Rock bits are employed for drilling wells, blast holes, or the like in subterranean formations for oil, gas, geothermal steam, minerals, and the like. Such drill bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and/or the bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled. High temperatures and pressures are often encountered when such rock bits are used for drilling in deep wells.
When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the service life of a drill bit in a rock formation. Prolonging the time of drilling minimizes the time lost in "round tripping" the drill string for replacing the bits. Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation.
One reason for replacing the rock bits include failure or severe wear of the journal bearings on which the cutter cones are mounted. These bearings can be friction- or roller-type bearings, and can be subject to high loads, high hydrostatic pressures in the hole being drilled, high temperatures due to drilling, elevated temperatures in the formation being drilled, as well as harmful abrasive particles originating from the formation being drilled. The journal bearings are lubricated with grease adapted to such severe conditions. Such lubricants are a critical element in the life of a rock bit. A successful grease should have a useful life longer than other elements of the bit so that premature failures of bearings do not unduly limit drilling.
The grease is retained within the rock bit to lubricate the journal bearings by a journal bearing seal, typically an O-ring type of seal. The seal must endure a range of temperature and pressure conditions during the operation of the rock bit to prevent the grease from escaping and/or contaminants from entering the bearing and, thereby ensure that the journal bearings are sufficiently lubricated. Elastomer seals known in the art are conventionally formed from a single type of rubber or elastomeric material, and are generally formed having identically configured dynamic and static seal surfaces, i.e., having a symmetric axial cross-sectional geometry. The rubber or elastomeric material selected to form such a seal has particular hardness, modulus of elasticity, wear resistance, temperature stability, and coefficient of friction. Additionally, the particular geometric configuration of the seal surfaces produces a given amount of seal deflection that defines the degree of contact pressure or "squeeze" applied by the dynamic and static seal surfaces against respective journal bearing and cone surfaces.
The wear, temperature, and contact pressure conditions that are encountered at the dynamic seal surface are different than those encountered at the static seal surface. Therefore, the type of seal material and seal geometry that is ultimately selected to form both seal surfaces represents a compromise between satisfying the operating conditions that occur at the different dynamic and static seal surfaces. Because of the different operating conditions at each seal surface, conventional seals formed from a single-type of material, having symmetric axial cross-sectional geometries, often display poor wear resistance and poor temperature stability at the dynamic seal surface where wear and temperature conditions, under high-temperature operating conditions, are the most aggressive. Accordingly, the service life of rock bits that contain such seals are defined by the limited capability of the seal itself.
U.S. Pat. No. 3,765,495 discloses a drill bit seal configured having a greater radial cross section than axial cross section by a ratio of at least 1.5:1. The seal has a symmetrical generally rectangular axial cross section and is made from a single type of elastomer. The seal has identically configured dynamic and static surfaces, and is formed from a single type of elastomeric material, reflecting a compromise between meeting the different operating conditions at each seal surface. Accordingly, a high-aspect ratio seal constructed in this manner is less than optimized to perform satisfactorily under the high temperature and pressure conditions encountered in a rock bit.
U.S. Pat. No. 5,362,073 discloses a composite rock bit seal formed from two or more different materials selected to provide a desired degree of wear resistance at the dynamic seal surface, and to provide a desired degree of seal contact at the static seal surface. The seal has a dynamic seal surface formed from a single type of elastomeric material, and has an inner and outer static seal surfaces that are each formed from different materials. Further, the dynamic seal surface has a radius of curvature less than that of each static seal surface. Although the seal surfaces are constructed from materials having different hardnesses, the seal geometry at each dynamic and static surface does not produce a desired degree of contact pressure at each respective rock bit surface.
It is, therefore, desired that journal bearing seals be constructed having a static seal surface and a dynamic seal surface that is each designed to accommodate the particular wear and temperature conditions that exist at each interfacing surface. It is desired that such journal bearing seals be designed to provide a desired degree of contact pressure at each static and dynamic surface.