Science corner

22nd April 2005 at 01:00
Sophie Duncan looks at how gears generate spin and speed

Gears are found in lots of household devices. Your class can explore how to make their own gears and find out how they work.

To make a set of gears you will need some foam-backed board or two sheets of corrugated cardboard stuck together. Cut three circles from the cardboard or foam-backed board, measuring 10.2cm, 5.1cm and 2.55cm in diameter.

Take the smallest circle and draw lines across it to divide it into eighths. Cut out alternate sections from the edge to make a cog shape, then lay the smallest cog next to the middle-sized circle.

Carefully rotate it around the circumference of this circle and mark where each of the teeth touch the edge. Use these marks to cut out the pieces on the middle-sized cog. Repeat with the largest circle. Make a hole in the centre of each cog and mark one "tooth" with a dot.

To test the cogs place them on a piece of foam-backed board so their teeth interlock. Secure in place using a split-pin. Start by using just two gears and experiment by turning each one in turn.

Encourage your students to notice how fast the gears rotate and, using the marked teeth as a reference, count how many times the smaller gears rotate when you turn the largest gear.

You should find that for each revolution of the large gear, the medium-sized gear turns around twice, and the smallest gear turns around four times. The gear ratio is described as 1:2:4.

You can encourage your students to work out the relationship between the diameters of the circles. This is the same as the gear ratio, because the circumference of a circle is proportional to the diameter.

Once students have explored the gears you can look at some of the things that use gears to function, and challenge students to think about what size cogs might be used.

For example, a salad-spinner spins salad really quickly. For each turn of the handle the basket containing the salad goes round several times. What size cogs would you expect to find inside? Check to see if you are right.

Other examples of machines that use gears include clocks and bicycles. Can your students work out how they work?

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