Patent Publication Number: US-8967052-B2

Title: Railcar constant contact side bearing assembly

Description:
RELATED APPLICATION 
     This application is a Continuation-In-Part of copending and coassigned U.S. patent application Ser. No. 13/507,145; filed Jun. 7, 2012. 
    
    
     FIELD OF THE INVENTION DISCLOSURE 
     The present invention disclosure generally relates to railroad cars and, more specifically, to a constant contact side bearing assembly for a railroad car. 
     BACKGROUND 
     A typical railroad freight car includes a car body supported on a pair of wheeled trucks which are confined to roll on rails or tracks. Each truck includes a bolster extending essentially transversely of the car body longitudinal centerline. In the preponderance of freight cars, a pivotal connection is established between the bolster and railcar body by center bearing plates and bowls transversely centered on the car body underframe and the truck bolster. Accordingly, the truck is permitted to pivot on the center bearing plates under the car body. As the railcar moves between locations, the car body also tends to adversely roll from side to side. 
     Attempts have been made to control the adverse roll of the railcar body through use of side bearings positioned on the truck bolster outwardly of the center bearing plates. A “gap style” side bearing has been known to be used on slower moving tank/hopper railcars. Conventional “gap style” side bearings include a metal, i.e. steel, block or pad accommodated within an elongated open top pocket or recess defined on the truck bolster. An elongated and upstanding housing or cage, integrally formed with or secured, as by welding or the like, to an upper surface on the truck bolster defines the open top recess and inhibits sliding movement of the metal block relative to the bolster. As is known, a gap or vertical space is usually present between the upper surface of the “gap style” side bearing and the underside of the railcar body. 
     Other conventional “gap style” side bearings have included roller bearings carried for rolling movements within the elongated housing or carrier mounted on the upper surface of the railcar bolster. The roller extends above an uppermost extent of the housing or carrier and engages with an underside of the railcar body. Such side bearings are able to support the railcar body with respect to the bolster while at the same time permitting the bolster, and therefore the truck, freedom to rotate with respect to the car body as is necessary to accommodate normal truck movements along both straight and curved track. 
     Under certain dynamic conditions, coupled with lateral track irregularities, the railcar truck also tends to adversely oscillate or “hunt” in a yaw-like manner beneath the car body. The coned wheels of each truck travel a sinuous path along a tangent or straight track as they seek a centered position under the steering influence of the wheel conicity. As a result of such cyclic yawing, “hunting” can occur as the yawing becomes unstable due to lateral resonance developed between the car body and truck. Excessive “hunting” can result in premature wear of the wheeled truck components including the wheels, bolsters, and related equipment. Hunting can also furthermore cause damage to the lading being transported in the car body. 
     Track speeds of rail stock, including tank/hopper cars, continue to increase. Increased rail speeds translate into corresponding increases in the amount of hunting movements of the wheeled trucks. “Gap style” or those side bearings including roller bearings simply cannot and do not limit hunting movements of the wheeled trucks. As such, the truck components including the wheels, bolsters, and related equipment tend to experience premature wear. 
     The art has also contemplated constant contact side bearings for railcars. Constant contact railcar side bearings not only support a railcar body with respect to the bolster during relative rotational movements therebetween but additionally serve to dissipate energy through frictional engagement between the underside of the railcar body and a bearing element thereby limiting destructive truck hunting movements. Constant contact side bearings typically include a housing assembly including a base or housing and a cap. The housing usually has a cup-like configuration and includes at least two apertured flanges, extending in opposed radial directions relative to each other, permitting the housing to be fastened to the bolster. In one form, the cap is biased from the housing and includes an upper surface for contacting and rubbing against a car body underside. The cap must be free to vertically move relative to the side bearing housing. 
     Such constant contact side bearings furthermore include a spring. The purpose of such spring is to absorb, dissipate, and return energy imparted thereto during a work cycle of the side bearing assembly and resiliently position the upper surface of the cap, under a preload force, into frictional contact with the car body underframe. The spring for such side bearings can comprise either spring loaded steel elements or elastomeric blocks or a combination of both operably positioned within a cavity defined by the side bearing housing and the cap. An elastomeric block which has been found particularly beneficial is marketed and sold by the Assignee of the present invention under the tradename “TecsPak.” As will be appreciated, however, such an elastomeric block, by itself, lacks longitudinal stiffness and, thus, requires surrounding housing structure to provide added support and stiffness thereto. 
     There are several challenges presented in connection with the design of a constant contact side bearing assembly. First, and during the course of operation, clearance between sidewalls on the housing and cap of a constant contact side bearing housing assembly tend to become enlarged due to abrasion and wear. Such wear is a critical detractor to side bearing assembly performance. That is, any gap or space between the sidewalls on the housing and cap of the side bearing assembly adversely permits longitudinal or horizontal shifting movements of the cap relative to the housing thereby reducing the energy absorption capability for the side bearing assembly—a critical operating criteria for the side bearing assembly. Of course, if the gap or space between the housing and cap of the side bearing assembly reaches a critical limit, the side bearing assembly is no longer useful and can be condemned. 
     During operation of the railcar side bearing assembly, and while controlling the clearance or gap between the cap and housing of the side bearing assembly so as to limit horizontal shifting movements of the cap relative to the housing remains advantageous, the cap must remain able to vertically reciprocate relative to the housing. As will be appreciated, if the cap cannot vertically reciprocate during operation of the side bearing assembly, the primary purpose and function of the constant contact side bearing assembly will be lost. 
     Designing a side bearing assembly having a multipiece cap for controlling the gap or space between the cap and wall structure on the housing and which is biased into contact with an underside of the railcar body is also known in the art. Although beneficial in limiting the clearance or gap between the cap and housing, designing a constant contact side bearing assembly with a mulitpiece cap introduces other design problems and challenges. For example, the multipiece cap members tend to vertically separate as the railcar rolls from side-to-side. That is, after the car body rolls in a first direction, the cap members of one side bearing assembly are allowed to vertically separate relative to each other. When the railcar body again rolls in an opposite direction, the vertically separated cap members of the one side bearing assembly are vertically crushed against each other by the underside of the car body. Especially when the cap members are formed from a non-metal materials, this continuous rolling action of the car body can have an adverse affect on the cap members. Of course, any cracking or sticking of the cap members relative to the housing can and often does result in condemnation of the side bearing assembly. The ability to limit vertical separation of the cap members relative to each other, however, is complicated when considering the requirement such cap members must also maintain their ability to horizontal shift or slide relative to each other so as to limit or reduce the clearance between the cap members and outstanding wall structure on the side bearing assembly housing. 
     Another design challenge involved with those constant contact side bearings using an elastomeric spring relates to the buildup of heat in proximity to the elastomeric spring. During operation of the railcar, frictional contact between the railcar body and the side bearing assembly results in the development of heat buildup. Unless such heat buildup can be controlled, the elastomeric spring will tend to soften and deform, thus, adversely affecting the operable performance of the constant contact side bearing assembly. 
     The frictional sliding relationship between the side bearing assembly and the related railcar component can create temperatures within the side bearing assembly that can exceed the heat deflection temperature of the elastomeric spring thus causing the elastomeric spring to deform. As used herein and throughout, the term “heat deflection temperature” means and refers to a temperature level at the which the elastomeric spring, regardless of its composition, tends to soften and deform. Deformation of the elastomeric spring can significantly reduce the ability of the elastomeric spring to apply a proper preload force and, thus, decreases vertical suspension characteristics of the side bearing assembly which, in turn, results in enhanced hunting of the wheeled truck. Enhanced hunting and/or unstable cyclic yawing of the truck increases the resultant translation/oscillation of the railcar leading to a further increase in the heat buildup and further deterioration of the elastomeric spring. 
     Thus, there is a continuing need and desire for a railcar constant contact side bearing assembly including a multipiece cap design which allows the cap members to horizontally slide or shift relative to each other whereby optimizing energy absorption and related performance criteria for the side bearing assembly while maintaining vertical reciprocity of the cap members relative to the housing and which limits vertical separation of the cap members relative to each other 
     SUMMARY 
     According to one aspect, there is provided a constant contact side bearing assembly for a railcar including a housing and a multipiece cap arranged in operable combination with each other. The side bearing assembly housing includes upstanding wall structure defining a central axis for the side bearing assembly. The multipiece cap includes a first member arranged within the housing and having generally vertical wall structure arranged to slidably contact the wall structure of the housing arranged to one side of the central axis during operation of the side bearing assembly. The multipiece cap further includes a second member arranged at least partially within the housing and carried by the first member. The second cap member includes generally vertical wall structure arranged to slidably contact the wall structure of the side bearing housing arranged to an opposite or second side of the central axis of the side bearing assembly during operation of the side bearing assembly. A generally flat surface on the second member extends beyond the wall structure of the housing. A spring is arranged within the housing beneath both the first and second members of the multipiece cap for returning energy imparted to the spring during operation of the side bearing assembly. The members of the multipiece cap define non-vertical interengaging and slidable surfaces therebetween which are disposed at an acute angle relative to a horizontal plane for maintaining the wall structure on each cap member in sliding contact with the wall structure of the housing thereby limiting horizontal shifting movements of the multipiece cap relative to the housing while maintaining vertical reciprocity of the cap members relative to the housing. The first and second members of the multipiece cap are provided with interlocking instrumentalities for allowing the first and second cap members to horizontally slide relative to each other while limiting vertical separation of the first and second members relative to each other during operation of the side bearing assembly. 
     In one form, the spring of the side bearing assembly includes an elastomeric member having first and second axially aligned ends. Preferably, the generally flat surface of the second member of the multipiece cap establishes a coefficient of friction ranging between about 0.4 and about 0.9 with the railcar during operation of the side bearing assembly. 
     According to another aspect, there is provided a constant contact side bearing assembly for a railcar including a housing and a multipiece cap arranged in operable combination with each other. The housing includes generally vertical wall structure and defines a central axis for the side bearing assembly. The multipiece cap includes a first non-metal member arranged within the housing and a second non-metal member arranged at least partially within the housing and carried by the first member. A generally flat surface on the second non-metal. member extends beyond the wall structure on the housing. Each non-metal cap member defines wall structure. The wall structure on the first non-metal cap member is arranged to one side of the central axis for sliding contact with the wall structure of the housing during vertical reciprocatory movements of the multipiece cap relative to the housing. The wall structure on the second non-metal cap member is arranged to an opposite side of the central axis for sliding contact with the wall structure of the housing during vertical reciprocatory movements of the multipiece cap relative to the housing. A spring is arranged within the housing for returning energy imparted to the side bearing assembly. The cap members define non-vertical interengaging and slidable angled surfaces therebetween which are disposed at an acute angle relative to a horizontal plane for maintaining the wall structure on each non-metal cap member in sliding contact with the wall structure of the housing thereby limiting horizontal shifting movements of the multipiece cap relative to the housing. The first and second members of the multipiece cap carry interlocking instrumentalities for allowing the cap members to horizontally slide relative to each other while limiting vertical separation of the first and second members relative to each other during operation of the side bearing assembly. 
     An insert is preferably maintained in operable association with the generally flat surface on the second non-metal cap member for contacting an underside of the railcar thereby establishing a coefficient of friction ranging between about 0.4 and about 0.9 with the railcar during operation of the side bearing assembly. 
     According to another aspect, there is provided a constant contact side bearing assembly for a railcar including a housing, and a multipiece cap arranged in operable combination with each other. The housing includes generally vertical wall structure and defines a central axis for the side bearing assembly. The multipiece cap includes a first plastic member movably arranged within the housing and a second plastic member movably arranged at least partially within the housing and carried by the first plastic member. A portion of the second plastic member extends beyond the housing and defines generally flat surface. Each plastic cap member defines generally vertical wall structure. A spring is arranged within the housing for returning energy imparted to the side bearing assembly. The cap members define non-vertical interengaging and slidable angled surfaces therebetween which are disposed at an acute angle relative to a horizontal plane for urging and maintaining the generally vertical wall structure on each in sliding engagement with the wall structure of the housing while maintaining vertical reciprocity of both cap members relative to the housing during operation of the side bearing assembly. The first and second members of the multipiece cap are provided with interlocking instrumentalities for allowing the cap members to horizontally slide relative to each other while limiting vertical separation of the first and second members relative to each other during operation of the side bearing assembly. 
     To establish and maintain a coefficient of friction ranging between about 0.4 and about 0.9 with the railcar during operation of the side bearing assembly, the generally flat surface on the second plastic cap member is preferably provided with a metal insert. In one embodiment, the interlocking instrumentalities are formed as an integral part of the plastic cap members. In one form, the spring includes an elastomeric member having axially aligned ends. 
     According to another aspect, there is provided a constant contact side bearing assembly for a railcar including a housing, a non-metal spring seat and a non-metal top cap arranged in operable combination relative to each other. The side bearing assembly housing has generally vertical wall structure defining a central axis for the side bearing assembly. The non-metal spring seat is arranged within the housing for vertical reciprocatory movement. The non-metal top cap is at least partially arranged with the housing for vertical reciprocatory movement. The top cap has a generally flat surface spaced at least partially above the wall structure of the housing. The top cap is carried by the spring seat. A spring is arranged within the housing for returning energy imparted to the side bearing assembly. The spring seat and top cap define cooperating angled surfaces therebetween for urging the spring seat and top cap in opposed directions away from the central axis of the side bearing assembly such that non-metal wall structure, on each of the spring seat and top cap, is moved into sliding engagement with the wall structure on the housing in response to a vertical load acting on the side bearing assembly while maintaining vertical reciprocity of the spring seat and top cap relative to the housing. An apparatus is provided in operable combination with the top cap and spring seat of the multipiece cap for allowing the top cap and spring seat to horizontally slide relative to each other while limiting vertical separation of the top cap and spring seat relative to each other during operation of the side bearing assembly. 
     To allow the side bearing assembly to establish a coefficient of friction ranging between about 0.4 and about 0.9 with the railcar during operation of the side bearing assembly, a metallic insert is maintained in operable association with and is generally centered on the flat surface of the top cap. Preferably, the spring for the side bearing assembly includes an elastomeric member. Preferably, the apparatus for allowing the top cap and spring seat to horizontally slide relative to each other while limiting vertical separation of the top cap and spring seat relative to each other during operation of the side bearing assembly is formed integral with the top cap and spring seat. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a portion of a railroad car wheeled truck including one form of a constant contact side bearing assembly embodying principals of this invention disclosure; 
         FIG. 2  is an enlarged top plan view of the constant contact side bearing assembly illustrated in  FIG. 1 ; 
         FIG. 3  is a side elevational view of the constant contact side bearing assembly illustrated in  FIG. 2 ; 
         FIG. 4  is a view similar to  FIG. 3  with parts broken away to show additional details; 
         FIG. 5  is an enlarged sectional view taken along line  5 - 5  of  FIG. 2 ; 
         FIG. 6  is a top plan view of a first member or spring seat forming part of the present invention disclosure; 
         FIG. 7  is a side view of the spring seat illustrated in  FIG. 6 ; 
         FIG. 8  is a bottom plan view of the spring seat shown in  FIG. 6 ; 
         FIG. 9  is an end view of the spring seat shown in  FIG. 6 ; 
         FIG. 10  is a top plan view of a second member or top cap forming part of the present invention disclosure; 
         FIG. 11  is side view of the top cap illustrated in  FIG. 10 ; 
         FIG. 12  is an end view of the top cap illustrated in  FIG. 10 ; 
         FIG. 13  is a graph showing the enhanced vertical energy capability offered by a side bearing assembly according to the invention disclosure and a prior art type constant contact side bearing assembly; and 
         FIG. 14  is a graph representative of a force-displacement plot of hysteresis loops of both a prior art type constant contact side bearing assembly and an embodiment of a constant contact side bearing assembly according to this invention disclosure; 
     
    
    
     DETAILED DESCRIPTION 
     While this invention disclosure is susceptible of embodiment in multiple forms, there is shown in the drawings and will hereinafter be described a preferred embodiment of this invention disclosure, with the understanding the present disclosure is to be considered as setting forth an exemplification of the disclosure which is not intended to limit the disclosure to the specific embodiment illustrated and described. 
     Referring now to the drawings, wherein like reference numerals indicate like parts throughout the several views,  FIG. 1  shows a fragment of a railcar wheeled truck assembly, generally indicated by reference numeral  10 , for supporting and allowing a railcar body  12  defining a part of a railcar  13  ( FIG. 3 ) to ride along and over tracks T. Truck assembly  10  is of a conventional design and includes a side frame  14 , a bolster  16 , extending generally transversely relative to a longitudinal centerline  18  of the railcar body  12  ( FIG. 3 ), and a wheel set  20 . A conventional center bearing plate  22  is suitably mounted on the bolster  16  for pivotally supporting one end of the car body  12  ( FIG. 3 ). 
     A railroad car side bearing assembly embodying principals of this invention disclosure is generally indicated in  FIG. 1  by reference numeral  30  and is arranged in operable combination with each wheeled truck assembly  10 . More specifically, and as is conventional, a railroad car side bearing assembly is mounted on an upper surface  17  of the railcar bolster  16  on opposite lateral sides of the center bearing plate  22  to limit hunting movements and oscillation of the wheeled truck assembly  10  as the railcar moves over the tracks T. 
     The aesthetic design of assembly  30  illustrated in the drawings is merely for exemplary purposes. Whereas, the principals and teachings set forth below are equally applicable to other side bearings having different forms and shapes. Turning to  FIG. 2 , side bearing assembly  30  includes a housing or cage  40 , a multipiece cap  60  arranged for generally telescoping or vertical reciprocatory movements relative to the housing  40 , and a spring  100  ( FIG. 3 ). 
     In the embodiment shown in  FIGS. 2 ,  3  and  4 , housing  40  is preferably formed of a strong and wear resistant metal material, such as steel or the like, and includes upstanding wall structure  44  extending upwardly from a base  46  to define an axis  47  for the side bearing assembly  30 . The housing wall structure  44  extends upwardly from the base  46  for a predetermined distance. The wall structure  44  of the side bearing housing  40  defines an open-top cavity or internal void  48  having a predetermined inner surface configuration. 
     The housing base  46  is configured for suitable attachment to an upper surface  17  of the railcar bolster  16  as through any suitable means, i.e. threaded bolts or the like. In the illustrated embodiment, housing base  46  includes a pair of mounting flanges  50  and  50 ′ radially extending outwardly in opposed directions away from the side bearing assembly axis  47 . Each mounting flange  50 ,  50 ′ defines a bore or aperture  52 ,  52 ′, respectively, for allowing a suitable fastener to extend therethrough so as to permit housing  40  to be fastened to the upper surface  17  of the bolster  16 . Preferably, the bores or apertures  52 ,  52 ′ are aligned relative to each other along a longitudinal axis  54  such that, when housing  40  is secured to the bolster  16 , axis  54  extends generally parallel to the longitudinal axis  18  ( FIG. 1 ) of car body  12 . 
     Turning to  FIG. 3 , the multipiece cap  60  for assembly  30  preferably includes a first non-metal member or spring seat  70  and a second non-metal member or top cap  80  arranged in operable combination with each other. Preferably, and to enhance the vertical reciprocity of the multipiece cap  60  within housing  40 , the first cap member or spring seat  70  and the second member or top cap  80  are each formed from a high performance plastic material of the type sold by DuPont™ under the tradename Zytel® under Model Nos. 75LG50HSL BK031, 70G33HS1L BK031, ST801AHS BK010, and HTNFE8200 BK431 and equivalents thereto. Besides being less weight than steel, forming the first member or spring seat  70  and the second member or top cap  80  from such non-metal, high performance plastic material has also shown lower wear rates than steel which, in turn, increases the expectant life of the side bearing assembly  30 . 
     As shown in  FIG. 4 , spring seat  70  is positioned within the housing  40  for generally vertical movements and includes a generally horizontal or flat spring engaging surface  72 . Turning to  FIGS. 6 and 7 , spring seat  70  furthermore includes generally vertical wall structure  74  extending upwardly from one side of surface  72 . When arranged within the side bearing housing  40 , the wall structure  74  of member  70  is arranged to one side of the vertical axis  47  of the side bearing assembly  30  ( FIG. 2 ). Preferably, wall structure  74  is formed integral with the supporting plate  72 . Notably, and as shown in  FIG. 2 , an outer surface  75  on the wall structure  74  of the spring seat  70  complements an inner surface  45  of the side bearing housing wall structure  44  arranged to one side of the vertical axis  47  of the side bearing assembly  30 . In the embodiment illustrated for exemplary purposes, the side bearing housing inner surface  45  and the spring seat outer wall surface  75  each have a curved surface configuration which complement each other and promote sliding movement therebetween. 
     As shown in  FIGS. 4 and 5 , the second member or top cap  80  is at least partially positioned within the housing  40  for generally vertical movements and is operably carried by the first member or spring seat  70 . Turning to  FIGS. 10 and 11 , member  80  desirably includes an upper generally flat car engaging surface  82 . As shown in  FIG. 2 , when the side bearing assembly  30  is secured to the bolster  16 , the generally planar or flat surface  82  of member  80  is disposed above a terminal end of the upstanding wall structure  44  of the side bearing housing  40  for a predetermined distance. In the example shown in  FIG. 3 , the normal distance between surface  82  of member  80  and the top edge of the wall structure  44 , indicated by the distance “X”, is determinative of the permissible compressive movement of the side bearing assembly  30  and such that after the underside  15  of the railcar body  12  contacts the upper edge of the housing structure  44 , the side bearing assembly  30  functions as a solid unit and will prevent further rocking and relative movement between the bolster  16  and the railcar body  12 . 
     Cap member  80  furthermore includes generally vertical wall structure  84  which, when cap member  80  is assembled in operable relation with the side bearing assembly as shown in  FIGS. 2 ,  3  and  4 , is disposed to an opposite side of the axis  47  from the upstanding wall structure  74  of the spring seat  70 . Preferably, the wall structure  84  is formed integral with the generally planar surface  82  of cap  80 . As shown in  FIG. 2 , an outer surface  85  on the wall structure  84  of cap  80  complements the side bearing housing wall structure inner surface  45  disposed to an opposed side of the vertical axis  47  of the side bearing assembly  30  from surface  75  of member  70 . In the embodiment illustrated for exemplary purposes, the side bearing housing inner surface  45  and the wall structure outer surface  85  on member  80  each have a curved surface configuration which complement each other and promote sliding movement therebetween. 
     In the embodiment shown in  FIGS. 5 and 10 , the top cap  80  furthermore includes an insert  90  that is maintained in operable association with and preferably generally centered on the upper generally flat surface  82  on member  80 . The insert  90  is preferably formed from a metal material selected from the class of: steel and austempered ductile iron. The insert  90  is arranged in operable association with the top cap  80  so as to slidably interact and contact with the underside  15  of the car body  12  ( FIG. 5 ). In the embodiment illustrated by way of example, the insert  90  has a diameter of about 3 inches. The exact shape and design of the insert  90  can take any of a myriad of designs and configuration without detracting or departing from the spirit and scope of this invention disclosure. Suffice it to say, the insert  90  is engineered and designed whereby allowing the side bearing assembly  30  to establish a coefficient of friction ranging between about 0.4 and about 0.9 with the railcar  12  during operation of the constant contact side bearing assembly  30  and so as to limit hunting movements and oscillation of the wheeled truck assembly  10  as the railcar moves over the tracks. Attention is directed to coassigned and copending U.S. patent application Ser. No. 13/507,145; the applicable portions of which are incorporated herein by reference for a fuller understanding the design and functionality of insert  90 . 
     Preferably, the housing  40  and members  70 ,  80  comprising the multipiece cap  60  are configured relatively to each other so as to inhibit rotation of the cap members  70 ,  80  relative to the housing  40 . In the illustrated embodiment, the inner surface  45  of the side bearing housing wall structure  44  has an oblong-like configuration which, as mentioned, complements the exterior surface configurations on the wall structures  76 ,  86  of the cap pieces  70  and  80 , so as to inhibit rotation of the cap pieces  70 ,  80  relative to the housing  40 . Of course, with only slight redesign, other structure, i.e, channels and projecting ribs, would equally suffice to inhibit rotation of the cap pieces  70 ,  80  relative to the housing  40  without detracting or departing from the spirit and scope of the present invention disclosure. 
     One of the salient aspects of this invention disclosure relates to the ability to limit—if not eliminate—horizontal shifting movements of the side bearing assembly cap  60  relative to the side bearing assembly housing  40  whereby significantly enhancing operating performance characteristics of assembly  30 . To accomplish this desired end, and as illustrated in  FIGS. 3 and 4 , the first and second cap members  70  and  80 , respectively, define non-vertical interengaging and slidable planar surfaces  76  and  86 , respectively, therebetween for maintaining the outer surfaces  75  and  85  of the respective members  70  and  80  in frictional sliding contact with the inner surface  45  ( FIG. 2 ) of the side bearing housing  40 . That is, and in response to a vertical load being directed against assembly  30 , the cooperating angled surfaces  76  and  86  on the respective first and second members  70  and  80  of the multipiece cap  60  urge the spring seat  70  and top cap  80  in opposed directions relative to each other and away from the centerline or upstanding axis  47  of the side bearing assembly  30  such that the outer surfaces  75  and  85  on each of the first and second member  70  and  80 , respectively, are constantly urged toward and maintained in sliding engagement with the inner surface  45  ( FIG. 2 ) of the side bearing housing  40 . 
     In one form, the non-vertical surfaces  76  and  86  of the first and second members  70  and  80 , respectively, of the multipiece side bearing assembly cap  60  are disposed at a predetermined acute angle θ. In one form, the predetermined acute angle θ ranges between about 20° and about 30° relative to a horizontal plane. In a most preferred form, the cooperating angled surfaces  76  and  86  between the first and second members  70  and  80 , respectively, of cap  60  are disposed at an angle of about 25° relative to a horizontal plane. 
     Since the side bearing assembly  30  of the present disclosure is of a resilient type, it is essential some form of yieldable apparatus be incorporated therein. In this regard, spring  100  is arranged in operable combination with and for absorbing, dissipating and returning energy imparted to the multipiece cap  60 . As shown, spring  100  is arranged and accommodated within a chamber or cavity  48  formed by a combination of housing  40  and cap  60  for urging the multipiece cap  60  upwardly into contact with the underside  15  of the railcar body  12  ( FIG. 3 ). 
     Like the overall side bearing design, the exact shape or form of the spring  100  can vary or be different from that illustrated for exemplary purposes without detracting or departing from either the spirit or scope of this invention disclosure. In the embodiment illustrated in  FIGS. 3 ,  4  and  5 , spring  100  is comprised of a formed and resiliently deformable thermoplastic elastomer member  110  and, preferably, a thermal insulator  120 . 
     In the embodiment illustrated for exemplary purposes in  FIGS. 4 and 5 , member  110  of spring  100  has a configuration suitable for accommodation between base  46  of the side bearing housing  40  and an underside of the support plate  72  of the spring seat  70 . Member  110 , illustrated by way of example in  FIG. 4 , preferably embodies the teachings set forth in coassigned U.S. Pat. No. 7,338,034; the applicable portions of which are incorporated herein by reference. In the illustrated embodiment, member  110  defines a generally centralized bore  112  opening to axially aligned ends  113 ,  113 ′ of member  110 . It should be appreciated, however, member  110  could also be solidly configured. Suffice it to say, the thermoplastic member  110  preferably has an elastic strain to plastic strain ratio of about 1.5 to 1. Coassigned U.S. Pat. No. 4,198,037 to D. G. Anderson, the applicable portions of which are incorporated hereby by reference, better describes the composition and methodology for forming member  110 . 
     The thermal insulator  120  of spring  100  is preferably arranged at one end of and is intended to operably protect the thermoplastic member  110  from the adverse affects of heat generated by the sliding frictional movements between the underside  15  of the railcar body  12  ( FIG. 3 ) and the planar surface  82  on the side bearing cap  60  during movements of the railcar between locations. Suffice it to say, and in the illustrated embodiment, the thermal insulator  120  is operably carried at one end of the thermoplastic member  110  and is preferably of the type disclosed in coassigned U.S. Pat. Nos. 6,092,470; 6,892,999; and 7,044,061; the applicable portions of which are incorporated herein by reference. 
     In the embodiment illustrated for exemplary purposes in  FIGS. 3 and 4 , the base  46  of the side bearing assembly  40  supports that end of the spring  100  opposite from the thermal insulator  120 . Preferably, a spring guide or projection  42  ( FIG. 5 ) is provided and is centrally located on the base  46  of the side bearing housing  40 . In the embodiment illustrated in  FIG. 5 , the spring guide  42  fits within the bore or recess  112  defined by member  110  whereby operably locating at least the lower end of the spring  100  within the side bearing assembly housing  40 . Preferably, a spring guide  73  depends from the underside  72  of cap member and fits through the thermal insulator  120  and into the bore or recess  112  ( FIG. 5 ) defined by member  110  whereby operably locating the upper end of the spring  100  within the side bearing assembly housing  40 . 
     In the embodiment illustrated for exemplary purposes, the side bearing assembly  30  is configured to promote the dissipation of heat from the cavity  48  and away from the thermoplastic spring  100  thereby prolonging the usefulness of the side bearing assembly  30 . As shown in  FIGS. 3 and 4 , the wall structure  44  of the side bearing housing  40  preferably defines a pair of openings  45  (with only one being shown) disposed to opposite lateral sides of the longitudinal axis  47  of the side bearing housing  40  and extending through a thickness of the wall structure  44 . Each opening  45  is formed toward the base  46  or toward a lower end of the side bearing housing  40  in a vicinity of an intersection between wall structure  44  and base  46 . In the illustrated embodiment, the openings  45  are generally aligned along a line extending generally perpendicular or normal to axis  54  of housing  40 . As will be appreciated, the openings  45  provide a particular advantage when a thermoplastic spring is used to resiliently urge the cap  60  against and into frictional sliding contact with an underside  15  of the railcar body  12  ( FIG. 2 ) by allowing air to freely pass through the housing  40  and away from the spring  100 . 
     The multipiece cap  60  of the side bearing assembly  30  is furthermore preferably designed to reduce the adverse affects of heat on the thermoplastic spring  100  during operation of the side bearing assembly  30 . More specifically, in the embodiment illustrated in  FIGS. 2 ,  5  and  10 , the top cap or member  80  of the multipiece cap  60  includes a pair of diametrically opposed openings  83 ,  83 ′ arranged toward an intersection of the generally flat surface  82  and the wall structure  84  on member  80 . Preferably, the openings  83 ,  83 ′ are disposed such that they are not completely blocked when the generally flat surface  82  on cap member  80  is frictionally engages with the underside  15  of the railcar body ( FIG. 2 ). Although two openings in the top cap  80  are illustrated for exemplary purposes, more openings can be provided and their disposition relative to the wall structure  84  altered without detracting or departing from the spirit and scope of this invention disclosure. Suffice it to say, the purpose of the openings  83 ,  83 ′ is to direct heat away from the spring  100  thereby prolonging the usefulness of the spring and the effectiveness of the side bearing assembly. Any suitable structure for accomplishing those desirable ends would should be considered within the spirit and scope of this aspect of the invention disclosure. 
     An apparatus  130  is carried by the first member or spring seat  70  and the top cap  80  for allowing the spring seat  70  and top cap  80  to horizontally slide relative to each other while limiting vertical separation of the spring seat  70  and said top cap  80  relative to each other during operation of said constant contact side bearing assembly  30 . In the embodiment illustrated in  FIG. 5 , the first and second members  70  and  80 , respectively, of the multipiece cap  60  are provided with interlocking instrumentalities  140  and  150  for allowing the first and second cap members  70  and  80 , respectively, to horizontally slide relative to each other so as to maintain the wall structure  74  and  84  of the first and second cap members  70  and  80 , respectively, in frictional sliding contact with the interior wall structure  45  of housing  40  ( FIG. 3 ) while limiting vertical separation of the first and second members  70  and  80 , respectively, relative to each other during operation of the constant contact side bearing assembly  30 . The interlocking instrumentalities  140  and  150  for accomplishing such ends can take a myriad of configuration and designs without detracting or departing from the spirit and novel scope of this invention disclosure. 
     In the embodiment illustrated by way of example in  FIG. 5 , the instrumentalities  140  and  150  comprising apparatus  130  are preferably disposed in diametrically opposed relation relative to each other and on opposed lateral sides of the axis  47  of the side bearing assembly  30 . As may be deduced from  FIGS. 5 ,  6 ,  8 ,  9 ,  10   12 , components of the instrumentalities  140  and  150  are preferably disposed toward an outer side edge and radially inwardly of a generally arcuate segment defined by the outer surfaces  74  and  84  of pieces  70  and  80  of cap  60 . 
     In one form, the interlocking instrumentalities  140  and  150  are mirror images of each other. As shown by way of example in  FIG. 6 , and toward opposed lateral exterior sides thereof, the first cap member or spring seat  70  defines a pair of open-sided recesses or voids, generally indicated by reference numerals  141  and  151 . Each recess or void  141 ,  151  defined by cap member  70  has a predetermined marginal edge  142 ,  152 , respectively. Moreover, and toward opposed sides thereof, the first cap member  70  defines a pair of steps or supports  143 ,  153  laterally projecting in opposed directions relative to each other and away from the center of cap member  70 . Each step or support  143 ,  153  preferably has a generally linear side edge  144 ,  154 , respectively, extending generally parallel relative to each other. Moreover, and as shown in  FIGS. 5 and 7 , each step or support  143 ,  153  also has a generally flat or planar underside or undersurface  145 ,  155 , respectively, extending generally parallel to surface  17  on the bolster  16  ( FIG. 1 ) and which opens to the generally flat spring engaging surface  72  of cap member  70 . 
     Preferably, each step or support  143 ,  153  extends for a predetermined portion of the longitudinal length of the respective opening  141 ,  151 . As such, an entry port  146 ,  156  extends between and opens to both the slanted planar surface  76  of cap member  70  and to the underside or surface  145 ,  155  of the each projection  143 ,  153  longitudinally between a distal end of each lateral step or support  143 ,  153  and the marginal edge of the opening  142 ,  152 , respectively. In one form, each entry port  145 ,  155  has a predetermined width defined between a distal end of each lateral step or support  143 ,  153  and the marginal edge of the respective opening  142 ,  152 . 
     As shown by way of example in  FIGS. 5 ,  11 , and  12 , and toward opposed exterior sides thereof, the second member or top cap  80  carries a pair of depending arms, generally indicated by reference numerals  147  and  157  which are adapted to operably cooperate with the steps or supports  143 ,  153  on the first cap member or spring seat  70 . As shown in  FIGS. 5 and 12 , each arm  147 ,  157  includes a vertically depending arm section  148 ,  158 , respectively, along with a generally horizontal arm section  149 ,  159 , respectively, extending generally normal or perpendicular to the vertically depending arm section  148 ,  158 , respectively, and inwardly toward the center of cap member  80 . Notably, in a preferred embodiment, the arm sections  148 ,  158  on cap member  80  embrace and capture the projections  143 ,  153  on cap member  70  therebetween. Each generally horizontal arm section  149 ,  159  defines a generally flat or planar surface  149 ′,  159 ′, respectively, extending generally parallel to and, when the pieces  70 ,  80  of the multipiece cap  60  are arranged in operable combination with each other, in confronting relation with the generally flat or planar underside or undersurface  145 ,  155 , respectively, on cap member  70 . As such, the interlocking instrumentalities  140  and  150  comprising apparatus  130  readily allow the spring seat  70  and top cap  80  to horizontally slide relative to each other while limiting vertical separation of the spring seat  70  and said top cap  80  relative to each other during operation of said constant contact side bearing assembly  30 . 
     In one form, the generally horizontal arm section  149 ,  159  of each arm  147 ,  157 , respectively, has a predetermined width and preferably extends the full width of the vertically depending arm section  148 ,  158 , respectively. Moreover, and in a preferred form, the predetermined width of the generally horizontal arm section  148 ,  158  is greater than the size of the respective entry port  146 ,  156  on the second cap member or spring seat  70 . As such, and during assembly of the multipiece cap  60 , the cap pieces  70  and  80  need be angled or tilted relative to each other to allow the generally horizontal arm section  149 ,  159  on the respective arm  147 ,  157  to fit within and through the respective entry port  146 ,  156  on the first cap member or spring seat  70  whereby allowing the generally horizontal arm section  149 ,  159  of each arm  147 ,  157  to fit under and into confronting relation relative to the respective generally flat or planar underside or undersurface  145 ,  155  on cap member  70 . As will be appreciated from an understanding of this disclosure, this design furthermore inhibits the cap pieces  70  and  80  from inadvertently becoming completely separated from each other during operation of the railcar constant contact side bearing assembly  30  regardless of the horizontal sliding position of the cap pieces  70  and  80  relative to each other. 
     The advantages provided by a side bearing assembly embodying principals of this invention disclosure are illustrated by way of example in  FIG. 13  which schematically illustrates a calculated longitudinal force-displacement hysteresis loop of the present disclosure wherein the outer parallelogram defined by points ABCDEFA represents a cycle length of a side bearing assembly embodying principals of the present disclosure as the bolster  16  of truck assembly  10  oscillates or “hunts” between extreme positions of travel about the center bearing plate  22  ( FIG. 1 ). It should be noted, however, the schematic illustration in  FIG. 13  is intended for illustrative purposes only and should not be interpreted or construed, directly or indirectly, as representing actual measurements of loads applied to or movements associated with components parts of the side bearing assembly  30 . 
     The area of the graph shown in  FIG. 13  and defined by points ABZJKDEVLMA illustrates a calculated force-displacement hysteresis loop of a conventional side bearing assembly wherein a gap or space is required between the top cap and side bearing housing to allow for vertical displacement of the cap relative to the side bearing housing. More specifically, in the graph shown in  FIG. 13 , points ABZJKDEVLMA represent a cycle length of a conventional side bearing assembly having a gap or space between the side bearing housing and cap and the effects on longitudinal loading of the side bearing assembly caused by such space or gap between the side bearing housing and cap as the truck assembly bolster  16  oscillates or “hunts” between extreme positions of travel about the center bearing plate  22  ( FIG. 1 ). 
     Point A on the graph illustrated in  FIG. 13  schematically represents the increased longitudinal loading on the side bearing assembly when the truck assembly bolster  16  ( FIG. 1 ) is urged toward an extreme rotational position and the sidewalls of a conventional side bearing assembly are pressed into contact relative to each other by the longitudinal loads placed on the side bearing assembly as a result of the truck assembly “hunting” or yawing between positions as the railcar moves between locations. The distance between points A and B in  FIG. 13  schematically represents the reduced longitudinal loading on the side bearing assembly as the truck assembly bolster  16  traverses in a first rotational direction away from one extreme rotational position. 
     Point B on the graph illustrated in  FIG. 13  schematically represents the longitudinal loading on the side bearing when the railcar bolster is arranged toward a position, proximate to its extreme rotational position, but wherein the sidewalls of the side bearing housing and cap of the side bearing assembly have deflected as a result of the reduced longitudinal loads being removed therefrom. Points B and Z on the graph in  FIG. 13  schematically illustrate the relatively constant longitudinal loading on the side bearing assembly as the truck assembly bolster  16  moves away from a position, proximate to its extreme rotational position, wherein longitudinal loads are lessened on and deflection has occurred to the sidewalls of the side bearing housing and cap, to a neutral or centered position. The relatively constant longitudinal loading of the railcar side bearing assembly remains as the cap longitudinally shifts in the gap between it and the side bearing housing is represented by the distance between points B and Z. 
     As shown in  FIG. 13 , between points Z and J, the longitudinal loading on the side bearing assembly loading remains relatively constant as the gap between the cap and side bearing assembly continues to collapse as the truck assembly bolster  16  continues to rotate about the center bearing plate  22  ( FIG. 1 ) from the neutral position toward an opposite extreme rotational position. Point J on the graph shown in  FIG. 13  represents the longitudinal loading on the side bearing assembly when the sidewalls of the side bearing housing and cap of a conventional side bearing assembly again contact relative to each other. The distance between points J and K on the graph shown in  FIG. 13  schematically represents the increase in longitudinal loading on the side bearing assembly as the sidewalls of the side bearing housing and cap of a conventional side bearing assembly deflect as the bolster  16  continues to rotate or move toward the extreme rotational position during hunting movements of the truck assembly  10 . 
     With the sidewalls of the side bearing housing and cap of a conventional side bearing assembly in contact relative to each other (point K), the longitudinal loading on the side bearing assembly remains relatively constant as indicated on the graph illustrated in  FIG. 13  between points K and D. Between points K and D on the graph illustrated in  FIG. 13 , the railcar underside  15  slides relative to the side bearing assembly as the bolster continues to traverse toward an extreme rotational position. 
     Point D on the graph illustrated in  FIG. 13  schematically represents the increased longitudinal loading on the side bearing assembly when the truck assembly bolster  16  ( FIG. 1 ) is urged toward an extreme rotational position (opposite from the position represented in the graph shown in  FIG. 13  by point A and the sidewalls of the side bearing assembly are pressed into contact relative to each other by the increased longitudinal loads placed on the side bearing assembly as a result of the truck assembly “hunting” or yawing between positions as the railcar moves between locations. Between points D and E on the graph illustrated in  FIG. 13 , the longitudinal loading on the side bearing assembly is again reduced as a result of the truck assembly bolster  16  traversing in a second rotational direction away from one extreme rotational position toward a position arranged proximate the extreme rotational position but wherein deflection of the sidewalls of the side bearing housing and cap have occurred as a result of the longitudinal loads being removed therefrom. Points E and V on the graph in  FIG. 13  schematically illustrate the relatively constant longitudinal loading on the side bearing assembly as the truck assembly bolster  16  moves away from a position, proximate to its extreme rotational position, wherein longitudinal loads are removed from the sidewalls of the side bearing housing as the cap moves to a neutral or centered position. The relatively constant longitudinal loading of the railcar side bearing assembly remains as the cap longitudinally shifts in the gap between it and the side bearing housing is represented by the distance between points E and V. 
     As shown in  FIG. 13 , and between points V and L, the longitudinal loading on the side bearing assembly remains relatively constant as the gap between the cap and side bearing housing continues to collapse as the truck assembly bolster  16  continues to rotate about the center bearing plate  22  ( FIG. 1 ) from the neutral position toward an opposite extreme rotational position and through a position (point L) wherein the sidewalls of the side bearing housing and cap of a conventional side bearing again come in contact relative to each other. The distance between points L and M on the graph shown in  FIG. 13  schematically represents the increase in longitudinal loading on side bearing assembly as the sidewalls of the side bearing housing and cap, of a conventional side bearing assembly deflect as the bolster  16  continues to rotate or move toward the extreme rotational position during hunting, movements of the truck assembly  10 . 
     With the sidewalk of the side bearing housing and cap of a conventional side bearing assembly being in contact relative to each other (point M), the longitudinal loading on the side bearing assembly remains relatively constant as indicated on the graph illustrated in  FIG. 13  between points M and A. Between points M and A on the graph illustrated in  FIG. 13 , the railcar underside  15  slides relative to the side bearing assembly as the bolster continues to traverse toward an extreme rotational position. 
     The adverse affects of the spacing between the top cap and housing of a conventional side bearing assembly are illustrated in  FIG. 13  by the distance between points B and J along with the distance between points E and L. That is, as the truck assembly bolster  16  rotates during “hunting” movements thereof, the rotational movement of the truck assembly bolster  16  places a force or longitudinal load on the side bearing assembly whereby causing the top cap of the side bearing assembly to longitudinally shift relative, to the side bearing housing until the distance separating the wall structure of the top cap and the wall structure of the side bearing housing collapses. The collapse of the distance separating the wall of the top cap from the wall of the side bearing housing is schematically represented in  FIG. 13  by the distance between points B and J along with E and L. It is important to note, the distance separating the wall of the top cap from the wall of the side bearing housing on a conventional side bearing assembly progressively worsens with wear. That is, the distance separating the wall of the top cap from the wall of the side bearing housing, schematically represented in  FIG. 13  by the distance between points B and J along with E and L, continues to increase with wear. Increased wear between the cap and side bearing housing reduces the energy absorption capability of the side bearing assembly. 
     Notably, the side bearing assembly of the present disclosure is furthermore designed to be self-adjusting. That is, during operation of the side bearing assembly embodying features of the present disclosure, the interengaging and sliding surfaces on the side bearing housing and the multipiece top cap automatically adjust to wear therebetween and, thus, are maintained in constant contact relative to each other. Accordingly, and with the present disclosure, there is substantially no lost motion between the top cap and side bearing housing when the truck assembly  10  shifts from one rotational position to the other. Accordingly, and as schematically represented in  FIG. 13  those shaded areas marked with diagonal lines in the graph shown in  FIG. 13  are advantageously available for energy absorption by the side bearing assembly  30  during operation of the constant contact side bearing assembly. As noted above, those shaded areas marked with diagonal lines in the graph shown in  FIG. 13  schematically illustrating the enhanced ability of the side bearing assembly of the present disclosure to absorb energy will only increase when considering wear between the cap and side bearing housing of a conventional side bearing assembly. 
     The advantages of a side bearing assembly embodying principals and teachings of the present disclosure are further exemplified in  FIG. 14 . The solid line or hysteresis loop  170  in the graph illustrated in  FIG. 14  represents the vertical energy absorption capabilities of the side bearing assembly embodying principals and teachings of the present invention disclosure. The dash line or hysteresis loop  180  in the graph illustrated in  FIG. 14  represents the vertical energy absorption capabilities of a conventional side bearing assembly. The enhanced ability of the side bearing assembly embodying principals of this invention disclosure to absorb, dissipate and return energy to the railcar as compared to a conventional side bearing design is readily apparent when the two hysteresis loops  170  and  180  are compared. 
     From the foregoing, it will be observed that numerous modifications and variations can be made and effected without departing or detracting from the true spirit and novel concept of this invention disclosure. Moreover, it will be appreciated, the present disclosure is intended to set forth an exemplification which is not intended to limit the disclosure to the specific embodiment illustrated. Rather, this disclosure is intended to cover by the appended claims all such modifications and variations as fall within the spirit and scope of the claims.