Fairfield Software Question 2

[cfordyce05]

I'm working on a spur gear design and would like to get a rough check of the gear strength so I downloaded the Fairfield software and entered my data. I have a question on the material strength entries. I would like to use 17-4PH in an H900 condition so under Rating Stress for bending I have 77,500psi. From the datasheet the UTS is 240ksi. What am I suppose to put in the surface stress box? The default is 225000psi. Is it actually wanting the UTS there? The directions when you click on the bending stress box are:

"Enter a Rating Stress for Bending. This is the material allowable stress, also known as the Endurance Limit Stress. The default value is 65,000 PSI, which is for carburized and hardened gearing per AGMA Grade 2 requirements."

The directions when you click on the surface stress are:
"Enter a Rating Stress for Surface Contact. This is the material allowable stress, also known as the Endurance Limit Stress. The default value is 225,000 PSI, which is for carburized and hardened gearing per AGMA Grade 2 requirements."

They both say that it is the Endurance Limit Stress but show two different numbers.

Thanks,

Casey

 

[dinjin]

You might try looking at a 13 tooth pinion on a 14 tooth blank
and center distance for a 14 tooth/ 35 tooth center distance
while you are checking out calculations

 

[cfordyce05]

Sorry for the delayed response. Dislocated my knee and haven't been able to work on this project. I should have quite a bit of time to devote to it while I'm recovering from surgery...

dinjin,

Does doing what you're saying basically provide the tip relief that Terry mentioned?

 

[tbuelna]

cfordyce05- What dinjin suggests is similar to an addendum modification. It makes the pinion teeth a bit sturdier, but it does not provide much tip relief. If your gears require tip relief, then that is produced by modifying the flank right around the start of active profile. With gear teeth of average size (say around 8-12 DP), the amount of tip relief modification would only be .002" or .003" at most. However, due to the extremely small size of your gear teeth the amount of tip relief modification would likely be just a few microinches, which would be extremely difficult to accurately produce and measure.