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CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 20 2008 000 911.1, filed Jan. 22, 2008. 
     FIELD OF THE INVENTION 
     The invention relates to a cylinder lock with plate tumblers and to a key for the lock, which can be used in particular as a motor vehicle door lock. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     DE-U 72 03 658 discloses a cylinder lock which has a cylinder housing in which a cylinder core is rotatably mounted. The cylinder core has, in its central region, an axially extending key channel and guide slots which extend radially therethrough in a direction perpendicular to a longitudinal axis of the cylinder core. In each of the guide slots, two spring-loaded plate-shaped tumblers, so-called plate tumblers composed of sheet-metal punched parts, are seated therein so as to bear against one another in pairs, or axially adjacently, and so as to be alternately and radially movable. When the key is removed from the key channel, the plate tumblers radially engage in their blocking position (i.e., on account of the spring loading) into radially opposing blocking grooves which extend in the axial direction of the cylinder housing, such that the cylinder core is seated in a rotationally fixed manner in the cylinder. 
     The plate tumblers are of solid-frame-type design and have in each case two frame longitudinal webs and two frame transverse webs which form a substantially rectangular recess which is aligned axially with the key channel. Inwardly protruding control cams are arranged at inner edges of the frame longitudinal webs. 
     The appropriate flat key has a key bit or middle web which is substantially rectangular in cross section and which has two wide side surfaces and two narrow side surfaces. In each case, one groove-like recess is provided mirror-symmetrically oppositely on the wide side surfaces. The recesses extend in the longitudinal direction of the key bit and have groove walls which are aligned at right angles to the groove base, wherein the recesses are designed as control edges or control paths which interact with the correspondingly arranged control cams of the plate tumblers. 
     The paired arrangement makes it possible to use reversible keys which can be inserted into the key channel to actuate the closing mechanism by rotating the keys 180° about their longitudinal axis of the key bit. Here, the plate tumblers are pulled back into the interior space of the cylinder core counter to the respective spring force. On the basis of this known design, it is possible to use high quantities of different plate tumblers and thereby ensure high numbers of variations of locks. Furthermore, a shorter, more space-saving design of the closing cylinder is possible. On the other hand, a cylinder lock of said type still requires a relatively large cylinder core diameter of more than 15 mm, for example, as a result of which the spatial requirement in this respect still does not meet various needs and desires, such as those of the automobile industry. 
     Cylinder core diameters of less than 15 mm are possible using cylinder locks with plate tumblers which have closing cams on the inner edges of the frame transverse webs, with the control paths being formed on the narrow side surfaces of the key bit (DE-U 76 37 619). A disadvantage of such an arrangement, however, is the possibility of simple reproduction and a weakening of the key bit by means of excessively deep milled portions of the control paths, for which reason said cylinder lock type with plate tumblers is no longer common in motor vehicle doors despite its small design. 
     A further cylinder lock type with plate tumblers and with a particularly high level of variation diversity of the control edges and particularly high strength of the flat key is known for example from DE 27 42 165 C2. The cylinder core of the closing cylinder has plate tumblers which are alternately spring-loaded towards one another and the flat key is provided with oppositely-aligned, stepped incision paths or control paths which are open to opposite narrow sides of the flat profile and which are separated from one another by a continuous key bit or middle web of the flat profile. In said known cylinder lock type, the flat key has a Z-shaped basic profile with the middle web, and two incision paths or control paths are arranged on the inner surfaces, wherein each faces towards the opposite narrow side of the flat profile of the two Z-limbs. To be able to operate said cylinder lock type with a reversible key which facilitates use, the control paths are formed so as to be mirror-symmetrically identical to one another, though this restricts the variation diversity. Furthermore, when using the reversible key version, it is not easily possible to ensure a closing cylinder diameter of less than 15 mm. 
     It is an object of the invention to increase the variation diversity of a cylinder lock type known from DE 27 42 165 C2 and the like, while simultaneously permitting the use of a reversible key for this purpose. Furthermore, it is also an object to produce a closing cylinder having a diameter of less than 15 mm, yet capable of accommodating up to 8 plate tumblers, for example. 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below by way of example on the basis of the drawings, in which: 
         FIG. 1  shows a cross section through a cylinder lock according to the invention with 6 plate tumblers; 
         FIG. 2  shows a plan view of the cylinder core of the cylinder lock according to  FIG. 1 ; 
         FIG. 3  shows a perspective view of a flat key for the cylinder lock according to  FIG. 1 ; 
         FIG. 4  shows a plan view of a cylinder core of a further embodiment of the invention; 
         FIG. 5  shows a side view of the cylinder core according to  FIG. 4 ; 
         FIG. 6  shows a plan view of another embodiment of a cylinder core for a cylinder lock according to the invention; 
         FIG. 7  shows a side view of the cylinder core according to  FIG. 6 ; and 
         FIG. 8  shows a view of the wide side of a key bit according to yet another embodiment of the key according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , a cylinder lock according to the invention has a cylinder housing  1  with a cylinder wall  2  which may be of any desired external design and which may be composed of various materials known in the art, such as, but not limited to metals, plastics, and the like. The cylinder wall  2  surrounds an axial cylindrical lock core recess  3 . In addition, the cylinder wall  2  includes at least one blocking groove  4  therein. The blocking groove  4  extends axially parallel in the cylinder wall  2  and is open in the direction of the recess  3 . If two blocking grooves  4  are provided, said blocking grooves  4  are arranged diametrically oppositely, as shown in  FIG. 1 . 
     A cylinder core  6  is arranged in the cylindrical lock core recess  3  of the cylinder housing  1  so as to be rotatable to a limited extent about the central longitudinal axis  5  of said cylindrical lock core recess  3 . As best shown in  FIGS. 1 and 2 , the cylinder core  6  is provided with an axial key channel  9 . In the region of the axial key channel  9 , the cylinder core includes a plurality of continuous chamber slots  7  aligned transversely with respect to the longitudinal axis  5 . While there are six chamber slots  7  depicted in  FIG. 2 , it should be understood to those of ordinary skill in the art that more or less chamber slots  7  may be provided. The chamber slots  7  are separately spaced from one another by wall webs  8 , so as to be axially aligned with one another. Each chamber slot  7  has, on one side, a widened portion such as a blind hole  10 , for example, with a bearing step  11  on which a pressure spring  12  is mounted. 
     A substantially rectangular-frame-shaped plate tumbler  13  is slidably seated in each chamber slot  7 , wherein the plate tumbler  13  is composed, for example, of a sheet-metal punched part having two transverse frame webs  14 ,  15  and two longitudinal frame webs  16 ,  17 . The two transverse frame webs  14 ,  15  and two longitudinal frame webs  16 ,  17  collectively form a substantially rectangular interior space having a height “h” and a width “b”, such that the interior space surrounds the key channel  9 . On the outside of the longitudinal frame web  16  which faces towards the blind hole  10 , an abutment lug  18 , which projects into the sheet-metal plane, is arranged above the bearing step  11  such that the pressure spring  12  presses against the abutment lug, wherein the abutment lug  18  can be supported against the inner wall  19  of the cylinder wall  2  of the cylinder housing  1 . 
     An assembly slot  20  is expediently situated, above the key channel  9  on the side at which the blind hole  10  opens, in the wall of the cylinder core  6 . The assembly slot  20  extends parallel to the longitudinal axis  5  up to the key channel  9  and through the wall webs  8 . The assembly slot  20  facilitates the assembly of the plate tumblers  13  and of the pressure springs  12 . 
     At one end, the cylinder core  6  has—as is conventional—a driver  21  for a lock mechanism (not illustrated) and, at the other end, a collar  22  which serves in particular as a stop on the cylinder housing  1 . 
     Each rectangular interior space of the frame of the plate tumblers  13 , which rectangular interior space is aligned with the key channel  9 , has a control step  24 . The control step may either be provided on a side beyond the bisecting line of the height “h”, or on one side of a central longitudinal plane  25  (e.g., below the longitudinal plane  25  in  FIG. 1 ) which encompasses the longitudinal axis  5  and which lies at right angles or perpendicular to the frame longitudinal webs  16 ,  17 . The control step  24  projects into the interior space of the frame in a corner region  23  situated diagonally opposite the abutment lug  18  between a transverse frame web  14  or  15  and a longitudinal frame web  16  or  17 , with the control step  24  extending at right angles to the inner edge of the longitudinal frame web  16  or  17 . 
     In the case of plate tumblers  13  which are adjacent in the direction of the longitudinal axis  5 , the control step  24  for one plate tumbler may be situated on one longitudinal frame web  17  and, for the following plate tumbler, on the other longitudinal frame web  16  in the laterally opposite corner region  23  on the same side of the plane  25 .  FIG. 1  shows said configuration with six plate tumblers  13  which are seated one behind the other in the axial direction of the longitudinal axis  5 , with three control steps  24  of three plate tumblers  13  being provided on the one side (the left-hand side in  FIG. 1 ), and likewise, three control steps  24  of three further plate tumblers  13  being provided on the opposite side (the right-hand side in  FIG. 1 ), by means of stepped punched portions, with the control steps  24  being arranged at different heights as measured from the inner edge of a transverse frame web  14  or  15 . Nevertheless, it is to be understood by those of ordinary skill in the art that the present invention is not so limited, as it falls within the scope of the invention for control steps  24  to also be arranged on cams which project into the interior space of the frames. 
     The blind holes  10  of adjacent chamber slots  7 , and therefore also the abutment lugs  18  of adjacent plate tumblers  13 , are expediently arranged diagonally oppositely, as a result of which the chamber slots  7  can be arranged closer together and the cylinder core  6  can be of short design. 
     On account of the design of the cylinder core  6 , each plate tumbler contains one control step  24 , wherein the control steps  24  are arranged laterally at both sides in the key channel  25 , yet only at one side with respect to the plane  25 , with a spacing to the plane  25  of the frames of the plate tumblers  13  and so as to slide in the same direction during the actuation with a key (arrow  40  in  FIG. 1 ), it is possible to design a reversible key  43  having a central, continuous middle web or key bit  30 . As best shown in  FIG. 3 , the key bit  30  is rectangular in cross section and has a central longitudinal central axis  41 , a height “h” and a width “b”, two narrow edges  31 ,  32 . two wide side surfaces  33 ,  34 , and an end edge  35 . 
     On one side of a longitudinal plane  42  which extends perpendicular or at right angles to the wide side surfaces  33 ,  34  and which encompasses the longitudinal central axis  41  of the middle web  30  or the middle of the height “h”, a control path  37  which projects from said longitudinal plane  42  at right angles and extends in the longitudinal direction of the middle web  30  is formed on the wide side surface  33  with a spacing to the longitudinal plane  42 , and a control path  36  which projects from said control plane  42  at right angles and extends in the longitudinal direction of the middle web  30  is formed on the wide side surface  34  with a spacing to the longitudinal plane  42 . The control paths  37 ,  36  may be generated for example by an abutting milled portion and are situated on the one side of the longitudinal plane  42  with a spacing to the latter. 
     On the other side of the longitudinal plane  42 , a control path  36   a  which projects from said longitudinal plane  42  at right angles and extends in the longitudinal direction of the middle web  30  is arranged on the wide side surface  33  with a spacing to the longitudinal plane  42 , and a control path  37   a  which projects from said longitudinal plane  42  at right angles and extends in the longitudinal direction of the middle web  30  is arranged on the wide side surface  34  with a spacing to the longitudinal plane  42 . 
     The contours of the control paths  36  and  37  differ from one another, and the contours of the control paths  36   a  and  37   a  also differ from one another, but the contours of the control paths  36   a  and  36  and the contours of the control paths  37   a  and  37  are identical. 
     The control paths  36  and  37  are situated on one side of the longitudinal plane  42 , and the control paths  36   a  and  37   a  are situated on the other side of the longitudinal plane  42 , wherein the control paths  36 ,  36   a  and  37 ,  37   a  have the same spacing to the longitudinal plane  42 . The control paths  36 ,  36   a  and the control paths  37 ,  37   a  are therefore situated mirror-symmetrically diagonally opposite one another with respect to a mirror plane which passes through the longitudinal central axis  41  of the middle web  30 . 
     Accordingly, the key  43 , as a reversible key, provides identical control paths  36 ,  37  or  36   a ,  37   a  to the control steps  24  of the plate tumblers  13  after having been rotated through 180° about its longitudinal axis  41 . 
     The control paths  36 ,  36   a ,  37 ,  37   a  are adapted, in a manner known per se, to the control steps  24  of the plate tumblers  13  in such a way that, as the key  43  is inserted into the key channel  9 , the plate tumblers  13  are pulled out of a blocking groove  4  into the interior of the cylinder core  6 . Said position of the plate tumblers  13  is shown in  FIG. 1 . The cylinder core  6  can thereafter be rotated about its longitudinal axis  5 . If the key  43  is pulled out, the springs  12  force a portion of the plate tumblers  13  into a blocking groove  4 , such that the cylinder core  6  is locked in the cylinder housing  1 . According to the invention, only one blocking groove  4  is required. Yet a second blocking groove may be provided to facilitate assembly by making it possible for the cylinder core  6  to be pushed into the cylinder housing  1  during assembly even after having been rotated 180° about its longitudinal axis  5 . 
     Referring now to  FIGS. 4 and 5 , an alternative embodiment of the invention provides a paired arrangement of plate cylinder tumblers  13 , with two plate tumblers  13  being arranged in one chamber slot  7 , such that the structural length of the cylinder core  6  can be kept very short. By way of example,  FIG. 5  shows the distribution of the plate tumblers  13 . The plate tumblers  13  correspond to the design shown in  FIG. 1  and the design of the cylinder housing  6  corresponds to the design likewise shown in  FIG. 1 . 
     The invention provides the use of at least three plate tumblers  13  and the corresponding configuration of the cylinder core  6  can be seen in  FIGS. 6 and 7 . As shown in the Figures, the blind holes  10  may be arranged on the same narrow side of the chamber slots  7 . The greater the number of plate tumblers  13 , the more expedient it is for the blind holes  10  of adjacent chamber slots  7  to be arranged diagonally oppositely, as shown in  FIG. 2 . 
     The invention permits for example up to eight plate tumblers  13  without it being necessary for the diameter of the cylinder core  6  to exceed 15 mm. 
     Referring now to  FIG. 8 , a further advantageous embodiment of the invention is shown wherein the reversible key  43  also has two longitudinal or centering grooves  44  which each extend parallel to the longitudinal central axis  41 . Here, one centering groove  44  is provided on each side of the key  43  in relation to the wide side surfaces  33 ,  34 , with the centering grooves  44  being arranged in each case between the two control paths  37 ,  36   a  and  36 ,  37   a  which, respectively, are situated on the same side of the key  43  in relation to the wide side surfaces  33 ,  34 . Here, each of the centering grooves  44  expediently extend, proceeding from the end edge  35 , into the reversible key  43  as viewed perpendicular to the wide side surfaces  33 ,  34 , such that said centering grooves  44  are open in the direction of the end edge  35 . In particular, between two control paths  37 ,  36   a  and  36 ,  37   a  which, respectively, are situated on the same side of the key  43  in relation to the wide side surfaces  33 ,  34 , a wall  45  is provided in each case which connects the control paths  37 ,  36   a  and  36 ,  37   a , respectively, and which adjoins the wide side surfaces  33 ,  34 . The walls  45  each have one wall side surface  46  which is parallel to the wide side surfaces  33 ,  34  and into which the centering grooves  44  are formed. 
     The centering grooves  44  are also preferably arranged and formed centrally (not illustrated) or slightly eccentrically ( FIG. 8 ) in relation to the height “h”. 
     The centering grooves  44  serve to guide and centre the key  43  in the cylinder core  6 . For this purpose, the cylinder core  6  has at least one centering pin  50  which projects into the key channel  9 , wherein the centering pin  50  is guided in a sliding fashion in a centering groove  44 , and thereby guides and centers the key  43 , as the key  43  is inserted. 
     Furthermore, according to one aspect of the embodiment depicted in  FIG. 8 , the control paths  36 ,  36   a ,  37  and  37   a  and therefore the walls  45  end with a certain spacing before the end edge  35 , such that the latter protrudes slightly beyond the control paths  36 ,  36   a ,  37  and  37   a  and the walls  45  in the direction of the longitudinal central axis. The free end edge  35  forms a key stop which delimits the insertion travel of the key  43  into the cylinder lock. 
     Furthermore, the end edge  35  and the narrow edges  31 ,  32  preferably merge into one another via rounded edges  48 . 
     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of the principles of this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.

Summary:
A cylinder lock is disclosed. The cylinder lock can be used on a motor vehicle door lock. The disclosed cylinder lock has plate tumblers and also a key for the cylinder lock. The key for the cylinder lock could be a reversible key.