Because of the large area of sliding contact between the moving and stationary threads, screws typically have large frictional energy losses. Even well-lubricated jack screws have efficiencies of only 15% - 20%, the rest of the work applied in turning them is lost to friction. When friction is included, the mechanical advantage is no longer equal to the distance ratio but also depends on the screw's efficiency. From conservation of energy, the work "Win" done on the screw by the input force turning it is equal to the sum of the work done moving the load "Wout", and the work dissipated as heat by friction "Wfric" in the screw