A hoisting boom assembly comprising:
- an upright column rotatable around a vertical axis and a first and a second boom;
- said first boom having a first end pivotally mounted on said upright column and a second end pivotally connected to said second boom;
- said second boom being made up of a first and a second section, said first section being pivotable in a vertical plane and said second section being journalled to said first section and pivotable in said vertical plane;
- the operating range of said second boom being defined between a first extreme position in which longitudinal directions of its first and second section are substantially parallel to each other and, a second extreme position in which the longitudinal directions of its first and second section are substantially opposite to each other;
- an actuator fitted between said upright column and said first boom for pivoting said first boom in a vertical plane relative to said upright column;
- a horizontal shift mechanism and a horizontal shift actuator for shifting a free end of said second boom in a horizontal direction, said horizontal shift mechanism operating between said first and second sections of said second boom independently of the position of said first boom;
- compensating means for equalizing forces created by the movement of said hoisting boom assembly at least when said second boom is close to said first and second extreme position, said compensating means being formed by combining a horizontal shift actuator, included in said horizontal shift mechanism and connected at one end to said first section of said second boom, and at the other end to said first boom at a first pivoting point substantially spaced from a second pivoting point linking a first transmission rod forming a part of said horizontal shift mechanism and said first boom;
- an auxiliary frame connected to said hoisting boom assembly for movably supporting a drilling device;
- tilting means including a first means for tilting said auxiliary frame due to the movement of the hoisting boom assembly;
- said first means being formed of a lever arm arrangement for connecting said auxiliary frame by a first pivotal lever arm to a first end of a lever member which is rotatably connected to a middle part of said second section of said second boom, the lever member being further connected at its other end by a second pivotal lever arm to the end of said first section of said second boom.