Calc Gear Ratio: Helical/Spur Gears

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Discussion Overview

The discussion revolves around calculating the gear ratio for a system involving helical and spur gears, specifically focusing on a gearbox arrangement. Participants explore the implications of gear teeth, diameters, and the relationships between different gears in the system.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about calculating the gear ratio for two gears on the same shaft, questioning the relevance of the number of teeth.
  • Another participant states that when gears are on the same axis, the gear ratio is 1, suggesting that the number of teeth is not useful for this calculation.
  • A participant proposes that to find the overall reduction ratio, one must multiply the individual ratios of the gears, although they express uncertainty about this method.
  • Clarification is sought regarding the meaning of "i," which is identified as the number of teeth in the gears.
  • One participant notes that for gears to mesh, they need to have the same module, indicating a potential issue with the provided dimensions.
  • A detailed breakdown of the ratios for different gear pairs is provided, leading to an overall reduction ratio calculation of approximately 80.07.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement regarding the calculations and assumptions about gear ratios. Some participants agree on the method of multiplying individual ratios, while others raise questions about the coherence of the dimensions and the relationships between gears.

Contextual Notes

There are unresolved questions about the assumptions regarding gear dimensions and the validity of the ratios provided, as well as the implications of using teeth counts versus diameters in calculations.

namith11
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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!
View attachment 66498
 
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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 won't 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 can't 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 won't 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 can't 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 don't have the reduction ratio and hence I am 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 don't have the reduction ratio and hence I am 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
 
Last edited:

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