Contact pattern problem on hypoid with shot peening

[Gearen]

Hi,
We have been trying shot peening on our hypoid gears to increase their fatigue life. However, a big problem in manufacturing has turned up. With shot peened gears we get very bad and fluctuating contact pattern when the gears are assembled. This seems to be due to a bulge at the tip of the tooths from the shot peening that cause flank interference.
I have made some sketches to show what I mean.

The picture below shows the contact pattern at testing that we have without shot peening the pinion and ring gear and with shot peened gears (a little exaggerated).

We have measured the gear flanks with a profilometer and noticed that we have a bulge at the tip of the tooths which seems to make the flank interference. (see picture below)

The gears are face milling gears that are cut, hardened and lapped. If the gears should be shot peened, it is done before the lapping.
Does anyone have any suggestions how we can resolve this problem?

 

[swall]

How much retained austenite do you have?

 

[FoMoCoMoFo]

have you read the thread below this one titled..heat treating OE gears a waste ?

 

[Gearen]

swall
I am not so good when it comes to material science but I think 20-30 % is what we have.

 

[swall]

Here is what could be happening. Case depth will tend to build a corner, and you will have higher retained austenite at that location. The peeing transforms the retained austenite to martensite and you get a volume increase, i.e. the bulge. To prove if this is happening, get some gears subzero treated prior to peening and see if that makes a difference.

 

[Gearen]

Oh my, that was really an interresting theory.

You say case depth build a corner and thats why you get more retained austenite. Is that because you get more carbon atoms to that place (coming both from the flank and the top) and more carbon gives more retained austenite?

The subzero treatment I have almost never heard of (same as cryogenic?). What will happen then if we shot peen? Isnt cryogenics used with race cars to improve the gears also?

However, if it should turn out that you are right, is there any other way to solve the problem? I guess subzero treatment is expensive.

 

[NickE]

The Austenite -> martensite reaction can be competed by a quench to extremely low temps, doesnt always need to be LN2, but thats cheap and easy.

Yes Carbon is an austenite stabilizer.

I like swall's answer.

 

[adamant]

Perhaps you need to modify the lapping treatment for the shot peened flanks.

 

[Gearen]

I found the AGMA information sheet about shotpeening of gears

http://webstore.ansi.org/ansidocstore/product.asp?sku=AGMA+938-A05

It is a very nice text which I can recommend. It says that sharp corners need to be rounded before shot peening to avoid edge rollover which I guess is my problem. We'll see if we can get those corners rounded of in some way. Any ideas?

 

[Tmoose]

even a 45-ish degree chamfer/bevel makes a corner less vulnerable.

How much of the tip is involved in the meshing? According to your contact pattern it gets real close, so maybe you can't afford to lose any flank surface.

Are you really after a shotpeened flank? Maybe most of the flank and dip need to be masked

 

[sera00]

Gearen,
Until today I saw your comment.
I do apply shot peening process several types of gears. The condition you describe, based on my experience is that your nozzle are not properly directed to the tooth root where you want to have the shot peening process to be applied.
It don make sense to hit the tooth flank, the critical area that will help you to increase your fatigue life is the tooth root not the flank.