The screw Simple machines in technical theatre

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# The screw Simple machines in technical theatre - PowerPoint PPT Presentation

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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Delbert Hall and Stew Ives

### The screwSimple machines in technical theatre

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
• 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
• 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

References:

• http://www.cosi.org/files/Flash/simpMach/sm1.html
• http://www.uark.edu/depts/aeedhp/agscience/simpmach.htm
• http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/incline.html