Abstract:
A bearing structure including first and second tapered bearing surfaces for accommodating on-bottom and off-bottom thrust and radial forces on a drive shaft in a down hole drilling environment and further including a radial bearing mounted between the first and second tapered bearings for restricting bending of the drive shaft.

Description:
The subject invention pertains generally to drilling tools and more particularly to a bearing assembly for accomodating the forces generated in a down hole motor used for example, in the oil drilling arts. 
     The design of bearing assemblies which will withstand the hostile environments and force loads typically encountered in down hole well drilling present a continuing challenge to the industry. The extremely high temperatures encountered preclude the successful application of sealed bearing technology as it currently exists. Thus, the prior art has turned to bearing structures exposed to the drilling fluids passing through the motor. 
     In the prior art, a pair of bearings have typically been provided to accomodate the drive shaft and attendant forces thereon. The bearings have been separated such that one bearing accomodates radial forces only and the second accomodates thrust loads only. Typical bearing materials have included poly-crystalline diamonds, ceramics, and tungsten carbide matrix. 
     It has appeared desirable to the inventor to improve over the prior art bearing structures in order to extend the lifetime of bearings in such apparatus. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an improved bearing structure. 
     It is another object of the invention to provide a bearing structure capable of handling both the radial and thrust loads created in a down hole motor. 
     It is yet another object of the invention to provide a bearing assembly with a longer life than those currently available in the state of the art. 
     These and other objects are achieved by a bearing assembly having tapered or canted bearing surfaces which absorb both radial and thrust loads. The design provides for a certain amount of fluid flow through the bearing structure with a central radial bearing serving to restrict fluid flow and to eliminate drive shaft deformation in the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiment for implementing the just summarized invention will now be described in conjunction with the drawings of which: 
     FIG. 1 is a diagramatic illustration of a down hole drill motor according to the prior art; 
     FIG. 2 is a cross sectional drawing illustrating the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a prior art down hole motor structure wherein the preferred embodiment finds application. As known in the art, the structure includes a progressive cavity motor 13, a coupling 15, a drive shaft 17, and a drill bit 19. The drive shaft 17 is born by a bearing assembly comprising bearings 21 and 23. Typically one of these bearings is a radial bearing and the other is a thrust bearing. In the structure of FIG. 1, a radial force arise from the eccentric motion of the coupling 15 as it is driven by a progressive cavity motor and from the drilling force on the drill bit 19. Thrust forces are generated for example by the pressure differential between points P2 and P3 in FIG. 1. The pressure differential across the bearing assembly is for example, on the order of 200 to 2,000 pounds per square inch (PSI). 
     FIG. 2 illustrates the preferred embodiment of a tapered friction bearing assembly. The assembly includes an off-bottom thrust and radial bearing 25, 27; a flow restrictor radial bearing 29, 31; and an on-bottom thrust and radial bearing 33 and 35. Each inner bearing element 25, 29, 35, is attached to the drive shaft 17 of the down hole motor. The outer bearing elements 27, 31, 33 are attached to a housing 39 which is threadably connected to a bottom nut 41. Locks 43 and a lock ring 45 serve to attach the bearings. The inner bearing elements 25, 29, and 35 are further held in position by a shaft cap 47 and are locked to the drive shaft 17 by the eccentric lock surface 50. 
     The opposing bearing surfaces 49 may be of any suitable bearing surface construction. Such constructions include poly-crystalline diamond, ceramic and tungsten carbide matrix. Flow grooves 53 are formed in the surfaces 49, 51. These grooves 53 have a width selected to provide sufficient flow of drilling fluid (&#34;mud&#34;) to cool and lubricate the bearings. The flow gap 54 between the middle radial bearing elements 29, 31 determines the maximum flow possible. Sufficient pressure drop across the bearings, on the order of minimum 200 PSI, is required to assure sufficient flow of cooling fluid. 
     In operation, the off-bottom thrust and radial bearing 25, 27 accomodates the forces on the drive shaft 17 when the drive shaft is driven while the drill bit 19 is off-bottom, i.e. disengaged from a drilling operation. The on-bottom thrust and radial bearing 33, 35 accomodates the forces on the drive shaft 17 when the drill bit 19 is on-bottom, engaged in a drilling operation. The radial bearing 29, 31 serves to prevent bending of the drive shaft 17 which would otherwise occur as a result of the varying force pattern to which the drive shaft 17 is exposed. Prevention of bending increases the fatigue lifetime of the system. 
     The angle of the taper of the bearing elements 25, 27 is determined by the angle with respect to vertical of the resultant force vector on the drive shaft 17 in the off-bottom position. Similarly, the angle of the on-bottom thrust and radial bearing 33, 35 with respect to vertical is determined to match the angle of the resultant force vector typically expected to be experienced by the drive shaft 17 in its running, on-bottom mode. It will be observed that the drive shaft 17 will ride up or down with respect to the housing 39, depending on whether the drill string is in the off-bottom or on-bottom mode and otherwise in response to the forces experienced by the drive shaft 17. The optimum force balance of course if where the drilling parameters are such that the bearings are required to carry no load, i.e. where the drive shaft &#34;floats.&#34; 
     The structure just described thus accomodates both radial and thrust forces, while preventing bending of the drive shaft 17. The structure is usable in a wide variety of applications including both turbine driven and progressive cavity down hole motors. As will be appreciated, the just described preferred embodiment is susceptible to numerous modifications and adaptations without departing from the scope and spirit of the invention. Therefore, it is to be understood, that within the scope of the appended claims, the invention may be practiced other than as specifically described herein.