1. You can make gears with fewer teeth without undercutting on a 20 deg. gear than a 14-1/2 deg. gear.
2. 20 deg. gears require a closer tolerance on the gear-to-gear center distance to obtain the same backlash as 14-1/2 deg. gears.
These are the two basic rules. So if you need small gears without undercutting the teeth, you need a larger pressure angle. If you have relatively large gears with lots of teeth you'd probably like to use a smaller pressure angle since it will be a little more difficult to hold tolerances on the larger center distance and still minimize backlash.
Hi Don,
Thanks for your response. Our gears have 48 or more teeth. What I am wondering about the noise level at 20 deg vs. 14-1/2 deg. We are wondering about changing the pressure angle to 20 deg from present 14-1/2 deg. If noise is not going to be an issue, we would like to try 20 deg PA gears.
The smaller PA will be quieter as the contact ratio is increased, however they are relatively weaker (offset maybe by the larger contactact ratio)
20 is the industry standard, 14.5 may cost more or be harder to get.
the load on bearings increases with the pressure angle, for quiet running use a plastic gear (the biggest one)if this is possible (check for stresses).
Generally 14 1/2 degree compared to 20 deg pressure angles
14 1/2 degree PAs will have:
Higher contact ratio
Lower Bending Strength
Higher Shear Strength
More undercut
Lower bearing stresses and separation loads
20 degree PA will have:
Lower contact ratios
Higher Bending Strenth
Lower Shear Strenght
Less Undercut
Higher Bearing stress and separation loads
Most people are worried about Bending Strength in their designs - hence use 20 deg PA as a starting point.
Where are 14 1/2 deg PA an advantage?
- worm drives where the primary failure mode is augering of the worm thru the gear - esp if the gear is plastic
- designs that are challenging to achieve a contact ratio of greater than 1
- designs where bearing life is marginal
