Long Addendum Teeth Profile 3

[smtsrc]

Hello All,

We came by the knowledge of some competitor companies (agricultural tractor transmissions) use high addendum gear profiles like 1.35xM. Even the gears are have high numbers of teeths, the tips are very small unlike the normal profile.
I could not understand the benefits of such so-called high depth teeth and was looking for articles about it but could not find a good source. Does any of you familiar with such profile gears? Do you know the advantages of such profiles? Do they give better tooth root strength or flank safety?

Best regards

 

[tbuelna]

It sounds like these are "extended addendum" gears. They typically use standard pitch centers, tooth thickness, and backlash. The benefit of this approach is higher contact ratio, smoother meshing, reduced noise, etc. The drawback is reduced tooth bending strength. The practical limit to extending the addendum is minimum acceptable width of the tooth top lands.

Don't know the reason this particular type of gear geometry was used for your tractor transmission.

 

[Windward]

tbuelna, why is there a minimum acceptable width for the tooth top lands? I have read that the narrower the land, the more likely the tooth will chip there. If this is true, what would cause it to chip? I suppose the shear stress when the mesh begins, but I don't know anything about it. I would like to have your comments. This subject recently became important to me.

Also, why does the bending strength decrease?

 

[mfgenggear]

Having apex at the gear tips, causes week tooth bending loads.
it's not a good scenario, thus profile shift + at the pinion teeth to
increase pinion tooth bending ratio, - profile shift at the gear.
to have proper tooth contact.
good attributes are the following
contact ratio 1.5 + depending the loads.
contact stress
high tooth bending stress ratio

there is a good video to help with under standing the above
AGMA tooth stress

https://www.youtube.com/watch?v=VaUsmBspl58

The gears of the competitors might be an issue, but not enough data
to really make a proper answer.
Follow proper design factors.
scuffing or scoring also must be considered.

 

[mfgenggear]

The other reason for profile shifting of the pinion
if the number of low number of teeth , under cut will result
and again cause week tooth bending ratio.
if the pressure angle is to high apexing of the teeth will result.
this is not good. a good gear program would be
on that calculates all of the previous post and the above.

 

[spigor]

why is there a minimum acceptable width for the tooth top lands?

A tooth tip that is too narrow for a given case depth will get through- carburized, will have unfavorable residual stress distribution and therefore be very brittle and easy to break off.
Both high addendum and long addendum (a positive profile shift) decrease the top land width.

Also, why does the bending strength decrease?

The bending strength decreases as the addendum gets higher because the meshing forces that act at the tip of the tooth are further away from the tooth's base; a force acting further and further away from the critical section creates more and more bending stress in it.

 

[Windward]

Good explanation, spigor. I was about to design something using gears that had zero land width, for a good reason. But I see now that it wouldn't work. I also see how I might get around the problem.

Now that I have a little understanding of it, I speculate that even if the gears are not case-carburized or otherwise hardened, decreasing the land width still increases the chance of failure. At the start of the mesh, all of the force is at the tip, where the cross-section taking the load is the least. Shear and bending stress at that point might be enough to cause a failure even if the bending stress at the root is safe.

mfgenggear, that is a long video, but a good one.

 

[mfgenggear]

it is a long video, but the professor is good.

some times it's unavoidable to have tooth tips that are on the thin side.
when this does happen do not carburize the tooth tips and end faces to minimize
the effect as spigor stated.

a good program will calculate the tooth tips land width.
example of that, is tooth tips are .070 inches, case depth is .030.
unavoidable thru case hardening will happen.
this is why it is important not only for stress loads, but for geometry
to pick the correct Module or diametral Pitch, and pressure angle.

when I process parts my personnel written programs looks for this.