Rotary steerable tool

A rotary steerable tool adapted to be mounted in a downhole drilling apparatus for adjusting the direction of drilling of the apparatus, the rotary steerable tool comprising:

• a tubular outer housing;
• at least one steering pusher slidably mounted to the housing for movement between an extended position, in which the steering pusher engages a wall of a borehole formed by the drilling apparatus, and a withdrawn position, in which the steering pusher does not engage the wall of the borehole;
• a tubular sleeve mounted inside the housing and adapted to be connected at first and second ends thereof to a drill string to transmit rotary drive to a drilling bit, wherein the sleeve defines a passage for passage of drilling fluid to the drilling bit;
• a pressure chamber defined between the sleeve and the housing and communicating with at least one of said steering pushers for enabling the steering pusher to move from the withdrawn to the extended position thereof;
• and
• a piston slidably mounted in the tubular sleeve and adapted to be moved by predetermined changes in drilling fluid pressure between a first axial position, in which the interior of the sleeve communicates directly with the pressure chamber to cause at least one of said steering pushers to move to the extended position thereof to engage the wall of the borehole and adjust the direction of drilling of the drilling apparatus by bending the drilling apparatus, and a second axial position, in which the interior of the sleeve does not communicate directly with the pressure chamber to prevent each of the said steering pushers from moving to the extended position thereof.