A new continuous dynamic system for measuring the sphericity error of bearing balls

Abstract

The sphericity error is one of the most important tolerances to evaluate the dimensional accuracy of bearing balls. A continuous dynamic measurement system based on an industrial camera and a Z-shaped transparent track is developed to measure the sphericity error of bearing balls. The Z-shaped track, which is considered for the parameter setting of length, width and inclination, is designed to achieve a pure rolling of the ball around three perpendicular axes. During the whole measurement process, 15 images are dynamically captured by an industrial camera and divided into three groups according to the three mutually perpendicular directions. Then, a sub-pixel edge contour of the bearing ball from each image is extracted by an image-processing step, such as image de-noising, contrast enhancement, region-of-interest extraction and edge detection. Finally, the roundness error and sphericity error, as defined by ISO 12181-1 and ISO 3290-1, are acquired from the extracted contour of each image. Three different diameter bearing balls with a precision grade of G1000, representing a sphericity error of approximately 25 μm, are applied as experimental objects. The two times standard deviation roundness error obtained from the contour of each image are all less than 7 μm after six repeated measurements. In addition, the two measurement results respectively acquired from the proposed method and a commercial projection measuring instrument have good consistency.