Patent Publication Number: US-4730681-A

Title: Rock bit cone lock and method

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved earth boring rotary rock bit in which wear-induced cone loss is prevented by automatically causing a rotational lockup between the cone and its supporting journal pin when a predetermined degree of wear therebetween occurs. 
     Rotary rock bits are well known in the drilling art and typically comprise a bit body having three or more support arms depending therefrom and carrying journal pins which project downwardly and radially inwardly from the support arms. Cutting cones are coaxially supported on the journal pins for rotation relative thereto, and they are captively retained on the pins, by annular arrays of ball bearings disposed within facing annular races. Sealing means maintain lubricant in the bearing areas and prevent entry of borehole fluids and detritus therein. During rotation of the bit body within the earth the cutting cones are caused to rotate relative to their supporting journal pins to thereby perform the cutting function of the bit. 
     Rock bits of this general type are exemplified in U.S. Pat. Nos. 2,885,185; 3,207,241; 3,381,968; 3,489,421; 3,628,616; 3,656,764; 3,680,873; 3,721,306; 3,823,789; 3,917,361; 3,995,917; 4,006,788; 4,021,084; 4,061,376; 4,067,406; 4,068,731; 4,150,728; 4,161,223; 4,181,377; 4,185,706; 4,189,014; 4,193,464; 4,204,437 and 4,276,946. 
     A longstanding problem heretofore associated with conventional rock bits of this type is that when a bearing seal fails the inner surface of the cone and the loaded side of the journal pin begin to wear away, thereby progressively widening the gap between the top of the journal and the cone. At a certain point in time the width of such gap can increase to an extent such that the ball bearings can escape and permit the cone to fall off its journal pin. 
     This is not to say that such gap-widening in every instance causes cone separation in conventional rock bits. Sometimes the ball bearings will jam in their races and lock the cone on its journal pin before the gap widens enough to allows cone loss. However, such cone lockup in conventional rock bits is wholly a fortuitous event, and cannot be relied upon to prevent cone separation. 
     Cone loss must be avoided since a cone in the bottom of a drill hole can render further drilling extremely difficult, if not impossible, if the lost cone cannot be successfully fished out. The fishing-out of a separated cone is usually a laborious, time-consuming and expensive endeavor. 
     Heretofore, the prevention of cone loss is commonly accomplished by estimating the drilling time to which a given rock bit may be exposed, and by carefully monitoring the penetration rate, rotary torque and drill string action during this time. 
     The accuracy of this time estimate is unavoidably dependent upon a wide variety of factors. If this time estimate is overly conservative, unnecessary drill bit replacement costs may be incurred. On the other hand, if the time estimate is overly optimistic, and bit failure signals are not observed, cone loss can occur. 
     From the foregoing it can be seen that it would be highly desirable to provide improved rotary rock bit apparatus and associated drilling methods which eliminate or minimize above-mentioned and other limitations and disadvantages typically associated with rock bits of conventional construction. Accordingly, it is an object of the present invention to provide such apparatus and methods. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, an improved rotary rock bit is provided in which wear-induced loss of any of its cutting cones is prevented by automatically causing a rotational lockup between one of the cones and its supporting journal pin in response to a predetermined degree of surface wear therebetween. The rotational lockup of one of the cones, which indicates that the rock bit has failed causes an easily detectable increase in rotary table torque and decrease in the bit&#39;s penetration rate during the drilling operation. When these &#34;dull bit&#34; signals are sensed, the worn bit is pulled up and replaced. 
     Each of the cutting cones of the bit is retained on its journal pin by an annular array of ball bearings in laterally facing annular races formed on the journal pin and within the cutting cone. Circumscribing the journal pin axially inwardly of the races is an annular floating journal bushing which serves to maintain the cone and pin in axial alignment and functions as a bearing element interposed between the surfaces of the pin and cone. An O-ring or other type of seal circumscribing the journal pin adjacent the base of the cone is used to prevent entry of bore hole fluid into the cone. 
     The automatic cone lockup feature of the present invention is provided by means of a lockup groove formed on the upper surface of the journal pin at the juncture of the ball race and the main journal bearing surface. 
     During normal operating of the rock bit, with the cone rotating on its journal pin, the ball bearings are maintained in the pin and cone races, function in a conventional manner, and cannot enter the groove. 
     However, when a seal fails the upward reactive drilling force on the cone in the presence of drilling fluid progressively wears away the bushing or bearing surfaces of the cone and the pin and causes an increasingly widening gap between the cone and pin adjacent the lockup groove. Continued drilling with extraordinary clearance between journal and cone causes the cone to wobble erratically and skews the pin and cone axes. 
     When a predetermined degree of such surface wear has occurred, the upper pin-cone gap is widened sufficiently to permit entry into the lockup groove of one or more of the ball bearings. As the cone rotates one or more of the ball bearings then is forced into the lockup groove by cone wobble and becomes trapped therein. Additional ball bearings cannot be forced past the trapped ones, and become wedged between the trapped balls and the cone. The trapped and wedged balls rotationally lock the cone on its journal pin or at least substantially inhibit rotation of the cone about the pin. 
     The ball bearings in the rotationally locked cone still retain the cone on its journal pin, thus preventing cone loss while at the same time preventing further pin-cone surface wear which could cause such loss. 
     The lockup groove thus uniquely functions to prevent wear-induced cone loss and automatically creates a signal indicating that the dulled bit needs to be replaced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a rotary rock bit which embodies principles of the present invention and is connected to the lower end of a schematically depicted drill pipe string rotationally driven by a rotary table; 
     FIG. 2 is an enlarged scale, somewhat simplified cross-sectional view through a cutting cone portion of the bit taken along line 2--2 of FIG. 1; 
     FIG. 3 is a perspective view of the journal pin used to rotatably support the cutting cone in FIG. 2; 
     FIG. 4 is a cross-sectional view similar to that in FIG. 2 but depicting the worn cone being rotationally locked on its journal pin by several ball bearings trapped and wedged between the cone and pin; and 
     FIG. 5 is a cross-sectional view through the trapped and wedged ball bearings, and adjacent portions of the pin and cone, taken along line 5--5 of FIG. 4. 
    
    
     DETAILED DESCRIPTION 
     Illustrated in FIG. 1 is a rotary rock bit 10 which embodies principles of the present invention and uniquely eliminates the cone loss problem heretofor associated with rock bits of conventional construction. The bit 10 has a body 12 having a threaded upper end portion 14 adapted to connect the bit to the lower end of a drill string 16 rotationally driven by a rotary table 18. Dependent from the body 12 are three support arms 20 each of the arms having a journal pin 22 (FIG. 3) which projects downwardly and radially inwardly therefrom. It will be appreciated that while a three cone bit is illustrated, other drill bits having differing numbers of cones are also within the scope of my invention. In other words, my invention is applicable to bits having at least one cutting cone that is supported by a journal pin. Rotatably and coaxially secured to each of the journal pins 22, in a manner subsequently described, is a cutting cone 24. The cutting cone can have a variety of teeth or other cutting elements as a part of the cone. During rotation of the bit 10 the cones 24 rotate relative to their supporting journal pins to thereby provide the bit with its earth-cutting action. 
     As will be seen, when a seal failure occurs and a predetermined degree of pin-cone surface wear occurs, one or more of the cones 24 is automatically caused to be rotationally locked on its journal pin. It will be appreciated that instead of complete lockup of the cone there may still be some inhibited rotation. This lockup of the cone causes a readily detectable increase in the torque of rotary table 18, and a decrease in the bit&#39;s penetration rate, thereby signalling the driller of the necessity to replace the worn bit. 
     Importantly, this cone lockup automatically occurs prior to an amount of surface wear between any of the cones and their journal pins which might otherwise permit one or more of the cones to be separated from the bit. 
     Referring now to FIG. 3, each of the journal pins 22 has a journal bearing surface 26, a longitudinal axis 28, an outer end 30, a pilot pin journal 32, and an annular depression in the form of a ball race 34. 
     As illustrated in FIG. 2, each of the cutting cones 24 has a rotational axis 36, a bore 38 which is complementarily configured relative to the journal pin 26, a cylindrical inner surface 40 defined by the opening 38 and an annular ball race 42. The cone opening 38 coaxially receives the journal pin 22 as illustrated in FIG. 2, the pin and cone races 34, 42 being aligned. In the races 34, 42 is an annular array of hardened-steel ball bearings 44 which are loaded into the races through a ball insertion passage 46 extending through the journal pin into the pin race 34 (only an inner end portion of insertion passage 46 being depicted in the drawings). After the balls are loaded in the races, the opening 46 is plugged in a conventional manner. 
     The ball bearings 44 retain the cone 24 on the journal pin 22. Adjacent the base 48 of the cone 24 is an O-ring seal 50 carried in facing annular grooves 52, 54 formed respectively in the journal pin 22 and the cone 24. O-ring seal 50 functions to prevent bore hole fluid from entering the interior of the cone. 
     The diameter of the cylindrical inner cone surface 40 is large enough that an annular gap 47, having a width &#34;X&#34;, is formed between the pin and cone surfaces 26, 40. Positioned in this gap is an annular floating journal bushing 56 which is formed from a suitable bearing material, such material being softer than the hardened-steel ball bearings. Bushing 56 maintains the pin and cone in axial alignment. 
     Referring again to FIG. 3, the automatic cone lockup feature of the present invention is provided by means of a small lockup groove or channel 58 formed in the journal pin 22 along the upper portion of the ball race 34 and the journal surface 26. The illustrated groove 58 has a maximum radial depth less than the maximum radial depth of pin race 34 and laterally communicates with the pin race 34. In operation, the groove or channel is on the &#34;unloaded&#34; side of journal pin 22. 
     During normal operation of the rock bit, the lockup groove 58, which forms a depression in the upper journal pin surface, in no way interferes with or hinders the rotation of the cutting cone 24 relative to its supporting journal pin 22. 
     However, as the bit accumulates drilling time and the seal 50 fails, the upward drilling reactive force &#34;F&#34; and borehole fluid and detritus causes the bushing 56 to progressively wear away, while also causing the cone axis 36 to skew downwardly from the journal pin axis 28 as illustrated in FIG. 4. Eventually the bushing 56 is completely worn away so that lower circumferential portions of the pin and cone surfaces 26, 40 come into contact and wear each other away. This, in turn, progressively widens the gap depth &#34;X&#34; around the upper circumferential portion of the pin 22. 
     When a predetermined amount of such surface wear has occurred (and the axes 28, 30 concomitantly skew to a predetermined degree), an upper circumferential portion of the races 34, 42 separate to an extent which permits one or more of the balls 44 (e.g., balls 44 a  in FIG. 5) to shift axially into and be trapped in the lockup groove 58. Additional balls 44 b  cannot be forced passed the trapped balls and become wedged between the trapped balls 44a and the cone 24. The trapped and wedged balls 44 a , 44 b  rotationally lock or materially inhibit the rotation of cone 24 relative to its supporting journal pin 22. The lockup of one or more of the cones 24, as previously mentioned, increases the rotary table torque and decreases the bit&#39;s penetration rate, thereby signalling the drill operator that the worn bit needs to be replaced. 
     It is important to note that after cone lockup the balls 44, which function as lockup members, still retain the cone 24 on its journal pin 22 and prevent cone loss. It can be seen in FIG. 4 that although the upper gap depth &#34;X&#34; has widened sufficiently to permit the axial shifting of balls 44 a  into the lockup channel 58, this upper gap depth is not wide enough to permit any of the trapped balls 44 a  to escape from between the cone and pin. Accordingly, the balls 44 still retain the cone 24 on the journal pin 22. Stated in another manner, the trapping and wedging action of the balls 44 a , 44 b  automatically occurs prior to an amount of surface wear between the pin and cone sufficient to permit escape of the balls 44. 
     It should also be emphasized that this rotational cone lockup, unlike cone lockups in conventional rock bits, is not simply a fortuitous event--it automatically occurs in a very consistent and uniformly predictable manner due to the unique incorporation into the bit of the lockup groove 58. 
     From the foregoing it can be seen that the present invention provides an improved rotary rock bit having incorporated therein a unique automatic cone lockup feature, provided via the lockup grooves 58, which simply and inexpensively eliminates the cone loss problems commonly associated with rock bits ov conventional construction. 
     The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.