Calculate Gear Ratio - Helical/Spur Gears

[namith11]

Hey,
I want to know how to calculate the gear ratio of the given arrangement in the picture. I am not sure of how to calculate the gear ratio of two gears on the same shaft. Also the input gear is helical and the output gear is a spur gear. I have mentioned the number of gear teeth, i hope that is enough. I am very confused. Please help!

 

[Baluncore]

If I read it correctly, the ratios of gear pairs are;
driver : driven
7 : 58 = 0.12069
19 : 51 = 0.3725
15 : 54 = 0.2777
The result is ( 7 * 19 * 15 ) / (58 * 51 * 54 ) = 0.0125
or 80.066 : 1

Helical gears behave the same as spur gears. Helical gears are quieter at high speeds so they are used here for the high speed, low torque, input shaft.

 

[Baluncore]

The diameters in your diagram appear to be tip diameters not pitch circle diameters which explains the slight ratio difference between tooth count and diameters.