Gear-Principles.pdf

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Gear

Teacher’s Notes

Mechanisms: Gear

Gears are wheels with teeth that mesh with each other. Because the teeth lock together, they can

efficiently transfer force and motion.

Did you know?

Not all gears are round.

Some gears are square,

triangular and even

elliptical.

Driven gear

Drive gear

The drive gear is the gear that is turned by an outside effort, for instance your hand or an engine.

Any gear that is turned by another gear is called a driven gear. The drive gear provides the input

force and the driven gear delivers the output force.

Using a gear system can create change in speed, direction and force. But there are always

advantages and disadvantages. For example, you can not both have more output force and an

increase in speed at the same time.

To predict the ratio of which two meshed gears will move relative to each other, divide the number

of teeth on the driven gear by the number of teeth on the drive gear. This is called the gear ratio.

If a driven gear with 24 teeth is meshed with a drive gear with 48 teeth, there is a 1:2 gear ratio.

Meaning that the driven gear will turn twice as fast as the drive gear.

Gears are found in many machines, where there is the need to control the speed of rotary

movement and turning force. Common examples include power tools, cars and egg beaters!

Mechanisms: Gear

Teacher’s Notes

This model shows a 1:1 gear ratio. he speeds of the drive

gear and the driven gears are the same, because they have

the same number of teeth. The drive and driven gears turn in

opposite directions.

Drive gear

Driven gear

This model shows gearing up. The larger drive gear turns

the smaller driven gear, resulting in increased speed, but

reduced output force.

Drive gear

Driven gear

This model shows gearing down. The smaller drive turns the

larger driven gear, resulting in reduced speed, but increased

output force.

Drive gear

Driven gear

Mechanisms: Gear

Teacher’s Notes

This model shows an idler gearing. The small gear is an idler

gear. The idler gear does not affect the speed or output

force of either the drive or the driven gears. The drive and

the driven gears turn in the same direction and at the same

speed.

Drive gear

Idler gear

Driven gear

This model shows an example of compound gearing.

Because of the way this compound gearing is arranged,

the turning speed is significantly reduced and the output

force greatly increased. The smaller drive gear slowly turns

the larger driven gear. The smaller gear on the same axle

as the driven gear is now set in motion and slowly turns

the second large driven gear, making it turn even more

slowly.

Driven gear

Drive gear

Driven gear

Drive gear

This model shows a gearing set up for periodic movement,

i.e. the driven gear turns for a short while and then stops for

a moment. Speed is significantly reduced as movement only

occurs when the driven gear is meshed with one of the two

drive gears.

Mechanisms: Gear

Teacher’s Notes

This model shows an angle gearing. The two meshed bevel

gears transfer the speed and force unchanged, but at an

angle of 90º.

This model shows a differential gearing. The input force is

transferred to two output forces at an angle of 90º When

one output pointer is stopped the other will double its

original speed. When both output pointers are stopped

the handle cannot be turned.

This model shows a worm gear. It reduces speed

significantly as it takes a complete turn of the worm gear to

move the gear above by a single tooth. It changes direction

by 90º. The output force is increased significantly.

Worm gears can only be used as a drive gear.

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