... gear through the method of quenching, so that the teeth become harder and the transmitting noise becomes smaller. Small conical helical ... box is 20GrMnTi. After specially processed,the precision and hardness become higher,so that the service life of screws becomes longer ... . 4.6 Screw and barrel are made of 38CrMoAlA, which become hard, wear-resisting and anti-corrosive by precision work and ...