Abstract:
A centrifugal blood pump has an impeller with a hub, a blade support ring and alternating long and short blades. A tapered seal between a pump housing wall and a hub of an impeller which provides a fluid tight seal interface surrounding a shaft. The long blades of the impeller have rear edges which are closer to the wall than is the seal interface and inner edges which extend from the rear edges to the hub. This provides high flow in the vicinity of the seal interface to enhance heat dissipation from the seal interface.

Description:
REFERENCE TO COPENDING APPLICATION 
     Reference is made to our copending application Ser. No. 628,727 filed on even date entitled &#34;Centrifugal Blood Pump with Tapered Shaft Seal&#34;. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to centrifugal blood pumps. 
     2. Description of the Prior Art 
     Centrifugal pumps have been used for many years to pump a wide variety of different fluid materials. In general, a centrifugal pump includes a pumping chamber with an inlet aligned with a rotational axis of the pump, an outlet adjacent the periphery of the pumping chamber, and an impeller mounted within the pumping chamber for rotation about the axis. The impeller in such pumps can be mounted on a drive shaft which extends outside the pumping chamber to a rotational drive source or the shaft can be mounted within the pumping chamber as a spindle about which the impeller rotates (rotatably driven by means other than the rotation of the shaft, such as a magnetic drive arrangement). In any case, as the impeller is rotated, it imparts centrifugal force and velocity to the fluid, thus pumping the fluid from the pump inlet to the pump outlet. 
     In recent years, centrifugal pumps have been used extensively for pumping blood during open heart surgery. Examples of centrifugal blood pumps are shown in the following U.S. patents: Rafferty et al U.S. Pat. No. Re. 28,742; Dorman et al U.S. Pat. No. 3,608,088; Rafferty et al U.S. Pat. No. 3,647,324; Kletschka et al U.S. Pat. No. 3,864,055; Rafferty et al U.S. Pat. No. 3,957,389; Rafferty et al U.S. Pat. No. 3,970,408; Rafferty et al U.S. Pat. No. 4,037,984; and Reich et al U.S. Pat. No. 4,135,253. 
     The pumping of blood requires great care to avoid any damage to the red corpuscles, or any of the other constituents of blood. Any practical blood pump useful as part of heart/lung bypass equipment during open heart surgery must deliver the requisite flow volumes under pressure, without damaging the blood being pumped. 
     In a centrifugal pump, and in particular in a centrifugal pump for pumping liquids such as blood, a fluid tight seal between the drive shaft and the housing is an important factor in the performance of the pump. Friction at the seal produces heat which can damage both the components of the pump and the blood being pumped if not dissipated. 
     In prior art centrifugal pumps, the rotation of the impeller can lead to generation of an air bubble surrounding the shaft. This air bubble tends to seek the smallest shaft diameter, which is adjacent the drive shaft seal. In prior art centrifugal pumps, the area adjacent the drive shaft seal has also been a relatively stagnant or low flow area in terms of fluid flow within the pumping chamber. The air bubble tends to insulate the seal from the flow of the fluid within the pump chamber, thus decreasing the dissipation of heat generated by friction at the seal interface. 
     SUMMARY OF THE INVENTION 
     The present invention is an improved centrifugal blood pump which has a seal between the pump housing wall and the hub of the impeller for providing a fluid tight seal interface which surrounds the shaft at an intermediate position between the wall and the hub. The impeller has a plurality of first blades which are attached to and extend outward from the hub. Each of these blades has a rear edge which is closer to the wall than is the seal interface, and has an inner edge which extends from the rear edge to the hub and which is generally parallel to the seal. This arrangement of the plurality of first blades causes the seal interface to be located in a high flow area, thus enhancing the dissipation of heat generated by friction at the seal interface. 
     In preferred embodiments the impeller includes an annular blade support ring which is supported by the first blades at a position which is spaced radially outward from the hub and the seal. A plurality of second shorter blades are supported by the blade support ring at positions between the longer first blades. The short blades increase the pumping efficiency, while maintaining a small hub diameter since the short blades are not attached to the hub. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of the centrifugal pump of the present invention. 
     FIG. 2 is a side view of the centrifugal pump. 
     FIG. 3 is a sectional view of the centrifugal pump along section 3--3 of FIG. 1. 
     FIG. 4 is an exploded perspective view of the centrifugal pump. 
     FIG. 5 is a view of the rotor along view 5--5 of FIG. 3. 
     FIG. 6 is a view of the drive plate along view 6--6 of FIG. 3. 
     FIG. 7 is an exploded view, partially in section, of the tapered shaft seal of the centrifugal pump. 
     FIG. 8 is a sectional view of the shaft seal along section 8--8 of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the preferred embodiment shown in the Figures, centrifugal pump 10 of the present invention includes a three-part housing 12 formed by front housing section 14, center wall housing section 16, and rear housing section 18. Front and center sections 14 and 16 are sealed to define pumping chamber 20. Center and rear sections 16 and 18 are sealed to define rotor chamber 22. 
     Front housing section 14 (which is preferably transparent so that operation of the pump can be visually monitored) includes axially aligned pump inlet 24 and tangential pump outlet 26. Blood or other biological fluid is received at inlet 24 from inlet tubing 27 and is pumped to outlet 26 and outlet tubing 28 by rotation of impeller 30 within pumping chamber 20. 
     Impeller 30 is mounted on a threaded outer end 32A of shaft 32, and is rotated about an axis defined by shaft 32. Impeller 30 includes a conical shaped impeller hub 34 (with internal threads 34A for engaging threaded outer end 32A), a plurality of long blades 36, a plurality of short blades 38, and circular flange 40. 
     Long blades 36 are attached at their inner ends to impeller hub 38. Flange 40 is attached to and is supported by long blades 36. Short blades 38 are supported by flange 40. In the particular embodiment shown in the Figures, long and short blades 36 and 38 are alternatively spaced about the circumference of impeller 30. 
     Large diameter impellers require a greater number of blades in order to achieve pumping efficiency. By use of short blades 38 supported by flange 40, impeller 30 achieves pumping efficiency while retaining a small hub diameter, since only long blades 36 are attached to hub 34. 
     Shaft 32 is mounted for rotation by a pair of axially aligned ball bearings 42 and 44. Ball bearing 42 is press fitted into center wall section 16, while ball bearing 44 is press fitted into rear housing section 18. 
     Rotor 46 is connected to shaft 32, so that as rotor 46 rotates within rotor chamber 22, shaft 32 and impeller 30 are rotated. In the particular embodiment shown in the Figures, pump 10 is a magnetically driven pump. Rotor 46 carries a plurality of small magnets 48. Each magnet 48 has the same pole orientation (which in the particular embodiment shown has the north (N) pole closest to drive console 50). Magnets 48 are equally spaced around the circumference of rotor 46 and, in the particular embodiment shown in FIG. 3, five magnets 48 spaced at 72° intervals (center-to-center) from one another are carried by rotor 46. 
     Drive console 50 includes drive plate 52 which is rotated by motor 54 about an axis which is aligned with the axis of shaft 32. Clip 55 and spring-loaded latch 56 engage flange 18A of rear housing section 18 to hold pump housing 12 in position adjacent drive console 50. Pump housing 12 can be quickly removed from engagement with drive console 50 by lifting latch 56. 
     Drive plate 52 carries five equally spaced south (S) pole magnets 58 and five equally spaced north (N) pole magnets 60. Magnets 58 and 60 are arranged alternatively (as shown in FIG. 6). This gives both attractive and repelling force to magnets 48 carried by rotor 46. This magnetic drive allows the use of small, discrete magnets in pump 10, rather than a single large magnet with multiple poles. This provides a significant cost reduction which is of particular advantage since pump housing 12, when used for pumping blood or other biological fluids, must be disposed of or resterilized after a single use. 
     In the present invention, leakage of fluid from pumping chamber 20 into rotor chamber 22 is prevented by a tapered shaft seal 62 formed by seal stator 64, seal rotor 66, and resilient elastomer spring 68. Tapered seal 62 is tapered to conform to the taper of impeller hub 34 so that an air bubble (which seeks the smallest shaft diameter within pumping chamber 20) will not insulate the seal interface edges from fluid flow. Tapered seal 62 provides a seal interface 70 between seal stator 64 and seal rotor 66 which is generally perpendicular to the axis of shaft 32 and which is located at an intermediate position between wall 16 and hub 34. The location of the seal interface 70 is in a high fluid flow area, which increases cooling effects and improves dissipation of heat caused by friction at seal interface 70. 
     In the preferred embodiment of the present invention shown in FIG. 1, seal stator 64 is fixed to center wall section 16, seal stator 64 is a high thermal conductivity material (such as nickel-plated aluminum). Seal stator 64 has a central passage 72 which is axially aligned with shaft 32 and is of sufficient diameter so that shaft 32 does not contact seal stator 64. Front face 74 of seal stator 64 defines the location of seal interface 70 and is, in the preferred embodiment shown in FIGS. 1 and 2, generally perpendicular to the axis of shaft 32. 
     Seal stator 64 has a flange 64A at its rear end which extends outward in a radial direction and generally conforms to the surface of wall 16 at the rear end of pumping chamber 20. Flange 64B provides a large surface area for seal stator 64, thus increasing the ability of seal stator 64 to transfer heat generated at seal interface 70. 
     Seal rotor 66 is positioned on shaft 32 adjacent to seal stator 64. Rear face 76 of seal stator 66 engages front face 74 of seal stator 64 to provide seal interface 70. Front face 78 of seal rotor 66 faces and is engaged by spring 68. Seal rotor 66 has a pair of inwardly projecting keys 80 which engage axially extending keyways 82 on shaft 32 so that seal rotor 66 can move in the axial direction and yet rotates with shaft 32. Such a keyed arrangement may not be necessary if, by friction fit or bonding, the seal rotor 66 is driven by the spring 68 to rotate therewith. In a preferred embodiment, seal rotor 66 is a low friction polymer material such as nylon. 
     Spring 68 is an elastomer (such as silicone rubber) ring which is mounted coaxially on shaft 32 between impeller hub 34 and seal rotor 66. Rear face 84 of spring 68 engages front face 78 of seal rotor 66, and front face 86 of spring 68 engages rear face 88 of hub 34. Elastomer spring 68 is maintained under compression by hub 34, which is threaded on outer end 32A of shaft 32, so that it urges seal rotor 66 in an axial direction into engagement with seal stator 64. Spring 68 preferably has an annular rib 90 which is positioned in annular groove 92 in front face 78 of seal rotor 66 and has an annular rib 94 which is positioned in annular groove 96 in the rear face 88 of hub 34. Ribs 90 and 94 help to maintain an axial alignment of spring 68 so that an essentially uniform axial force is applied to seal rotor 66. In another embodiment (not shown), the resilient elastomer spring is positioned between the seal stator and wall section with which it is mounted (rather than between the seal rotor and hub) and the seal rotor is fixed to the hub to effectuate the sealing of the pumping chamber about the shaft. 
     To increase fluid flow in the area of seal interface 70, each of the long blades 36 of impeller 30 has a rear edge 36A which is closer to center wall 16 than are impeller hub 34 and seal interface 70. Each long blade 36 has an inner edge 36B which extends from rear edge 36A to impeller hub 34, and which is closely spaced and generally parallel to the outer surface of tapered seal 62. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the particular embodiment of pump 10 shown in the Figures utilizes a synchronous magnetic drive, the shaft seal is equally applicable to pumps in which other forms of coupling (including direct coupling) between shaft 32 of pump 10 and motor 54 of console 50 are provided. In addition, where the shaft is a fixed spindle about which the impeller is rotated by other means such as a magnetic drive arrangement wherein magnets are mounted directly on the impeller), the shaft seal seals the pumping chamber from the bearings and lubricants between the shaft and impeller hub. 
     As a further example, although as shown in the Figures flange 40 is attached to long blades 36 near their rear edges, in other embodiments flange 40 is connected to long blades 36 at other points such as their front edges or a location between the front and rear edges.