Abstract:
A piston rolling thrust bearing construction is constituted of a pair of bearing plates with opposing faces disposed in a spaced alignment with a rolling ball assembly positioned between the opposing faces to support relative movement between the bearing plates. The piston rolling thrust bearing is mounted to the open end of the skirt of a piston disposed in a cylinder to compensate for non-axial motion relative to the cylinder axis due to the articulating motions of connecting elements or any structural misalignments within an engine drive train assembly.

Description:
PRIORITY 
       [0001]    This application claims benefit of U.S. provisional application for patent 61/337,370, filed Feb. 3, 2010 and to U.S. provisional application for patent 61/337,372, filed Feb. 3, 2010. 
     
    
     BACKGROUND 
       [0002]    The field relates to thrust bearings, particularly to rolling thrust bearings. The field also relates to rolling thrust bearings for pistons used in internal combustion engines. More particularly, this field covers piston rolling thrust bearing constructions for coupling a piston to connecting linkage of an internal combustion engine. 
         [0003]    Opposed-piston diesel engines have an acknowledged potential for superior performance according to standard measures of output power and fuel efficiency. For example, the Rootes-Lister diesel engine (also known as the Commer ‘TS3’ diesel) illustrated in  FIG. 1  advanced two-stroke engine construction by way of an engine configuration which included three pairs of opposed pistons driving a single crankshaft. Each piston was coupled to a respective crankpin by a rocker assembly. Each rocker assembly included a rocker arm pivoted between two ends, a piston rod connected to a first end of the rocker arm and to a wrist pin located inside the piston, and a connecting rod connecting the second end of the rocker arm to a crankpin. All of the rocker assemblies were identical, with each rocker arm end being pivoted to the engine frame. The architecture of the rocker assemblies significantly reduced side forces acting on the pistons, thereby making the engine very durable. However, at least one construction feature severely compromised the performance of the Rootes-Lister engine. 
         [0004]    In the Rootes-Lister engine wrist pins (“gudgeon” pins, UK) were mounted inside the pistons, which limited the size of the bearings for the pins, and therefore the ultimate load bearing capacity of the pistons. As a result of this and other constraints, the engine was limited to operating at very low power levels (about 38 HP/liter). 
         [0005]    Examples of opposed piston engine constructions that remove wrist pins from inside pistons are found in Great Britain Patent 558,115 and in U.S. Pat. Nos. 7,156,056 B2 and 7,360,551 B2. In each case, there is no articulation of the piston-to-crankshaft linkage that is internal to the piston. Instead, joints external to the pistons couple the linear motions of the pistons to each of a pair of crankshafts located above and below the cylinders. The axes of the crankshafts lie in a plane that is normal to the axes of, and that bisects, the cylinders. Both crankshafts are connected to each pair of opposed pistons through multiple connecting rods. Consequently, very close tolerances must be maintained during manufacturing to avoid, or at least mitigate, misalignment between the connecting rods and external wrist pins that could result in undesirable side forces exerted on the pistons. A consequence of coupling both crankshafts to the single wrist pin of each piston is an over constraint condition whereby unequal elastic deformation of the coupling components can lead to significant deflection of the wrist pin in a direction orthogonal to the piston motion that produces undesirable side forces acting on the piston. 
         [0006]    Accordingly, the potentially high power levels in two-stroke, opposed-piston engines have not been fully achieved by single crankshaft constructions with rocker assemblies because wrist pins are located inside the pistons. However, dual-crankshaft constructions in which the wrist pins have been removed from, and relocated outside of, the pistons have also not achieved full power potential due to side forces resulting from over constraint of the wrist pins. 
       SUMMARY 
       [0007]    An object of this invention is therefore to provide a connecting linkage construction enabling opposed pistons to operate at high power levels. 
         [0008]    Another object of this invention is to couple a piston to connecting linkage without limiting the load bearing capacity of the piston due to bearing size constraints. 
         [0009]    Yet another object is to eliminate forces orthogonal to piston motion that are produced by over-constraint of multiple crankshafts with common connections to the pistons of an opposed-piston engine. 
         [0010]    In general, these and other objects are achieved by a construction in which a piston is connected to a connecting linkage with a rolling thrust bearing which does not limit the load bearing capacity of the piston resulting from bearing size constraints. 
         [0011]    In general, these and other objects are achieved by a construction in which a piston is connected to a connecting linkage by a rolling thrust bearing which transmits only those linkage forces that are parallel to the piston&#39;s motion. 
         [0012]    In general, these and other objects are achieved by a construction in which a piston is connected to a connecting linkage by a rolling thrust bearing that eliminates forces that otherwise would be imparted to the piston skirt by movement of the upper end of a rocker arm. 
         [0013]    In general, these and other objects are achieved with piston rolling thrust bearing constructions constituted of a pair of bearing plates, opposing linearly-grooved faces of which are disposed in a spaced alignment, with a rolling ball assembly positioned between the opposing faces to support relative movement between the bearing plates. The piston rolling thrust bearing is mounted to the open end of a piston skirt to compensate for non-axial motion relative to the piston axis due to the articulating motions of connecting elements and/or any structural misalignments within the drive train assembly. 
         [0014]    In general, these and other objects are achieved with a piston rolling thrust bearing constituted of a pair of bearing plates having opposed, complementarily curved, linearly-grooved faces disposed in a spaced relationship with a rolling ball assembly positioned between the opposed faces to support relative movement between the plates. The rolling thrust bearing is mounted to the open end of a piston skirt with the curved face of the first bearing plate facing the interior of the piston. 
         [0015]    When connecting linkage motion transverse to the piston axis is small, these and other objects are achieved with a piston rolling thrust bearing constituted of a pair of parallel flat bearing plates having opposed, flat linearly-grooved faces with a rolling ball assembly positioned therebetween to support relative movement between the plates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a side sectional schematic illustration of a prior art opposed piston engine. 
           [0017]      FIG. 2  is a side view of a piston rolling thrust bearing assembly installed in a piston. 
           [0018]      FIG. 3  is a cross sectional view of a first piston rolling thrust bearing embodiment which illustrates how the thrust bearing operates as a virtual wrist pin when mounted in a piston. 
           [0019]      FIG. 4  is a side view of the first piston rolling thrust bearing embodiment installed in a piston. 
           [0020]      FIG. 5  is a side view of the first piston rolling thrust bearing embodiment installed in a piston and attached to the upper end of a rocker arm. 
           [0021]      FIG. 6A  is a cross sectional view of a piston showing the first piston rolling thrust bearing embodiment when the piston is at TDC (top dead center) and BDC (bottom dead center) positions. 
           [0022]      FIG. 6B  is a cross sectional view of the piston showing the first piston rolling thrust bearing embodiment when the piston is at 90° and 270° positions. 
           [0023]      FIGS. 7A through 7F  depict assembly of the first piston rolling thrust bearing embodiment. 
           [0024]      FIG. 8  is an enlarged plan view of the curved, linearly-grooved face of a flexible bearing plate. 
           [0025]      FIG. 9  is a side view of a second piston rolling thrust bearing embodiment installed in a piston and attached to a connecting linkage. 
           [0026]      FIGS. 10A-10F  depict assembly of the second piston rolling thrust bearing embodiment. 
           [0027]      FIG. 11  is an exploded view of a reverse-thrust bearing assembly in the second piston rolling thrust bearing embodiment. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    The piston rolling thrust bearing embodiments illustrated and described herein all incorporate a pair of spaced-apart bearing plates with a rolling ball assembly positioned therebetween that supports relative movement between the plates. The piston rolling thrust bearings are adapted to be mounted to a piston at the open end of the piston skirt. A rear side of a piston rolling thrust bearing includes a yoke for attaching the piston rolling thrust bearing to connecting linkage that connects the piston to at least one crankshaft. Although specific piston rolling thrust bearing embodiments with curved and planar bearing plate faces are disclosed, these are not intended to limit the application of the invention just to two bearing plate constructions. 
         [0029]    This specification and the accompanying drawings are directed to constructions for piston rolling thrust bearings that connect pistons to rocker arms, connecting rods, and other connecting linkage elements. In this regard, with reference to  FIG. 2 , a piston  10  with a crown  11  and a skirt  12  has a piston rolling thrust bearing  14  mounted to the open end of the skirt. A yoke  16  secured to the rolling thrust bearing provides an attachment point for connecting linkage. With reference to  FIG. 5 , the yoke  16  provides a bearing structure where a connecting linkage can be coupled to the piston by means of a bearing (or gudgeon) pin  19 . 
       First Embodiment of a Piston Rolling Thrust Bearing 
       [0030]    The conventional wrist pin bearing of four-stroke engines is typically a plain bearing that relies on hydrodynamic and squeeze film effects to prevent metal-to-metal contact. In two-stroke engines, the bearing interface is often under a unidirectional load that does not support entrainment of lubrication oil into the interface to supply this separation. Therefore two-stroke engines typically use roller or needle bearings that do not require unloading for their operation. But these bearings are difficult to size small enough to fit within the piston and cylinder while still carrying the peak loads of such applications as internal combustion compression-ignition engines where 200 bar combustion pressures are not uncommon. By relocating the bearing interface far away from the wrist pin axis, even outside the piston skirt and cylinder envelope, much larger and more plentiful rolling elements can be applied to the bearing function. But simply moving the wrist pin outside the skirt would radically change the skirt loading such that it would appear as a torque on the piston with the skirt edges supporting the loads rather than the skirt surfaces. By using a bearing sector whose rollers and race surfaces reside remotely from the piston crown while still locating a virtual axis of rotation of the bearing near the back surface of the crown, the kinematics of the conventional wrist pin can be preserved while gaining a degree of freedom in using larger bearing elements for handling higher loads.  FIG. 3  is a schematic representation of a “virtual bearing” assembly  50  used to illustrate a first embodiment of the piston rolling thrust bearing. The virtual bearing assembly  50  occupies an arcuate sector  52  of a bearing very much larger than would fit within a piston  60 . It should be clear from this figure that the axis of rotation  54  of the virtual bearing wrist pin  56  is at a location very near the back surface  63  of the crown  61 , inside the skirt  62 . 
         [0031]      FIG. 4  shows the piston rolling thrust bearing assembly  100  fully assembled. The assembly includes a yoke  101  and an elongated bearing retainer mount  102  that supports the assembly  100 . A bearing plate  103  includes a concave, face  104  with a series of linear ball-race grooves that constrain roller bearing balls of a curved rolling ball assembly  105  to move on an arc about a piston thrust-bearing axis; these ball-race grooves  104   g  are clearly seen in  FIG. 7A . The rolling balls of the curved rolling ball assembly  105  are held captive by a curved metal or plastic cage  108 . The rolling balls of the rolling ball assembly  105  are further held captive by a bearing plate  106  having a convex face  107  in opposed alignment with the concave face  104 . A series of linear ball-race grooves that match the grooves of the bearing plate face  104  are formed in the convex face  107 ; these ball-race grooves  107   g  are clearly seen in  FIG. 7C . The opposing complementarily curved faces  104  and  107  and the ball-race grooves  104   g  and  107   g  constrain the rolling balls of the curved rolling ball assembly  105  to roll along an arc centered on the axis of the piston  200 . The ball-race grooves  104   g  and  107   g  with the curved rolling ball assembly  105  constitute a curved linear roller bearing. A disc-shaped backing plate  106 T is secured by threaded screws to the flat outer face of the bearing plate  106 . The circumference of the backing plate  106 T is threaded so as to seat the piston thrust bearing assembly  100  by engagement of threads  213  on the inner surface of the piston skirt  212 , near the open end of the skirt. The diameter of the bearing plate  106  is larger than the diameter of the backing plate  106 T so that as the backing plate engages the threads, the bearing plate is secured against motion by an annular ridge  214  formed in the bore of the piston skirt  212 . A hydrodynamic retainer bearing  109  secured to the end of the elongated bearing retainer mount holds the piston rolling thrust bearing assembly  100  together. 
         [0032]    As per  FIG. 4 , the piston thrust bearing assembly  100  is located remotely from the piston crown  211 ; it represents a virtual wrist pin whose axis of rotation  54  is located within the piston skirt  212 , near the back surface of the crown  211 . As per  FIG. 5 , presume that the piston  200  is one of a pair of opposed pistons disposed in a cylinder  400  of an opposed piston engine. The piston  200  is disposed in the exhaust end of the cylinder  400  where an exhaust port  402  is formed. The engine includes a connecting linkage to connect the piston  200  to a crankshaft. The connecting linkage includes a connecting rod  410  and a rocker arm  430 . One end of the connecting rod  410  is attached to a crankshaft  450  and the opposite end to the rocker arm  430 . The upper end  433  of the rocker arm  430  is coupled to the piston thrust bearing assembly  100  by the pin  19 . The piston  200  reciprocates in the bore of the cylinder  400  between top dead center (TDC) and bottom dead center (BDC) positions. The piston  200  is shown at or near BDC. Preferably, when the piston  200  is at this position, the axis  54  of the virtual wrist pin bearing intersects the axis of the cylinder  400  at a position between the exhaust port  402  and the corresponding cylinder end  403 . 
         [0033]    As per  FIG. 6A , when the piston  200  passes through TDC (near where combustion forces are at a maximum) and BDC (near where inertial forces are at a maximum), the piston rolling thrust bearing  100  is in straight alignment, (rotated 0°), with reference to the piston axis which perpendicularly intersects the virtual axis of rotation of the virtual wrist pin. As per  FIG. 6B , when the piston  200  moves from TDC or BDC the rocker arm  430  (best seen in  FIG. 5 ) urges the piston rolling thrust bearing  100  to pivot from the 0° centered position on an arcuate path towards a displaced angle. At crank angles of 90° and 270°, the axis  54  of the virtual wrist pin bearing is at the same point in the piston  200 , but the piston rolling thrust bearing  100  has rotated off the axis of the piston. However, because of the rolling bearing structure, all forces acting upon the piston  200  are transmitted to the axis  54  of the virtual wrist pin bearing. In this regard, the yoke  101 , elongated retainer mount  102 , bearing plate  103 , and curved rolling ball assembly  105  all rotate while the bearing plate  106  remains secured to the bore of the piston skirt  212 . Therefore, with reference to  FIG. 5 , side forces that otherwise would be imparted directly to the piston skirt  212  by movement of the upper end  433  of the rocker arm  430  are directed to the axis  54  of the virtual wrist pin bearing by the moving parts of the rolling thrust bearing  100 . 
         [0034]      FIGS. 7A-7F  illustrate the assembly of the piston rolling thrust bearing assembly  100 . In  FIG. 7A  an elongate bearing retainer mount  102  is formed on or secured to a yoke  101 . The first bearing plate  103  with concave face  104  is received on the retainer mount  102  and secured to the yoke  101  with the concave face  104  facing the end  112  of the retainer mount. The concave face  104  has formed in it a set of elongate spaced ball-race grooves  104   g  and a central slot through which the retainer mount  102  extends.  FIG. 7B  shows the curved rolling ball assembly  105  received on the retainer mount  102 , against the concave face  104 , with the rolling balls oriented in place by the ball-race grooves  104   g.  As per  FIG. 7C , the second bearing plate  106  has a convex face  107  oriented to oppose the concave face  104  of the first bearing plate  103 . The concave face  107  has formed in it a set of elongate linear ball-race grooves  107   g  and a central slot through which the retainer mount  102  extends. The backing plate  106 T is secured to a flat outer surface of the second bearing plate  106 . As per  FIG. 7D , the second bearing plate  106 , with the backing plate  106 T secured thereto, is received on the elongate bearing retainer mount  102 . The first and second bearing plates  103  and  106  are mutually oriented with the sets of ball-race grooves in the concave and convex faces  104  and  107  in opposing alignment, and the backing plate  106 T facing the end  112  of the retainer mount  102 . The opposed sets of ball-race grooves constrain the rolling balls for rolling movement in an arc centered on the axis of the piston to which the rolling thrust bearing assembly is mounted. As per  FIG. 7E , the piston rolling thrust bearing assembly  100  is held together by a hydrodynamic retainer bearing  109  which is secured to the end  112  of the elongate bearing retainer mount  102 , for example, by screws. The hydrodynamic retainer bearing  109  holds the piston rolling thrust bearing assembly  100  together under reverse inertial load to keep the rolling balls of the rolling ball assembly  105  loaded. A curved surface  120  (best seen in  FIG. 7D ) formed in the outer face of the backing plate  106 T conforms to the bearing surface of the retainer bearing  109 , allowing the retainer bearing to slide against the backing plate  106 T. As per  FIG. 7F , when assembled as illustrated in  FIG. 7E , the rolling thrust bearing assembly  100  is secured to the piston  200  by engagement of the threaded portion of the backing plate  106 T with the inner surface of the piston skirt  212 , with the concave face  104  of the first bearing plate  103  facing toward the interior of the skirt. Although the backing plate  106 T is configured as a disc, this bearing seating construction is not meant to be limiting. In another bearing seating construction, the seating element can be configured as a truncated cone with a wide end attached to the outer face of the bearing plate  106  and a narrow end threaded on a post fixed to the back surface of the piston crown. 
         [0035]      FIG. 8  shows details not seen in  FIG. 7C  of a preferred construction of the second bearing plate  106 . In  FIG. 8  roller balls of the rolling ball assembly  105  are engaged in the set of elongate linear ball-race grooves  107   g  formed in the concave face  107  of the second bearing plate  106 . Standard manufacturing processes introduce tolerances in the dimensions of the roller balls, the grooves, and the second bearing plate that can cause loss of contact between roller balls and grooves due to uneven loading of the roller balls. Accordingly, elongate linear slits  190  and  191  through the second bearing plate  106  are provided in interleaved radial patterns in order to permit to bearing plate to flex so as to ensure that all of the roller balls remain in contact with the grooves. The slits  190  extend outwardly from a central opening of the bearing plate  106  toward the circumferential periphery of the bearing plate  106 , at least partly across outer grooves. The slits  191  extend inwardly from the circumferential periphery of the bearing plate  106 . The interleaved patterns of slits define zones of the bearing plate  106  that can flex independently of each other in response to pressure of the roller balls. In order to accommodate flexion of the bearing plate zones, a shallow depression can be formed in the surface of the backing plate  106 T that faces the bearing plate  106 . 
       Second Embodiment of a Piston Rolling Thrust Bearing 
       [0036]      FIGS. 9-11  illustrate a second embodiment of a rolling thrust bearing assembly  300 . In  FIG. 9 , the piston rolling thrust bearing assembly  300  is shown attached to a piston  200 . The assembly  300  includes a wrist pin-bearing frame  301  that includes one half of a wrist pin-bearing support, an end plate and a yoke, all identified as  301  in this figure. The frame  301  includes an end plate  301   a.  A first flat bearing plate  302  with a linearly-grooved planar face  303  is secured to the end plate  301   a , with the planar face  303  facing the interior of the piston skirt  212 . A flat rolling ball assembly  304 , including a set of rolling balls  305 , is sandwiched between the first flat bearing plate  302  and a second flat bearing plate  306  having a linearly-grooved planar face  307  opposing the planar face  303 . The planar faces  303  and  307  retain the rolling ball assembly  304  for rolling movement along a rolling track that perpendicularly intersects the piston axis. A thrust bearing retainer assembly  312  is mounted onto a thrust bearing retainer mount  301   m  to secure the entire rolling thrust bearing assembly  300  during thrust reversal. 
         [0037]    Assembly of the rolling thrust bearing assembly  300  is shown in  FIGS. 10A-10E . In  FIG. 10A , an elongate bearing retainer mount  301   m  formed on or secured to the end plate  301   a  receives the first flat bearing plate  302 , which is secured to the end plate  301   a  with the linearly-grooved planar face  303  facing outwardly. The ball-race grooves  303   g  formed in the face  303  are visible in this figure. In  FIG. 10B , the flat rolling ball assembly  304  is received on the bearing mount  301   m , with the rolling balls  305  aligned with the grooves  303   g  in the planar face  303 . As per  FIG. 10C , ball-race grooves  307   g  formed in the linearly-grooved planar face  307  of the second bearing plate  306 . As per  FIG. 10D , the second bearing plate  306  is received on the bearing mount  301   m,  so that the grooves in the planar face  307  are aligned with the rolling balls  305 . As per  FIG. 10E , the thrust bearing retainer assembly  312  is received on the end of the bearing mount  301   m  to secure the entire flat-thrust bearing assembly during thrust reversal. The bearing retainer assembly  312  is seated in the groove  317  (best seen in  FIG. 10D ) and thereby secured on the bearing mount  301   m.  As per  FIG. 10F , when assembled as illustrated in  FIG. 10E , the rolling thrust bearing assembly  300  is secured to the piston  212  by, for example, engagement between threads on the periphery of the second bearing plate and the inner surface of the piston skirt. 
         [0038]      FIG. 11  is an exploded view of the thrust bearing retainer assembly  312 . The bearing retainer assembly includes a Belleville spring washer  309  sandwiched between two end plates  308  and  310 . This spring washer  309  assures that constant pressure is exerted in opposing directions when the thrust bearing retainer assembly  312  is secured to the bearing retainer mount  301   m . A retainer thrust bearing  313  provides an interface between the flat surface of the second bearing plate  306  and the flat surface of the end plate  308  to compensate for any up or down motion at the bearing retainer mount  301   m . The bearing retainer assembly  312  is secured to the bearing retainer mount  301   m  by seating split ring retainers  311  in the groove  317  on the end of the bearing retainer mount  301   m  ( FIG. 10D ), where the Belleville spring washer  309  is depressed so as to maintain pressure against the retainers  311 . 
       Piston Cooling Provisions 
       [0039]    The rolling thrust bearing assemblies described and illustrated herein may be utilized in an opposed-piston configuration wherein the pistons are cooled by application of a liquid coolant such as lubricating oil. In these configurations, the rolling bearing assembly constructions are adapted to accommodate the delivery of liquid coolant into the pistons and to enable outflow of the liquid coolant along the inside surfaces of the pistons. In this regard, the opposed-piston engines include elongated jets that extend through the open ends of the skirts and spray the liquid coolant into the pistons to cool the crowns. As  FIGS. 7A and 9A  show, the elongate bearing retainer mounts  102  and  301   m  are tubular with central bores  325  and  326  through which the jets extend. The central bores  325  and  326  of the retainer mounts  102  and  301   m  are wide enough to accommodate the jets while the pistons move between TDC and BDC. Liquid coolant exits the pistons by flowing along the inside surfaces of the pistons, and passing the rolling bearing assemblies via grooves on the inside surfaces and notches in the edges of the second bearing plates. For the first embodiment rolling thrust bearing, the notches  130  in the second bearing plate  106  are best seen in  FIG. 7C ; for the second embodiment rolling thrust bearing, the notches  330  in the second bearing plate  306  are best seen in  FIG. 10C . In  FIG. 6B , grooves on the inside surface of the piston skirt  212  are indicated by reference numeral  230 . 
         [0040]    The scope of patent protection afforded the novel constructions described and illustrated herein may be practiced in the absence of any element which is not specifically disclosed in the specification, illustrated in the drawings, and/or exemplified in the embodiments of this application. Moreover, although the invention has been described with reference to preferred embodiments, it should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.