$fn = 200;

module shaft() {
    difference() {
        cylinder(h = 5, d = 6);
        translate([2, -2.5, 0])
            cube(5);
    }

    translate([0, 0, -5])
        cylinder(h = 5, d = 7);
}

difference() {
    cylinder(h = 16, d1 = 18, d2 = 15);
    translate([0, 0, 5])
        shaft();
}