Hydraulic drive unit for driving a drilling tool

A drive unit comprising:

• a frame;
• a driven shaft mounted to rotate with respect to the frame about an axis of rotation;
• a first pressurized fluid motor presenting a lobed cam fixed with respect to said frame;
• a cylinder block fixed, with respect to rotation, with said driven shaft;
• a plurality of cylinders arranged in the cylinder block and disposed radially with respect to the axis of rotation;
• a plurality of pistons mounted to slide in the cylinders and capable of abutting said cam;
• the shaft being capable of being driven by the first motor at at least one first speed of rotation and presenting a recess which constitutes a passage which conveys drilling fluid to a drilling tool;
• and, the cylinder block of said first motor being coaxial to the axis of rotation and is disposed about said shaft, wherein:
• A) said drive unit comprises a second motor which is coupled in rotation to said shaft and which is capable of driving said shaft at a second speed of rotation substantially higher than said at least one first speed of rotation;
• B) a first pinion gear is fixed, with respect to rotation, with the shaft, surrounds said shaft and is coaxial to the axis of rotation;
• C) the second motor presents an output shaft with which is fixed, with respect to rotation, with a second pinion gear which meshes with said first pinion gear,
• D) the first pressurized fluid motor is disengageable and is provided with a disengagement mechanism capable of controlling withdrawal of said pistons out of abutment with the lobed cam.