  or on the plunger  10 . This will be discussed further below. 
     Furthermore, hydraulic sealing elements on the shower screen  9 / on  the plunger  10  can also be used specifically as spindle re-engagement devices, which will also be explained in more detail below. 
     Another way of implementing the spindle re-engagement device may be provided by means of a driver  37 , which will be described in further detail in connection with  FIGS. 19 to 30 . 
     The spindle re-engagement devices may also include electromagnets, as discussed below. 
     In the exemplary embodiment shown in  FIGS. 1 to 18 , the spindle re-engagement devices comprise the first stop unit  18  and the second stop unit  19 . 
     A stop unit  18 ,  19  is arranged on the outer spindle  3  in each of its sections which are close to the base plates  5 ,  6 . The stop unit  18  is located at the output section  3   d  of the outer spindle  3  and is associated with the first base plate  6 . The stop unit  19  is located at the bearing end  3   c  of the outer spindle  3  associated with the second base plate  5 . 
     Each stop unit  18 ,  19  comprises a respective stop element  18   a,    19   a,  a compression spring  18   b,    19   b  and a disc  18   c,    19   c . The compression springs  18   b,    19   b  are each arranged between the disc  18   c,    19   c  and the stop element  18   a,    19   a,  which are also formed here as discs, and form respective force storage elements. The disc  18   c  is axially fixed against the output section  3   d  of the outer spindle  3 . The other disc  19   c  is in axially fixed contact with the second base plate  6 . The compression springs  18   b,    19   b  and the stop elements  18   a,    19   a  are each axially slidable on the outer spindle  3 . 
     As soon as the brewing slide  2  moves in the direction of movement BR 2  towards the first stop unit  18  in  FIG. 7  on the first base plate  5  into the associated end position and reaches the first stop unit  18 , the stop section  16  of the drive section  2   d  of the brewing slide  2  first comes into contact with the stop element  18   a  of the stop unit  18 . Then, upon further movement, the compression spring  18   b  is pretensioned against the stationary disc  18   c  and the outer spindle nut  15  is disengaged from the movement thread  3   e  of the outer spindle  3 . 
     During “spindle disengagement”, the movement thread  3   e  and the outer spindle nut  15  disengage, and then a thread runout  3   e - 2  of the movement thread  3   e  is moved out of the outer spindle nut  15 . 
     At this moment, the brewing slide  2  is in rest position, i.e. the speed of the brewing slide  2  is zero, even if the outer spindle  3  continues to rotate (“grounds ejection position”). This moment is shown in  FIG. 7 . However, in this case, the inner spindle  7  continues to move until the beginning of the spindle section  7   e  comes into abutment with the stationary output gear  27  at a thread runout  7   b - 1  of the movement thread  7   b  of the inner spindle  7  (“open position”). This will be further described in connection with  FIGS. 9, 10 ;  17 ,  18 . 
     If the direction of movement of the outer spindle  3  is now reversed, the spring force of the compression spring  18   b  ensures that immediate spindle re-engagement of the brewing slide  2  is carried out, i.e. the thread runout  3   e - 2  of the movement thread  3   e  of the outer spindle  3  comes into engagement with the outer spindle nut  15  again. 
     In the second end position shown in  FIG. 8 , the spindle disengagement and spindle re-engagement of the movement thread  3   e  of the outer spindle  3  and outer spindle nut  15  are performed in a similar manner. 
     As soon as the brewing slide  2  moves in the direction of movement BR 1  toward the second end position in  FIG. 8  on the second base plate  6 , first the stop section  17  of the drive section  2   d  of the brewing slide  2  comes into contact with the stop element  19   a  of the stop unit  19 . Then, with further movement, the compression spring  19   b  is pretensioned against the stationary disc  19   c  and the outer spindle nut  15  is disengaged from the movement thread  3   e  of the outer spindle  3 . The movement thread  3   e  comes out of engagement with the outer spindle nut  15 , wherein a thread runout  3   e - 1  of the movement thread  3   e  is moved out of the outer spindle nut  15 . 
     Also in this case, the brewing slide  2  is in rest position, i.e. the speed of the brewing slide  2  is zero, even if the outer spindle  3  continues to rotate (“immersion position”). At the same time, the inner spindle  7  also continues to rotate until the tab at the first spindle end  7   c  of the inner spindle  7  comes into abutment with the stationary first base plate  5  (“closed position”). This will be further explained in connection with  FIGS. 13 to 16 . 
     When the direction of movement of the outer spindle  3  is reversed again, the spring force of the compression spring  19   b  ensures that immediate spindle re-engagement of the brewing slide  2  is carried out, i.e. the thread runout  3   e - 1  of the movement thread  3   e  of the outer spindle  3  comes into engagement with the outer spindle nut  15  again. 
     In order to reduce the frictional noise and the wear behavior of the outer spindle nut when it overlatches during “spindle disengagement” in the respective end position of the brewing slide  2 , an additional friction element (e.g. O-ring) can be used which acts between the shower screen  9  and the inner wall of the interior  12   a  of the brewing cylinder  12 . Since the shower screen  9  never leaves the interior  12   a  of the brewing cylinder  12  and both components are always driven synchronously in the same direction of movement BR, this additional friction element as a damping element reduces the contact force of the outer spindle nut  15  on the movement thread  3   e  of the outer spindle  3  by the respective compression spring  18   b,    19   b . Only when the direction of rotation is reversed can the respective compression spring  18   b,    19   b  develop its full effect for re-spindle engagement. Disadvantages of this additional friction element are additional friction losses when moving to the different positions and stick-slip effects when the brewing cylinder  12  is dry and wetted with coffee grounds. 
     Friction elements on the movable inner spindle  7  in cooperation with the brewing slide  2 , i.e. with its brewing chamber  12   b,  may also be useful for spindle re-engagement, in particular on the shower screen  9  (or on the plunger  10  if this is movable instead of the shower screen  9  and the shower screen  9  is stationary). As a result of the frictional force between the shower screen  9  and the brewing chamber  12  or the brewing slide  2 , which is provided by the friction element, the outer spindle nut  15  of the brewing slide  2  is forced back into engagement with the associated external thread (movement thread  3   e ) of the outer spindle  3  for spindle re-engagement as soon as a reversal of movement occurs at the inner spindle  7 . In this constellation, the first spindle re-engagement device comprises at least one such friction element, for example in the form of an O-ring. 
     Such a friction element can also be formed as a hydraulic sealing element. The already existing seal  34  is suitable for this purpose. These seals  34  on the plunger  10 / shower  screen  9  can be used specifically for this function. By a time-delayed relief of the seals  34  after reversal of the direction of the inner spindle  7 , a high frictional force is transmitted to the brewing slide  2  and this is briefly synchronized with the axial movement of the inner spindle  7  (for example in the direction of movement BR 2  in  FIG. 23 ). This results in a movement impulse on the brewing slide  2  as a spindle re-engagement. It is advantageous that the increased friction is no longer present in the remaining course of movement after seal relief. This avoids additional wear of the moving components. 
     The application of a suitable water pressure to the seals  34  may be performed jointly or separately via controlled valves, for example solenoid valves. A control device of the brewing unit  1 , which is not shown, can perform a suitable control of the solenoid valves and thus of the seals  34 . 
     A further possibility for avoiding the described overlatching of the outer spindle nut  15  is to hold the brewing slide  2  in a respective end stop by means of a respective electromagnet, which is switched on to hold the brewing slide  2 . Advantageously, the inner spindle  7  can then be moved independently of the outer spindle  3  even when the direction of rotation is reversed. The brewing slide  2  can be released to a defined position point of the inner spindle  7  when the respective electromagnet is switched off. Such electromagnets are not shown, but can be easily imagined with the following description in connection with  FIGS. 7 and 8 . 
     Such an electromagnet may be arranged between an inner side of the first base plate  5  and the end face  13  of the brewing slide  2  for the end position shown in  FIG. 7 , and may be attached to the first base plate  5 . Similarly, this applies to the electromagnet for the end position shown in  FIG. 8 . Here, the electromagnet is arranged between an inner side of the second base plate  6  and the other end face  14  of the brewing slide  2  and is attached to the second base plate  6 . 
     It is also conceivable that two or more electromagnets are provided per end position. Ring-shaped electromagnets, the diameter of which corresponds to that of the respective end face  13 ,  14  of the brewing slide  2 , could also be used. 
     Furthermore, it is also conceivable that the respective electromagnet actuates a mechanical locking of the brewing slide  2 , for example with the respective base plate  5 ,  6  in the respective end position. Such a mechanical locking device of the brewing slide  2  may be designed in such a way that, when it is released, it is moved by the electromagnet which has triggered the locking operation, in such a way that the mechanical locking device, for example by means of a lever or a movable cam, executes a movement pulse on the brewing slide  2  for spindle re-engagement. This can be done, for example, by a spring tensioned during locking or/and by another electromagnet. Here, the spindle re-engagement devices comprise the electromagnet(s) and the locking device. 
     A further embodiment of the spindle re-engagement devices has at least two electromagnets per end position of the brewing slide  2 . Here, one of the two electromagnets is fixedly mounted, for example on the base plate  5 ,  6 . The other of the two electromagnets is in each case attached to the brewing slide  2  opposite the one stationary electromagnet. For the electrical supply of the electromagnets, a DC voltage is provided which can be reversed in polarity, for example via a relay changeover contact or semiconductor switch, in order to act on one of the two electromagnets. For this purpose, when the brewing slide  2  assumes the end position, the two opposing electromagnets are switched by their current supply in such a way that poles of opposite polarity are opposite each other for attraction. For spindle re-engagement, the polarity of one of the two electromagnets is reversed so that like poles of the two electromagnets oppose and repel each other, thereby applying the motion pulse to the brewing slide  2  for spindle re-engagement. During the adjustment operation of the brewing slide  2  outside the end positions, the power supply to the electromagnets may be switched off to save energy. 
     It is also conceivable that an electromagnet with a movable, for example longitudinally displaceable or/and rotatable/pivotable core is attached to the base plate  5 ,  6 . When the end position is assumed, the electromagnet is switched on, the core is retracted against a spring force and the brewing slide  2  is held in the end position by the switched-on force of the electromagnet. For the spindle re-engagement operation, the electromagnet is switched off and the force of the tensioned spring presses on the movable core, which in turn exerts the movement impulse on the brewing slide  2  for spindle re-engagement operation. In this case, the spindle re-engagement devices comprise the electromagnet(s) with the movable core. 
     As a result of the two toothed belt wheels (input gear  26 , output gear  27 ) of the second gearing mechanism  22  (see  FIGS. 4, 5 ), the transmission ratio of the second gearing mechanism  22  between the outer spindle  3  and the inner spindle  7  can be further adjusted. If the diameter of the output gear  27  is smaller than a diameter of the input gear  26 , when coffee powder is pressed between the shower screen  9  and the plunger  10  in the brewing chamber  12   b  (this will be described further below), the transmission ratio at the inner spindle  7  is more powerful, so that a gearless drive motor  20  is able to implement the required pressing forces between the shower screen  9  and the plunger  10 . The comparably low torque for moving the outer spindle  3  is also sufficient for positioning the brewing slide  2 . At this point, a greater speed is required. 
     It may be advantageous if the first gearing mechanism  21  is arranged between the drive motor  20  and the outer spindle  3  as a reduction gear. In the exemplary embodiment shown (e.g.  FIG. 5 ), this first gearing mechanism  21  has one gear stage. Of course, two or more gear stages may also be possible. In this way, very many transmission ratios are constructively possible by suitable selection of the spindle pitches of the movement thread  3   e  of the outer spindle  3  and the movement thread  7   b  of the inner spindle  7 , and also of the transmission ratios of the gear units  21 ,  22 . The selection of a drive motor  20  for the brewing unit  1  is thus very flexible. With a comparatively reduced-power drive motor  20 , both high travel speeds and contact forces between the shower screen  9  and the plunger  10  which can be adjusted over a large span can be realized. 
     The embodiment described in the document EP 2 907 427 A1 describes a non-linear gear section in which the compromise between high pressing forces and simultaneously short travel cycles is achieved. For moving to the different positions, the functional elements are moved with large pitches. When compacting the coffee powder, the drive motor is reduced with a significantly smaller pitch in order to transmit large forces with the same guide. The decisive difference to the brewing unit  1  according to the invention shown here is that the transmission ratio between the drive motor  20  and the functional groups of the brewing unit  1  changes non-linearly. All transmission ratios in the double-spindle principle according to the invention with the outer spindle  3  and inner spindle  7  are constant, i.e. the movement threads  3   e ,  7   b  of the spindles  3 ,  7  have constant pitches, and the transmission ratios of the gear mechanisms  21 ,  22  (possibly also gearwheels) are constant. As a result of the aforementioned spindle disengagement of the brewing slide  2 , the standstill of the brewing slide  2  and thus of the brewing cylinder  12  is realized, so that in this case no transmission process takes place. 
       FIGS. 9 to 18  show schematic views of the brewing unit  1  according to the invention in various positions. Thereby,  FIGS. 9, 11, 13, 15, 17  each show top views of the brewing unit  1  in which the positions of the brewing slide  2  are clearly shown.  FIGS. 10, 12, 14, 16, 18  are respectively arranged thereunder and each show a horizontal sectional view thereof. In these sectional views, the positions of the brewing slide  2 , the shower screen  9  and the plunger  10  can be seen in relation to each other. 
       FIGS. 9 and 10  show an “open position”.  FIGS. 11 and 12  show a “filling position”. An “immersion position” is shown in  FIGS. 13 and 14 .  FIGS. 15 and 16  show a “closed position”, and  FIGS. 17, 18  show a “grounds ejection position”. 
     These various positions of the brewing unit  1  characterize the overall functionality of the brewing unit  1 . The “open position” ( FIGS. 9, 10 ) is of particular importance, because the control system of the brewing unit  1  can clearly reference the brewing function elements to each other here. Here, the inner spindle  7  reaches its end stop at the thread runout  7   b - 1  on the spindle section  7   e  in the associated inner spindle nut  27   a . In this case, the brewing slide  2  is disengaged on the piston drive side, i.e. the thread runout  3   e - 2  of the movement thread  3   e  of the outer spindle  3  is disengaged from the outer spindle nut  15  and the drive section  2   d  of the brewing slide  2  is in contact with the stop element  18   a  of the first stop unit  18  with its stop section  16 . This has already been described in detail above in connection with  FIG. 7 . Here, the piston drive side is to be understood as the side of the brewing unit  1  with the gear mechanisms  21 ,  22 . 
     In the “open position”, the shower screen  9  closes the filling opening  2   e  in the brewing slide  2 . The plunger  10  is free to stand in its place fixed by the plunger holder  11 . 
     From this starting position, the “filling position” ( FIGS. 11, 12 ) can be repeatedly approached with positional accuracy via a certain angle of rotation of the outer spindle  3 . In this case the brewing slide  2  lies approximately centrally between the two base plates  5 ,  6 . The brewing slide  2  has moved with the opening of the brewing cylinder  12  over the edge of the pressure side of the plunger  10 , which thus forms an incipient closure of the brewing chamber  12   b . The shower screen  9  has simultaneously continued to move in the direction of movement BR 1  towards the plunger  10 . The brewing chamber  12   b  is now accessible through the filling opening  2   e  by coffee powder to be filled in. 
     In the subsequent “immersion position” ( FIGS. 13, 14 ), the brewing slide  2  is already disengaged on the opposite drive side, as already described in detail in connection with  FIG. 8  above. In this case, the shower screen  9  has been retracted into the brewing cylinder  12  to such an extent that its seal  34  and the seal  34  of the plunger  10  both lie within the brewing cylinder  12  and seal the closed brewing chamber  12   b . If the seals  34  are not simple O-rings, they may now be subjected to water pressure to activate the sealing function. In this position, the distance between the plunger  10  and the shower screen  9 , with the correspondingly formed brewing chamber  12   b,  limits the maximum possible coffee powder weight for slightly compressed dry powder without major compaction. For larger weights of more than, for example,  15   g,  there is the possibility of approaching the “immersion position” with a coffee powder portion and then grinding in a second time in the “filling position” before coffee extraction begins. 
     During the extraction of the ground coffee, the density of the coffee grounds increases and this can be pressed out with the maximum possible piston force of the shower screen  9  after completion of the brewing process. The maximum pressing-out position (“grounds ejection position”— FIGS. 17, 18 ) is always reached, because when the direction of the drive or the outer spindle  3  is reversed, the brewing slide  2  overtakes the shower screen  9  and a defined grounds ejection occurs. Overfilling of the brewing unit  1  is thus avoided. The difference in speed between the brewing slide  2  and the plunger  10 / shower  screen  9  and the shear forces acting on the coffee cake result in a defined ejection position in the “grounds ejection position”. 
     In the “grounds ejection position”, the brewing slide  2  is in the end position on the first base plate  5 , with the shower screen  9  still in the brewing cylinder  12 , but with the screen element  33  and part of the second shell element  32  protruding slightly from the brewing cylinder  12 . Thereby, the coffee cake (coffee grounds) falls slightly from the screen element  33  of the shower screen  9  and can be collected in a container (not shown) of the coffee machine (not shown). This grounds container is arranged, for example, in an area below the space between the plunger  10  and the end face  14  of the brewing slide  2 . This is easily understood in connection with  FIGS. 17, 18 . This “grounds ejection position” also simultaneously forms a “grind-through position” of coffee powder directly into the grounds container. 
     Finally, there is the “closed position” ( FIGS. 15, 16 ), in which the plunger  10  and the shower screen  9  are moved together to the maximum extent, provided that no coffee powder has been ground in. 
     Advantageously, the pitch of the inner spindle  7  is chosen so that it acts in a self-locking manner during the brewing process. This avoids a backward movement of the shower screen  9  (or the plunger  10 , depending on the construction) due to the brewing pressure, so that a special current supply to the drive motor  20  (counter-holding) is not necessary during this process. Consequently, the drive motor  10  has less energy losses. 
     A referencing of the positions of the brewing slide  2 , the shower screen  9  or the plunger  10  (depending on the arrangement) can, in the case that the drive motor is a stepper motor, be effected by its number of steps or angle of rotation by means of the associated control system. It is also possible that the different positions are defined by electromechanical limit switches, proximity switches, light barriers or Hall sensors (also additionally). A further possibility is formed by rotary encoders of various designs, which are connected, for example, to the outer spindle  3  and/or the inner spindle  7  in a rotationally fixed manner. 
     In  FIGS. 19-20 , schematic perspective views of a variation of the exemplary embodiment according to  FIG. 1  with a driver device as a component of the first spindle re-engagement device are shown. 
     The variant of the brewing unit  1  is shown in  FIGS. 19-20  in the second end position. 
     In contrast to the exemplary embodiment according to  FIG. 1 , the drive motor  20  is arranged on the second base plate  6  together with the first gearing mechanism  21 , wherein the second gearing mechanism  22  remains on the first base plate  5 . The drive motor  20  is located centrally above the outer spindle  3  and guide rod  4 . 
     The first spindle re-engagement device is provided here with a driver device. The driver device is shown in  FIG. 21  in a schematic sectional view according to  FIGS. 19-20 . 
     The driver device includes a driver  37  having a first retaining element and a second retaining element corresponding to the first retaining element. 
     The driver  37  is arranged on the side of the movable inner spindle  7  as a friction element on the entrained securing rod  7   f  (anti-rotation device) and on the guide rod  4  so as to be displaceable. By means of the driver  37 , the necessary movement impulse is exerted on the brewing slide  2  for spindle re-engagement of the outer spindle nut  15  and the movement thread  3   e  of the outer spindle  3 . This will be further described below. The driver  37  can be moved in the other positions of the brewing slide  2  without blocking the brewing slide  2 . 
     The driver  37  comprises an annular retaining section  37   a  and a guide section  37   b , approximately semi-circular in section, which here has two quarter-circular elements each connected to the retaining section  37   a  by a connection  37   c.    
     In the example shown, a permanent magnet, for example in the form of a bar magnet, is inserted as a first retaining element in a pocket  37   e  within the connection  37   c  in the driver  37 . The bar magnet is symbolically indicated here by an axis  37   d.    
     An oppositely poled permanent magnet, for example also formed as a bar magnet, is arranged as a second retaining element in a receptacle  38  in the securing rod  7   f . This magnet is symbolically indicated by an axis  38   a.    
     The driver  37  is slid with its annular retaining section  37   a  onto the guide rod  4  and is arranged to be displaceably guided on the latter in its longitudinal direction (guide axis  4   a ) with a certain frictional resistance. Thereby, the guide section  37   b  is in contact with the securing rod  7   f . The two quarter-circular elements of the guide section  37   b  are thereby also displaceably guided with a certain frictional resistance on the side of the securing rod  7   f  which faces the guide rod  4 . 
     In a rest position of the driver  37 , which is shown in  FIG. 21  in a sectional view, the axis  37   e  of the bar magnet in the driver  37  and the axis  38   a  of the magnet in the receptacle  38  in the securing rod  7   f  are aligned. In this rest position, the frictional resistance or a retaining force between the driver  37  and the securing rod  7   f  is increased due to the attractive force of the magnets, i.e. greater than that outside the rest position. Thereby, the driver  37  is held in this rest position by the magnetic force of the two magnets attracting each other. The term rest position means here that the driver  37  does not perform any relative movement with respect to the securing rod  7   f  and does not change its position with respect to the securing rod  7   f.    
     When the securing rod  7   f  moves axially during an adjustment, the driver  37  continues to be held in the rest position due to the two magnets attracting each other and is moved along with the securing rod  7   f . At the same time, the guide section  37   b  slides on the guide rod  4  and is thus slidably guided thereon. 
       FIGS. 22-27  show schematic views of the variation according to  FIGS. 19-20  in various positions.  FIGS. 28-30  represent enlarged schematic sectional views of the driver device (circle markings) in the various positions according to  FIGS. 22-27 . 
       FIGS. 22, 24, 26  each show a side view of the brewing unit  1  in different positions, with  FIGS. 23, 25, 27  each showing a longitudinal sectional view of the brewing unit  1  in a common plane of the outer spindle  3 , guide rod  4 , inner spindle  7  and securing rod  7   f  for this purpose. 
     In each case, only the first spindle re-engagement device with the driver device is shown. The second spindle re-engagement device can be provided here, for example, by the seal  34  of the shower screen  9  (as already described above). Of course, other embodiments as described above, also in combinations, are possible. 
     The position of the variant of the brewing unit  1  in  FIGS. 22 and 23  corresponds to the second end position of the brewing unit  1  shown in  FIG. 8 . The outer spindle nut  15  and the movement thread  3   e  are disengaged. The driver  37  of the driver device of the first spindle re-engagement device is in the rest position, and the axes  37   e  and  38   a  are aligned. This is shown enlarged in  FIG. 28 . Spindle re-engagement can be performed by the second spindle re-engagement device, which is not shown. 
     In  FIGS. 24 and 25 , the first end position (see  FIG. 7 ) of the variant of the brewing unit  1  is shown. The outer spindle nut  15  and the movement thread  3   e  are disengaged. The driver  37  of the driver device of the first spindle re-engagement device is initially in the rest position, with the axes  37   e  and  38   a  aligned (see enlarged illustration in  FIG. 29 ). The driver  37  is close to the first base plate  5 , but does not contact it. 
     As soon as the shower screen  9  is now moved in the direction of movement BR 2  towards the opposite second base plate  6 , the inner spindle  7  and the securing rod  7   f  coupled thereto are adjusted in the direction of movement BR 2 . Since the outer spindle nut  15  and the movement thread  3   e  are still disengaged, the brewing slide  2  remains in the first end position. In this case, the driver  37  located on the securing rod  7   f  is carried along by the securing rod  7   f  due to the magnetic adhesion and is pressed against the stop section  17  of the brewing slide  2 . Until then, the driver  37  remains in its rest position due to the magnetic adhesion. Upon further movement of the securing rod  7   f , the magnetic adhesion of the driver  37  now resting against the still immobile brewing slide  2  is overcome. The securing rod  7   f  then continues to slide in the retaining section  37   a  of the now retained driver  37 . At this moment, the driver  37  exerts a movement impulse on the brewing slide  2 , allowing the spindle re-engagement of the outer spindle nut  15  and the movement thread  3   e  to take place. This can be seen in  FIGS. 26 and 27 . 
       FIG. 30  shows the situation of the driver device. The axes  37   e  and  38   a  are no longer aligned. The driver  37  is in contact with the stop section  17  of the brewing slide  2  via end faces of guide section  37   a  and retaining section  37   b.    
     Instead of the magnetic adhesion in the rest position of the driver  37  on the securing rod  7   f , another frictional adhesion can also be realized, for example, by a ball pressure element subjected to spring force. The ball pressure element is then in contact in the rest position of the driver  37  with a corresponding indentation or groove instead of the receptacle  38  in the securing rod  7   f . The adhesive force of this connection is large enough for the movement impulse for spindle re-engagement of the outer spindle nut  15  and the movement thread  3   e , but small enough to prevent jamming of the brewing unit  1 . 
     The spindle re-engagement device(s) can be arranged on one or both sides. The spindle re-engagement devices can have the functional units described above, also in different combinations. 
     The invention is not limited by the above exemplary embodiment, but is modifiable within the scope of the claims. 
     It is conceivable that the movement functionality of the shower screen  9  and the plunger  10  in the brewing unit  1  can also be interchanged with each other, i.e. arranged laterally reversed. 
     At this point, reference should be made to constructions which are referred to by the applicant as “German piston”. In reference to the filter coffee typical for Germany, a crema is deliberately omitted so that the piston coffee obtains filter coffee-like properties. If both brewing outlets of the plunger  10  are connected with a 2/2-way valve, it is possible to produce a product-specific crema. It is possible to define products that are dispensed either with or without crema. 
     Instead of the two base plates  5 ,  6 , a frame could also be used. 
     For the ejection of the coffee grounds, it is conceivable that a kind of pusher with brush and water spray can be used. 
     It is also conceivable that the driver  37  is also slidably attached to the inner spindle  7  or to a component connected to the inner spindle  7  other than the securing rod  7   f.    
     LIST OF REFERENCE SIGNS 
     
         
           1  Brewing unit 
           2  Brewing slide 
           2   a  Brewing slide axis 
           2   b  Housing 
           2   c  Guide bearing 
           2   d  Drive section 
           2   e  Filling opening 
           3  Outer spindle 
           3   a  Outer spindle axis 
           3   b  Drive end 
           3   c  Bearing end 
           3   d  Output section 
           3   e  Movement thread 
           3   e - 1 , 3   e - 2  Thread runout 
           4  Guide rod 
           4   a  Guide axis 
           4   b,    4   c  Rod end 
           5 ,  6  Base plate 
           5   a  Retaining plate 
           5   b,    5   c ,  6   a  Bearing 
           7  Inner spindle 
           7   a  Inner spindle axis 
           7   b  Movement thread 
           7   b - 1  Thread runout 
           7   c,    7   d  Spindle end 
           7   e  Spindle section 
           7   f  Securing rod 
           7   g  Rod axis 
           7   h,    7   i  Rod end 
           8  Securing tab 
           9  Shower screen 
           10  Plunger 
           11  Plunger holder 
           11   a,    11   b  Plunger end 
           12  Brewing cylinder 
           12   a  Interior 
           12   b  Brewing chamber 
           13 ,  14  End face 
           15  Outer spindle nut 
           16 ,  17  Stop section 
           18 ,  19  Stop unit 
           18   a,    19   a  Stop element 
           18   b,    19   b  Compression spring 
           18   c,    19   c  Disc 
           20  Drive motor 
           20   a  Motor axis 
           20   b  Motor output 
           21 ,  22  Gearing mechanism 
           23 ,  26  Input gear 
           24 ,  27  Output gear 
           27   a  Internal spindle nut 
           25 ,  28  Traction means 
           29 ,  29   a ;  30 ,  30   a  Hose 
           29   b  Connection 
           31 , 32  Shell element 
           33  Screen element 
           34  Seal 
           34   a  Seal connection 
           35 ,  35   a,    35   b  Connection 
           36  Valve 
           37  Driver 
           37   a  Retaining section 
           37   b  Guide section 
           37   c  Connection 
           37   d  Pocket axis 
           37   e  Pocket 
           38  Receptacle 
           38   a  Receptacle axis 
         BR, BR 1 , BR 2  Direction of movement