Two-cone bit with non-opposite cones

Abstract: A two-cone earth boring bit having non-opposite cones that minimize the tendency for off-center rotation or rough running. The bit is composed of two cones, each having a cantilevered bearing shaft (19) with an axis extending inwardly and downwardly. A rotatable, generally conical cutter (21, 39) is mounted on each bearing shaft, each cutter having a conical gage surface to engage and define a borehole with a wall (29) of select gage diameter. The axis (37) of one cutter is skewed relative to the other (43) to cause the conical gage surface (35, 41) of the two cones to engage the wall of the hole at points (A, B) that are other than 180 degrees apart as compared to nonskewed cutters. These points are separated by a distance less than the selected gage diameter. A line between these points is separated from a line extending from one point through the rotatable axis (47) on the bit by a selected angle (α). The body of the bit and/or stabilizers are separated from the wall of the hole by a distance less than the selected gage diameter. A line between these points is separated from a line extending from one point through the rotatable axis on the bit by a selected angle. The body of the bit and/or stabilizers are separated from the wall of the borehole by a distance in a range from preferably one-fourth to one inch.

Description: This invention relates in general to earth boring and in particular to an earth boring bit comprising, a body composed of two sections, a cantilevered bearing shaft with an axis extending inwardly and downwardly on each section, and a rotatable, generally conical cutter on each bearing shaft, said cutter having a conical gage surface to engage and define a cylinder with a wall of selected gage diameter surrounding the earth boring bit. Earth boring bits fall generally into two categories: (1) drag bits with a variety of forms including those with synthetic or natural diamond used for cutting elements; (2) those with rotatable cutters having earth disintegrating teeth formed of steel and other suitable metals, such as sintered tungsten carbide. The rotatable cone type bits have generally two or three cones. The three-cone bit (see e.g. US-A-4 763 736 and US-A-3 696 876) has enjoyed the greater commercial success for a number of reasons, including the fact that they "run smooth". Two-cone bits (see e.g. US-A-4 067 406) tend to run rougher, a condition that generates vibrations in the bit and drill string that impedes drilling and tends to be detrimental to the drilling rig and equipment. US-A-4 285 409 discloses a hybrid rock bit having a pair of cone cutters mounted to legs 120° apart (as in a three cone bit) with an extended drag bit leg occupying the remaining 120° segment. US-A-2 340 492 discloses a non-hybrid drill bit including two cutters the axes of which are at an angle of zero degrees one with respect to the other. DE-A-1 223 779 discloses a drill bit including two cutters and a helix section, the axes of the cutters being skewed by a angle of 60° one with respect to the other. It is advantageous to utilize two-cone bits, if they can be made to run smooth, in some types of earth formations. The softer formations can be effectively drilled with two-cone bits, which usually have longer teeth or cutting elements. It is the general object of the invention to provide an earth boring bit of the two-cone type with features that minimize rotation off-center, rough running and reduce torque. This object is achieved according to the invention in the above identified earth boring bit in that the axis of one cutter is skewed by an angle α greater than zero, but less than or equal to 13 degrees, relative to the axis of the other cutter to cause the conical gage surfaces of the two cutters to engage the wall of the cylinder at points A,B that are spaced apart by an angle other than 180 degrees, a line through said points A,B being separated from a line through point A and the center of the bit by an angle equal to half of the angle α. An advantageous embodiment of the invention are described in the dependent claim. The earth boring bit according to the invention may be summarized as a two-cone earth boring bit having non-opposite cones that minimize the tendency for off-center rotation or rough running. The bit is composed of two cones, each having a cantilevered bearing shaft with an axis extending inwardly and downwardly. A rotatable, generally conical cutter is mounted on each bearing shaft, each cutter having a conical gage surface to engage and define a cylinder with a wall of select gage diameter. The axis of one cutter is skewed relative to the other to cause the conical gage surface of the two cones to engage the wall of the cylinder at points that are other than 180 degrees apart as compared to nonskewed cutters. These points are separated by a distance less than the selected gage diameter. A line between these points is separated from a line extending from one point through the rotational axis on the bit by a selected angle. The body of the bit and/or stabilizers are separated from the wall of the hole by a distance in a range preferably 0.62 cm to 2.54 cm (one-fourth to one inch). Fig. 1 is a fragmentary longitudinal sectional view of a portion of one section of a two-cone earth boring bit which embodies the principles of the invention. Figure 2 is a schematic view of the two-cone bit of Fig. 1 as seen from above to show the relationship of the non-opposite cones. The numeral 11 in the drawings represents a portion of a section of a two-cone bit having a shank threaded at 13 for connection to the drill string (not shown). This section includes a lubrication and pressure compensator means 15, the description of which may be seen with reference to United States Patent 4,727,942, "Compensator for Earth Boring Bits", March 1, 1988. Lubricant is introduced through passages 17 to the surfaces of a bearing shaft 19, which is cantilevered from the section 13 to extend inwardly and downwardly. A rotatable cutter 21, with rows of disintegrating teeth 23, 25 is secured to the bearing shaft 19 by a resilient snap ring 27. The description or construction of the preferred snap ring may be seen with reference to United States Patent 4,344,658, "Earth Boring Bit with Snap Ring Cutter Retention", August 17, 1982. Lubricant is retained within the bearing surfaces of each cutter and bearing shaft by an o-ring seal 22 as described in United States 3,397,928, "Seal Means for Drill Bit Bearings". Fig. 2 illustrates schematically a borehole with a wall 29 of selected gage diameter. Two sections 13 are welded to form a bit body, portions of which are represented by the numerals 31, 33, through which drilling fluid flows through passages and nozzles (not shown) to flush cuttings from the borehole to the surface of the earth. Cutter 21 has conical gage surface 35 that engages the borehole wall 29 at point A as it rotates about its axis 37. Cutter 39 has conical gage surface 41 that engages the borehole wall 29 at point B as it rotates about its skewed axis 43. Both the cutters 21 and 29 are "offset" to be tangent with a circle 45 surrounding the centerline 47 which defines the rotational axis of the bit. Cutter 39 is not only offset but is skewed at an angle α relative to the axis 37 of the cutter 21. The portion 33 of the body of the cutter is separated by a distance C from the Wall 29 of the bore hole. Point A is the contact of gage surface 35 with the borehole wall 29. Point B is the point of contact of surface 41 with the borehole wall 29. These points define the perimeter or gage of the borehole as the bit rotates about axis 47. The gage diameter of the borehole is the sum of the distances between the axis 47 and point B plus axis 47 and point A. The purpose of skewing the cutters is to cause the conical surfaces 35 of cutter 21 and 41 of cutter 39 to engage the wall 29 of the borehole at points A and B so that they are other than 180 degrees apart. As a result, the points A and B are separated by a distance which is less than the selected gage diameter of the borehole, but cut a full diameter borehole when ratated about axis 47. Thus a line 48 through points A and B is separated from a line 49 by an angle α/2. Portion 33 of the body is separated from the wall of the borehole by the distance C. During rotation, the bit tries to rotate intermittently about point A or point B because of varying forces acting on the cutters. For example, differences in forces occur because the rock is not homogeneous and the wall of the hole is not always a smooth cylinder. When it rotates about point A, point B rotates along path b, which is inside the borehole wall 39 within an arc of α. Therefore, point B will not cut gage until the bit starts rotating about axis 47. Rotation about point A is also limited by the clearance C. When the surface D engages the wall 29 of the hole, further rotation about point A will cease. The surface D also acts as a stabilizer that forces the bit to rotate around axis 47. Since point B moves on an arc inside the gage diameter, there is no damage to the borehole wall or cutting overgage hole. In the prior two-cone bits with cones 180 degrees apart, the distance between points A and B is the gage diameter. Rotation about point A causes point B to move on an arc outside the hole gage diameter. This produces an oversized hole and causes rough, off-center running. The conventional way to overcome this problem is to use a close-fitting stabilizer on the bit, which increases drilling torque and chances for getting stuck in the borehole. Examples of successful dimensions and angles discussed above are as follows for a 20cm (7-7/8 inch) bit: α = 13° C = 0.793 cm (5/16 inch) Offset = 0.476 cm (3/16 inch) (radius of circle) It should be apparent from the foregoing description that an invention having significant advantages has been produced. One advantage of the bit is the reduction of the tendency to damage the borehole wall due to running off-center. Therefore, the borehole will not be oversized, and the bit will tend to run smoother. While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not thus limited, but is susceptible to various changes and modifications without departing from the spirit thereof.

Claims: An earth boring bit (11) comprising, a body composed of two sections (13), a cantilevered bearing shaft (19) with an axis extending inwardly and downwardly on each section (13), and a rotatable, generally conical cutter (21,39) on each bearing shaft (19), said cutter (21,39) having a conical gage surface (35,41) to engage and define a cylinder with a wall (29) of selected gage diameter surrounding the earth boring bit (11), characterized in that the axis (43) of one cutter (35) is skewed by an angle α greater than zero, but less than or equal to 13 degrees, relative to the axis (37) of the other cutter (21) to cause the conical gage surfaces (35,41) of the two cutters (21,39) to engage the wall (29) of the cylinder at points (A,B) that are spaced apart by an angle other than 180 degrees, a line (48) through said points (A,B) being separated from a line (48) through point (A) and the center (47) of the bit (11) by an angle equal to half of the angle α. An earth boring bit as claimed by claim 1 wherein the body of the bit is separated from the wall (29) of the cylinder by a distance in a range from 0.62 cm to 2.54 cm (one-fourth to one inch).

Inventors: PESSIER RUDOLF CARL OTTO, PESSIER, RUDOLF CARL OTTO
Applicants: BAKER HUGHES INC, BAKER HUGHES INCORPORATED
Application year: 1990
Main classification: E21B10
IPC codes: E21B10