A coupling device in which a shaft or pipe is coupled to a boss member in which balls, disposed in radial holes in the boss, engage a groove on the shaft or pipe. A set ring slidable on the boss selectively forces engagement of the balls with the groove. The set ring is biased in both direction toward the apertures. The bias can be provided by one or two spring rings engaging two oppositely inclined surfaces on either the boss or the set ring or by a coil spring set between two spring supports that are each moved by both the boss and the set ring.