Abstract:
A method of providing a consistent preload on thrust bearings in a bearing assembly. A first step involves placing against an inner race and an outer race of a bearing stack of tlirust bearings, deformable shims made from a material having a relatively flat stress-strain curve after its yield stress has been exceeded. A second step involves preloading the deformable shims beyond their yield point in situ until a predetermined preload tolerance is reached.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method of providing a consistent preload on thrust bearings in a bearing assembly, and a down hole bearing assembly constructed in accordance with the teachings of the method.  
       BACKGROUND OF THE INVENTION  
       [0002]     A common problem with bearing assemblies is having a consistent preload force on the inner and outer bearing races of thrust bearings. If the bearing preload is not consistent, the outer races will deform more or less than the inner bearing races. This results in non-uniform load distribution which, in turn, results in lower load handling and lift capacity of the thrust bearings. Down hole drilling fluid lubricated bearing assemblies rely upon accurate measurements being made by service technicians. If they make an error in measurement of only a few thousands of an inch, the change in the preload on the bearing stack can change significantly.  
       SUMMARY OF THE INVENTION  
       [0003]     According to the present invention there is provided a method of providing a consistent preload on thrust bearings in a bearing assembly. A first step involves placing against an inner race and an outer race of a bearing stack of thrust bearings, deformable shims made from a material having a relatively flat stress-strain curve after its yield stress has been exceeded. A second step involves preloading the deformable shims beyond their yield point in situ until a predetermined preload tolerance is reached. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:  
         [0005]      FIG. 1  is a side elevation view, in section, of a down hole bearing assembly constructed in accordance with the teachings of the present invention.  
         [0006]      FIG. 2  is a detailed side elevation view, in section, of a portion of the down hole bearing assembly illustrated in  FIG. 1 , showing deformable shims.  
         [0007]      FIG. 3  is a detailed side elevation view, in section, of a portion of the down hole bearing assembly illustrated in  FIG. 1 , showing a mandrel jacking section.  
         [0008]      FIG. 4  is a detailed side elevation view, in section, of a portion of the down hole bearing assembly illustrated in  FIG. 1 , showing a deformable overload protection ring.  
         [0009]      FIG. 5  is a side elevation view, in section, of the down hole bearing assembly, with the housing jacking section engaged.  
         [0010]      FIG. 6  is a side elevation view, in section, of the down hole bearing assembly illustrated in  FIG. 1 , with the housing removed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]     The preferred embodiment, a down hole bearing assembly generally identified by reference numeral  10 , will now be described with reference to  FIG. 1  through  6 . There are several aspects of the present invention that will hereinafter be described.  
         [0000]     Deformable Shims  
         [0012]     Structure and Relationship of Parts:  
         [0013]     Referring now to  FIG. 1 , down hole bearing assembly  10  includes an outer housing  12  with an inner surface  14  defining an interior bore  16 . An inner mandrel  18  is supported for rotation within interior bore  16  of outer housing  12 . Inner mandrel  18  has an outer surface  20 . A bearing stack  22  of thrust or radial bearings  24  is positioned between inner surface  14  of outer housing  12  and outer surface  20  of inner mandrel  18 , where each thrust bearing  24  has an inner race  26  and an outer race  28 . Referring to  FIG. 2 , deformable shims  30  are positioned against inner race  26  and outer race  28  of at least one of the thrust bearings  24  in bearing stack  22 . Deformable shims  30  are made from a material, such as a soft steel or other metal material, that has a relatively flat stress-strain curve after its yield stress has been exceeded, and are preloaded beyond their yield point in situ to a predetermined preload tolerance. Bushings  31  above bearing stack  22  and bushings  33  below bearing stack  22  facilitate rotation of inner mandrel  18  with respect to outer housing  12 .  
         [0014]     Operation:  
         [0015]     Referring to  FIG. 2 , down hole bearing assembly  10  is provided as described above, with shims  30  positioned against inner race  26  and outer race  28  of one of the thrust bearings  24  in bearing stack  22 . Shims  30  are then loaded beyond their yield point, such that they are more deformable with additional loading. For example, Graphs  1  and  2  below show the stress-strain curve for two different alloys. In Graph  1 , the yield point of the alloy is just under 600 MPa, while in Graph  2 , the yield point of the alloy is just over 300 MPa. After these points, it can be seen that the alloys defonn more easily with increased pressure, and in a relatively constant mauler. This creates a very consistent and repeatable preload force to help ensure a uniform load distribution to prolong the life capacity of thrust bearings  24 . For example, referring to Graph  1 , if a shim is used that is 1″ long and made from UNS31803 alloy, a preload deformation of 0.1″ would result from 780 MPa of pressure, and a preload deformation of 0.3″ would result in a preload stress of less than 800 MPa, the net difference being 20 MPa. This provides a preload force that is substantially the same over a large tolerance of preload deformation. Graph  1  is an example used solely for the purposes of illustration. Other suitable materials will have a similar profile, but will exhibit the profile at different values. v, 1 / 2  v, 2 / 2   
         [0000]     Torque Overload Protection  
         [0016]     Structure and Relationship of Parts:  
         [0017]     Referring to  FIG. 4 , inner mandrel  18  also has a threaded motor connection  32  adapted for threaded connection to a down hole motor assembly (not shown), and includes a U-joint  68  that connects to the power section of the down hole motor. A deformable overload protection ling  36  is included in the make up of motor connection  32 , where deformable overload protection ring  36  is made from a material that has a predictable yield strength that is lower than that of inner mandrel  18 , such that deformable overload protection ring  36  defonns to buffer inner mandrel  18  when momentary overload torque is transmitted through motor connection  32 . In the illustrated embodiment, overload will also result in deformation of shims  30 . It will be appreciated that deformable shims  30  are not essential to the operation of this aspect of the invention.  
         [0018]     Operation:  
         [0019]     Down hole bearing assembly  10  is provided as described above and depicted in  FIG. 1 , with deformable overload protection ring  36  positioned below motor connection  32 . Ring  36  is made of a metal that has a predictable yield strength which is lower than the bearing mandrel or bottom adapter. This ring is intended to permanently deform when momentary overload torque is transmitted through the drill bit and motor assembly.  
         [0000]     Low Positive Oil Pressure Innovation  
         [0020]     Structure and Relationship of Parts:  
         [0021]     Referring to  FIG. 1 , a sealed and lubricant filled bearing chamber  38  is formed between inner surface  14  of outer housing  12  and outer surface  20  of inner mandrel  18 . Bearing chamber  38  has a first end  40  and a second end  42  with a stationary seal  44  positioned at second end  42  and a floating seal piston  46  at first end  40 , although more than one seal  44  may be used. Referring to  FIG. 6 , floating seal piston  46  has a lubrication face  48  acting against lubricant in bearing chamber  38  and a drilling fluid face  50  against which drilling fluid acts, and a preload spring  52  is provided which acts against drilling fluid face  50 . A flow port  54  is positioned upstream of drilling fluid face  50  of floating seal piston  46 , such that drilling fluid passes through flow port  54  and applies pressure to act against drilling fluid face  50  of floating seal piston  46 . Bearing stack  22  of thrust bearings  24  is positioned in bearing chamber  38 .  
         [0022]     Operation:  
         [0023]     Referring to  FIG. 1 , down hole bearing assembly  10  is provided as described above, with floating seal piston  46  positioned at first end of bearing chamber  38 . Referring to  FIG. 6 , drilling fluid flows through flow port  54  and acts against drilling fluid face  50  of floating seal piston  46 , with spring  52  acting against drilling fluid face  50  as well. The force due to spring  52  and drilling fluid pressure acting against drilling fluid face  50  causes lubrication face  48  to push against the lubricant within bearing chamber  38  to induce a positive pressure on the lubricant. Since spring  52  applies a force even in the absence of drilling fluid pressure, the change in pressure when the drilling fluid does apply pressure allows the lubricant to be under a greater pressure than the drilling fluid pressure in a variety of operating conditions. For example, if drilling fluid pressure at motor collection  32  is 500 psi and decreases to 470 psi at drilling fluid port  54 , there would be a pressure differential of 30 psi between the two. If, however, the force applied by spring  52  increases lubricant pressure by 40 psi, then the pressure on the lubricant 510 psi, or 10 psi greater than the highest drilling fluid pressure of 500 psi.  
         [0000]     Servicing Enhancements  
         [0024]     Structure and Relationship of Parts:  
         [0025]     Referring now to  FIG. 5 , inner mandrel  18  is made in sections  1   8 A and  18 B, each with mating threads  60  for ease of assembly. Referring to  FIG. 3 , section  18 A acts as a mandrel jacking section, and has a shoulder  64  that engages those components that are positioned along outer surface  20  of inner mandrel  18 . Referring to  FIG. 5 , outer housing  12  is also made in sections  12 A and  12 B, with a stabilizer  61  positioned over section  12 B. Section  12 A acts as a housing jacking section with shoulder  66 . During the housing jacking process, section  12 A is backed onto shoulder  67 , such that, upon rotation of section  12 B, shoulder  69  applies a force to and helps loosen components that are stuck to inner surface  14  of housing  12 . Shoulder  66  is used during the mandrel jacking process to apply a force against the components stuck to section  18 A as section  18 B, and hence section  12 A, is rotated. While shoulder  66  is on section  12 A, it may equally be on section  18 B. The important aspect is that the movement of section  18 B engages shoulder  66  and the stuck components.  
         [0026]     Operation:  
         [0027]     Referring to  FIG. 3  and  6 , down hole bearing assembly  10  is provided as described above, with sections  18 A and  18 B making up inner mandrel  18 . Refining to  FIG. 3 , section  18 A has a shoulder  64  that engages components along outer surface  20  of inner mandrel  18 . During disassembly, mating threads  60  for mandrel jacking section  18 A and  18 B have sufficient travel such that mandrel jacking section  1   8 A serves as a screw jack to exert a jacking force upon those components that have become stuck to outer surface  20  of inner mandrel  18 . Referring to  FIG. 5 , mandrel jacking section  12 A serves as a screw jack to exert a jacking force upon those components that have become stuck to inner surface  14  of outer housing  12 . As section  12 B is rotated relative to section  12 A, section  12 A is pushed against shoulder  67  of section  18 B, which prevents further movement in that direction. Upon further rotation, shoulder  69  applies a force to those components which may be stuck on inner surface  14  of outer housing  12 B to allow section  12 B to be removed. Refining to  FIG. 6 , once section  12 B has been removed, the mandrel jacking process can be used. Section  18 B and therefore section  12 A as well is rotated such that shoulder  66  contacts the components stuck on inner mandrel  18 . This results in a tensile force along mandrel section  18 A between threads  60  and shoulder  66 , and a force against the components to help in disassembly.  
         [0028]     In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.  
         [0029]     It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the claims.