Patent ID: 7878272

Claim:
A downhole tool comprising: a downhole tool body disposed to be rotatably coupled with a drill string; at least three blades fixed to and extending radially outward from the tool body; at least one piston deployed in each of the blades, the pistons disposed to displace between radially opposed retracted and extended positions; at least one spring member deployed in each piston, said spring member disposed to exert a spring force that elastically spring biases the piston radially inward towards the retracted position; a hydraulic actuation module disposed to exert a hydraulic force that urges each piston radially outward against the spring force towards the extended position; and the spring force and the hydraulic force being preselected such that they balance external forces acting on the piston during rotation of the tool in a subterranean borehole, said balance of forces causing the tool to be continually urged towards a center of the borehole during rotation of the tool in the borehole.