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A method and assembly to produce data pulses in a drilling fluid in a drill string. The assembly comprises a shear valve that includes a valve member mounted in a valve passage in fluid flow communication with a fluid flow conduit of a drill string to which the assembly is connectable. The valve mem

A method and assembly to produce data pulses in a drilling fluid in a drill string. The assembly comprises a shear valve that includes a valve member mounted in a valve passage in fluid flow communication with a fluid flow conduit of a drill string to which the assembly is connectable. The valve member is connected to a reciprocation mechanism comprising a rocker, a driven crank arrangement, and a slider member that provides a sliding coupling between the crank arrangement and the rocker. The slider member is pivotally connected to the crank arrangement, is keyed to the rocker for angular displacement about a valve axis, and is radially slidable relative to the rocker, so actuation of the crank arrangement causes angular reciprocation of the rocker, and hence of the valve member, about the valve axis, to produce data pulses in the drilling fluid.

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1. An assembly to produce data pulses in a drilling fluid in a drill string, the assembly comprising: a housing having a hollow interior, the housing being connectable to the drill string to place a valve passage defined by the hollow interior of the housing in fluid flow communication with a drilli

1. An assembly to produce data pulses in a drilling fluid in a drill string, the assembly comprising: a housing having a hollow interior, the housing being connectable to the drill string to place a valve passage defined by the hollow interior of the housing in fluid flow communication with a drilling fluid conduit defined by the drill string;a shear valve mounted in the valve passage to produce data pulses in the drilling fluid by varying obstruction by the shear valve of the valve passage, the shear valve comprising a valve member which is angularly displaceable about a valve axis that is longitudinally aligned with the drill string, to vary obstruction of the valve passage; anda reciprocation mechanism operatively connected to the valve member to actuate angular reciprocation of the valve member about the valve axis, the reciprocation mechanism comprising: a rocker that is drivingly connected to the valve member, the rocker being mounted to be substantially coaxial with the valve axis and to be angularly displaceable about the valve axis,a driven crank arrangement mounted to rotate about a crank axis substantially parallel to and transversely spaced from the valve axis, anda slider member which extends radially between the driven crank arrangement and the rocker, the slider member coupled to provide a sliding coupling between the driven crank arrangement and the rocker, the slider member pivotally connected to the driven crank arrangement, the slider member keyed to the rocker for angular displacement about the valve axis, and the slider member coupled to be radially slidable relative to the rocker, so that angular reciprocation of the slider member about the valve axis due to rotation of the driven crank arrangement results in angular reciprocation of the rocker and hence of the valve member. 2. The assembly of claim 1, wherein the rocker defines a radial bore that extends radially therethrough, the slider member comprising a complementary mating shank which is slidingly received in the radial bore, the complementary mating shank and the radial bore having complementary peripheral outlines in cross-section. 3. The assembly of claim 1, wherein the driven crank arrangement comprises a crank wheel mounted to rotate about the crank axis, the slider member being pivotally connected to the crank wheel at a pivot axis which is parallel to and radially spaced from the crank axis, the pivot axis orbiting the crank axis upon driven rotation of the crank wheel. 4. The assembly of claim 1, further comprising a motor operably connected to the driven crank arrangement to drive the driven crank arrangement, the motor being mounted in the hollow interior such that the motor is located more or less centrally in the drilling fluid conduit defined in part by the hollow interior of the housing, when the housing is viewed in cross-section. 5. The assembly of claim 4, further comprising a torsionally resilient torsion member coaxial with the rocker and operatively connected to the rocker to transmit torque thereto, the torsionally resilient torsion member being anchored against rotation at a fixed end thereof furthest from the rocker, to exert torque on the rocker responsive to angular displacement of the rocker relative to the fixed end of the torsionally resilient torsion member. 6. The assembly of claim 5, wherein the torsionally resilient torsion member is connected to an end of the rocker furthest from the valve member, the torsionally resilient torsion member extending through a passage defined by the motor. 7. The assembly of claim 1, wherein the shear valve comprises a stator that defines a circumferentially extending series of ports, the valve member comprising a circumferentially extending series of blades that are complementary to the ports, such that angular displacement of the valve member about the valve axis displaces the valve member between an open position in which the respective blades clear corresponding ports to allow the flow of drilling fluid therethrough, and a closed position in which the respective blades are in register with the corresponding ports to obstruct the flow of drilling fluid through the ports. 8. The assembly of claim 7, wherein the blades and the ports are substantially identical in size and shape. 9. The assembly of claim 7, wherein the shear valve and the reciprocation mechanism are arranged such that a particular blade obstructs two or more of the series of ports in a single cycle of its angular reciprocation. 10. The assembly of claim 9, wherein angular spacing of the blades and the ports respectively, and a reciprocation angle of the valve member about the valve axis, are selected such that the particular blade is in register with one of the series of ports at one extremity of its angular reciprocation, and is in register with another one of the series of ports at the opposite extremity of its angular reciprocation. 11. The assembly of claim 7, further comprising an amplitude modification arrangement to dynamically vary an axial spacing between the stator and the valve member, thereby to vary an amplitude of data pulses produced by reciprocation of the valve member. 12. The assembly of claim 7, further comprising a torque assist arrangement to effect the exertion of torque on the valve member by the drilling fluid, to urge the valve member to the closed position, the torque assist arrangement comprising an impingement surface defined by the valve member and an opening extending axially through the stator to direct drilling fluid on to the impingement surface when the impingement surface is brought in to register with the opening in the stator. 13. The assembly of claim 12, wherein the opening and the impingement surface are positioned such that angular displacement of the valve member towards the closed position brings the opening to register with the impingement surface prior to reaching the closed position. 14. A valve mechanism comprising: a stator defining at least one fluid flow port therethrough;a valve member mounted adjacent the stator, and coupled to be pivotally displaceable about a valve axis between a closed position in which the valve member obstructs the at least one of a port of the stator, and an open position in which the valve member substantially clears the at least one fluid flow port of the stator;a rocker that is drivingly connected to the valve member to transmit torque and/or pivotal displacement of the valve axis to the valve member, the rocker being pivotally displaceable about the valve axis;a driven crank arrangement mounted to rotate about a crank axis substantially parallel to and transversely spaced from the valve axis; anda slider member which provides a sliding coupling of the driven crank arrangement to the rocker, the slider member connected to the driven crank arrangement to pivot about a pivot axis that orbits the crank axis upon rotation of the driven crank arrangement, the slider member slidably received in a complementary mating formation forming part of the rocker such that the slider member intersects the valve axis, the slider member keyed to the rocker for pivotal displacement about the valve axis, so that rotation of the driven crank arrangement causes actuation of reciprocating pivotal displacement of the rocker about the valve axis. 15. The valve mechanism of claim 14, wherein the complementary mating formation of the rocker comprises an elongated bore extending diametrically through the rocker, the slider member comprising a complementary mating shank which is slidingly received in the elongated bore. 16. The valve mechanism of claim 14, wherein the driven crank arrangement comprises a crank wheel mounted to rotate about the crank axis, the pivot axis about which the slider member is pivotally connected being to and radially spaced from the crank axis. 17. The valve mechanism of claim 14, wherein the stator defines a circumferentially extending series of fluid flow ports, the valve member being coaxial with the stator and comprising a circumferentially extending series of blades that are complementary to the ports, such that pivotal displacement of the valve member about the valve axis displaces the valve member between the open position in which the respective blades clear corresponding ports to allow the flow of fluid therethrough, and the closed position in which the respective blades are in register with the corresponding ports to obstruct the flow of drilling fluid through the ports. 18. The valve mechanism of claim 17, wherein the driven crank arrangement and the rocker are arranged such that a particular blade obstructs two or more of the series of ports in a single cycle of its pivotal reciprocation. 19. The valve mechanism of claim 14, further comprising a torque assist arrangement to effect the exertion of torque on the valve member by fluid flowing under pressure through the stator, to urge the valve member to the closed position, the torque assist arrangement comprising an impingement surface defined by the valve member and an opening extending axially through the stator to direct drilling fluid on to the impingement surface when the impingement surface is brought in to register with the opening in the stator. 20. A method to produce data pulses in a drilling fluid flowing through a drill string, the method comprising: mounting a shear valve in a valve passage that forms part of a drilling fluid conduit provided by the drill string, the shear valve comprising a valve member which is angularly displaceable about a valve axis longitudinally aligned with the drill string, to vary obstruction of the valve passage;mounting a reciprocation mechanism in the drill string such that the reciprocation mechanism is connected to the shear valve, the reciprocation mechanism comprising: a rocker that is drivingly connected to the valve member to drive angular reciprocation of the valve member, the rocker mounted to be substantially coaxial with the valve axis and to be angularly displaceable about the valve axis,a driven crank arrangement mounted to rotate about a crank axis substantially parallel to and transversely spaced from the valve axis; anda slider member providing a sliding coupling between the driven crank arrangement and the rocker, the slider member connected to the driven crank arrangement to pivot about a pivot axis parallel to the crank axis, the slider member slidably received in a complementary mating formation forming part of the rocker such that the slider member intersects the valve axis, the slider member keyed to the rocker for angular displacement about the valve axis; anddriving rotation of the driven crank arrangement such that the pivot axis orbits the crank axis, thereby actuating angular reciprocation of the rocker about the valve axis, to produce data pulses in the drilling fluid by varying obstruction of the valve passage by the valve member. 21. The method of claim 20, further comprising varying speed of rotation of the driven crank arrangement, to modulate a frequency of the data pulses. 22. The method of claim 20, wherein the shear valve comprises a stator defines a circumferentially extending series of fluid flow ports, the valve member being coaxial with the stator and comprising a circumferentially extending series of blades that are complementary to the ports, such that pivotal displacement of the valve member about the valve axis displaces the valve member between an open position in which the respective blades clear corresponding ports to allow the flow of fluid therethrough, and a closed position in which the respective blades are in register with the corresponding ports to obstruct the flow of drilling fluid through the ports. 23. The method of claim 22, wherein the driven crank arrangement and the rocker are arranged such that a particular blade obstructs two or more of the series of ports in a single cycle of its pivotal reciprocation. 24. The method of claim 22, further comprising exerting a flow assisted torque on the valve member by means of the drilling fluid flowing, to urge the valve member to the closed position. 25. The method of claim 24, wherein the exerting the flow assisted torque comprises bringing an impingement surface defined by the valve member into register with an opening extending axially through the stator, to direct drilling fluid flowing through the opening in the stator on to the impingement surface.