The screw Simple machines in technical theatre

1
The screwSimple machines in technical theatre
Delbert Hall and Stew Ives
2
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.

3
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.

4
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.

5
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 Revolutions Distance Pitch of Screw

6
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

7
Comparison of Mechanical Advantage
8
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.)

9
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.

10
How else can we use the Screw in Theatre?

• We can use the same concept behind
• an adjustable piano bench to lift a platform.

11
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

12

• References
• http//www.cosi.org/files/Flash/simpMach/sm1.html
• http//www.uark.edu/depts/aeedhp/agscience/simpmac
  h.htm
• http//hyperphysics.phy-astr.gsu.edu/hbase/mechani
  cs/incline.html