big_diameter=20;
small_diameter=3;
thick=small_diameter/2;

 rotate_extrude(convexity = 10, $fn=100)
 translate([big_diameter/2+small_diameter/2,0,0])

  union(){
    circle(r=small_diameter/2, $fn=50);
    translate([-thick/2,0,0])
    square(size=[thick,small_diameter],center=true);
  }