I am having an issue with Inventor and I know it has to be something stupid that I am doing or overlooking. I am trying to use the spur gear generator to create a pair of helical gears. I thought that was possible by changing the helix angle, but when I do, my center to center distance is thrown off.
I am trying to create a geartrain of a 25 tooth pinion and a 60 tooth gear. They are 20 DP gears, so their PD is 1.25" and 3" respectfully. They have a 45° helix angle. Based on my understanding of gear train design (admittadly limited), the C-C should be 2.125". Inventor tells me it should be 3.005"... If I change the helix angle to 0°, Inventor and I agree it should be 2.125"
What am I missing?
The numbers Inventor gave you are correct. A spur gear set (the teeth are cut parallel to the gear axis) has a standard center distance of 2.125". A helical gear set with 45° helix angle will have a standard center distance of 3.005". Changing the helix angle will result in a change in the standard center distance. A spur gear set is mathematically a helical gear set with a helix angle of 0°.
45° is quite high for a helix angle. Strength of helical gearing is at a maximum between 10°-20°. Keep in mind also that the higher the helix angle the higher the axial loads generated by the gearing.
There are several variables and factors that can and/or need to be tweaked to optimize a gear mesh design. I don't know what standards Inventor uses to calculate gearing but the American Gear Manufacturers Association (AGMA) is the usual reference for gearing in the United States. I haven't been able to consistently duplicate gearing I've designed using AGMA standards and practices using the Spur Gear Generator in Inventor. As a result, I don't use Inventor for design work. AGMA 917-B97, Design Manual for Parallel Shaft Fine-Pitch Gearing, is a good reference. AMGA 2001-D04, Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth. addresses how to determine the load carrying capacity of helical gearing. AGMA standards are working towards agreement with ISO standards but I believe there are still some differences in them and the same gear set may have different ratings depending upon which standard is used. In general I believe the AGMA standards tend to be more conservative (rate gears slightly lower) than the ISO standards.
Something you may want to also consider is, in general, gear drawings are not "literal". A cross section of a helical gear is usually shown as the same as a cross section of a spur gear. Modelling a helical gear with helical teeth and then doing a cross section on a drawing will result in a section that does not look like what is usually drawn for a helical cross section. Because of this I don't even model the teeth on my gears. It is best to not confuse the guys doing the actual manufacturing.
PS: Thank the heavens for the autosave feature on this forum. I lost all of the above and was starting to recreate it when I noticed the option to load the autosave data, whew!
Thanks for the info! Obviously, I haven't worked much with helical gears and it turns out we are going to proceed with spur and hope the noise isn't too bad... They are cheaper and more readily available. Strength is not a huge concern since we have enough room to play with to compensate with a larger gear set. The only thing I am concerned about is the sound, but the general rule of thumb I have found is if the PD speed does not exceed 1200 FPM, we should be OK... We are at about 980 FPM.
This is just a prototype, so if it is unacceptable, we can adjust!
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