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Delbert Hall and Stew Ives. The screw Simple machines in technical theatre. Introduction. The screw is a modified version of the inclined plane.

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  • The screw is a modified version of the inclined plane.
  • While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp similar to that of a spiral staircase.
calculating pitch
Calculating Pitch
  • The distance between two adjacent screw threads is called the pitch of a screw.
  • For example, if you place a ruler parallel to a screw and count 10 threads in a distance of one inch, then the pitch of the screw would be 1/10.
the mechanical advantage of the screw
The Mechanical Advantage of the Screw
  • The screw is often turned by another simple machine such as a lever or a wheel and axle.
  • In this case, the total mechanical advantage is equal to the circumference of the simple machine to which the effort force is applied divided by the pitch of the screw.
  • This is sometimes referred to as the “Idea Mechanical Advantage” because is does not take into account any friction.
converting linear motion
Converting Linear Motion
  • Turning the screw one complete revolution will move it into or out of an object a distance to the pitch of the screw, or .1” in this case. Therefore, a screw can be used to convert circular motion into linear motion
  • 1 Revolution’s Distance = Pitch of Screw
mechanical advantage
Mechanical Advantage
  • The formula for determining the ideal mechanical advantage of a screw is:

IMA= 2pL/P

  • Where L = length of the lever and P = 1 pitch
achieving mechanical advantage
Achieving Mechanical Advantage
  • The ideal mechanical advantage is of little meaning since there is typically a lot of friction.*
  • However, the screw is an enormously useful in when pressure is needed to seal the lid onto a jar, a top on a bottle, hold a light bulb in its socket or hold two pieces of wood or metal together.
  • *(Note: Ball screws are used in some machines to greatly reduce friction.)
other uses
Other Uses
  • Because of the mechanical advantage of a screw, it is also useful for moving heavy loads. Such screw driven devices include garage door openers and jacks.
how else can we use the screw in theatre
How else can we use the Screw in Theatre?
  • We can use the same concept behind

an adjustable piano bench to lift a platform.

raising a platform with the screw
Raising a Platform with the Screw
  • Applying force to the lever rotates the long Screw
  • Using a longer lever increases our Mechanical Advantage
  • This Expands the Scissor Jacks and makes the Platform Rise