Thank you so much for creating video! It's very educational for me. 6:07 It seems the Fusion360 SpurGear script uses `dedendum = (6 / 5) * module` (=1.2) while what you and various other sources suggest `dedendum = (5 / 4) * module` (=1.25). Presumably 1.2 slightly increases tooth robustness while having a minor effect on the risk for additional friction in the tooth root but it likely all depends on the use case, fillet and so on.
After some research it seems to be known as dedendum ratio and is a parameter that most often in textbooks is fixed to 1.25, has a minimum value to allow for valid meshing and can be traded-off depending on the use case.
I have to correct myself about what the F360 script uses. It's not a fixed 1.2 ratio. In fact depending on the gear you design it takes 1.25. Here's the complete calculation copied from the Python script: if (diametralPitch < (20 *(math.pi/180))-0.000001): dedendum = 1.157 / diametralPitch else: circularPitch = math.pi / diametralPitch if circularPitch >= 20: dedendum = 1.25 / diametralPitch else: dedendum = (1.2 / diametralPitch) + (.002 * 2.54)
Thank you for your comments and research ! At the time of making this video I chose the most common dedendum value seen which I assumed to be the "normalized" and then the "correct" value, so ((1 + 1/4) * module). Since, I saw various values for the tooth root clearance factor like 1/5 or 1/6. Now I consider (1/4 * module) clearance as being the meshing safer value but not the best compromise. As you say it's a trade-off. Even the addendum could be shortened at the same time as the dedendum for a stronger tooth, while keeping an eye on the contact ratio which shall never drop below 1.0, ideally always above 1.1 (but that's another topic). And thanks for python script extract !
Thank you so much for creating video! It's very educational for me.
6:07 It seems the Fusion360 SpurGear script uses `dedendum = (6 / 5) * module` (=1.2) while what you and various other sources suggest `dedendum = (5 / 4) * module` (=1.25). Presumably 1.2 slightly increases tooth robustness while having a minor effect on the risk for additional friction in the tooth root but it likely all depends on the use case, fillet and so on.
After some research it seems to be known as dedendum ratio and is a parameter that most often in textbooks is fixed to 1.25, has a minimum value to allow for valid meshing and can be traded-off depending on the use case.
I have to correct myself about what the F360 script uses. It's not a fixed 1.2 ratio. In fact depending on the gear you design it takes 1.25. Here's the complete calculation copied from the Python script:
if (diametralPitch < (20 *(math.pi/180))-0.000001):
dedendum = 1.157 / diametralPitch
else:
circularPitch = math.pi / diametralPitch
if circularPitch >= 20:
dedendum = 1.25 / diametralPitch
else:
dedendum = (1.2 / diametralPitch) + (.002 * 2.54)
Thank you for your comments and research !
At the time of making this video I chose the most common dedendum value seen which I assumed to be the "normalized" and then the "correct" value, so ((1 + 1/4) * module).
Since, I saw various values for the tooth root clearance factor like 1/5 or 1/6. Now I consider (1/4 * module) clearance as being the meshing safer value but not the best compromise. As you say it's a trade-off. Even the addendum could be shortened at the same time as the dedendum for a stronger tooth, while keeping an eye on the contact ratio which shall never drop below 1.0, ideally always above 1.1 (but that's another topic).
And thanks for python script extract !