Abstract:
A mixing fin assembly adapted to mix and guide materials in a rotatable mixing drum of a type used in a transit concrete mixing vehicle includes a spiral mixing fin designed to extend transversely into the drum constructed of a lightweight, moldable polymeric material which is flexible and resistant to abrasion, the mixing fin includes a plurality of conjoined sections that are fit together, preferably utilizing fin locking tabs and mating fin locking sockets. A preferred way of mounting the mixing fin into mixing drums uses a spiral fin mount rail track adapted to be fixed to the interior surface of the mixing drum. A locking tab locks the upper part of the rail track into the fin structure.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
       [0001]    Not applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    I. Field of the Invention 
         [0004]    The present invention relates to mobile systems carrying rotatable mixing drums for mixing and dispensing concrete. More specifically, this invention relates to a helical mixing fin assembly for mixing and moving materials within concrete mixing drums that involves a superior fin structure and system for mounting the helical mixing fin assembly within rotatable drum mixers. 
         [0005]    II. Related Art 
         [0006]    Concrete mixing trucks are widely used in the construction industry for preparing and transporting concrete mix to desired construction sites. Such trucks feature a large rotatable mixing drum which includes fins or agitators mounted inside for mixing and directing the movement of a concrete mixture therein. Such fins typically have a helical configuration which will tend to mix concrete when the mixing drum is rotated in a first direction and urge the concrete towards a discharge chute when the mixing drum is turned in an opposite direction. 
         [0007]    The mixing or agitation of materials such as concrete in a rotary mixing environment creates a great deal of abrasive friction between the mixing fins and the various abrasive components of the concrete mixture which is being transported. As a result, mixing fins have typically worn out long before the outer wall of the mixing drum itself and so the drums have had to be rebuilt or replaced sooner than desired. Accordingly, polymer-coated metal fins and mixing fins constructed of polymeric materials, particularly with internal reinforcing materials, have been developed. Mixing fin structures of the class have been shown, for example, in U.S. Pat. Nos. 5,056,924; 5,178,457; 5,378,061; and 5,427,449. While a degree of success has been achieved with such mixing fins, there remains a need for a helical or spiral mixing fin assembly that includes an improved mounting structure for use in concrete mixing drums of various materials. 
       SUMMARY OF THE INVENTION 
       [0008]    By means of the present invention, there is provided a mixing fin assembly adapted to mix and guide materials contained in a rotatable mixing drum of a type used in a transit concrete vehicle mixing system. The mixing fin assembly includes a spiral mixing fin designed to extend transversely into a drum mixing space. The fin is constructed of a lightweight, moldable polymeric material which is flexible and resistant to abrasion. The spiral mixing fin includes a plurality of conjoined sections that are fit together, preferably utilizing fin locking tabs and mating fin locking sockets at the ends of the sections which can be used to lock the plurality of sections together, preferably using an amount of adhesive. 
         [0009]    The assembly includes a mounting system. A preferred way of mounting the mixing fin into a metal mixing drum involves the use of a spiral fin mount rail track adapted to be fixed to the interior surface of the mixing drum as by welding and which fits into a track locking socket or slot recess in the fin structure which accommodates the fin mount rail track. The rail track also preferably includes an enlarged top or locking tab which is accommodated on the fin and which locks the upper part of the rail track into the fin structure. The spiral mixing fin preferably has aligned weep openings when assembled, which enable free flow of material along the inside surface of the mixing drum. Mixing holes may also be provided on the fin structure. 
         [0010]    The mixing fin assembly also may include an amount of adhesive and/or composite material applied to the bottom of the fin sections to secure the mixing fin to the interior surface of the mixing drum. Alternatively, the mixing fin assembly may be fixed to the mixing drum using bolts welded to the drum. 
         [0011]    The mixing fin assembly of the present invention is further adapted to be used in conjunction with drums made of polymeric material themselves in which the fin can be applied by adhesive or made as an integral part of the mixing drum structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a mixing drum with the shell partially cut away to expose an internal helical mixing fin assembly in accordance with the invention; 
           [0013]      FIG. 2  is a perspective view of the helical fin assembly of  FIG. 1  showing assembled fin sections without the drum shell; 
           [0014]      FIG. 3  is a view of a mixing drum with the side partially cut away similar to that of  FIG. 1  showing a fin mount rail track system which can be used to mount fin sections into the drum; 
           [0015]      FIG. 4  is a view of a segment of a helical mixing fin assembly showing section lines for  FIGS. 6A and 6B ; 
           [0016]      FIGS. 5A and 5B  are enlarged fragmentary views depicting one assembly technique for a helical or spiral mixing fin in accordance with the invention; and 
           [0017]      FIGS. 6A and 6B  are greatly enlarged cross-sectional views of a fin structure in accordance with the section lines in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following description details one or more exemplary embodiments illustrating the present invention. It should be noted that the detailed descriptions are intended by way of example only and are not intended to limit the scope of the invention in any respect. It will be further understood that the embodiments of the invention can be modified by those skilled in the art while remaining in keeping with the inventive concepts. 
         [0019]    In  FIG. 1 , there is shown a concrete mixing drum generally at  10  of a type designed to be carried by a transit concrete mixing truck. Such drums are conventionally mounted for rotation on a front support frame and a rear support frame in a well-known manner. Mixing drums can be formed in varying sizes, usually holding 3-20 cubic yards. As seen in  FIGS. 1 and 3 , the mixing drum  10  includes a head cone portion  12 , a belly or crossover portion  14 , a big cone portion  16  and a rear tail cone portion  18 . A support collar is shown at  19 . 
         [0020]    A helical or spiral mixing fin or flight shown assembled at  20  is made up of a plurality of adjoining sections that may be of varying lengths and heights assembled together to form a complete helix or spiral which is mounted to the inside surface  22  of the drum shell  24 . 
         [0021]    The helical or spiral mixing fin serves several functions. The main purpose of the mixing fin, of course, is to agitate and mix materials contained within the drum as the drum is rotated. In addition, however, the fin is configured to move the materials being mixed toward the front or head cone of the drum during mixing with the drum rotating in a first direction and to move the contents of the drum toward the tail or discharge end when the rotation is in a second direction where mixed concrete can be dispensed. The orientation of the fin is generally at a slight angle from orthogonal with respect to the inside wall  22  of the drum or shown in  FIGS. 6A and 6B . 
         [0022]    In  FIG. 3 , there is depicted a fin mount rail track  30  which represents one technique for fastening the helical mixing fin assembly  20  into the mixing drum  10 . The rail track includes spaced drainage openings or weep holes as at  32  which are designed to match spaced openings or weep holes  34  in the fin structure when the system is assembled. The top of the rail track includes a locking tab  36  ( FIGS. 6A and 6B ) which aids in locking mounted fin sections to the rail track. The fin mount rail track is preferably welded to the inside surface  22  of the drum shell  24  as at  37 , but a bolting assembly (not shown) can be provided. 
         [0023]    As shown in the figures, the helical or spiral mixing fin structure is made up of a plurality of conjoined sections as at  40  and  42  ( FIGS. 5A and 5B ). The embodiment shown includes a rather wide base  44  for stability and better mounting adhesion, and a continuous groove or track locking socket  46  for receiving the fin mount rail track  30  with locking tab  36 . A layer of adhesive  48  is applied to the bottom of the fin sections  44  to secure the fin sections to the inside surface  22  of the drum wall  24 , as shown best in  FIGS. 6A and 6B . Adhesive and/or composite material may also be applied to the fin mount rail track prior to locking the fin sections to the rail track. If the drum shell is of steel, the rail track is normally welded to the drum shell. 
         [0024]    As shown best in  FIGS. 5A and 5B , the fin sections themselves are locked together in a snap-fit-type arrangement using fin locking tabs as at  50  and mating locking sockets  52 . Seams are shown at  53 . Adhesive may be applied to the snap-fitting elements prior to locking the fin sections together. As shown in  FIG. 6B , the fin structure may include a series of spaced thin indicator disks  54 , overmolded in the fin structure, which are used to indicate maximum fin wear when exposed to show that replacement is required. 
         [0025]    The fin sections also include mixing holes as at  56  to aid in the blending of materials as they are mixed. The weep holes  34  in the fin structure are aligned with the openings  32  in the fin mount rail track to provide continuous drainage through the fin structure. As shown particularly in  FIGS. 6A and 6B , the fin structure is disposed to list somewhat toward the head cone portion of the drum rather than being orthogonally mounted relative to the drum surface. The fin has an enlarged upper portion  60  and a curved surface  62 . The fin profile shown in  FIGS. 6A and 6B  has proven to provide excellent mixing and agitation and the wide base  94  provides good stability. 
         [0026]    The fin sections are preferably molded from a “polymeric material” and may contain a plurality of such materials. As used herein, the term “polymeric material” is meant to be construed in a broad sense so as to include any composition suitable for such a mixing fin. Thus, this would include any such highly abrasion resistant, suitably flexible, non-metallic relatively light weight material that has a low coefficient of friction and which wears well and can be mold-processed readily. 
         [0027]    Preferably, the fin sections are fabricated from a thermoset polymeric material which is relatively light-weight, resilient and which will tend to remain smooth after wear. The most preferred polymeric composition is a polyurethane-based polymer blended with a filler material. The filler material is preferably a polyethylene material such as PRIMAX™ UH-1000 Series Particles (Air Products and Chemicals, Inc.) which is based on ultrahigh molecular weight polyethylene resin. The filler material provides for increased wear resistance, increased tear resistance, a lower coefficient of friction, and increased stiffness and hardness (Shore D) of the polymeric composition. The polymeric material used in making the fin sections preferably comprises a polyurethane and polyethylene blend which has about 70 to 95 wt-%, preferably about 80 to 90 wt-% of polyurethane and about 30 to 5 wt-%, preferably about 20 to 10 wt-% of polyethylene. It is to be understood that the polyurethane component can contain a curing agent and minor amounts of other components such as chain extenders, catalysts, pigments, etc. 
         [0028]    The polymeric material used to form the plastic fin sections can also have other filler materials such as various reinforcing fibers. Non-limiting examples of such fibers include glass fibers, carbon fibers, metallic fibers, polymeric fibers such as aramid fibers, coated fibers, etc. It is important that the outer surfaces of the fin sections be smooth, so as to facilitate efficient removal of concrete from the fins after use. 
         [0029]    The various fin sections of fin assembly  20  can be made in a typical molding operation such as, for example, by cast molding or reaction-injection molding (RIM). In that molding operation, the fin sections are formed by rapid injection of two metered liquid streams, one stream containing a polyurethane prepolymer (polyol and isocyanate) blended with the filler material such as polyethylene, and the other stream containing a typical curing agent. These two streams are mixed and poured into the mold to form the mixing fin. Alternatively, a third stream containing the filler material can be used to mix the filler material with two other streams containing the prepolymer and curing agent before they enter a mold. The curing agent is employed to provide cross-linking of the polymers. Once the molding has occurred, the fin section is placed in an oven to cure the polymeric material which causes final cross-linking to take place. Of course, other processes may be employed depending on the composition and internal structure of the fins. 
         [0030]    In operation, the flexible plastic mixing fin according to the invention has a smooth, slippery surface which prevents concrete from adhering to the fin and forming hardened buildup deposits. The polymeric material should have good wearability and strength characteristics relative to its weight. A further advantage to the plastic mixing fins according to the invention is that they can be easily molded to any shape and thickness. This allows them to be molded thicker at points where greater strength and stiffness are needed. 
         [0031]    While the above description details the use of a fin mount rail track  30  fixed to a metallic drum surface  22  to secure the mixing assembly structure in place, it will be appreciated that the fin structure of the invention can be secured in place by adhesive bonding, bolting and other techniques as well. The mixing fin assembly of the present invention can also be installed in mixing drums made of composite polymeric materials themselves, which may contain fiberglass, or the like, using any of the described techniques, but may preferably be integrally bonded to the shell. 
         [0032]    Lighter weight mixing drums capable of processing larger concrete loads using lightweight gas-hardened or otherwise lightweight hardened abrasion resistant steel shells can be used in combination with the mixing fins of the present invention. Gas hardening enables steel shells as thin as 1-3 mm to outlast heavier models having much thicker shells. 
         [0033]    This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the example as required. 
         [0034]    However, it is to be understood that the invention can be carried out by specifically different devices and that various modifications can be accomplished without departing from the scope of the invention itself.