Activity 6 Calculate RPM
Copy this table into your book and calculate the RPM speeds for each gear system:
25 20 80 40 75
Input (effort) Output (load) Input RPM 75
120 20
200 25
Bevel gears
Bevel gears are sometimes known as crown-and-pinion gears. The two gears intermesh at an angle and so change the turning force through 90°. They also change the direction of rotation. If different sizes of bevel gears are meshed together, they can change both the speed and the force.
Worm gears Worm gears are gears cut on a rotating shaft. They intermesh with a spur gear in order to change direction.
Rack-and-pinion gears
A rack-and-pinion gear is used to change rotational movement into linear movement. The gear is cut on a long straight bar and the spur gear then meshes with it to push it in the direction of rotation. They are often used to operate sliding gates on driveways or security complexes.
Materials used to make gears
Gears are made from various materials depending on where they are going to be used. Gears used in children’s toys are made of plastic. However, plastic gears would not be suitable for use as high-performance gears in a car engine. Car gears are made of high-carbon steel. Gears that have to be run without lubrication are often made of brass as this metal has self-lubricating properties. When choosing a material to make a gear the designer would consider various factors such as weight, the load being driven by the gear, the life expectancy of the machine and the type of lubrication that can be used. For example, small idler gears are usually made of a harder material as they revolve faster than larger gears and are therefore subject to more wear.
New word
lubrication: when a substance such as oil is put onto or into something to make it work smoothly
Figure 16 Bevel gears
120 120 200 150 180
Ratio Output RPM
Unit 1 Revise levers and gears 107
TERM 3
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