This paper has brought forth the method of determining the factors of position shaft by diagram for the same bending stress, and authors hava drwed the diagram for above aim.

Through enlarging the top fillet arcs radii and meanwhile reducing the addenda of the tools, an improvement on basic profiles of rack kind tools is proposed to improve the tooth bending strength of the external involute cylindrical gears.

The specific features of acting pressure and stress of involute scroll teeth were analyzed and the criterion for calculating bending fatigue-strength of scroll tooth roots was determined with a load model based on tooth root bending-fatigue strength being obtained.

The root stress and root stress intensity factor with the crack initiating position and expansion direction are studied and computed by the finite element method.