Shaftless screws from S2S Industries use a thick inside profile for stress resistance. The maximum stress imposed on the screw is by the gear drive along the inside center axis of the screw. Screws are available in a thicker center axis cross section and a thinner outer cross section, or as a one-piece section. They can be manufactured in a variety of standard and exotic materials. 514/228-1660; www.s2sindustries.com.