CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 2004/000565 filed on Mar. 19, 2004. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an improved actuator unit comprised of a piezoelectric actuator. 
     2. Description of the Prior Art 
     Actuator units of the type with which this invention is concerned are use among other things in fuel injection systems and in particular in fuel injection valves since the switching times of such actuator units are very short. The short switching times permit a more exact metering of the injected fuel quantity and permit an improved shape of the injection curve over time. In connection with the present invention, the generic term “fuel injection valve” is understood to mean all types of fuel injection values, for example, the injectors for common rail injection systems or injection nozzles of conventional fuel injection systems. A fuel injection valve with a piezoelectric actuator is triggered by applying an electric voltage to the piezoelectric actuator; this causes a rapid expansion of the piezoelectric actuator due to known physical effects of piezoelectric ceramic and causes a valve-closure member to lift away from a valve seat. The piezoelectric actuator has a certain mass that is accelerated in the course of this. If the voltage applied to the actuator is reduced, this causes the actuator to contract. As a function of the triggering speed, the inertia of the previously accelerated mass of the actuator generates damaging tensile forces in the piezoelectric actuator, in particular causing fractures in the soldered connections between the individual layers of the piezoelectric actuator. In order to prevent this kind of damage, it has become common practice to use a cylindrical hollow body in the form of a spring to prestress the piezoelectric actuator in the axial direction. A device of this kind is known, for example, from WO 00/08353 (Siemens). This hollow body is bent from a flat metal sheet and welded at the first joint thus produced. The first joint extends parallel to the longitudinal axis of the hollow body. 
     Among other things, the welding of the first joint has the following disadvantages: the welding causes a generally undesirable structural change to the hollow body in the immediate vicinity of the welding seam. A second problem is the spatters produced during the welding, which can lead to difficulties in assembly of the actuator unit or can even lead to functional failures of the fuel injection valve when one or more spatters come loose during operation. A third problem is a sinking-in of the welding seam (seam sinkage) at the beginning and end of the welding seam and the resulting notch effect and voltage spikes. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     The actuator unit according to the present invention has a hollow body and a piezoelectric actuator. The hollow body is elastically embodied, prestresses the actuator, is provided with apertures recesses, has a joint extending parallel to the longitudinal axis, has a bridge piece between each pair of recesses, and has a first end and a second end. According to the present invention, the recesses adjacent to the joint are smaller than the rest of the recesses. 
     Alternatively, it is also possible according to the present invention for the bridge piece between a recess adjacent to the joint and another recess adjacent to that recess to be wider than the bridge pieces between the rest of the recesses. 
     A disadvantage in actuator units with hollow bodies whose joints are not closed is that the spring rigidity in the axial direction is not constant over the entire circumference. As a rule, the spring rigidity of the hollow body is reduced in the region of the joint. This causes the hollow body to act on the piezoelectric actuator with forces in the axial and radial direction in addition to bending moments. This results in the uneven exertion of forces and bending moments on the piezoelectric actuator, which is undesirable. 
     The embodiments according to the present invention, which can be comprised in embodying the recesses adjacent to the joint as smaller than the rest of the recesses of the blank and/or embodying the bridge pieces in the region of the joint as wider than in the rest of the blank, serve to intentionally reinforce the hollow body in the regions in the immediate vicinity of the joint so as to compensate for the reduction in the spring rate in the region of the joint. It is therefore possible to achieve a spring rate of the hollow body that is constant and/or rotationally symmetrical over its entire circumference so that the piezoelectric actuator that the spring force of the hollow body acts on is loaded with forces exclusively the axial direction and not with lateral forces or bending moments. This can significantly extend the service life of actuator units equipped with a hollow body according to the present invention. It has turned out to be advantageous if the ratio of the width of a bridge piece between a recess adjacent to the joint and a recess adjacent to that recess to the width of the remaining bridge pieces of the blank has a value between 1.3 and 1.9, preferably 1.6. This means that the bridge pieces in the immediate vicinity of the joint are wider, for example by a factor of 1.6, than the rest of the bridge pieces of the blank. 
     In special cases, it can also be helpful to embody the width of the bridge pieces as a function of the load; the widths of the bridge pieces can differ from one another by up to a factor of 3. 
     The recesses in the blank are advantageously disposed so that when the blank is formed into a hollow body, they are disposed in planes and the planes extend parallel to one another. This improves the behavior of the hollow body and makes it easier to manufacture. 
     It is particularly advantageous if the recesses are disposed in an odd number of planes in the axial direction. In exemplary embodiments that were tested in practice, 15 or 17 turned out to be an advantageous number of planes. Providing an odd number of planes in the blank assures that the uppermost and lowermost planes are the same so that the behavior of the hollow body at its upper end is the same as the behavior of the hollow body at its lower end. This measure also improves the behavior of the hollow body in that at its end faces, the hollow body only transmits axially oriented spring forces to the piezoelectric actuator, a booster piston of a hydraulic coupler, or another component of the injector. 
     It has also turned out to be advantageous if a number of recesses are disposed one after another in a plane and this plane forms a right angle with the longitudinal axis of the hollow body. It is particularly advantageous if there is an even number of recesses in a plane. This arrangement results in the fact that the spring rate is constant over the circumference of the hollow body and consequently, no lateral forces are transmitted to the actuator. 
     For reasons involving the manufacture and durability of the hollow body, it has turned out to be advantageous if the recesses are embodied as bone-shaped and extend lateral to a longitudinal axis of the hollow body. 
     The “bone-shaped” geometry of the recesses can be described in that the recesses are comprised of a middle piece and two head pieces; the head pieces have at least a first radius, the middle piece has a second radius, and the recesses have a length. Various trials have shown various proportions to be favorable among the principle measurements of the first radius (R 1 ), the second radius (R 2 ), and the length (L), as well as the width of the bridge piece at the joint in relation to the width of the rest of the bridge pieces: 
     In a favorable embodiment form, the radius R 1  of a recess adjacent to the joint is smaller by a factor of 0.867 than the radius R 1  of the rest of the recesses. In addition, the second radius R 2  of a recess adjacent to the joint is larger by a factor of 1.317 than the radius R 2  of the rest of the recesses of the blank. Moreover, the length of a recess adjacent to the joint is shorter by a factor of 0.984 than the length of the rest of the recesses. The width of the bridge piece at the joint is expressed by the equation b&gt;a/2; in particular b=1.4·a/2. A detailed description of the related values, in particular the values “a” and “b,” is given below in conjunction with the drawings. 
     In another exemplary embodiment, it has also turned out to be advantageous if the recesses adjacent to the joint have the following dimensions:
         R 1 =0.35 mm-0.43 mm, in particular 0.39 mm   R 2 =4.0 mm-8.9 mm, in particular 5.0 mm to 7.9 mm   L=3.5 mm-4.5 mm, in particular 4.0 mm.       

     In another embodiment form, the recesses adjacent to the joint have the following dimensions:
         R 1 =0.41 mm-0.49 mm, in particular 0.45 mm   R 2 =5.5 mm-6.5 mm, in particular 6.0 mm   L=3.7 mm-4.7 mm, in particular 4.2 mm.       

     For the rest of the recesses that are not adjacent to the joint, the following dimensions have turned out to be favorable:
         R 1 =0.43 mm-0.51 mm, in particular 0.47 mm   R 2 =4.0 mm-4.8 mm, in particular 4.4 mm   L=4.5 mm-5.5 mm, in particular 5.0 mm.       

     In another advantageous exemplary embodiment, the recesses that are not adjacent to the joint have the following dimensions:
         R 1 =0.4 mm-0.5 mm, in particular 0.45 mm   R 2 =5.5 mm-6.5 mm, in particular 6.0 mm   L=4.0 mm-4.5 mm, in particular 4.255 mm.       

     It has also turned out to be advantageous if the first radii of the head pieces of a recess adjacent to the joint are different from each other, which will be explained by way of example below in conjunction with  FIG. 8   c.    
     It is also advantageous if the recesses of two adjacent planes are offset from one another. It is particularly advantageous if the offset of the recesses of two adjacent planes is equal to half the repeat pattern of the recesses of a plane. The term “repeat pattern” will be explained in greater detail below in conjunction with  FIG. 2 . It is particularly advantageous if the hollow body has a circular cross section or if the cross section of the hollow body is the shape of a regular polygon. 
     According to the present invention, the hollow body can also have a region that is not perforated by recesses at its first end and/or at its second end. As a result of this, the spring force transmitted by the hollow body to a cover plate or another component of the injector is comparatively uniform since the hollow body is intentionally reinforced in the region of its ends. This translates into a reduction in the maxima of the spring force over the circumference of the hollow body and further alleviates the problem of lateral forces introduced into the piezoelectric actuator by the hollow body. 
     The hollow body according to the present invention can be used in actuator units in which the piezoelectric actuator is disposed inside the hollow body and in which the prestressed hollow body acts on the piezoelectric actuator with compression. This means that the hollow body itself is loaded with tension. 
     The hollow body according to the present invention can, however, also be used in actuator units in which the piezoelectric actuator is disposed outside the hollow body and the prestressed hollow body acts on the piezoelectric actuator with compression. In this case, the hollow body is usually loaded with compression. 
     In order to be able to transmit the prestressing force of the hollow body to the piezoelectric actuator in the best possible way, it is advisable for the first end of the hollow body to be connected to an upper cover plate or an adjusting disk and for its second end to be connected to a lower cover plate or a coupler housing. These connections can be produced, for example, by means of welding or crimping. 
     If only a radial fixing of the hollow body is required, then this can occur by means of an annular groove or a shoulder in the upper and/or lower cover plate or in the adjusting disk and coupler housing. This can be sufficient, for example, if the hollow body is loaded not with tension but with compression. In these embodiment variants, it is particularly advantageous that the annular groove and the shoulder center the hollow body in relation to the piezoelectric actuator or to the hydraulic coupler. This effect can be further improved if the annular groove and shoulder are dimensioned so that they cause the hollow body to flare slightly during assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will become apparent from the description herein below, taken with the drawings, in which: 
         FIG. 1  shows a first exemplary embodiment of an actuator unit according to the present invention, 
         FIG. 2  shows a second exemplary embodiment of an actuator unit according to the present invention, 
         FIG. 3  shows an example for a blank from which a hollow body is bent, 
         FIG. 4  shows a perspective view of a first exemplary embodiment of a hollow body, 
         FIG. 5  shows an exemplary embodiment of a blank from which a hollow body according to the present invention is bent, 
         FIG. 6  shows a perspective view of a hollow body that has been bent from a blank according to  FIG. 5 , 
         FIG. 7  shows another exemplary embodiment of a blank for manufacturing a hollow body according to the present invention, 
         FIG. 8  shows another exemplary embodiment of a blank for manufacturing a hollow body according to the present invention, 
         FIG. 9  depicts the forces that can be transmitted to a hollow body according to the present invention according to  FIG. 8 , 
         FIG. 10  shows another exemplary embodiment of a blank for manufacturing a hollow body according to the present invention, 
         FIG. 11  depicts the forces that can be transmitted to a hollow body according to the present invention according to  FIG. 10 , 
         FIG. 12  shows another exemplary embodiment of a blank for manufacturing a hollow body according to the present invention, and 
         FIG. 13  schematically depicts a fuel injection system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a first exemplary embodiment of an actuator unit according to the present invention. The actuator unit is comprised of a piezoelectric actuator  1 , which can be comprised of a number of stacked individual piezoelectric elements (not shown). The piezoelectric actuator  1  is triggered via contacting pins  2  that are disposed alongside the actuator  1  and are connected to the actuator  1  in an electrically conductive fashion. The application of a voltage between the contact pins  2  produces a longitudinal expansion of the piezoelectric actuator  1 , which is used, for example, to control an injection valve in an internal combustion engine. The piezoelectric actuator  1  with the contact pins  2  is disposed inside a hollow body  4  embodied in the shape of a tubular spring. The ends of the piezoelectric actuator  1  each rest against a respective cover plate  5 ,  6 , the upper cover plate  6  being provided with feedthroughs  61  through which the contact pins  2  extend. The upper and lower cover plate  5 ,  6  are each connected to the hollow body  4  in a form-locked manner and/or by frictional engagement, preferably by means of welding. The welding seams between the upper and lower cover plate  5 ,  6  and the hollow body  4  are not shown in  FIG. 1 . Alternatively, the connection between the hollow body and the two cover plates  5 ,  6  can also be produced, for example, by means of a crimp, the crimped upper and lower edge regions of the hollow body  4  each engaging with the cover plates  5 ,  6  (not shown). 
     The hollow body  4  and the cover plates  5 ,  6  act on the piezoelectric actuator  1  with compression by means of a prestressing force. This means that before being welded to the upper and lower cover plate  5 ,  6 , the hollow body  4  is prestressed and then welded. 
     The hollow body  4  is preferably made of spring steel. The hollow body  4  is provided with a multitude of apertures, recesses  7  in order to be able to set a desired spring rate with a predetermined wall thickness “s.” For the sake of clarity, not all of the recesses in  FIG. 1  are provided with reference numerals. Since the multitude of recesses  7  can best be produced by means of punching, the hollow body  4  is as a rule comprised of sheet metal. First, a blank with the recesses  7  is stamped out of the metal sheet. Then, the blank is bent until it has a circular cross section, for example, or a cross section in the shape of a regular polygon. This produces a first joint where the two ends of the bent blank meet each other (not shown in  FIG. 1 ). 
       FIG. 2  shows a second exemplary embodiment of an actuator unit according to the present invention, which is integrated into a piezoelectrically actuated injector  71 . 
     Since the present invention essentially relates to an actuator unit and a hollow body  4  associated with it, not all of the details the injector  71  are explained; instead, essentially only the connection of the actuator unit to the injector  71  is described. The remaining functionalities of the injector  71  are already known to those skilled in the art in the field of injection technology and therefore require no further explanation. 
     The injector  71  has a high-pressure connection  73 . Highly pressurized fuel (not shown) is supplied to the injector  71  via the high-pressure connection  73 . If an injection into the combustion chamber, not shown, of an internal combustion engine is to take place, a nozzle needle  75  lifts away from its seat, not shown, and unblocks injection orifices that are also not shown. A piezoelectric actuator  79  actuates a control valve  77 , which controls the nozzle needle  75 . Between the piezoelectric actuator  79  and the control valve  77 , a hydraulic coupler  81  is provided, an enlargement of which is depicted on the right side of  FIG. 2 . 
     The hydraulic coupler  81  is essentially comprised of a valve piston  83  and a booster piston  85  that are guided in a coupler housing  86 . Between the valve piston  83  and the booster piston  85 , there is a coupler gap  87 , which is filled with fuel (not shown). This coupler gap  87  is required, among other reasons, because the thermal expansion coefficients of the piezoelectric actuator  79  and the metallic components of the injector  71  differ greatly from each other. 
     By means of its valve piston  83 , the hydraulic coupler  81  actuates the control valve  77 , while a projection  89  of the booster piston  85  rests against the piezoelectric actuator  79 . A hollow body  4  according to the present invention, which is prestressed with compression, presses the booster piston  85  against the piezoelectric actuator  79 , thus acting on it with a compressive prestressing force. The first end  15  of the hollow body  4  rests against a shoulder  91  of the coupler housing  86 . The second end  17  of the hollow body  4  rests against an adjusting disk  93 . The adjusting disk  93  transmits the spring force of the hollow body  4  to the projection  89  of the booster piston  85  and therefore to the piezoelectric actuator  79 . 
     In order to position the hollow body  4  concentric to the hydraulic coupler  81  and therefore also concentric to the piezoelectric actuator  79 , the diameter D 1  of the shoulder  91  is matched to the inner diameter of the hollow body  4  so that the hollow body  4  flares slightly when it is slid onto the shoulder  91 . Since the hollow body  4  according to the present invention has a first joint  31  (not shown) extending over the entire length of the hollow body  4 , this allows the hollow body  4  to flare relatively easily so that it fits onto the shoulder  91 . 
     If, as in the exemplary embodiment according to  FIG. 1 , the hollow body  4  is acted on with a compressive prestressing force, then it is sufficient for it to be supported in the axial direction at its ends  17  and  15 , as shown in  FIG. 1 . In order to further improve the radial fixing of the hollow body  4 , an annular groove (not shown) can alternatively or additionally be provided in the shoulder  91  and/or in the adjusting disk  93 . 
       FIG. 3  shows a blank  9  that can be curved to form a hollow body  4  according to the present invention. A multitude of recesses  7  are stamped out of the blank  9 . For the sake of clarity, reference numerals are not provided for all of the recesses  7 , which are bone-shaped in the exemplary embodiment according to  FIG. 2 . The blank  9  is rectangular; two opposite edges  11  and  13  of the blank  9  are interrupted by the recesses  7 , while the opposite edges  15  and  17  extend in straight lines, uninterrupted by the recesses  7 . 
     The blank  9  is curved or bent to form a cylindrical or polygonal hollow body so that the edges  15  and  17  constitute the first end  15  and the second end  17  of the hollow body  4  (see  FIG. 4 ), i.e. the longitudinal axis  35  not shown in  FIG. 2  (see  FIG. 4 ) of the hollow body  4  extends parallel to the edges  11  and  13 . 
     When the blank  9  is bent in the above-mentioned fashion to form a cylinder or polygon, the edges  11  and  13  touch each other and form a first joint  31  (see  FIGS. 4 and 5 ), which extends parallel to the longitudinal axis  35  of the hollow body  4 . 
     The blank  9  contains groups of recesses  7 , each of which comprises a number of recesses in a row. The recesses are separated from one another by bridge pieces  19 . Here, too, for the sake of clarity, not all of the bridge pieces  19  of the blank  9  have been provided with reference numerals. When the blank  9  is bent to form a hollow body in the manner described above, the recesses  7  disposed in a row lie in a plane. By way of example, a line  20  in  FIG. 3  indicates a row of recesses  7  that are disposed one after another. In the exemplary embodiment of a blank  9  shown in  FIG. 3 , sixteen rows of six recesses  7  are disposed between the edge  15  and the edge  17 . 
     As is clear from  FIG. 3 , the recesses  7  of two adjacent rows are offset from one another. The offset is selected so that it corresponds to half of the length of one recess  7  and one bridge piece  19 . This measurement for one recess and two half bridge pieces  19  is indicated by way of example in  FIG. 3  by the double arrow  21 . This measurement is also referred to as the “repeat pattern.” The offset between the recesses  7  of two adjacent rows of recesses is labeled with the reference numeral  23  in  FIG. 3 . 
     When the blank  9  is rolled to form a hollow body  4  (see  FIG. 4 ) and the ends of this hollow body  4  are acted on with a compressive force via an upper cover plate  5  (see  FIG. 1 ) and a lower cover plate  6  (see  FIG. 1 ), then the force F acting between the upper cover plate  5  and the edge  15  over the circumference of the hollow body  4  has the curve qualitatively depicted by the line  25  (see  FIG. 5 ). The circumference angle φ begins with 0° at the edge  13  and ends with 360° at the edge  11 . 
     It is clear that wherever a bridge piece  19  “supports” the edge  15 , a large force F can be transmitted, as indicated by the maxima  27  of the line  25 . The sole exception is where the edges  11  and  13  abut each other. The “cut” recess  7  there, with its parts  7 ′ and  7 ″, weakens the structure of the blank  9  so that the force F transmitted between the upper cover plate  5  and the hollow body  4  is weaker at this point. This fact is indicated in  FIG. 3  by the maxima  27  of significantly lower value for the force F at φ=0° and at φ=360°. 
     The edge  17  behaves similarly. As is clear from  FIG. 3 , in the immediate vicinity of the edge  17  at φ=0° and 360°, there is a cut recess comprised of the parts  7 ′ and  7 ″ whereas in the immediate vicinity of the edge  15  at φ=0° and 360°, there is a split bridge piece  19  with the halves  19 ′ and  19 ″. This results in a somewhat different force curve over the circumference of the edge  17 . 
     As is clear from the lower F/φ graph in  FIG. 3 , there are four maxima  27  and two local maxima  29  in the vicinity of the edges  11  and  13  at the angles φ=30° and 330° that are significantly lower than the maxima  27 . 
     As a result of this circumferentially uneven transmission of force between the upper cover plate  6  and the edge  15  on the one hand and between the lower cover plate  5  and the edge  17  on the other hand, the hollow body  4  produces a bending moment that acts on the upper cover plate  6  and the lower cover plate  5  when the hollow body  4  is attached with a prestressing force to the upper and lower cover plates  6 ,  5 . This bending moment is naturally also transmitted to the piezoelectric actuator  1 , which has an unfavorable effect on its operational reliability and lifespan. This bending moment is also undesirable in hydraulic valve elements actuated by the actuator unit. 
       FIG. 4  shows a perspective view of a hollow body  4 , which has been manufactured from a blank  9  shown in  FIG. 3 . The rows of recesses  7 , which are not individually labeled in  FIG. 4 , constitute sixteen planes E 1  to E 16  that extend perpendicular to the longitudinal axis  35  of the hollow body  4 . To illustrate this, one plane E 2  is indicated in  FIG. 4 . The wall thickness s of the hollow body  4  is also indicated in  FIG. 4 . 
       FIG. 5  shows a blank  9  that can be used to manufacture a hollow body  4  according to the present invention. It is clear from the full depiction of the blank  9  that a total of seventeen rows of recesses  7  are provided. When the blank  9  is formed into a hollow body, these seventeen rows constitute seventeen planes in which the recesses  7  are disposed. The edges  11  and  13  constitute the joint  31  in the hollow body. The edges  17  and  15  constitute a first end and a second end in the finished hollow body  4 . This is why in connection with the finished hollow body  4 , the reference numeral  17  is used for the first end of the hollow body  4  and the reference numeral  15  is used for the second end of the hollow body  4 . 
     According to the present invention, in the blank  9 , the recesses  7   a  and  7   b  adjacent to the edges  11  and  13  have a geometry that has been altered in comparison to the rest of the recesses  7 , not all of which have been provided with reference lines. The different geometries of the recesses  7 ,  7   a , and  7   b  will be explained in greater detail below in conjunction with the detail A from the blank  9 . In this exemplary embodiment, the recesses  7   a  and  7   b  have the same geometry. As is clear from  FIG. 4 , the recesses  7 ,  7   a , and  7   b  are “bone-shaped.” Each recess  7 ,  7   a ,  7   b  is comprised of a middle portion  37  and two head portions  39  adjoining this. The reference numerals  37  and  39  have been attached by way of example to only a single recess  7 . The head portion  39  can be quantitatively described by a first radius R 1  while the middle portion  37  can be quantitatively described by a second radius R 2 . Another important geometric value of the recesses  7 ,  7   a , and  7   b  is the length L. It has turned out to be advantageous here if the first radius of the recesses  7   a  and  7   b  is smaller by a factor of 0.867 than the first radius of the recesses  7 . It has also turned out to be advantageous if the second radius R 2  ( 7   a ,  7   b ) of the recesses  7   a  and  7   b  is greater by a factor of 1.317 than the second radius R 2  of the recesses  7  and if the length L of the recesses  7   a  and  7   b  is shorter by a factor of 0.984 than the length of the recesses  7 . 
     There is a bridge piece  19  between each pair of recesses  7 . The first row of recesses  7  that are disposed in the immediate vicinity of the edge  17  is comprised of six recesses  7 . The six recesses  7  of the first row are disposed so that one recess is split. This recess  7  is divided into two symmetrical halves by the edges  11  and  13 . 
     The second row contains four recesses  7  and one each of recesses  7   a  and  7   b . The recesses  7   a  and  7   b  are disposed so that they are in the immediate vicinity of the edges  11  and  13 . Since the recesses  7   a  and  7   b  are smaller than the recesses  7 , the hollow body  4  is reinforced at a circumference angle φ of 30° and a circumference angle φ of 330°, namely in those places where the recesses  7   a  and  7   b  influence the spring rate of the hollow body  4 . This reinforcing in the region of the circumference angles of φ=30° and 330° compensates for the weakening of the hollow body  4  by the joint  31  disposed between the edges  11  and  13  (see  FIG. 4 ). The result of this measure is clearly shown in the F/φ graph shown above the blank  9 . In comparison to  FIG. 3 , in which there is a significant drop in the transmittable force in the vicinity of the circumference angles φ=30° and 330°, in the F/φ graph in  FIG. 5 , there are six maxima  27 , that all represent the same amount. This means that a hollow body  4  manufactured from the blank  9  according to  FIG. 5  has a uniform spring rate over the circumference of its ends  15  and  17  so that the spring force transmitted by the hollow body  4  to an upper or lower cover plate and/or a shoulder  91  or  93  acts exclusively in the axial direction and does not exert any lateral forces or bending moments on the components on which the spring force of the hollow body  4  acts. A blank  9  according to  FIG. 5  can therefore attain the object according to the present invention. 
     With regard to the width of the bridge pieces  19 , which is labeled with “a” in detail A, and the width “b” of the half bridge pieces  41  between a recess  7   a  and the edge  11  and between a recess  7   b  and the edge  17 , respectively, the following quantitative relationship has turned out to be advantageous. The width b of the half bridge piece  41  should be greater than a/2, in particular, should reflect the equation b=1.4·a/2. 
       FIG. 6  is a perspective view of a detail from a hollow body  4  according to the present invention. It is clear from this depiction that the recesses  7   a  and  7   b  are disposed in the immediate vicinity of the joint  31 . 
       FIG. 7  depicts a blank  9  and a detail of the blank  9 , which show the dimensions of the recesses  7  and of the entire blank. This blank  9  has only recesses  7  and no recesses with the different geometry ( 7   a  and  7   b ). 
       FIGS. 8   a,    8   b,  and  8   c  show blanks  9  or details of blanks  9 , with a dimensional depiction of the recesses  7   a  and  7   b  adjacent to the joint  31 . These exemplary embodiments are also able to attain the object underlying the present invention, which is essentially comprised in achieving a prestressing of the piezoelectric actuator  1  and  79  in the axial direction without exerting any lateral forces. 
     The embodiment forms whose details are depicted in  FIGS. 8   b  and  8   c  have also turned out to be advantageous. A detailed explanation of this has been omitted here since the dimensions furnished in the above-mentioned figures are self-explanatory and the principle design of such a blank  9  has been described in detail in conjunction with  FIGS. 3 and 5 . 
     However, reference is made to the lower detail B in  FIG. 8   c . In it, the first radius R 1  of the recesses  7   a  and  7   b  at the end oriented toward the edges  11  and  13  (not shown) is composed of three arc segments  43 . In the middle, there is a first arc segment  43  with a radius of 0.6 mm, which is adjoined at both ends by two second arc segments  45  with a radius of 0.25 mm. The recesses  7   a  and  7   b  whose geometry is described in conjunction with  FIG. 8   c  are exemplary embodiments for recesses in which the first radii of the head portions of a recess  7   a  or  7   b  adjacent to the joint differ from each other. 
       FIG. 9  is an F/φ graph of a hollow body  4  manufactured from a blank according to  FIG. 8 , in various load states. Three lines that correspond to three different forces F 1 , F 2 , and F 1  depict the load states. It is clear from  FIG. 9  that the spring rate of the hollow body  4  is constant over the circumference in a wide range of load states. 
       FIG. 10  shows another exemplary embodiment of a blank  9  for manufacturing a hollow body  4  according to the present invention. The blank  9  has the following differences from the blanks described above: 
     The blank  9  is not perforated in the region of the edges  15  and  17  that correspond to a second end and a first end of the hollow body  4 . This reinforces the hollow body  4  in the region of its first end  17  and in the region of its second end  15 , which reduces the value of the maxima  27  (see  FIG. 3 ,  FIG. 5 , and  FIG. 9 ). 
     A second essential measure for improving the hollow body  4  is comprised in individually adapting the width a of the bridge pieces  19  to the loads that occur. The bridge piece  19 . 1  in the first row of recesses  7  that are disposed in the immediate vicinity of the edges  11  and  13  is thus wider than a bridge piece  19 . 2  that is disposed in the blank, farther away from the edges  11  and  13 . In the exemplary embodiment shown, the width a 1  of the bridge piece  19 . 1  adjacent to the edges  11  and  13  is 1.2 mm, whereas the other bridge pieces  19 . 2  have a width a 2  of only 0.75 mm. Depending of the dimensioning of the bridge piece widths a 1  and a 2 , there can even be an overcompensation for the weakening of the hollow body  4  due to the presence of the joint  31 . This effect is demonstrated in  FIG. 11 , which is an F/φ graph. If the bridge piece width a 1  is selected as shown in  FIG. 10   b , then all six maxima  27  are of the same amount. This design is indicated in  FIG. 10   b  by the “bridge piece width a 1 =1.2”. If the bridge piece width in the immediate vicinity of the edges  11  and  13  is further increased, then the spring rate of the hollow body at the circumference angles φ=30° and φ=330° is greater than in the angle regions between them. This results in a superelevation of the curve in the vicinity of the circumference angles 30° and 330°, which is indicated in  FIG. 11  by the line “bridge piece width  3 .” 
       FIG. 12  shows another exemplary embodiment of a blank  9  according to the present invention in which the bridge piece widths have been individually determined as a function of the load situation. The blank  9  is symmetrical in relation to a symmetry axis  47  so that the dimensioning of the detail A, which depicts a quadrant of the blank  9 , represents by reflection the overall dimensions of the entire blank  9  (not shown). The reference numerals  7 , R 1 , R 2 , L,  19 ,  21 , and others have been omitted from  FIG. 12  for the sake of clarity. It should also be noted with regard to  FIG. 12  that the same bridge piece widths are provided in the first row of recesses and in the 15th row of recesses. In addition, the bridge piece widths are the same in the second, fourth, sixth, eight, tenth, and fourteenth row of recesses. The bridge piece widths are also the same in the third, fifth, seventh, ninth, eleventh, twelfth, and thirteenth rows of recesses. 
     In conjunction with  FIG. 13 , the description below is intended to describe how the fuel injection valve  116  according to the present invention is integrated into a fuel injection system  102  of an internal combustion engine. The fuel injection system  102  has a fuel tank  104  from which an electrical or mechanical fuel pump  108  delivers fuel  106 . It feeds the fuel  106  via a low-pressure fuel line  110  to a high-pressure fuel pump  111 . From the high-pressure fuel pump  111 , the fuel  106  travels to a common rail  114  via a high-pressure fuel line  112 . A number of fuel injection valves  116  are connected to the common rail and inject the fuel  106  directly into combustion chambers  118  of an internal combustion engine that is not shown. 
     As a matter of course, each of the characteristics described in the specification, illustrated in the drawings, or recited in the claims can be essential to the present invention either individually or in combination with other characteristics. 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.
An actuator unit suitable for actuating a fuel injection valve of an injection system for internal combustion engines is comprised of a piezoelectric actuator and a hollow body embodied in the form of a spring. Embodying the hollow body according to the present invention can extend the service life of the actuator unit.