The invention achieves better use of a machine tool when mounting various types of drills or drill crowns. A safety clutch 14 provided to a drive system contains two clutch elements for transmission of torque centering around a rotating axial line. The clutch elements are coupled through the medium of an engaging mechanism equipped with an engaging element 22. When the engaging element exceeds limit torque to release an engaging state, the limit torque can be adjusted to various values by using one or more operation means 36. |