Abstract:
A plastic rotationally molded polyethylene buoyancy tank for boat lifts allows connection of steel to lengthwise grooves molded into each side of the buoyancy tank. Preferably, a plastic bulkhead is formed within the tank during its molding process. A segment of framing steel is bracketed between a pair of inwardly tapered compressible opposed grips. The compressible grips each have outwardly tapered slots with mating tapered rigid inserts. With the inserts partially seated on the grips, the grips are slipped lengthwise or pressed laterally into their respective grooves. Bolts extending through the grips into threaded holes in the inserts are tightened to pull the grips over their tapered inserts. This spreads and compresses the grips in their respective grooves, locking the brackets and connecting steel in place on top of the tanks.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to lifts for watercraft and more particularly concerns buoyancy tanks for boat lifts and the like.  
           [0002]    Corrosion is a major concern in the manufacture and maintenance of boat lifts. Typically, boat lift buoyancy tanks are encompassed by steel which secures the tanks and permits connection of other components to the lift. In use, at least parts of the steel are always submerged and constantly exposed to a corrosive environment which leads to shortened lift life. One method used by the industry to deal with this problem is the use of steel inserts, such as specially threaded nuts, which are molded into plastic parts. Since the steel must be fixed to the lift in the specific locations and orientations dictated by the inserts, design flexibility is limited. Furthermore, these inserts are normally longitudinally spaced on the buoyancy tanks and molded in place perpendicular to the shrinkage plane. In the cooling phase of the molding process, as the plastic shrinks over the length of the tank, the inserts can&#39;t move with the shrinkage. This results in tank deformation and other problems.  
           [0003]    Bulkheads are another factor impacting the manufacture and quality of boat lift buoyancy tanks. Bulkheads are currently used in some existing buoyancy tanks as a divider or baffle to control air and water. Because buoyancy tanks are made by rotational molding, the molded tank is a finished product. Access to the inside of the tank to install a bulkhead is impossible without violating the integrity of the tank. As a result, in known manufacturing processes, bulkhead installation is a time consuming and tedious process.  
           [0004]    It is, therefore, an object of this invention to provide a boat lift buoyancy tank which supports all non-plastic connections above the water line when the lift is raised. Another object of this invention is to provide a boat lift buoyancy tank which permits universal selection of connection points for the lift components along the length of the tank. A further object of this invention is to provide a boat lift buoyancy tank which eliminates the need for multiple single position inserts to accommodate connection of the lift components. Yet another object of this invention is to provide a boat lift buoyancy tank which has contours adapted for connection of lift components which will not deform as a result of shrinkage during the molding process. It is also an object of this invention to provide a boat lift buoyancy tank which has an integral internal bulkhead formed during the tank molding process. Still another object of this invention is to provide a boat lift buoyancy tank which eliminates the need for installing bulkheads after the tank is molded.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention, a plastic rotationally molded polyethylene buoyancy tank for boat lifts allows connection of steel to lengthwise grooves molded into each side of the buoyancy tank. The grooves are located proximate the top of the buoyancy tank so that the steel anchored to the grooves is fully above the water line when the lift is in its raised position. Therefore, the steel is not constantly exposed to the corrosive environment of the water. The lengthwise grooves allow the steel to be attached anywhere along the length of the tank, affording a high degree of flexibility in the design of the boat lift structure.  
           [0006]    A segment of framing steel is bracketed between a pair of inwardly tapered compressible opposed grips. The compressible grips each have outwardly tapered slots with mating tapered rigid inserts. With the inserts partially seated on the grips, the grips are slipped lengthwise or pressed laterally into their respective grooves. Bolts extending through the grips into threaded holes in the inserts are tightened to pull the grips over their tapered inserts. This spreads and compresses the grips in their respective grooves, locking the brackets and connecting steel in place on top of the tanks.  
           [0007]    Preferably, a plastic bulkhead is formed within the tank during its molding process. The bulkhead is made from a plastic that has a slightly higher melting point than the tank&#39;s plastic. The plastic bulkhead is held in place in the mold during the tank molding process. The temperature in the process melts the powdered plastic for the tank and almost melts the plastic bulkhead. The two plastics fuse together around the perimeter of the inner mold surface, resulting in a molded-in-place bulkhead.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0009]    [0009]FIG. 1 is a perspective view of a preferred embodiment of the molded plastic buoyancy tank with a plastic bulkhead molded in place;  
         [0010]    [0010]FIG. 2 is a top plan view of the buoyancy tank of FIG. 1;  
         [0011]    [0011]FIG. 3 is a side elevation view of the buoyancy tank of FIG. 1;  
         [0012]    [0012]FIG. 4 is an end elevation view of the buoyancy tank of FIG. 1;  
         [0013]    [0013]FIG. 5 is a perspective view of a preferred embodiment of the grips used to secure float framing components to the buoyancy tank;  
         [0014]    [0014]FIG. 6 is a perspective view of a preferred embodiment of the inserts used with the grips of FIG. 5;  
         [0015]    [0015]FIG. 7 is an end elevation assembly view of the grips and inserts of FIGS. 5 and 6;  
         [0016]    [0016]FIG. 8 is a perspective view of a preferred embodiment of a bracket assembly for use with the grips and inserts of FIGS. 5 and 6;  
         [0017]    [0017]FIG. 9 is a cross-sectional view taken in a plane transverse to the length of the tank at one stage of float assembly;  
         [0018]    [0018]FIG. 10 is a cross-sectional view in a plane transverse to the length of the tank at a subsequent stage of float assembly;  
         [0019]    [0019]FIG. 11 is a perspective view of a float using the tanks, grips and inserts of the present invention; and  
         [0020]    [0020]FIG. 12 is an exploded sectional view illustrating the bracket assembly of FIG. 8 mounted on the buoyancy tank. 
     
    
       [0021]    While the invention will be described in connection with a preferred embodiment and method, it will be understood that it is not intended to limit the invention to that embodiment or method. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0022]    Turning first to FIGS.  1 - 4 , a preferred embodiment of the rotationally molded polyethylene buoyancy tank is illustrated. The tank  10  is an elongated shell having a substantially flat top wall  11  with a raised portion  12 . The flat outer portions of the top wall  11  are desirable for abutment with brackets as will be hereinafter explained. The raised portion  12  of the tank  10  affords an irregular cross-section for the top wall  11 , thus affording additional strength to the tank  10 . The tank  10  also has side walls  13  and  14  and an arcuate bottom wall  15 . The shell is completed by end walls  16  and  17 . Each side wall  13  and  14  is provided with a lengthwise groove  18  and  19 , respectively, extending substantially for the entire length of the sidewalls  13  and  14  and proximate the top wall  11 . The tank  10  is also provided with one or more air inlet/outlet passages  21  and water inlet/outlet passages (not shown). As shown in FIG. 1, one or more bulkheads  22  may be provided within the tank  10  for the purpose of controlling the flow of air and water in the tank  10 .  
         [0023]    Preferably, the tank is molded of polyethylene or other suitable plastic by a rotational molding process. In the molding process, the bulkhead  22  is secured in the tank mold at the beginning of the process. The plastic powder for the tank  10  has a melting temperature which is lower than the process temperature at which the tank  10  is molded. The bulkhead  22 , on the other hand, is preformed from plastic having a melting temperature slightly higher than the process temperature. Therefore, as the tank  10  is molded at the process temperature, the tank  10  and the bulkhead  22  are fused together along the perimeter  23  of the bulkhead  22 . As shown, the bulkhead  22  is transverse to the length of the tank  10  and disposed approximately at its midpoint. However one or more bulkheads can be used in any position or orientation.  
         [0024]    Looking at FIGS. 1, 4 and  12 , the configuration of the grooves  18  and  19  is best seen. The grooves  18  and  19  have top  24  and  25  and bottom  26  and  27  walls, respectively. The grooves  18  and  19  are opposed to each other so that the top  24  and  25  and bottom  26  and  27  walls extend inwardly into the tank  10  to side walls  28  and  29 , respectively. Preferably, as shown, the groove top  24  and  25  and bottom  26  and  27  walls are narrowingly tapered or converging toward the groove side walls  28  and  29 , respectively. A taper angle of approximately two degrees relative to horizontal is suitable.  
         [0025]    Looking now at FIGS. 5, 7,  8  and  12 , a grip  30  for use with the grooves  18  and  19  is illustrated. The grip  30  is a resiliently compressible member having a back wall  31  with a rigid plate  32  inset in the back wall  31  along the length of the grip  30 . The side walls  33  and  34  of the grip  30  are inwardly narrowingly tapered from the back wall  31  to the front wall  35  so that, in exterior cross section, the grip  30  is substantially trapezoidal. A recess  36  is provided in the grip  30  through the front wall  35 . The recess  36  is wideningly or outwardly tapered toward the back wall  31 . The recess taper angle of two to three degrees relative to horizontal is suitable. The end walls  37  and  38  have slots  39  and  41  extending from the front wall  35  toward the back wall  31  to permit the front wall  35  and back wall  31  to be more manipulable relative to each other. One or more smooth holes  42  and  43  extend through the plate  32  and the back wall  31  into the recess  36 .  
         [0026]    Looking at FIGS. 6, 7,  8  and  12 , a rigid insert  44  having a trapezoidal cross-section is tapered for insertion into the recess  36  in the grip  30 . Preferably, the taper of the cross-section of the insert  44  is at a steeper angle than the taper of the recess  36 . This can best be seen in FIG. 7. The insert  44  is provided with one or more threaded holes  45  and  46  which align with the smooth holes  42  and  43  in the grip  30  when the insert  44  is inserted into the recess  36  in the grip  30 . Bolts  47  and  48  extending through the holes  42  and  43  in the grip  30  and threadedly engaged in the holes  45  and  46  in the insert pull the rigid insert  44  into the recess  36  as they are rotated. Preferably, the plate  32  and the insert  44  are made of aluminum and the grip  30  is made of molded polyurethane.  
         [0027]    Looking now at FIG. 8, the use of the grips  30  to secure a segment of the framing steel  51  of the boat lift to the tank  10  is illustrated. As shown, a section of framing steel  51  such as an angle iron is sufficiently long to extend substantially across the top wall  11  of the tank  10 . A 3″×2″×{fraction (3/16)}″ steel angle iron is suitable. The ends of the framing steel  51  are connected to brackets  52  and  53 , possibly formed from ¼″ steel. As shown, spacers  54  and  55  are welded between the brackets  52  and  53  and the framing steel  51  if necessary to permit the framing steel  51  to ride over the raised portion  12  of the tank  10  when the horizontal portions  56  and  57  of the brackets  52  and  53  are seated on the flat top wall  11  of the tank  10 . The brackets  52  and  53  extend downwardly to grip mounting portions  58  and  59 . The grip mounting portions  58  and  59  have holes (not shown) for the bolts  47  and  48  to extend through the brackets  52  and  53  into the rigid plates  32  in the grip  30 .  
         [0028]    Looking now at FIGS. 9, 10 and  12 , the manner of mounting the segment of framing steel  51  to the tank  10  will be explained. Inserts  44  are inserted into the recesses  36  in grips  30  so as to hold these components together without significantly deforming the grips  30 . One grip  30  with its insert  44  is then inserted into each groove  18  and  19  in the tank  10  with the inserts  44  against the side walls  28  and  29  of the grooves  18  and  19 . The grips  30  are aligned lengthwise in the grooves  18  and  19  at the position at which it is desired to secure the segment of framing steel  51  to the tank  10 . The brackets  52  and  53  welded to the framing steel  51  are aligned with the grips  30  seated in the grooves  18  and  19 . As shown in FIG. 9, the holes in one of the brackets  53  are aligned with the holes  42  and  43  in its grip  30 . The bolts  47  and  48  are inserted through the bracket  53 , the rigid plate  32  and the back wall  31  of the grip  30  into the threaded holes  45  and  46  in the insert  44 . The bolts are then tightened to pull the insert  44  deeper into the recess  36  until the front wall  35  of the grip  30  is brought into contact with the side wall  29  of the groove  19 . The insert  44  compresses the side walls  33  and  34  of the grip  30  against the top and bottom walls  25  and  27  of the groove  19  to lock the grip  30  in the groove  19 . As seen in FIG. 10, the holes in the other bracket  52  are then aligned with the holes  42  and  43  in the other grip  30  and the process repeated to secure the other grip  30  in place in its groove  18 . With the grips  30  so secured, the segment of framing steel  51  is secured in position atop the top wall  11  of the tank  10 . As shown, the segment of framing steel  51  is provided with a plurality of holes  61  to facilitate connection of additional framing steel to the tank  10 .  
         [0029]    Turning to FIG. 11, a boat lift is illustrated in which two tanks  10  in parallel alignment have framing steel secured thereto by use of the parallel grooves  18  and  19  in the upper side walls  13  and  14  of the tanks  10 . All of the steel is fixed at or above the level of the grips  30  so that all of the steel will be above the water line when the boat lift is in a raised condition. Since the grooves  18  and  19  extend for substantially the entire length of the tanks  10 , the grips  30  and therefore the framing steel  51  can be positioned at any desirable location along the length of the tanks  10 . This affords a great deal of flexibility in the design of the lift so that the lift structure can be easily contoured to suit any configuration of water vehicle using the same tanks  10 .  
         [0030]    Thus, it is apparent that there has been provided, in accordance with the invention, a boat lift buoyancy tank and a method for making the tank that fully satisfy the objects, aims and advantages set forth above. While the invention has been described in conjunction with a specific embodiment and method, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art and in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit of the appended claims.