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FIELD OF THE INVENTION 
     The present invention relates to a bearing assembly for wellbore drilling and, in particular, an oil-sealed bearing assembly for use in a wellbore drilling operation. 
     BACKGROUND OF THE INVENTION 
     In the drilling of bore holes into the earth, as in the case of drilling oil and/or gas wells, it is common to drive the drill bit by a downhole mud motor located at the end of a drill string. In particular, drilling fluid, generally referred to as drill mud, is circulated to drive the motor by positive hydraulic displacement or turbine action. The mud then passes through the ports in the drill bit and carries material loosed by the drill bit back to the surface through the annular space between the drill pipe and the resulting bore hole. 
     Bearing assemblies for wellbore drilling are mounted between the drill bit and the drill string to permit rotation of the drill bit. The drill bit is attached to a hollow drive shaft, also known as a mandrel that is located within a bearing housing. The mandrel is rotatably driven by the mud motor while the bearing housing is fixed to the drill string and remains relatively stationary. In its position behind the drill bit, the bearing assembly is subject to significant radial and axial loading. Radial and thrust bearings are thus located along the bearing assembly to react (absorb) radial and axial loads. 
     Lubrication between the rotator mandrel and stator housing may be achieved by oil or mud located in the annular space between those components. In the case of oil lubrication, an oil-sealed bearing chamber is formed by seals such as PolyPak® or KALSI SEAL®. The seals are acted upon by downhole drilling fluid pressures, including pump pressures and hydrostatic pressures, resulting in higher pressures above the sealed bearing chamber as compared to below the sealed bearing chamber. Such pressure differential results in damage to the seals, leading ultimately to seal failure. To reduce the pressure differential, a flow restrictor located above the sealed chamber is used to reduce the fluid flow in the annular passageway between the mandrel and housing. 
     In mud lubricated bearing assemblies, it is desirable to reduce the fluid flow through he bearings to prevent premature wear due to the effects of high drilling fluid pressures, as well as the abrasive actions of the mud itself. In this case, radial bearings serve as flow restrictors. 
     Despite the use of flow restrictors/radial bearings to increase the durability of the bearing assembly, the flow restrictors/radial bearings are themselves limited by their ability to withstand damage resulting from lateral and radial loads incurred during drilling. Carbide-containing flow restrictors are very expensive and damage thereto is of great concern. 
     The mandrel component of the bearing assembly is also susceptible to damage by drilling loads, as well as by the severe shock and vibration incurred during drilling applications. In particular, the mandrel is engaged to the housing by a split ring, also called a saver ring. The split ring includes two semi-cylindrical halves having annular grooves in their inner surfaces. The machined grooves engage into annular recesses formed on the surface of the mandrel. During assembly, the halves of the split ring are fit over the mandrel. This form of assembly requires that the fit between the mandrel and the split ring to be somewhat loose. This loose fit permits some vibration between the mandrel and the split ring, thereby causing mandrel failure by cracking. 
     There is a need, therefore, for improved construction of bearing assemblies which provides for a longer operational life of the assembly over current constructions. 
     SUMMARY OF THE INVENTION 
     A bearing assembly for wellbore drilling has been invented. In one embodiment, the bearing assembly has an extended life without repair by providing support for the flow restrictor/radial bearing (which will be termed herein, the flow restrictor). In another embodiment, a compression nut is threadedly engaged onto the mandrel to replace the split ring. 
     In accordance with a broad aspect of the present invention there is a bearing assembly comprising: a first elongate member adapted for connection to a drill bit assembly; a second elongate member adapted for connection to a drill string, the first elongate member and the second elongate member being telescopically disposed and being rotatable about a long axis of the bearing assembly one relative to the other; a first bearing disposed between the first elongate member and the second elongate member; a flow restrictor positioned between the first elongate member and the second elongate member to restrict flow between the members; and a second bearing adjacent the flow restrictor and on a side of the flow restrictor opposite the first bearing. 
     In accordance with another broad aspect of the present invention there is a bearing assembly for use in a drill string to accommodate rotation of the drill bit comprising a first elongate member and a second elongate member, the second elongate member, being telescopically disposed over the first elongate member and the first elongate member being rotatable within the second member and about the long axis of the tool; a flow restrictor disposed between the first elongate member and the second elongate member to restrict fluid flow therebetween; a bearing on each side of the flow restrictor, the bearing assemblies being disposed to act between the first elongate member and the second elongate member. 
     The first elongate member and the second elongate member can be the mandrel and outer housing of the bearing assembly, respectively. 
     The bearings are preferably oil-lubricated requiring sealed oil chambers. A sealed, lubricated bearing offers an extended useful life over a mud-lubricated bearing. In a bearing assembly having lubricant-containing sealed bearings, preferably means are provided to balance the seals of the chamber. In one embodiment, at least one fluid flow channel is provided past the upper bearing assembly to provide for balancing the pressures at the seals of the lubricant-filled chamber. In another embodiment, openings are provided through the housing such that fluids at external pressure are in contact with chamber seals. 
     Preferably, the bearing assembly includes radial and thrust bearings. In one preferred embodiment the thrust bearings include on-bottom bearings for accommodating load in one direction and off-bottom bearings for accommodating load in an opposite direction to the on-bottom bearings. These thrust bearings are positioned on the bearing assembly to be adjusted simultaneously. Preferably, these thrust bearings are contained within the same lubricant-filled chamber. 
     In a preferred bearing assembly, there are at least three radial bearings spaced along the bearing assembly with, for example, two on one side of the flow restrictor and one on the other side. Preferably, greater radial support is provided between the flow restrictor and the bit end of the bearing assembly where greater lateral and sideways bending loads are experienced. Preferably radial bearings are positioned on either side of the flow restrictor and on either side of the thrust bearings. All radial bearings are preferably selected to maintain the mandrel concentric within the bore of the housing. 
     In accordance with another aspect of the present invention, there is provided a bearing assembly for use in a drill string to accommodate rotation of the drill bit comprising a first elongate member and a second elongate member, the second elongate member being telescopically disposed over and rotatable about the first elongate member; a flow restrictor disposed between the first elongate member and the second elongate member to restrict fluid flow therebetween; at least one bearing disposed to act between the first elongate member and the second elongate member and a compression nut threadably engaged to the first elongate member and disposed between the first elongate member and the second elongate member, the compression nut abutting against a shoulder on the inner diameter of the second elongate member and supporting the at least one bearing. 
     Preferably, the bearing is a thrust bearing. In one embodiment, the compression nut is positioned to maintain the second elongate member in telescopic arrangement over the first elongate member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the orientation of FIGS. 1A to  1 D. 
     FIGS. 1A to  1 D are together a sectional view along the axis of a bearing assembly according to the present invention. 
     FIG. 2 is a section through an upper bearing along line  2 — 2  of FIG. 1B with the mandrel, Du bearing and floating piston removed. 
     FIG. 3 is a partial sectional view of another bearing assembly according to the present invention. 
     FIG. 4 is an enlarged view of a bearing having mounted thereon a compression nut. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the figures, a bearing assembly  10  is shown. Bearing assembly  10  includes a mandrel  12 , a mandrel adapter  13  secured by a threaded connection to the mandrel and a housing  14 . Housing  14  is telescopically disposed and rotatable about mandrel  12 . Box end  16  of mandrel  12  is adapted for connection directly or indirectly to a drill bit (not shown). Mandrel adapter  13  is adapted for connection to the power section of the mud motor. Holes  13   a  engage the motor drive shaft (not shown). Open end  17  of housing  14  is adapted for connection into a drillstring of tubulars (not shown). 
     In operation, bearing assembly  10  is connected between a drill bit and a drill string of tubulars. Generally, drilling fluid is pumped through the drill string into bore  18  of the housing. Thereafter, the fluid passes into the inner bore  19  of mandrel adapter and then into the inner bore  19   a  of the mandrel. This fluid then passes out through the ports in the drill bit and back up the outside of the housing on its way back to surface. Some of the fluid also passes through the annular space  42  between the mandrel and the housing. 
     The drilling fluid is under pressure as it passes through this route. In particular, in the bores of the housing and the mandrel, fluid is pressurized by hydrostatic pressure as well as pump pressure. Once the fluid passes through the drill bit ports, any pump pressure is dissipated leaving only hydrostatic pressure acting on the drilling fluid. Thus, generally, the fluid inside the bearing assembly is at a greater pressure than the fluid outside the bearing assembly. 
     A lubricant-filled bearing chamber  20  is disposed between mandrel  12  and housing  14  to support rotation of the mandrel relative to the housing. The lubricant is preferably oil but can be other substitutes such as silicone, grease, etc. Bearing chamber  20  is filled with oil through fill ports  21  that are sealed off before use by metal threaded plugs or welded caps. Bearing chamber  20  is sealed by upper sealing elements  22   a ,  22   a ′ on a floating piston  23  and a lower sealing element  22   b . These seals maintain the oil within the chamber about the bearing members contained therein. The sealing elements are for example O-rings or pressure deformable seals such as PolyPac or Kalsi seals. Sealing element  22   a  seals between piston  23  and mandrel  12 . Sealing elements  22   a ′ seal between piston  23  and housing  14 . Piston  23  is movable axially through the annular space between the housing and the mandrel to permit expansion and contraction in the chamber volume, as caused by change in external pressure and temperature. A plated surface or a piston housing sleeve  24 , for example formed of chromium iron, is secured to the surface of the housing adjacent piston  23  to provide a smooth durable surface over which the piston can move. 
     The seals  22   a ,  22   a ′ and  22   b  at either end of bearing chamber  20  are pressure balanced to improve bearing operation and useful life. In particular, sealing element  22   b  is exposed to external pressure and openings  25  are formed through housing  14  to permit communication of fluids at external pressure to sealing elements  22   a  and  22   a ′. Bearing chamber  20  is positioned between a flow restrictor  26  and the box end  16  of the mandrel. 
     Flow restrictor  26  includes a stationary flow restrictor  27   a  secured within the housing and a rotatable flow restrictor  27   b  on the shaft. A stopper ring  29  supports and retains the rotatable flow restrictor on the shaft. A suitable flow restrictor is one adapted to lose no more than 10% pumping pressure, although other flow restrictors could be used, as desired. Flow restrictor  26  restricts fluid flow therepast in the annular space between the mandrel and the housing. This reduction in flow effectively reduces the differential pressure of the fluid that comes into contact with upper seals  22   a  and  22   a ′ of bearing chamber  20 . In particular, fluid jetting against floating piston  23  is substantially eliminated. Thus, flow restrictor  26  tends to substantially equalize the pressures acting against seals  22   a ,  22   a ′ and seal  22   b . This increases the useful life of bearing chamber  20 . 
     Bearing chamber  20  contains two radial bearing surfaces  46 ,  48  and axial thrust bearings  49   a ,  49   b . Radial bearings  46 ,  48  are positioned on either side of the thrust bearings to provide lateral support for them. As will be appreciated, all of these bearings need not be contained in the same oil-filled chamber. 
     A compression nut  50  is mounted on mandrel  12  and is positioned within bearing chamber  20 . Compression nut  50  includes inner facing threads  51  for threaded engagement with a threaded portion  52  on mandrel  12 . A shoulder  54  is formed on mandrel  12  against which compression nut  50  is torqued. Compression nut  50  is threaded onto mandrel with substantially zero tolerance such that once the compression nut is torqued onto the threaded portion of the mandrel and locked against shoulder  54 , the threads are in tension and load is uniformly distributed along the interface of the threads. Any force applied to the compression nut is directly transmitted into the mandrel without vibration therebetween. Preferably the threaded engagement is through a left-hand thread, when viewed from the top. A left hand thread provides that during use, the rotation of the drill bit and circulation of the drilling fluid will cause the compression nut to be constantly torqued up on the mandrel. 
     The compression nut when torqued onto the mandrel acts as a part of the mandrel and increases the life span and axial thrust loading capacity of the mandrel. Thrust loading distribution on the bearing mandrel is reduced and the formation of cracks in the mandrel about the nut is prevented. The compression nut acts as a thrust bearing race providing a strong and stable support for dissipation of axial loads from the thrust bearings on either side of the nut including the on bottom  49   a  and off bottom  49   b  bearing stacks. As such, compression nut  50  provides true axial contact for the thrust bearings eliminating transversal movement and vibration between the nut and the bearing stacks and the mandrel. The compression nut also serves to effectively enlarge the outer diameter of the mandrel such that it is retained in the housing by abutment against shoulder  54  of housing. 
     On bottom bearing stack  49   a  is adjacent off bottom  49   b  stack. As such, these bearing stacks beneficially operate under similar pressure conditions. As will be appreciated, when one of these thrust bearings is under load preferably the other is totally free. This arrangement is termed endplay. Adjustment is necessary to accomplish endplay. By positioning the on bottom and off bottom thrust bearings in adjacent position, adjustment of endplay is facilitated. In particular, the width of setting shim  53  is selected and positioned between off bottom bearings  49   b  and a shoulder on housing  14  to control the space in which bearings act. Selection of shim  53  controls endplay for both thrust bearings  49   a ,  49   b  simultaneously. 
     According to the present invention, the bearing assembly includes a second bearing section  28  positioned between flow restrictor  26  and end  17  of housing  14 . In general, bearing section  28  provides support for radial and end loading of the bearing assembly in drilling applications using drilling mud motors. Bearing  28  also protects the flow restrictors from sideways and bending loads. The components of flow restrictors are very brittle and subject to cracking and failure when subject to such forces. Bearing  28  holds concentric the flow restrictors about the mandrel thereby increasing their useful life. 
     Bearing  28  can be any radial bearing suitable for downhole use such as for example, a mud lubricated bearing or an oil lubricated bearing. Because of the longer useful life of oil-filled bearings in most downhole conditions, oil-filled bearings are preferred. In one embodiment as shown in FIG. 3, the bearing is a roller bearing  29  in an oil-filled chamber. In the embodiment illustrated in FIGS. 1, a du bearing  30  is used. Du bearing  30  is positioned in a chamber  32  filled with oil through fill ports  34 . Du bearing  30  includes a housing  35  positioned in the annulus between housing  14  and mandrel  12 . Housing  35  includes longitudally extending channels  37  that permit lubricant flow along the radial bearing surface. The chamber is sealed by upper seals  36   a  and lower seals  36   b . Upper seals  36   a  are carried on a floating piston  38  that moves depending on the fluid pressures and temperatures in which the bearing assembly is operating. 
     To equalize the pressures acting on upper seals  36   a  and lower seals  36   b , and thereby to prevent damage to the seals such as by extrusion thereof, channels  40  are provided for flow of drilling fluid past bearing section  28 . Channels  40  are formed in the outer surface of housing  35  and extend along its length. Thus, chambers  42 ,  44 , above and below, respectively, bearing  28  are maintained at substantially equal pressures. While eight channels are shown in FIG. 2, other numbers of channels can be used as desired. 
     Channels  40  by pass the bearing chamber but do not affect the concentricity of the mandrel within the housing inner bore. In particular, the channels are formed in a spaced apart manner on housing  35  maintaining a maximum outer diameter approximately equal to that of the inner diameter of the housing  14 . 
     Since turbulence in the drilling fluid can damage the bearing assembly, preferably the channels are formed to minimize turbulence in the drilling fluid passing therethrough. In one embodiment, to reduce the formation of turbulence the channels are preferably oriented substantially parallel to the long axis  10 x of the bearing assembly. By forming substantially axially oriented channels past the second bearing, axial flow of the drilling fluid is maintained. For use in shallow wells, where pressures of drilling fluid are not so great, these channels can be omitted, if desired. 
     To facilitate assembly the housing and mandrel can be formed in separate parts that are secured together during assembly. 
     Although preferred embodiments of the present invention have been described in some detail hereinabove, those skilled in the art will recognize that various substitutions and modifications may be made to the invention without departing from the scope and spirit of the appended claims.

Summary:
A bearing assembly is disclosed for use with a mud motor. The bearing assembly includes at least three radial bearings to protect the flow restrictor of the bearing assembly against harmful sideways and lateral loading. The bearing assembly also includes a compression nut to eliminate cracks on the mandrel at the split ring.