THE SCREW SIMPLE MACHINES IN TECHNICAL THEATRE Delbert Hall and Stew Ives

Introduction  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  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 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  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  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

Comparison of 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  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?  We can use the same concept behind an adjustable piano bench to lift a platform.

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

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