THE SCREWSIMPLE 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

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