Abstract:
An internal gear set comprising an inner rotor having a number of radially projecting cylindrical tooth members engaging a conjugate internally toothed outer rotor. The latter has one more tooth than there are tooth members on the inner rotor and is mounted eccentrically to the inner rotor so that the rotors move conjugately relative to one other.

Description:
This is a divisional of Application Ser. No. 09/666,041, filed Sep. 20, 2000 now U.S. Pat. No. 6,617,367, and claims the benefit of U.S. Provisional Application Ser. No. 60/154,847, filed Sep. 20, 1999, all of which are incorporated herein by reference. 
     Applicants claim the light to priority based on Provisional Patent Application Ser. No. 60/154,847 filed Sep. 20, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to pumps, and in particular to a rotary pump having inner and outer rotors, wherein the inner rotor drives the outer rotor. Further, the invention relates to a rotary pump having inner and outer rotors for use in low viscosity and abrasive metering applications. 
     BACKGROUND OF THE INVENTION 
     Rotary pumps having pumping elements consisting of a driving inner rotor and a driven outer rotor are generally referred to as internal gear pumps. A particular class of internal rotary gear pumps commonly known as internal gerotor pumps are often used in chemical metering applications, for example, when pumping the components of two-part polyurethane foam. 
     Gerotor type pumping elements are characterized by an inner member having one less tooth than the outer member and by the fact that each tooth of one member is always in contact with some portion of the other member. This interaction between the members results in continuous driving contact, and when the gears are rotated, a series of expanding and contracting chambers are formed which, when connected with appropriate passages, provides pumping action. In the case of the conventional externally generated gerotor, the outer member has a series of inwardly protruding circular teeth such that the set has the aforementioned properties. 
     The limitations of this approach are apparent in the foam-in-place packaging industry where two-part polyurethane is used to make the packaging materials. The two-part polyurethane foam packaging material is based upon the reaction of two precurser components, which when mixed will react to form a polymer foam and gaseous by-products. In particular, and most commonly, an isocyanate containing component is mixed with a polyol containing component and these components react to produce a urethane polymer (polyurethane), steam, and carbon dioxide. 
     As the two-part polyurethane foam requirements have become more specialized in the foam-in-place industry, the constituent parts, i.e., the isocyanate containing component and the polyol containing component, have become more abrasive and less viscous. Because of the inherent sliding action in a conventional externally generated gerotor set, pump life in this particular application has been reduced from over 1000 hours to about 100-200 hours. 
     A lesser known form of the conventional gerotor is the IGR or Internally Generated Rotor Set. In this device, the inner rotor has a number of circular externally protruding teeth and the outer rotor is internally generated such that it has the same characteristics as an externally generated rotor set, i.e., an inner member having one less tooth than the outer and where each tooth of one member is always in contact with some portion of the other member, thus resulting in continuous driving contact. In this case, however, the circular teeth of the inner member can be replaced by rolls which are contained in recesses in the inner member, which recesses are of substantially the same diameter as the rolls. Properly designed, this allows the rolls to operate hydrodynamically within the recesses. Further, the combination of both centrifugal and pressure forces drive the roll into intimate rolling contact with the outer member, thus providing fluid tight sealing as well as the elimination of the sliding contact that has led to the reduction in pump life in conventional gerotor pumps as the pumped chemicals have become more abrasive and less viscous. It should be noted that because of the pressure loading of the rolls, the IGR will accommodate a certain amount of tooth wear without a loss in pumping performance. An example of an internally generated rotor set is disclosed in U.S. Pat. No. 3,623,829. 
     Rotary pumps using internally generated rotor sets of the foregoing types are characterized by the fact that the lobe outline of the inner rotor is centered on an axis spaced from and parallel to the axis on which the recess outline of the outer rotor is centered, this spacing being termed the “eccentricity.” One cycle is defined as the rotation required for the inner rotor to advance one lobe in relation to the outer rotor, and the total volumetric expansion (or contraction) of the spaces between gear lobes of a specified thickness in one cycle is termed the “displacement” of the rotor set. Internally generated rotor sets are not known to have been previously used in chemical metering applications. 
     In a practical device using an internally generated rotor set of the foregoing type, there are a number of ways of supporting the rotors. Both rotors may be rotated about fixed axes, or either of the rotors may be held fixed while the other rotor is rotated and orbited in relation to it. As between these alternatives the choice is determined to some extent by end use considerations. In a chemical metering application it is generally desired that rotors rotate about fixed axes. 
     SUMMARY OF THE INVENTION 
     As embodied and broadly described herein, the invention is a rotary fluid displacement device for pumping low viscosity or abrasive fluids such as are currently used in the two-part polyurethane foam industry, comprising a pump housing having a fluid inlet port and a fluid outlet port, and an internally generated rotor set located within the pump housing. The internally generated rotor set further comprises an inner rotor including a support having a predetermined number of fragmental cylindrical recesses equally spaced about its periphery and a rigid cylindrical tooth rotor received in each recess in rotational sliding contact therewith. Each tooth member has substantially the same diameter as the corresponding recess and a portion of each tooth member is projecting from the periphery. An internally toothed outer rotor is formed as the conjugate of the inner rotor, and is in simultaneous rolling engagement with all of the tooth members. Moreover, the outer rotor has a number of teeth one greater than that of the inner rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a longitudinal elevation in section on a plane through the central axis of a rotary pump having an internal gear set embodying the invention. 
         FIG. 2  is a transverse elevation in section taken on line  2 — 2  of  FIG. 1  showing the construction of the novel internal gear set. 
       FIG.  3 . is a transverse elevation of an inner and outer rotor set of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the present preferred embodiments of the invention, an example of which is illustrated in the accompanying drawings. 
     In accordance with the invention, the rotary pump has an inner rotor and an outer rotor, with the inner rotor being located within the outer rotor. 
     As shown in  FIG. 1 , the pump has a mounting plate  12  of annular shape with a central bore in which a bearing  14  is fitted to receive a drive shaft  16 . A seal  18  of a suitable type is received in an annular enlargement of the bore at one end of the sleeve bearing. The plate  12  also has a flat mounting surface  20  and tapped holes  22  for mounting the pump on a suitable support. 
     Two blind diametral holes are drilled and tapped in the plate  12  to define an outlet port  24  and an inlet port  26 . These ports respectively receive the pressure and suction lines of the pump. The outlet port  24  preferably communicates with a hole  28  that is aligned with a hole  30  in the sleeve bearing  14  that permits the fluid to enter the clearances between the bearing  14  and the shaft  16 , thus providing lubrication between the two. 
     Each of the ports  24  and  26  communicates with one face of the plate  12  through an arcuate kidney-shaped aperture or port  32  or  34 , respectively, the outlines of these ports being shown by broken lines in FIG.  2 . 
     A housing  36  is bolted on the plate  12  by bolts  38 . To ensure accuracy of positioning transversely to the shaft axis, the housing has an accurately machined annular shoulder  40  fitting within a flange  42  formed by an accurate counterbore on the mating face of the plate. To ensure accuracy of positioning angularly about the shaft axis the housing and plate have holes to receive the ends of a locating pin  44 . A circular O-ring seal  46  is also provided between the housing and plate. 
     The housing has an accurate blind central bore  48  concentric with the shaft axis. In this bore a sleeve bearing  50  is fitted to receive the shaft  16 . The shaft, which has its axis fixed in relation to the support on which the pump is mounted, receives and is keyed by means of cylindrical keys  52  to a support member  54  forming a part of an inner gear  56 . 
     The housing  36  has a counterbore  57  machined eccentrically to the axis of the shaft  16 . In  FIG. 2  the axis of the shaft is shown at  58  and that of the counterbore  57  is shown at  60 . Within this counterbore is fitted an outer rotor or internally generated rotor  62 . A vent  64  is drilled diagonally through the housing  36  from a point on the end of the counterbore inward of the outer rotor  62 , to the root of the bore  48 , and serves to prevent pressure loading of the shaft. 
     The inner and outer rotors are shown in further detail in FIG.  3  and are constructed as follows. Each of the rotors is formed from flat plate stock having precisely parallel surfaces. The inner rotor is formed by machining a number of parallel holes of equal diameter equally spaced radially from and angularly about the axis  58 . In a preferred embodiment, there are seven 0.1875 inch diameter holes on a 0.396 inch pitch circle. The edges of the plate are then cut to produce surfaces  66  intersecting the machines holes, leaving fragmental cylindrical recesses or pockets  68  defining the roll spaces of the inner rotor and opening through the outer periphery thereof. Solid metal cylindrical tooth members or lobes  70  are slidably received, preferably with a slip fit in the recesses or pockets  68 . These lobes  70  have substantially the same diameter as the holes from which the recesses  68  are formed. The length of the tooth members or lobes  70  is substantially equal to the inner rotor thickness. Thus, the lobes  70  project radially beyond the openings through the outer periphery of the inner rotor to provide the teeth  72  for the inner rotor. 
     The outer rotor  62  is formed on a standard Fellows gear shaper or any other known machine for producing the desired shape, thereby producing teeth  72  and spaces  74 . The number of teeth  72  is one greater than the number of cylindrical tooth members or lobes  70  on the inner rotor which in the case of the preferred embodiment is seven. The form of the teeth  72  and spaces  74  is thereby generated as the conjugate of the inner rotor. 
     The outer rotor shaping operations may be understood by considering parts of  FIG. 2  as a plan view of a commercially available Fellows gear shaper, wherein the part  36  represents the horizontal bed of the machine rotating about a fixed axis represented by  60  in the drawing, and the part  62  represents the workpiece which is initially a blank ring mounted on the bed so as to be rotatable about its own axis, this axis being coincident with the axis  60 . One of the lobes  70  in the figure may be considered as representing a circular metal cutting tool having an axis c and a diameter equal to one of the lobes  70 . The cutting tool is mounted on the cutter spindle of the gear shaper which has a fixed axis s at right angles to the plane of the sheet. The spindle moves in axial strokes in the manner characteristic of gear shapers. Instead of mounting the circular cutting tool coaxially with its spindle, it is secured to the spindle eccentrically with its axis c spaced from the axis s of the spindle by the eccentricity which is equal to the distance between the axes  58  and  60  in the finished pump. The axis s of the spindle is also spaced from that of the blank (represented by  60 ) by the pitch radius of the rolls or lobes of the inner rotor. In the case of the preferred embodiment, the eccentricity is 0.035 inches and the pitch radius of the inner rolls is 0.198 inches. 
     During the rotor cutting operation the axis c rotates at constant speed around the fixed axis s. Also, the blank rotates around its fixed axis  60  at a constant speed synchronized therewith and in the same sense or direction. This is accomplished by suitable adjustment of the gear train on the shaper between the cutter spindle and the bed. It will be seen that one tooth of the outer rotor blank is formed in each revolution of the axis c, and therefore the gearing is such that in one complete revolution of the blank there are as many revolutions of the axis c as there are teeth to be cut in the outer gear, namely, one more than the number of lobes in the inner rotor  56 . 
     Therefore, it will be evident that in operation of the pump, every tooth member or lobe  70  of the inner rotor will theoretically remain continuously in contact with the surface of the outer gear, thereby creating as many expansible and contractable interstitial spaces or chambers as there are lobes on the inner rotor. In  FIG. 2  these chambers are designated  76 ,  78 ,  80 ,  82 ,  84  and  86 . If the shaft  16  is rotated in the direction of the arrow, the outer rotor  62  is constrained to rotate about the axis  60  at a somewhat lower velocity which bears the same ratio to that of the shaft as the number of tooth members on the inner gear bears to the number of teeth on the outer gear. The chambers therefore progress counterclockwise as viewed in FIG.  2 . It will be seen that the chambers in communication with the aperture or port  32  are contracting in volume, while those in communication with the aperture or port  34  are expanding. Therefore, the device pumps fluid from the suction port  26  to the pressure port  24 , each chamber progressing through a complete cycle of expansion and contraction in one revolution of the shaft. The total volume displaced by one chamber per cycle, times the number of chambers, equals the theoretical pump displacement per cycle. 
     The rotor set herein described is characterized by rolling action of the individual tooth members or lobes  70  on the internally generated surface of the outer rotor  62 , as contrasted to sliding action that takes place in many of the commonly used internal gear sets of this general type. This rolling action entails rotational sliding of each tooth member or lobe  70  within its individual recess or pocket  68 . Because of this rolling contact, the load capability is greatly increased through the elimination of wear resulting from galling, welding and scoring associated with sliding friction. Because of both centrifugal and pressure forces, the rolls are forced into intimate contact with the outer rotor, thus providing fluid tight sealing and allowing compensation for wear. 
     Alternative structures of the inner gear may be employed, in addition to which the number and diameter of the lobes  70  may be chosen to conform to particular operational specifications. Thus the support member  54  may be constructed of various materials and may take various forms consistently with the provision of recesses for the tooth members. 
     Also, the lobes  70  may be of tubular or sleeve form, that is, of hollow cylindrical form, thereby reducing weight. They may also be in the form of cylinders over which wear sleeves of the same or a different material are fitted. The outer surfaces may be treated to resist wear, particularly when light weight materials such as aluminum are employed. 
     In a preferred embodiment, the invention is a rotary fluid displacement device for dispensing fluids having low viscosities and/or abrasive properties. The fluids can be the constituent parts of a two-part polyurethane foam, or any other fluid having these properties. In this embodiment, the rotary fluid displacement device has an inlet port and an outlet port, wherein the inlet port is in fluid communication with a container housing a fluid. In addition, the pump has inner and outer rotors. When the pump is activated, the movement of the inner and outer rotors draws the fluid into the pump cavity and forces it out the outlet port. Fluids exhibiting low viscosities and/or abrasive properties are often used when making two-part polyurethane for foam-in-packaging. In a two-part polyurethane foam process, an isocyanate containing component and an polyol containing component are pumped separately through different pumps and are mixed after leaving the pump outlet. 
     Because of fixed internal clearances, volumetric efficiency on conventional gerotor forms drops rapidly when fluid viscosities are less than 10 centipoise (Cp.). Further, because of the sliding action of these devices, operation with abrasive fluids or slurries will cause rapid and permanent performance deterioration. In the case of the IGR, there is rolling and sealing contact between the rolls and the generated outer rotor surface and hydrodynamic action between the rolls and their respective recesses in the inner rotor. Initial IGR testing on fuels such as kerosene which has a viscosity of about 1.5 Cp. and water, with a viscosity of less than 1 Cp., demonstrated little performance deterioration due to internal leakage. Further, life testing with abrasive fluids has demonstrated that even with as much as 0.010 inches of wear on the rolls, performance was not materially affected. These tests clearly substantiate the compensating ability of the device. 
     The present invention also includes the method of pumping low viscosity and/or abrasive fluids with a rotary fluid displacement device. In a preferred embodiment, the method comprises the steps of pumping the fluid from a container to the fluid inlet, through the pump motor, and out the fluid outlet. The pumping action is accomplished by using an internally generated rotor set as described above. 
     The use of an internally generated rotor set in this type of low viscosity and/or abrasive metering application has resulted in substantial improvements as compared to the use of externally generated rotor sets. Due to the low viscosity and increased abrasiveness of the chemicals now being used in making two-part polyurethane foam, the life of rotary pumps having externally generated rotor sets has decreased from about 1,000 hours to about 100-200 hours. This has led to increased costs in equipment maintenance, as well as increased down time for equipment. Preliminary results for rotary pumps using internally generated rotor sets in making two-part polyurethane foam have shown that failure of the rotor set does not occur until the unit has been used for 1000-2000 hours, or more. 
     In a two-part polyurethane foam making process, the rotor set is considered to be in a state of failure when the efficiency of the pump reaches less than 80%. Efficiency is determined by comparing the actual output flow against the theoretical output flow of the fluid. In an experiment conducted with a pump having an externally generated rotor set (EGR) and a pump having an internally generated rotor set (IGR), where the pumps were pumping fluid from the same drum at the same time, the following data was collected: 
                                                   HOURS OPERATED   IGR EFFICIENCY   EGR EFFICIENCY                                0   95   92       238   93   79       281   93   72       303   93   66       542   92   no data       843   93   60       998   93   65       1889   89   no data       2332   89   no data                    
As evidenced by the data, the pump having an internally generated rotor set operated 2,332 hours without failing, i.e., the pump operated with efficiencies greater than 80%. The pump having an externally generated rotor set, on the other hand, failed in less than 238 hours. Thus, in this experiment, the internally generated rotor set had a life of about ten times that of the externally generated rotor set.
 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention and in construction of this invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only.