This invention refers to a speed reducer made up of a chain carrying nuts and of a screw, i.e., a bail screw in the firts embodiment, a roller screw in the second embodiment. The ball screw or the roller screw drives the nuts when these are aligned on one of the straight straigth stretches of chain. The ball screw, or the roller screw, can be almost as long as the distance betweew the centers of the output sprockets on wich the chain is mounted, given these features and because the nuts wrap the ball screw, or the roller screw in a greatsurface, the load capacity is high. Given that in both embodiments the contact is rolling, friction is before therefore low and as consequence, efficiency is high, so high indeed that it allows this invention also the server as an amplifier.