Definition of rotary drill motor

The space available in the casing of a pneumatic rotary rock or coal drill is necessarily limited and precludes the use of a reciprocating engine. The power unit used instead is similar in design to the vane compressor. The rotor runs at a very high speed, between 3,500 rpm and 4,000 rpm, and this is reduced by gearing to give a drill spindle speed of about 650 rpm.