Gear ratio is the relationship between the numbers of teeth on two gears that are meshing together or are connected.
To calculate the ratio of gears use the following formula
Gear Ratio= The number of teeth on the output gear / The number of teeth on the input gear
So if one gear had 10 teeth and the second gear 5 teeth the gear ration would be 2 – 1.
This is ok but what if you don’t or can’t count the number of teeth on a gear. If this is the case then you can work out the gear ratio by measuring the distance between the centre of each gear to the point of contact.
Example
Gear A - 2" diameter and Gear B - 4" diameter. So from the centre to the edge of Gear A is 1 and from the centre to the edge of Gear B is 1. Giving us a gear ratio of 2 – 1.
If you are using three gears the middle gear is known as an idler gear and is used when you want to keep the input and output shafts turning in the same direction. An idler gear does not affect the gear ratio.