Abstract:
A lift mechanism for use in raising ground trench covers includes an axially extending housing, a cam located at one end of the housing, and a latch located at the other end of the housing. Rotational cam movement drives movement of the latch between retracted and actuated positions. The cam is connected to a latch driving mechanism, which may include springs and/or cables of unequal strength and/or unequal length. A method for using the lift mechanism is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a lifting mechanism. More particularly, the invention relates to an adaptor or attachment used in combination with a cable or chain to lift relatively heavy plates used to cover trenches containing electrical power and/or data lines. 
   In recent years, many utilities have employed ground trenches for running electrical power and data lines. These trenches may be fabricated from steel, concrete, or composite materials and are intended to contain and protect the electrical lines, while affording ready access to them. Typically, each trench is covered by a series of plates which are placed end to end over the length of the trench and rest in recess running along the top edges of the trench sidewalls. Like the trenches, the cover plates may be fabricated from steel, concrete, or various composites and each may weigh as much as 50 to 100 pounds. The weight of the cover plates, alone, makes it difficult to place, remove and replace the plates when installing the trenches or servicing the electrical lines contained in them. In addition, sand or gravel may become lodged in the small spaces separating the individual plate segments or between the plate segments and trench walls, making removal of the covers even more difficult. When the covers are lodged or wedged in place due to a tight spacing between adjacent cover(s) and/or the lodging of sand or gravel, for example, the initial lifting force required to dislodge even a 50–100 pound cover may be as much as hundreds of pounds. In such an instance, the use of manual tools, such as crowbars or hand held lift hooks, has obvious limitations. 
   The present invention is directed to the problem of removing these ground trench cover plates. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes disadvantages of prior cover lifting methods and mechanisms, while providing new advantages not previously obtainable. The present invention is directed to a lift mechanism for use in raising or moving trench covers, comprising an axially extending housing, a cam and a latch. The cam is located adjacent a first end of the housing and capable of moving between a first cam position and a second cam position. The latch is located adjacent a second end of the housing and is operatively associated with the cam and capable of rotational movement between a latch retracted position when the cam is in the first cam position and a latch actuated position when the cam is in the second cam position. The lift mechanism may be inserted into or removed from a cover aperture when the latch is in the latch retracted position, and the lift mechanism may engage and support the cover when the latch is in the latch actuated position. 
   In accordance with one preferred embodiment of the invention, the cam interacts with the latch through a pair of springs, one acting to urge the latch to its retracted position and the other to urge the latch to its actuated position. However, other cam and latch interaction mechanisms are contemplated. 
   In accordance with another preferred embodiment of the invention, the cam may move from its first position to its second position simply by applying a lifting force to the lift mechanism itself. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and attendant advantages thereof, will be best understood by reference to the following description taken in connection with the accompanying drawings, in which: 
       FIG. 1  is a perspective view, showing a cross-section of a typical ground trench with its overlying cover plates and illustrating the general use of the cover lift mechanism of the present invention; 
       FIG. 2  is a perspective view illustrating various components of one preferred embodiment of the present invention; and 
       FIGS. 3 and 4  are side elevational views, in partial cross-section, illustrating the operation of the preferred embodiment shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Set forth below is a description of what is currently believed to be the preferred embodiment and/or best example of the invention claimed. Alternatives or modifications to this preferred embodiment are contemplated. Any alternatives or modifications which make insubstantial changes in function, in purpose, in structure, or in result are intended to be covered by the claims of this patent. 
   Referring first to  FIG. 1 , cover lift mechanisms, each generally designated by reference numeral  10 , may be engaged to a trench cover  15 , for example, and lifted in tandem using a fork lift or other powered device to lift and remove the cover. As illustrated, each trench cover is typically provided with apertures  16  which receive lift mechanisms  10 . 
   In the illustrated embodiment of the invention, the lift mechanism  10  includes a housing  20 , a cam  25 , a connector  30 , an actuation piston  35 , and latch  50  as major components. Housing  20  may be a cylindrical or have other cross-sectional shapes. Cam  25  is rotatably mounted relative to actuation piston  35 , while connector  30  is rigidly connected to cam  25  by pin  40 . Pin  40  may have an ovoid or other non-concentric, cross-sectional shape. Pin  40  also passes through the upper extension  35   a  of piston  35  allowing the pin  40 , cam  25 , and connector  30 , to rotate relative to piston  35 , as best shown in  FIGS. 3–4 . Assembly pin  36  is insertable through slot apertures  22  in housing  20 , is fixed within actuation piston  35 , and may be locked by a washer  37  and tie pin  38  connection as shown or by other means, securing the piston  35  in relative, movable position near a top portion of the housing  20 . Pin  36  may move vertically along the axis of mechanism  10  (“axially”) by sliding within slot  22 , thus permitting slidable axial movement of piston  35  within housing  20 . A lower portion  35   b  of actuation piston  35  is connected to latch  50  using various methods, as further described below. 
   In the example shown in the illustrated embodiments, piston  35  is connected to opposing portions of latch  50  about latch pivot point  53 , at points  50   a ,  50   b , by two extension springs having different spring rates. Spring  51  has a relatively low spring rate and is termed here a “latch retraction spring.” Spring  52 , has a relatively high spring rate and is termed here a “latch actuation spring.” The specific spring dimensions and spring rates will depend upon the size and materials of the particular device and will be well known to or readily selected by those of ordinary skill in. Latch  50  is rotatable about latch pin  53  under the action of either of springs  51  and  52 , as explained below. 
   The operation of the preferred embodiment of the cover lift mechanism  10 , shown in the drawings, will now be described. When connector  30  and cam  25  are non-axially aligned with housing  20 , or “unloaded” as shown in  FIG. 3 , latch  50  is in its axially-aligned or “retracted” position. This is due to the fact that latch retraction spring  51 , the weaker spring, is actuated, while latch actuation spring  52  having a longer body length is not. Thus, spring  51  urges latch  50  to rotate about latch pin  53  in the clockwise direction of the arrow shown in  FIG. 3 , moving latch  50  into its retracted position. At this point, cover lift mechanism  10  may be inserted into or removed from the aperture  16  of cover plate  15 . 
   To engage the latch, connector  30  and cam  25  (which may be of 2-piece or 1-piece construction) are brought into axial alignment with housing  20 , that is “loaded,” as shown in  FIG. 4 . Due to the geometry of cam  25 , as it rotates pin  40  moves away from the upper end of housing  20  and piston  35  moves upwardly, as permitted by housing slot  22 , tensioning spring  52 . The spring rates and body lengths of the springs are selected such that now, given the raised position of piston  35 , the latch actuation spring  52  applies a greater force to the latch, causing the latch to rotate in a counterclockwise direction, as shown by the arrow in  FIG. 4 , to its open, actuated position. The weight of mechanism  10  is sufficient to counteract the force necessary to rotate cam  25  to move the latch into a retracted position. This permits connector  30  and cam  25  to be easily and manually moved into the loaded position. 
   Once the mechanism  10  has been inserted into the cover aperture  16  and latch  50  is oriented as illustrated in  FIG. 4 , the powered device joined to mechanism  10  via connector  30  may be operated to bring latch  50  into engagement with the underside of cover  15  and lift the cover off of the trench. 
   It should now be understood from the foregoing description that alternative mechanisms may be employed to connect the cam/piston arrangement to the latch. Such mechanisms include but are not limited to rack-and-pinion gears, elastic cables, chains, spring-and-cable, or spring-and-chain combinations, etc. 
   The above description is not intended to limit the meaning of the words used in the following claims that define the invention. Rather, it is contemplated that future modifications in structure, function, or result will exist that are not substantial changes and that all such insubstantial changes in what is claimed are intended to be covered by the claims.