Abstract:
An air displacement pipette has a tubular pipette tip with an upper section surrounding a locking chamber, and a body section leading from the upper section and tapering downwardly to a reduced diameter end. A tubular mounting shaft on the pipette has a distal end configured and dimensioned for axial insertion into the locking chamber of the pipette tip. Coacting surfaces on the distal end of the mounting shaft and the upper section of the pipette tip establish an axially interengaged relationship between the pipette tip and the mounting shaft in response to insertion of the distal end of the mounting shaft into the locking chamber. A sleeve is axially shiftable on the mounting shaft between a retracted position accommodating the establishment of the axially interengaged relationship, and an advanced position disrupting the axially interengaged relationship to thereby accommodate axial ejection of the pipette tip from the mounting shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Provisional Application Ser. No. 60/543,742 filed Feb. 11, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to air displacement pipettes, and is concerned in particular with an improvement in pipette tips and the manner in which they are releasably retained on and ejected from the tubular mounting shafts of the pipettes. 
     2. Description of the Prior Art 
     It is known to detachably retain a pipette tip on the tubular mounting shaft of an air displacement pipette. The pipette is equipped with a manually operable ejection mechanism for disengaging and releasing the thus retained pipette tip once it has served its purpose. Retention is commonly achieved by effecting a friction fit between coacting surfaces on the pipette tip and the mounting shaft. 
     This leads to certain difficulties in that users are often uncertain as to the level of force required to achieve a secure friction fit. An inadequate force can result in the pipette tip becoming prematurely dislodged, whereas an excessive force can result in the pipette tip being jammed in place, which in turn disadvantageously increases the force that must be exerted by the manually operable ejection mechanism when dislodging the pipette tip from its retained position. These problems are exacerbated in multi channel pipettes. 
     It is also known to provide the cylindrical walls defining the upper ends of the pipette tips with interiorly projecting circular ribs or ridges designed to coact in snap engagement with mating surfaces on the tubular mounting shafts of the pipettes. 
     However, this also leads to certain difficulties in that in order to achieve a snap engagement, the upper walls of the pipette tips must be radially expanded, which in turn requires the user to exert unacceptably high forces when axially inserting the tubular mounting shafts into the pipette tips. Comparable forces are required to disengage the tips from the mounting shafts. Moreover, slight dimensional variations can have a significant impact, e.g., by either additionally increasing the forces required to engage and release the pipette tips if their internal wall diameters are too small, or resulting in unacceptably loose connections if their internal wall diameters are too large. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a tubular pipette tip has an upper section surrounding a locking chamber. A tubular mounting shaft on an air displacement pipette has a distal end configured and dimensioned for insertion into an axially interengaged relationship with the upper section. A spring loaded ejection sleeve is manually shiftable on the pipette mounting shaft between a retracted position accommodating establishment of the aforesaid axially interengaged relationship, and an advanced position disrupting that relationship to thereby accommodate axial ejection of the pipette tip from the pipette mounting shaft. 
     In accordance with another aspect of the present invention, a spring loaded collar on the ejection sleeve serves to forcibly eject the pipette tip from the mounting shaft when the axially interengaged relationship is disrupted. The spring loaded collar also serves to eject a pipette tip that has not been fully inserted to establish its axially interengaged relationship with the mounting shaft. 
     In accordance with still another aspect of the present invention, a tubular pipette tip has a body section tapering downwardly from its upper section to a reduced diameter end. The upper section of the pipette tip is provided with at least one and preferably a plurality of integral circumferentially spaced resilient fingers that project inwardly into the locking chamber to coact in snap engagement with a complimentary surface on the distal end of the mounting shaft. 
     These and other aspects, features and advantages of the present invention will now be described in greater detail with reference to the accompanying drawings, wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a manually operable air displacement pipette incorporating the concepts of the present invention; 
         FIG. 2  is an enlarged vertical sectional view through the tip mounting and ejection assembly of the pipette illustrated in  FIG. 1 , with the pipette tip separated therefrom; 
         FIG. 3  is a further enlarged vertical sectional view of the end portion of the tip mounting and ejection assembly shown in  FIGS. 1 and 2 ; 
         FIG. 4  is a side view of the pipette tip shown in  FIGS. 1 and 2 ; 
         FIG. 5  is a vertical sectional view of the pipette tip taken on line  5 - 5  of  FIG. 4 ; 
         FIG. 6  is a top plan view of the pipette tip; 
         FIG. 7  is a perspective view of the crown section of the pipette tip; 
         FIGS. 8-11  are views similar to  FIG. 3  showing successive stages in the tip mounting and ejection sequence; 
         FIGS. 12A ,  13 A,  14 A and  15 A are side views of alternative pipette tip embodiments; 
         FIGS. 12B ,  13 B,  14 B and  15 B are vertical sectional views, respectively, of the pipette tip embodiments shown in  FIGS. 12A ,  13 A,  14 A and  15 A; and 
         FIG. 16  is a partial sectional view showing the pipette tip of  FIGS. 15A and 15B  axially interengaged with the mounting shaft of the pipette. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference initially to  FIGS. 1-3 , a manually operable air displacement pipette incorporating concepts of the present invention is generally depicted at  10 . The pipette includes a housing  12  with a manually operable push button  14  at its upper end. The push button is connected via internal components (not shown) to a piston  16  projecting from the lower end of the housing. The piston  16  extends through a seal assembly  18  contained in the enlarged diameter head  20  of an aspirating and dispensing cylinder  22 . The cylinder is threaded into the lower end of the housing and communicates with an integral tubular mounting shaft  24  with a distal end configured and dimensioned to removably retain a disposable pipette tip  26 . 
     As can best be seen in  FIG. 3 , the mounting shaft  24  is threaded into the cylinder end as at  28 , with its reduced diameter coacting with the end of the cylinder  22  to form a circular shoulder  30 . The distal end of the mounting shaft  24  is externally configured with an enlarged diameter shoulder  32  optionally having a chamfered leading edge  34 . An intermediate surface  36  tapers inwardly from shoulder  32  to a circular groove  37  containing a resilient O-ring seal  38 . A cylindrical section  40  extends from the groove  37  to an end surface  42 . 
     A sleeve  44  surrounds the aspirating and dispensing cylinder  22  and its tubular shaft extension  24 . As can best be seen in  FIG. 2 , the upper end of sleeve  44  is spaced radially from the exterior surface of cylinder head  20  to define an annular space containing a first coiled compression spring  46 . The spring  46  is axially confined between an external shoulder  48  on cylinder head  20  and a spring retainer  50  snap fitted into the upper sleeve end. Spring  46  resiliently urges sleeve  44  into a retracted position at which an internal sleeve shoulder  44 ′ contacts the shoulder  30 . 
     Sleeve  44  includes a cylindrical press fitted insert  54  formed with an enlarged diameter end  56  having a chamfered or radiused leading edge  58 . A collar  60  surrounds and is axially shiftable on the sleeve insert  54 . 
     The lower interior of sleeve  44  is spaced radially from the exterior of insert  54  to define an annular spaced containing a second coiled compression spring  62 . Spring  62  is axially confined between an internal shoulder  64  on sleeve  44  and the collar  60 . The spring  62  serves to resiliently urge the collar  60  against the enlarged diameter end  56  of sleeve insert  54 . 
     As can best be seen in  FIGS. 4-7 , the pipette tip  26  has a tubular configuration with an upper section having an upper wall segment  67  surrounding a locking chamber  68  and a lower wall segment  69  surrounding a sealing chamber  70 . A body section  72  extends downwardly from the upper section  66  to a reduced diameter open end  74 . The upper wall segment  67  of section  66  is formed with at least one and preferably a plurality of circumferentially spaced resilient fingers  76 . Preferably, as shown, a pair of resilient fingers  76  are provided in an oppositely disposed relationship. The fingers  76  border and project inwardly in cantilever fashion from an upper chamfered rim  78  into the locking chamber  68 . The lower wall segment  69  is interiorly provided with an entry section  80  tapering inwardly to a cylindrical section  82 . A stop surface in the form of a circular ledge  81  is located between the locking chamber  68  and the sealing chamber  70 . As can best be seen in  FIGS. 5 and 7 , the lower wall segment is reinforced by external circumferentially spaced ribs  86  extending from ledge  80  to the body section  72 . The lower ends  84  of external vertical ribs  86  lie on a plane demarcating the upper crown section  66  from the body section  72 . 
     A tip mounting sequence will now be described with initial reference to  FIG. 8  where a pipette tip  26  is shown supported on the lower ends  84  of ribs  86  in the aperture of a support plate  88  or the like. The pipette  10  is first aligned with the tip  26  and then lowered, causing the cylindrical end  40  of the mounting shaft  24  to pass axially through the locking chamber  68  into the sealing chamber  70 . The shoulder  32 , aided by its chamfered leading edge  34 , makes initial contact with the resilient fingers  76  and begins to deflect them outwardly. 
       FIG. 9  shows an intermediate stage in the mounting sequence at which axial insertion of the mounting shaft  24  has progressed to the point where the resilient fingers  76  are now fully expanded, the O-ring seal  38  is about to enter into sealing engagement with the cylindrical section  82  of the sealing chamber  70 , and the collar  60  has encountered the upper rim  78  of the pipette tip and has begun to shift axially against the compressive force of spring  62  and away from the enlarged diameter end  56  of sleeve insert  54 . 
       FIG. 10  shows the final stage in the mounting sequence. As indicated by the arrows  90 , the resilient fingers  76  have now snapped inwardly behind and in locked interengagement with the shoulder  32  on mounting shaft  24 . Spring  62  has been compressed and loaded to an elevated level between shoulder  64  and collar  60 . A fluid-tight seal has been established between the O-ring seal  38  and the cylindrical section  82  of the sealing chamber  70 , and the shoulder  32  has bottomed out against the circular ledge  81 . The ledge  81  thus establishes a positive stop, which in combination with the audible sound of the fingers  76  snapping into interlocked engagement, provides the user with a reliable indication that the pipette tip has been securely mounted. Because of the angle α of inward inclination of the interlocked fingers, any attempt to pull the pipette tip off of the mounting shaft  24  will only serve to further deflect the fingers inwardly, thus enhancing the interlocked relationship between the pipette tip and the cylinder extension. 
     With reference again to  FIGS. 1 and 2 , it will be seen that the pipette  10  further includes an ejection button  92  connected via a mechanical linkage (not shown) contained in housing  12  to a link  94  bearing against the spring retainer  50 . Tip ejection is effected by manually pushing button  92  in the direction of arrow  96 , resulting in a corresponding axial shifting of link  94 , causing sleeve  44  to shift axially in the same direction on cylinder  24  against the compressive force of springs  46  and  62 . 
       FIG. 11  shows that as the sleeve  44  and its insert  54  shift in the direction of arrow  96 , the resilient fingers  76  are biased outwardly by the enlarge diameter end  56  of insert  54 . When the fingers are deflected outwardly beyond the shoulder  32 , the axially interengaged relationship between the pipette tip  26  and the mounting shaft  24  is disrupted, allowing the spring  62 , now loaded to an elevated level, to act via collar  60  to forcibly eject the tip  26  from the end of the mounting shaft. The spring loaded collar will also serve to forcibly eject a pipette tip that has not been fully inserted, e.g., inserted only to the extent shown in  FIG. 9 . 
     It thus will be seen that in order to effect tip ejection, a user need only press button  92  with a force necessary to overcome the resistance of springs  46  and  62 . Appropriate spring selection will insure that this force is modest and ergonomically friendly. 
     In light of the foregoing, those skilled in the art will appreciate that the tip mounting and ejection assembly of the present invention is not limited in use to manually operable pipettes of the type herein disclosed, and that the concepts of the present invention are applicable to a wide range of mechanically and/or automatically driven pipette types and designs. 
     It should also be understood that various pipette tip designs may be employed with the above described mounting and ejection assembly. For example, in the tip embodiment shown at  26   a  in  FIGS. 12A and 12B , although the upper section  66   a  of the tip again surrounds a locking chamber  68   a , it is formed separately from and assembled as an insert into the upper end of the body section  72   a . The resilient fingers  76   a  project in cantilever fashion upwardly from a circular base at the bottom of the locking chamber, and an internal shelf  98  has a through bore  100  surrounded by a raised bead  102  projecting upwardly into the locking chamber  68   a . With this embodiment, the end surface  42  of the mounting shaft  24  will coact in sealing engagement with the raised bead  102 , making it unnecessary to employ an O-ring seal  38 . 
     In another pipette tip embodiment  26   b  shown in  FIGS. 13A and 13B , the upper section  66   b  includes a locking chamber  68   b  and a lower sealing chamber  70   b , and is again formed separately and assembled as an insert into the upper end of body section  72   b . The resilient fingers  76   b  project downwardly and inwardly in cantilever fashion from a top rim into the locking chamber  68   b , and the internal shelf  98   b  is located at the bottom of the upper section. 
     In  FIGS. 14   a  and  14   b , the pipette tip  26   c  is similar to that shown in  FIGS. 13A and 13B , except that here the internal shelf  98   c  is formed as a thin apertured membrane designed to coact in sealing engagement with the end surface  42  of the mounting shaft  24 . 
     In  FIGS. 15A and 15B , the pipette tip  26   d  is similar to that depicted in  FIGS. 4-7 , except that here the sealing chamber  70   c  is bordered by an angled ledge  104  positioned to coact in sealing engagement with the O-ring seal  38  on the tubular shaft extension  24 . 
     As shown in  FIG. 16 , the O-ring  38  coacts in a “face sealing” relationship with the angled ledge  104 , without disadvantageously increasing frictional resistance to subsequent ejection of the tip from the mounting shaft. 
     In light of the foregoing it will now be understood by those skilled in the art that the mounting shaft  24  of the pipette and each of the several pipette tip embodiments  26   a - 26   d  are respectively configured and dimensioned to effect an axially interengaged relationship and a snap connection between a shoulder  32  or the like on the former and resilient fingers on the crown sections of the latter. A positive stop on the pipette tip limits the extent of mounting shaft insertion required to achieve the snap connection, and this, together with the audible nature of the snap connection, provides the user with a reliable indication that an adequate insertion force has been exerted, and that the pipette tip has been reliably and securely retained on the mounting shaft. 
     Tip ejection requires only a modest force exerted on button  92  and transmitted to sleeve insert  54  to spread the resilient fingers  76  sufficiently to disrupt their interengaged relationship with the mounting shaft  24 . The pipette tip is then freed for forcible ejection by the spring loaded collar  60 .