A motor assembly includes a working part, a synchronous motor having a shaft, and a mechanical coupling joining the working part to the motor. The coupling has two driving teeth fixed relative to the shaft, two driven teeth fixed relative to the working part, and two middle members. The coupling provides a predetermined range of angular movement between the motor and the working part. The middle members are circumferentially distributed between the two driving teeth and between the two driven teeth, thereby each of the middle members is movable by the driving teeth to contact the driven teeth and provides an interference between a driving tooth and a driven tooth to drive the working part.