Helical question

[Steve West]

Hi, just a little confused over helical gears,
Are there two ways to to make them ?.

I made a simplified image to show what i mean.

The top gear is what GT outputs, and has a sort of twist in the
teeth from rotating all the bottom by the same amount.

the bottom one everything is parallel, from rotating everything proportionately
according to radius.
Looking at images on the web, both seem to be around.

Are both correct ?
I would guess the bottom one would be easier to cut.

I will be 3D printing, so manufacturing either makes no difference,
so are there any advantages or disadvantages in each method ?.
thanks,
Steve.

[ArtF]

Hi Steve:

  I believe what your describing is flank twist. Its a natural occurance from hobbing a
helical gear with a norm hob that is basically a spur hob. I have seen in the literature
some design consideration to create nonlinear corrections to get rid of the twist, but Im
not so sure I understand all the considerations. To my mind the shape of the tooth must
remain an involute. If you rotate based on radius, that involution is compromised, so
I don't think rotation based on radius is sufficient to make a parallel helical properly.

  Id like to hear from anyone with experience in this area, I know the helical flank twist
in the accepted method of making helicals, and I know some work has been done to make
them in parallel for in order to increase the root pressure tolerance.

In short though, I woudn't worry too much about the twist, and Id use the twisted form as it
seems the vast majority of the literature calls it normal. Ill keep my eyes out for information on
it, though I suspect manufacture would be very difficult ..but again perhaps that my ignorance. :)

 

Art

[JustinO]

For gears with parallel axes to "work", every mating pair of "slices" perpendicular to the axes has to work. If you make a hundred really thin, identical spur gears and make two stacks of fifty, you can mesh them and arrange them to look like helical gears, or herringbone gears, or pretty much any weird thing you want. They will "work". This is how the typical helical gear works (although they do vary the geometry a tad to avoid pressure at edges). There is nothing that says every pair of slices has to be identical, but they do have to "work", and have the same pitch circles.

The bottom gear doesn't look like anything I'm familiar with. It looks like something a drawing program might generate if you twist a model of a spur gear. It looks like the middle slices are smaller than the outer slices. This could not possibly "work" with parallel axes. Perhaps I am being confused by your "simplified image".

"Looking at images on the web, both seem to be around."
Can you link a couple images of the type your lower drawing depicts?

Thank you.

[JustinO]

If you hold a worm wheel next to a helical gear, the teeth look very twisted on the worm wheel, and hardly twisted at all on the helical gear -- could that be what we're looking at?

[ArtF]

>>If you make a hundred really thin, identical spur gears and make two stacks of fifty, you can mesh them and arrange them to look like helical gears, or herringbo ne gears, or pretty much any weird thing you want. They will "work".

  I second that, its basically what GM does. The spurs that you stack are not normal spurs though, they have been changed to be larger
so the profile is correct as a spur when viewed from the helical angle. In other words for normal helicals you can run a standard same module
spur gear on them turned to the helical angle.

  After checking a few documents, I am pretty sure the twist must exist to be a proper conventional mesh, but it IS possible for a conjugate
type of mesh if some process were applied to do a radius based correction to get rid of twist. Id be interested if you find any information
on such a process.


Art

[JustinO]

Is the idea of these "untwisted tooth" gears to cut a "helical" gear without coordinated motions? Just a simple linear pass across a stationary blank?

My hand is on the trap door lever...

[ArtF]

Im not so sure youd get anything that way.. It'd take at least 3 axis working in coordination to cut that pass, a linear
pass would leave different root depths across the face.

Art

[JustinO]

So what is their attraction?

Talking about these gears, my mind kept drifting off thinking about generating hyperboloidal gears...

[ArtF]

Id have to think their a mathematical curiosity only..

Art

[Steve West]

Thanks for the answers,
on looking again more closely could not find my "imagined" gears,

But interesting to think that with the advances in 3D printing, many things
that we accept as standards, partly came about due to manufacturing limitations,
and now we need to go back and reinvent.

[ArtF]

Steve:

  Always worth thinking about, and I do all the time. In this case though, Im pretty sure the math
ensures the anti-twist helical isnt really a gear..more a graphic design. :)
  It IS good to always consider such things though, its where innovation comes from..

Art