Abstract:
A bushing for a lift axle assembly for a heavy vehicle. The lift axle assembly selectively lowers and raises supplemental wheels of the vehicle into and out of engagement with a support surface and forms a part of the support system for the vehicle when the supplemental wheels are lowered. The assembly comprising at least one bushing. The bushing includes a hard polymeric sleeve having a wall thickness of about 0.1″ to about 0.3″ (0.25-0.76 cm) and a hardness of greater than Shore 65D. An elastomeric polyurethane sleeve surrounds the hard polymeric sleeve and is fixed to it. A metal shaft is rotatably mounted in the hard polymeric sleeve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     TECHNICAL FIELD 
     This invention relates to elastomeric bushings, and in particular to an improved bushing for use in heavy off-road and over-the-road vehicles. The bushings of the present invention are particularly, but not exclusively, useful in replacing rubber and polyurethane bushings in a lift axle suspension system for such heavy vehicles. 
     BACKGROUND OF THE INVENTION 
     It is common to provide lift axles for certain heavy hauling vehicles such as dump trucks, refuse haulers, grain trucks, ready-mix cement transit mixers, block haulers, construction vehicles, and other special application vehicles. Such vehicles are adapted for both roadway and off-road operation. The lift axles form a supplemental suspension system for the vehicle and provide additional support for on-road use when the vehicles are heavily laden, to assist in spreading the vehicle weight and to minimize road and bridge damage. An example of such a vehicle is the dump truck shown diagrammatically in FIG.  1 . Lift axles are also used on heavy over-the-road tractors and trailers to meet government weight-per-axle limitations when the truck is heavily laden. 
     Lift axle suspension systems selectively lower and raise the supplemental wheels of a vehicle into and out of engagement with the road or other support surface and form a part of the support system for the vehicle when the supplemental wheels are lowered. The supplemental wheel assemblies are of various constructions, but they generally include an arm mounted to each side of the vehicle frame via one or more pivot connectors. An air bag (also called an air spring or air bellows) is commonly interposed vertically between the arm and the hanger bracket for selectively lowering the axle, wheel, and tire assembly into engagement with the road surface and providing the support for it. Another mechanism such as a second air bellows or a spring is provided for lifting the axle. The lift mechanism may also include a pivot connector. The pivot connectors are generally in the form of elastomeric bushings. 
     Numerous commercial lift axle suspension systems are available. They include, for example, ReycoGranning Suspensions Models LT80, LT120, T100AT, T300A, LT225, and T350AX, Granning Air Suspensions Models L120, L200, L225, BL100, T200AX, and LT120, Watson &amp; Chalin Models LWSL-1100-SR, SL-1800, WCAL-1300, and AL2200, and Spring Valley Models AB1000, AB1000CL, AB1000AL, AB2250AL, SS120, AB1300AL, AB800CL, and TR225AR. Current commercial air lift axle assemblies have a capacity ranging from about 8,000 pounds to about 25,000 pounds. The assemblies themselves are heavy, weighing on the order of nine hundred to two thousand pounds. Although details of their constructions vary, most have either a single arm and a single bushing on each side of the frame or a two-arm pariallelogram construction having four bushings on each side of the frame. 
     The patent literature also contains numerous examples of lift axle assemblies, such as Narahari, U.S. Pat. No. 3,960,389, Gibson, U.S. Pat. No. 4,000,913, Taylor, U.S. Pat. No. 4,293,145, Vandenberg, U.S. Pat. No. 4,300,787, VanDenberg et al., U.S. Pat. No. 4,573,704, Hermann, U.S. Pat. No. 5,018,593, VanDenberg et al., U.S. Pat. No. 5,505,481, and Hauri, U.S. Pat. No. 5,549,322. 
     It has been found that existing elastomeric bushings for lift axle assemblies wear out quickly and thus need frequent replacement. Rubber, non-rotating bushings have been used traditionally. They have too much “wind-up” (built-up tension as they rotate) and do not allow the suspension to cycle through its entire range of motion. The substitution for conventional rubber bushings of rotatable polyurethane bushings like those described in co-assigned patents to Sturmon, U.S. Pat. Nos. 4,840,395 and 5,988,614, has not been an entirely satisfactory solution. These bushings usually require more torque to rotate than the lift axle lift spring can provide to consistently raise the axle. 
     All of the patents mentioned herein are incorporated by reference. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, generally stated, a lift axle assembly is provided for a heavy vehicle for selectively lowering and raising supplemental wheels of the vehicle into and out, of engagement with a support surface, the assembly forming a part of the support system for the vehicle when the supplemental wheels are lowered, the assembly comprising at least one bushing, the bushing comprising a hard sleeve having a hardness of greater than Shore 65D, an elastomeric sleeve surrounding the hard sleeve, and a shaft rotatably mounted in the low friction sleeve. The shaft is typically in the form of a bolt, a solid pin, or a hollow tube. The low friction sleeve has a coefficient of friction lower than the elastomeric sleeve. In a preferred embodiment, the low friction sleeve is a self-supporting tube. In a particularly preferred embodiment the low friction sleeve is made of ultra-high molecular weight polyethylene. 
     In accordance with another aspect of the inventions, a heavy vehicle having a frame is provided with a moveable arm, and a pivot connecting the moveable arm to the frame, the pivot comprising a bushing having a first part operatively attached to the frame and a second part attached to the arm, the second part being coaxial with the first part, wherein one of the first and second parts comprises a hard polymeric sleeve and an elastomeric sleeve surrounding the hard sleeve, and the other of the first and second parts comprises a metal shaft rotatably mounted in the hard sleeve. The sleeve is preferably self-supporting and made of a hard polymer. 
     Typically, but not necessarily, the shaft is rigidly connected to the vehicle body and the sleeves are mounted in the rotatable arm. The arm has a substantial angular travel from the assembly&#39;s retracted position to its lowered position. In most designs it rotates at least ten degrees but less than fifty degrees, usually on the order of fifteen to thirty degrees. It has been found that the use of the bushing of the present invention greatly reduces the effort required to rotate the lift axle from its road-engaging position to its retracted position. It has been found that as little as ten foot pounds of torque, and typically no more than fifty foot pounds of torque, is required to turn the sleeve with respect to the shaft, even when the bushing is clamped tightly or after long continuous use in the road-engaging position. The bushing also increases bushing life by reducing torque-induced stress on the elastomeric component. 
     Preferably, the bushing is substantially free of lubricant. 
     Preferably, and in accordance with one aspect of the invention, the elastomeric sleeve is made of polyurethane. As used herein, except as otherwise indicated, the term “polyurethane” includes polyurethanes, polyureas, and blends thereof. The elastomer preferably has a Shore (durometer) hardness in the range of 65A to 95A. The elastomeric sleeve, for many applications, typically has a hardness of about Shore 85A to 95A. A particularly preferred material has a tensile strength of at least 4,000 psi, a tear (die C) strength of at least 525 pli, a 100% modulus of over 2,500, and an ultimate elongation of at least 100%. An ultimate elongation in the range of 100% to 300% is preferred. Other elastomeric materials may also be used, but are not preferred. 
     The hard sleeve is made of a material having a much higher Shore hardness, in the range of about 65D or higher. It also has a lower coefficient of friction than the elastomeric sleeve. The preferred material is a polyolefin, such as a polyethylene or a polypropylene; particularly preferred is an ultra-high molecular weight (UHMW) polyethylene having a weight average molecular weight of 3.1 million or greater. Although more expensive, a self-lubricating acetal or a polytetrafluoroethylene sleeve is believed also to be operable in the invention. Although less preferred, other polymers such as hard polyurethane (65D or greater), nylon and phenolic (Garolite) resins may also be workable. For some extreme applications, the hard sleeve may be a metal, such as bronze, although this modification is generally not preferred. 
     The hard polymeric sleeve is preferably fixed to the elastomeric sleeve. In the preferred embodiments, the elastomeric sleeve is heated sufficiently to adhere to the hard plastic sleeve. The inner sleeve is preferably about 0.1″ to 0.5″ (0.25-1.27 cm) thick. The outer elastomeric sleeve may be 0.2″ to 2.5″ (0.5-6.35 cm) thick. 
     The bushing may optionally include an outer metal shell surrounding the elastomeric sleeve. The elastomeric sleeve may be bonded to the shell, but need not be. 
     Preferably, the arm is part of a lift-axle assembly carrying a wheel. The lift axle assembly may be either a pusher or a tag configuration. In this environment, the bushing of the present invention has been found to provide remarkable decreases in the forces required to rotate the bushing or shaft and to provide exceptionally long life, while providing vibration dampening, self-alignment, and axial stress relief at least comparable to that of known bushings. 
     The bushing of the present invention may also be used in other heavy vehicle applications, such as the assemblies used for raising and lowering cement mixer mixing thimbles, and heavy vehicle steering axles. The bushing of the present invention, however, is not believed to be suitable for all uses in heavy vehicles. It will not tolerate large side loads without support, and its direct loading capacity is limited. Therefore, although it is highly suited to uses in which maximum loads on the bushing are on the order of ten thousand pounds, it is not believed to be highly suitable for use in such vehicles for torque rod bushings or center or end bushings, because they may experience loads up to eighty thousand pounds in off-road use. 
     Polyurethane has been found to be a particularly advantageous elastomer. The hard sleeve can be formed by cutting sections from a standard rigid polymer tube and forcing the tube into a precast polyurethane sleeve. Preferably, the hard sleeve is a rigid tube positioned in a mold, and the elastomeric is poured and cured around it. The need for machining either the elastomeric sleeve or the hard sleeve is eliminated, although the hard sleeve may be machined if desired. 
     Other aspects of the invention will be apparent to those skilled in the art in light of the following description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a view in side elevation of a heavy truck having lift axles in which the bushing of the present invention may be used. 
     FIG. 2 is a view in side elevation of a lift axle assembly using four bushings of the present invention. 
     FIG. 3 is a view in side elevation of a lift axle assembly using one bushing of the present invention. 
     FIG. 4 is a view in perspective of one embodiment of bushing of the present invention. 
     FIG. 5 is a view in end elevation of the bushing of FIG.  4 . 
     FIG. 6 is a cross-sectional view taken along the line  6 — 6  of FIG.  5 . 
     FIG. 7 is a view in side elevation of the bushing of FIGS. 4-6. 
     FIG. 8 is a cross-sectional view taken along the line  8 — 8  of FIG.  7 . 
     FIG. 9 is a view in end elevation, corresponding to FIG. 5, of another embodiment of bushing of the present invention. 
     FIG. 10 is a cross-sectional view taken along the line  10 — 10  of FIG.  9 . 
     FIG. 11 is a view in side elevation of the bushing of FIGS. 9-10. 
     FIG. 12 is a cross-sectional view taken along the line  12 — 12  of FIG.  11 . 
     Corresponding reference numerals will be used throughout the several figures of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what I presently believe is the best mode of carrying out the invention. 
     Referring now to the drawings, and in particular to FIG. 1, reference numeral  1  indicates a heavy vehicle of a type for which the bushing of the present is particularly adapted, illustratively a dump truck. The vehicle  1  includes a frame  3  and wheels  5 , attached to the frame through a standard suspension system. Also attached to the frame  3  are two lift axle assemblies  7   a  and  7   b . Each lift axle assembly  7  includes at least one wheel  9 , including tire  11 , on each lateral side of the frame  3 . The lift assemblies  7   a  and  7   b  also include a lift mechanism, not shown in FIG. 1, each of which includes, on each lateral side of the frame  3 , at least one arm, an air bag for lowering the arm and supporting the arms (and axle) when the arms are lowered, a spring for raising the arm and absorbing shock when the arms are lowered, and at least one bushing in accordance with the present invention attached to each arm. 
     FIGS. 2 and 3 illustrate typical lift axle lift mechanisms in which the bushing of the present invention may be used. 
     The lift mechanism  120  of FIG. 2 is a parallelogram mechanism. It includes a first fixed depending bracket  121  attached to one of the longitudinal rails  103  of vehicle frame  3 . A first arm  123  and a second arm  125  are attached to the first bracket  121  by pivots  127  and  129 , respectively. The free ends of the arms  123  and  125  are connected to a moveable bracket  131  by pivots  133  and  135 , respectively. The moveable bracket  131  carries one end of the axle for the wheel  9 . A first, vertically expandable, air bag  137  is mounted between the moveable bracket  131  and a second fixed bracket  137  mounted to the frame rail  103 . A second, horizontally expandable, air bag  141  is mounted between the first fixed bracket  121  and the first arm  123 . In this arrangement, expanding the first air bag and relieving pressure in the second air bag lowers the wheel  9  into engagement with the road or other support surface. In this condition, the first air bag  137  acts as a support for the wheel  9  and its axle, while the second air bag  141  acts as a shock absorber and controls the rate of rebound of the wheel  9 . Expanding the second air bag  141  and letting air out of the first air bag  137  raises the wheel  9  out of engagement with the support surface. The total travel of arms  123  and  125  about their pivots  127  and  129  between the lowered and raised positions of the wheels is about twelve degrees. 
     In accordance with the present invention, the pivots  127 ,  129 ,  133 , and  135  are all composite bushings  301  or  401  of the present invention. 
     A second common form of lift mechanism  220  is shown in FIG.  3 . In this embodiment, a single bracket  221  mounted to the longitudinal rail  103  includes a first depending leg  223  and a second depending leg  225 . The first leg  223  carries a pivot  227  at its lower end. The pivot  227  connects a moveable arm  229  to the first leg  223 . The arm  229  carries at its free end an axle  231  for wheel  9 . An air bag  233  between the arm  229  and the second leg  225  is inflated to lower the arm  229 . An air bag or leaf spring (not shown), carried by the first leg  223 , raises the arm  229  when the air bag  233  is not inflated. The air bag  233  and the spring perform generally the same functions as the airbags  137  and  141 , respectively, of the lift mechanism  120  of FIG.  2 . 
     In accordance with the present invention, the pivot  227  is a composite bushing  301  or  401  of the present invention. 
     The foregoing lift mechanisms are well known in the art and are merely illustrative of the types of lift mechanisms of the present invention. 
     In accordance with the preferred embodiments of the present invention, the pivot or pivots of the lift mechanism are formed as shown in FIGS. 4-6 or in FIGS. 7 and 8. 
     A first preferred embodiment of bushing  301  of the present invention is shown in FIGS. 4-6. The bushing  301  is a jam-style bushing and is formed of an outer polyurethane sleeve  313 , an-inner polyethylene sleeve  315 , and a steel shaft  317 . The outer sleeve  313  is made of a polyurethane elastomer. The preferred elastomer is a modified diphenylmethane diisocyanate (mdi) terminated polyether prepolymer-based polyurethane having a durometer of 85A to 95A. A particularly preferred material has a tensile strength of at least 4,000 psi (ASTM D-412), a die C tear strength of at least 525 pli (ASTM D-624), a 100% modulus of over 700 psi, a 300% modulus of over 1500 psi, and an ultimate elongation in the range of 510% to 380% (ASTM412), a coefficient of friction of 0.3 to 0.5 (ASTM D-1894), a bashore resilience of 50 to 40% (ASTM 2632), a 5% compression modulus of 210 to 505 psi and a 25% compression modulus of 955 to 2370 psi (ASTM D-575), and an abrasion resistance (NBS″) of 145 to 300 (ASTM D-1630), a melting point of 400 to 450° F., a continuous service temperature of 140 to 160° F., and a specific gravity of 1.09 to 1.13. 
     One embodiment of outer sleeve  313  is illustratively about 8.5 cm in outer diameter, about 6 cm in inner diameter, and about 12 cm long. 
     The inner sleeve  315  is made of ultrahigh molecular weight polyethylene. The preferred sleeve has a molecular weight of about 4,200,000, a density at 73° F. of 0.941 g/cc. (ASTM D792), a Rockwell “R” hardness of 64 R (ASTM D785), a Durometer “D” hardness of 67 D (ASTM D2240), an ultimate tensile strength at 2″/min. of 6400 psi (ASTM D638), a tensile yield strength at 2″/min. of 3400 psi (ASTM D638), a coefficient of linear thermal expansion of 9.1×.00001 in./in./F° (ASTM D696), and a relative coefficient of friction of 0.14. 
     The inner sleeve of this embodiment is illustratively about 6 cm in outer diameter, about 5 cm in inner diameter, and about 12 cm long. The inner and outer sleeves are adhered to each other, but are presently not bonded. 
     The steel shaft  317  is typically made of 1020 to 1026 steel tube meeting ASTM A512, A513, or A519. It illustratively has an outer diameter of 5 cm, an inner diameter of 2.6 cm, and a length of 13.5 cm. The shaft  317  fits snugly but easily rotatably in the inner sleeve  315 , such that a force of about ten pounds is required to turn it. 
     The bushing  301  is manufactured as follows: A pin having the same outside diameter as the shaft  317  is rinsed in release agent. The outside of the inner sleeve  315  is optionally coated with a bonding agent. The cylindrical inside of a mold is coated with release agent. The pin and inner sleeve  315 , and outer sleeve  313  are placed in a mold. Liquid uncured polyurethane is poured into the mold and allowed to pre-cure. The bushing is then removed from the mold, the mold pin is removed from the center of the bushing, and the bushing is post cured for 12-16 hours. The pour side of the polyurethane is trimmed to the correct dimensions. 
     It will be understood by those skilled in the art that the shaft  317  will typically be held between legs of a mounting bracket on a heavy vehicle (illustratively corresponding to the brackets  121 ,  131 , and  227 ) by means of a bolt passing through the hollow shaft  317 . The bolt locks the shaft to the bracket, allowing the bushing, including sleeves  313  and  315 , to rotate. The outer sleeve  313  is press fitted into an eye in a moveable beam (illustratively corresponding to arms  123  and  125  or moveable arm  229 ). It will also be understood by those skilled in the art that the dimensions of the bushing  301  will vary widely, depending on the application. 
     An alternative construction of bushing  401  in accordance with the present invention is shown in FIGS. 7 and 8. As shown in these drawings, the bushing  401  includes an outer steel shell  411 , an outer polyurethane sleeve  413 , an inner polyethylene sleeve  415 , and a steel shaft  417 . The steel shell is typically made of 1020 to 1026 steel tube meeting ASTM A512, A513, or A519. It illustratively has an outer diameter of 3.125″, an inner diameter of 2.875″, and a length of 4.125″. The remainder of the bushing  401  corresponds to the bushing  301  except in its dimensions. The outer sleeve  413  is made of polyurethane. It is illustratively about 2.875″ in outer diameter, about 2.0″ in inner diameter, and about 4.125″ long. The inner sleeve  415  is made of ultrahigh molecular weight polyethylene. The inner sleeve is illustratively about 2.0″ in outer diameter, about 1.74″ in inner diameter, and about 4.125″ long. The inner and outer sleeves are presently adhered to each other, but are not bonded. 
     The steel shaft  417  illustratively has an outer diameter of 1.735″, an inner diameter of 1.006″, and a length of 4.75″. The shaft  417  fits snugly but easily rotatably in the inner sleeve  415 , such that a force of about ten pounds is required to turn it. 
     The bushing  401  is manufactured as follows: A pin having the same outside diameter as the shaft  417  is rinsed in release agent. The outside of the inner sleeve  415  is optionally coated with a bonding agent. The inside of the outer shell  411  is coated with a bonding agent. The pin, inner sleeve  415 , and outer shell  411  are placed in a mold. Liquid uncured polyurethane is poured into the mold between the inner sleeve  415  and outer shell  411  and allowed to pre-cure. The bushing is then removed from the mold, the mold pin is removed from the center of the bushing, and the bushing is post cured for 12-16 hours. The pour side of the polyurethane is trimmed to the correct dimensions. 
     Tests on a bushing having an outer shell and made in accordance with Sturmon, U.S. Pat. No. 4,840,395, and on bushings  301  and  401  have been conducted as follows, with the following results: 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Torque 
               
             
          
           
               
                   
                 U.S. Pat. No. 
                   
                   
               
               
                   
                 4,840,395 
                 Bushing 301 
                 Bushing 401 
               
               
                   
                   
               
             
          
           
               
                 Free state 
                  75 to 245 in-lbs 
                 &lt;5 in-lbs 
                 &lt;5 in-lbs 
               
               
                 Pressed 
                   
                  8 to 14 in-lbs 
                  5 to 10 in-lbs 
               
               
                 into beam 
               
               
                 Pressed into 
                  95 to 290 in-lbs 
                 10 to 18 in-lbs 
                  8 to 12 in-lbs 
               
               
                 beam under 
               
               
                 2000 lbs load 
               
               
                 Pressed into 
                 105 to 360 in-lbs 
                 13 to 24 in-lbs 
                 10 to 18 in-lbs 
               
               
                 beam under 
               
               
                 5000 lbs load 
               
               
                   
               
             
          
         
       
     
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. As previously noted, the dimensions, materials, and proportions of the parts will be varied to suit the application. Further, the bushings of the invention may be used in other heavy vehicle uses, particularly in those uses which require free rotation of at least about five degrees and which do not place large side or conical loads on the bushing, as for example in the lifting mechanisms of cement mixers. The preferred bushings may be used in other applications. The bushings  301  could be formed by cutting sections of the inner sleeve from a length of tubing and forcing it into a preformed elastomeric outer sleeve. Other variations, within the scope of the following claims, will be apparent to those skilled in the art in light of the foregoing disclosure.