Patent Publication Number: US-7721904-B2

Title: Hydraulic tank access cover with self-centering and anti-rotation device

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/550,598 filed Mar. 5, 2004. 

   BACKGROUND OF THE INVENTION 
   The invention relates generally to covers for the access holes of hydraulic tanks and, more specifically, to a hydraulic tank access cover that has a self-centering and anti-rotation feature. 
   Mobile construction equipment use hydraulic systems for many uses such as ground drives, hydraulic cylinder activation and hydraulic motor power. These hydraulic systems typically operate at high pressure making them very sensitive to contamination. The hydraulic reservoirs used in these systems need to be thoroughly cleaned before filling with hydraulic oil, otherwise severe contamination can result in excessive pump wear, valve sticking, cylinder scoring, seal tearing, and ultimately total system failure. Having an access hole to be able to clean out the tank before use or even after a system failure has happened can prevent costly future failures. 
   Hydraulic tank access holes need to be able to be sealed completely such that they do not leak hydraulic oil even when under a given pressure load. Common in the industry are sealing systems which include welding a tapped flange around the hole with blind tapped holes that can hold a gasket and lid bolted down. A second common method is to weld nuts on the underside of the tank surface around the perimeter of the hole to bolt down a lid. Still another form is an outer cover that bolts through the center to a threaded receptacle bolted inside the tank to a gusset or baffle that is welding across the center of the hole inside the tank. All of these methods require expensive machined parts, excess fabrication time, and welding to the inside of the tank. The welding process itself generates a very undesirable form of metal contaminates in the form of weld splatter, weld slag, and flaked off mill scale that are extremely hard on hydraulic systems to the point that a new machine will fail as it drives off the assembly line. An access cover that eliminates welding altogether to the tank has a huge advantage in not generating metal contaminates to begin with. 
   SUMMARY OF THE INVENTION 
   The access cover system of the present invention has the advantage that no internal or external welding is required to the tank itself, thus eliminating the possibility of weld generated contamination. The invention has an additional advantage in that it self centers and has an internal anti-rotation feature that allows fast assembly times with air impact guns. 
   A hole is generated in the tank during the fabrication stage by laser, water jet, or plasma cutting method. It is during this same cutting stage that a tab is cut without introducing any extra material or pierce time, which would negatively add cost. These tabs are later bent inward before final assembly to provide the anti-rotation feature. The cover assembly consists of an outer plate, a gasket and an inside cover plate. A center hole in all parts allows a flanged bolt to pass through the outer cover gasket and thread into the inner cover plate. The inside cover plate is inserted into the hole of the tank between the tabs and moved backwards until the side tabs of the inner plate come into contact with the tabs of the tank itself. At this point the outer cover is centered over the tank hole and the bolt is tightened down. The gasket seals around the outer edge of the outer cover. A captured O-ring completes the seal under the flange of the bolt. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  and  1   b  are a plan view and a side view, respectively, of a hydraulic tank showing an access hole modified in accordance with the present invention. 
       FIGS. 2   a - c  are plan, side section, and end section views, respectively, of an inside cover plate of the present invention. 
       FIGS. 3   a  and  3   b  are a plan view and a side view, respectively, of an outside cover plate of the present invention. 
       FIG. 4   a  is a side view of a bolt for use with the present invention showing in hidden line a recess for receiving an O-ring; 
       FIGS. 4   b  and  4   c  are end views of the bolt. 
       FIG. 5  is a plan view of the completed assembly of the present invention showing the inside cover plate and the outline of the hydraulic tank access opening shown in hidden line. 
       FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 5 . 
       FIGS. 7   a  and  7   b  are a plan view and a side view, respectively, of a hydraulic tank showing an access hole modified in accordance with an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   Illustrated in  FIGS. 1   a  and  1   b , generally at  10  is a section of a hydraulic tank for use in holding hydraulic fluid for a hydraulic system such as on an earth moving machine. The tank  10  includes an access hole  12  having a generally circular profile. The opening  12  includes a plurality of tabs  14  spaced around the periphery of the access opening. The tabs  14  are cut out of a sheet of metal during fabrication of the access opening  12  having the profile as illustrated in  FIG. 1   a , and then bent downwardly so that the end portions  16  extend below the underside surface of the tank  10 , as shown in  FIG. 1   b . This arrangement of the tabs  14  leaves the opening  12  essentially round which facilitates effective sealing of the opening  12 . 
   The invention includes an inside cover plate  18  ( FIG. 2 ). The inside cover plate  18 , in a preferred embodiment, has a profile that includes a rectangular portion  20  and a curvilinear portion  22 . The rectangular portion  20  transitions into the curvilinear portion  22  by way of a pair of shoulder sections  24   a  and  24   b . A central portion  26  of the inside cover plate  18  projects downwardly in the form of a truncated, four-sided pyramid, including a lower, flat portion  28  that is perforated by a bolt hole  30 . 
   An outside cover plate  32  ( FIG. 3 ) is generally circular in profile having a planar outer peripheral flange  34  of an annular shape. A central portion  36  of the outside cover plate  32  projects upwardly and forms a frustoconical shape having a flat top portion  38  that is perforated by a bolt hole  40 . A bolt  42  ( FIG. 4 ) is used in assembling the inside cover plate  18  and outside cover plate  32  to seal the access opening  12  of the hydraulic tank  10 . The bolt  42  includes a threaded end portion  44 , a hexagonal head  46  the underside of which includes a circular recess  48  in which is received a sealing gasket, such as an O-ring, for sealing the bolt hole  40 . The recess  48  assures that the O-ring will be retained inside the recess  48  and will not be damaged by the bolt  42  as it is tightened to seal the opening  12 . The recess  48  is of a size to accommodate an O-ring that is slightly larger than the bolt hole opening  40  so that the combination of the bolt  42  and O-ring will seal the bolt hole opening  40 . 
   The assembled cover is illustrated in  FIGS. 5 and 6 , generally at  54 . A gasket  50  is positioned between the inside cover plate  18  and the outside cover plate  32  and the bolt holes  30  and  40  are aligned. In the preferred embodiment, a threaded nut  52  has been secured to the underside of flat portion  28  of the inside cover plate  18  in coaxial alignment with the bolt hole  30 . The bolt  42  is then inserted through the bolt hole  40  and the threaded end portion  44  is received in threaded engagement with the nut  52 . The bolt  42  is tightened to pull the two cover plates  18  and  32  toward each other to provide a seal. 
   In assembling the cover  54  to seal an access opening  12 , cover  54  is assembled as shown in  FIGS. 5 and 6 , with the bolt  42  loosened so that there is a sizeable gap between the two cover plates  18  and  32 . The gasket  50  and cover plates  18  and  32  should be inspected for any cuts or loose material that could be allowed into the tank  10 . Slip one corner of the inside cover plate  18  with the rectangular portion  20  in between two of the locating tabs  14  of the opening  12  in the tank  10 . The cover  48  is then moved to bring the inside cover plate  18  fully inside the tank  10 . The cover  48  is moved to bring a pair of the tabs  14  into contact with the shoulders  24  of the inside cover plate  18 . The spacing of the tabs  14  and the contact of the tabs  14  with the shoulders  24  provide both a self-centering function and will restrain the inside cover plate  18  against rotation as the bolt  42  is tightened. Once the inside cover plate  18  is in position, the bolt  42  is tightened to bring the inside cover plate  18  into contact with the underside of the hydraulic tank  10  and the outside cover plate  32  into contact with and pressing on the gasket  50  into sealing engagement with the top surface of the hydraulic tank  10 . The O-ring of the bolt  42  will also engage and seal the perimeter of the bolt hole  40  of the outside cover plate  32 . It is to be noted that the shape of the cover plates  18  and  32  permit compression of the cover plates  18  and  32  toward each other by tightening of the bolt  42  and thus act as a spring to preload the bolt  42  and thereby improve the seal created and help to prevent creep of the gasket  50 . In this way, a single bolt with O-ring seal is used to create enough compressive load to seal the outer gasket perimeter. In addition, the O-ring containment system provided by the recess  48  allows a metal to metal contact of the flange bolt  42  with the outside cover plate  32  to achieve a bolt torque clamp load to hold bolt tension over time. The prior art uses a nylon crush washer which creeps over time due to compression set of the high compressive stress and then leaks. Further, the formed outer edge of the inner cover plate  18  increases the stiffness of the plate  18  to resist a much greater load induced on the plate  18  by the center flange bolt  42  than that which could be resisted by a flat plate alone. This allows a thinner material to be used, which reduces both cost and weight. 
   In an alternative embodiment, an opening  12 ′ of a hydraulic tank  10 ′ is formed with tabs  14 ′ that are not only bent downwardly as in the first embodiment, but also are bent inwardly, as illustrated in  FIG. 7  at  16 ′. The free end portions  16 ′ of the tabs  14 ′ will provide a support of the inside cover plate  18  so that if, during assembly or disassembly, the bolt  42  slips out of threaded engagement with the inside cover plate  18 , it will not drop into the hydraulic tank  10 ′.  15 . 
   The foregoing description and drawings comprise illustrative embodiments of the present inventions. The foregoing embodiments and the methods described herein may vary based on the ability, experience, and preference of those skilled in the art. Merely listing the steps of the method in a certain order does not constitute any limitation on the order of the steps of the method. The foregoing description and drawings merely explain and illustrate the invention, and the invention is not limited thereto, except insofar as the claims are so limited. Those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.