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PRIORITY APPLICATION 
   This Application claims priority from U.S. Provisional Application, Ser. No. 60/356,193 filed on Feb. 12, 2002. 

   FIELD OF THE INVENTION 
   The present invention is related, in general, to devices for driving posts into the ground during the construction of fences, guard railing, and the like. More particularly, the invention relates to a post driver which may be fitted to a vehicle for mobility, and which is adjustable to allow driving a post at any desired angle corresponding to the contours of the ground in which it is driven. The mobile post driver of this invention further includes a means for retaining a post being driven in place, thereby eliminating the need for the user to manually hold the post in place during the initial phases of the driving operation. 
   BACKGROUND OF THE INVENTION 
   It is known to use mechanical post driving devices to achieve the driving of posts of relatively large cross-section into the ground during the construction of fences, guard railings, and the like. These prior art devices generally operate by use of a large, heavy hammer assembly which is raised and dropped on the post, striking the post, and thereby driving it into the ground at a desired site. Prior art post driving devices often are designed to be affixed to a vehicle, improving ease of use and allowing the user to quickly and efficiently place a large number of posts at the desired location during fence construction. 
   Prior art post driving devices, including those for which operation of the hammer assembly is achieved by means of hydraulic pressure or by a cable-type assembly, place the hammer lifting means at the rear of the hammer assembly between the post driver and the vehicle to which it is mounted. Prior art devices also often require the user to manually hold the post being driven in place during the initial strikes of the post driving operation, thereby increasing risk to the user. 
   Such devices, in general, are effective for their intended purpose. However, placement of the hammer lifting means at the rear of the hammer assembly results in certain disadvantages. In order to efficiently drive a post of relative large cross-section, the hammer assembly must be of relatively large weight and mass. Lifting and dropping such a hammer assembly using only a rear-mounted hydraulic cylinder or cable may result in significant tilting of the post driver during the striking operation due to the offset placement of the hammer assembly relative to the hammer lifting means. 
   Additionally, in order to be effective, the hammer assembly must be offset relative to the vehicle to which it is mounted, further contributing to the instability of the post driver. Accordingly, there is need in the art for a post driving device which overcomes this limitation in the prior art devices. There also is a need for a mobile post driver which obviates the need for a user to manually hold a post in place. 
   SUMMARY OF THE INVENTION 
   The post driver of the present invention is constructed to be supported by a vehicle, such as a tractor or large truck. The device of this invention is particularly suited for driving posts having a large cross-section into the ground such as, for example, large wooden fence posts used in constructing plank fencing. 
   The mobile post driver of the present invention includes a hammer assembly constructed of a material of sufficient strength and weight to drive a post into soils of varying consistency and resistance. The device includes lifting means for lifting the hammer assembly to a desired height prior to dropping the hammer on a post, thereby driving the post into the ground by impact forces. The device further includes a means for retaining a post in place during the driving operation as well as an adjustable stabilizing means for stabilizing the post driver during operation. 
   In a preferred embodiment of the present invention, the mobile post driver comprises a frame for supporting a hammer assembly, a swiveling means for mounting the frame to a vehicle, a lifting means for raising the hammer assembly to a desired height, an adjustable stabilizer for stabilizing the post driver during operation, and a means for retaining a post in place during the driving operation. Placed on either side of the hammer assembly are two means for lifting the hammer assembly. The hammer lifting means are provided interior to the frame, affording increased stability during the raising and lowering phases of a post driving operation. The frame also includes a stabilizing means comprising a beam and foot assembly which may be raised or lowered to the ground to provide an additional stabilizing point, thereby improving the stability of the device during operation. 
   The post driver of the present invention further includes a retention means for retaining a post in place during the driving operation. In a preferred embodiment, the retention means comprises a cap of sufficient size to accommodate the post being driven. The cap includes a recessed portion for contacting the upper surface of the post, thereby keeping the post in a desired alignment. The cap fiber may include metal studs or pins in the recessed portion, securing the post in the desired alignment. 
   The post driver of the invention additionally includes a mounting means for mounting the driver to a vehicle, thereby rendering the post driver mobile. Advantageously, the mounting means allows the alignment of the post driver to be altered in accordance with the contours of the ground into which the post is to be driven. Regardless of the specific contour of the ground into which the post is driven, it will be appreciated that this feature allows the driving of posts into the ground in a substantially perpendicular orientation to the ground. 
   Additional advantages and other novel features of the invention will be set forth in part in the description that follows and will become apparent to those skilled in the art from the Detailed Description of the Invention to follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a frontal view of the post driver of the invention. 
       FIG. 2  is a section view of the post driver of the invention taken along line  2 — 2  of  FIG. 1   
       FIG. 3  is a section view of the post driver of the invention taken along line  3 — 3  of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the post driver  10  of this invention comprises a frame  12 , a hammer assembly  14 , and a post retaining cap  16 . The frame  12  comprises a left support  18 , right support  20 , and rear support  22 . Left support  18 , right support  20 , and rear support  22  are connected by cross-members  24 . As will be appreciated from  FIG. 2  and the description below, the left support  18 , right support  20 , rear support  22 , and cross-members form a cage  25  into which hammer assembly  14  and post retaining cap  16  are slidably inserted. Accordingly, hammer assembly  14  and post retaining cap  16  are slidably retained for vertical movement within frame  12 . In the preferred embodiment best visualized in  FIG. 2 , the frame  12  is substantially “C-shaped,” with the hammer assembly  14  and post retaining cap  16  slidably retained within the frame  12 . 
   The hammer assembly  14  is of standard design. In a presently preferred embodiment, a rectangular hammer assembly  14  is provided having sufficient weight and mass to effectively drive a post (not shown) of desired dimensions into soil of varying consistency and resistance. However, it should be appreciated that any desired shape of hammer may be employed such as, for example, a cylindrical hammer. The hammer assembly  14  is operably connected to hammer lifting means  26  as will be described below. The hammer assembly  14  further includes a peg or bolt  28  for supporting he post retaining cap  16 . 
   In the presently preferred embodiment, the post retaining cap  16  is a rectangular block large enough to substantially overlay a post  17  of desired size, as best seen in  FIGS. 1 and 3 . The post retaining cap  16  is releasably supported on the hammer assembly  14  by means of a cable  30  affixed to the post retaining cap  16  at either end and releasably connected to peg or bolt  28  at the middle. The post retaining cap  16  includes a recess  32  which is designed to fit over the top surface of a post  17  to be struck. The recess  32  also may include studs or pins  31  which engage the surface of the post  17  to be struck. 
   In a preferred embodiment of the present invention, hammer lifting means  26  are incorporated into the left frame support  18  and the right frame support  20 . Using left frame support  18  as an example, the exterior of the left frame support  18  comprises a hollow tube  34  having an upper portion  36  and a lower portion  38 . Upper portion  36  contains a channel  40  therethrough which allows the cross member  44  or ear  44  of hammer  14  to engage the hollow interior of tube  34 . Moreover, it will be appreciated that channels  40  therefore provide a path of travel for the hammer assembly  14  to slide vertically along a partial length of the left frame support  18  and the right frame support  20 . The hammer lifting means  26  further includes hydraulic cylinders  27  and interior metal rods  42 , which are slidably inserted into the interior of the hollow tubes  34 . The interior metal rods  42  include an ear  44  projecting therefrom, not only oriented along the longitudinal axis of the rods  42  but also configured to be cooperatively received in channels  40 . The ears  44  are connected to the hammer assembly  14  by any desired means, such as by welding. The tubes  34  are illustrated as cylindrical but also may be rectangular or square in cross-section. 
   The interior metal rods  42  and, thereby, the hammer assembly  14  connected thereto are raised and lowered by hydraulic cylinders  27  located in the lower portion  38  of the hollow tubes  34 . Interior metal rods  42  are separated from the hollow tubes  34  by a plurality of bushings  46 . It will be appreciated that any suitable means for reducing friction may be utilized as lubricated bearings between the interior metal rods  42  and the hollow tubes  34 . In a presently preferred embodiment, friction between the interior metal rods  42  and the hollow tubes  34  is minimized by use of a plurality of bushings  46  composed of a lubricant-impregnated plastic, thereby eliminating the need for periodically lubricating the bushings  46 . Such lubricant-impregnated plastics are known in the art and are sold under various trade names such as, for example, NYOIL™. 
   As best seen in  FIG. 3 , the frame  12  also includes incorporated into the rear support  22 , a stabilizing means  48  which has a top portion  50  and a bottom portion  52 . The rear support  22  comprises a substantially “I-shaped” member  54  (best seen in  FIG. 2 ) having a front portion  56  and a back portion  58 . The stabilizing means  48  is slidably retained within the back portion  58  of the I-shaped member  54  of rear support  22  by cross-pieces  60 . Stabilizing means  48  comprises a beam and foot assembly  62  slidably retained by the bottom portion  52  of the rear support  22 . In a presently preferred embodiment, stabilizing means  48  is connected to and, therefore, may be raised and lowered by a hydraulic cylinder  49  located in the top portion  50  of the rear support  22 . As will be described in greater detail below, during operation of the post driver  10 , the stabilizing means  48  is lowered into supporting contact with the ground into which the post  17  is being driven, which thereby provides an additional point of support for the driver  10  and enhances stability of this device during operation. 
   The post driver  10  of the present invention further includes a mounting means  64  for mounting to a vehicle (not shown). Depending upon user preference, it will be appreciated that the post driver  10  may be mounted to the front, back, or side of a vehicle. In the presently preferred embodiment of the invention, best seen in  FIG. 3 , the mounting means  64  comprises at least one slide channel  66  configured to cooperatively receive a correspondingly sized bar (not shown) attached to a vehicle and extending laterally therefrom. A vertical beam  68  extends from the slide channel  66  and is connected to a swivelling connector  70 . 
   As seen in  FIG. 3 , the swivelling connector  70  comprises a cylindrical swivel mount  72  and a rear attachment bracket  74 . Cylindrical swivel mount  72  connects to rear support  22  by means of a first bracket  76  and includes a hollow external tube  78  as well as an internal tube  80  rotatably received within hollow external tube  78 . The hollow external tube  78  and internal tube  80  are separated by plastic bushings  82  to reduce friction as described supra. Cylindrical swivel mount  72  further includes a rocker arm  84  attached to internal tube  80  which allows the bracket  76  and, thereby, the rear support  22  and post driver  10  to be tilted from side to side. In the presently preferred embodiment, movement of rocker arm  84  and, subsequently, the desired side-to-side tilting of post driver  10  is accomplished by means of a hydraulic cylinder (not shown). 
   The rear attachment  74  of the swivelling connector  70  is operably connected to the rear support  22  at a second bracket  76 , thereby allowing the post driver  10  to be tilted forwards and backwards, also by means of a hydraulic cylinder (not shown). The hydraulic cylinder (not shown) may be attached to bracket  86 . It will be appreciated that this arrangement permits orientation of the post driver  10  relative to the ground to be adjusted as needed regardless of the contours of the ground into which a post is to be driven. 
   Reference is now made to use of a preferred embodiment of the present invention. A vehicle with the post driver  10  mounted thereon is positioned such that the post driver  10  is located directly above the site wherein a post is to be driven. As needed, the post driver  10  is tilted by means of the swivelling connector  70 , either side to side, and forward or backwards by the hydraulic cylinder (not shown) attached to the second bracket  86  attached to the rear support  22 , or both, to ensure that the post will be driven into the ground in a substantially perpendicular orientation. Stabilizing means  48  then is lowered until the beam  40  and foot  62  assembly is in contact with the ground. 
   The post to be driven is positioned under hammer assembly  14 . Hammer assembly  14  then is lifted using hammer lifting means  26  such that the path of the hammer assembly  14  is defined by channels  40  in the left frame support  18  and the right frame support  20 . Of course, the post retaining cap  16 , which is releasably attached to the hammer assembly  14  by cable  30 , also is raised. In the presently preferred embodiment of the invention, the hydraulic pressure by which the hammer assembly  14  is lifted then is released. Gravity causes the hammer assembly  14  and post retaining cap  16  to fall into contact with the post. However, it will be appreciated that alternative means for accelerating the hammer assembly  14  into contact with a post may be employed as a feature of the present invention. For example, a spring arrangement could be employed to accelerate the downstroke of the hammer assembly  14  rather than relying only on gravity. 
   At this first strike of the hammer assembly  14  and post retaining cap  16 , the post is fitted into the recess  32  and brought into contact with the studs or pins  31  therein. In the case of a wooden post, the studs or pins  31  arc driven a short distance into the wood of the post. Cable  30  is automatically released from contact with the peg or bolt  28  on hammer assembly  14 ; now the hammer assembly  14  is free to operate independently of the post retaining cap  16 . Accordingly, the post retaining cap  16  will retain the post in place during the remainder of the post driving operation. The hammer assembly  14  then is lifted and dropped as described supra until the post has been driven a desired distance into the ground. At that point, the cable  30  is manually reattached to the peg or bolt  28  on the hammer assembly  14 , and the hammer assembly  14  and the post retaining cap  16  are removed from contact with the post. The user then may position the post driver  10  at the site where the next post is to be driven with the process repeated. 
   It will be appreciated that the combination of the stabilizing means  48  and the dual hammer lifting means  26  allow the post driver  10  of the present invention to provide a more stable platform for operating the hammer assembly  14  in comparison to post drivers having a single rear-mounted hammer lifting means. In the preferred embodiment of this invention, a control panel (not shown) is provided to control lifting of the hammer assembly  14 , tilting of the post driver  10 , as well as raising and lowering of the stabilizing means  48  using hydraulic pressure. For example, whenever the post driver  10  of the present invention is mounted to a tractor, it will be appreciated that the hydraulic pressure required to operate the post driver  10  may be provided by the tractor&#39;s own hydraulic system. It also should be appreciated that the post retaining cap  16  obviates the need for a user to manually hold a post in place, thereby increasing user safety and convenience. 
   The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to enable one of ordinary skill in the art thereby to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention whenever interpreted in accordance with the breadth to which it is fairly, legally and equitably entitled.

Summary:
A post driver is powered by a pair of hydraulic cylinders, one on either side of the hammer to provide a balanced lifting force to the hammer. A post retaining cap is formed to provide a recess for the post. The orientation of the driver may be controlled about a horizontal axis so the post may be driven vertically without regard to the contour of the ground. The post driver is further provided with an extensible beam and foot which is extended to engage the ground and further support the driver.