A bi-directional actuator is disclosed which comprises a frame, rotatable about a principal longitudinal axis. The frame mounts two or more partially spherical drive elements. The centers of the partially spherical elements are located on the longitudinal axis of the frame, and the elements are mounted for rotation about individual axes which are perpendicular to but intersect with the longitudinal axes. The individual axes of the respective drive elements are also angularly oriented with respect to each other. The cross sectional outline of the frame is contained within a cylindrical envelope of less diameter than the spheres. Driving devices, such as a belt, are provided to engage at least one of the drive elements in any rotary position of the frame. Directly opposite the belt or other driving device, the driven element or elements engages a material to be driven, typically a piece of fabric. Rotation of the drive elements by the belt provides for material advancement along a first (longitudinal) axis, rotation of the frame about its own axis serves, through the spherically contoured drive elements, to advance the material along a second axis, at right angles to the first.