Abstract:
A push button connection for an injection device comprising a push button ( 10 ) and a driving part ( 20 ). The two parts of the push button connection are mounted to each other and is relatively rotatable to each other. In order to minimize the friction occurring between the push button and the driving part when relatively rotated forces are transmitted via a pivot bearing ( 18, 22 ). In order also to minimize the effect of forces appearing dislocated from the center line a number of radial bearings ( 13, 23; 14, 25 ) having a little friction diameter is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. §371 national stage application of International Patent Application PCT/EP2008/050624 (published as WO2008/095762), filed Jan. 21, 2008, which claimed priority of European Patent Application 07101729.7, filed Feb. 5, 2007; this application further claims priority under 35 U.S.C. §119 of U.S. Provisional Application 60/899,977, filed Feb. 7, 2007. 
    
    
     THE TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a push button connection for an injection device and especially to such connection where a push button is rotated relatively to a driving member to which it is connected. 
     DESCRIPTION OF RELATED ART 
     U.S. Pat. No. 6,235,004 discloses an injection device in which according to FIG. 15-16 a dose is set by rotating the scale drum out of the housing in a threaded connection. When injecting the set dose the user pushes on the push button which forces the scale drum and the bushing to rotate together back into the housing. During this rotation of the bushing to which the push button is attached, the push button and the bushing rotates relatively to each other. The friction occurring between these relatively rotatable parts contributes to the force a user needs to apply in order to push back the bushing and the scale drum in order to inject the set dose. 
     U.S. Pat. No. 7,427,275 discloses an injection device in which the push button is formed with a bore encompassing a stem on a sleeve member. The push button and the stem are welded together such that the push button and the sleeve member are axially and rotatably fixed to each other. 
     DESCRIPTION OF THE INVENTION 
     It is an object of the present invention to provide a dose button connection for an injection device which minimizes the forces a user must apply to inject a dose. 
     When a user pushes on the injection button, the force applied is directed to the forward movement of the driving part, however, since the push button and the driving part rotate relatively to each other a friction between these rotating parts will occur. The user therefore also has to apply a force large enough to overcome this friction. One way of minimizing the force a user must apply in order to perform an injection is therefore to minimize this friction. By forming a pivot bearing between the two parts, the surface area of interaction between the two objects can be minimized and the radius of the resulting friction force can be kept at a minimum. 
     In order to secure the fit between the push button and the driving part and on the same time direct forces applied on the periphery of the push button to the driving part at least one radial bearing between the push button and the protrusion is formed. 
     Preferably one radial bearing is formed in the upper area and one is formed in the lower area both having the least possible radius of friction. In this way forces applied at in the periphery area of the push button and causing tilting of the push button on the protrusion of the driving part is properly transferred. 
     If a user applies a force eccentric to the centre axis of the push button i.e. on a peripheral area of the button, the push button will tilt and a reaction torque will occur at the radial bearings. In order to minimize this force pair, which in this load case is located at the distance from the radial bearing surface to the centre axis of the system, this distance, which again equals the radius of the protrusion, must be as little as possible and the distance between the bearings as long as possible. However, in order not to make the protrusion too narrow and fragile it is preferred to balance the radius of the bearings, such that the upper bearing has the smallest diameter and the lower bearing at the root of the column shaped protrusion has a diameter large enough to resist the bending force as a result of the offset applied push button forces. 
     In order to assemble the push button in an irreversible manner making it difficult to dissemble, it is preferred to secure the push button at the intended position by adding a track into which a rim on the harder part is forced during the manufacture of the injection device. The necessary compliance of the push button for the assembly snap-on can be secured by selection of a soft material and/or a vertical slit in the hollow section of the geometry. 
     Further the materials used for the push button and the protrusion on the driving part could be materials having low internal friction, or the materials could be surface treated in order to lower the internal friction. 
     The push button used in the connection has a central bore dedicated to engage the protrusion provided on the driving part. The bottom of the bore is preferable formed with a pivot. This pivot bears on a surface of the protrusion thus forming a pivot bearing. 
     DEFINITIONS 
     An “injection pen” is typically an injection apparatus having an oblong or elongated shape somewhat like a pen for writing. Although such pens usually have a tubular cross-section, they could easily have a different cross-section such as triangular, rectangular or square or any variation around these geometries. 
     As used herein, the term “drug” is meant to encompass any drug-containing flowable medicine capable of being passed through a delivery means such as a hollow needle in a controlled manner, such as a liquid, solution, gel or fine suspension. Representative drugs includes pharmaceuticals such as peptides, proteins (e.g. insulin, insulin analogues and C-peptide), and hormones, biologically derived or active agents, hormonal and gene based agents, nutritional formulas and other substances in both solid (dispensed) or liquid form. 
     All references, including publications, patent applications, and patents, cited herein are incorporated by reference in their entirety and to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     All headings and sub-headings are used herein for convenience only and should not be constructed as limiting the invention in any way. 
     The use of any and all examples, or exemplary language (e.g. such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. The citation and incorporation of patent documents herein is done for convenience only and does not reflect any view of the validity, patentability, and/or enforceability of such patent documents. 
     This invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which: 
         FIG. 1  Show a cross section view of the connection between a push button and a driving part. 
         FIG. 2  Show a cross section view of the connection and the forces occurring. 
     
    
    
     The figures are schematic and simplified for clarity, and they just show details, which are essential to the understanding of the invention, while other details are left out. Throughout, the same reference numerals are used for identical or corresponding parts. 
     DETAILED DESCRIPTION OF EMBODIMENT 
     When in the following terms as “upper” and “lower”, “right” and “left”, “horizontal” and “vertical”, “clockwise” and “counter clockwise” or similar relative expressions are used, these only refer to the appended figures and not to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as there relative dimensions are intended to serve illustrative purposes only. 
     In that context it may be convenient to define that the term “distal end” in the appended figures is meant to refer to the end of the injection device carrying the injection needle whereas the term “proximal end” is meant to refer to the opposite end pointing away from the injection needle. 
       FIG. 1  discloses the connection between the push button  10  and the driving part  20 . 
     When a user wants to inject a dose, which he or she has first selected, the user pushes the push button  10  which then moves the driving part  20  axially forward in the injection device. During this forward movement of the driving part  20  it also rotates usually because it is interfaced with a dose dial drum which is threadedly connected to a housing. Such injection device is described in details in EP 1.003.581. The combined axial and rotatable movement of the driving part  20  drives the set dose out from the injection device. 
     As the users finger pushes on the push surface  11  of the push button  10  it is unable to rotate due to the friction between the users finger and the push button  10  whereas the driving part  20  is forced to rotate due to its interface, therefore a relative rotation between the push button  10  and the driving part  20  takes place. 
     The push button  10  which could be manufactured from a suitable polymeric material being softer that the material from which the driving part  20  is manufactured comprises at the proximal end a push surface  11  which is contacted by the user&#39;s finger when a dose is to be injected and an opposite located cylindrical bore  12  with a circular cross section. The most proximal part  13  of the bore  12  has a smaller diameter than the remaining part  14  of the bore  12 . At the distal end of the bore  12 , a radial extending track  15  is provided. 
     The push surface  11  could be provided with a tactile cut-out  16  informing visual impaired users on the content of the injection device and the most proximal bottom surface  17  of the bore  12  is formed with a raised pointer forming a pivot  18 . 
     The driving part  20  is provided with a protrusion  21  having a circular cross section and a top surface  22 . This protrusion  21  has at its proximal end a top part  23  with a decreased diameter compared to the remaining part  26  of the protrusion  21 . Further the protrusion  21  is provided with a radial extending rim  24  at its distal end. In the area around this rim  24 , the protrusion  21  is provided with a belt  25  with a slightly raised diameter. 
     When the push button  10  is mounted on the protrusion  21  of the driving element  20  it is simply clicked on such that the extending rim  24  enters the track  15 . This forms a connection almost impossible to disconnect since the polymeric material of the push button  10  is softer than the material from which the protrusion  21  is produced. In this position the pivot  18  formed in the most proximal bottom surface  17  of the bore  12  bears on the top surface  22  of the protrusion  21  thus forming a pivot bearing  22 ,  18 . Further the push button  10  is radially supported by the protrusion  21  at the top part  23  forming a radial top bearing  23 ,  13 . The belt  25  on the protrusion  21  bears on an area of the remaining part  14  of the bore  12  thus forming a radial bottom bearing  14 ,  25 . 
     In  FIG. 2  the push button  10  connection is disclosed with the various forces occurring when a user applies an injection force in the peripheral area of the push button  10 . 
     When the user applies an injection force A at the peripheral area of the push button  10  a vertical reaction force B will appear at the pivot point  22 ,  18 , at the same time a radial force C will occur at the upper radial bearing  13 ,  23 . Since the upper radial bearing  13 ,  23  are located at the top part  23  having the smaller diameter, the resulting torque is relatively small. Further, a radial force D will occur at the lower radial bearing  14 ,  25 , however due to the distance between the upper radial bearing  13 ,  23  and the lower radial bearing  14 ,  25 , the force resulting on the lower radial bearing  14 ,  25  is relatively small. 
     Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject matter defined in the following claims.