Abstract: An electrical machine, such as an electric motor, generator or torquer-generator, includes a relatively rotatable cylindrical armature and magnetic pole structure separated by an annular gap into which a surrounding fluid may flow during operation of the machine. In order to prevent the build-up of deposited solid material in the gap between the components, one of the components is provided at its surface with a narrow elongate projection which extends helically along and around the surface. The height of the projection is slightly less than the gap between the components so that it remains out of contact with the surface of the other component so as not to frictionally engage it and interfere with relative rotation between the components.

Abstract: A preform element includes a polycrystalline facing table having a front face, a peripheral surface, and a rear surface bonded to the front surface of a tungsten carbide substrate. The front surface of the substrate has a flat central region and an outer peripheral region which is inclined away from the front face of the facing table as it extends outwardly. The outer peripheral region is formed with a number of spaced semi-circular recesses into which extend projections of diamond material formed on the rear surface of the facing table. The outer edge of each recess lies on the periphery of the substrate and the curved inner edge is tangential to the central region of the substrate. Smaller recesses are formed in the bottom surfaces of the semi-circular recesses and at other locations over the front surface of the substrate.

Abstract: A preform cutting element for a rotary drag-type drill bit includes a facing table of polycrystalline diamond having a rear surface bonded to a tungsten carbide substrate. The cutting element is formed with an angled chamfer to form a straight cutting edge on the facing table. The junction between one end of the cutting edge and the remainder of the peripheral surface of the facing table is chamfered in a part-circular arc to provide a smooth transition between the cutting edge and the rest of the peripheral surface. Such transition may reduce the spalling or other damage which may be caused by steady state and impact loads in this region.

Abstract: A rotary drill bit comprises a bit body, a shank for connection to a drill string, a plurality of cutters mounted on the bit body, and a gauge structure which extends around the bit body and, in use, engages the surrounding formation forming the sides of the borehole being drilled. At least a section of the gauge structure is rotatably mounted on the bit body so that, in use, the gauge section may remain substantially non-rotating in engagement with the formation while the bit body rotates relative to it. The external surface of the non-rotating gauge section may be formed with longitudinal grooves to permit the flow of drilling fluid past the gauge section to the annulus. Alternatively the outer surface of the gauge section may be generally cylindrical, in which case internal passages are provided through the gauge section, and/or the bit body, for the flow of drilling fluid past the gauge section to the annulus.

Abstract: A system for controlling the rotation of a roll stabilizable control unit in a steerable rotary drilling assembly comprises an instrument carrier rotatably mounted on a support connected to the drill string. A first rotatable impeller is mounted for rotation by a flow of drilling fluid over the impeller and is coupled to the instrument carrier so as to transmit a torque to it. Sensors carried by the instrument carrier sense the rotational orientation of the instrument carrier and produce a control signal indicative of its rotational orientation, and the torque applied to the instrument carrier by the impeller is controlled, at least partly in response to said signal, so that the instrument carrier can, for example, be roll stabilized if required. A second rotatable impeller is coupled to the instrument carrier for transmitting to it a second torque, which may also be controlled, in the opposite direction to the torque transmitted by the first impeller.

Abstract: A steerable rotary drilling system comprises a bottom hole assembly which includes, in addition to a drill bit, a modulated bias unit and a control unit including an instrument carrier which is rotatable relative to the bias unit. The bias unit comprises a number of hydraulic actuators spaced apart around the periphery of the unit, each having a movable thrust member which is displaceable outwardly for engagement with the formation. Each actuator can be connected, through a rotatable control valve, to a source of drilling fluid under pressure, the control valve comprising a first part, rotatable with the instrument carrier, which cooperates with a second part which is rotatable with the bias unit. Means are provided to roll stabilize the instrument carrier so that relative rotation between the bias unit and instrument carrier, as the bias unit rotates, causes the valve to operate the actuators in synchronism with rotation of the bias unit so as to apply a lateral bias thereto.

Abstract: A modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations, comprises a number of hydraulic actuators spaced apart around the periphery of the unit. Each actuator comprises a movable thrust member which is hydraulically displaceable outwardly and a pivotally mounted formation-engaging pad which overlies the thrust member. An inlet passage supplies fluid under pressure to the chamber, and an outlet passage delivers fluid from the chamber to a lower pressure zone. A selector control valve modulates the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit so that, as the drill bit rotates, each pad is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling.

Abstract: A rotary drilling tool has a plurality of cutters mounted on the tool body and formed with cutting edges defining a cutting profile. The cutters include at least two concentric arrays of primary preform polycrystalline diamond cutters which are radially spaced so as to define an annular groove in the cutting profile, between the two arrays. The deepest part of the groove in the cutting profile is defined by secondary preform polycrystalline diamond cutters located at a radial position which is intermediate the radial positions of the two arrays of primary cutters, so that in use the secondary cutters remove the tops of annular ridges of formation left between the arrays of primary cutters.

Abstract: A method of manufacturing a rotary drill bit includes the steps of forming the bit body with a plurality of sockets, inserting in each socket a projecting carbon former, applying a fusible hard facing material to the surface of the bit body, and around the formers, by the localised application of heat to the solid material so as to melt it and weld it to the surface of the bit body, and then replacing the formers with cutters. During the application of the hard facing material, each former is also heated by the application of heat directly to it, so as to raise the temperature of the former to a level greater than the temperature to which it would otherwise be raised solely by conduction of heat from the hard facing material.

Abstract: A cutter assembly for a rotary drill bit comprises a preform cutting element mounted on a carrier. The cutting element includes a thin cutting table of polycrystalline diamond, defining a front cutting face and a cutting edge bonded to a less hard substrate which is in turn bonded to the carrier. The substrate and/or the carrier comprises a first portion of high erosion resistance, formed for example from tungsten carbide, and a second portion of lower erosion resistance, formed for example from tungsten metal, which is located in the vicinity of the cutting edge of the cutting element so that, in use, the wear flat is mostly formed in the material of lower erosion resistance.

Abstract: A rotary drill bit for drilling holes in subsurface formations comprises a bit body having a shank for connection to a drill string, a plurality of preform primary cutting elements mounted on the bit body and defining a primary cutting profile having a downwardly convex nose portion. There are associated with at least certain of the primary cutting elements respective secondary elements which are spaced inwardly of the primary profile. The distance of the secondary elements from the primary profile, when measured in a direction perpendicular to said profile, is generally greater for secondary elements nearer the nose portion than it is for secondary elements further away from the nose portion, and is preferably such that the vertical distance of the secondary elements from the profile is substantially constant.

Abstract: A rotary drill bit comprises a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted on the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit. Certain cutter assemblies on the bit body are adapted to exhibit a volume factor which is significantly greater than the volume factor of other cutter assemblies on the bit body, with increase of rate of penetration, and at least the majority of said cutter assemblies of higher volume factor are better adapted for cutting softer formations than at least the majority of said other cutter assemblies. The bit therefore tends to act as a "heavy set" drill bit at lower rates of penetration in hard formations, and as a "light set" drill bit at higher rates of penetration in softer formations, and therefore tends to drill each formation more efficiently.

Abstract: A rotary drill bit comprises a bit body carrying a plurality of primary preform cutting elements defining a primary cutting profile. The bit includes means to apply a lateral imbalance force to the bit as it rotates, and a portion of the outer periphery of the bit body includes a low friction bearing surface so located as to transmit the lateral force to the formation. There are associated with some of the primary cutting elements respective secondary elements spaced inwardly of the cutting profile defined by the primary cutting elements, but the portion of the periphery of the bit body where the bearing surface is located is substantially free of such secondary elements. The arrangement enhances the anti-whirl characteristics of the bit.

Abstract: A rotary drill bit comprises a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of polycrystalline diamond preform cutting elements. The cutting elements are mounted on a single radially extending blade so that the bit is imbalanced and a resultant sideways force is applied to the bit as it rotates during drilling. The gauge of the bit body is provided with a low friction bearing pad which extends rearwardly of the blade and transmits the resultant sideways force to the sides of the bore hole. Since the bearing pad is low friction, it slides around the surface of the formation and any tendency for bit whirl to be initiated is reduced.

Abstract: A rotary drill bit comprises a bit body carrying a plurality of preform cutting elements, and a plurality of circumferentially spaced gauge pads which, in use, engage the surrounding formation. Some of the gauge pads carry cutting elements and others are free of cutting elements. To reduce cost, each gauge pad which is free of cutting elements has an outer bearing surface which is more abrasion resistant than the outer surfaces of the gauge pads which carry cutting elements.