US Patent 7261169 Shank for a rotary/percussion tool

A shank for a working tool has at least three recesses (5) provided within the lockable region (3) and arranged in a cross-section (c-c) to the tool axis (A) at least at angles of 0°, 120°, and 240° within the radial reference dimension (R) of the shank, with at least one of the recesses (5) being axially closed toward a free end of the shank (1), and at least one protrusion (6) arranged circumferentially between two adjacent recesses (5) outside of the radial reference dimension, with the sum of central recess angles (α) of the recesses amounting to at least 150°, and a central protrusion angle (β) of the at least one protrusion (6) being smaller than a central recess angle (α).