A platform moving mechanism for effecting movement of platform means, such as the deck segments of a tire-building machine having an annular expandable drum defined by a plurality of circumferentially arranged deck segments each having a plurality of supporting members extending circumferentially of the drum in spaced apart side-by-side relation, and which are in end to end interleaved relation between adjacent deck segments, and in which the deck segments are movable radially to effect relative circumferential movement of the supporting members to define substantially cylindrical supporting surfaces of different radii, in which the platform moving mechanism is formed by opposed pusher arms lying in planes extending axially of the drum pivoted at their inner ends to the deck segments and pivoted at their outer ends to pusher rings movable axially toward and away from each other, and in which the inner ends of the pusher arms are provided with meshing gear teeth so that as the pusher rings are moved axially toward and away from each other, the deck segments are moved conjointly radially inwardly and outwardly to provide for the desired radial positioning of the deck segments and consequently the circumferential positioning of the supporting members so that the outer surfaces of the latter define substantially concentric supporting surfaces at any position of the pusher rings with respect to each other.