求翻译：As shown in Fig. 1, the ball joint consists of an outer part, a cage, an inner part, and six balls. The internal surface of the outer part is built with a sphere centred at O and six circular tracks centred at O1 presenting gothic arc profile. This centre O1 is shifted with respect to the sphere centre O by an offset e. Similarly, the external surface of the inner part is built with a sphere centred at O and six circular tracks centred at O2 presenting gothic arc profile; the centre O2 is shifted with respect to the sphere centreO by the same offset e.是什么意思？

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