Centralizer for wireline tools

A centralizer apparatus comprising:

• a central mounting rod (20);
• two floating spring mechanisms (30, 90) mounted on the central mounting rod and free to move along at least part of its length;
• three or more jointed arms (120, 140), spaced around the central mounting rod, each arm being made up of two arm sections (60, 80) of equal length connected by a hinge (70), the end of each arm section distant to the hinge being connected to a pivot (50) on a respective floating spring mechanism, such that the jointed arm is suspended between the two floating spring mechanisms and can open or close in a lateral direction;
• each jointed arm being adapted proximate to the hinge for engaging the wall of a borehole;
• the floating spring mechanisms comprising a spring (36, 96), and means (42, 62) to transfer an axial force from the spring to a rotational force about the pivot on each of the jointed arm sections, such that they are biased to open laterally;
• the centralizer apparatus further comprising:
• blocking means (22, 24) attached to the mounting rod;
• and
• a linkage (99) connected to the spring of the floating spring mechanism, the linkage being disposed to engage the blocking means of the mounting rod before the floating spring mechanism is engaged by the blocking means, thereby transferring an axial force acting on the mounting rod to a force acting on the spring of the floating spring mechanism.