page_title: Simple machine
section_title: Self-locking machines
hierarchical_section_title: Simple machine / Self-locking machines
caption_attribution_description: Dansk: en bolt og en skrue English: a bolt and a screw Polski: śruba i wkręt Čeština: šroub (s matkou) a vrut
context_page_description: A simple machine is a mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage to multiply force. Usually the term refers to the six classical simple machines that were defined by Renaissance scientists:
  Lever
  Wheel and axle
  Pulley
  Inclined plane
  Wedge
  Screw
  A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force. The machine can increase the amount of the output force, at the cost of a proportional decrease in the distance moved by the load. The ratio of the output to the applied force is called the mechanical advantage.
  Simple machines can be regarded as the elementary "building blocks" of which all more complicated machines are composed. For example, wheels, levers, and pulleys are all used in the mechanism of a bicycle. The mechanical advantage of a compound machine is just the product of the mechanical advantages of the simple machines of which it is composed.
context_section_description: In many simple machines, if the load force Fₒᵤₜ on the machine is high enough in relation to the input force  Fᵢₙ, 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:
  The most common example is a screw.  In most screws, applying torque to the shaft can cause it to turn, moving the shaft linearly to do work against a load, but no amount of axial load force against the shaft will cause it to turn backwards.
  In an inclined plane, a load can be pulled up the plane by a sideways input force, but if the plane is not too steep and there is enough friction between load and plane, when the input force is removed the load will remain motionless and will not slide down the plane, regardless of its weight.
  A wedge can be driven into a block of wood by force on the end, such as from hitting it with a sledge hammer, forcing the sides apart, but no amount of compression force from the wood walls will cause it to pop back out of the block.
  A machine will be self-locking if and only if its efficiency η is below 50%:
  Whether a machine is self-locking depends on both the friction forces (coefficient of static friction) between its parts, and the distance ratio dᵢₙ/dₒᵤₜ (ideal mechanical advantage).  If both the friction and ideal mechanical advantage are high enough, it will self-lock.
caption: The screw's self-locking property is the reason for its wide use in threaded fasteners like bolts and wood screws
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