When hobbing gears with rack type props, when the reference line of the rack cutter is tangent to the dividing circle of the gear blank, the gear cut is a standard gear; When the reference line of the rack cutter is not tangent to the dividing circle of the wheel blank, the gear cut out is a modified gear.
The distance between the reference line of the tool and the dividing circle of the wheel blank is called the displacement amount, expressed in xm, and X is called the displacement coefficient. When the tool leaves the center of the wheel blank, X takes a positive value (called positive deflection), otherwise X takes a negative value (called negative deflection). For helical gears, the relationship between end face modification coefficient and normal modification coefficient is X1=XN.
After gear modification, its tooth profile and standard gear belong to the same involute, but their application sections are different. By using this feature, favorable involute sections can be obtained by selecting modification coefficient X, so that the gear transmission performance can be improved. The application of modified gear can avoid undercutting and improve the contact strength of tooth surface and bending strength of tooth root. In addition, the modified gear can also be used to match the center distance.
