Gear measurement method and gear measurement apparatus

A gear measurement apparatus that measures a shape of a tooth groove of a gear, the gear measurement apparatus comprising:

• a measurement ball configured to be rolled along the tooth groove of the gear such that the ball simultaneously contacts both of opposing surfaces of the tooth groove, the gear being rack teeth formed on a shaft member;
• a measurement device configured to measure an actual movement locus of the ball at a center point of the ball as the ball is rolled along the tooth groove of the gear, the actual movement locus being a shape evaluation index for the gear, a reference point of each tooth groove based on a measurement of an arc outer peripheral surface positioned on a back side of each tooth groove of each rack tooth of the gear, the actual movement locus of the ball for each rack tooth being measured by setting the decided reference point of each tooth groove as an origin point and an axial direction of a rack shaft is determined as a Y-axis, a two-dimensional coordinate system of a plane that is orthogonal to the Y-axis for each tooth groove is set based on a measurement of a tooth tip of each rack tooth of the gear, the actual movement locus of the ball for each rack tooth being measured in the set two-dimensional coordinate system;
• a design locus acquisition device configured to acquire a design locus, which is a movement locus in design, of the ball where the ball is moved along the tooth groove of the gear;
• and
• a computing device configured to compute an error between the measured actual movement locus and the acquired design locus.