What are Gears

1
What are Gears?

• Gears are wheels or cylinders with teeth that
  mesh with the teeth of other gears to transmit
  motion
• Gears are used in everything from automobiles to
  small toys

2
Functions of Gears

• Transfer Motion
• Change Direction
• Increase/Decrease Speed
• Increase/Decrease Torque
• Torque is turning or twisting force. Ex A
  longer wrench gives you more torque and makes it
  easier to loosen a bolt or screw.
• Change Planes of rotation

3
Drivers Followers
A driver is the gear which is being turned by a
power source (motor or your hand). A follower is
turned by the driver.
motor
motor
Follower
Driver
Follower
Idle
Driver
4
Spur Gears

• Transfer motion within a plane (a straight line)
• Most common gear
• Spur gears are made in many different sizes and
  from a number of different materials
• Steel, plastic, etc.

• Spur gears are generally identified by the number
  of teeth they have

5
Spur Gears

• Gears are attached to axles which run through the
  center of the gear
• Gears must be positioned just right, so that they
  do not push on each other but the teeth will
  still turn the other gear

Meshing gears turn in opposite directions.
6
Lego Spur Gears

• Common sizes are 8, 16, 24, and 40 tooth
• Correct Spacing
• Horizontal
• 40 and 24- 3 holes between axles
• 40 and 8- 2 holes between axles
• Vertical
• 40 and 24- 2 studs and 1 plate between studs
  holding axles
• 24 and 16- 1 stud between studs holding axles
• Gears can be placed in a number of other
  combinations diagonally, but none other
  vertically or horizontally

7
Gears Mesh

• The teeth on gears mesh together so that 1 tooth
  on 1 gear pushes past 1 tooth on a second gear.
• 1 turn of the driver will turn the follower
  around 1 time.

Driver 40 tooth gear Follower 40 tooth gear
8
Gearing Down

• Smaller gear is the driver
• Larger gear is the follower
• The driver (smaller gear) has to turn around
  multiple times to get the follower (larger gear)
  to turn around once
• Gearing down gives you a slower speed (because it
  takes multiple turns of the driver to turn the
  follower once)
• Gearing down also gives you more torque

Driver 8 tooth Follower 40 tooth The driver has
to turn around 5 times to get the follower to
turn once
9
Gearing Up

• Larger gear is the driver
• Smaller gear is the follower
• The driver (larger gear) has more teeth so the
  follower (smaller gear) turns around multiple
  times for each full turn of the driver
• Gearing up gives you higher speed (because it
  takes only 1 turn of the driver to make the
  follower turn multiple times)
• Gearing up is like 21st gear on a 21 speed bike -
  you go faster but more torque is necessary

Driver 40 tooth Follower 8 tooth When the
driver turns around 1 time the follower turns
around 5 times.
10
Gear Ratios

• Each gear tooth on the driver turns one tooth on
  the follower
• We want to be able to talk about the speed of
  gears in mathematical terms
• Gear Ratios tell us how many times faster or
  slower the follower is rotating with respect to
  the driver
• Gear Ratios are expressed in terms of the number
  of teeth on the follower over the number of teeth
  on the driver.
• Gear Ratio

11
Gear Ratios 2 of the same size gears
Driver 40 tooth gear Follower 40 tooth gear
Which reduces to which is
sometimes expressed as 11 No Change in Speed
12
Gear Ratios Gearing Down
Driver 8 tooth Follower 40 tooth
Which reduces to which can also be
expressed as 51 The follower is 5 times slower
than the driver
13
Gear Ratios Gearing Up
Driver 40 tooth Follower 8 tooth
Which reduces to which can also be
expressed as 15 The follower is 5 times faster
than the driver
14
Idle Gears

• When a gear is placed directly between two
  others, that gear does not affect the gear ratio.
  
• The gear ratio is computed using the driver and
  the last gear in the gear train.
• The idle gear has no effect on a gear train
  except to change the direction of rotation

Which gear is the idler? If a motor is attached
to the axle with the green gear on it, which gear
is the driver? Which is the follower?
15
Other Gears

• Bevel Gears
• Meshing at 90 degree angles
• Rack and Pinion
• Linear motion

16
Double Bevel Gears

• Work as Spur Bevel Gears
• Sizes 12, 20, 36 tooth

17
Crown Gear

• Transfer motion in between planes (at angles)
• Lego crown gear changes axis of rotation by 90
  degrees
• Crown gears can mesh with other crown gears and
  spur gears as well
• Used in gumball machines, egg beaters

18
Worm Gear

• Inclined plane
• 1 turn of the work gear moves by 1 tooth of any
  other gear
• Spur Gear cannot turn worm gear

19
Knob Wheel

• Not a true gear
• Only works with another knob wheel
• Can function like a gear transfer rotary motion

20
Compound Gearing

• Can change the speed of your follower by changing
  the gear configuration
• Use two gears on an axle to get a larger gear
  ratio in your gear train
• When a gear train similar to the one at right is
  used, the ratios of the meshing gears are
  multiplied to find the overall gear ratio

21
Compound Gear Ratios

• The Lego gear train on the last slide had two
  sets of gears, each had a ratio of 408 (or 51).
  What is the ratio of the gear train?
• Answer
• Try this one
• 8 to 40, then 24 to 40, then 24 to 24

What is the overall gear ratio of this Gear
train? If the 8 tooth gear is the driver and
the last 24 tooth gear the follower, is the
gear train geared up or down?
22
Gear Ratios

• Answer to the last question is 253
• This means that for every 25 times you turn the
  driver, the follower will turn three times
• If your driver is larger, your follower will turn
  more. If your driver is smaller your follower
  will turn fewer times
• Remember the idle gears? They can be ignored when
  computing gear ratios