section_path: Self-locking machines
section_name: Self-locking machines
paragraph_text: Self-locking machines
  
  In many simple machines, if the load force F out {\displaystyle F_{\textrm {out}}} on the machine is high enough in relation to the input force F in {\displaystyle F_{\textrm {in}}}, the machine will move backwards, with the load force doing work on the input force. So these machines can be used in either direction, with the driving force applied to either input point. For example, if the load force on a lever is high enough, the lever will move backwards, moving the input arm backwards against the input force. These are called reversible, non-locking or overhauling machines, and the backward motion is called overhauling.
  
  However, in some machines, if the frictional forces are high enough, no amount of load force can move it backwards, even if the input force is zero. This is called a self-locking, nonreversible, or non-overhauling machine. These machines can only be set in motion by a force at the input, and when the input force is removed will remain motionless, "locked" by friction at whatever position they were left.
  
  Self-locking occurs mainly in those machines with large areas of sliding contact between moving parts: the screw, inclined plane, and wedge:
  
  A machine will be self-locking if and only if its efficiency η {\displaystyle \eta } is below 50%:
  
  η ≡ F out / F in d in / d out < 0.5 {\displaystyle \eta \equiv {\frac {F_{\text{out}}/F_{\text{in}}}{d_{\text{in}}/d_{\text{out}}}}<0.5}
  
  Whether a machine is self-locking depends on both the friction forces (coefficient of static friction) between its parts, and the distance ratio d in / d out {\displaystyle d_{\textrm {in}}/d_{\textrm {out}}} (ideal mechanical advantage). If both the friction and ideal mechanical advantage are high enough, it will self-lock.