When to use 25 degree pressure angle? 1

[nsmith40]

I am working on a small planetary gearbox design (5 inch ring) to transmit approximately 8 HP. I have done some gear stress calculations and found that I can decrease the gear stress by increasing the pressure angle from 20 degrees to 25 degrees. What disadvantages are associated with a 25 degree pressure angle gears? Is efficiency lower?

 

[dimjim]

You may have greater strength with the 25 degree
pressure angle but I assume the contact stresses
would be higher. If strength is your main criteria,
then go for it. The 25 degree pressure angles have
worked well in heavy equipment design where slow
speeds are encountered. No experience with high
speed equipment.

 

[tbuelna]

the 25deg pressure angle will give you a lower contact ratio, higher dynamic loads and slightly noisier gears. It will also give you better bending strength. A tooth bending failure in a gear is usually a catastrophic failure, so you want to make sure that you have adequate rim strength and bending stress below the endurance limit for the gear material you have selected.

The efficiency for a 20deg vs. a 25deg pressure angle should be similar.

 

[plasgears]

25 deg pressure angle will boost the separating force in the gear set. This means that loads on gear pins or trunnions will be higher than with 20 deg PA.

 

[Spurs]

nsmith40

One significant issue is that your application is a planetary gear box. Higher pressure angles are usually easier to use in planetaries because it tends to help reduce the likelihood of involute interference in internal designs.

The higher pressure angle will increase the bearing loads as separation forces will go up.

Generally contact ratios go down as the pressure angles increase, however in cases of pinions with small numbers of teeth, undercutting may be avoided which in fact may contribute to higher contact ratios as well; - so we have to be careful not to over generalize.

Dont be afraid to go with the higher pressure angle if the bearing loada are not a big issue. I have gone as high as 54 degrees with great success!

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