Abstract:
The present disclosure relates to a device for pressure control, including a rod and a plunger. The rod has a first end region delimiting a pressurized space and is movable along an axis between a top dead center and a bottom dead center. The plunger has a traverse substantially perpendicular to a plunger axis transmitting kinetic energy from a plunger drive to the rod in a contact region between a traverse surface and a second end region of the rod arranged opposite the first end region. The rod includes a calotte-shaped end region in the contact region of the rod and the traverse includes a calotte-shaped recess in the contact region of the traverse.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2015/064309 filed Jun. 24, 2015, which designates the United States of America, and claims priority to DE Application No. 10 2014 216 173.8 filed Aug. 14, 2014, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to pumps and, more specifically, to a high-pressure fuel pump and/or an engine valve for pressurizing a fuel. 
       BACKGROUND 
       [0003]    Both in the case of engine valves and in the case, for example, of piston pumps that are used as high-pressure fuel pumps for the pumping of fuel, a rod is commonly provided which is driven by a plunger. The plunger itself is driven, for example in the case of a piston pump as a high-pressure fuel pump, by a camshaft of an internal combustion engine. 
         [0004]      FIG. 12  shows a diagrammatic illustration of a rod  12  that is driven by a plunger  10 . The arrangement illustrated in  FIG. 12  may be used both in, for example, a piston pump  14  as a high-pressure fuel pump  16  and in engine valves  18 . In both cases, high-pressure fuel pump  16  and engine valve  18 , a movement of the rod  12 , which in the case of the piston pump  14  constitutes a piston  20 , influences a pressure in a space (not illustrated) which is arranged above the piston  20  in  FIG. 12  and which is situated at a first end region  22  of the rod  12 . 
         [0005]    In the case of the piston pump  14 , fuel is pressurized by way of the movement of the piston  20  along a piston axis  24 . 
         [0006]    In the case of an engine valve  18 , the movement of the rod  12  along a rod axis  26  causes the engine valve  18  to be opened and closed, and thus, upon opening, a pressure is discharged, and upon closing of the engine valve  18 , pressure is built up. Altogether, therefore, the arrangement shown in  FIG. 12  constitutes a pressure-influencing device  28  both in the case of use in a piston pump  14  and in the case of use in an engine valve  18 . 
         [0007]    The pressure-influencing device  28  in  FIG. 12  has a rod guide  30  for guiding the rod  12  and has a plunger guide  32  for guiding the plunger  10 . The plunger  10  is constructed from a plunger skirt  34  and a traverse  36 , and the traverse  36  is in contact, by way of the plunger skirt  34 , with a roller  38 . A camshaft moves the roller  38  upward and downward along a plunger guide axis  50 , which in  FIG. 12  coincides with the rod guide axis  52 , wherein the roller  38  transmits said upward and downward movement to the traverse  36 . The traverse  36  is in turn in contact with the rod  12  at a second end region  42  of the rod  12 , and transmits the upward and downward movement to the rod  12 , such that the latter can, by way of its first end region  22 , influence a pressure in a space (not shown) arranged above the first end region  22  of the rod  12 . 
         [0008]    Also schematically illustrated in  FIG. 12  is a flange  44 , by way of which the pressure-influencing device  28  can be fastened for example to an engine housing. 
       SUMMARY 
       [0009]    In general, in the case of a rod  12  driven by the plunger  10 —for example in engine valves  18  or in piston pumps  14 —considerable contact forces are generated at a contact point  46  between a rod end  48  in the second end region  42  of the rod  12  and the traverse  36  of the plunger  10 . This is caused firstly by the axial load F a  but also by way of geometrical tolerances of the individual components of the pressure-influencing device  28  and the respective play of the individual elements in the pressure-influencing device  28 . 
         [0010]    In detail, the following forces act:
       The Hertzian stress or the Hertzian contact (F a , see  FIG. 12 ) owing to the axial force F a , which effects a flattening of the surfaces that are in contact with one another, such that a contact surface of enlarged contact area exists rather than ideal punctiform contact;   Transverse forces (see  FIG. 13 ) which result from an angle error α between a plunger guide axis  50  and the rod axis  26 ;   Transverse forces resulting from the contact angle β 1  between the rod axis  26  and the normal at the contact point between the traverse  36  and the rod  12  (see  FIG. 13 );   Transverse forces resulting from the contact angle β 2  between the plunger axis  40  and the normal at the contact point between the traverse  36  and the plunger  10  (see  FIG. 13 );   Contact moments as a product of the axial load F a  and the spacings a 1  and a 2  of a contact point K between traverse  36  and rod  12  to the plunger guide axis  50  and to a rod guide axis  52  respectively (cf.  FIG. 13 ). The contact moments arise owing to the contact angles β 1  and β 2 , the concentricity error of the two guide axes  50 ,  52 , that is to say the angle error α, and the spacing between the plunger guide axis  50  and an intersection point S of a flange surface  54  of the flange  44  with the rod guide axis  52 .       
 
         [0016]    All of these forces lead to considerable bearing reaction forces both in the plunger guide  32  and in the rod guide  30 , which bearing reaction forces can lead to wear and ultimately to abrasion of the linear or sliding guides. The maximum admissible bearing reaction forces in the guides  50 ,  52  determine the maximum admissible errors of the overall system. 
         [0017]    Until now, to improve the system, close tolerances, and associated high production costs, and/or an increase of the guide lengths, have/has been implemented. Here, the individual forces are influenced as follows:
       To be able to compensate the Hertzian stress and the angle error α between the guide axes  50 ,  52 , a spherical rod end  48 , in particular of calotte-shaped form, is used. Here, the expression “calotte” encompasses all segments on dome-shaped bodies. The calotte-shaped rod end  48  is, as shown in  FIG. 13 , placed against a planar traverse  36 . The planarity of the traverse  36  permits both a convex and a concave surface, which leads to considerable scatter of the Hertzian stress. To achieve admissible Hertzian stresses, either the tolerances for the planarity and/or the tolerances for the shape of the calotte-shaped rod end  48  must be reduced, which is associated with an increase in production costs. Furthermore, it is also possible for the radius of the calotte-shaped rod end  48  to be increased, though this increases the contact moment. For compensation, it is therefore necessary in turn to limit the tolerances, which likewise leads to an increase in production costs.   Transverse forces resulting from the angle error α can be reduced only by restricting the tolerances, which is associated with higher manufacturing costs. The resulting transverse forces may also be reduced by way of a lower stiffness or transverse spring rate of the rod  12 , which can normally be achieved only with difficulty owing to the axial load F a  and the required component strength.   The angle error is, overall, the sum of the angle error α between the guide axes  50 ,  52 , the guide clearances (that is to say tilting of the plunger  10  in the plunger guide  32  or of the rod  12  in the rod guide  30 ), and the perpendicularity γ of the traverse  36 , that is to say the angle error of the traverse  36  with respect to the guide diameter of the plunger  10 , that is to say of the plunger skirt  34 . The sum of said angle errors are the contact angles β 1  and β 2 . The resultant transverse force on the rod  12  is calculated using the term sin β 1 ×F a . The resultant transverse force on the plunger  10  is calculated using the term sin β 2 ×(F a ×1/cos α). Said transverse forces can be reduced only by reducing the tolerances and/or, to a limited extent, by increasing the guide lengths. Both however lead to an increase in production costs.   The lever arms a 1  and a 2  to the guide axes  50 ,  52  result from the concentricity errors of the guides  50 ,  52  with respect to one another and the contact angles β 1  and β 2 , which result from the angle errors α, γ and the radius of the calotte-shaped rod end  48 . This leads to the radial migration of the contact point K, and generates the lever arms a 1  and a 2 . To reduce the lever arms a 1  and a 2 , it is possible, on the one hand, to restrict the tolerances of the concentricity errors or of the radius of the calotte-shaped rod end  48 . This however does not lead to a significant improvement, but does lead to increasing production costs. Alternatively, the nominal value of the radius of the calotte-shaped rod end  48  may be reduced, which is however normally possible only with difficulty owing to the Hertzian stresses.       
 
         [0022]    Altogether, therefore, the considerable contact forces that prevail in a construction according to the prior art as per  FIG. 12  and  FIG. 13  in the case of the contact of a calotte-shaped rod end  48  with a planar traverse  36  can be lessened only with a considerable increase in production costs, and only to an unsatisfactory extent. 
         [0023]    In accordance with the teachings of the present disclosure, a high-pressure fuel pump for pressurizing a fuel has a piston which is arranged so as to be movable along a piston axis between a first, top dead center and a second, bottom dead center, and a plunger with a traverse which is arranged substantially perpendicular to a plunger axis and which serves for transmitting kinetic energy from a plunger drive to the piston in a contact region between a traverse surface and an end region of the piston. In the contact region, the piston has a calotte-shaped end region, and the traverse has a likewise calotte-shaped recess. 
         [0024]    The “top dead center” is to be understood to mean a position of the rod in which the rod is, by a drive, for example a camshaft, pushed to its highest deflection point along the rod axis relative to an axis of, for example, the camshaft. Analogously, the expression “bottom dead center” is to be understood to mean the point at which the rod is situated closest to the axis of, for example, the camshaft. 
         [0025]    Correspondingly, a pressure-influencing device for influencing a pressure in a medium has a rod with a first end region for delimiting a space which has the medium, wherein the rod is arranged so as to be movable along a rod axis between a first, top dead center and a second, bottom dead center. Also provided is a plunger with a traverse which is arranged substantially perpendicular to a plunger axis and which serves for transmitting kinetic energy from a plunger drive to the rod in a contact region between a traverse surface and a second end region of the rod, said second end region being arranged opposite the first end region. In the contact region, the rod has a calotte-shaped end region, and the traverse has a likewise calotte-shaped recess. 
         [0026]    Thus, the second end region of the rod is formed by the calotte-shaped end region. 
         [0027]    Here, the pressure-influencing device may be a high-pressure fuel pump or an engine valve. In the case of the high-pressure fuel pump, the rod is then formed by the piston. 
         [0028]    By way of the described arrangement, it is now the case that the rod, by way of its calotte-shaped rod end, moves no longer on a planar traverse but in a calotte-shaped depression, that is to say the previous “calotte/surface contact” is replaced with “calotte/calotte contact”. Here, a calotte, in particular a spherical calotte, is formed into the previously planar surface of the traverse. In this way, for the same Hertzian stress, it is possible to select a smaller radius on the calotte-shaped end region of the rod. The angle error y is thereby eliminated entirely. Only a slight concentricity error remains between a rod axis and a central point of the calotte shape. This has a positive effect on the transverse forces and the resulting moments, because the contact angles β 1  and β 2 , and the lever arms a l  and a 2 , are reduced. 
         [0029]    This is because, owing to the calotte-shaped recess in the traverse, a contact point K between the traverse and the rod is shifted from an outer edge region of the calotte-shaped end region of the rod toward the rod axis. In this way, the described lever arms a 1  and a 2 , which define spacings between the contact point K and a plunger guide axis and rod guide axis respectively, and the contact angles β 1 , β 2 , which define angles in each case of a normal to the traverse at the contact point K with respect to a rod axis and a plunger axis respectively, are considerably reduced. 
         [0030]    In this way, the contact forces acting between the elements can be considerably reduced, but without changing tolerances and guide lengths to an excessive extent, such that altogether, an improved transmission of kinetic energy from the plunger to the rod can be achieved, without the production costs being excessively increased in the process. The traverse preferably has, in regions adjoining the calotte-shaped recess, a traverse surface which is of planar form substantially perpendicular to the plunger axis. Thus, that region of the traverse surface which comes into contact with the calotte-shaped end region of the rod is preferably not entirely of calotte-shaped form but additionally still has planar sub-regions. This is advantageously conducive to reinforcing the traverse overall. Furthermore, it may however also be advantageous for further measures to be implemented for stiffening the traverse, for example if the traverse is of thicker form parallel to the plunger axis, or is formed from a stiffer material, in relation to a traverse from the prior art. 
         [0031]    It is possible for the calotte-shaped recess to be generated in the traverse surface by being formed into a planar traverse surface by stamping. An inexpensive realization of the traverse surface geometry is possible in this way. 
         [0032]    In some embodiments, the calotte-shaped recess is arranged symmetrically about an axis which bisects the traverse perpendicularly to the longitudinal axis thereof. This means that the calotte-shaped recess is arranged, overall, symmetrically on that side of the traverse which comes into contact with the calotte-shaped end region of the rod. In this way, it is possible for a defined position of a central point of the calotte-shaped recess on the traverse to be generated, which in turn leads to defined guidance of the rod by the traverse. 
         [0033]    The traverse may be arranged so as to be movable radially with respect to the plunger axis, wherein the traverse is inserted into the plunger without radial fastening. In this way, it is possible for the concentricity errors to be compensated by way of the radially movable traverse. This is because the concentricity errors constitute only a very small fraction of the lever arms a 1  and a 2 ; they constitute a static position error of the calotte shape. In the case of an traverse that is movable radially with respect to the plunger axis, it is thus the case that the traverse finds its position within the initial strokes of the rod, and can thus compensate the static position error. 
         [0034]    A recess radius of the calotte-shaped recess of the traverse is greater than a rod radius of the calotte-shaped end region of the rod. This yields the advantage that the rod is, in all operating states, reliably situated with its calotte-shaped end region in the calotte-shaped recess of the traverse. 
         [0035]    Some embodiments include a rod guide having a rod guide axis, wherein a rod end radius of the calotte-shaped end region of the rod is smaller than or equal to a spacing, which exists at the top dead center of the rod, between a tangent to a rod calotte surface at the rod axis and an intersection point of the plunger axis and the rod guide axis. 
         [0036]    The spacing between the tangent to the calotte-shaped end region of the rod, at the point at which the rod axis intersects an outer surface of the rod, and an intersection point of the plunger axis with the rod guide axis changes during the operation of the rod. The spacing is smaller at the top dead center of the rod than at the bottom dead center and in all operating states in between. This means that the radius of the calotte-shaped end region of the rod is selected to be smaller than or equal to the smallest spacing between the intersection point of the guide axes and a smallest protrusion of the rod end—in the position at top dead center. This has the effect that the contact angles β 1  and β 2  are smaller than or equal to the angle error α, and it is thus the case that only low transverse forces act. 
         [0037]    If, for construction-related reasons, it is not possible for the rod end radius of the calotte-shaped end region of the rod to be designed to be smaller than the described minimum spacing at top dead center, the recess radius of the calotte-shaped recess may b considerably greater than the radius of the calotte-shaped end region. Here, a rod guide having a rod guide axis is provided, wherein a rod end radius of the calotte-shaped end region of the rod is greater than a spacing, which exists at the top dead center of the rod, between a tangent to a rod calotte surface at the rod axis to an intersection point of the plunger axis and the rod guide axis, wherein a recess radius of the calotte-shaped recess of the traverse is greater than a rod end radius of the calotte-shaped end region of the rod, to such an extent, in the case of identical materials being used, that the Hertzian stress is situated in the region of contact between a planar traverse surface and a calotte-shaped end region of the rod. 
         [0038]    This means that, if the radius of the calotte-shaped end region of the rod cannot be realized for example owing to Hertzian stress values having increased to too great an extent owing to the very small radius of the end region, the values of the Hertzian stress should be compensated by way of a larger radius of the calotte-shaped recess. This is because, the greater the radius of the calotte-shaped recess of the traverse is, the smaller the contact surface between the end region of the rod and traverse surface becomes, owing to the Hertzian stress. In relation to an arrangement in which no calotte-shaped recess is provided in the traverse, it should be the case that at least similar values for the Hertzian stress are realized. 
         [0039]    The pressure-influencing device may be a high-pressure fuel pump, though may alternatively also be an engine valve. An example embodiment of the invention will be discussed in more detail below on the basis of the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    In the drawings: 
           [0041]      FIG. 1  shows a detail of an internal combustion engine having a pressure-influencing device, wherein the pressure-influencing device is a high-pressure fuel pump which is fastened by way of a flange in the internal combustion engine according to teachings of the present disclosure; 
           [0042]      FIG. 2  shows a detail of an internal combustion engine having a pressure-influencing device without flange fastening according to teachings of the present disclosure; 
           [0043]      FIG. 3  shows the pressure-influencing device from  FIG. 1  and  FIG. 2 , with a calotte-shaped recess in a traverse of a plunger; 
           [0044]      FIG. 4  shows the pressure-influencing device from  FIG. 3 , with angle error positions; 
           [0045]      FIG. 5  shows the pressure-influencing device from  FIG. 1  and  FIG. 2 , wherein the traverse does not have a calotte-shaped recess; 
           [0046]      FIG. 6  shows the pressure-influencing device from  FIG. 1  and  FIG. 2 , with a calotte-shaped recess in the traverse; 
           [0047]      FIG. 7  is a schematic geometrical illustration of the pressure-influencing device from  FIG. 5 , for illustrating the contact angles and lever arms; 
           [0048]      FIG. 8  is a schematic geometrical illustration of the pressure-influencing device from  FIG. 6 , for illustrating the contact angles and lever arms that exist; 
           [0049]      FIG. 9  is a schematic geometrical illustration of the pressure-influencing device from  FIG. 6 , for illustrating ideal radius relationships of the calotte-shaped recess and of a calotte-shaped end region of a rod; 
           [0050]      FIG. 10  is a further schematic geometrical illustration of the pressure-influencing device from  FIG. 6 , for illustrating ideal radius relationships of the calotte-shaped recess and of the calotte-shaped end region; 
           [0051]      FIG. 11  shows a diagram which illustrates the radial forces, which prevail in different geometrical arrangements of the pressure-influencing device, in a manner dependent on the force acting on a rod axis according to teachings of the present disclosure; 
           [0052]      FIG. 12  shows a pressure-influencing device according to the prior art, without geometrical errors; and 
           [0053]      FIG. 13  shows a pressure-influencing device according to the prior art, with geometrical errors. 
       
    
    
     DETAILED DESCRIPTION 
       [0054]    Below, the expressions “rod” and “piston” are synonymous with one another. The same applies to the expressions “pressure-influencing device”, “engine valve” and “high-pressure fuel pump”. 
         [0055]      FIG. 1  shows an internal combustion engine  56  to which a pressure-influencing device  28  in the form of a high-pressure fuel pump  16  is fastened by way of a flange  44 . The pressure-influencing device  28  has a plunger  10  with a plunger guide  32 , with a plunger skirt  34  and with a traverse  36 . Furthermore, the pressure-influencing device  28  has a rod  12  in the form of a piston  20  and a rod guide  30 . 
         [0056]      FIG. 2  shows a pressure-influencing device  28  with plunger  10  and plunger guide  32  and plunger skirt  34  and with rod guide  30  and rod  12 . In the case of the internal combustion engine  56  shown in  FIG. 2 , no flange  44  is provided. 
         [0057]      FIG. 3  schematically illustrates the pressure-influencing device from  FIG. 1  with flange  44 , which forms a flange plane  58 . The pressure-influencing device  28  in the form of the high-pressure fuel pump  16  has the plunger  10  with plunger guide  30 , plunger skirt  34  and traverse  36 , and the rod  12  with rod guide  30 . The rod  12  of the traverse  36  is driven along a rod axis  26  between a first, top dead center  60  and a second, bottom dead center  62 , that is to say is moved up and down. The traverse  36  is in turn driven by way of a roller  38 , which is arranged underneath the traverse  36 , along a plunger axis  40 , which coincides with the rod axis  26  in the idealized illustration of the pressure-influencing device  28  shown in  FIG. 3 . The roller  38  is driven by way of a camshaft  65  of the internal combustion engine  56 . 
         [0058]    The roller  38  and the camshaft  65  thus jointly form a plunger drive  66 . 
         [0059]    In the idealized illustration in  FIG. 3 , not only the plunger axis  40  and the rod axis  26  but also a plunger guide axis  50 , that is to say the axis of the plunger guide  32 , and a rod guide axis  52 , that is to say the axis of the rod guide  30 , coincide. 
         [0060]    As can also be seen in  FIG. 3 , the rod  12 , or the piston  20 , has a clearance in the rod guide  30 , and the plunger  10  also has a clearance in the plunger guide  32 . Furthermore, the traverse  36  is mounted movably in the plunger skirt  34 , as indicated by the arrows P, and is movable radially relative to the plunger axis  40  in all directions. 
         [0061]    In the ideal embodiment of the pressure-influencing device  28 , the traverse  36  and the rod  12  make punctiform contact in a contact region  68  of a traverse surface  70  and of a second end region  42 , which is situated opposite a first end region  22 , of the rod  12 . In the contact region  68 , the traverse has a calotte-shaped recess  72 , and the rod  12  has a calotte-shaped end region  74 . The calotte-shaped recess  72  does not span the entire traverse surface  70 , but rather the traverse  36  has, adjacent to the calotte-shaped recess  72 , a traverse surface which is of planar form perpendicular to the plunger axis  40 . The calotte-shaped recess  72  may be formed into the traverse surface  70  for example by stamping. The calotte-shaped recess  72  is arranged symmetrically on the traverse surface  70 , such that the lowest point of the calotte-shaped recess  72  is intersected by the plunger axis  40 , which runs perpendicular to a longitudinal axis  76  of the traverse  36 . 
         [0062]      FIG. 3  shows merely an idealized illustration of the pressure-influencing device  28 , whereas  FIG. 4  illustrates, overlaid thereon, the conditions that actually prevail. In reality, the plunger guide axis  50  and the rod guide axis  52  and/or the plunger axis  40  and the rod axis  26  do not coincide, such that transverse forces act in addition to an axial force F a  acting perpendicularly on the rod  12 . Said transverse forces can be minimized by way of the combination of calotte-shaped recess  72  in the traverse surface  70  and the calotte-shaped end region  74  on the second end region  42  of the rod  12 . 
         [0063]    This is shown by a comparison between a pressure-influencing device according to the prior art, as shown in  FIG. 5 , and the example pressure-influencing device  28  as shown in  FIG. 6 . Comparing the two illustrations in  FIG. 5  and  FIG. 6 , it can be seen that, for the same inclination of the rod axis  26  about the plunger guide axis  50 , a contact point K between the calotte-shaped end region  74  and traverse  36  is considerably further remote from the rod axis  26  in the case of a pressure-influencing device  28  as per  FIG. 5  than in the pressure-influencing device  28  as per  FIG. 6 . Said relatively large spacing also yields greater contact angles β 1 , β 2  and increased acting transverse forces. 
         [0064]      FIG. 7  illustrates the situation of the pressure-influencing device  28  from  FIG. 5  schematically in a geometrical arrangement. For better understanding, the clearance in the guides  30 ,  32  and the concentricity error at an intersection point S between rod axis  26  and plunger axis  40  have not been illustrated, because said errors are generally very small in relation to the errors illustrated. 
         [0065]    As can be seen in  FIG. 7 , the traverse  36  may have an angle error γ both in a positive direction and in a negative direction. Furthermore, the tilting of the rod  12  away from the plunger axis  40  yields the angle error α. The contact angles β 1 , β 2  result from the sum of α and γ. 
         [0066]    This means that the angle error γ may, in expedient situations, hereinafter referred to as “best case”, compensate the angle error α, depending on sign. Said angle error γ may however also further increase the angle error α, this being referred to hereinafter as “worst case”. 
         [0067]    The sum of α and γ results in the contact points, illustrated in  FIG. 7 , for the “worst case” (contact point  78 ), a “neutral case” (contact point  80 ) and for the “best case” (contact point  82 ). For the case of the contact point  78 , the contact angles β 1 , β 2  are shown, which are relatively large. Also shown are the acting axial force F a  on the rod axis  26  and the lever arms a 1  and a 2 , which constitute the spacing of the respective contact point  78 ,  80 ,  82  from the plunger axis  40  or from the rod axis  26 . The greater the contact angles β 1 , β 2 , and thus the greater the lever arms a 1  and a 2 , the greater the transverse forces acting on the pressure-influencing device  28 . 
         [0068]      FIG. 8  geometrically illustrates the situation of the pressure-influencing  28  as per  FIG. 6 . Here, owing to the calotte-shaped recess  72  in the traverse  36 , the angle error γ of the traverse  36  becomes irrelevant. This means that the contact angle β can only be as great as the angle error α. As a result, it is also the case that only the lever arm a 2  exists, that is to say a spacing between contact point K and rod axis  26 , the lever arm a 1 , is omitted. 
         [0069]    Altogether, this yields considerably lower transverse forces acting on the pressure-influencing device  28 , which leads to considerably lower loads and considerably less wear of the pressure-influencing device  28 . 
         [0070]    In some embodiments, the Hertzian stresses may be kept constant without restriction of the production tolerances. This can be realized through selection of the radius relationships of calotte-shaped recess  72  and calotte-shaped end region  74 . Here, a distinction is made between two cases. The distinguishing criterion is the condition that the Hertzian stress should not be increased in relation to an arrangement of the pressure-influencing device  28  as shown in  FIG. 5 . This determines whether a rod end radius  84  of the calotte-shaped end region  74  of the rod  12  can be designed to be smaller than or equal to a minimum spacing a min , at the top dead center  60  of the rod  12 , between a tangent T to a rod calotte surface  86  at the point of the rod axis  26  and the intersection point S of the plunger axis  40  and the rod guide axis  52 . 
         [0071]    In the first case, it is possible for the rod end radius  84  to be designed to be smaller than the spacing a min , as illustrated in  FIG. 9 . 
         [0072]    Owing to Hertzian stresses becoming too large, however, it may also not be expedient to design the rod end radius  84  to be smaller than the spacing a min . Said situation—second case—is illustrated in  FIG. 10 . 
         [0073]    In all operating states, however, it is advantageous for a recess radius  88  of the calotte-shaped recess  72  of the traverse  36  to be greater than the rod end radius  84 . 
         [0074]    In some embodiments, the dimensions ensure adequate stiffness of the traverse  36 . In this way, the contact point K is always situated between the axes  50 ,  52  and a very small variance between “worst case” and “best case” tolerances can be realized. 
         [0075]      FIG. 9  illustrates various situations of the rod end radius  84  for the first case. The illustration shows rod ends  48  with three different rod end radii  84 . Furthermore, a stroke  90  of the rods  12  is indicated. As can be seen, the contact point  82  of the rod  12  with the largest rod end radius  84  is spaced apart from the rod axis  26  to a considerable extent. The smaller the rod end radius becomes, the smaller said spacing a 2  also becomes. With a reduction of said spacing a 2 , the contact angle β and thus the transverse forces acting on the pressure-influencing devices  28  are simultaneously also reduced. As can be seen, in  FIG. 9 , the situation is at its best if the rod end radius  84  is smaller than a min . 
         [0076]    Owing to the Hertzian stresses, it may however also be expedient for the rod end radius  84  to be selected to be greater than a min , This configuration also constitutes a significant improvement in relation to the situation in  FIG. 5 , as long as the recess radius  88  has a minimum radius which is considerably greater than the rod end radius  84 . 
         [0077]    The situation—second case—is illustrated in  FIG. 10  for two different recess radii  88 . The illustration likewise shows two rods  12  with different end radii  84  in a range greater than a min . It can be seen that, in the case of the relatively small recess radius  88  for the relatively large rod end radius  84 , a contact point K is realized which is spaced apart from the rod axis  26  to a considerable extent. In the case of the relatively large recess radius  88 , however, the contact points K both for the relatively small rod end radius  84  and for the relatively large rod end radius  84  are situated relatively close to the rod axis  26 . 
         [0078]      FIG. 11  shows a diagram illustrating the transverse force, which acts on the pressure-influencing device  28 , as a function of the axial load F a . The forces for four different arrangements of the pressure-influencing device  28  are plotted. Diagram A illustrates the force conditions for a pressure-influencing device  28  without calotte-shaped recess  72  in the traverse  36  for the “best case” situation, which is shown in  FIG. 7  with the contact point  82 . 
         [0079]    By contrast, the Diagram C illustrates the situation for a pressure-influencing device  28  without calotte-shaped recess  72  for the “worst case” scenario—contact point  78  in  FIG. 7 . 
         [0080]    Diagram B shows the force conditions for a pressure-influencing device  28  which has a calotte-shaped recess  72  in the traverse  36 . In the diagram B, the traverse  36  exhibits radial mobility relative to the plunger axis  40 . 
         [0081]    Diagram D shows the situation of a pressure-influencing device  28  with the calotte-shaped recess  72 , but in the case of the traverse  36  being fixed and not being radially movable relative to the plunger axis  40 . 
         [0082]    It can be clearly seen that the arrangement with calotte-shaped recess  72  and movable traverse  36  provides considerably better force conditions than the “worst case” scenario of the pressure-influencing device  28  without calotte-shaped recess  72 . Since the achievement of “worst case” and “best case” cannot be controlled, and the force profile in Diagram B closely resembles the “best case” situation, more effectively controllable force conditions are obtained in a pressure-influencing device  28  with calotte-shaped recess  72 . At the same time, the differences between Diagrams B and D show that a radially movable  36  may be very much favored. 
         [0083]    Altogether, the calotte-shaped recess  72  generates direction-independent transverse forces which lie at a low level between “best case” and “worst case” of the pressure-influencing device  28  according to the prior art. This corresponds to a general reduction of the acting transverse forces. 
         [0084]    Altogether, the transverse forces arising from the axial forces F a  owing to geometrical discontinuities of the components can be reduced by up to 40% in relation to the “worst case” configuration from the prior art. The detrimental influences of the transverse forces owing to the contact angles β 1 , β 2  can be largely eliminated, leading to a reduction of the transverse forces. At the same time, the perpendicularity of the traverse  36  with respect to the plunger axis  40  is virtually irrelevant, which leads to a reduction in production costs. The calotte-shaped recess  72  of the traverse  36  can be generated by way of simple stamping, which is particularly inexpensive. Altogether, the angle error γ is eliminated entirely, and the variance and magnitude of the overall angle error β 1  and β 2  is considerably reduced, such that, for the design process, virtually constant loads can be expected, and the “best case” and “worst case” advantageously lie close together. Additionally, with skilled pairing of the rod radius  84  and of the recess radius  88 , it is even possible for β 1  and β 2  to be kept smaller than the inevitable angle error α between the axes  50 ,  52  of the guides. 
         [0085]    These advantages can be utilized in order to increase the axial load F a  overall, to improve the service life of the guides  30 ,  32 , that is to say increase robustness, to reduce the required guide lengths, which is associated with a reduction in costs and reduction in size of structural space, and, altogether, to increase the tolerances of the components, which likewise contributes to a reduction in costs in the production process. 
         [0086]    In some embodiments, the calotte-shaped recess  72  may be provided in a separate slide shoe which is arranged in the plunger  10 . 
       REFERENCE DESIGNATIONS 
       [0000]    
       
           10  Plunger 
           12  Rod 
           14  Piston pump 
           16  High-pressure fuel pump 
           18  Engine valve 
           20  Piston 
           22  First end region 
           24  Piston axis 
           26  Rod axis 
           28  Pressure-influencing device 
           30  Rod guide 
           32  Plunger guide 
           34  Plunger skirt 
           36  Traverse 
           38  Roller 
           40  Plunger axis 
           42  Second end region 
           44  Flange 
           46  Contact point 
           48  Rod end 
           50  Plunger guide axis 
           52  Rod guide axis 
           54  Flange surface 
           56  Internal combustion engine 
           58  Flange plane 
           60  First, top dead center 
           62  Second, bottom dead center 
           65  Camshaft 
           66  Plunger drive 
           68  Contact region 
           70  Traverse surface 
           72  Calotte-shaped recess 
           74  Calotte-shaped end region 
           76  Longitudinal axis of traverse 
           78  Contact point “worst case” 
           80  Contact point “neutral case” 82  Contact point “best case” 
           84  Rod end radius 
           86  Rod calotte surface 
           88  Recess radius 
           90  Stroke 
         α Angle error (plunger guide axis—rod axis) 
         β 1  Contact angle (rod axis—normal to traverse at contact point) 
         β 2  Contact angle (plunger guide axis/plunger—normal to traverse at contact point) 
         γ Angle error of traverse (angle of traverse relative to plunger guide) 
         A “Best case” without calotte-shaped recess 
         B Movable traverse with calotte-shaped recess 
         C “Worst case” without calotte-shaped recess 
         D Fixed traverse with calotte-shaped recess 
         K Contact point between rod and traverse 
         P Arrow 
         S Intersection point of plunger axis/rod axis 
         T Tangent 
         F a  Axial load/Hertzian stress/axial force 
         a 1  Spacing of contact point to plunger guide axis/plunger axis 
         a 2  Spacing of contact point to rod guide axis/rod axis 
         a min  Spacing of tangent to rod calotte surface to intersection point of plunger axis/rod axis