A friction drive tool adapter uses a planetary speed-increasing mechanism of which the rollers 10 are supported in a rotary input portion 1-3 of the adapter, the annular raceway 22 is in a housing 20 and the sun roller 32 is on a tool mount 30.
According to the invention the tool adapter is further provided with a first angular speed sensor 51A provided on the fixed portion 20 for detecting the rate of rotation of the rotary portion, a second angular speed sensor 51A provided on the fixed portion for detecting the rate of rotation of the tool mount 30, and an abnormal rotation monitor 50 which receives inputs from the first and second sensors and generates an abnormality signal when the step-up ratio falls below a predetermined level, owing to slippage in the mechanism caused by an overload; damage to the adapter and poor machining can therefore be avoided.
The adapter also includes a built-in generator 52 consisting of a rotor 52b provided on the outer circumferential part of the rotary potion and a stator 52a provided in the interior of the fixed portion 30 so as to provide a power source for the abnormal rotation monitor 50.