Abstract:
A method for restoring the registry of opposed shafts that carry meshing pump teeth inside a polymer pump that contains polymer without removing polymer from within the pump, the method using an asymmetric pattern of apertures in the shaft ends and a template having a pattern of holes that matches the pattern of apertures, and dowels that closely fit the holes and apertures.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to timing the registry of meshing teeth in a polymer pump that contains polymer. 
         [0003]    2. Description of the Prior Art 
         [0004]    Pumping apparatus that pumps molten polymer (polymer) and pressurizes that polymer can contain a pair of opposed shafts, each shaft carrying teeth that force viscous polymer from the inlet of the pump to its outlet. The pressure under which the polymer exists at the outlet of the pump is substantially elevated above the pressure existing at the inlet of the pump. For example, with high density polyethylene (HDPE), the inlet pressure can be from about 30 to about 40 psig at from about 3500 to about 5500 Fahrenheit (F), whereas the outlet pressure can be from about 2,000 to about 3,000 psig at from about 375° to about 575° F. 
         [0005]    The polymer fills the space between the teeth at the inlet side and is conveyed to the outlet side of the pump, after which the teeth are brought to their point of closest approach, i.e., cyclically into meshing engagement with one another, the engagement serving to exclude the polymer and generate pressure. The design of the teeth is such that the clearance between adjacent surfaces is minimized in part to prevent back flow of polymer from the high pressure outlet side of the pump back into the lower pressure inlet side. The greater this back flow of polymer, the less efficient the operation of the pump, causing the pump&#39;s turning speed to be increased to compensate, and wasting energy in the operation of the pump. 
         [0006]    Accordingly, to prevent this undesired back flow of polymer, the registry of the pump teeth relative to one another when in meshing engagement must be timed to be very close, but without any actual physical contact of the meshed teeth. If the teeth contact one another when meshed, premature and undesired wear of the teeth occur thereby not only allowing back flow of polymer, but also requiring shutdown of the pump and an expensive, premature reworking of the worn teeth. Since each shaft of such a pump can cost as much as $100,000, it is desirable to maintain the non-touching registry of the teeth on these opposed shafts for as long as possible. For example, when the desired non-touching teeth registry is maintained, the operating life of such a pump can extend for up to 5 years, whereas if touching during pumping occurs, this life span can be reduced to 2 years at the very best. 
         [0007]    However, to prevent polymer back flow, the gap (tolerance) between adjacent teeth when in meshing engagement must be quite small, about 0.02 of an inch in the case of HDPE. The opposing teeth bearing shafts are fixed relative to one another to maintain this non-touching timing. 
         [0008]    When a pump is new and contains no polymer, the teeth are clean of polymer and the desired non-touching gap registry between adjacent meshed teeth can easily be achieved even in the field, e.g., when installed in the plant. This is so because one can readily obtain access to the interior of the pump and physically gauge the gap between adjacent meshed teeth before the opposing shafts are fixed to one another to maintain this registry while the pump is in operation. 
         [0009]    However, from time to time, maintenance of gear boxes, couplings, and the like must be carried out on any pump, and at such times it may be necessary to stop the operation of the pump. This leaves the pump full of polymer, and its teeth covered with polymer. During such maintenance work, it may be necessary to remove the equipment that keeps the shafts and their teeth registry constant thereby causing the loss of the desired non-touching tolerance between adjacent meshed teeth. Since the pump is full of molten polymer, access to the interior of the pump to re-set the timing (registry) of the pump teeth is much more problematic. The polymer could be removed from the interior of the pump and from around the meshed teeth, but this is a time-consuming and costly approach. 
         [0010]    It is much more desirable, and cost effective, to be able to re-set the timing of the pump teeth registry from outside the pump without requiring access to the interior of the pump, so that maintenance procedures can be completed. This invention provides such a method. 
       SUMMARY OF THE INVENTION 
       [0011]    This invention provides a method for timing the registry of meshing polymer pumping teeth relative to one another while those teeth are immersed in molten polymer by employing a pattern of apertures on the ends of the shafts carrying those teeth and a template with holes there through that matches the pattern of apertures. The shafts are rotated until the pattern of shaft end apertures matches the pattern of template holes, and dowels having a tolerance relative to such apertures and holes of not more than about 0.001 of an inch are inserted into each matching aperture/hole set. 
         [0012]    The dowels and template are then removed, and the thus registered pump shafts, and their teeth, are re-fixed relative to one another in conventional manner, and pumping resumed. 
         [0013]    By this method, the gap between meshed teeth that are surrounded by molten polymer can be reliably set remotely from the interior of the pump thereby eliminating the need for emptying the pump of its polymer load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a plan view of the inlet side of a polymer pump. 
           [0015]      FIG. 2  shows a plan view of the outlet side of the pump of  FIG. 1 . 
           [0016]      FIG. 3  shows a side view of the normally exposed opposing shaft ends of the pump of  FIG. 1 . 
           [0017]      FIG. 4  shows a view of  FIG. 1  with its opposed, teeth carrying shafts exposed. 
           [0018]      FIG. 5  shows a close-up view of the pumping teeth shown in  FIGS. 1 and 4 . 
           [0019]      FIG. 6  shows a cross-section of the pump of  FIG. 1  with its opposed teeth carrying shafts. 
           [0020]      FIG. 7  shows the ends of the opposed shafts that normally extend outside of the interior of the pump of  FIG. 1 , and a pattern of apertures in those shaft ends pursuant to this invention. 
           [0021]      FIG. 8  shows a template useful in the process of this invention. 
           [0022]      FIG. 9  shows an exploded view of the template of  FIG. 8  when employed relative to the shaft ends of  FIG. 7  with dowels fitting the pattern of shaft end apertures and the pattern of template holes after those patterns are matched with one another. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Although, for sake of clarity and brevity, this invention is described in detail herein with respect to pumping HDPE, it can be used in the pumping of any molten polymer. 
         [0024]      FIG. 1  shows polymer pump  1  having a closed top  2  and closed bottom  3 . Upstanding, spaced apart sides  15  and  16  support internally of pump  1  a pair of parallel, opposed shafts  11  and  13  that extend fully across the pump&#39;s interior ( FIG. 4 ). Upstanding side  4  and an opposing upstanding side  20  ( FIG. 3 ) complete the enclosure of the interior of pump  1 . 
         [0025]    Side  4  is the inlet side of pump  1 . Side  4  has an opening  5  through which polymer is introduced into the interior of pump  1  to be forced to the outlet  21  ( FIG. 2 ) of the pump. Through opening  5  pumping teeth (teeth) carried by shafts  11  and  13  can be seen. Shaft  11  carries chevron style teeth  7  and  9 , while shaft  13  carries chevron style teeth  6  and  8 . These teeth are shown in a more spaced apart configuration than actual for sake of clarity. The teeth shown are chevron style, but can be any style, including spur gear or single helical. Line  10  denotes the demarcation line between shafts  11  and  13  and is the line (point) of closest approach for teeth carried by opposing shafts  11  and  13  when those teeth are at their closest approach and in a meshed configuration ( FIG. 6 ). Shafts  11  and  13  carry key ways  12  and  14  respectively so that shafts  11  and  13  can be fixed to one another by conventional apparatus (not shown) that maintains, while the pump is in operation, the non-touching registry between adjacent opposing teeth when at their point of closest approach. 
         [0026]      FIG. 2  shows the outlet side  20  of pump  1  to carry an opening  21  to allow pressurized polymer to issue from the interior of the pump.  FIG. 2  shows a pair of opposing chevron teeth  22  and  23  carried respectively by shafts  13  and  11  after they have pushed polymer toward opening  21  and as they near their line of closest approach  10  for meshing engagement thereof. Again, although a plurality of teeth are present around the entire periphery of both shafts  11  and  13  ( FIG. 6 ), only two pairs of teeth are shown only for sake of clarity. 
         [0027]      FIG. 3  shows the side  16  of pump  1  wherein shaft ends  11  and  13  are exposed outside the interior of pump  1 . Shaft ends  11  and  13  have center points  32  and  31 , respectively. The shafts rotate about their respective center points in the directions shown by arrows  33  and  34 . Entering polymer shown by arrow  35  passes into inlet  5  ( FIG. 1 ) wherein it is picked up by moving pumping teeth and forced to outlet  21  as shown by arrow  36 , and as shown in greater detail in  FIG. 6 . 
         [0028]      FIG. 4  shows the view of  FIG. 1  with side  4  removed to reveal that shafts  11  and  13  extend across the full interior of the pump. Shaft  11  has an exposed end face  41  outside of the pump, while its opposing end  44  is carried in side  15  journaled in circular bearing  45 . Similarly, shaft  13  has end face  40  that is exposed outside the interior of the pump, and an opposed end  42  journaled in side  15  in circular bearing  43 . 
         [0029]      FIG. 4  shows that shafts  11  and  13  are of substantially larger diameter inside pump  1 , these larger diameter portions  47  and  48  being the part of the shafts that carries the pumping teeth. In this Figure, the pumping teeth spaces  49  and  50  for shaft parts  47  and  48 , respectively, are shown to be relatively larger than normal for sake of clarity only, the teeth being relatively small compared to the diameter of parts  47  and  48 . This is better shown in  FIG. 6 . 
         [0030]      FIG. 5  shows a plurality of teeth in general and a close-up of teeth  6  through  9  on the inlet side  5  ( FIG. 1 ) in particular.  FIG. 5  shows these teeth as they are rotated away from meshing along line of closest approach  10 . In this mode, the teeth pickup additional polymer (not shown) and move it in a pumping mode. On the inlet side of the pump, upper teeth  6  and  8  on shaft part  47  are moving upwardly (and carrying polymer upwardly) as shown by arrow  51  while lower teeth  7  and  9  on shaft part  48  are moving downwardly (and carrying polymer downwardly) as shown by arrow  52 . All of the moving teeth are carrying incoming polymer with them in the direction of their movement, whether up (arrow  51 ) or down (arrow  52 ). In this Figure, for example, when in the meshing configuration at line  10 , tooth  7  was in between teeth  6  and  8 , and, when so disposed, with proper pump timing registry, tooth  7  is maintained at its 0.02 inch tolerance with teeth  6  and  8 . Thus, tooth  7  did not physically contact either of teeth  6  and  8 , the tolerance being filled with polymer. 
         [0031]      FIG. 6  shows vertical cross-section A-A of  FIG. 5 .  FIG. 5  shows inlet opening  60  to be of substantially larger area and volume than outlet opening  61 . Polymer entering at  35  is forced by its conveying teeth into progressively smaller volumes  67  and  68 , and thereby put under substantially greater compressive forces when delivered to outlet  61 . Thus, exiting polymer  36  is under a substantially higher pressure, e.g., 3000 psig, than entering polymer  35 , e.g., 30 psig. This pressure differential can cause flow back in the direction of arrow  73  if the teeth carried by shaft parts  47  and  48  become worn by repeated physical contact between the opposing teeth when in their point of closest approach  10  ( FIG. 4 ). 
         [0032]    Cross-sectional  FIG. 6  shows that after teeth  6 ,  7 , and  8  have delivered their conveyed polymer through restricted passage ways  69  and  71  to outlet  61 , these teeth then move into the meshing configuration of closest approach shown in  FIG. 6 . In this inter-meshing configuration, tooth  7  is physically disposed between and adjacent to teeth  6  and  8 , but not physically touching either of those teeth. This is the point of closest approach  10  for these three teeth. The gaps  65 , between teeth  6  and  7 , and  66 , between teeth  7  and  8 , are both desirably maintained at the 0.02 inch registry tolerance mentioned hereinabove for HDPE. This prevents premature wear of these teeth when repeatedly put into and out of this meshed configuration during the pumping life of pump  1 . 
         [0033]    Initially, for example, when new, pump  1  is timed in a conventional manner well known in the art. After some operation of pump  1  so that it contains polymer in its interior, template  80  is prepared so that it is unique to the particular shafts of pump  1 . Once made, the template can be used to restore pump  1  to its timed state at any time over the service life of that pump. If the teeth carrying shafts of pump  1  are re-used in another pump, template  80  could be used to establish proper timing for those shafts. 
         [0034]      FIG. 7  shows the first step in carrying out this invention. In this step, when pump  1  is not in operation and shafts  11  and  13  are in proper timing registry to maintain the desired 0.02 inch tolerance, shaft faces  40  and  41  are exposed, i.e., separated from the apparatus (not shown) that causes shafts  11  and  13  to stay in the desired registry during the operation of pump  1 . Each exposed shaft face  40  and  41  has at least two spaced apart apertures drilled there into. In the case of face  40 , apertures  76  and  77  are drilled a finite distance into the body of shaft  13 . In this example, apertures  76  and  77  are placed asymmetrically on face  40  in that aperture  76  is further from center point  31  and closer to outer periphery  75  of shaft  13  than is aperture  77 . Apertures  78  and  79  are shown in this example to be drilled symmetrically into the body of shaft  11 . That is, apertures  78  and  79  are each located an equal distance above and below center point  32  (an equal distance from outer periphery  74 ). 
         [0035]      FIG. 8  shows a separate, unitary template member  80  that is employed in this example of the process of this invention. Template  80  is co-extensive with shafts  13  and  11  in that it essentially covers at least a substantial area of shaft end faces  40  and  41 . 
         [0036]    A pattern of holes  81  through  84  is provided which holes extend fully through template  80 . This pattern of holes is made to match the pattern of apertures  76  through  79  in end faces  40  and  41  ( FIG. 7 ) 
         [0037]    With a symmetrical aperture pattern  78 / 79  such as that shown for shaft  11  there is more than one way (front or back side) template  80  can be held up to shaft ends  40  and  41  and the hole pattern  83 / 84  matched (aligned). However, with asymmetrical aperture pattern  76 / 77  there is only one orientation in which template  80  can be held up to shaft ends  40  and  41  and hole pattern  81 / 82  matched to pattern aperture  76 / 77 . Thus, pursuant to this invention, at least one of shafts  11  and  13  will have an asymmetrical aperture pattern. If desired, both shafts can have an asymmetrical shaft pattern with their asymmetries the same or different. 
         [0038]      FIG. 9  shows an exploded view in respect of template  80  being held adjacent (abutting) faces  40  and  41  in order to match the aperture patterns of faces  40  and  41  to the hole patterns of template  80 . If pump  1  is out of timing, the patterns cannot be made to match. In such a case, one or both of shafts  11  and  13  are rotated until the patterns can be made to match exactly. Dowels are then inserted through the template holes into the shaft apertures. 
         [0039]    To ensure that the desired gap, e.g., 0.02 of an inch for HDPE, is obtained between the teeth then meshing inside the pump, the tolerance between the dowel inserted and the hole/aperture pair in which it is inserted should not be greater than about 0.001 of an inch. 
         [0040]      FIG. 9  shows template  80  essentially up against, but not touching shaft faces  40  and  41  for sake of clarity only. In practice, template  80  will be firmly touching faces  40  and  41 . This can be achieved in any desired manner known in the art such as drilling and tapping either or both of center points  31  and  32  to form a threaded opening  97  and  98  to receive a holding bolt (not shown) that temporarily affixes template  80  to shafts  11  and  13 . With template  80  in place abutting faces  40  and  41 , dowels  91 ,  92 ,  93 , and  94  are inserted, respectively, through holes  81 ,  82 ,  83 , and  84 , and fully into apertures  76 ,  77 ,  78 , and  79 . When template  80  is firmly abutting faces  40  and  41  with the template hole/shaft aperture patterns matching, and dowels firmly inserted through the holes into the apertures, the desired timing registry between the meshing teeth inside the pump is achieved even though those teeth are covered with polymer. 
         [0041]    Dowels  91  through  94  are then removed from their apertures, and template  80  removed from contact with faces  40  and  41 . Shafts  11  and  13  are then re-attached to the apparatus (not shown) that is normally used during pump operation to maintain these shafts in their desired registry, and operation of the pump begun. 
         [0042]    A matching template hole/shaft end aperture pair can be straight sided or tapered. In a specific embodiment, hole/aperture pairs can be straight sided, tapered, or a combination of such pairs. If a hole/aperture pair is tapered, the taper should be uniform from the start of the hole to the end of the aperture so that the mating dowel, with its close tolerance, can tightly and uniformly follow the taper angle from the start of the hole to the end of the aperture. 
         [0043]    The cross-section of the dowels used can be curvilinear, polygonal, or any desired combination thereof. 
         [0044]    All apertures need not be drilled to the same depth in the shafts. If desired, apertures can be drilled to differing depths with dowels being sized in length to match those depths in order to give an added dimension of asymmetry. More than two apertures can be employed on a given shaft face. 
         [0045]    The cross-sectional distance across a shaft aperture and/or template hole, e.g., the diameter for a straight sided matching aperture/hole pair that is round, can be at least ⅛th of an inch, and preferably not more than about 1 inch. The apertures in the shaft ends can vary in depth from about ½ to about 1 inch. 
         [0046]    The template itself can be any rigid member such as carbon steel plate at least ½ inch in thickness. The dowels can be solid metal members and should not be semi-rigid or otherwise flexible such as are hollow roll pins and the like.