Design a gear train

[PhilipFry]

Well, I think I'm starting to catch on with this gear stuff. I've been trying to design a gear train off the seat of my pants using a spreadsheet that I've been assembling to calculate bending and shear stresses. I've been comparing my answer to Fairfield's program, but this has been a disaster so far. My stresses are off as much as 10 percent in either direction and I haven't figured out how I could be wrong.

First, as I understand it, when taking into consideration gear velocities, you divide the modified lewis formula by the dynamic factor (In my case, Kv=50/(50+sqrt(V)). Using this equation, it is impossible to get a value greater than 1, but that's exactly what I get with the package...something like 1.026. Huh? Is there another version of the stress equation that I'm not aware of that multiplies Kv?

Second, does anyone know how Fairfield comes up with the geometry factor, J? I noticed that there's an equation for the lewis tooth form factor, but is there one for the J factor? I would like to be able to change the number of teeth in excel and the geometry factor change occordingly.

Third, how do the pin diameters come into play with the stress equation? I've never seen anything that takes those into account, but one of the prompts in the Fairfield package asks for that dimension.

Any suggestions?

 

[PhilipFry]

Pinion
Pitch angle: 20 degrees
# of teeth:18
Diametral Pitch: 12
Face Width: 2.00 in
Outline Factor: .38
RPM of Pinion: 18
Torque at Pinion: 10,000 in-lb


Gear
Pitch angle: 20 degree
# of teeth:54
Diametral Pitch: 12
Face Width: 1.5 in
Outline Factor: .45


I've calculated a pitch line velocity of 7.07 ft/min, a load distribution factor of 1.15, a dynamic constant of .97702 using (70.72/[70.72+SQRT(Velocity)])^0.63.

Gear Yield Stress = 56,750 psi
Pinion Yield Stress = 50,291 psi

In case you can check for shear as well,
I'm using a material with a modulus of elasticity of 20305 ksi.

Elastic coefficient = 2318.9
I Factor = .1205

Pinion shear stress = 98809
Gear Shear Stress = 114095.8

Thanks for the help.

 

[carburized]

Mae1778

Getiing to durability and bending stresses in gear teeth largely depends on quality of gear teeth, I factor and J factor. I factoer is very easy to calculate. Also, addendum correction plays a very large part in determining the stresses. J factor on the other hand, needs to be interpolated and really almost impossible to calculate by hand. So, there are programs available like you said, Fairfield's or some body else. Fairfield is a legend in its field and be rest assured what they calculate is correct. If you calculate by Lewis formula, you will be off and probably be conservative. I work for Gear company ( not Fairfield by the way) and know what you are talking about. One of the replies I read above, you said outline factor, I am not familiar with this term. Tell me what this means? Is this a addendum correction or what. If you tell me that and what center distance you gears operate, I could design a gear set for you. I wll give you a website address of mine, and you could take a look at it, It has a sample problem.

http://www.gearsandbearings.com

Thanks