Abstract: The equation of asymmetric rackcutter in the normal plane was established based on gear machining principle. Through matrix transformation with the homogeneous coordinate， the profile equation of generating gear was derived. The tangent vector of undercutting point on the surface of generating gear tooth is zero. The range of addendum modification coefficient when undercutting doesn’t occur was obtained， providing the basis for parameter selection in gear design. Based on the finite element model of gear engagement， the change of the tooth root acceleration and Von Mises stress within a meshing cycle was analyzed. In addition， the influences of different torque and speed on tooth root impact stress were investigated. The relationship between the impact stress and gear top modification was achieved. The results showed that for asymmetric helical gear with lownumber teeth， three directions of x， y and z are all important to gear vibration. Rotating speed affects gear impact more significantly than torque， and appropriate modification will reduce the impact stress.

Key words: lownumber teeth, asymmetric helical gear, undercutting, meshing impact, modification