Thread: Helical gear confusion

Hi,
Gears have always fascinated me, especially once my uncle gave me some old car gearbox gears at about age 8. The way the helical teeth just seem to flow around the gear amazed me.

Any way, whilst looking through some stuff (the same place i found the hard cast iron sprocket) i found a couple of helical gears, and something just didn't seen right with them. The helix angle is fairly low, about 15 deg, but still something was just not right.
It took until today for it to click- the gears are not in fact helical, but spur gears cut at an angle, ie the root and tip of each tooth is flat, not rounded.

I would think that they would have several of the helical gear advantages- like wear and noise, but would be fairly easy to cut, really no harder than spur gears.

Needing to cut a full set of gears for the conrik (or even attempt a norton style box) my mind wondered off to have a good hard think about it. Could it be worth the extra effort?

Any thoughts?

Ewan

hi Ewan

Your post prompted me to take these pics . This gear setup raises/lowers the column on the T&G grinder .

One gear has straight teeth , the other angled teeth .

MIKE

At such a small angle, can you be sure they're not helical? In his book Ivan Law talks about two kinds of improvised worm wheels, one using using a simple spur gear set at an angle, the other using a gear with straight, angled teeth as you describe. From memory both options will work but with reduced smoothness and increased wear. That's the only reference I can recall. Wikipedia mentions lots of weird & wonderful gear types but not those. Maybe you should cut some and see?

single helical gears are great but they cause thrust in one direction. from memory double helical gears are noisy but dont have the end thrust problem. also im not sure if helical gears are needed unless they they need to transmit high loads. the only advantage i can see is they run smoother.
So higher loads and quieter sounds good as long as the end thrust produced can be delt with. this is all based on memory and no real knowledge of the subject
aaron

Originally Posted by Ueee

Hi,
Needing to cut a full set of gears for the conrik (or even attempt a norton style box) my mind wondered off to have a good hard think about it. Could it be worth the extra effort?
Any thoughts?
Ewan

Yes, stick with the spur gears!

I absolutely do not claim to have any expertise in gear design and manufacture, but a friend of mine and I have been trying to work out how he can cut a new helical timing gear for his vintage Chrysler, so I dug around to gather some information. Yes, he could probably buy one, but he likes the challenge. So far progress has been almost non-existent. He has produced several nice looking gears which don't mesh with the original gear, and has now resorted to cutting trial gears in Jelutong rather than cast iron or aluminium because of the cost.

Besides the side thrust issue, there are several problems in cutting helicals which do not occur when cutting spur gears, among others:
. The standard numbered cutters for a given DP are not the same as the ones used for the same diameter spur gear.
. Calculation of helical gear centre distances is different from spur gear calculations

Here are the gear equations for both spur and helical gears.
Gear Formulae - Engineers Edge

I have a copy of the Cincinnati "Treatise on Milling and Milling Machines" which goes into enormous (and largely incomprehensible to me) detail on the cutting of helical gears. I think it is available on the internet somewhere, so would be worth a look if you decide to persevere with the helicals.

Here's a video of cutting a helical gear on a vertical mill - note the protective safety gear!

[ame=http://www.youtube.com/watch?v=vyg0Emz4Vx4&playnext_from=TL&videos=wLkwI6JIu8M]Cutting helical gears - YouTube[/ame]

Good luck, and let us know how you did it.

Frank.