Friction drive spindle unit

A spindle unit for rotating a rotatable tool, said spindle unit comprising:

• a spindle assembly including
• a housing having a forward end;
• a spindle rotatable in the housing about a spindle axis and having an enlarged head that is exposed at the forward end of the housing and is configured to drive a rotatable tool, a reduced end located within the housing, and an intermediate section located between the head and the reduced end, the reduced end and intermediate section being smaller in transverse cross section than the head;
• a head bearing supporting the spindle in the housing at the forward end of the housing, and being configured to transfer radial loads between the spindle and housing and also axial loads that urge the spindle into the forward end;
• and
• a tail bearing also supporting the spindle in the housing and being offset axially from the head bearing;
• an electric motor including a housing and a rotor that revolves in the housing about a motor axis, the housing of the spindle assembly being fixed in position with respect to, but being a separate component from, the housing of the motor;
• and
• a transmission located between the motor and the spindle assembly and including
• a sun roller driven by the rotor of the motor;
• an outer ring located around the sun roller and coupled as a separate component to the spindle at the reduced end of the spindle, the outer ring being eccentric to the sun roller so that a wedge gap exists between the sun roller and the outer ring;
• support rollers located between the sun roller and the outer ring;
• and
• a loading roller located between the sun roller and the outer ring such that the wedge gap is located to the side of the loading roller, the loading roller being displaceable so that it fits tightly in the wedge gap when torque is transferred from the sun roller to the outer ring through the loading roller.