Earth boring bit with protected seal means

Abstract: An earth boring bit having a body, pressure compensating lubrication system and cantilevered bearing shaft with an annular seal surface and a radial seal surface. A cutter is rotatably secured to the bearing shaft, with an annular seal recess having radial and cylindrical seal surfaces to oppose the seal surfaces on the body and bearing shaft to define a seal gland. A resilient and annular packing ring is compressed between the cylindrical seal surfaces, but has a length less than the minimum length of the gland. A shroud extends inwardly near the outer backface a selected distance behind the packing ring to bias it inwardly. The shroud extends inwardly toward the bearing not more than about 30% of the cross-sectional thickness of the O-ring in the relaxed condition. The O-ring is compressed in a range of about 10 to 15 percent.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to an earth boring bit, according to the preamble of Claim 1. An earth boring bit of this kind is known from US-A-3,397,928. 2. Background Information One of the most successful seal means used in earth boring bits of the type having rotatable cutters is the O-ring seal. It successfully confines lubricant to the bearing area while excluding detritus for long periods of time before failure. There are two general categories of O-ring seals utilized in earth boring bits: (1) those permitting axial motion or play of the O-ring; and (2) those utilizing a recess to retain the seal in one position relative to either the cutter or bearing shaft, usually the cutter. An example of those earth boring bits with seals permitting axial motion relative to the bearing shaft or cutter are U.S. Patent Nos. 4,014,595 and 4,516,641. Document US-A-3,397,928 reveals a seal means for drill bit bearings, particularly seal means of a packing ring type containing the feature of that the annular packing ring has a compressed length less than the miminum length between the radial surface of the annular seal recess and the radial surface carried by the main part of the drill bit. The reduced length with respect to the annular seal recess ensures that sealing by the annular packing ring is still effective, when lubrication pressure peaks or relative movement of the single part of the drill bit occurs. What has to be regarded as a serious drawback with respect to the above document is the fact that the annular packing ring is able to move within the recess while being pressed against the vertical sealing surface of the main body of the drill bit or of the cutter which, in turn, will lead to heat generation and excessive wear of the annular packing ring, lessening the time-span of these sealing means. In accordance with the invention this drawback is overcome by the characterising features of Claim 1. Examples of those seals which are confined in one position relative to the cutter are U.S. Patent Nos. 3,765,495 and 4,372,624. Document US-A-4,554,985 reveals a rotary drill bit with annular sealing means in which the contact pressure at the top of the seal is considerably smaller than the contact pressures at each of the top sides of the seal so that there can be no resulting pressure or force which will bias the seal in any direction along the bearing surfaces. The consideration of the parts receiving the sealing means in this document results in that the sealing means is squeezed into a triangular shape, keeping the annular packing ring in its initial position without the possiblity of relative movement along the axis of the bearing surfaces. A sealing arrangement following this document will not be capable of compensating for lubrication pressure pulsation by means of elastic deformation of the sealing means. The primary advantage of those O-ring seals permitting axial motion is the minimization of pressure pulses in the lubricant. Pressure pulses adjacent the seal can be detrimental and can result in leakage of lubricant or damage to the seal unless there is sufficient play of the seal in the recess to avoid extrusion. A sufficient amount of play reduces the magnitude of the pressure pulses to prevent extrusion of the O-ring from the seal recess (commonly called the "gland"). The advantages of those seals systems in which the seal is confined in one position, usually in the cutter, are: (1) protecting the seal from abrasives that cause wear and deterioration, 2) biasing the seal to keep the seal with the cutter to minimize pressure pulses and (3) preventing the pressure pulses in the lubricant from pushing the O-ring with excessive force against a stationary seal surface on the body of the bit. SUMMARY OF THE INVENTION An improved earth boring bit having a body, pressure compensating lubrication system and cantilevered bearing shaft with a cylindrical seal surface and a radial seal surface. A cutter is rotatably secured to the bearing shaft, with an annular seal recess having radial and cylindrical seal surfaces to oppose the seal surfaces on the body and bearing shaft. A resilient and annular packing ring is compressed between the cylindrical seal surfaces but has a clearance from the radial seal surfaces to permit pressure compensating movement of the seal in the recess. A shroud extends inwardly at the outer end of the annular seal recess to bias the packing ring inwardly to protect the ring. The seal recess has a length about 30 percent greater than the relaxed thickness of the O-ring. Additional features, objects and advantages of the invention will be come apparent with the following description. DESCRIPTION OF THE DRAWINGS Figure 1 is a longitudinal cross-section of one leg of an earth boring bit that embodies the features of the invention. Figure 2 is a fragmentary longitudinal section of a portion of the Figure 1 embodiment, showing one region of a bearing shaft extended from the head. Figure 3 is a fragmentary, longitudinal cross-section showing an enlarged view of the seal means and recess configuration of Figure 1. Figure 4 is a fragmentary, longitudinal section of the recess area in the cone of the Figure 1 embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT The numeral 11 in Fig. 1 of the drawing designates an earth boring bit having a threaded upper portion 13 for connection to a drill string member (not shown). A fluid passage 15 directs drilling fluid to a nozzle (not shown) that impinges drilling fluid against the borehole bottom to flush cuttings to the surface of the earth. A pressure compensating system 17 is contained within each section of the body, there usually being three which are welded to form the composite body. The lubrication system is preferably similar to that shown in U.S. Patent No. 4,727,942. In each section of the body is a lubrication passage 19 that extends downwardly into intersection with another lubrication passage 21 in the upper portion of a ball plug 23 which is secured to the body by a plug weld 25. A third lubrication passage 27 carries lubricant to a cylindrical bearing surface 29 on a bearing shaft 30 which is cantilevered downwardly and inwardly from an outer and lower region of the body of the bit. The ball plug 23 retains a series of ball bearings 31 that rotatably secure the cutter 33 to the bearing shaft 30. Dispersed in the cutter are a plurality of rows of earth disintegrating teeth 35 that are constructed of a sintered tungsten carbide secured by interference fit into mating holes in the cutter 33. As shown in Fig. 3, a cylindrical seal surface 39 is formed near the base of the bearing shaft 30 to adjoin a radial seal surface 41, these surfaces being joined by a suitable radius. The seal surfaces 39, 41 are opposed by a radial seal surface 43 in the cutter 33, which also contain a cylindrical seal surface 45, these surfaces being joined by a suitable radius. As indicated best in Fig. 4, a shroud 47 extends a selected distance inwardly of the cylindrical seal surface 45 in the cutter 33 toward the cylindrical surface 39 in the bearing shaft to bias the resilient packing ring, here an O-ring, 37 inwardly. Preferably, this shroud 47 extends inwardly not more than about 30 per cent of the cross-sectional thickness of the O-ring 37 in its relaxed condition before assembly on the bearing shaft 30. The O-ring 37 is preferably compressed between the cylindrical seal surfaces 39, 45 in a range of about 10 to 15 percent. As indicated in Fig. 3, the shroud 47 has an inward oblique surface 49 adapted to engage the O-ring 37 to bias it inwardly and away from radial seal surface 41. The shroud 47 also has an outward oblique surface 51 which opposes a parallel oblique surface 53 in the head, as indicated in Fig. 2. The cone backface 52 of Fig. 3 opposes the last machined surface 54 in the body of the bit. The resilient O-ring 37, when compressed as indicated in Fig. 3, does not span the distance between radial seal surfaces 41, 43 of the head and cutter, but rather, has a length less the minimum length of the composite gland by distance "C". The composite recess should be approximately 30 percent greater in width than that of the O-ring when in its relaxed condition. The nominal squeeze of the O-ring is preferably 12 percent but in practice varies in a range from about 10 to 15 percent. Thus, the clearance C is sufficient to permit the O-ring 37 to move back and forth within the composite seal recess to compensate for pressure differences in the lubricant and minimize pressure pulses that otherwise may tend to push the O-ring outwardly in a manner to cause excessive heat, wear or extrusion. With reference to Fig. 3, the preferred dimensions are as follows: C = .15D X = 1.4D Y = 1.2D Z = .15D α = 30° β = 30° Where: C is the difference in the length of the compressed seal and the minimum length of the composite gland or distance between radial seal surfaces. X is the distance from the cutter backface to the radial seal surface 43. Y is the distance from the cutter backface 42 to the minimum diameter of the shroud. Z is the distance the shroud extends inwardly from a cylindrical seal surface 45. α and is the angle of the outward oblique surface. β is the angle of the inward oblique surface. D is the cross-sectional thickness of the O-ring in its relaxed condition. Bits constructed as indicated above have less wear than comparison bits of the metal forming the radial seal surface 41 in the head. This is the result of the shroud 47, which prevents the O-ring 37 from being urged forcefully against the radial seal surface 41 and the mating surface of the seal. Also, the configuration of the shroud tends to prevent abrasives from the drilling fluid from reaching the O-ring and seal surface 41.

Claims: Earth boring bit comprising a body, i.e. the main part of the head of the earth boring bit, a pressure compensating lubrication system (17) carried by the body, a cantilevered bearing shaft (30) extending from an outer region of the body, a cutter (33) rotatably secured to the bearing shaft (30), a cylindrical seal surface (39) carried by the body, an annular seal recess in the cutter (33) having radial (43) and cylindrical (45) seal surfaces to oppose the seal surface (39) carried by the body, a resilient and annular packing ring (37) compressed between the cylindrical seal surfaces (39, 41, 43, 45), the length of the compressed annular packing ring (37) being the minimum length between the radial surface (43) of the annular seal recess and a radial seal surface (41) carried by the body minus a selected distance (C) to permit pressure compensating movement of the annular packing ring (37) in the recess, characterized in that a shroud (47) is provided on the cutter (33) and adjacent the annular seal recess extending inwardly a selected distance not more than about 30 % of the cross-sectional thickness of the annular packing ring (37) in its relaxed condition, to bias the packing ring (37) away from the radial seal surface (41) of the body. Earth boring bit defined by claim 1, further characterized in that the packing ring (37) is of the O-ring type. Earth boring bit defined by claim 1, further characterized in that the shroud (47) extends inwardly from the cutter backface and includes an oblique surface (49) to bias the packing ring (37) inwardly. Earth boring bit defined by claim 2, further characterized in that the O-ring type packing ring (37) is compressed between the cylindrical surfaces (39, 45) in a range of about 10 to 15 percent.

Inventors: HALL JAMES R, MAURSTAD CARY A, HALL, JAMES R., MAURSTAD, CARY A.
Applicants: BAKER HUGHES INC, BAKER HUGHES INCORPORATED
Application year: 1992
Main classification: E21B10
IPC codes: F16J15, E21B10