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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
   The present invention relates to rotating platform members and mechanisms for limiting the rotation of such members. In greater particularity, the present invention relates to rotatable platforms mounted on carriages such as a rail mounted chassis or frame and having a working element mounted to the platform mechanism. In still greater particularity, the present invention relates to a mobile track crane or similar vehicle wherein the crane is mounted on a rotatable upper assembly supported on a track engaging carriage. Specifically, the present invention is related to a stop mechanism to limit the rotational movement of a track crane to prevent the crane from interfering with equipment on an adjacent track. 
   BACKGROUND OF THE INVENTION 
   Proper maintenance of railroad track requires the periodic removal and replacement of worn or degraded ties from beneath the rails. Modern railroad maintenance utilizes tie cranes to move the ties to and from the track bed. A stand alone tie crane is exemplified by the Model 12-12 Tie Crane available from the Kershaw Division of Progress Rail, owner of the present application. Such tie cranes are self propelled and include a lower frame and an upper platform mounted to the lower frame on a large diameter slewing ring which allows the upper deck to rotate a full 360 degrees. The crane boom is mounted to the upper platform and moves with the platform. In environments where two tracks are adjacent, care must be taken that the crane boom does not extend beyond the clearance guideline of the adjacent track. Known technology requires the operator to dismount from the cab to place fixed mechanical stops in position to arrest the movement of the platform and boom towards the adjacent track. The stops provide a sudden stop that is jarring to the operator and the equipment with associated wear on both. 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention to insure that the tie crane boom does not foul the clearance guidelines of an adjacent track while the operator is working. 
   Yet another object of the invention is to enable the operator to set the stops to arrest rotation of the crane boom from within the operator cabin. 
   Still another object of the invention is to reduce the shock and wear on the operator and the equipment during engagement with the stops. 
   These and other objects, which will become apparent from a reading of the specification, are accomplished using an operator actuated retractable engagement bar mounted to the rotatable platform. When extended the bar will engage a sliding stop mounted for movement between two resilient members mounted to the support carriage. As the platform turns, the bar moves the sliding stop into engagement with one of the resilient members which arrests movement of the platform in angular direction it had been moving. A second sliding stop and pair of resilient members are mounted on the opposite end of the carriage such that rotation of the platform on the work side of the carriage is unobstructed but also such that the platform is prevented from rotating in either angular direction a distance sufficient to foul the adjacent track. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a pair of tie cranes on a track adjacent a second track showing the range of motion provided by the present invention; 
       FIG. 2  is a perspective view of an embodiment of the present invention mounted to the platform and frame which are only partially shown. 
       FIG. 3 . is a plan view of the components of an embodiment of the present invention in cooperative positioning 
       FIG. 4  is a perspective view of the components of an embodiment of the present invention in cooperative position. 
       FIG. 5  is a side elevation of one embodiment of the present invention 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , it may be seen that adjacent railroad tracks A and B are depicted. In practice the centerline of such adjacent tracks would be as little as fourteen feet apart, therefore it is clear that very little clearance exists on such tracks when one track is occupied by a work crew and the other is in service. In  FIG. 1 , for purposes of illustration two tie cranes  11  and  12  are shown working on the side of Track A opposite Track B. As may be seen in the illustration, the cranes may be seen to have a rotatable platform  13  carrying a crane boom  14  with a grapple  15  attached to the free end thereof. A support carriage  17  is mounted on the rails R and carries the platform  13  along the rails, the platform is shown partially in phantom to show carriage  17  and the slewing ring  18  which allows the platform to rotate. The drive mechanism and rotational mechanism is conventional and will not be discussed. However, inside the operator cab  19  is a control lever  20  for the present invention. 
   Referring to  FIG. 2 . The underside of platform  13  and a portion of the carriage  17  is shown in relation to the present invention. A slidable stop  21  is supported on a rod  22  carried by mounting weldments  23  and  24  which are bolted or otherwise affixed to the front plate  25  of the carriage. Identical structure is affixed to the rear plate  26  of the carriage but not visible in the drawing. A pair of elastomeric members  27  and  28 , illustrated as coaxially mounted compression springs, are mounted at each end of rod  22 . The elastomeric members could be any other suitable spring, elastomeric solid block, or bladder that would absorb energy, mounted at the ends of the rod. Stop  21  is free to slide along rod  22  and includes a cylindrical sleeve  31  mounted coaxially about rod  22  and an upright post  32  extending orthogonally from the rod. Guide plates  33  and  34  insure that post  32  remains upright as the stop slides along rod  22 . 
   As seen in  FIG. 2 to 5 , affixed to the bottom of platform  13 , approximately along the centerline thereof and near the arcuate rear portion thereof is a bracket  36 , which slidably receives a stop bar  41 . Stop bar  41  is linked to an actuator  42  mounted to the underside of platform  13  and controlled from the operators cabin by lever  20 . Actuator  42  is illustrated as a fluidic cylinder, however, it may be a pneumatic cylinder, hydraulic cylinder, worm gear, or any other mechanism capable of moving stop bar  41  linearly within bracket  36  such that the bar is extendable and retractable relative to the end of the bracket. Although a hydraulic control circuit  43  is generally depicted between actuator  42  and lever  19 , any appropriate control circuit could be used. 
   With the stop bar  41  retracted, the platform  13  is capable of rotating 360 degrees on slewing ring  18 . When the tie crane needs to operate in a mode that will prevent the crane from interfering with an adjacent track, the operator moves the crane to the side of the track he will work on, namely opposite the adjacent track, and extends stop bar  41 . If the platform rotates toward the other track, stop bar  41  engages post  32  and urges sliding stop  21  along rod  22  until stop  21  engages the elastomeric member  27  or  28  closest to the adjacent track. The elastomeric member aborbs the energy of the moving platform and stops the platform near the end of rod  22 . As shown in  FIG. 1 , the extended stop bar  41  will engage the sliding stop mounted on either end of the carriage to arrest movement toward the adjacent track regardless of which direction of rotation the crane boom takes. 
   While the present invention is shown in a single embodiment, it is to be understood that various components may be implemented in various ways without departing from the scope or spirit of the invention which is set forth in the appended claims.

Summary:
An improvement for a railroad tie crane utilizes a movable stop mounted on an elongated member between a pair of cushioned terminals to reduce impact stress on equipment and personnel in situations where the tie crane must be prevented from fouling an adjacent track. The movable stop is selectively engaged by a retractable bar mounted on the rotatable platform carrying the crane boom.