52 to transmit the rotation of the reduction gear unit 30 to the kneading tool 18 preferably the worm wheel 40 is made of low heat resistant material such as resin preferably the first member 51 is made of a resin which can withstand higher temperatures the second member 52 is made of metal the main shaft 70 comprises a metal body 72 and a thermal insulating portion 74 formed on one end of the metal body 72 adjacent the output unit 50 the first member 51 and the second member 52 are sleeved on the thermal insulating portion 74 preferably the thermal insulating portion 74 is made of heat resistant plastic for example a thermosetting plastic and over molded on the metal body 72 an aperture 76 for discharging the gas generated during the forming of the thermal insulation portion 74 is formed in the thermal insulating portion 74 a sealing ring 37 is received inside the circular disc 53 and tightly sleeved on the main shaft 70 in this embodiment the cost and weight of the motor assembly 20 are relatively low as the worm wheel 40 and the first member 51 are made of resin moreover the.