an air - mechanical expanding chuck for supporting a core for winding webs . a cylindrical chuck body defines a chamber which contains a piston with its piston rod extending out of the chamber for insertion into the core . the piston rod forms a tapered arbor at its end . three gripping elements bear against flat surfaces formed on the tapered arbor . by applying air to one side of the piston , the arbor is moved inboard relative to the core and the gripping elements are thereby expanded to engage the core &# 39 ; s interior . the chuck body &# 39 ; s inward face is provided with radially extending t - grooves to receive corresponding t - formed outer ends of the gripping elements which are movable radially in the t - grooves . the gripping elements are readily removable from the grooves and replaceable by elements of other sizes for different diameter cores . to disengage the gripping elements , compressed air is applied to the inboard side of the piston thereby moving the piston , piston rod and arbor outboard relative to the core whereby the gripping elements are released from the core .