Transcript Simple Machines In Technical Theatre The Screw Delbert

THE SCREW
SIMPLE MACHINES IN
TECHNICAL THEATRE
Delbert Hall and Stew Ives
Introduction
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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
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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
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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
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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
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The formula for determining the ideal mechanical
advantage of a screw is:
IMA= 2pL/P
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Where L = length of the lever and P = 1 pitch
Comparison of Mechanical Advantage:
Achieving Mechanical Advantage
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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
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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?
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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:
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http://www.cosi.org/files/Flash/simpMach/sm1.html
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http://www.uark.edu/depts/aeedhp/agscience/simpmach.htm
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http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/incline.html