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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     This invention relates generally to manhole or manway covers, and more particularly to a device to assist in the lifting of such cover. 
     Because of the weight of manway lids, various efforts have been made to make it easier to open them. Usually such devices are associated with the lid hinge assembly. U.S. Pat. Nos. 4,137,669 and 5,038,520 use conical compression springs pulling a rod to assist. U.S. Pat. No. 5,184,422 also uses a compression spring, but vertically oriented to assist the limited distance left of a lid which swings away from the manway in a horizontal plane. U.S. Pat. No. 5,394,650 uses torsion springs around a horizontal hinge pin to assist the lid lifting. Spring balanced lids have been available in the marine industry for at least twenty years. A variety of Cen-Tex brand hatches with spring-assisted lids is available from Timco Industries, Inc. of New Albany, Ind. These use torsion springs to balance round, rectangular, and elongate oval shapes of lids. Some aspects of inconvenience in the mounting or use of one or more of these devices are the necessity of attaching one or more components of the balancing assembly to the manway frame or cover, the inability to hold the cover open adequately, the inability or difficulty in adjusting the balancing force to prevent the cover from dropping, and the necessity to use welding equipment for some installations to manway frames. The difficulty of adapting the balancing assembly to various styles of covers and arrangements of hinging the cover to the manway frame has been an ongoing problem. 
     SUMMARY OF THE INVENTION 
     Described briefly, according to a typical embodiment of the present invention, a balancer mount is connected to the manway hinge and engages the manway frame to locate the lifter assembly. A cam is situated in the mount for pivotal action on the hinge and has a portion to engage the lid, and a portion to receive a force applied by a balancing spring seated in the mount. A spring adjustment device is provided at a readily accessible location on the mount. 
     In one embodiment, the lid lifter assembly is arranged to provide the lid lifting assist at a location between the hinge arms of the lid. In another embodiment, the lifter assembly is mounted outboard of the hinge arms and applies the lift assist at a location outboard of the hinge arms but adjacent one of the hinge arms. In both instances, the lifter assembly has a housing with at least one arm portion engaging the lid hinge mounting bracket of the manway to hold and control the lifter assembly housing relative to the manway. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a manway/lid assembly with the lid lifter assembly mounted thereto. 
     FIG. 2 is a bottom plan view showing only the manway frame, hinge mounting brackets and the lifter assembly. 
     FIG. 3 is a section through the assembly of FIG. 1 taken at line  3 — 3  in FIG.  1  and viewed in the direction of the arrows. 
     FIG. 4 is a section like FIG. 3 but showing the lid in the open and neutral balanced position. 
     FIG. 5 is a pictorial view of the lifter assembly separate from the manway assembly. 
     FIG. 6 is an exploded view thereof. 
     FIG. 7 is a pictorial view of an alternate embodiment of the invention. 
     FIG. 8 is an exploded view of the alternate embodiment. 
     FIG. 9 is an exploded view of a manway/lid assembly with the lid lifter assembly of the alternate embodiment. 
     FIG. 10 is a top plan view thereof, with a portion of the manway lid broken away to show the location of the cam relative to the lid. 
     FIG. 11 is a section therethrough taken at line  11 — 11  in FIG.  10  and viewed in the direction of the arrows. 
     FIG. 12 is an enlarged fragmentary section through the lifter assembly taken at  12 — 12  in FIG.  10  and viewed in the direction of the arrows. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring now to the drawings in detail, and particularly FIGS. 1-6, the manway assembly  11  is mounted to the top of a compartment  12  (shown only fragmentarily and not in section) of a marine vessel, railroad tank car, or stationery installation, for example. It includes the frame  13  which is normally welded in place on the compartment, and a lid  14  connected by a hinge pin  16  to a couple of brackets  17  welded to the frame  13 . This is a fairly conventional type of installation. In the illustrated embodiment, the hinge arm portion  18  of the lid has a slot  19  therein and has a couple of buttresses  21  to strengthen the lid at the hinge mounting arms. 
     According to this first embodiment of the present invention, a mount for the lid lifter assembly  22  is connected to the hinge pin  16  and has a portion engaging the manway frame to position and stabilize the mount. While the mount may have different forms, the illustrated example includes a downwardly-opening U-shaped channel  23  having apertures  24  therein receiving flanged bushings  26  through which the hinge bolt  16  extends. A cotter pin  27  at each end retains the hinge pin in the assembly as shown in FIG.  1 . The U-channel  23  has a stop plate  28  affixed by welding or otherwise to the front end  29  of the channel. The stop plate  28  has laterally extending arms  28 A bearing on brackets  17 . The U-channel  23  also has a door clip  31  fixed to the bottom edges  32  by welding, and a door  36  whose lower edge is received within the door clip. The door extends upwardly along the rear edges  37  of the channel to the top  38  where it is fastened by a screw to a spring adjustment block  39  welded in the top of the U-channel. A spring adjustment screw  41  is threaded into block  39  and has a lower end received in a top retainer  42  for a compression spring  43  whose lower end is received on the spring bottom retainer  44 . The spring bottom retainer is connected by a threaded stud  46  to the upper end of a yoke member  47 , whose lower end is pinned in aperture  48  at one end of a cam  49 , which has an aperture  50  pivotally mounted on the bushings  26  received on the hinge pin  16  whereby the cam is rockably mounted to the hinge pin  16 . The opposite end of the cam  49  has a bar  51  affixed thereto and which is engageable with the lower edges  52  of arms  18 . Thus, the compression spring is in position to act along axis  55  and urge the cam in a clockwise direction (FIG. 3) to operate as a first class lever to cam the lid in an opening direction of arrow  56 . 
     The lid lifter according to the present invention can be mounted to a variety of conventional manway and lid assemblies, by simply removing the hinge pin, putting the mount and cam in place and re-installing the hinge pin through the cam and mount and the lid and manway hinge apertures. Then the amount of assist can be adjusted by simply turning the screw at the top to compress the spring as desired. No special tools or techniques are needed. No fastener (other than the binge pin) or welding, is required. 
     To demonstrate the adaptability to another type of conventional manway and lid assembly, FIGS. 7-12 are provided and will be described now. In these figures, components having the same or similar characteristics in the description of the first embodiment will be given the same reference numerals, where convenient, but with the additional digit “1”. Accordingly, the lifter assembly  122  is provided to assist the person to lift the lid assembly  114  which is pivotally mounted by the hinge  116  to the manway brackets  117  fixed on the manway frame  113  as by welding, for example. But in this manway assembly, the lid has only a single hinge arm  118  pivotally mounted on the hinge bolt  116 , rather than the arm  18  slotted at  19  in the manway assembly of FIGS. 1-4. Therefore, the lift assembly according to FIGS. 1-6 cannot be mounted in the plane of the centerline of the manway assembly  113 / 114 . Instead, this alternate embodiment  122  is mounted outboard of bracket  117 L as best shown in FIG.  10 . For that purpose, the inverted U-channel  123  is provided with a stop plate  128  only on the channel wall  123 N that is adjacent the bracket  117 L. The arm  128 A extends laterally from the plate  128  affixed to the front edge  129  of channel wall  123 N, and is positioned to bear downward on the top  117 T (FIGS. 9 and 11) of bracket  117 L when the lift assembly  122  is mounted on the hinge pin  116 . Similar to the relationship shown in FIG. 3 for the first embodiment, the plate  128  of the alternate embodiment is positioned immediately adjacent the wall of frame  113 . The bracket engaging arms  28 A and  128 A prevent the spring force from turning the housing counterclockwise under load. 
     Because of the offset location of the lift assembly  122  with respect to the center line of the manway assembly, the lid contacting portion of the cam  149  has a horizontal offset at  149 A which brings the lid-contacting top portion  151  of the cam closer to the center of the lid hinge arm  118 . Thus, portion  151  is clearly under the tab  161  at one side of the lid hold-down bolt slot adjacent the hinge arm  118 , whereby the cam portion  151  is able to engage the tab for lifting, as best shown in FIG.  12 . 
     A few additional differences between the second embodiment and the first embodiment may be noted, such as the shape of the bottom plate  131 , considering the different lengths of the walls  123 N and  123 F of the U-channel  123 . Another is the notch  123 F in the front of the wall  123 N and the notched washers  162  shown in FIG. 9 to accommodate the lid stop rod  163  normally found in this particular style of manway/lid assembly and which is in position to engage the underside  164  of the hinge arm  118  to provide a stop when the lid of the conventional lid assembly is opened. With the lifter assembly of the present invention, the stop and rod  163  are not required, as the spring adjusting screw  141  can be adjusted to hold the lid open at a 60° angle from the closed position, and it is not necessary to lift the lid all the way back to the stop. Of course, for those users who would be inclined to open the lid as far as it would go, the stop rod  163  and hinge abutment  164  are beneficial. 
     Apart from the differences and some details described specifically above, the other features of the lift assembly can be identical to those described above for the first embodiment. These include the door  136 , the spring adjustment block  139 , the spring adjustment screw  141 , top retainer  142 , compression spring  143 , bottom retainer  144 , threaded stud  146 , yoke member  147  and pin  148 . The arrangement of the spring  143  and adjusting screw  141 , spring rate, dimensions from pivot pin axis to the axis of connector pin  148 , and the contact point of bar  51  in the first embodiment or cam top  151  in the second embodiment with the lid, can be determined based upon the size and weight of the lid, the amount of lift assist desired, and the angle of the lifter assembly load applying axis  55  extending through aperture  48 , relative to horizontal. Good lift assistance consistent with size and space requirements of the lift assembly according to the present invention can be achieved with an angle of axis  55  up from horizontal at 53°. 
     The manway/lid assembly of FIGS. 9-12 is constructed somewhat different from that shown in FIGS. 1,  3  and  4 , but the installation of the complete lift assembly  122  is just as easy for this embodiment as for the first. The cam  149  will already be pivotally mounted on bushings  126  received in apertures  125  in the U-channel  123 . The rest of the components  139 - 148  will be in place in the channel  123 . Then it is only necessary to remove the hinge pin  116 , install it through the bushings  126  and then through the manway bracket  117 L, washer  162 L, hinge arm  118 , washer  162 R, bracket  117 R, the flat washer, lock washer, and install the nut on the hinge pin bolt. The lid stop rod  163  can remain in place during this entire procedure. There is usually a weld at the outside face of each of the hinge brackets  117 L and  117 R where the stop rod had been welded to the bracket. The notch  123 F in the front of wall  123 N is large enough to accommodate the bulge of the weld, and still permit the wall  123 N to be placed snug against the outside face of the bracket  117 L without interference. In cases where the manway lid will accommodate the center-mounted lift assembly of FIGS. 1-6, and if the manway happens to have a stop rod such as  163  in FIG. 11, the stop rod can be cut out and discarded to admit the stop plate  28  and front end of the assist assembly to place it against the manway wall  13 . In that case, if the user desires to open the lid entirely, it can be rested on the top of the U-channel in the absence of the stop rod. 
     The present invention will work regardless of whether the hole in the hinge arm  18  or  118  is round, or elongated to accommodate some vertical travel while being clamped down closed as standard hold-down clamps (not shown) are applied. Additional washers such as  162 L can be used to avoid lateral tilting of the lid by the outboard-mounted lift assembly of FIGS. 7-12 if the spacing between the hinge arm and brackets  117 L and  117 R of the manway assembly is excessive. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.

Summary:
A lid lifter mount is installed on the hinge pin of a conventional manway lid, and has a compression spring assembly engaging a lever which is also installed on the hinge pin. The lever has a bar engaging a lid hinge arm. A spring force adjustment screw connected to the mount enables adjustment of the amount of lift assist the bar applies to the hinge arm by cam action against the arm as the operator lifts the lid. Two embodiments are shown for attachment to two types of lid hinges.