Robotics
Gears
St. Mary´s School, Panama
6
Table of Contents
Objectives .................................................................................................................2
Spur gears.................................................................................................................3
Bevel gears................................................................................................................3
Worm gears ...............................................................................................................4
Rack and pinion gears ...............................................................................................4
Slip clutch gear .........................................................................................................4
Idler gear ..................................................................................................................5
Gear box ...................................................................................................................5
Gear ratios................................................................................................................6
Compound gear ratio..........................................................................................7
Ch. 6: Gears, by: Ernesto E. Angulo J.
1
St. Mary´s School, Panama
Robotics
6
Gears
Objectives of the Chapter:
To learn the different types of gears;
To learn the purpose of the different types of gears;
To learn how to calculate gear ratios.
Gears are wheels with teeth and are used to change the speed and power of a motor.
gears to:
1. Speed up or slow down your robot.
2. Make your robot stronger or weaker.
You can use
There are many different types of gears. Spur gears are wheels with teeth.
Bevel gears mesh at right angles, so they change the
direction of rotation.
Worm gears look like screws. They have many special properties.
Rack & pinion gears turn rotational motion into straight-line motion.
Ch. 6: Gears, by: Ernesto E. Angulo J.
2
St. Mary´s School, Panama
Robotics
A slip clutch is a special type of spur gear that lets the axle continues
to rotate if the gear gets stuck.
The gear connected to the power source is the primary gear or driver or
input gear.
The gear that receives the power is the secondary gear or follower or
output gear.
S
Power source is
what makes the
system to move.
G
Spur gears are wheels with teeth that mesh. Spur gears are used to
change the speed and force of a rotating axle.
Spur gears change the direction of rotation. If the input axle rotates
clockwise, then the output axle will rotate counter clockwise.
B
G
Bevel gears are just like spur gears, except they mesh at right angles.
This causes the output axle to be perpendicular to the input axle.
There are 2 types of Lego bevel gears.
The 12 tooth gears can only mesh with themselves.
The 24 tooth gears, which are also called crown gears, can mesh with
themselves as well as with other spur gears.
Ch. 6: Gears, by: Ernesto E. Angulo J.
3
St. Mary´s School, Panama
Robotics
W
G
The worm gear is a special type of gear that looks like a screw.
Worm gears have some unique properties.
1. Worm gears change the direction of rotation. The output
axle is perpendicular to the input axle. This is similar to
bevel gears.
2. Worm gears provide a very large gear ratio. This means
that worm gears can produce a very large increase in
force, but at the cost of a slowdown in speed. Every time
the worm gear makes one revolution, it advances just 1
tooth on the spur gear that is meshed with it. That means
if a 24 tooth spur gear is meshed with a worm gear, the
gear ratio is 24 to 1!
3. Worm gears can only be driven in one direction. A worm
gear can turn the spur gear, but the spur gear cannot turn the worm gear. The worm gear can only be
on the input axle. This makes the worm gear act like a ratchet. This is useful if you need something
like a robot arm to stay in one place, even though gravity might be trying to pull it down.
R
P
G
Pinion
Rack
S
C
The rack and pinion is a special pair of gears. The rack looks
like the outside of a spur gear that has been uncurled and laid
flat. The pinion is the small spur gear that meshes with the
rack.
When the pinion rotates, it makes the rack move back and
forth. Or, if the rack moves back and forth, that makes the
pinion rotate. So the rack and pinion turns rotational motion
into straight line motion and vice versa.
G
The slip clutch is a special type of gear. It has 24 teeth, just like a
spur gear, but the difference is that if the slip clutch is stuck, the axle
can continue rotating. This prevents the axle from breaking.
Ch. 6: Gears, by: Ernesto E. Angulo J.
4
St. Mary´s School, Panama
Robotics
Look at this example. The output wheel is connected to the motor
through a slip clutch. If the output wheel stops rotating, the motor
can continue to turn and not stall. Stalling a motor is bad for it
and could damage it. You should use a slip clutch in situations
where the output axle might get stuck. The slip clutch is similar to
belts and pulleys by not allowing the motor to stall, but it can
transmit more force because it has teeth like a gear.
I
G
An idler gear is any gear inserted between two
other gears. Idler gears do not affect the gear ratio between the input and output
gears. The gear ratio would be computed just the same if there were no idler
gears.
However, idler gears DO affect the direction of rotation. Recall that when
using spur gears, the output axle rotates in the opposite direction as the
input axle. By adding an idler gear between the two, the output axle
rotates in the same direction as the input axle. You can also use idler
gears to change the spacing between the input and output axles.
Remember, idler gears do not change the gear ratio.
G
B
An important component of any robot that you build is the gearbox.
Typically, you will have to use more than one pair of gears to achieve
a desired speed, strength, or accuracy. It is important to build a good
gearbox so your robot works reliably. Here are
some tips to help when building a gearbox.
1.
Make sure there is enough support for the axles. Axles should be
supported in at least 2 places. Here is an example of a poor gearbox. The axles
bend and the gears do not mesh very well.
Ch. 6: Gears, by: Ernesto E. Angulo J.
5
St. Mary´s School, Panama
Robotics
Look at the next page to see an example of an improved gearbox design.
Now the axles have more support, and the gears mesh better. Flat plates
have been added to make a square frame.
2. Don't press the gears or bushings too tightly against the beams. If there
is too much friction, then the gears will not spin very well. When building
a gear box, see if you can turn the gears with your hand. They should
spin freely with as little resistance as possible.
3. Make sure the teeth of the gears do not mesh too tightly. Sometimes,
odd gear spacing can cause the teeth not mesh properly. If your gears
make a lot of noise as they turn, that probably means the gears are
meshed together too tightly.
Gear Ratios
In math, ratio is the relation between two quantities and is expressed as the quotient of one divided by
the other, or separated by a colon. (1:2)
How much the speed and force change, depends on the gear ratio.
To calculate the ratio, divide the teeth of the follower by the teeth of the driver.
[number of teeth of the follower] : [number of teeth of the driver]
.
For example, this picture shows an 8 teeth spur gear meshed with a 40 teeth spur gear. If the 8 teeth
gear is on the driver, and the 40 teeth gear is the follower, then the gear ratio is:
40 to 8. Simplifying, the ratio is 5 to 1 (5:1)
40
5
=
8
1
This means that the driver should turn five times to make the follower turn
one. As the follower turns slower than the driver does, the system transmits
power.
Ch. 6: Gears, by: Ernesto E. Angulo J.
6
St. Mary´s School, Panama
Robotics
If the 40 teeth gear is the driver, the ratio will be:
8 to 40. Simplifying, the ratio is 1 to 5 (1:5)
Here, the driver turns one time while the follower turns five. Therefore, the system transmits speed since
the .follower is faster than the driver.
C
G
R
When more than one pair of gears is used together, that is called a compound gear train. The gear
ratios for each individual gear pair are shown in fractions and multiplied together to compute the overall
compound gear ratio for the gear train.
First Gear Pair Second Gear Pair Let's look at an example. The gearbox at the left has two pairs
of gears. The first pair of gears has an input gear with 8 teeth
and an output gear with 40 teeth. The gear ratio of this gear
pair is 40 to 8 or, by simplifying, 5 to 1.
Input Axle
The second pair of gears has an 8 teeth input gear meshed
with a 24 teeth output gear. The gear ratio of this pair is 24 to
8 or, again simplifying, 3 to 1. Notice that the 8 teeth gear of
the second gear pair is on the same axle as the the 40 teeth
gear of the first gear pair. The output axle from the first gear
pair becomes the input axle for the second gear pair.
Output
Let's compute the
gearAxle
ratio for the entire compound gear train. This is the ratio between the last output
axle and the first input axle. To do this, we multiply the gear ratios of the individual gear pairs.
The total gear ratio is 15 to 1. That means the input axle must make 15 revolutions for the output axle to
make 1.
You can combine as many gear pairs as you want in a compound gear train. There is no limit. By
combining gears you can make almost any gear ratio that you want!
Ch. 6: Gears, by: Ernesto E. Angulo J.
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