The apparatus of claim 1 wherein the axis of rotation of the first valve member is offset from the second member such that rotation of the first member moves the openings out of alignment.

Downhole flow pulsing apparatus for providing a percussive effect, the apparatus comprising:</claim-text> a housing for location in a string, the housing defining a throughbore to permit passage of fluid therethrough; a valve located in the bore ing a flow passage and including a valve member, the valve member being movable to vary the area of the flow passage to, in use, provide a varying fluid flow therethrough; a fluid actuated positive displacement motor operatively associated with the valve for driving the valve member; and a pressure responsive device which expands or retracts in response to the varying fluid pressure created by the varying fluid flow, the expansion or retraction providing a percussive effect. view more

The apparatus of claim <HIL><BOLD>1</BOLD></HIL> wherein the speed of the motor is directly proportional to the rate of flow of fluid through the motor.

The apparatus of claim <HIL><BOLD>1</BOLD></HIL> the positive displacement motor operates using the Moineau principle and includes a lobed rotor which rotates within a lobed stator, the stator having one more lobe than the rotor.

The apparatus of claim <HIL><BOLD>1</BOLD></HIL>, wherein the valve includes first and second valve members each defining a respective axial flow opening and which openings are aligned to collectively define an open axial drilling fluid flow port first member being rotatable about a longitudinal axis of the housing to vary the alignment of the openings and thus vary the open area of said port between a minimum open area and a maximum open area to, in use, provide a varying flow therethrough and variation of the fluid pressure. view more

Downhole flow pulsing apparatus comprising:</claim-text> a housing for location in a string, the housing defining a throughbore to permit passage of fluid therethrough; a valve located in the bore and including first and second valve members each ing a respective axial flow opening and which openings are aligned to collectively define an open axial drilling fluid flow port through the valve, the first member being rotatable about a longitudinal axis of the housing to vary the alignment of the openings and thus vary the open area of said port between a maximum open area and a minimum open area to, in use, provide a varying flow therethrough and variation of the fluid pressure; and a fluid actuated positive displacement motor operatively associated with the valve for driving the valve member. view more