Why Do Spur Gear Pairs Have Different Face Widths?

[cherish]

TL;DR Summary

Why does face width of a spur gear pair normally not equal?

Dear Physicsforum Mentors,

For a pinion and gear wheel, the face width instead of 12mm for both, it should be 14mm and 12mm.

The explanation I got is, when there's 0.2mm deviation in axial, then only 11.8mm width will be in meshing. later, when the system be worn out, that 0.2mm will be in meshing and making noise.

I just wonder if there's any other critical reason, rather than just to avoid of making noise as described above. Since by searching, I still see there're many spur gear pairs with the same face width?

Thank you for your taking time on reading my thread :)

 

[Lnewqban]

I have never heard of that, but engineers and manufacturers have tried many things to reduce noise coming from spur gears, especially in high speed applications.

Please, see:
https://www.machinedesign.com/motors-drives/article/21828196/driving-out-gearbox-noise

https://khkgears.net/new/gear_knowledge/gear_technical_reference/how_to_reduce_gear_noise.html

 

[Baluncore]

Summary: Why does face width of a spur gear pair normally not equal?

How have you come to that conclusion? Are the gears you surveyed made from metal or from plastic? When correctly lubricated, metal gears will NOT wear significantly.

Higher speed spur gears will throw lubricant from the pinion. Is the lubrication by oil bath, or open and exposed, greased once only when manufactured?

A wider gear may be needed where the manufacturing tolerance on axial position is wide. Gears may be positioned axially by a simple bush, by deep groove bearings, or by opposed tapered roller bearings.

 

[jrmichler]

What @Lnewqban said.

Also, while some people may design gear pairs with unequal face width in certain applications, normal practice is to make both gears in a pair the same face width. If you are involved with gear design, I can highly recommend that you have a copy of Dudley's Handbook of Practical Gear Design and Manufacture on your shelf. Everything that you want to know about gear life and gear noise is in that book.

I once read an earlier edition of that book, and it convinced me that I should confine my gear design efforts to low speed gear trains with low loads. But I did get enough out of it to be able talk intelligently to a real gear expert.

 

[cherish]

Dear all,

I am very appreciated with all of your guidance and especially comment from Baluncore are very clear for me now. I have came back to the guy who made that statement to get more clarify and he assumed the noise might come out during rotation without carrying load (the one he described is in a motorbike gearbox), so no fix rule for a pinion should be wider than ring gear.
Thanks Lnewqban and jrmichler, I have visited these links for reading, and will stick the Handbook for later if I have chance to go with a gear design for a certain case :)

 

[Lnewqban]

... I have came back to the guy who made that statement to get more clarify and he assumed the noise might come out during rotation without carrying load (the one he described is in a motorbike gearbox), so no fix rule for a pinion should be wider than ring gear.
...

You are welcome, cherish

As you can see in this video, the spur gears of a motorcycle transmission have relaxed axial tolerances regarding movement.
All I have seen have gear wheels of exactly same width.
They work in a bath of oil and abnormal noise could be caused by incorrect grade of oil or excessive wear or improperly adjusted shifting mechanism.

 

[cherish]

Hi lnewqban,

I got a picture from Honda that looks a bit different, maybe it varies from manufacturers, or the illustration will not be an exact like between 12mm and 13mm