# Calculating Gear ratio!

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Hey,
I want to know how to calculate the gear ratio of the given arrangement in the picture. Im 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. Im very confused. Please help!!!!!
View attachment 66498

## Answers and Replies

maajdl
Gold Member
Gear ratio is an angular rotation ratio.
When on the same axis, rotations are the same and therefore gear ration is 1 .
Numbers of teeth is not useful for this calculation, but should be coherent.

Hey,
So if i want to calculate the overall reduction ratio of the system, i will have to multiply the individual ratios of the gears wont i? I can then get my overall ratio. I am not if this is the right way of going about it though! This way, i got the overall reduction to be 83 to 1. But i cant be suree :(

Hey,
So if i want to calculate the overall reduction ratio of the system, i will have to multiply the individual ratios of the gears wont i? I can then get my overall ratio. I am not if this is the right way of going about it though! This way, i got the overall reduction to be 83 to 1. But i cant be suree :(

Did you draw this or is it a real question?

I'm assuming you have the diameter, but what is i?

hey Chris,

Its a real Gear box. I drew the sketch up to calculate the gear ratio. Its an old motor. I know what the motor specs are, but i dont have the reduction ratio and hence im trying to calculate it. i is the number of teeth. In the diagram, the diameter and number of teeth for each gear is specified.
Thanks

hey Chris,

Its a real Gear box. I drew the sketch up to calculate the gear ratio. Its an old motor. I know what the motor specs are, but i dont have the reduction ratio and hence im trying to calculate it. i is the number of teeth. In the diagram, the diameter and number of teeth for each gear is specified.
Thanks

Well something doesn't make sense then, as the i and d should be the same ratio between a gear pair. Or it's a totally unrelated diameter.

For gears to mesh they need to have the same module.
http://www.tech.plymouth.ac.uk/sme/desnotes/gears/nomen1.htm

Each shaft must turn at the same speed, and the 2nd and 3rd inputs are compund gears. This is done to allow larger ratios without having physically huge gears.

There are only 3 ratios:
Input shaft ti layshaft 1
layshaft 2 to layshaft 2
layshaft 2 to output shaft

Ignoting the diameters and using the teeth as correct:

Gear Pair 1: 7:58
Ratio 1 = 8.28
Gear Pair 2: 19:51
Ratio 2 = 2.684
Gear Pair 3: 15:54
Ratio 3 = 3.6

Overall Reduction Ratio = R1*R2*R3 = 80.07

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