Junk sub is an effective drilling tool forcatching the heavy scrap iron or metal debriswhich are  too  heavy  to  be  completely  circulated  out  of  the  hole  in  normal  drilling  operation.Its cup  OD  is  big,smaller  annular  gap  with  the  wellbore,but  the  mandrel  is  small  at  the  rim of cup,larger  annular  gap  with  wellbore.So  the  fluid  speed  dropped  promptly  and  form  a  vortex  at  the  rim  of  cup,meanwhile,the  carry  ability  of  fluid  also  dropped.So  the  heavy objects in the fluid will fall into the cup and carried out in the trip process.