Abstract:
A pump includes a housing and a shaft rotatable about an axis. The shaft has a shaft diameter and has a portion disposed within the housing. A plurality of retractable vanes is coupled to the shaft and each vane includes a first end that contacts the housing. A seal element is positioned to define a seal between the shaft and the housing. The seal element includes a first bushing, a second bushing, and a dry seal assembly including a wiper that defines a diameter that is smaller than the shaft diameter.

Description:
RELATED APPLICATION DATA 
     This application claims benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Application No. 61/086,655, filed Aug. 6, 2008, which is fully incorporated herein by reference. 
    
    
     BACKGROUND 
     The invention relates to a shaft seal for a pump. More particularly, the present invention relates to a shaft seal for a rotary vane pump. 
     Rotary vane pumps can be used in aircraft to provide vacuum for the various instruments used for navigation and control. Typically, aircraft rotary vane pumps are positioned within the engine compartment where they are exposed to hydrocarbons and hydrocarbon vapors such as oil, engine exhaust, fuel, etc. 
     SUMMARY 
     The present invention provides a shaft seal assembly for a rotary vane pump. The shaft seal assembly includes a wiper that contacts the shaft seal and maintains a seal therebetween. The shaft seal assembly also includes an O-ring that contacts the pump housing to bias the wiper into contact with the shaft and also provides a seal between the shaft seal assembly and the pump housing. The shaft seal assembly can be used in new pumps and can also be installed in prior art pumps including a conventional shaft seal bushing. 
     In one construction, the invention provides a pump that includes a housing and a shaft rotatable about an axis. The shaft has a shaft diameter and has a portion disposed within the housing. A plurality of retractable vanes is coupled to the shaft and each vane includes a first end that contacts the housing. A seal element is positioned to define a seal between the shaft and the housing. The seal element includes a first bushing, a second bushing, and a dry seal assembly including a wiper that defines a diameter that is smaller than the shaft diameter. 
     In another construction, the invention provides a pump that includes a housing that defines a pump space and a seal space and a shaft that includes a seal portion positioned adjacent the seal space. A rotor is coupled to the shaft and is rotatable about a rotational axis with the shaft and a plurality of vanes is retractably supported by the rotor. Each vane includes a first end that contacts the pump space. A first bushing is positioned between the shaft and the housing within the seal space and a second bushing is positioned between the shaft and the housing within the seal space. The second bushing is spaced a non-zero distance along the axis from the first bushing. A wiper is disposed between the first bushing and the second bushing. The wiper includes an innermost surface that contacts the shaft and an outermost surface spaced a non-zero distance from the housing. A biasing member is positioned between the wiper and the shaft and is in contact with the outermost surface of the wiper and the housing to bias the wiper toward the shaft. 
     In yet another construction, the invention provides a method of repairing a rotary vane pump. The method includes removing a bearing disposed in a bearing space having a first axial length, the bearing positioned adjacent a shaft having an outer surface and a housing having an inner surface. The method also includes inserting a first bushing having an inside surface that substantially seals against the outer surface of the shaft and an outer surface that seals against the inner surface of the housing and inserting a second bushing having an inside surface that substantially seals against the outer surface of the shaft and an outer surface that seals against the inner surface of the housing. The method further includes positioning a wiper between the first bushing and the second bushing, the wiper having an innermost surface in contact with the outer surface of the shaft and an outermost surface spaced from the inner surface of the housing, the first bushing, the second bushing, and the wiper cooperating to define a second axial length that is about equal to the first axial length, and biasing the wiper toward the shaft to compensate for wear of the wiper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a rotary vane pump including a standard shaft seal; 
         FIG. 2  is an exploded perspective view of a portion of the pump of  FIG. 1 , including a shaft seal assembly embodying the invention; 
         FIG. 3  is a section view of the portion of the pump of  FIG. 2  showing the shaft seal assembly in an operating position; 
         FIG. 4  is a section view of the shaft seal assembly of  FIG. 2 ; 
         FIG. 5  is a perspective section view of the shaft seal assembly of  FIG. 2 ; 
         FIG. 6  is a front view of a bushing of the shaft seal assembly of  FIG. 2 ; 
         FIG. 7  is a section view of the bushing of  FIG. 6  taken along line  7 - 7  of  FIG. 6 ; 
         FIG. 8  is a perspective view of a dry seal assembly of the shaft seal assembly of  FIG. 2 ; and 
         FIG. 9  is a section view of the dry seal assembly of  FIG. 8  taken along line  9 - 9  of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
       FIG. 1  illustrates a rotary vane pump  10  that includes a conventional shaft seal  15 . The rotary vane compressor  10  includes a housing  20  having a front end  25 , a pump cavity  30 , and a rear end  35 . A rotor  40  includes a plurality of retractable vanes  45  that are positioned within the pump cavity  30 . The rotor  40  is coupled to a prime mover such as an aircraft engine such that the rotor  40  rotates within the pump cavity  30  to pump air as required. In preferred constructions, the rotor vanes  45  are formed from a carbon composite or metallized carbon that provides for increased life and reduced wear. 
     The illustrated pump  10  is a dry pump that operates without internal lubricants. In addition, the metallized carbon material expands when exposed to hydrocarbons. The expansion can cause increased wear and reduced vane life. As such, the shaft seal  15  is provided to reduce the likelihood of hydrocarbons entering the pump cavity  30 . 
     A three-finger drive  50  includes a shaft  55  and three fingers  58  that engage the rotor  40  to drive the rotor  40 . The fingers  58  are arranged to impart rotational movement to the rotor  40  while still allowing axial movement between the three-finger drive  50  and the rotor  40 . 
     The conventional shaft seal  15  includes a single bushing  60  positioned adjacent the shaft  55  and inside of the front end  25  of the housing  20 . In this position, the bushing  60  is designed to inhibit leakage between the bushing  60  and shaft  55  and between the bushing  60  and front end  25  of the housing  20 . However, the design of the bushing  60  is such that any wear of the bushing  60  can reduce the effectiveness of the bushing  60 . 
       FIG. 2  is an exploded view of a rotary vane pump  65  similar to that of  FIG. 1  and including a shaft seal assembly  70  of the present invention in place of the conventional shaft seal  15 . The shaft seal assembly  70  is positioned on the shaft  55  of the three-finger drive  50  and provides a seal between the seal assembly  70  and the shaft  55  and the seal assembly  70  and the front end  25  of the housing  20 . 
       FIG. 3  illustrates the shaft seal assembly  70  in an operating position. It should be noted that the shaft seal assembly  70  includes portions that interfere with the front end  25  of the housing  20  and the shaft  55 . These interferences will be discussed in detail below. 
     As illustrated in  FIGS. 4 and 5 , the shaft seal assembly  70  includes a first bushing  75 , a dry seal assembly  80 , and a second bushing  85 . In preferred constructions, the first bushing  75  and the second bushing  85  are similar or even identical with one possible arrangement illustrated in  FIGS. 6 and 7 . However, other constructions could employ bushings of differing designs. The first bushing  75  and the second bushing  85  are circular members having a substantially rectangular cross-section as illustrated in  FIG. 4 . The inside diameter  90  of the first bushing  75  and the second bushing  85  are sized to seal against the shaft  55 . Similarly, the outside diameter  95  of the first bushing  75  and the second bushing  85  are sized to seal against the front end  25  of the housing  20 . In one construction, the bushings  75 ,  85  are made from a carbon material that is similar to the material used in the conventional shaft seal bushing  60 . Of course other materials could be employed as desired. 
     The dry seal assembly  80 , illustrated in  FIGS. 8 and 9 , is positioned between the first bushing  75  and the second bushing  85  and includes a wiper  100  and an o-ring  105 . The wiper  100  includes a tongue  110  and an outer portion  115  having a width that is selected such that the width of the wiper  100 , the first bushing  75  and the second bushing  85  cooperate to occupy about the same space as the conventional bushing  60 . The outer portion  115  also includes a channel  120  positioned to receive the O-ring  105 . 
     With reference to  FIGS. 4 and 5 , the tongue  110  extends from a first end  125  of the outer portion  115  and includes an extension portion  130  that extends toward a second end  135  of the outer portion  115 . Thus, the tongue  110  and outer portion  115  cooperate to define a U-shaped channel  140  that is open in a direction away from the pump cavity  30 . In other words, the U-shaped channel  140  is open in a direction from which likely contaminates will enter the pump cavity  30 . The tongue  110  is sized to define an inside diameter  145  that is smaller than the outside diameter of the shaft  55  such that when positioned adjacent the shaft  55 , the tongue  110  must deflect. Thus, as illustrated in  FIG. 3 , the tongue  110  will interfere with the shaft  55  if it does not deflect. The deflection assures that the tongue  110  will contact the shaft  55  during all operating conditions and even as the tongue  110  wears, contact will be maintained. In addition, the tongue  110  is formed and sized to provide a desired contact pressure against the shaft  55 . 
     As mentioned above, the O-ring  105  fits within the channel  120  and is sized to interfere with the front end  25  of the housing  20  such that it must deflect to be installed properly. Thus, as illustrated in  FIG. 3 , if the O-ring  105  does not deflect, it interferes with the front end  25  of the housing  20 . 
     When positioned as described, the O-ring  105  functions to provide a positive seal between the shaft seal assembly  70  and the front end  25  of the housing  20 . In addition, the force between the O-ring  105  and the front  25  of the housing  20  forces the wiper  100  inward until a balance is reached and the desired force between the tongue  110  and shaft  55  is reached. As the tongue  110  wears, the force between the tongue  110  and the shaft  55  is reduced and the O-ring  105  is able to push the wiper  100  inward until the forces once again balance. In this way, the dry seal assembly  80  is able to maintain contact between the tongue  110  and the shaft  55  even as the tongue  110  wears. 
     In some constructions, the wiper  100  is formed from a PTFE material (polytetra fluoroethylene, TEFLON). However, preferred constructions employ a composite material such as a graphite PTFE combination. In one construction, the wiper  100  is formed from a material that is 10 percent graphite and 90 percent PTFE. Of course other materials could be employed if desired. 
     The shaft seal assembly  70  is configured such that it can be used to replace the prior shaft seal bushing  60  if desired. In one use, an existing pump  10  is disassembled and the bushing  60  is removed. The first bushing  75 , the dry seal assembly  80 , and the second bushing  85  are positioned in the front end  25  of the housing  20  and the pump  10  is reassembled with the new shaft seal assembly  70 . The shaft seal assembly  70  provides positive contact with the shaft  55  and with the front end  25  of the housing  20  to improve the seal at the shaft  55  and reduce the likelihood of contaminates entering the pump cavity  30 . 
     Thus, the invention provides, among other things, a new and useful shaft seal assembly  70  for use in a rotary vane pump  10 ,  65 . The shaft seal assembly  70  can be used, among other things, to replace a conventional shaft seal bushing  60  to enhance seal performance.