Abstract:
A spring element has a substantially cylindrical housing ( 125 ) and at least one spring ( 128 ) arranged therein. At least one guidance member ( 129 ) that radially braces the spring ( 128 ) is arranged between an outer side of the spring ( 128 ) and an inner wall of the housing ( 125 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority of the German utility model application 202 04 321.5 filed Mar. 19, 2002 which is incorporated by reference herein.  
         FIELD OF THE INVENTION  
         [0002]    The invention concerns a spring element, in particular for use in a stand for surgical microscopes.  
         BACKGROUND OF THE INVENTION  
         [0003]    Surgical microscopes supported by stands must be easily pivotable over a predefined range, and must retain the position that is set. A stand having a spring apparatus for weight compensation is known from DE 37 39 080 A1, in which weight compensation is implemented by combining Bowden cables with springs. This, however, involves energy assistance of a displacement movement exerted by an operator on a handle. There is no teaching in this existing art as to holding a load in a counterbalanced “floating” state, as is desirable in the case of surgical microscopes.  
           [0004]    DE 197 42 050 A1 (1999) discloses a stand assemblage having a pivotable parallelogram carrier that is weight-compensated, by way of a Bowden cable and a weight compensation spring, in such a way that the counterweights which are additionally present can be made particularly small. In this assemblage, the Bowden cable is guided in a particular configuration in order to minimize, over a wide pivoting range of the pivot arm, the weight compensation errors resulting from the finite deflection radius. The weight compensation errors are not eliminated by this feature, however, so that in certain pivot positions it is still necessary to adjust the counterweights.  
           [0005]    U.S. Pat. No. 6,070,839 (2000) discloses a further assemblage having a pivot arm and a Bowden cable/spring design that makes possible pure weight compensation—in the manner of the weight compensation with diagonal bracing springs mentioned above—but without contributing equalization torques in order to improve tipover resistance.  
           [0006]    U.S. Pat. No. 5,253,832 (1999) describes a stand having a centrally arranged tension spring for weight compensation. This assemblage does not offer an easy adjustment capability for modified loads, so that changes to a microscope or to its accessories cannot readily result in an adjustment of the compensating forces. It is additionally disadvantageous that the tension spring has a different degree of compression and expansion depending on the pivot position of the carrier arm, which because of the spring characteristic results in differing compensation forces and thus in compensation behavior that differs over the pivot range of the carrier arm and is thus unusable for a user in the surgical context.  
           [0007]    Springs are ideal for particularly high-performance weight compensation in a small space. Both tension and compression springs are suitable, depending on the installation location. Springs that apply the same force over a specific compression or expansion travel would theoretically be ideal. Such springs are not usable, however, in a comparable assemblage having comparable parameters. Conventional springs are therefore preferably used, but by means of counter-compensation in the form of cams or the like they can tolerate a movement (up-down pivoting) of the load in order always to apply the same compensation force or compensation torques.  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore the object of the present invention to implement a stand assemblage having a conventional spring, and in that context also to make possible the use of long springs.  
           [0009]    This object is achieved, according to the present invention, by the features of claim 1. Advantageous developments of the invention are the subject matter of the dependent claims.  
           [0010]    In order to prevent buckling of the spring under compressive load in the case of long compression springs, at least one guidance member that radially braces the spring is arranged between the outer side of the spring and the inner wall of the housing. The guidance member is advantageously sleeve-shaped.  
           [0011]    In order to reduce friction and thus losses, it is advisable to equip the guidance member with rolling bearing elements, for example ball-bearing bushings, or to configure it as a plain bearing.  
           [0012]    In order to influence the characteristic of the spring elements, it is advantageous if an axial stop that limits the displacement travel of the guidance member is provided.  
           [0013]    In order to achieve nonlinear resulting spring characteristic curves, at least two compression springs, arranged axially behind one another and positioned in series, are provided, the springs being braced against one another via the axially displaceable guidance member and having identical or different spring constants.  
           [0014]    With identical spring constants there is then an inflection in the spring characteristic curve, since when the one spring is in the unlocked state, twice the travel can be enabled for a given force, while in the locked state for the same given force, only the distance itself can be traveled. In a further advantageous embodiment, the housing is embodied as a pneumatic cylinder having a piston axially displaceable in the cylinder, the spring enclosing at least a portion of the piston. The piston can also simultaneously be embodied as a guidance element. As a result, pneumatic spring forces and/or damping elements, as in the case of a shock absorber, can be utilized in addition to the spring force. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention is presented in exemplary embodiments and will be explained in more detail with reference to the schematic drawings, in which:  
         [0016]    [0016]FIG. 1 shows a spring element having a compression spring braced radially by a guidance element;  
         [0017]    [0017]FIG. 2 shows a variant of the spring element depicted in FIG. 1, having two compression springs axially braced against on another via a guidance member; and  
         [0018]    [0018]FIG. 3 shows a spring element combined with a pneumatic pressure cylinder.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The spring element evident from FIG. 1 substantially comprises a housing  125  and a plunger  126  mounted displaceably therein. A compression spring  128  is arranged between rear end wall  127  and plunger  126 . For clearer elucidation of the spring element, compression spring  128  is shown in shortened fashion. If a longer spring travel of plunger  26  is desired, compression spring  128  can also be dimensioned considerably longer. The risk then exists that compression spring  128  may buckle out radially and come into contact with the inner wall of housing  125 . To prevent this, an approximately cylindrically configured, axially displaceable guidance member  129  is arranged in the central region of compression spring  128 . Guidance member  129  can be made of plastic or of a bearing metal, e.g. bronze, so that defined friction conditions exist with the spring element in any service position. Guidance member  129  has two lugs  130  that engage into the turns of compression spring  128 . Guidance member  129  is thus entrained upon axial contraction of compression spring  128 , so that it is always located in the predefined region of compression spring  128  regardless of the compression of compression spring  128 , and optimum guidance thus results. With one guidance member  129 , guidance member  129  is located in the middle of spring  128 ; and with multiple members  129 , said guidance members  129  are distributed symmetrically along the longitudinal axis of spring  128 .  
         [0020]    The spring element shown in FIG. 2 comprises a housing  131  having a tension rod  132 , displaceable therein, having a head  133  of enlarged diameter. Between front end wall  134  of housing  131  and head  133 , two compression springs  135  and  136  are axially braced against on another via a guidance member  137 . Guidance member  137  is guided in housing  131  by way of rolling bearing elements  138 , for example ball-bearing bushings, which are characterized by very low friction. Compression springs  135 ,  136  can have identical or different spring properties. A stop  139  is provided in housing  131 . The action of compression spring  135  is limited by the arrival of guidance member  137  against stop  139 . A nonlinear spring characteristic over the entire spring travel of compression springs  135 ,  136  can thus be achieved. Stop  139  can, however, also be releasable, so that the spring characteristic curve of the system can be influenced.  
         [0021]    The spring element evident from FIG. 3 comprises a cylinder  140  having a pneumatic piston  141 . The spring element thus constitutes a system similar to a bicycle pump. Piston  141  has a head  142  and a shaft  143  with reduced diameter as compared to the latter. Shaft  143  is surrounded by a compression spring  144 , head  142  of piston  141  being braced in the axial direction against compression spring  144 . In its central region, compression spring  144  is radially braced in cylinder  140  via a sleeve-shaped guidance member  145 . Upon compression of compression spring  144 , guidance member  145  can co-move axially as a result of friction or, as in FIG. 1, can be connected via lugs  130  or other driver elements to the turns of compression spring  144 . Cylinder  140  and piston  141  coact in the manner of a gas spring. The desired spring characteristic curve of the overall spring element can be achieved by coordinating the spring characteristic of this gas spring with the properties of compression spring  144 .  
         [0022]    The elements of the springs shown in FIGS. 1 through 3 are, of course, interchangeable with one another, so that the spring element of FIG. 1 can also have rolling bearing  138  or plain bearing  129 .  
         [0023]    Parts List  
         [0024]    [0024] 125  Housing  
         [0025]    [0025] 126  Plunger  
         [0026]    [0026] 127  Rear end wall  
         [0027]    [0027] 128  Compression spring  
         [0028]    [0028] 129  Guidance member  
         [0029]    [0029] 130  Lug  
         [0030]    [0030] 131  Housing  
         [0031]    [0031] 132  Tension rod  
         [0032]    [0032] 133  Head  
         [0033]    [0033] 134  Front end wall  
         [0034]    [0034] 135  Compression spring  
         [0035]    [0035] 136  Compression spring  
         [0036]    [0036] 137  Guidance member  
         [0037]    [0037] 138  Rolling bearing element  
         [0038]    [0038] 139  Stop p 1   140  Cylinder  
         [0039]    [0039] 141  Piston  
         [0040]    [0040] 142  Head  
         [0041]    [0041] 143  Shaft  
         [0042]    [0042] 144  Compression spring  
         [0043]    [0043] 145  Guidance element