Lesson 3: An Engineering Introduction to Machines–Gears (December 11 & 12)

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

INSTRUCTIONS:

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

Based on Mr. Tunon’s notes and powerpoint1

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.

A machine is an object or mechanical device that receives an input amount of work and transfers the energy to an output amount of work. For an ideal machine, the input work and output work are always the same. Remember that work is force times distance; even though the work input and output are equal, the input force does not necessarily equal the output force, nor does the input distance necessarily equal the output distance.2

 

Gears

  • A gear is just a wheel with teeth, sometimes called a cog. To do any work with a gear, you need to have at least two cogs with their teeth fitting into each other. Because the teeth fit together, when you turn one gear, the other one turns too!
  • Intermeshing gears are used to transmit motion and force. Two or more intermeshing gears is called gear train.
QUESTION 1 out of 20 (Write the question and answer): IN YOUR OWN WORDS (don’t just copy the definition above), write a definition for a “gear.”
  • In gear trains, we have two types of gears; one is the Driver and the other one is the follower.
  • The Driver gear is the gear that has the force or motion input.
  • The Follower gear is the gear that results in the force or motion output.
  • If we were to change the size of the driver and the follower gears we can increase the gear ratio.
  • If the gear ratio of a gear train is 18/8 or 9/4, this means that for every 9 turns of the driver the follower will make 4 complete rotations

The purpose of Gears:

  • Increase speed: If you connect two gears together and the first one has more teeth than the second one (generally that means it’s a bigger-sized wheel), the second one has to turn round much faster to keep up. So this arrangement means the second wheel turns faster than the first one but with less force. Looking at our diagram below, turning the red wheel (with 24 teeth) would make the blue wheel (with 12 teeth) go twice as fast but with half as much force.3
Gears increase speed, increase force, and change direction
  • Increase force: If the second wheel in a pair of gears has more teeth than the first one (that is, if it’s a larger wheel), it turns slower than the first one but with more force. (Turn the blue wheel and the red wheel goes slower but has more force.)
  • Change direction: When two gears mesh together, the second one always turns in the opposite direction. So if the first one turns clockwise, the second one must turn counterclockwise.

QUESTION 2 out of 20 (Write the question and FILL IN THE BLANKS):   The _____________ gear is the gear that has the force or motion INPUT; and the ______________  ___________ is the gear that results in the force or motion OUTPUT.  A gear ratio of  4/1 means that for every ____ turns of the driver the follower will make ____ complete rotations.

QUESTION 3 out of 20 (Write the question and FILL IN THE BLANKS):  The main purposes of gears are to increase ________________, increase _______________, and to change ____________________.

How Gears are measured

  • The size of a gear is measured by the number of teeth, by it’s circumference, by it’s diameter, or it’s radius.
  • It a gear has 12 teeth and the width of each tooth is 1 inch wide, then the circumference of the gear is 12 inches long.
  • The circumference of a circle is π times the diameter.  (C = π * d )
  • The circumference of a circle can also be determined by multiplying 2π times the radius.  (C = 2 π *r)
  • The number of teeth on the driver gear to the number of teeth on the driven gear is called the Gear ratio. The larger the gear ratio, the slower the driven gear will turn in relation to the driver gear speed.
  • A gear train is formed by mounting multiple gears on a frame so that the teeth of the gears engage.

    Gear Train

QUESTION 4 out of 20 (Write the question and FILL IN THE BLANKS): A ________  __________ is formed by mounting multiple gears on a frame so that the teeth of the gears engage.

  • Because gear friction reduces power, engineers never use more gears than are needed.
  • To get larger gear ratios, engineers avoid the use of larger gears by teaming the gears in pairs.

  • By placing the driven gear of the first pair of gears on the same axle with the drive gear of the second pair of gears we create a new gear train called a compound gear train.

QUESTION 5 out of 20 (Write the question and FILL IN THE BLANKS): Engineers try to minimize the number of gears because gear friction reduces ______________.  One way to accomplish this goal is to put a pair of gears on the same axle in a gear train called a _____________  ___________  _______________.

QUESTION 6 out of 20 (Write the question and answer): It is considered a good engineering design to put more than on gear on the same axle? (TRUE or FALSE).

QUESTION 7 out of 20 (Write the question and answer, or incorporate question in your answer): Explain your answer to Question Number 6 (previous question).  That is, how could putting more than one gear on the same axle be either harmful or beneficial to a machine?

SPUR GEARS

  • Spur gears are the most common type of gears. They have straight teeth, and are mounted on parallel shafts. Sometimes, many spur gears are used at once to create very large gear reductions.
Spur gears
  • Spur gears are used in many devices like the electric screwdriver, oscillating sprinkler, windup alarm clock, washing machine and clothes dryer. But you won’t find many in your car.
  • This is because the spur gear can be really loud. Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth.

Advantages of Spur Gear:4

  • Spur gears have high power transmission efficiency.
  • They are compact and easy to install.
  • They offer constant velocity ratio.
  • Spur gears are highly reliable.
  • They can be used to transmit large amount of power.
  • They are relatively inexpensive to make and install, when compared to other types of gears.

Disadvantages of Spur Gear:

  • Spur gears produce a lot of noise when operating at high speeds.
  • Gear teeth experience a large amount of stress.

QUESTION 9 out of 20 (Write the question and answer, or incorporate the question in your answer):   Assume you are the engineer building a machine in a large new residential building.  This machine will be in a location that will be difficult to access once the building is finished.  However, since this is a residential building, silence is absolutely critical.  Explain why or why not spur gears would be a good choice.  If you can’t give a reasonable response based on the question, indicate what additional information you would need in order to make a reasonable response.

QUESTION 10 out of 20 (Write the question and answer, or incorporate the question in your answer):  Explain why you think spur gears where historically a popular choice for use in mechanical alarm clocks, and other mechanical clocks.

QUESTION 11 out of 20 (Write the question and answer, or incorporate the question in your answer):  Explain why you think spur gears are not a popular choice for use in automobile transmissions.

Helical Gears

  • To reduce the noise and stress in the gears, most of the gears in your car are helical.

  • The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement.
  • This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions.
  • Because of the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that can support this thrust load.
  • One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees.
helical gears, rotated 90 degrees

QUESTION 12 out of 20 (Write the question and answer, or incorporate the question in your answer):   IN YOUR OWN WORDS (don’t just copy the definition above), write a definition for a “helical gear.”

QUESTION 13 out of 20 (Write the question and FILL IN THE BLANKS):  The gradual engagement makes helical gears operate much more _____________and _____________ than spur gears. 

Bevel Gears

bevel gears

Bevel gears are useful when the direction of a shaft’s rotation needs to be changed. They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well.
The teeth on bevel gears can be straight, spiral or hypoid. Straight bevel gear teeth actually have the same problem as straight spur gear teeth — as each tooth engages, it impacts the corresponding tooth all at once.

spiral bevel gear

QUESTION 14 out of 20 (Write the question and answer, or incorporate the question in your answer):   Bevel gears are usually mounted on shafts that are _____ degrees apart, but can be designed to work at other angles as well.

Worm Gears

Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

worm drive

Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.

Typical worm drives can only be turned in one direction, which means that conveyor belts and security gates lock-up when not being used and will not run backwards. Automatic security gates will often adopt two worm drives, one to open and another to close. This means the gate can be locked in each direction and cannot be breached or forced.

QUESTION 15 out of 20 (Write the question and FILL IN THE BLANK:   Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the _________ cannot turn the ____________.

QUESTION 16 out of 20 (Write the question and FILL IN THE BLANK:   Typical worm drives can only be turned in ________ direction.

Draw AND LABEL a sketch of each of the following types of gears:

QUESTION 17 out of 20 — Draw SPUR GEARS.

QUESTION 18 out of 20 — Draw a GEAR TRAIN.

QUESTION 19 out of 20 — Draw BEVEL gears.

QUESTION 20out of 20 — Draw WORM gears.

  1.   John Tunon J. P. Taravella High School
  2. CK-12 website (https://www.ck12.org/physics/simple-machines-1501903848.65/lesson/Simple-Machines-PHYS/)
  3.   ExplainThatStuff website(https://www.explainthatstuff.com/gears.html)
  4.   MechTeacher.com (http://mechteacher.com/spur-gear-advantages-disadvantages-applications/)