Abstract:
A shaft seal is interposed between a tank and a rotating shaft extending through the tank and having a non-rotatable stator fixed to the tank and a rotor fixed for rotation to the shaft. A multiple labyrinth structure is formed between the rotor and the stator for preventing the ingress of contaminants from the tank and the egress of lubricant from the shaft. A self-aligning arrangement is constructed and arranged to permit the stator to shift axially along the shaft and compensate for various forces inflicted on the shaft as the shaft rotates.

Description:
FIELD OF THE INVENTION 
     The present invention relates broadly to shaft sealing structure and, more particularly, pertains to a sealing and feed through arrangement for a rotating shaft used in mixing equipment to prevent the ingress of contaminants and control the egress of lubricant regardless of the various forces subjected upon the shaft. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Spiral blade horizontal drum mixers are used to provide highly viscous, wet mixtures of concrete, mortar and the like. Such horizontal drum mixers are commonly equipped with a low speed rotating shaft which extends across a mixing tank, and protrudes or feeds through the opposed end walls of the tank with one end of the shaft coupled to a drive arrangement and the other end of the shaft supported by a bearing structure. As is well known, sealing arrangements surrounding the shaft are positioned on the interior and exterior surfaces of each end wall. More particularly, a rotor is connected for rotation with the shaft and is mounted adjacent to a liner on each end wall, while a non-rotatable stator is fixed on the outside of each end wall. The rotor and stator cooperate to form a primary labyrinth seal into which grease is periodically delivered in a manner which will keep the shaft lubricated and, at the same time, trap contaminants so that they are restricted from entering the seal. In most cases, the labyrinth seal is formed by a single flange-like baffle extending into a formed recess to create a tortuous path that makes it difficult for contaminants to invade the seal to degrade lubricant effectiveness. 
     In the course of normal operation of the horizontal drum mixture, it has been found that the rotating shaft is subject to radial, axial and angular forces which can cause the labyrinth seals to bind and fail. One way to avoid this problem is to provide larger, more tolerable clearances between the baffle and its recess. However, such alternative is not desirable because it results in higher costs of machining the rotor and stator. Even if it were cost permissible, the larger labyrinth pathways would allow grease to flow more quickly into and out of the seal, such that the lubricating and trapping functions of the seal would be negatively affected. 
     A further problem with prior art seals is the exposure of the inner portions of the seal within the mixing tank to the setting or “freezing” of grout infiltrate during periods of mixer shut down at the interface between each rotor and end wall liner. Before mixing can resume, this problem must be rectified by breaking the newly formed grout seal such as by applying a grinding wheel thereto. Such remedy creates unwanted down time of the mixer and results in increased maintenance costs. 
     It should also be noted that the formation of shaft seals of the type described above involves machining methods that can create a troublesome gap between the rotor and stator which makes alignment between these components less than precise, so that the integrity of the seal is compromised. 
     Despite the existence of various shaft sealing devices, there remains a need for an enhanced sealing arrangement which will provide effective sealing action, whether the shaft is at rest or in rotation. Accordingly, it is desirable to provide a shaft seal which will effectively retain lubricant and prevent contamination therein. It is also desirable to provide a shaft seal in which a rotor and stator are manufactured and assembled in a more precise fashion. Likewise, it is desirable to provide a shaft seal which is unaffected by various forces applied to the shaft during the rotation thereof. Furthermore, it is desirable to provide a sealing device which will enable the holding of tighter tolerances in the formation of labyrinth seals. It is also desirable to provide a shaft seal which will improve the effectiveness of a horizontal drum mixing machine. 
     It is a general object of the present invention to provide an improved primary seal and feed through arrangement for mixing equipment having a mixing tank through which a rotatable shaft extends, such that a portion of the seal arrangement floats and self aligns to compensate for radial, axial, and angular misalignment between the rotating shaft and the seal arrangement. 
     It is also an object of the present invention to provide a double labyrinth type shaft seal which allows for the use of tighter tolerances along the labyrinth pathways minimizing wear and improving the sealing capabilities for a rotating shaft. 
     It is a further object of the present invention to provide a secondary face seal arrangement interposed between a seal rotor and a seal stator mounted along a low speed rotating shaft on a horizontal drum mixer. 
     Yet another object of the present invention is to provide a shaft seal of split construction which is easily manufactured, assembled and serviced. 
     Still another object of the present invention is to provide a shaft seal for a spiral blade mixer which is effective to control the setting of wet mixed material during periods of mixer shutdown. 
     Moreover, a further object of the present invention is to provide a shaft seal which employs a compressible gasket to facilitate set-up of labyrinth structure. 
     In accordance with one aspect of the present invention, a shaft seal is interposed between a tank and a rotating shaft extending through the tank, and has a non-rotatable stator fixed to the tank and a rotor fixed for rotation to the shaft. The seal is improved so as to include a multiple labyrinth structure formed between the rotor and the stator for preventing the ingress of contaminants from the tank and the egress of lubricant from the shaft. The invention is further improved by a self-aligning arrangement constructed and arranged to permit the stator to shift axially along the shaft and compensate for various forces inflicted on the shaft as the shaft rotates. In the preferred embodiment, the multiple labyrinth structure includes a pair of baffles oriented at substantially 90° to each other. The rotor is formed with an axially extending recess receiving one of the baffles and a radially extending groove receiving the other of the baffles. The tank includes an end wall and has a liner connected thereto. One of the baffles is defined by an axially extending flange formed on the stator, while the other of the baffles is formed by the liner. A compressible gasket is interposed between facing surfaces of the stator and rotor and is engaged against the end wall. A set of fastener assemblies extends through the tank liner, the tank end wall, the gasket and the stator for holding the rotor and stator together in a preload arrangement relative to the tank. A first rubber spring washer and ring are disposed for rotation between the rotor and the shaft, and a second rubber spring washer and a ring are fixedly disposed between the stator and the shaft. A stationary lip seal is also fixed between the stator and the shaft. The stator extends outwardly of the tank end wall and the rotor extends inwardly of the tank liner. A bearing device is supported on an end of the shaft outwardly of the stator. The tank liner is spaced from the walls forming the rotor groove. The self-aligning arrangement is defined by the spacing of the tank liner from the walls of the groove, the preload arrangement of the fastener assemblies and the compressibility of the gasket. 
     In another aspect of the invention, a seal is provided for use with a mixing tank provided with a rotating shaft extending across and protruding through opposed end walls, each end wall having an inner liner. The seal includes a stator fixed to one of the end walls and having a facing surface and an axially extending flange formed thereon. A rotor is connected to the shaft for rotation therewith, and has a facing surface as well as an axially extending recess for receiving the stator flange to define a first labyrinth passage. The rotor also has a groove extending radially inwardly from the periphery of the rotor for receiving the liner of the end wall to define a second labyrinth passage in communication with the first labyrinth passage for controlling the flow of lubricant between the shaft and the rotor. A compressible member is interposed between the facing surfaces of the stator and the rotor and is engaged with the end wall. The liner is spaced from the walls forming the groove and cooperates with the compressible member to enable the stator to float axially on the shaft and compensate for various forces inflicted upon the shaft as the shaft is rotating, while continuing to control the flow of lubricant between the shaft and the rotor. A mechanical face seal assembly is located between the stator and the rotor and includes a pair of facing steel rings, one of the rings being disposed for rotation in the rotor and the other of the rings being fixed in the stator. The mechanical face seal further includes a first rubber spring washer associated with one of the rings, and a second rubber spring washer associated with the other of the rings. 
     The invention also contemplates a method of constructing a seal for use with a rotating shaft extending across and protruding through a mixing tank having a hole formed in an end wall and a liner extending adjacent thereto. The method comprises the steps of sliding a rotor having a facing surface, a recess extending axially from the facing surface, and a groove extending radially inwardly from a periphery of the rotor through the end wall and onto the shaft, such that the liner projects into the groove in spaced relationship therewith; positioning the liner a predetermined distance from the walls forming the groove and clamping the rotor to the shaft; installing a compressible member against the facing surface of the rotor such that an outer periphery of the compressible member lies against the end wall of the tank; passing threaded fasteners through the liner, end wall and the compressible member, and then attaching primary nuts to the threaded fasteners so that the nuts engage the end wall; sliding a stator having a facing surface and an axially extending flange over the shaft and threaded fasteners, and against the compressible member and nuts, such that the flange extends into the rotor recess in spaced relationship therewith; and threading secondary nuts onto the threaded fasteners to non-rotatably fix the stator relative to the shaft and to preload the threaded fasteners such that the stator shifts axially upon the rotating shaft when various forces are imposed thereon. The step of sliding a rotor on the shaft includes a step of providing the rotor with a mechanical face seal. The step of sliding a stator on the shaft includes the step of providing the stator with a lip seal engageable with the shaft. The step of sliding the rotor onto the shaft includes a step of providing nesting halves of the rotor. The step of sliding the stator on the shaft includes the step of providing mating halves of the stator. The method includes the step of supporting an end of the shaft in a bearing structure. The step of positioning the liner a predetermined distance from the walls forming the rotor groove includes the step of spacing an inside wall of the liner in the range of 0.003 to 0.005 inches from an innermost wall of the groove. 
     Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. 
     In the drawings: 
     FIG. 1 is a cut-away, isometric view of a spiral blade mixer embodying the shaft seal of the present invention; 
     FIG. 2 is an enlarged, partial isometric view of one portion of the shaft seal as seen from the exterior of the spiral blade mixer shown in FIG. 1; 
     FIG. 3 is an enlarged, partial isometric view of another portion of the shaft seal as seen from the interior of the spiral blade mixer shown in FIG. 1; 
     FIG. 4 is an exploded view of the shaft seal; 
     FIG. 5 is a fragmentary end view of the shaft seal as taken on line  5 — 5  of FIG. 2; 
     FIG. 6 is a fragmentary, cross-sectional view of the shaft seal taken on line  6 — 6  of FIG. 5; and 
     FIG. 7 is a fragmentary, cross-sectional view of the shaft seal taken on line  7 — 7  of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIG. 1 discloses interior and exterior views of a seal arrangement  10  embodying the present invention as it appears when installed on opposite ends of a rotating shaft  12  extending across and protruding through suitable openings formed in opposed end walls  14 ,  16  of a typical horizontal drum mixer tank  18 . Each of the end walls  14 ,  16  has a multi-piece liner  20  bolted thereto by suitable fastener assemblies  22 . The liner  20  includes a pair of generally semi-circular liner plates  24  (FIG. 3) which surround the shaft  12  on the inside of mixer tank  18 . In the preferred embodiment, the shaft  12  is provided with a spiral blade or paddle configuration  26  and is coupled to a drive mechanism  28  for rotating the shaft  12  at relatively low speed (e.g. 20 rpm), and effectively mixing dry and wet mixtures of concrete, mortar and other grout-like products in the tank  18 . The shaft  12  is preferably provided with a wear plate assembly  30  (FIG. 3) which serves as a support structure for the spiral blade configuration  26 . The exterior surface of end wall  14  is equipped with a lubrication system (not shown) having hoses  32  for periodically delivering grease or other lubricants to the seal arrangement  10 . The grease is transported in a manner which will lubricate the seal arrangement  10  and the shaft  12 , and simultaneously prevent contaminants in the interior of mixing tank  18  from entering the seal arrangement  10  as will be appreciated hereafter. 
     As seen in FIG. 2, the end of shaft  12  protruding beyond end wall  14  is supported in a bearing structure  34  having an outer race  36  and an inner race  38  normally locked on shaft  12  by a cylindrical collar  40 . Another bearing structure (not shown) is provided on the other end of shaft  12  between end wall  16  and drive mechanism  28 . 
     Turning now to the exploded view in FIG. 4, the sealing arrangement, according to the present invention, is comprised of an annular, metallic rotor  42 , a stepped, metallic stator  44 , a compressible rubber gasket  46 , a mechanical face seal assembly  48  and a split elastomeric lip seal  50 , all of which are positioned about the shaft  12  relative to each of the end walls  14 ,  16  and the liner plates  24  of the particular end wall. 
     The metallic rotor  42  includes a pair of mating halves  52 ,  54  which are joined together in clamping relationship on the shaft for rotation therewith by a pair of shoulder bolts received in suitably threaded apertures formed in the halves  52 ,  54 . Only one shoulder bolt  56  is seen in FIG.  4 . The rotor  42  also has a facing surface  58 , a recess  60  extending axially from the facing surface  58 , and a groove  62  extending radially inwardly from the outer periphery of the rotor  42 . The inner periphery of the rotor  42  is provided with a first shallow, annular relief channel  64  projecting radially outwardly therefrom, and a first radially outwardly extending annular notch  66  which accommodates a first annular rubber spring washer  68 . A first annular, hardened steel ring  70  having an L-shaped cross section fits into the washer  68  and forms a seal face  72 . The washer  68  and ring  70  together rotate in the rotor  42  and define one half of the mechanical face seal assembly  48 , the other half being located in the stator  44  to be described below. 
     As shown in FIG. 6, the rotor  42  is adapted to project through the end wall hole defined by the radially innermost edge  74  of liner plates  24 , and the radially innermost edge  75  of adjacently disposed end wall  14 ,  16 , which is offset radially and outwardly from liner plate edge  74 . The liner plate edge  74  serves as a first baffle which extends into the rotor groove  62  such that a first labyrinth passage  76  is defined between the walls of the groove  62 , and an inside wall  78  of liner plates  24 , the liner plate edge  74  and an outer wall  80  of liner plate  24 . The first labyrinth passage  76  is connected with an axially extending passage  82  formed by the wall  80 , the edge  75  and an axially extending segment  84  of rotor outer periphery. 
     The metallic stator  44 , like the rotor  42 , includes a pair of mating halves  86 ,  88  which are joined together around shaft  12  by a pair of shoulder bolts  90  received in suitably threaded apertures in the halves  86 ,  88 . Stator  42  has a four-sided base plate portion  92 , and an annular portion  94  stepped radially inwardly from the base plate portion  92 . The base plate portion  92  has a facing surface  96  and four corners, each of which is formed with in inwardly extending U-shaped notch  97  (FIG.  4 ). A flange  98  projects axially from the facing surface  96  (FIG. 4) and serves as a second baffle adapted to be received in rotor recess  60  such that a second labyrinth passage  100  is defined between the walls of recess  60 , and a radially outermost wall  102  of flange  98 , an end  104  of flange  98  and a radially innermost wall  106  of flange  98 . 
     The inner periphery of stator  44  is formed with a second radially extending annular notch  108  (FIG. 4) which receives a second annular rubber spring washer  110 . A second annular hardened steel ring  112  having an L-shaped cross section is nested inside the washer  110 , and forms a seal face  114  engageable with seal face  72 . Washer  110  and ring  112  define the other half of mechanical face seal assembly  48 . The inner periphery of stator  44  is also provided with a second shallow, annular relief channel  116  and a third radially outwardly extending notch  118  which maintains the lip seal  50  therein. As seen in FIG. 4, lip seal  50  preferably has a C-shaped split construction having free ends  119 ,  120 . 
     The compressible gasket  46  is similar in shape to the stator base plate portion  92  and is formed with a large central opening  121  through which the shaft  12  and flange  98  pass. The gasket  46  has four corner portions, each of which is provided with a circular opening  122  to be aligned with a respective notch  97  in each corresponding corner of base plate portion  92 . Gasket  46  is adapted to be interposed between rotor and stator facing surfaces  58  and  96  (FIGS.  4  and  6 ), respectively, such that the gasket outer edges are engageable against the exterior surface of end wall  14 ,  16  as shown in FIGS. 2 and 6. 
     With reference to FIGS. 2,  5  and  7 , the stator  44  and gasket  46  are fixedly secured to the exterior surfaces of each end wall  14 ,  16  by a set of four fastener assemblies  124 . Each of the fastener assemblies  124  includes a threaded screw  126  having a conical head  128  which is received in a corresponding recess  130  formed in liner plate  24 . The shaft of threaded screw  126  passes through an enlarged opening  131  in each end wall  14 ,  16  and through one of the openings  122  formed in the corners of gasket  46 . A first nut  132  is then threaded upon the screw  126  for engagement against the exterior surface of end wall  14 , 16 . The threaded shaft  126  also passes through one of the notches  97  of stator base plate portion  92 , first washer  134  and a second washer  136  smaller in diameter than washer  134 . Second and third nuts  138  and  140 , respectively, are threaded on the end of screw  126  in a manner to be further described, such that this fastener assembly  124  is preloaded to enable a floating behavior of the stator  44  as various forces are subjected to the shaft  12 . 
     A pair of diametrically exposed threaded grease holes  142  are formed on the exterior face of stator  44  for transporting grease via hoses  32  into and around facing notches  66  and  108  so that the rings  70 , 112  of face seal assembly  48  and the shaft  12  are properly lubricated. The labyrinth passages  76  and  100 , passage  82  and a passage  144  between rotor facing surface  58  and gasket  46  form a double labyrinth seal which retards the outgoing flow of grease G (FIG. 3) from the shaft  12  to an interface  146  inside tank  18  between rotor  42  and the inside wall  78  of liner plates  24 . The grease purged through the sealing arrangement  10  also acts as a trap to prevent the ingress of contaminants from tank  18 . It is noted that the baffles defined by the liner plate edges  74  and the flange  98  are preferably oriented at 90° relative to each other to create a particularly tortuous path for restricting the egress of lubricant and the ingress of contaminants. 
     Before assembly begins, all facing surfaces of the seal components should be adequately lubricated. To assemble the sealing component  10 , the four components  68 ,  70 , 110 , 112  of face seal assembly  48  are positioned over the end of the shaft  12  before the bearing structure  34  is installed. Mating rotor halves  52 ,  54  are loosely connected around shaft  12  between end wall  14  or  16  and bearing structure  34  using shoulder bolts  56 . The loosely connected rotor halves  52 ,  54  are then slid through the hole in end wall  14  or  16  until the innermost surfaces of the halves  52 ,  54  contact the square assembly  30  surrounding the shaft  12 . At this point, the liner plate edges  74  are loosely placed in the rotor groove  62 , and the liner plates  24  are secured to end wall  14  or  16  using threaded screw  126  and nut  132  as detailed above. Then, using a straight edge (not shown), a predetermined gap  148  (FIG.  6 ), preferably in the range of 0.003 to 0.005 inches, is set between an innermost vertical wall  150  of groove  62  and the inside wall  78  of liner plates  24 . Once gap  148  is set, shoulder bolts  56  are torqued to clamp rotor halves  52 ,  54  on shaft  12 . It is important that gap  148  is equally set with respect to bolt halves  52 ,  54 . First spring washer  68  and first ring  70  are installed in the formed rotor  42 , such that the ring seal face  72  faces the bearing end of the shaft  12 . Next, the compressible gasket  46  is installed over the threaded screws  126  and nuts  132  as also detailed above. The stator halves  86 ,  88  are then tightly connected together around shaft  12  by shoulder bolts  90  such that formed stator  44  is non-rotatable relative to shaft  12 . Second spring washer  110  and second ring  112  are installed in the formed stator  44  such that seal face  72  matingly engages seal face  114 . The formed stator  44  is then slid along shaft  12  so that the notches  97  pass over the threaded screws  126  and nuts  132 , and the base plate portion  92  on stator  44  is moved against gasket  46  until flange  98  bottoms in recess  60  so that gasket  46  is compressed. Washers  134  and  136  are installed over each screw  126  and nut  138  is tightened to initially preload the fastener assembly  124 . To attain the desired preload, the nut  138  is then backed off one half turn, which is specifically designed to correspond to the gap  148  set previously. The nut  138  of preloaded fastener assembly  124  thus functions as a simple micrometer which acts together with the compressibility of gasket  46  and the structure of groove  62  to create a self-aligning feature which allows the stator  44  to slide incrementally or “float” along shaft  12  to compensate for radial, axial and angular misalignment forces imposed upon shaft  12  during rotation thereof. Once nut  138  is set, the nut  140  is threaded on each screw  126  so as to “jam” or lock each nut  138  in place. The one-piece split lip seal  50  is then installed in the stator notch  118 . The last assembly step involves attaching the lubrication hoses  32  with the grease holes  142  formed in stator  44 . 
     With the seal arrangement  10  in place, grease is periodically delivered to the shaft  12  and the face seal assembly  48  and purged with the double labyrinth seal, slowing the exit of lubricant at interface  146  so as to prevent the entry of contaminants into the seal. A particularly salient feature of the invention is the capability of the purged grease at the interface  146  to prevent the setting of wet concrete-like mix and the resulting lockup of shaft  12  inside the tank  18  during mixer shut down. 
     In operation, the rotor  42 , as well as spring washer  68  and ring  70 , rotate together with the shaft  12 , while the stator  44  and its spring washer  110  and ring  112  remain fixed to end wall  16  and non-rotatably mounted on shaft  12 . Rings  70 ,  112  float on the spring washers  68 , 110 , respectively, to allow them to align with each other. Lip seal  50  prevents grease from migrating out of stator  44 . Because the rotor halves  52 ,  54  are tightly clamped to shaft  12  by shoulder bolts  56 , no grease will escape from the interface  152  between the rotor  42  and the shaft  12  inside tank  18  nor will any contaminants enter this interface  152 . 
     It should be appreciated that the present invention provides a shaft seal  10  in which a rotor  42  and a stator  44  are manufactured and assembled in a more precise manner. That is, shaft seal  10  of the present invention employs a split construction for the rotor  42  and stator  44  which enables a significant improvement in assembly and service in the field, and avoids the gap problems between previously machined stators and rotors. The arrangement described above sets forth a shaft seal  10  with a primary double labyrinth seal, in which tighter clearances may be maintained between the liner plates  24  and the groove  62 , and between the flange  98  and the recess  60 , thereby minimizing wear and increasing seal capability. The compressible gasket  46  is employed to facilitate set-up of the labyrinth structure in the seal array. “Floating” of the stator  44  in response to shaft forces is achieved with the enlarged groove  62 , fastener assemblies  124  set to a slight preload, and the compressibility of gasket  46 . 
     Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.