In this paper, the concept and the principle of virtual modification( mothed are intraduced, The calculation) formula of normal backlash for internal involute gear driving presented.

• 提出了在内啮合渐开线齿轮法隙计算中的虚拟变位概念,阐述了虚拟变位的基本原理。并通过虚拟变位的方法,推导出了内啮合渐开线齿轮传动法向齿侧间隙的计算公式。