Abstract:
A tubular spring is provided for preloading a piezoelectric or magnetostrictive actuator for the actuation of fuel injection valves for fuel injection systems of internal combustion engines. The tubular spring has at least two assembly engagements for uniform preloading of the tubular spring.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a tubular spring for an actuator and a method for assembling the tubular spring.  
       BACKGROUND INFORMATION  
       [0002]     Piezoelectric or magnetostrictive actuators for the actuation of fuel injection valves for fuel injection systems of internal combustion engines are usually preloaded by way of a spring in order to eliminate tensile and shear forces. The residual load of the fuel injection valve&#39;s return spring, present even when the fuel injection valve is closed, is usually used to generate the preload.  
         [0003]     An actuator, e.g., for the actuation of fuel injection valves for fuel injection systems of internal combustion engines, that has multiple layers made of a piezoelectric or magnetostrictive material arranged in stacked fashion, is described in published German patent document DE 199 51 012. The actuator is preloaded by way of a tie rod disposed in a central recess, by an opposite-direction thread. Immobilization and energy transfer are accomplished via a cover plate and a base plate.  
         [0004]     This configuration is disadvantageous in particular because of the need to configure the stacked actuator in hollow fashion so that the tie rod can be accommodated in the recess. This makes the actuator even more susceptible to damage during preassembly and assembly.  
         [0005]     Described in published international patent document WO 99/08330, is a piezoelectric actuator that is slid into a spring sleeve and joined, in preloaded and in frictionally or positively engaged fashion, to two ends of the spring sleeve. The result is to produce a basic unit in which the preload force of the piezoelectric actuator is permanently defined.  
         [0006]     The particular disadvantage of this actuator is that the preload of the actuator is defined solely by the elasticity of the spring sleeve. Progressive characteristics in order to compensate for short activation times cannot thereby be achieved.  
       SUMMARY  
       [0007]     The tubular spring according to the present invention for an actuator, and the method according to the present invention for assembly, have the advantage that assembly engagements in the tubular spring make possible uniform preloading of the tubular spring and of the actuator, and stress-free welding of the tubular spring to the actuator head and actuator foot.  
         [0008]     Advantageously, the assembly engagements are disposed in at least one of the otherwise continuous end regions of the tubular spring. The result, on the one hand, is that the spring characteristics of the tubular spring are not negatively influenced. A positive influence, with a longer possible actuator stroke, is possible as a result of introduction of the assembly engagements in only one end region.  
         [0009]     It is additionally advantageous that the assembly engagements are disposed on the tubular spring symmetrically and in paired fashion, or equidistantly at identical angular spacings, so that uniform loading is possible.  
         [0010]     It is furthermore advantageous that a preassembly of the actuator module in the load-free state is first performed before the actuator module is inserted into an assembly apparatus for preloading and final assembly. As a result, the actuator is already protected from damage due to shear forces, and can be stored and transported until further processed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic cross-sectional view of a fuel injection valve suitable for being fitted with a tubular spring according to the present invention.  
         [0012]      FIG. 2  is a plan view of a sheet-metal blank for manufacture of the tubular spring, according to a first example embodiment.  
         [0013]      FIG. 3  is a schematic cross-sectional view illustrating the assembly of a tubular spring configured according to the present invention as shown in  FIG. 2 .  
         [0014]      FIG. 4  is a perspective view of the completely assembled tubular spring configured according to the present invention as shown in  FIGS. 2 and 3 .  
         [0015]      FIG. 5  is a plan view of a sheet-metal blank for manufacture of the tubular spring, according to a second example embodiment.  
         [0016]      FIG. 6  is a side view of the rolled-up sheet-metal blank shown in  FIG. 5 , of the tubular spring configured according to the present invention.  
         [0017]      FIGS. 7A and 7B  depict, in a sectional view and a side view, respectively, the completely assembled tubular spring configured according to the present invention as shown in  FIGS. 5 and 6 . 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  shows, in a highly schematic depiction, a fuel injection valve suitable for being fitted with a tubular spring configured according to the present invention. Fuel injection valve  1  depicted in  FIG. 1  is suitable, in particular, as a fuel injection valve  1  for direct injection of fuel into a combustion chamber (not depicted further) of an internal combustion engine.  
         [0019]     Fuel injection valve  1  encompasses a nozzle body  2  in which a valve needle  3  is guided. The latter has at one outflow end a valve closure element  4  that forms a sealing fit with a valve seat surface  5 . Fuel injection valve  1  is embodied as an outward-opening fuel injection valve  1 . A return spring  6 , disposed in nozzle body  2 , impinges upon valve needle  3  in such a way that fuel injection valve  1  is held closed in the idle phase, and valve needle  3  is returned back to its idle position after the opening phase.  
         [0020]     Nozzle body  2  opens into a housing  7  in which a hydraulic coupler  8  and an actuator module  9  are disposed.  
         [0021]     Actuator module  9  encompasses a piezoelectric stacked actuator  10  that, for protection against shear forces, is preloaded with a tubular spring  11  configured according to the present invention. In a present example embodiment, actuator  10  is additionally protected by an injection-molded sheath  12 . Actuator  10  has an actuator foot  13  and an actuator head  14  which are, for example, welded to tubular spring  11 . Tubular spring  11  and the components of actuator  10  are described in more detail below with reference to  FIGS. 2 through 4 . Hydraulic coupler  8 , which is disposed on the inflow side of actuator module  9 , encompasses a working piston  15  that is preloaded by a coupler spring  16  with respect to housing  7 . Hydraulic coupler  8  converts the short stroke of piezoelectric actuator  10  into a longer stroke of valve needle  3 . Coupler  8  and actuator module  9  are encapsulated in an actuator housing  17 . Flowing around the latter is the fuel, delivered centrally via an inflow fitting  18 , that flows through housing  7  and through nozzle body  2  to the sealing seat. Fuel injection valve  1  is actuatable via an electrical plug contact  19 .  
         [0022]     In order to ensure, during the assembly and operation of actuator  10 , that shear forces which may result in damage to actuator  10  are prevented, actuator  10  is encapsulated in tubular spring  11 . The latter preloads actuator  10  so that it is stabilized by the exclusively axial forces acting on it.  
         [0023]     Tubular spring  11  is manufactured in the manner depicted in  FIGS. 2 and 5  for two different example embodiments, respectively. Firstly, a basic shape is produced by punching out of a sheet-metal blank  20 , and is rolled up as depicted in  FIG. 6  for the second example embodiment. In an abutting region in which edges  21  of tubular spring  11  are located next to one another, those edges are joined to one another by longitudinal welding so that a closed tube is created.  
         [0024]     During manufacture of the basic shape, cutouts  22  are introduced into sheet-metal blank  20 . Cutouts  22  are regularly distributed over a portion of the surface of tubular spring  11 , cutouts  22  being embodied approximately in the shape of an “8”. Cutouts  22  are produced by punching or similar methods. Webs  23  are left behind between cutouts  22  as a result of the machining process, and are responsible for the supporting and resilient action of tubular spring  11 . As a result of the “8”-shape of cutouts  22 , tubular spring  11  is flexible in terms of its axial extension: it can easily be extended or compressed. The spring constant of tubular spring  11  is influenced by the number, shape, size, and placement of cutouts  22 . According to the first example embodiment depicted in  FIG. 2 , cutouts  22  are embodied in a central region  32  of the tubular spring, whereas end regions  33  remain free of cutouts  22  and, aside from assembly engagements  24  described in more detail below, have a continuous surface. In the second example embodiment depicted in  FIGS. 5 and 6 , cutouts  22  are embodied both in central region  32  and in one of end regions  33 .  
         [0025]     Assembly engagements  24  that are required for the assembly of actuator  10  in tubular spring  11  are provided according to the present invention. In the first example embodiment depicted in  FIG. 2 , assembly engagements  24  are provided in both end regions  33  of tubular spring  11 , whereas according to the second example embodiment depicted in  FIGS. 5 and 6 , assembly engagements  24  are embodied in only one of end regions  33 . The region in which cutouts  22  are embodied is larger as a result, so that tubular spring  11  becomes more elastic.  
         [0026]     As is apparent from the first example embodiment depicted in  FIG. 2 , for uniform force introduction, assembly engagements  24  are provided here at least in paired fashion opposite one another. In the second example embodiment depicted in  FIGS. 5 and 6 , on the other hand, at least three assembly engagements  24  are present, distributed equidistantly over the circumference of tubular spring  11  at identical angular spacings. Between assembly engagements  24 , further cutouts  34  can be provided which, like the additional number of cutouts  22 , make tubular spring  11  softer and thus enable a longer stroke for actuator  10 . The function of assembly engagements  24  is explained in more detail in the description relating to  FIGS. 3, 7A  and  7 B.  
         [0027]      FIG. 3  depicts the assembly of actuator  10  in a tubular spring  11  shown in  FIG. 2 .  
         [0028]     Actuator  10  is advantageously assembled in tubular spring  11  in such a way that a preload is generated in actuator module  9  by a controlled-force displacement. Despite production and stiffness tolerances, the preload force can be adjusted very accurately without the use of further components. The welding of actuator head  14  and actuator foot  13  to tubular spring  11  can be accomplished in a region unaffected by the preload force, so that the weld can be created in stress-free fashion and exhibits considerably better stability. This is made possible by the stiffness of tubular spring  11  in the region of assembly engagements  24 .  
         [0029]     Assembly is performed in the following sequence: First, tubular spring  11  is fitted onto actuator foot  13  and welded to it with a first weld seam  25 . Then actuator  10  is inserted into tubular spring  11 . While actuator  10  is held in contact against actuator foot  13 , centering pins  26  are inserted between electrical lines  19  and tubular spring  11  in order to center actuator  10  in tubular spring  11 . Actuator head  14  is then inserted, and slid into tubular spring  11  until it rests against actuator  10 . Actuator module  9  preassembled in this fashion is transportable without difficulty, since the sensitive actuator  10  is located in tubular spring  11  and protected from shear forces. Actuator module  9  is inserted into an assembly device  27  that is depicted schematically in  FIG. 3 , and clamping jaws  28  engage into outflow-end assembly engagements  24  of tubular spring  11 . A load cell  29  rests against actuator head  14 . Load cell  29  is first calibrated, and then clamping jaws  28  along with tubular spring  11  are displaced toward load cell  29 , as indicated by arrows  30 . The force F with which tubular spring  11  is pressed against actuator head  14  can be read off at any time directly from load cell  29 . Tubular spring  11  can then be displaced until the desired preload on actuator  10  is achieved. The magnitude of the force is based on requirements regarding the stroke of actuator  10 , and the desired activation characteristics. A progressive spring constant can also be attained with a suitable configuration of tubular spring  11 .  
         [0030]     Lastly, tubular spring  11  is welded to actuator head  14  via a second weld seam, 31 . The force path in tubular spring  11  is such that all components that are later to be under load are also stressed during preloading. Only end region  33  of tubular spring  11  in which assembly engagements  24  are embodied is not located in the force path, so that second weld seam  31  can be created in a stress-free state.  
         [0031]      FIG. 4  shows the completely assembled actuator module  9 . Tubular spring  11  is welded by way of weld seams  25  and  31  to actuator foot  13  and actuator head  14 , respectively. Assembly engagements  24  are required only in end region  33  of tubular spring  11  located closer to actuator head  14 , but may nevertheless be provided at both ends of tubular spring  11  as also depicted in  FIGS. 2 and 3 , both for reasons of symmetry in order to avoid any distortion of the spring characteristic and for cost reasons.  
         [0032]      FIGS. 7A and 7B  depict a completely assembled actuator module  9  having a tubular spring  11  as shown in  FIG. 6 , in a sectional view and a side view, respectively.  
         [0033]     The assembly process for a tubular spring  11  according to the second example embodiment, depicted in  FIGS. 5 through 7 , encompasses the following assembly steps. First, actuator  10  is fitted onto a fuel inlet  18  joined to an actuator foot  13 . Tubular spring  11  is then likewise fitted onto actuator foot  13 , and joined to it in stress-free fashion via a first weld seam  25 . Tubular spring  11  is then preloaded by tension on assembly engagements  24 . Tubular spring  11  is first overextended beyond the necessary tensile force, and the tensile force is then reduced to the necessary tensile force of approximately 600 N.  
         [0034]     Lastly, tubular spring  11  is welded to actuator head  14  via a second weld seam  31 . The force path in tubular spring  11  is such that all components that are later to be under load are also stressed during preloading. Only end region  33  of tubular spring  11  in which assembly engagements  24  are embodied is not located in the force path, so that second weld seam  31  can be created in a stress-free state.  
         [0035]     The present invention is not limited to the example embodiments depicted, and is applicable in particular to a plurality of fuel injection valve designs. All the features of the example embodiment may also be combined with one another.