BACKING RING FOR SMOOTH BEARING DESCRIPTION OF THE INVENTION This invention relates to plain bearings for sealed shafts and more particularly to an improved bearing assembly, which provides a seal or sealing stop between a shaft shoulder and a backup ring. Roller bearing assemblies are known which incorporate two rows of tapered roller bearings pre-assembled into a complete pre-lubricated package for assembling on trunnions at the ends of shafts or shafts. The bearing assemblies are used as box bearings with rail assembled on trunnions at the ends of the box axes with rail. Bearings of this type typically employ two rows of tapered roller bearings each fitted within each end of a common bearing cup with their respective bearing cones having an internal diameter which is dimensioned to provide an interference fit with the bearing. axle stump and with a hub or cylindrical separator placed between the cones that provides a precise separation in the stump. The seals mounted within each end of the bearing cup provide a sealing contact with the bearing of the wear rings against the outer ends of the respective bearing cones at each end of the assembly. In a typical rail box installation, the axle journal is machined with a chamfer at the end towards the inside, and a backup ring having a surface complementary to the contour of the chamfer and a splice surface for coupling the end towards the inside of the internal wear ring that accurately places the bearing assembly on the stump. An end cap mounted to the end of the shaft by screws threaded into holes in the end of the shaft engages the wear ring on the outside and holds the entire assembly at the end of the shaft. Wear rings typically have an internal diameter that is dimensioned to provide an interference fit with the trunnion over at least a portion of its length so that the entire assembly is tightened as a unit at the end of the trunnion shaft. The bearing assembly is a roller bearing that includes a track or inner cone fitted around a portion of the axle or tree stump. The inner liner includes a rolling surface facing outwards. An external track or cup has an inwardly directed bearing surface. The roller elements are located between and in contact with the internal and external bearing surfaces. A backup ring has a contoured surface complementary to and engaging with the contoured surface of a chamfer formed in the shaft. The chamfer leads from the stump to the shoulder of the tree. The contoured surfaces cooperate to secure the backup ring against axial movement along the shaft. An annular wear ring is interposed between and engages with the inner race and the backup ring. In one embodiment, the backup ring includes a shoulder that receives a corner of the wear ring. An annular bead is interposed between and engages with the tree shoulder and the backup ring. In another embodiment, a first ring of sealing paste is interposed between and engages with the tree shoulder and the backup ring. A second ring of sealing paste is interposed between and engages the wear ring and the backup ring. The sealing bead in the sealing rings forms a seal to prevent water, abrasives and other contaminants from entering the sealed bearing from near the tree and between the tree and the backup ring. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a sectional view of a tree stump having mounted thereon a tapered roller bearing assembly according to the present invention; Figure 2 is a sectional view of a tree stump having mounted thereon a tapered roller bearing assembly according to a second embodiment of the present invention. Referring now to Figure 1 of the drawings, a bearing assembly indicated generally by the reference numeral 10 in Figure 1 is shown mounted on a journal 12 at the free cantilever end of a shaft or shaft 14, typically an axle. of box with rail. The stump 12 is machined in very restricted tolerances and terminates at its inner end in a contoured chamfer 22 leading to a cylindrical shoulder 18 of the shaft 14. At the free end of the shaft, the stump portion 12 terminates in a guide portion 24 slightly conical or tapered that is dimensioned to facilitate the installation of the bearing assembly in the stump. A plurality of threaded holes 26 is formed at the end of the shaft 14 to receive cap bolts or threaded bolts 28, to mount a bearing retainer cap 30 at the end of the shaft to hold the bearing in position as described more fully in the present below. The bearing assembly 10 is pre-assembled before it is assembled and held on the journal 12 by the cap 30. The bearing assembly includes a unitary bearing cup or external bearing surface 32 having a pair of bearing surfaces 34, 36 formed at each end adjacent thereto cooperating with a pair of bearing cones 38, 40, respectively, to support the two rows of tapered rolls 42, 44, respectively, between them. A central spacer 46 is placed between the cones 38, 40 to hold the cones in a precisely spaced relationship with each other. The bearing cup 32 is provided with recessed holes 48, 50 cylindrical at their opposite ends on the outside of the bearing surfaces 34, 36, and a pair of seal assemblies 52, 54 are pressed into each of the holes reaming 48, 50 cylindrical. The seals 52, 54 include elastomeric sealing paste members 56, 58, respectively, which are rubbed on and form a seal with a pair of seal wear ring bushings 60, 62, respectively, having their ends directed inwardly. in engagement with the ends directed outwards from the bearing cones 38, 40, respectively. The other end of the wear ring bushing 60 is received in a recessed hole 64 cylindrical at the end directed axially outwardly of an annular backup ring 66 which, in turn, has a recessed hole 68 at its other end which is dimensioned to receive within it an interference and non-interference ratio in the cylindrical shoulder 18 of the shaft 14. The counterbore 64 and the outer diameter of the wear ring bushing 60 are also sized to provide an interference fit so that the ring of wear is constricted within the backing ring 66 which is machined precisely to provide a contoured internal surface 70 which is complementary to and which engages with the contour of the chamfer 116 when the bearing is mounted on the shaft. The outwardly directed end of the bushing 62 bears against the retaining cap 30. The obturator bead 75 is observed to be located between the backing ring 66 and the shoulder portion of the chamfer 22. The obturator bead 75 is very often a silicone seal such as Dow Corning 737. The plug 74 can be formed by applying a seal. coating of paste to seal at the intersection of the chamfer 22 and the counterbore 168 and then after a pressure adjustment of the backing ring 66, a seal is formed. The sealing bead 75 forms a seal for water and abrasives that can enter the bearing 10 sealed along the outer edge of the shaft 14. Referring now to Figure 2 of the drawings, a bearing assembly indicated generally by the number Reference 110 in Figure 2 is shown mounted on a journal 112 at the cantilevered free end of a shaft or shaft 114, typically a box axis with rail. The stump 12 is machined in very restricted tolerances and terminates at its inner end in a contoured chamfer 122 leading to a cylindrical shoulder of the shaft 114. At the free end of the shaft, the stump portion 112 terminates at a slightly guiding portion 124. tapered or tapered to be dimensioned to facilitate the installation of the bearing assembly in the stump. A plurality of threaded holes 126 is formed at the end of shaft 112 to receive cap bolts or threaded pins 128 to mount a bearing retainer cap 130 at the end of the shaft to hold the bearing in position as described more fully in the present below. The bearing assembly 110 is pre-assembled before it is assembled and held in the journal 112 by the cap 130. The bearing assembly includes a unitary bearing cup or external bearing surface 132 having a pair of bearing surfaces 134, 136 formed at each end adjacent thereto cooperating with a pair of bearing cones 138, 140, respectively, to support the two rows of tapered rollers 142, 144, respectively, between them. A central separator 146 is placed between the cones 138, 140 to maintain the cones in a precisely spaced relationship with each other. The bearing cup 132 is provided with recessed holes 148, 150 cylindrical at its opposite ends on the outside of the bearing surfaces 134, 136, and a pair of seal assemblies 152, 154 are pressed into each of the holes reaming 148, 150 cylindrical. Seals 152, 154 include elastomeric sealing paste members 156, 158, respectively, which are rubbed on and form a seal with a pair of seal wear ring bushings 160, 162, respectively, having their ends directed inwardly. in engagement with the ends directed outwardly from the bearing cones 138, 140, respectively. The directed end of an annular backup ring 66 which, in turn, has a counterbore hole 168 at its other end that is dimensioned to be received in an interference relationship or non-interference in the cylindrical shoulder 118 of the shaft 114. The reamed hole 164 and the outer diameter of the wear ring bushing 160 are also sized to provide an interference fit so that the wear ring is pressed into the. backing ring 166 that is machined accurately to provide a contoured internal surface 170 that is complementary to and engages the contour of the chamfer 116 when the bearing is mounted on the shaft. The outwardly directed end of the bushing 162 bears against the retaining cap 30. A first sealing ring or seal 175 is observed to be located between the backing ring 166 and the shoulder portion of the chamfer 122. A second sealing ring or seal 177 is observed to be located between the corner of the ring. 160 of wear and the shoulder 164 of the backup ring 166. The first ring 175 of sealing paste and the second ring 177 of sealing paste usually consist of a nitrile rubber or other suitable rubber or a synthetic rubber. Seal rings 175 and 177 form a seal for water and abrasives that can enter sealed bearing 110 along the outer edge of shaft 114.