Lesson 6: An Engineering Introduction to Machines–Screws (January 10, 2019)

Objectives: (1) To introduce students to the basic concepts of simple machines, as used by engineers; and (2) to specifically learn the most basic principles of the SCREW.

INSTRUCTIONS:

  • Read the article below.1
  • Answer the questions in the blue boxes. PLEASE NOTE: There are 20 questions in blue boxes inserted throughout the article.

 

REVIEW:

Types of machines
Types of machines

In Engineering, there are five main types of simple machines:

    • levers,
    • wheels and axles (which count as one),
    • pulleys,
    • Incline planes, and
    • screws.
REVIEW QUESTIONS:

Question 1 of 20 (incorporate the question in your answer):  What are the 5 main types of simple machines, as discussed in this class?

Question 2 of 20 (incorporate the question in your answer):  In an ideal simple machine, does the work-in generally equal the work-out?

The Screw

  • People always count screws as a separate kind of simple machine. But really screws are just one kind of inclined plane. They are such an important kind that we give them their own category.
  • A screw is simply an inclined plane around a cylinder. To describe this better you can view many types of screws as a cylinder with a head (solid top) at one end and a pointed tip (like a nail) at the other end. More importantly, it has ridges winding around it. The correct term for the ridges (or grooves) around the shaft or cylinder is the thread.

Question 3 of 20 (Write the question and fill in the blank): A screw is simply an _________  ______ around a cylinder.

Question 4 of 20 (Write the question and fill in the blank): The ridges winding around the shaft or cylinder of the screw is the _______.

History

  • People invented wheels about 3700 BC, but the earliest possible example of a screw that we know of was from about 700 BC. That’s when the Assyrian king Sennacherib may have used a screw pump to lift water for the Hanging Gardens of Babylon, the famous gardens of his palace. After that, there is no more evidence of people using screws until about 250 BC, in Sicily. Then the Greek inventor Archimedes certainly made a wooden screw press.
A cider press with big wooden screws
  • After Archimedes, farmers started using screw presses more and more often. They used screw presses to press apples into cider or grapes into wine, or olives into olive oil.  By the late Middle Ages, more than a thousand years later, printers used screw presses to print books and newspapers. But until there were machines to make small metal screws, people didn’t use small screws. The screwdriver, which is a kind of lever, wasn’t invented until about 1800 AD. Even then, screws didn’t really become popular about 1860, when Henry Bessemer invented a way to make good steel cheaply. So modern screws with their screwdrivers are really a combination of two simple machines – the inclined plane and the lever. And they’re really pretty new.

Question 5 of 20 (Write the question and fill in the blank):  The screw was invented about _______ years ago.

Question 6 of 20 (Write the question and fill in the blank):  In about _________ BC, the Greek inventor Archimedes,  made a wooden screw press.

Question 7 of 20 (Write the question and fill in the blank):  After Archimedes, farmers started using screw presses more and more often. They used screw presses to press apples into ________ or grapes into _________, or olives into _____________.

Question 8 of 20 (Write the question and fill in the blank):  Until  about 1860 when there were machines to make ________  ___________  _______________, people didn’t use small screws.

 

Parts of the screw

Diagram 1 – a screw
  • The distance between threads are the same for each screw but are different on other screws. The distance between the threads is called Pitch.
  • Screws are very useful for holding things together. They can pull or push an object together. They can be used to lift very heavy objects and tighten things too.

Question 9 of 20 (Write the question and fill in the blank):  The distance between the threads on the  screw is called the ________.

Question 10 of 20 (Write the question and fill in the blank):  Screws are very useful for holding things _____________.

Question 11 of 20 (Write the question and fill in the blank):  Screws are very useful for ______  very heavy objects.

Nail and Screw

  • These two are not the same. Unlike the nail, a screw has ridges around the shaft. It is harder to drive a screw into a piece of wood because the ridges on the screw create a lot of friction and resistance. To drive a screw into the wood, it has to turn in a circular motion by a screw-driver.

Other kinds of screws

Diagram 2 – bolt and drill bit
  • Bolt:  A bolt is a kind of screw, but does not have a pointed tip. It is not drilled into place, but rather, a hole is made for the bolt to go through. Then a nut is placed at the end to screw the bolt through. Bolts are very powerful in holding things together.
  • Drill Bit: This is a type of screw that can make holes in wood, plastic, metal and stones when attached to an electric drill. Like the regular drill, it is pointed at one end, and it has threads too. The drill bit has deeper grooves that carry pieces of the wood (or other material) from the hole to the surface as the drill bit turns.

Question 12 of 20 (Write the question and fill in the blank):  A _______ is a type of screw without a pointed tip.  A nut is placed at the end to hold onto it.

Question 13 of 20 (Write the question and fill in the blank):  A _______  ________ is a type of screw that can make holes in wood, plastic, metal and stones.

Mechanical Advantage

  • Mechanical advantage depends on the space between the threads and the length (and thickness) of the screw. The closer the threads are, the greater the mechanical advantage. It is easier to drive a screw into an object if the thread spacing is smaller. This takes less effort but more turns. If the spaces between the threads are wider, it is harder to drill a screw into an object. It takes more effort but fewer turns.
  • A screw bites into wood when you turn it around. Imagine you’re an ant and you want to climb from the bottom of a screw to the top. If you climb vertically up the outside, you go a relatively short distance but it takes an awful lot of climbing force. If you walk up the screw thread, winding around and around, you’re really walking up a kind of spiral staircase—a ramp wrapped around in a circle. Yes, you walk much further—but it’s a whole lot easier.

Question 14 of 20 (Write the question and fill in the blank):  The mechanical advantage  depends on the space between the ___________ and the length and ___________ of the screw.

Question 15 of 20 (Write the question and fill in the blank):  The ____________ the threads are, the greater the mechanical advantage.

Question 16 of 20 (Write the question and fill in the blank):  A screw is easier to drive  into an object if the thread spacing is smaller. This takes less effort but more ___________.

Examples of screws

  • Some good examples of screws are bolts, screws, bottle tops, guitar tuners, light bulbs, faucet taps and cork openers.

Questions 17 through 20.  For each one of the objects shown below, list:

(A) The name of the object.
(B) How a screw is utilized in the object or device.
(C) What design feature(s) of the screw gives the object or device the desired characteristic (Example:  large diameter of screw and small thread spacing gives great lifting power)

#17. Jacks to lift a house

 

#18.  A 2.5 ton scissor jack

 

#19.  Dental implant
#20.  Cider press

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  1. SOURCES:  Quatr.us from Professor Carr website (https://quatr.us/physics/screw-simple-machines-physics.htm), viewed 1-6-2019;   Mr. Tunon’s powerpoint, J. P. Taravella High School; eSchool Today, Simple Machines, (https://eschooltoday.com/science/simple-machines/what-is-a-screw.html), viewed 1-9-2019