Now let’s look at three more important factors to keep in mind when selecting the “right” helical gears for your applications: ratio, rotation and thrust.
RATIO
The ratio of a pair of helical gears may be determined from the shaft speed or the number of teeth in the two gears.
Ratio = RPM of Driving Gear ÷ RPM of Driven Gear
Example: Ratio = 900 ÷ 900 = 1
Ratio = No. of Teeth in Driven Gear ÷ No. of Teeth in Driving Gear
Example: Ratio = 12 ÷ 12 = 1
ROTATION
In a helical gear train with an even number (2, 4, 6, 8, etc.) of gears in mesh, the first gear (the driver) and the last gear (the driven) will always rotate in opposite directions. All even numbers of gears will rotate in opposite directions in relation to the pinion or driver. In a helical gear train with an odd number (1, 3, 5, 7, etc.) of gears in mesh, the first gear (the driver) and the last gear (the driven gear) will always rotate in the same direction.
All odd numbers of gears will rotate in the same direction in relation to the pinion or driver.
THRUST
The chart on Figure above illustrates the thrust (the driving force or pressure) of helical gears when they are rotated in various directions, as well as where the bearings should be placed to absorb the thrust in each example. Use it to help determine the correct hand helical gears (right or left) for various applications, as well as the thrust of helical gears at right angles (90 degrees) or parallel to one another.
