Patent Publication Number: US-2016245291-A1

Title: Self-priming centrifugal pump

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to pumps and, in particular, to self-priming pumps with enhanced performance, efficiency and/or serviceability. 
     2. Description of the Related Art 
     Self-priming centrifugal pumps generally include a spinning impeller positioned inside an annular volute, which in turn is positioned within a pump casing. The volute forms an eye at the center where liquid enters the pump and is directed into the center of the impeller. Rotation of the impeller accelerates the liquid outward to the perimeter of the impeller where it is collected in the volute and discharged from the pump casing at an elevated pressure. As the liquid is driven outward by the centrifugal force of the rotating impeller, a vacuum formed at the eye is used to draw source fluid through the inlet and into the pump. 
     In a “wet prime” type pump, a centrifugal pump is arranged in a casing designed to retain water when the pump is not operating. When the pump is started, the impeller in the pump casing starts to mix the retained water with the air in the case. Inside the casing, a “P-trap” is utilized to allow the air to be expelled from of the pump cavity via the pump outlet, while the water remains available to the impeller. This air expulsion continues until enough air has been removed from the piping connected to the pump suction inlet so that the impeller eye becomes substantially flooded. This point, the pump achieves prime. 
     In such wet prime pumps, the pump casing may include a partition to separate the suction (i.e., inlet) side from the pressure (i.e., outlet) side so that the air/water mixture discharges exclusively toward the outlet side of the casing. In the outlet-side chamber of the casing during the self-priming operation, air is expelled via the outlet and is prevented from flowing back into the inlet-side chamber by the partition, while liquid water remains available to flow back to the suction side around the submerged or partially submerged pump impeller. 
     Self-priming centrifugal pumps are employed in applications where the source liquid may not be uniform. For example, so-called “trash pumps” may be self-priming centrifugal pumps in which solids suspended in the fluid are allowed to be cycled through the pump. Trash pumps are used for, e.g., wastewater treatment, lift stations for municipal sewage, and waste handling for food processing plants. 
     SUMMARY 
     The present disclosure provides a self-priming centrifugal pump with enhanced efficiency and performance characteristics and/or features which facilitate installation, inspection and maintenance of the pump. For efficiency and performance, the pump may include a smooth fluid flow path which enhances pump output for a given input power, including one or more of a specially shaped and directed volute discharge, a lack of internal stiffening ribs on the pump casing walls, a necked inlet and a rounded, flow-channeling outlet aperture. For maintenance and serviceability, the pump may include one or more of a coarse-threaded drive shaft and impeller with a concentricity feature, a combination port for both filling the casing and accessing the inlet flapper valve, and a drive disassembly system which facilitates attachment or removal of the drive system from the pump. Any combination of the aforementioned features may be utilized in accordance with the present disclosure. 
     In one form thereof, the present disclosure provides a centrifugal pump including: a drive mechanism; an impeller drivingly connected to the drive mechanism; a casing having an inlet and an outlet. The casing includes: an inlet-side wall having an inlet aperture formed therein; an outlet-side wall joined to the inlet-side wall to form a cavity within the casing, the outlet-side wall having an outlet aperture; a volute disposed in the casing and in fluid communication with the inlet aperture and the outlet aperture, the volute having a central opening sized to receive the impeller and a spiral-shaped fluid channel such that the fluid channel progresses radially outwardly toward a volute discharge opening, the volute discharge opening defining a longitudinal discharge axis which extends through the outlet aperture. The volute is adapted to receive fluid accelerated outwardly by the impeller, direct the fluid radially outwardly through the spiral-shaped fluid channel, and discharge the fluid along the longitudinal discharge axis toward the outlet aperture. 
     In another form thereof, the present disclosure provides a centrifugal pump including: a drive mechanism; an impeller drivingly connected to the drive mechanism; a flapper valve; a casing having an inlet and an outlet. The casing includes: an inlet-side wall having an inlet aperture formed therein, the flapper valve positioned at the inlet aperture to admit a flow of fluid into the casing via the inlet aperture while preventing a flow of fluid out of the casing via the inlet aperture; an outlet-side wall joined to the inlet-side wall to form a cavity within the casing, the outlet-side wall having an outlet aperture; a partition wall interposed between the inlet-side wall and the outlet-side wall to form an inlet pump chamber and an outlet pump chamber, the partition wall having an inner drive aperture positioned to allow fluid communication between the inlet chamber and the outlet chamber via the inner drive aperture; a combination port formed in the casing near the flapper valve, the combination port sized and positioned to allow access to the flapper valve by a maintenance person, and to allow fluid to be added to the inlet pump chamber; and a fill vent formed through the casing on an opposite side of the partition wall as the combination port, such that the fill vent allows fluid communication between the outlet pump chamber and the ambient environment, whereby liquid added to the inlet pump chamber is allowed to flow to the outlet pump chamber via the inner drive aperture while air contained in the outlet pump chamber vents to atmosphere via the fill vent. 
     In yet another form thereof, the present disclosure provides a centrifugal pump including: a drive shaft having a first coarse thread and a first centering feature; an impeller drivingly connected to the drive shaft, the impeller having a second coarse thread and a second centering feature, the second coarse thread engageable with the first coarse thread of the drive shaft to selectively rotatably fix the drive shaft to the impeller, and the second centering feature engageable with the first centering feature to concentrically align the impeller with the drive shaft. 
     In still another form thereof, the present disclosure provides a centrifugal pump comprising: a drive mechanism; an impeller drivingly connected to the drive mechanism; a casing having an inlet and an outlet, and a drive disassembly system. The casing includes: an inlet-side wall having an inlet aperture formed therein; an outlet-side wall joined to the inlet-side wall to form a cavity within the casing, the outlet-side wall having an outlet aperture; a bore extending inwardly from the exterior of the outlet-side wall whereby the bore is accessible to a user of the pump. The drive disassembly system includes: a guide rail sized to be snugly received within the bore of the stiffener; and a rail guide having a bearing and a flange fixed to the bearing, the bearing sized to be slidingly received on the guide rail while the flange is fixed to the drive mechanism, such that the drive mechanism can be assembled into or removed from the casing while being supported by the guide rail. 
     In still another form thereof, the present disclosure provides a method of disassembling a drive mechanism from a centrifugal pump, the method including: inserting a rail into a bore formed in a casing of the pump, such that the rail fits snugly within the bore; sliding a rail guide over the rail and into engagement with the pump; affixing the rail guide to the drive mechanism while maintaining the rail guide in sliding engagement with the rail; and disconnecting the drive mechanism from the casing and sliding the drive mechanism away from the casing using the support of the rail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features of the disclosure, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of embodiments of the disclosure taken in conjunction with the accompanying drawings. These above-mentioned and other features of the invention may be used in any combination or permutation. 
         FIG. 1  is an elevation, cross-section view of a centrifugal pump made in accordance with the present disclosure, taken along the line I-I of  FIG. 8  but with the drive disassembly system of  FIG. 8  removed; 
         FIG. 2  is an enlarged view of a portion of  FIG. 1 , illustrating a drive shaft assembly connection to the pump impeller; 
         FIG. 3  is a perspective view of the pump shown in  FIG. 1 , illustrating a flapper access port and fill vent; 
         FIG. 3A  is a perspective view of an alternative pump casing in accordance with the present disclosure; 
         FIG. 3B  is another perspective view of the alternative pump casing shown in  FIG. 3A ; 
         FIG. 4  is an elevation, cross-section view of the casing of the pump shown in  FIG. 1 , taken along the line IV-IV of  FIG. 1 ; 
         FIG. 5  is an elevation, cross-section view of the casing of the pump shown in  FIG. 1 , taken along the line V-V of  FIG. 4 ; 
         FIG. 6  is a bottom plan, cross-section view of the casing of the pump shown in  FIG. 1 , taken along the line VI-VI of  FIG. 1 ; 
         FIG. 7  is an elevation, partial cross-section view of the pump shown in  FIG. 1 , taken along the line VII-VII of  FIG. 8 , illustrating the pump outlet; 
         FIG. 8  is a perspective view of the pump shown in  FIG. 1 , and including a drive disassembly system attached thereto; 
         FIG. 9  is another perspective view of the pump shown in  FIG. 8 , illustrating removal of the drive mechanism via the drive disassembly system; 
         FIG. 10  is a perspective view of the drive shaft and impeller shown in  FIG. 1 ; 
         FIG. 11  is an exploded, partial cross-section view of the pump shown in  FIG. 1 , illustrating an impeller inspection port; 
         FIG. 12A  is a perspective, cross-section view of the casing of the pump shown in  FIG. 1 ; 
         FIG. 12B  is a perspective view of the pump shown in  FIG. 1 , illustrating features on the inlet side of the pump; and 
         FIG. 13  illustrates centralizing single-start Acme threads. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the disclosure and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     The present disclosure provides a self-priming centrifugal pump, shown as pump  10  in, e.g.,  FIGS. 1, 3 and 8 , which includes various features providing increased pump efficiency and/or facilitating installation, inspection and maintenance, among other benefits. 
     For example, as shown in  FIG. 1  and further described in detail below, centrifugal pump  10  includes volute  34  having a geometry and configuration which tends to “aim” pressurized fluid toward outlet aperture  20  to aid in efficient fluid discharge. Outlet aperture  20  has a rounded, gradual transition area  102  leading to outlet adapter  98  to further facilitate discharge of pressurized fluid with minimal losses. Fastener bosses  104  are similarly rounded and shaped to minimize eddying and turbulence in the vicinity of outlet aperture  20  and direct the flow efficiently through outlet aperture  20 . 
     Further, both inlet pump chamber  30  and outlet pump chamber  32  are substantially free of stiffening ribs, which also promotes a smooth and laminar fluid flow through chambers  30 ,  32  and minimizing turbulence. More particularly, inlet and outlet pump chambers  30 ,  32  are each substantially defined by respective inner surfaces of casing  12 , and by respective surfaces of partition wall  24  as shown in  FIG. 5  and further described below. These surfaces are substantially free of stiffening ribs such that no stiffening ribs are disposed within the fluid flow paths through chambers  30 ,  32 . In order to provide strength to casing  12 , stiffening ribs  100  are located at the outside surface of the pump casing  12  as shown, e.g., in  FIG. 3 . An alternative design of stiffening ribs  100 A is shown in  FIGS. 3A and 3B . 
     Still further efficiency and performance is realized by locating drain plugs  130 ,  134  ( FIGS. 11 and 12B , respectively) and their associated drain channels  132  ( FIG. 11 ) and  136  ( FIG. 12A ) at locations outside the flow path of volute  34 , in order to provide for gravitationally draining the pump casing  12  without introducing any features in the vicinity of volute  34  which can cause turbulence or eddying and thereby mitigating abrasive wear during pump operation. 
     With regard to serviceability, pump  10  includes combination port  82  ( FIG. 3 ) which doubles as a flapper access portion and a fill port for adding liquid (e.g., water) to casing  12  for pump priming. Fill vent  92  facilitates this priming functionality, while combination port cover  84  provides a single integral unit for covering both port  82  and vent  92 . Combination port  82  both reduces manufacturing cost and complexity by requiring only one aperture through casing  12  for two functions, while also facilitating installation and maintenance of pump  10  as described below. 
     Pump  10  may also be used in conjunction with drive disassembly system  50  ( FIGS. 8 and 9 ) to facilitate removal of drive mechanism  40  from pump casing  12  for service or inspection. Reinstallation of drive mechanism  40  is also made easier by drive disassembly system  50 , as described in detail below. 
     Within drive mechanism  40 , drive shaft  46  couples to impeller  44  via coarse threads  72 ,  76  ( FIG. 2 ), which promotes ease of installation and prevents cross-threading. In order to maintain a high level of concentricity between drive shaft  46  and impeller  44 , the coarse threads are supplemented with a tight-tolerance fit between distal nubbin  70  formed on drive shaft  46  and bore  74  formed in impeller  44 , as shown in  FIG. 2 . 
     Pump  10  further includes provisions for inspecting and maintaining impeller  44  from the inlet side of casing  12 , by removal of inspection cover  110  and inspection side wear plate  112 , as shown in  FIG. 11 . 
     Various features of centrifugal pump  10  are described in turn below. The embodiment disclosed below is not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiment is chosen and described so that others skilled in the art may utilize its teachings. Moreover, it is appreciated that a pump made in accordance with the present disclosure may include any one of the following features or any combination of the following features, and may exclude any number of the following features as required or desired for a particular application. 
     1. Smoothed Interior Surfaces 
     Centrifugal pump  10  includes several features related to pump casing  12  which, individually and in the aggregate, contribute to enhanced pump efficiency and performance by minimizing turbulent flows and eddying of fluid as it passes from inlet aperture  18  to outlet aperture  20  via inlet pump chamber  30  and outlet pump chamber  32 . 
     For example, beginning at inlet aperture  18  shown in  FIG. 1 , inlet adapter  90  may include a necked portion  91  with a gradually increasing flow area as fluid passes from inlet conduit  140  ( FIG. 3 ) through adapter  90  and toward inlet aperture  18  in casing  12 . For example and as shown in  FIG. 1 , the flow area may substantially constant through a cylindrical portion of adapter  90 , and may then gradually increase through a tapered (e.g., frustoconical) necked portion  91  which continuously increases the diameter of a circular flow area. This continuous and gradual increase provides a smooth, low-turbulence flow of fluid from inlet conduit  140  through inlet aperture  18  and into inlet pump chamber  30 , as necked portion  91  gradually relieves fluid pressure at inlet aperture  18  and allows fluid to more slowly and smoothly transition its flow direction downward through pump chamber  30  toward impeller  44 . 
     Moreover, providing necked portion  91  in inlet adapter  90  allows centrifugal pump  10  to be used with a variety of nominal sizes for inlet conduit  140  and outlet conduit  142  ( FIG. 3 ) for given sizes of inlet and outlet apertures  18 ,  20 . For example, adapters  90  and  98  may allow a given size of centrifugal pump  10  (e.g., a 3-inch, 4-inch or 6-inch pump, referring to the nominal size of outlet conduit  142 ) to be used with various sizes of inlet and outlet conduits  140 ,  142  by providing the appropriate set of adapters. For conduits  140  or  142  which do not match the size of apertures  18  or  20  respectively, a necked portion (e.g., necked portion  91 ) permits this size disparity while avoiding or minimizing a fluid efficiency penalty from an abrupt change in flow area from conduit  140 ,  142  and aperture  18 ,  20  respectively. It is contemplated that either, both or neither of inlet and outlet adapters  90 ,  98  may include a necked portion to facilitate smooth flow as required or desired for a particular application. 
     In an exemplary embodiment, inlet and outlet conduits  140  and  142  are provided with the same nominal size while aperture  18  is larger than aperture  20 . Necked portion  91  provides a gradual “step up” of the flow path area through inlet adapter  90  to accommodate an inlet fluid conduit  140  of a smaller flow area than inlet aperture  18 . At the same time, outlet adapter  98  may have a flow area substantially equal to outlet aperture  20 , in order to receive pressurized flow from volute  34  without posing an impediment to smooth flow. Outlet adapter  98  therefore may not need a necked portion similar to necked portion  91  of inlet adapter  90 . 
     After passing into inlet pump chamber  30 , fluid is drawn into channels  45  of impeller  44 , which rotates under power provided by drive shaft  46  to accelerate the fluid outwardly into fluid channel  36  of volute  34 , as best seen in  FIGS. 4 and 5 . In the illustrative embodiment of  FIGS. 1, 11 , impeller  44  is a “double curvature” design including two fluid channels  45  defining a fluid flow path which spirals radially outwardly. Although the illustrated design of impeller  44  is well suited to a “trash pump” application for centrifugal pump  10  (e.g., where pump  10  accepts fluids with solids in suspension or other non-uniform fluid characteristics), it is appreciated that other designs may be used for impeller  44  as required or desired for a particular application. 
     Pressurized fluid discharged from impeller  44  to volute  34  travels through the spiral-shaped volute fluid channel  36  to discharge opening  38 , which defines discharge axis A V  “aimed” to pass directly through outlet aperture  20  as further described below. The pressurized fluid is directed by discharge opening  38  along volute discharge axis A V , such that the fluid passes directly through outlet pump chamber  32  and toward outlet aperture  20 , as shown in  FIGS. 4 and 5  and further described in detail below. 
     As the pressurized fluid approaches outlet aperture  20 , outlet transition area  102  and fastener bosses  104  provide rounded and smooth transition surfaces to facilitate smooth fluid flow from outlet pump chamber  32  to outlet adapter  98  and ultimately to outlet conduit  142  ( FIG. 3 ). Specifically, referring to  FIG. 5 , the transition from the substantially horizontal top wall of outlet side wall  16  of casing  12  to the substantially vertical side wall of outlet adapter  98  (i.e., the “internal edge” of outlet aperture  20 ) is a radiused (also known as “filleted”) transition in which the radius of the fillet is generally commensurate with the thickness of the adjacent portion of outlet side wall  16 . In an exemplary embodiment, for example, the radius of the fillet varies from as little as equal to the minimum thickness of outlet side wall  16  to as much as 1.3 the minimum thickness. In the illustrated embodiment, for example, the radius of the fillet around outlet aperture  20  is 0.75 inches while the wall thickness is 0.57 inches, though of course these nominal values will vary depending on the size and power of pump  10 . In exemplary embodiments, the nominal fillet radius is at least 131% of the minimum wall thickness. 
     Fastener bosses  104  may be provided at the interior surfaces of casing  12  (i.e., within inlet and/or outlet pump chambers  30 ,  32 ) adjacent inlet and/or outlet apertures  18 ,  20 . Fastener bosses  104  provide for sufficient material to be available for threaded engagement of fasteners  105  with casing  12  to connect adapters  90 ,  98  to inlet and outlet side walls  14 ,  16  respectively, as shown in  FIG. 1 . Referring to the depiction of bosses  104  adjacent to outlet aperture  20  in  FIGS. 5-7 , it can be seen that bosses  104  have a rounded profile which facilitates smooth flow from outlet chamber  32  to outlet adapter  98  via outlet aperture  20 . 
     As best seen in  FIGS. 1 and 7 , fastener bosses  104  provide a smoothly rounded, convex distal surface at their respective ends, and transition to an annular concave surface which forms the junction between the convex end surface and the adjacent inner surface of outlet side wall  16 . This concave-to-convex transition avoids abrupt corners or other sharp features within the fluid flow path in outlet chamber  32 , and particularly avoids such sharp features in the fluid flow path along volute discharge axis A V . In this way, rounded bosses  104  prevent or minimize turbulence in the fluid flow which might otherwise compromise pump efficiency and performance. 
     Although the lower-pressure space in inlet chamber  30  is less susceptible to adverse performance impacts relating to the shape of bosses  104  around inlet aperture  18  or any other threaded aperture in casing  12 , the same rounded shape of bosses  104  is provided for maximum pump efficiency. 
     Referring now to the bottom plan view of fastener bosses  104  in  FIG. 6 , three fastener bosses  104  closest to the volute discharge opening  38  are illustrated. In the context of  FIG. 6 , the three bosses  104  in question are shown along right side portion of outlet aperture  20  and within outlet chamber  32 . As illustrated, these bosses  104  are contoured in a “tear drop” shape, in which a pointed end of the tear drop is pointing toward outlet aperture  20 . This tear drop shape for fastener bosses  104  promotes a substantially laminar flow over the outer surface of bosses  104  as fluid discharged from volute  34  advances toward outlet aperture  20 . 
     Turning now to  FIGS. 3 and 3A-3B , yet another flow-enhancing feature is illustrated in the form of exterior ribs  100  and  100 A respectively, which are integrally and monolithically provided as a portion of the exterior of casings  12  and  12 A respectively. Ribs  100  and  100 A both serve to strengthen and rigidify casing  12  in order to prevent or minimize any potential bulging or flexing of the material of casing  12  from the substantial pressures (positive or negative) which may be developed in pump chambers  30 ,  32 . 
     In  FIG. 3 , a central vertical rib  100  extends from a lower base  106  upwardly to outlet aperture  20  and outlet adapter  98  on either side of casing  12 . For purposes of the present discussion, the “bottom” of pump  10  is base  106  while outlet aperture  20  and adapter  98  is at the “top” of pump  10 . “Vertical” is the direction extending from bottom to top. In addition, a plurality of front-to-back stiffening ribs  100  extend from the inlet side of casing  12 , along inlet side wall  14 , and terminate at the central vertical rib  100 . A further set of front-to back-ribs  100  extend from the drive side of casing  12  along outlet side wall  16 , and terminate at the vertical central rib  100  at staggered vertical positions as compared to the inlet-side ribs  100  such that each of drive-side ribs  100  intersect the central vertical rib  100  at a different vertical position than each of the inlet-side ribs  100 , as shown in  FIG. 3 . For purposes of the present discussion, the “front” of pump  10  is considered as the side from which drive shaft  46  projects, and the “back” of pump  10  is the side including inlet aperture  18  and adapter  90 . The “front-to-back” direction is substantially perpendicular to the “vertical” direction as illustrated in  FIG. 3 . 
     On the vertical face of outlet side wall  16 , ribs  100  all emanate radially outwardly from a common center, as best seen in  FIG. 8 . In particular, nine ribs  100  extend radially outwardly along the vertical face from outer drive aperture  22  ( FIG. 9 ), round the corner at the junction between the vertical and side faces of outlet side wall  16 , and extend back to vertical rib  100  as noted above. A similar radially outwardly extending set of six ribs  100  are formed on the vertical face of inlet side wall  14 , as best seen in  FIG. 12B . 
     An alternative casing  12 A having a different arrangement of ribs  100 A is shown in  FIGS. 3A and 3B . For purposes of the present disclosure, casings  12 ,  12 A having ribs  100 ,  100 A respectively are interchangeable with the other components of pump  10 . Accordingly, a reference to casing  12  herein is also a reference to casing  12 A, unless otherwise specifically stated. Moreover, casing  12 A is substantially similar to casing  12  described herein, with reference numerals of casing  12 A corresponding to the reference numerals of casing  12 , except with an “A” appended thereto. Structures of casing  12  correspond to similar structures denoted by corresponding reference numerals of casing  12 A, except as otherwise noted. 
     As best seen in  FIG. 3A , five ribs  100 A extend generally radially outwardly from a central area of outlet side wall  16 , similar to the radially arranged ribs  100  described above. However, only the two lowermost ribs  100  extend horizontally from the rim around outer drive aperture  22 , round the corner at the junction between the vertical and side faces of outlet side wall  16 , and extend backwardly toward inlet side wall  14 . An uppermost rib  100 A extends vertically along the vertical face of outlet side wall  16  but, unlike the lowermost ribs  100 A, does not join the rim around outer drive aperture  22 . Two intermediate ribs  100 A are disposed between the lowermost and uppermost ribs  100 A, and extend radially outwardly from the central area of outlet side wall  16 . Like the uppermost rib  100 A, the intermediate ribs  100 A do not join the rim around outer drive aperture  22 . 
     As shown in  FIGS. 3A and 3B  and in contrast to casing  12  described above, casing  12 A lacks a vertical rib and does not have any stiffening ribs on inlet side wall  14 . In the illustrated embodiment, ribs  100 A are provided only on the high-pressure (i.e., outlet) side of casing  12 A, to which provides resistance to bulging or flexing of outlet side wall  16 . However, the size, number and extent of ribs  100 A are optimized, as shown in  FIG. 3A  and described above, to provide this resistance with a minimum of added material and expense. The low-pressure (i.e., inlet) side of casing  12 A has no ribs because, in the illustrated application, inlet side wall  14  alone may be sufficient to avoid excessive material flex from the relatively lower (and negative) pressures experienced in inlet pump chamber  30  ( FIG. 5 ). 
     The disposition of ribs  100  and  100 A only on the exterior surface of casings  12  and  12 A allows their strengthening function to be met without introduction of stiffening ribs inside pump chambers  30  and  32 . More particularly, the portion of inlet pump chamber  30  extending from inlet aperture  18  to impeller  44  is free of stiffening ribs along the interior surfaces of inlet side wall  14 , as well as along the surface of partition wall  24  which cooperates with such interior surfaces to form inlet chamber  30 . Similarly, the portion of outlet pump chamber  32  disposed generally between volute discharge opening  38  and outlet aperture  20  is also free of interior ribs along the interior surfaces of outlet side wall  16  and the adjacent portion of partition wall  24  which cooperates with such interior surfaces to form outlet chamber  32 . Accordingly, the portions of pump chambers  30  and  32  directly disposed in the flow path of fluid passing through centrifugal pump  10  are free from any stiffening ribs or other features designed for selective strengthening of inlet side or outlet side walls  14 ,  16 . 
     Advantageously, the lack of ribs or other stiffening features in the flow paths within casings  12  and  12 A facilitates flow with a minimum of turbulence and eddying, which reduces wear from fluid and solids in suspension while preserving hydraulic efficiency. Meanwhile, the provision of external ribs  100 ,  100 A as shown in  FIGS. 3 and 3A-3B  respectively (described in detail above) provide the strength and rigidity to casings  12 ,  12 A associated with such strengthening features. 
     2. Volute Discharge 
     In  FIG. 4 , volute discharge axis A V  is illustrated from the front, i.e., from a perspective facing a “spin plane” of impeller  44  that is perpendicular to its axis of rotation. In  FIG. 5 , volute discharge axis A V  is illustrated from the side, i.e., from a perspective facing a vertical center plane containing the axis of rotation of impeller  44 . 
       FIG. 4  illustrates that the spiral-shaped pathway of volute  34  does not terminate in a discharge opening defining a vertical discharge axis, but rather, continues its spiral-shaped pathway to produce the illustrated axis A V  which directs fluid flow from discharge opening  38  across the center plane of casing  12  and back toward outlet aperture  20 , which resides on the opposite side of the center plane. Channel  36  is a spiral-shaped structure as illustrated, and defines a correspondingly spiral-shaped flow axis centrally located in channel  36  and extending through the entire extent of channel  36 . As fluid flows through channel  36 , it follows this spiral-shaped flow axis until it is discharged at discharge opening  38 . 
     In the illustrated embodiment, discharge axis A V  is tangent to this spiral-shaped flow axis at discharge opening  38 , and is oriented or “aimed” to pass directly through outlet aperture  20 . In an exemplary embodiment, axis A V  is also perpendicular to a plane defined by discharge opening  38 . This angled and aimed arrangement for axis A V  directs pressurized fluid flowing from discharge opening  38  directly toward outlet aperture  20 , thereby minimizing turbulence, deceleration or eddying of fluid along the side walls of outlet side wall  16  of casing  12  as the fluid flows toward and through outlet aperture  20 . 
     Turning to the side view of  FIG. 5 , axis A V  is also shown to be forwardly angled with respect to a vertical direction, i.e., angled with respect to the substantially vertical walls of inlet and outlet side walls  14  and  16 , while also being non-perpendicular with the substantially horizontal base  106  and opposing top portions of inlet and outlet side walls  14 ,  16 . Moreover, axis A V  is generally aimed toward outlet aperture  20 , as viewed in the side section view of  FIG. 5 , to promote discharge from discharge opening  38  with a maximum volume of fluid received at outlet aperture  20  and a minimum volume of fluid traveling at high speed along partition wall  24  disposed adjacent volute  34 . Directing flow from discharge opening  38  along a path angled away from the adjacent partition wall  24  avoids frictional interaction between the fluid and partition wall  24 , and thereby promotes efficient operation of centrifugal pump  10 . 
     3. Drain Channels 
     Turning now to  FIGS. 11, 12A and 12B , drain channels  132  ( FIG. 11 ) and  136  ( FIG. 12A ) passing through selected locations within casing  12  are illustrated. Drain channels  132 ,  136  are both in direct fluid communication with respective lower portions of outlet pump chamber  32 , such that drain plugs  130 ,  134  ( FIG. 12B ) respectively can be removed to allow fluid trapped in outlet pump chamber  32  to be drained from casing  12  by gravity and without inverting centrifugal pump  10 . In particular, both drain channels  132 ,  136  are in direct fluid communication with a sump region  138  formed in a lower portion of casing  12 . 
     In the illustrated embodiment, centrifugal pump  10  is a self-priming “wet prime” pump design. In the illustrated self-priming pump design, casing  12  is designed to retain water or other liquid within sump region  138  when pump  10  is not operating. Impeller  44  can draw fluid stored in sump region  138  upon activation of pump  10 , and can expel any entrapped air from the outlet aperture  20  while picking up additional liquid until a vacuum at inlet aperture  18  is created to draw additional liquid into casing  12  from the source. At this point, the pump is “primed” and ready for regular service. As described in detail below, the liquid in sump region  138  may be initially introduced into casing  12  via a combination fill port and flapper access port  82  ( FIG. 3 ). In the context of the present disclosure, the “air” in the casing is the non-pumpable fluid (i.e. gas) which resides in the casing during normal operation. 
     As best seen in  FIG. 12A , sump region  138  has a central portion which is in direct fluid communication with impeller  44 , while the remainder of the sump region is separated from impeller  44  by the wall forming volute  34 . As illustrated in  FIGS. 11 and 12A  respectively, drain channels  132 ,  136  are arranged outside the volute flow path and on opposite sides of volute  34  and impeller  44 , and are therefore in indirect fluid communication with the central portion of sump  138  accessed by impeller  44 . That is, the draining of the central portion of sump  138  via drain channels  132  and/or  136  would require the fluid to first migrate to the other portions of sump region  138  (i.e., the portions not in direct fluid communication with impeller  44 ), and then enter channel  132  or  136 . 
     In this way, drain channels  132 ,  136  do not form any apertures or other features which are in direct fluid communication with, or form any part of, volute  34 . Therefore, drain channels  132 ,  136  do not interrupt or otherwise affect the fluid mechanics of impeller  44 . For purposes of the present disclosure, two distinct fluid areas are in “direct” fluid communication if fluid exchange between the two areas does not require the fluid flow path to change direction or otherwise “turn a corner.” By contrast, two distinct fluid areas are in “indirect” fluid communication if fluid exchange between the two areas does require the fluid flow path to change direction or otherwise “turn a corner.” 
     4. Combination Fill/Inspection Port 
     Turning now to  FIG. 3 , port  82  is shown in an upper end of inlet side wall  14  of casing  12 . Port  82  serves as a combination port, accomplishing two functions: access to flapper valve  80  and related structures for, e.g., installation, replacement or maintenance; and as a fill port for adding liquid to casing  12 , and particularly for adding liquid to sump region  138 , shown in  FIGS. 1 and 11  and described above. 
     Turning to  FIG. 1 , flapper valve  80  is shown in its installed, seated position upon inlet adapter  90 . In an exemplary embodiment, flapper valve  80  is formed as a resilient polymer or rubber material which bears against the annular inner surface of inlet adapter  90  (i.e., adjacent necked portion  91 ) to prevent flow of fluid from inlet pump chamber  30  back through inlet aperture  18  and inlet adapter  90 , while resiliently bending or “flapping” away from its seated position about a living hinge  81  ( FIG. 3 ) so that liquid can be freely admitted to inlet pump chamber  30  via inlet adapter  90  and inlet aperture  18 . Living hinge  81  connects flapper valve  80  to a valve mount portion  83 , which is attached to adapter  90  by fasteners  88  and retainers  86 A,  86 B as illustrated. 
     When centrifugal pump  10  is in service, inlet conduit  140  and outlet conduit  142  may both be rigidly affixed to adapters  90 ,  98 , respectively. In addition, base  106  of casing  12  may be affixed to the underlying surface, such as by mounting bolts  107  shown in  FIG. 3 . For these and other reasons, disconnection of inlet adapter  90  to access flapper valve  80  and its associated structures may not be practical or time efficient. However, because flapper valve  80  may be made from a relatively soft and resilient material such as polymer or rubber, relatively frequent inspection, maintenance or repair may be necessary. Combination port  82  offers access to flapper valve  80  from the top portion of centrifugal pump  10 , which is typically the most accessible portion to a service person when pump  10  is mounted in a service location and configuration. 
     To allow or prevent access to port  82 , combination port cover  84  is provided. When cover  84  is affixed to casing  12  by fasteners  114 , fill port cover portion  94  provides a seal (in cooperation with an O-ring positioned about the periphery of port  82 ) around flapper access port  82 , which fluidly isolates inlet pump chamber  30  from the ambient environment and thereby allows vacuum or suction pressure to develop therewithin for proper operation of pump  10 . When removed, as shown in  FIG. 3 , port  82  allows a service person to remove fasteners  88 , retainers  86 A and  86 B, and flapper valve  80  for inspection, maintenance and/or repair. Additionally, because port  82  is offset along a front-to-back direction with respect to flapper valve  80  as shown in  FIG. 1 , removal of port cover  84  also allows for a visual inspection of flapper valve  80  and its associated structures without removal of the same from inlet adapter  90 . 
     Turning again to  FIG. 3 , casing  12  includes fill vent  92  which offers selective fluid communication between outlet pump chamber  32  and the ambient environment. Fill vent  92  facilitates the use of combination port  82  as a fill port for admitting liquid into casing  12 , and specifically to sump region  138  from the inlet side, by allowing displaced air to vent to the ambient atmosphere from outlet pump chamber  32  via vent  92  as water flows into sump  138  from the inlet side. Combination port cover  84  also serves to fluidly isolate outlet pump chamber  32  from the ambient environment when cover  84  is installed upon casing  12 , by covering vent  92  with fill vent cover portion  96  (and an O-ring positioned about the periphery of vent  92 ). As best seen in  FIG. 3 , fill vent cover portion  96  is formed as a forward extension of fill port cover portion  94  in order to pass over partition wall  24  and onto fill vent  92 . In the illustrated embodiment, fill port cover portion  94  and fill vent cover portion  96  are integrally and monolithically formed as a single component. 
     In one exemplary embodiment, fasteners  114  used to connect combination port cover  84  to combination port  82  include an enlarged flat fastener head having a fastener aperture  116  formed therethrough. For field inspections and maintenance, field surface fasteners  114  facilitate removal and installation of combination port cover  84  by engagement with a service person&#39;s hand, any wrench or clamp capable of engaging the flat head portion of fasteners  114 . Alternatively as shown in  FIG. 9 , rod R may be passed through fastener aperture  116  to gain leverage. 
     5. Assembly and Alignment of Drive Shaft and Impeller 
       FIGS. 1, 2 and 10  illustrate the connection between drive shaft  46  and impeller  44 . As described in further detail below, this connection facilitates initial assembly and subsequent reassembly by providing a coarse threaded engagement which is easy to thread and difficult to cross-thread. In order to maintain a desired concentricity between the rotational axis of impeller  44  and axis A D  drive shaft  46 , distal nubbin  70  formed on drive shaft  46  defines a tight clearance fit with a corresponding bore  74  formed in impeller  44 . 
     Referring to  FIG. 1 , drive shaft  46  protrudes from a front surface of casing  12  as part of drive mechanism  40  attached thereto. In addition to drive shaft  46 , drive mechanism  40  includes a plurality of bearings  47  supported by drive shaft housing  42  and rotatably supporting drive shaft  46 , such that drive shaft  46  can freely rotate with respect to housing  42 . Drive side wear plate  48  is connected to drive shaft  46  and biased by a biasing element (illustrated as a compression spring) into firm engagement with drive shaft housing  42  and away from contact with impeller  44 . Cover plate  49  connects to the front (i.e. outer) surface of housing  42  to retain and protect bearings  47  (which may be, for example, a ball bearing or roller bearing). Impeller  44  is fixed to drive shaft  46  (as described further below) and forms the final component of drive mechanism  40 . 
     When drive mechanism  40  is initially assembled or reassembled (e.g., after inspection or maintenance) as illustrated in  FIG. 10 , male threads  72  of drive shaft  46  are engaged with the correspondingly formed female threads  76  of impeller  44  to affix drive shaft  46  to impeller  44 , as best seen in  FIG. 2 . In the illustrated embodiment, threads  72  and  76  are coarse threads which promote easy initial thread alignment and engagement and correspondingly deter cross-threading or other mis-engagement of male threads  72  with female threads  76 . In one exemplary embodiment, best seen in  FIG. 2 , threads  72  and  76  are trapezoidal thread forms, sometimes referred to as “acme” threads, which provide a relatively loose thread engagement and a robust resistance to cross-threading. An exemplary embodiment of “coarse” trapezoidal threads useable in connection with the present disclosure are Acme “Centralizing Screw Threads” of tolerance class 4C as defined in ANSI/ASME B1.5-1997, the entire disclosure of which is hereby expressly incorporated by reference herein. The use of such coarse trapezoidal threads  72 ,  76  ensure that when drive shaft  46  is inserted through the other components of drive mechanism  40  and initially engaged with impeller  44 , rotation of drive shaft  46  with respect to impeller  44  in the tightening direction causes a reliably proper thread engagement. 
     Further detail regarding class 4C centralizing threads in accordance with the present disclosure is provided in Tables 7a, 7b and 8-11 below and  FIG. 13 . 
     
       
         
           
               
             
               
                 TABLE 7a 
               
             
            
               
                   
               
               
                 American National Standard Centralizing Acme Single-Start 
               
               
                 Screw Threads - Formulas for Determining Diameters (ASME/ANSI B1.5-1988) 
               
               
                 D = Nominal Size or Diameter in Inches 
               
               
                 P = Pitch = 1 ÷ Number of Threads per Inch 
               
            
           
           
               
               
            
               
                 No. 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Classes 2C, 3C, and 4C External Threads (Screws) 
               
               
                 1 
                 Major Diam., Max = D (Basic). 
               
               
                 2 
                 Major Diam., Min = D minus tolerance from Table 11, cols. 7, 8, or 10. 
               
               
                 3 
                 Pitch Diam., Max = Int. Pitch Diam., Min (Formula 9) minus allowance from Table 
               
               
                   
                 9, cols. 3, 4, or 5. 
               
               
                 4 
                 Pitch Diam., Min = Ext. Pitch Diam., Max (Formula 3) minus tolerance from Table 
               
               
                   
                 10. 
               
               
                 5 
                 Minor Diam., Max = D minus P minus allowance from Table 11, col. 3. 
               
               
                 6 
                 Minor Diam., Min = Ext. Minor Diam., Max (Formula 5) minus 1.5 × Pitch Diam. 
               
               
                   
                 tolerance from Table 10. 
               
               
                   
                 Classes 2C, 3C, and 4C Internal Threads (Nuts) 
               
               
                 7 
                 Major Diam., Min = D plus allowance from Table 11, col. 4. 
               
               
                 8 
                 Major Diam., Max = Int. Major Diam., Min (Formula 7) plus tolerance from Table 
               
               
                   
                 11, cols. 7, 9, or 11. 
               
               
                 9 
                 Pitch Diam., Min = D Minus Pl2 (Basic). 
               
               
                 10 
                 Pitch Diam., Max = Int. Pitch Diam., Min (Formula 9) plus tolerance from Table 
               
               
                   
                 10. 
               
               
                 11 
                 Minor Diam., Min = D minus 0.9P. 
               
               
                 12 
                 Minor Diam., Max = Int. Minor Diam., Min (Formula 11) plus tolerance from Table 
               
               
                   
                 11, col. 6. 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7b 
               
               
                   
               
               
                 Limiting Dimensions of American National Standard Centralizing Acme Single- 
               
               
                 Start Screw Threads, Classes 2C, 3C, and 4C (ASME/ANSI B1.5-1988) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 Nominal Diameter, D 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 ½ 
                 ⅝ 
                 ¾ 
                 ⅞ 
                 1 
                 1⅛ 
                 1¼ 
                 1⅜ 
                 1½ 
               
            
           
           
               
               
            
               
                   
                 Threads per Inch* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Limiting Diameters 
                 10 
                 8 
                 6 
                 6 
                 5 
                 5 
                 5 
                 4 
                 4 
               
               
                   
               
            
           
           
               
            
               
                 External Threads 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Classes 2C, 3C, and 4C, 
                   
                 Max 
                 0.5000 
                 0.6250 
                 0.7500 
                 0.8750 
                 1.0000 
                 1.1250 
                 1.2500 
                 1.3750 
                 1.5000 
               
               
                 Major Diameter 
               
               
                 Class 2C, Major Diameter 
                   
                 Min 
                 0.4975 
                 0.6222 
                 0.7470 
                 0.8717 
                 0.9965 
                 1.1213 
                 1.2461 
                 1.3709 
                 1.4957 
               
               
                 Class 3C, Major Diameter 
                   
                 Min 
                 0.4989 
                 0.6238 
                 0.7487 
                 0.8736 
                 0.9985 
                 1.1234 
                 1.2483 
                 1.3732 
                 1.4982 
               
               
                 Class 4C, Major Diameter 
                   
                 Min 
                 0.4993 
                 0.6242 
                 0.7491 
                 0.8741 
                 0.9990 
                 1.1239 
                 1.2489 
                 1.3738 
                 1.4988 
               
               
                 Classes 2C, 3C, and 4C, 
                   
                 Max 
                 0.3800 
                 0.4800 
                 0.5633 
                 0.6883 
                 0.7800 
                 0.9050 
                 1.0300 
                 1.1050 
                 1.2300 
               
               
                 Minor Diameter 
               
               
                 Class 2C, Minor Diameter 
                   
                 Min 
                 0.3594 
                 0.4570 
                 0.5371 
                 0.6615 
                 0.7509 
                 0.8753 
                 0.9998 
                 1.0719 
                 1.1965 
               
               
                 Class 3C, Minor Diameter 
                   
                 Min 
                 0.3704 
                 0.4693 
                 0.5511 
                 0.6758 
                 0.7664 
                 0.8912 
                 1.0159 
                 1.0896 
                 1.2144 
               
               
                 Class 4C, Minor Diameter 
                   
                 Min 
                 0.3731 
                 0.4723 
                 0.5546 
                 0.6794 
                 0.7703 
                 0.8951 
                 1.0199 
                 1.0940 
                 1.2188 
               
               
                   
                   
                 Max 
                 0.4443 
                 0.5562 
                 0.6598 
                 0.7842 
                 0.8920 
                 1.0165 
                 1.1411 
                 1.2406 
                 1.3652 
               
               
                 Class 2C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Min 
                 0.4306 
                 0.5408 
                 0.6424 
                 0.7663 
                 0.8726 
                 0.9967 
                 1.1210 
                 1.2186 
                 1.3429 
               
               
                   
                   
                 Max 
                 0.4458 
                 0.5578 
                 0.6615 
                 0.7861 
                 0.8940 
                 1.0186 
                 1.1433 
                 1.2430 
                 1.3677 
               
               
                 Class 3C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Min 
                 0.4394 
                 0.5506 
                 0.6534 
                 0.7778 
                 0.8849 
                 1.0094 
                 1.1339 
                 1.2327 
                 1.3573 
               
               
                   
                   
                 Max 
                 0.4472 
                 0.5593 
                 0.6632 
                 0.7880 
                 0.8960 
                 1.0208 
                 1.1455 
                 1.2453 
                 1.3701 
               
               
                 Class 4C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Min 
                 0.4426 
                 0.5542 
                 0.6574 
                 0.7820 
                 0.8895 
                 1.0142 
                 1.1388 
                 1.2380 
                 1.3627 
               
            
           
           
               
            
               
                 Internal Threads 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Classes 2C, 3C, and 4C, 
                   
                 Min 
                 0.5007 
                 0.6258 
                 0.7509 
                 0.8759 
                 1.0010 
                 1.1261 
                 1.2511 
                 1.3762 
                 1.5012 
               
               
                 Major Diameter 
               
               
                 Classes 2C and 3C, Major 
                   
                 Max 
                 0.5032 
                 0.6286 
                 0.7539 
                 0.8792 
                 1.0045 
                 1.1298 
                 1.2550 
                 1.3803 
                 1.5055 
               
               
                 Diameter 
               
               
                 Class 4C, Major Diameter 
                   
                 Max 
                 0.5021 
                 0.6274 
                 0.7526 
                 0.8778 
                 1.0030 
                 0.1282 
                 1.2533 
                 1.3785 
                 1.5036 
               
               
                 Classes 2C, 3C, and 4C, 
                   
                 Min 
                 0.4100 
                 0.5125 
                 0.6000 
                 0.7250 
                 0.8200 
                 0.9450 
                 0.0700 
                 1.1500 
                 1.2750 
               
               
                 Minor Diameter 
                   
                 Max 
                 0.4150 
                 0.5187 
                 0.6083 
                 0.7333 
                 0.8300 
                 0.9550 
                 1.0800 
                 1.1625 
                 1.2875 
               
               
                   
                   
                 Min 
                 0.4500 
                 0.5625 
                 0.6667 
                 0.7917 
                 0.9000 
                 1.0250 
                 1.1500 
                 1.2500 
                 1.3750 
               
               
                 Class 2C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Max 
                 0.4637 
                 0.5779 
                 0.6841 
                 0.8096 
                 0.9194 
                 1.0448 
                 1.1701 
                 1.2720 
                 1.3973 
               
               
                   
                   
                 Min 
                 0.4500 
                 0.5625 
                 0.6667 
                 0.7917 
                 0.9000 
                 1.0250 
                 1.1500 
                 1.2500 
                 1.3750 
               
               
                 Class 3C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Max 
                 0.4564 
                 0.5697 
                 0.6748 
                 0.8000 
                 0.9091 
                 1.0342 
                 1.1594 
                 1.2603 
                 1.3854 
               
               
                   
                   
                 Min 
                 0.4500 
                 0.5625 
                 0.6667 
                 0.7917 
                 0.9000 
                 1.0250 
                 1.1500 
                 1.2500 
                 1.3750 
               
               
                 Class 4C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Max 
                 0.4546 
                 0.5676 
                 0.6725 
                 0.7977 
                 0.9065 
                 1.0316 
                 1.1567 
                 1.2573 
                 1.3824 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Nominal Diameter, D 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1¾ 
                 2 
                 2¼ 
                 2½ 
                 2¾ 
                 3 
                 3½ 
                 4 
                 4½ 
                 5 
               
            
           
           
               
               
            
               
                   
                 Threads per Inch* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Limiting Diameters 
                 4 
                 4 
                 3 
                 3 
                 3 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                   
               
            
           
           
               
            
               
                 External Threads 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Classes 2C, 3C, and 4C, 
                   
                 Max 
                 1.7500 
                 2.0000 
                 2.2500 
                 2.5000 
                 2.7500 
                 3.0000 
                 3.5000 
                 4.0000 
                 4.5000 
                 5.0000 
               
               
                 Major Diameter 
               
               
                 Class 2C, Major Diameter 
                   
                 Min 
                 1.7454 
                 1.9951 
                 2.2448 
                 2.4945 
                 2.7442 
                 2.9939 
                 3.4935 
                 3.9930 
                 4.4926 
                 4.9922 
               
               
                 Class 3C, Major Diameter 
                   
                 Min 
                 1.7480 
                 1.9979 
                 2.2478 
                 2.4976 
                 2.7475 
                 2.9974 
                 3.4972 
                 3.9970 
                 4.4968 
                 4.9966 
               
               
                 Class 4C, Major Diameter 
                   
                 Min 
                 1.7487 
                 1.9986 
                 2.2485 
                 2.4984 
                 2.7483 
                 2.9983 
                 3.4981 
                 3.9980 
                 4.4979 
                 4.9978 
               
               
                 Classes 2C, 3C, and 4C, 
                   
                 Max 
                 1.4800 
                 1.7300 
                 1.8967 
                 2.1467 
                 2.3967 
                 2.4800 
                 2.9800 
                 3.4800 
                 3.9800 
                 4.4800 
               
               
                 Minor Diameter 
               
               
                 Class 2C, Minor Diameter 
                   
                 Min 
                 1.4456 
                 1.6948 
                 1.8572 
                 2.1065 
                 2.3558 
                 2.4326 
                 2.9314 
                 3.4302 
                 3.9291 
                 4.4281 
               
               
                 Class 3C, Minor Diameter 
                   
                 Min 
                 1.4640 
                 1.7136 
                 1.8783 
                 2.1279 
                 2.3776 
                 2.4579 
                 2.9574 
                 3.4568 
                 3.9563 
                 4.4558 
               
               
                 Class 4C, Minor Diameter 
                   
                 Min 
                 1.4685 
                 1.7183 
                 1.8835 
                 2.1333 
                 2.3831 
                 2.4642 
                 2.9638 
                 3.4634 
                 3.9631 
                 4.4627 
               
               
                   
                   
                 Max 
                 1.6145 
                 1.8637 
                 2.0713 
                 2.3207 
                 2.5700 
                 2.7360 
                 3.2350 
                 3.7340 
                 4.2330 
                 4.7319 
               
               
                 Class 2C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Min 
                 1.5916 
                 1.8402 
                 2.0450 
                 2.2939 
                 2.5427 
                 2.7044 
                 3.2026 
                 3.7008 
                 4.1991 
                 4.6973 
               
               
                   
                   
                 Max 
                 1.6171 
                 1.8665 
                 2.0743 
                 2.3238 
                 2.5734 
                 2.7395 
                 3.2388 
                 3.7380 
                 4.2373 
                 4.7364 
               
               
                 Class 3C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Min 
                 1.6064 
                 1.8555 
                 2.0620 
                 2.3113 
                 2.5607 
                 2.7248 
                 3.2237 
                 3.7225 
                 4.2215 
                 4.7202 
               
               
                   
                   
                 Max 
                 1.6198 
                 1.8693 
                 2.0773 
                 2.3270 
                 2.5767 
                 2.7430 
                 3.2425 
                 3.7420 
                 4.2415 
                 4.7409 
               
               
                 Class 4C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Min 
                 1.6122 
                 1.8615 
                 2.0685 
                 2.3181 
                 2.5676 
                 2.7325 
                 3.2317 
                 3.7309 
                 4.2302 
                 4.7294 
               
            
           
           
               
            
               
                 Internal Threads 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Classes 2C, 3C, and 4C, 
                   
                 Min 
                 1.7513 
                 2.0014 
                 2.2515 
                 2.5016 
                 2.7517 
                 3.0017 
                 3.5019 
                 4.0020 
                 4.5021 
                 5.0022 
               
               
                 Major Diameter 
               
               
                 Classes 2C and 3C, Major 
                   
                 Max 
                 1.7559 
                 2.0063 
                 2.2567 
                 2.5071 
                 2.7575 
                 3.0078 
                 3.5084 
                 4.0090 
                 4.5095 
                 5.0100 
               
               
                 Diameter 
               
               
                 Class 4C, Major Diameter 
                   
                 Max 
                 1.7539 
                 2.0042 
                 2.2545 
                 2.5048 
                 2.7550 
                 3.0052 
                 3.5056 
                 4.0060 
                 4.5063 
                 5.0067 
               
               
                 Classes 2C, 3C, and 4C, 
                   
                 Min 
                 1.5250 
                 1.7750 
                 1.9500 
                 2.2000 
                 2.4500 
                 2.5500 
                 3.0500 
                 3.5500 
                 4.0500 
                 4.5500 
               
               
                 Minor Diameter 
                   
                 Max 
                 1.5375 
                 1.7875 
                 1.9667 
                 2.2167 
                 2.4667 
                 2.5750 
                 3.0750 
                 3.5750 
                 4.0750 
                 4.5750 
               
               
                   
                   
                 Min 
                 1.6250 
                 1.8750 
                 2.0833 
                 2.3333 
                 2.5833 
                 2.7500 
                 3.2500 
                 3.7500 
                 4.2500 
                 4.7500 
               
               
                 Class 2C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Max 
                 1.6479 
                 1.8985 
                 2.1096 
                 2.3601 
                 2.6106 
                 2.7816 
                 3.2824 
                 3.7832 
                 4.2839 
                 4.7846 
               
               
                   
                   
                 Min 
                 1.6250 
                 1.8750 
                 2.0833 
                 2.3333 
                 2.5833 
                 2.7500 
                 3.2500 
                 3.7500 
                 4.2500 
                 4.7500 
               
               
                 Class 3C, Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Max 
                 1.6357 
                 1.8860 
                 2.0956 
                 2.3458 
                 2.5960 
                 2.7647 
                 3.2651 
                 3.7655 
                 4.2658 
                 4.7662 
               
               
                   
                   
                 Min 
                 1.6250 
                 1.8750 
                 2.0833 
                 2.3333 
                 2.5833 
                 2.7500 
                 3.2500 
                 3.7500 
                 4.2500 
                 4.7500 
               
               
                 Class 4C Pitch Diameter 
                  {open oversize brace}  
               
               
                   
                   
                 Max 
                 1.6326 
                 1.8828 
                 2.0921 
                 2.3422 
                 2.5924 
                 2.7605 
                 3.2608 
                 3.7611 
                 4.2613 
                 4.7615 
               
               
                   
               
               
                 *All other dimensions are in inches. The selection of threads per inch is arbitrary and for the purpose of establishing a standard. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 American National Standard Centralizing Acme Single-Start Screw Thread Data (ASME/ANSI B1.5-1988) 
               
            
           
           
               
               
               
            
               
                   
                 Diameters 
                 Thread Data 
               
            
           
           
               
               
               
               
            
               
                   
                 Centralizing, Classes 
                   
                 Lead Angle at Basic 
               
            
           
           
               
               
               
               
            
               
                 Identification 
                 2C, 3C, and 4C 
                   
                 Pitch Diameter* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Threads 
                 Basic 
                 Pitch 
                 Minor 
                   
                   
                 Basic 
                 Basic 
                 Centralizing 
               
               
                 Nominal 
                 per 
                 Major 
                 Diameter, 
                 Diameter, 
                   
                 Thickness at 
                 Height of 
                 Width 
                 Classes 2C, 
               
               
                 Sizes 
                 Inch,* 
                 Diameter, 
                 D 2  = 
                 D 1  = 
                 Pitch, 
                 Pitch Line, 
                 Thread, 
                 of Flat, 
                 3C, and 4C, λ 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 (All Classes) 
                 n 
                 D 
                 (D − h) 
                 (D − 2h) 
                 P 
                 t = P/2 
                 h = P/2 
                 F = 0.3707P 
                 Deg 
                 Min 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 ¼ 
                 16 
                 0.2500 
                 0.2188 
                 0.1875 
                 0.06250 
                 0.03125 
                 0.03125 
                 0.0232 
                 5 
                 12 
               
               
                  5/16 
                 14 
                 0.3125 
                 0.2768 
                 0.2411 
                 0.07143 
                 0.03571 
                 0.03571 
                 0.0265 
                 4 
                 42 
               
               
                 ⅜ 
                 12 
                 0.3750 
                 0.3333 
                 0.2917 
                 0.08333 
                 0.04167 
                 0.04167 
                 0.0309 
                 4 
                 33 
               
               
                  7/16 
                 12 
                 0.4375 
                 0.3958 
                 0.3542 
                 0.08333 
                 0.04167 
                 0.04167 
                 0.0309 
                 3 
                 50 
               
               
                 ½ 
                 10 
                 0.5000 
                 0.4500 
                 0.4000 
                 0.10000 
                 0.05000 
                 0.05000 
                 0.0371 
                 4 
                 3 
               
               
                 ⅝ 
                 8 
                 0.6250 
                 0.5625 
                 0.5000 
                 0.12500 
                 0.06250 
                 0.06250 
                 0.0463 
                 4 
                 3 
               
               
                 ¾ 
                 6 
                 0.7500 
                 0.6667 
                 0.5833 
                 0.16667 
                 0.08333 
                 0.08333 
                 0.0618 
                 4 
                 33 
               
               
                 ⅞ 
                 6 
                 0.8750 
                 0.7917 
                 0.7083 
                 0.16667 
                 0.08333 
                 0.08333 
                 0.0618 
                 3 
                 50 
               
               
                 1 
                 5 
                 1.0000 
                 0.9000 
                 0.8000 
                 0.20000 
                 0.10000 
                 0.10000 
                 0.0741 
                 4 
                 3 
               
               
                 1⅛ 
                 5 
                 1.1250 
                 1.0250 
                 0.9250 
                 0.20000 
                 0.10000 
                 0.10000 
                 0.0741 
                 3 
                 33 
               
               
                 1¼ 
                 5 
                 1.2500 
                 1.1500 
                 1.0500 
                 0.20000 
                 0.10000 
                 0.10000 
                 0.0741 
                 3 
                 10 
               
               
                 1⅜ 
                 4 
                 1.3750 
                 1.2500 
                 1.1250 
                 0.25000 
                 0.12500 
                 0.12500 
                 0.0927 
                 3 
                 39 
               
               
                 1½ 
                 4 
                 1.5000 
                 1.3750 
                 1.2500 
                 0.25000 
                 0.12500 
                 0.12500 
                 0.0927 
                 3 
                 19 
               
               
                 1¾ 
                 4 
                 1.7500 
                 1.6250 
                 1.5000 
                 0.25000 
                 0.12500 
                 0.12500 
                 0.0927 
                 2 
                 48 
               
               
                 2 
                 4 
                 2.0000 
                 1.8750 
                 1.7500 
                 0.25000 
                 0.12500 
                 0.12500 
                 0.0927 
                 2 
                 26 
               
               
                 2¼ 
                 3 
                 2.2500 
                 2.0833 
                 1.9167 
                 0.33333 
                 0.16667 
                 0.16667 
                 0.1236 
                 2 
                 55 
               
               
                 2½ 
                 3 
                 2.5000 
                 2.3333 
                 2.1667 
                 0.33333 
                 0.16667 
                 0.16667 
                 0.1236 
                 2 
                 36 
               
               
                 2¾ 
                 3 
                 2.7500 
                 2.5833 
                 2.4167 
                 0.33333 
                 0.16667 
                 0.16667 
                 0.1236 
                 2 
                 21 
               
               
                 3 
                 2 
                 3.0000 
                 2.7500 
                 2.5000 
                 0.50000 
                 0.25000 
                 0.25000 
                 0.1853 
                 3 
                 19 
               
               
                 3½ 
                 2 
                 3.5000 
                 3.2500 
                 3.0000 
                 0.50000 
                 0.25000 
                 0.25000 
                 0.1853 
                 2 
                 48 
               
               
                 4 
                 2 
                 4.0000 
                 3.7500 
                 3.5000 
                 0.50000 
                 0.25000 
                 0.25000 
                 0.1853 
                 2 
                 26 
               
               
                 4½ 
                 2 
                 4.5000 
                 4.2500 
                 4.0000 
                 0.50000 
                 0.25000 
                 0.25000 
                 0.1853 
                 2 
                 9 
               
               
                 5 
                 2 
                 5.0000 
                 4.7500 
                 4.5000 
                 0.50000 
                 0.25000 
                 0.25000 
                 0.1853 
                 1 
                 55 
               
               
                   
               
               
                 *All other dimensions are given in inches. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 American National Standard Centralizing Acme Single-Start 
               
               
                 Screw Threads - Pitch Diameter Allowances (ASME/ANSI B1.5-1988) 
               
            
           
           
               
               
            
               
                   
                 Allowances on External Threads† 
               
               
                   
                 Centralizing 
               
            
           
           
               
               
               
               
            
               
                 Nominal Size Range* 
                 Class 2C, 
                 Class 3C, 
                 Class 4C, 
               
            
           
           
               
               
               
               
               
            
               
                 Above 
                 To and Including 
                 0.008{square root over (D)} 
                 0.006{square root over (D)} 
                 0.004{square root over (D)} 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0 
                  3/16 
                 0.0024 
                 0.0018 
                 0.0012 
               
               
                  3/16 
                  5/16 
                 0.0040 
                 0.0030 
                 0.0020 
               
               
                  5/16 
                  7/16 
                 0.0049 
                 0.0037 
                 0.0024 
               
               
                  7/16 
                  9/16 
                 0.0057 
                 0.0042 
                 0.0028 
               
               
                  9/16 
                  11/16 
                 0.0063 
                 0.0047 
                 0.0032 
               
               
                  11/16 
                  13/16 
                 0.0069 
                 0.0052 
                 0.0035 
               
               
                  13/16 
                  15/16 
                 0.0075 
                 0.0056 
                 0.0037 
               
               
                  15/16 
                 1 1/16 
                 0.0080 
                 0.0060 
                 0.0040 
               
               
                 1 1/16 
                 1 3/16 
                 0.0085 
                 0.0064 
                 0.0042 
               
               
                 1 3/16 
                 1 5/16 
                 0.0089 
                 0.0067 
                 0.0045 
               
               
                 1 5/16 
                 1 7/16 
                 0.0094 
                 0.0070 
                 0.0047 
               
               
                 1 7/16 
                 1 9/16 
                 0.0098 
                 0.0073 
                 0.0049 
               
               
                 1 9/16 
                 1⅞ 
                 0.0105 
                 0.0079 
                 0.0052 
               
               
                 1⅞ 
                 2⅛ 
                 0.0113 
                 0.0085 
                 0.0057 
               
               
                 2⅛ 
                 2⅜ 
                 0.0120 
                 0.0090 
                 0.0060 
               
               
                 2⅜ 
                 2⅝ 
                 0.0126 
                 0.0095 
                 0.0063 
               
               
                 2⅝ 
                 2⅞ 
                 0.0133 
                 0.0099 
                 0.0066 
               
               
                 2⅞ 
                 3¼ 
                 0.0140 
                 0.0105 
                 0.0070 
               
               
                 3¼ 
                 3¾ 
                 0.0150 
                 0.0112 
                 0.0075 
               
               
                 3¾ 
                 4¼ 
                 0.0160 
                 0.0120 
                 0.0080 
               
               
                 4¼ 
                 4¾ 
                 0.0170 
                 0.0127 
                 0.0085 
               
               
                 4¾ 
                 5½ 
                 0.0181 
                 0.0136 
                 0.0091 
               
               
                   
               
               
                 All dimensions are given in inches. 
               
               
                 *The values in cols. 3 to 5 are to be used for any size within the range shown in cols. 1 and 2. These values are calculated from the mean of the range. 
               
               
                 It is recommended that the sizes given in Table 8 be ued whenever possible. 
               
               
                 †An increase of 10 percent in the allowance is recommended for each inch, or fraction thereof, that the length of engagement exceeds two diameters. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 American National Standard Centralizing Acme Single-Start Screw 
               
               
                 Threads - Pitch Diameter Tolerances 1  (ASME/ANSI B1.5-1988) 
               
               
                 (For any particular size of thread, the pitch diameter tolerance is obtained 
               
               
                 by adding the diameter increment from the upper half of the table to the 
               
               
                 pitch increment from the lower half of the table. Example: A 0.250-16- 
               
               
                 ACME-2C thread has a pitch diameter tolerance of 0.00300 + 0.00750 = 
               
               
                 0.0105 inch.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 Class of Thread 
               
            
           
           
               
               
               
               
            
               
                   
                 2C 
                 3C 
                 4C 
               
            
           
           
               
               
               
            
               
                   
                 Nom. 
                 Dimeter Increment 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Dia., 2  D 
                 .006{square root over (D)} 
                 .0028{square root over (D)} 
                 .002{square root over (D)} 
               
               
                   
                   
               
               
                   
                 ¼ 
                 .00300 
                 .00140 
                 .00100 
               
               
                   
                  5/16 
                 .00335 
                 .00157 
                 .00112 
               
               
                   
                 ⅜ 
                 .00367 
                 .00171 
                 .00122 
               
               
                   
                  7/16 
                 .00397 
                 .00185 
                 .00132 
               
               
                   
                 ½ 
                 .00424 
                 .00198 
                 .00141 
               
               
                   
                 ⅝ 
                 .00474 
                 .00221 
                 .00158 
               
               
                   
                 ¾ 
                 .00520 
                 .00242 
                 .00173 
               
               
                   
                 ⅞ 
                 .00561 
                 .00262 
                 .00187 
               
               
                   
                 1 
                 .00600 
                 .00280 
                 .00200 
               
               
                   
                 1⅛ 
                 .00636 
                 .00297 
                 .00212 
               
               
                   
                 1¼ 
                 .00671 
                 .00313 
                 .00224 
               
               
                   
                 1⅜ 
                 .00704 
                 .00328 
                 .00235 
               
               
                   
                 1½ 
                 .00735 
                 .00343 
                 .00245 
               
               
                   
                 1¾ 
                 .00794 
                 .00370 
                 .00265 
               
               
                   
                 2 
                 .00849 
                 .00396 
                 .00283 
               
               
                   
                 2¼ 
                 .00900 
                 .00420 
                 .00300 
               
               
                   
                 2½ 
                 .00949 
                 .00443 
                 .00316 
               
               
                   
                 2¾ 
                 .00995 
                 .00464 
                 .00332 
               
               
                   
                 3 
                 .01039 
                 .00485 
                 .00346 
               
               
                   
                 3½ 
                 .01122 
                 .00524 
                 .00374 
               
               
                   
                 4 
                 .01200 
                 .00560 
                 .00400 
               
               
                   
                 4½ 
                 .01273 
                 .00594 
                 .00424 
               
               
                   
                 5 
                 .01342 
                 .00626 
                 .00447 
               
               
                   
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                   
               
            
           
           
               
               
            
               
                   
                 Class of Thread 
               
            
           
           
               
               
               
               
            
               
                   
                 2C 
                 3C 
                 4C 
               
            
           
           
               
               
               
            
               
                   
                 Thds. per 
                 Pitch Increment 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Inch, n 
                 .030{square root over (1/n)} 
                 .014{square root over (1/n)} 
                 .010{square root over (1/n)} 
               
               
                   
                   
               
               
                   
                 16 
                 .00750 
                 .00350 
                 .00250 
               
               
                   
                 14 
                 .00802 
                 .00374 
                 .00267 
               
               
                   
                 12 
                 .00866 
                 .00404 
                 .00289 
               
               
                   
                 10 
                 .00949 
                 .00443 
                 .00316 
               
               
                   
                 8 
                 .01061 
                 .00495 
                 .00354 
               
               
                   
                 6 
                 .01225 
                 .00572 
                 .00408 
               
               
                   
                 5 
                 .01342 
                 .00626 
                 .00447 
               
               
                   
                 4 
                 .01500 
                 .00700 
                 .00500 
               
               
                   
                 3 
                 .01732 
                 .00808 
                 .00577 
               
               
                   
                 2½ 
                 .01897 
                 .00885 
                 .00632 
               
               
                   
                 2 
                 .02121 
                 .00990 
                 .00707 
               
               
                   
                 1½ 
                 .02449 
                 .01143 
                 .00816 
               
               
                   
                 1⅓ 
                 .02598 
                 .01212 
                 .00866 
               
               
                   
                 1 
                 .03000 
                 .01400 
                 .01000 
               
               
                   
                   
               
               
                   
                 All dimensions are given in inches. 
               
               
                   
                   1 The equivalent tolerance on thread thickness is 0.259 times the pitch diameter tolerance. 
               
               
                   
                   2 For a nominal diameter between any two tabulated nominal diameters, use the diameter increment for the larger of the two tabulated nominal diameters. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 American National Standard Centralizing Acme Single-Start Screw Threads - Tolerances 
               
               
                 and Allowances for Major and Minor Diameters** (ASME/ANSI B1.5-1988) 
               
            
           
           
               
               
               
               
            
               
                   
                 Allowance From Basic Major and 
                 Toler. 
                 Tolerance on Major Diameter Plus 
               
               
                   
                 Minor-Diameters (All Classes) 
                 on Minor 
                 on Internal, Minus on External Threads 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Minor 
                   
                 Diam.*** 
                 Class 2C 
                   
               
               
                   
                 Diam.† 
                 Internal Thread 
                 All 
                 External 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 All 
                 Major 
                 Minor 
                 Internal 
                 and 
                 Class 3C 
                 Class 4C 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Thds* 
                 External 
                 Diam.‡ 
                 Diam.† 
                 Threads, 
                 Internal 
                 External 
                 Internal 
                 External 
                 Internal 
               
               
                 Size 
                 per 
                 Threads 
                 (Plus 
                 (Plus 
                 (Plus 
                 Threads, 
                 Thread, 
                 Thread, 
                 Thread, 
                 Thread, 
               
               
                 (Nom.) 
                 Inch 
                 (Minus) 
                 0.0010{square root over (D)}) 
                 0.1P) 
                 0.05P) 
                 0.0035{square root over (D)} 
                 0.0015{square root over (D)} 
                 0.0035{square root over (D)} 
                 0.0010{square root over (D)} 
                 0.0020{square root over (D)} 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 ¼ 
                 16 
                 0.010 
                 0.0005 
                 0.0062 
                 0.0050 
                 0.0017 
                 0.0007 
                 0.0017 
                 0.0005 
                 0.0010 
               
               
                  5/16 
                 14 
                 0.010 
                 0.0006 
                 0.0071 
                 0.0050 
                 0.0020 
                 0.0008 
                 0.0020 
                 0.0006 
                 0.0011 
               
               
                 ⅜ 
                 12 
                 0.010 
                 0.0006 
                 0.0083 
                 0.0050 
                 0.0021 
                 0.0009 
                 0.0021 
                 0.0006 
                 0.0012 
               
               
                  7/16 
                 12 
                 0.010 
                 0.0007 
                 0.0083 
                 0.0050 
                 0.0023 
                 0.0010 
                 0.0023 
                 0.0007 
                 0.0013 
               
               
                 ½ 
                 10 
                 0.020 
                 0.0007 
                 0.0100 
                 0.0050 
                 0.0025 
                 0.0011 
                 0.0025 
                 0.0007 
                 0.0014 
               
               
                 ⅝ 
                 8 
                 0.020 
                 0.0008 
                 0.0125 
                 0.0062 
                 0.0028 
                 0.0012 
                 0.0028 
                 0.0008 
                 0.0016 
               
               
                 ¾ 
                 6 
                 0.020 
                 0.0009 
                 0.0167 
                 0.0083 
                 0.0030 
                 0.0013 
                 0.0030 
                 0.0009 
                 0.0017 
               
               
                 ⅞ 
                 6 
                 0.020 
                 0.0009 
                 0.0167 
                 0.0083 
                 0.0033 
                 0.0014 
                 0.0033 
                 0.0009 
                 0.0019 
               
               
                 1 
                 5 
                 0.020 
                 0.0010 
                 0.0200 
                 0.0100 
                 0.0035 
                 0.0015 
                 0.0035 
                 0.0010 
                 0.0020 
               
               
                 1⅛ 
                 5 
                 0.020 
                 0.0011 
                 0.0200 
                 0.0100 
                 0.0037 
                 0.0016 
                 0.0037 
                 0.0011 
                 0.0021 
               
               
                 1¼ 
                 5 
                 0.020 
                 0.0011 
                 0.0200 
                 0.0100 
                 0.0039 
                 0.0017 
                 0.0039 
                 0.0011 
                 0.0022 
               
               
                 1⅜ 
                 4 
                 0.020 
                 0.0012 
                 0.0250 
                 0.0125 
                 0.0041 
                 0.0018 
                 0.0041 
                 0.0012 
                 0.0023 
               
               
                 1½ 
                 4 
                 0.020 
                 0.0012 
                 0.0250 
                 0.0125 
                 0.0043 
                 0.0018 
                 0.0043 
                 0.0012 
                 0.0024 
               
               
                 1¾ 
                 4 
                 0.020 
                 0.0013 
                 0.0250 
                 0.0125 
                 0.0046 
                 0.0020 
                 0.0046 
                 0.0013 
                 0.0026 
               
               
                 2 
                 4 
                 0.020 
                 0.0014 
                 0.0250 
                 0.0125 
                 0.0049 
                 0.0021 
                 0.0049 
                 0.0014 
                 0.0028 
               
               
                 2¼ 
                 3 
                 0.020 
                 0.0015 
                 0.0333 
                 0.0167 
                 0.0052 
                 0.0022 
                 0.0052 
                 0.0015 
                 0.0030 
               
               
                 2½ 
                 3 
                 0.020 
                 0.0016 
                 0.0333 
                 0.0167 
                 0.0055 
                 0.0024 
                 0.0055 
                 0.0016 
                 0.0032 
               
               
                 2¾ 
                 3 
                 0.020 
                 0.0017 
                 0.0333 
                 0.0167 
                 0.0058 
                 0.0025 
                 0.0058 
                 0.0017 
                 0.0033 
               
               
                 3 
                 2 
                 0.020 
                 0.0017 
                 0.0500 
                 0.0250 
                 0.0061 
                 0.0026 
                 0.0061 
                 0.0017 
                 0.0035 
               
               
                 3½ 
                 2 
                 0.020 
                 0.0019 
                 0.0500 
                 0.0250 
                 0.0065 
                 0.0028 
                 0.0065 
                 0.0019 
                 0.0037 
               
               
                 4 
                 2 
                 0.020 
                 0.0020 
                 0.0500 
                 0.0250 
                 0.0070 
                 0.0030 
                 0.0070 
                 0.0020 
                 0.0040 
               
               
                 4½ 
                 2 
                 0.020 
                 0.0021 
                 0.0500 
                 0.0250 
                 0.0074 
                 0.0032 
                 0.0074 
                 0.0021 
                 0.0042 
               
               
                 5 
                 2 
                 0.020 
                 0.0022 
                 0.0500 
                 0.0250 
                 0.0078 
                 0.0034 
                 0.0078 
                 0.0022 
                 0.0045 
               
               
                   
               
               
                 *All other dimensions are given in inches. Intermediate pitches take the values of the next coarser pitch listed. 
               
               
                 **Values for intermediate diameters should be calculated from the formulas in column headings, but ordinarily may be interpolated. 
               
               
                 ***To avoid a complicated formula and still provide an adequate tolerance, the pitch factor is used as a basis, with the minimum tolerance set at 0.005 in. 
               
               
                 †The minimum clearance at the minor diameter between the internal and external thread is the sum of the values in columns 3 and 5. 
               
               
                 ‡The minimum clearance at the major diameter between the internal and external thread is equal to col. 4. 
               
               
                 Tolerance on minor diameter of all external threads is 1.5 × pitch diameter tolerance. 
               
            
           
         
       
     
     When threads  72 ,  76  are fully engaged, impeller  44  becomes rotatably fixed in the drive direction to the distal end of drive shaft  46 . That is to say, when impeller is rotated in the fluid-accelerating direction by drive shaft  46 , the engagement of threads  72 ,  76  tends to be tightened and the full engagement of threads  72 ,  76  is maintained. When impeller  44  is rotated in the opposite (i.e., non-functional) direction, threads  72 ,  76  will tend to disengage. Thus, to connect drive shaft  46  to impeller  44 , impeller  44  is immobilized and drive shaft  46  is rotated in the tightening direction until threads  72 ,  76  are engaged. Subsequent operation of pump  10  will ensure that this engagement is maintained, and therefore drive shaft  46  is selectively rotatably fixed to impeller  44 . To disconnect drive shaft  46  from impeller  44 , impeller  44  is immobilized and drive shaft  46  is rotated in the opposite direction to disengage threads  72 ,  76 . 
     As male threads  72  and female threads  76  approach full engagement as shown in  FIG. 2 , distal nubbin  70  formed at the end of drive shaft  46  encounters a correspondingly formed bore  74  formed in impeller  44 . Both nubbin  70  and bore  74  may be machined to a tight tolerance in order to concentrically align drive shaft  46  and impeller  44  with a precise and close-tolerance fit upon final assembly. In one exemplary embodiment, the total radial clearance between distal nubbin  70  and bore  74  is less than 0.004 inches, such as between 0.001 inches and 0.003 inches. Advantageously, the interaction between nubbin  70  and bore  74  reduces or eliminates any non-concentricity between drive shaft axis A D  and the intended rotational axis of impeller  44 . In an exemplary embodiment, the threaded connection formed by threads  72 ,  76  allows for a relatively large radial play of drive shaft axis A D  relative to the rotational axis of impeller  44 . That is, when drive shaft  46  is connected to by threads  72 ,  76  and not by nubbin  70  and bore  74 , the opposite end of drive shaft  46  is allowed to move radially such that drive shaft axis A D  becomes angled with respect to the rotational axis of impeller  44 . 
     In one exemplary embodiment, this radial play may be between 0.001 inches and 0.003 inches, as defined in ANSI/ASME B1.5-1997, the entire disclosure of which is hereby expressly incorporated by reference herein. By contrast, when nubbin  70  and bore  74  are engaged in addition to threads  72 ,  76  such that impeller  44  is tightened fully against the adjacent shoulder of drive shaft  46 , this radial play is eliminated and drive shaft axis A D  becomes substantially concentric with the rotational axis of impeller  44 . 
     Although drive shaft  46  includes the male features used to connect drive shaft  46  to impeller  44  (i.e., male threads  72  and nubbin  70 ) and impeller  44  includes the female features (i.e., female threads  76  and bore  74 ), it is contemplated that this arrangement can be reversed as required or desired for a particular design. That is, either component can be provided with male threads  72  and the other component can be provided with the corresponding female threads  76 . Similarly, either component can be provided with a male centering feature such as nubbin  70 , and the other component can be provided with the corresponding female feature such as bore  74 . 
     6. Drive Disassembly System 
     Turning now to  FIGS. 8 and 9 , drive disassembly system  50  used for disconnecting and connecting drive mechanism  40  from casing  12  and the remainder of pump  10  is illustrated. As described below, the components of drive disassembly system  50  may be connected to pump  10  when to facilitate removal and/or installation of drive mechanism  40 , and can be disconnected from pump  10  during regular operation. 
     Drive disassembly system  50  includes guide rail  52  selectively received within blind bore  66  ( FIG. 1 ) formed in a central stiffener  28 . Stiffener  28  extends along a front-to-back direction from the vertical portion of outlet side wall  16  to partition wall  24 , and provides a structural support which inhibits bulging or deflection of outlet side wall  16  under the high pressures developed within outlet pump chamber  32 . The strength and structural integrity afforded by stiffener  28  and its associated structures also firmly supports guide rail  52  within bore  66 . 
     In an exemplary embodiment, guide rail  52  is snugly received in bore  66 . For example, the total radial clearance between guide rail  52  and bore  66  may be between 0.0015 inches and 0.0055 inches. When so snugly received, guide rail  52  has minimal radial play and therefore firmly supports drive mechanism  40  during assembly and disassembly procedures as described further below. 
     In order to axially fix guide rail  52  in its fully received position in bore  66 , rail keeper  56  may be used to engage notch  54  formed in guide rail  52  ( FIG. 9 ). Rail keeper  56  may then be fastened to casing  12  in order to axially fix rail keeper  56  and guide rail  52  to casing  12 . 
     Rail guide  58  includes bearing  60  sized to be slidingly received over guide rail  52 , and flange  62  is fixed to bearing  60  (e.g., by welding). 
     When drive disassembly system  50  is used to remove drive mechanism  40  from casing  12 , guide rail is first installed as described above. A portion of the standard installation fasteners  43  holding drive mechanism  40  in place ( FIG. 1 ) are removed, such as the four fasteners  43  closest to guide rail  52 . Bearing  60  is then slid onto the previously installed guide rail  52  until flange  62  of rail guide  58  abuts casing  12 , as shown in  FIG. 8 . Fasteners  65  are passed through apertures  64  (not shown) formed in of flange  62  to bolt rail guide  58  to drive mechanism  40  at the locations where standard fasteners  43  were removed. In an exemplary embodiment, apertures  64  through flange  62  are oversized relative to fasteners  65 , which allows fasteners  65  to move slightly within apertures  64  such that alignment of drive disassembly system  50  relative to casing  12  can be controlled by interaction between guide rail  52  and bearing  60 , rather than between flange  62  and casing  12 . 
     Fasteners  65  used in connection with drive disassembly system  50  are larger than standard fasteners  43  used to secure drive shaft housing  42  to casing  12  ( FIG. 1 ). In this way, fasteners  43  are allowed pass through the threaded apertures  64  in the flange of drive shaft housing  42  (i.e., without threadably engaging threaded apertures  64 ), but fasteners  65  threadably connect to apertures  64 . In this way, drive mechanism  40  is fixed to casing  12  by fasteners  43 , while rail guide  58  is fixed to housing  42  by the larger threaded fasteners  65 . 
     With rail guide  58  affixed to drive shaft housing  42  and slidingly received upon guide rail  52 , drive mechanism  40  is ready to be removed from outer drive aperture  22  formed in outlet side wall  16  of casing  12 , as illustrated in  FIG. 9 . Any remaining fasteners  43  affixing drive shaft housing  42  to casing  12  are removed to free drive mechanism  40  from the remainder of pump  10 . 
     Turning to  FIG. 9 , drive mechanism  40  may then be pulled free of casing  12  using guide rail  52  for support of drive mechanism  40 . Advantageously, guide rail  52  accepts the weight of drive mechanism  40 , allowing the service person to focus on guiding drive mechanism  40  safely free of casing  12  without having to also support the weight manually. In addition, caster  68  may be affixed to a lower portion of drive shaft housing  42 , such as by caster bracket  69 , in order to cooperate with guide rail  52  to provide support for the weight of drive mechanism  40  during removal or installation in casing  12 . In the illustrated embodiment, standard fasteners  43  may be removed along the bottom portion of housing  42  to expose threaded apertures  64  (not shown), such that fasteners  65  can be threadably engaged with apertures  64  to affix bracket  69  to housing  42  in a similar fashion to rail guide  58  described above. 
     For installation or reinstallation of drive mechanism  40  via drive disassembly system  50 , the steps of removal are simply repeated in reverse. During final alignment of drive mechanism  40 , after it is received in outer drive aperture  22  and through inner drive aperture  26  ( FIG. 5 ), fasteners  65  may be loosened as needed to allow for any needed readjustment. 
     7. Impeller Inspection 
     Turning now to  FIG. 11 , inspection cover  110  and inspection side wear plate  112  are shown removed from their seated positions within casing  12  to expose outer inspection aperture  124  leading to inlet pump chamber  30 , as well as inner inspection aperture  126  leading to volute  34 , fluid channel  36  and impeller  44  seated in the central bore of volute  34 . 
     In an exemplary embodiment, inspection side wear plate  112  is fixed to inspection cover  110 , such as by fasteners. When so fixed, removal of inspection cover  110  also removes inspection side wear plate  112  as a single unit to allow access to inlet pump chamber  30 , volute  34  and impeller  44  for inspection, maintenance or repair. When inspection cover  110  and wear plate  112  are reinstalled to casing  12  through outer inspection aperture  124 , the previous spacing and configuration between the wear surface of wear plate  112  and the adjacent bearing surface of impeller  44  is maintained. 
     In order to set and maintain such proper spacing, fastener  114  may be used to affix inspection cover  110  and inspection side wear plate  112  to casing  12  via cannulated bolt  118  and bolt fixation plate  120 . Only one of this fastener arrangement is illustrated in  FIG. 11  for clarity, it being understood that the illustrative embodiment uses four such fastener arrangements for each of fastener apertures  116  formed in inspection cover  110 . 
     Fastener apertures  116  are threaded to receive the correspondingly threaded shaft of cannulated bolt  118 . The length of the threaded portion of cannulated bolt  118  is such that each bolt  118  may protrude beyond the distal end of aperture  116  to bear against the adjacent face of casing  12 , which prevents inspection cover  110  from fully seating against casing  12  because the distal end of bolts  118  contact casing  12  before cover  110 . In this way, the spacing of inspection side wear plate  112  from impeller  44  can be controlled by adjusting cannulated bolts  118  to protrude more or less beyond the distal end of fastener apertures  116 . 
     In order to rotationally fix cannulated bolts  118  in a desired position corresponding to proper axial spacing between wear plate  112  and impeller  44 , fixation plate  120  and fastener  122  are provided. Fixation plate  120  includes a bolt head receiving aperture  121  which is generally polygonal in order to rotationally fix cannulated bolt  118  to fixation plate  120  when the hexagonal head of bolt  118  is received within aperture  121 . In an exemplary embodiment, aperture  121  is a “twelve point” style of the type commonly used in wrenches and sockets and designed to rotatably fix to hex bolt heads. Aperture  121  is placed over the head of bolt  118 , such that fastener slot  123  aligns with fixation aperture  117  formed in inspection cover  110 . Fastener  122  is then passed through slot  123  and into threaded engagement with aperture  117 , rotationally and axially fixing bolt fixation plate  120  to inspection cover  110 , and therefore fixing the rotational orientation and axial adjustment of cannulated bolt  118 . Fastener  114  (described in detail above) is then passed through the central bore of cannulated bolt  118  and threadably engaged with the adjacent threaded aperture of casing  12  to affix inspection cover  110  thereto. 
     While this invention has been described as having an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.