Modify GearGen or is there an alternative?


#1

I have used GearGen written by Thomas Anagnostou with great success generating bevel gears up to about 24 in. in diameter. The design for the current iteration of the project requires a larger diameter bevel gear; the pitch diameter is now in the neighborhood of 51.20 inches. Given the 45:1 gear ratio and the 400 tooth limit in GearGen, I’m afraid that the pinion hasn’t enough teeth to take the stresses imposed.

Can GearGen be modified to handle such a large diameter, small tooth design? If not, is there another script that can? The link below is to a quick and dirty representation of what I’m working on.

Thanks,
David

https://1drv.ms/u/s!AvbRThajlgqAhhNEVi2U1peaCzDb

GearGen:

https://1drv.ms/u/s!AvbRThajlgqAhhT1fZ93C0tOEeqa


(David Cockey) #2

Are you doing any analysis of the stresses? My understanding (perhaps incorrect) is coarser gears with fewer teeth result in lower stresses for a given torque, pitch diameter and gear ratio.


#3

No, davidcockey, I’m not doing any analysis. My concern arises from intuition just by looking at the pinion. The teeth look like they’re prime candidates for Murphy’s Law.


#4

That would be my guess as well, since there is only a maximum of one tooth engaged at each point in time.

I’m also looking for a good gear generator (good quality, ready for production). Gear Generators doesn’t grow all over the place, it seems.

// Rolf


#5

Rolf, are you saying that your guess is that the coarser gear as drawn is better or that more teeth on the pinion would be better?

David W.


(David Cockey) #6

That’s my understanding also. The force on a tooth depends on the torque divided by the pitch circle, not the size and number of teeth. Stresses will be lower with larger and wider teeth. However vibration/noise may be greater with larger teeth.


#7

Thanks guys. I guess my concerns are ill-founded. I’m hoping the carbon-fiber teeth can handle it.


#8

More on tooth strength (from the article “Guerrilla guide to CNC machining”) :

In general, low pressure angles result in quiet-running, precise gears that are easy to machine due to generous clearances - but will be weaker, and have more pronounced undercuts. High pressure angles result in thicker tooth profiles which need to be machined with smaller tools, and can be more noisy and prone to backlash and slippage - but also survive a lot more abuse.”

http://lcamtuf.coredump.cx/gcnc/ch6/

// Rolf


#9

Awesome! Thanks Rolf.

Here’s what was generated by gearGen for the main gear:
Summary:[Pitch Diam=51.2] [Teeth=360] [Module=0.142] [CircPitch=0.447] [Pressure Angle=20] [Cone Angle=88.727] [Samples=5]

and for the pinion gear:
Summary:[Pitch Diam=1.135] [Teeth=8] [Module=0.142] [CircPitch=0.446] [Pressure Angle=20] [Cone Angle=1.273] [Samples=5]

Thanks again for the link.

David W.


(David Cockey) #10

Another consideration in gear design is to select a ratio which results in the all the teeth on one gear eventually meshing will all the teeth on the other gear. With a ratio of 45:1 and gears with 8 and 360 teeth, each tooth on the large gear will mesh with only 1 of the 8 teeth on the small gear, and each tooth on the small gear will mesh with only 45 of the 360 teeth on the large gear. That means if a tooth is damaged or a slightly different shape the wear will be be concentrated on a few teeth.

A better choice (if the ratio does not have to be exactly 45:1 would be to use 8 and 359 teeth with a ratio of 44.875, or 8 and 361 teeth with a ratio of 45.125. With either of those combination all teeth on the small gear will eventually mesh with all teeth on the large gear and vice-versa.


#11

david, that’s fantastic! That never occurred to me. I can certainly live with either of those ratios. Thanks!

David W.