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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 generally to shock-absorbing bumpers. More particularly, a relates to bumpers for booms on backhoes. 
     BACKGROUND OF THE INVENTION 
     Bumpers made of stacks of rubber plates are common in the art. They are used in a variety of places such as loading docks and pads for construction vehicle outriggers. 
     The stacks are usually quite large, consisting of from 8 rubber plates to as many as 50 or 100 rubber plates. In loading docks, for example, 50 or more rubber plates are threaded onto steel rods that extend several feet across the back of the loading dock. To make sure the plates retain their shape, they are often compressed between two steel plates fixed to the loading dock that are disposed on either end of the rubber plate stack. 
     The plates in each stack are typically fused together to provide longer life and better wear. To further enhance wear, the plates are typically compressed tightly between the angle iron or plates at each end. This prevents relative movement between adjacent plates and makes them collectively quite rigid. 
     Unfortunately, this rigidity is not desirable in many applications. For example, backhoe bumpers used to stop backhoe booms must be flexible in order to conform to the surface contours of a backhoe boom. They must also be flexible to absorb shocks when the backhoe boom is stowed and bangs against the bumper. 
     It is an object of this invention to provide a bumper for a backhoe boom that provides this flexibility and shock absorption. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a bumper for a backhoe boom comprises a mounting plate, at least two rods fixed to the plate, and two stacks of rubber plates mounted on free ends of the rods. 
     Nuts may be fixed to free ends of the rods to hold the stacks in place. The rods are parallel to one another and parallel to the surface of the backhoe on which the bumper is mounted. The plates in each stack of plates are not bonded or fused to each other. The two stacks may be mounted on opposite sides of the mounting plate. The rods may be cantilevered away from the mounting plate. The rods may be threaded on both ends. The rods preferably extend the same distance away from the mounting plate and are unsupported at their outer ends. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of the bumper fixed to a swing tower of a backhoe. 
         FIG. 2  is an end view of the bumper of  FIG. 1 . 
         FIG. 3  is a side view of the bumper of  FIGS. 1-2 . 
         FIG. 4  is a plan view of the plate support for the bumper of the foregoing Figures. 
         FIG. 5  is an end view of the plate support of  FIG. 4 . 
         FIG. 6  is a plan view of a rubber plate for the bumper of the foregoing Figures. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention is susceptible of being made in any of several different forms, the drawings show a preferred form of the invention. One should understand, however, that this is just one of many ways the invention can be made. Nor should any feature of the illustrated embodiment be considered a part of the invention, unless that feature is mentioned in the claims. In the drawings, like reference numerals refer to like parts throughout the several views. 
       FIGS. 1-6  show a bumper  100  for the boom of a backhoe. Bumper  100  includes a base plate  102 ; two threaded rods  104 ,  106  fixed to the base plate; a first stack  108  of rubber plates  109  fixed to one end of threaded rods  104 ,  106 ; a second stack  110  of rubber plates  109  fixed to the other end of threaded rods  104 ,  106 ; two retaining plates  112  disposed on both ends of threaded rods  104 ,  106 ; and a means for securing the rubber plates to the rods, shown here as four cap nuts  114  threaded onto the free ends of each of threaded rods  104 ,  106 . 
     Base plate  102  is generally planar, rectangular, and elongated vertically. It defines a mounting hole  116  passing through the middle of the plate from one side of the plate to the opposite side of the plate. Hole  116  is configured to receive a threaded fastener  118  that extends through plate  102  and into boom tower  120  to secure bumper  100  to the boom tower. The bottom surface of plate  102  mounts to tower  120  and the top surface of plate  102  supports rods  104 ,  106 . 
     Threaded rods  104 ,  106  are fixed to the surface of plate  102  generally parallel to the surface of boom tower  120 , parallel to each other, and parallel to the top and bottom surfaces of plate  102 . They have the same length. Rods  104 ,  106  are preferably welded to the surface of plate  102  by weldments  105 . Each of rods  104 ,  106  has two free ends. Both free ends of both rods are threaded to threadedly engage cap nuts  114 . Rods  104 ,  106  extend past the outer edges of plate  102  and are held by plate  102  in a spaced apart and parallel relation to the surface of boom tower  120 . All four free ends of rods  104 ,  106  extend the same distance away from plate  102 , having the same unsupported free length. Rods  104 ,  106  are spaced apart from one another a constant distance over their entire lengths. Rods  104 ,  106  are oriented parallel to the surface of boom tower  120  and are spaced apart from it. 
     The free ends  126 ,  128  of rods  104 ,  106  extending from a first side of plate  102  support a stack of rubber plates  108 . Each rubber plate  109  of plates  108  is generally rectangular and has two holes  122 ,  124  through which free ends  126 ,  128  extend. Free ends  126 ,  128  protrude through plates  108  sufficient to support a retaining plate  112  that is mounted on both rod ends  126 ,  128 , and a cap nut  114  threaded to each free end  126 ,  128  to retain the retaining plate  112  and stack  108  of rubber plates  109 . 
     Retaining plates  112  are mounted across the free end face of the last rubber plate. Plates  112  prevent the last rubber plate (and hence the other plates in the stack) from buckling, and distribute the pressure applied by the nuts across the entire face of the stack of rubber plates. 
     Rods  104 ,  106  also have second free ends  130 ,  132  extending from a second side of plate  102  opposite the first side of plate  102 . Free ends  130 ,  132  support another stack of rubber plates  110 . Each rubber plate  109  of plates  110  is generally rectangular and has two holes  122 ,  124  through which free ends  130 ,  132  extend. Free ends  130 ,  132  protrude through plates  110  sufficient to support a retaining plate  112  that is mounted to both rod ends  130 , 132 , and a cap nut  114  that is threaded to each free end  130 ,  132  to retain the retaining plate  112  and stack  110  of rubber plates  109 . 
     Rubber plates  109  are shown in  FIG. 6  in plan view. The plates themselves can be made of any elastomeric material, including natural and artificial elastomers, such as NBR and urethane. For reasons of economy and durability, however, plates formed from the salvaged carcasses of used truck or automobile tires are particularly suitable. The sidewalls of these carcasses are particularly well-suited and have thicknesses of between 0.25 and 0.50 inches. One feature that makes them suitable is their internal reinforcement. When the tires are manufactured, fabric or steel cords are embedded in the sidewalls and treads to give the tire additional strength. These embedded cords act as a fiber reinforcing matrix, which supports the rubber, enhancing wear resistance and tear strength. 
     Each of stacks  108 ,  110  comprise between 3 and 6 rubber plates  109 , and More preferably 4 rubber plates  109  (as illustrated in the Figures). The plates in each stack are preferably not fused together. Since the plates are not fused together, they can bend and flex more easily when the backhoe boom is seated against the free outer surfaces  134  of stacks  108 ,  110  (see  FIG. 2 ). This helps the plates accommodate small misalignments as well as irregularities in the surface of the backhoe boom such as draft angle and parting line. Additional flexibility can be provided by loosening the nuts. 
     One will appreciate that the present disclosure is intended as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims. 
     For example, the nuts may be replaced with spring pins; roll pins; keys; swaged, crimped, or crushed rod ends; swaged, crimped, or crushed fasteners coupled to the rod ends; weldments; and brazing material. As another example, the rods may be replaced with square rod, sheet, bar, and ribbon stock. As a further example, the round holes in the rubber plates may be replaced with square holes, oval holes and slots. As yet another example, the rods need not be parallel to one another, or parallel to the mounting plate. As a further example, the rods need not extend from both sides of the mounting plate, they need only extend from one side. As yet another example, there need be only one rod extending through each stack of rubber plates. As a further example, the stack of plates can be replaced with one or more urethane-based shock absorbers.

Summary:
A bumper for a backhoe boom includes a plate for mounting to the backhoe, two rods threaded at both ends that are cantilevered away from the mounting plate, and two stacks of rubber plates mounted on the threaded rods on either side of the mounting plate. The two stacks of rubber plates are secured to the threaded rods with nuts. The nuts abut two retaining plates that press against the stacks of rubber plates.