Through vs Case Hardening to Fix Root Fatigue Failure 4

[thetank8]

A spur gear exhibited a broken tooth during a vibration test. The vibration of the overall assembly translated to an unknown input torque on the shaft of this gear which resulted in it impacting its mating gear in both directions. The rotation of the mating gear is ultimately constrained by a non-backdrivable worm.

Material: S45C through-hardened to HRC 40
Module: 0.35mm
# Teeth: 38
Pressure Angle: 20 deg
Profile Shift: None
Mating Gear Material: S45C through-hardned to HRC 55

First, from the photos of the broken tooth below, is it a correct assumption that the failure mode was fatigue breakage at the root of the tooth?

Second, would changing the material to something like a low-carbon alloy steel and carburizing so it is hard on the surface but still ductile at its core improve its performance? Or should I simply try to increase the through-hardness of the S45C gear as much as possible?

Photo:

Cross Section:

Please let me know if there is more information needed. Thanks!

 

[israelkk]

Can you five more info what is the source of vibration in real life that you need to test the assembly under vibration?

 

[dinjin]

You might want to incorporate a full fillet radius in the root.
Did the deformation of the tooth occur because of some debree
in the pinion root? Nothing wrong with the hardening specs.
How many teeth in the mating pinion?

 

[thetank8]

Israelkk, the assembly is mounted on a vehicle so our vibration tests are based on ISO 16750-3: Road vehicles—Environmental conditions and electrical testing for electrical and electronic equipment, Mechanical Loads.

Dinjin, the mating pinion has 9 teeth and is profiled shifted by 1.1. There was no obvious debris but now that you mention it, it is suspicious that there is deformation at the top land of the gear. The center distance to its mating gear was kept very close to theoretical to try and minimize backlash. Could the top of the gears be jamming into the fillet of the mating gear?

You said there is nothing wrong with the hardening specs but is there a spec that would produce better fatigue resistance?

Thanks.

 

[desertfox]

hi

Shot peening after gear hardening can improve fatigue life:-

http://www.shotpeener.com/library/pdf/1982003.pdf

also a link to images of failed gear teeth by various mechanisms:-

http://www.xtek.com/pdf/wp-gear-failures.pdf

 

[dinjin]

The cleanliness of the steel, grain size, etc. all contribute.
You probably already know about vacuum degassed steels giving
exceptional life. If the pinion was hobbed, I would not expect
any root problem on it. I agree too that shotpeening would
help but may not be practical on this little module system.
The fact that we cannot see any other cracks propogating makes
me wonder if there was not a default in that tooth root or
subsurface. Thanks for the excellent pictures. The teeth
to the left also looked a bit deformed.

 

[thetank8]

Desertfox, the gear teeth are very small (0.35 module). Even the smallest shot size I can find (S-70, .007") would be much too large, right?

 

[desertfox]

Hi

I didn't realise how small the gears were so maybe they are to small, however there are different processes which shot peening can be applied, it might be worth a phone call to a vendor, the worst he can say is no.

 

[gearcutter]

Which load rating standard has the gear set been designed to and what are the application and safety factors that were used?

The fracture surface does not reveal any evidence of fatigue or ductile fracture.
It looks like a typical brittle fracture.
It also looks like the gears have been hard meshing.
I'm interested to know why there is heat discolouration associated with the deformation around the top lands. There's also heat discolouration in the root areas.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

I can't see that there was a fundamental root stress problem with this gear. Given that the mating pinion to this 38T gear only had 9T, and there seems to some indication of abnormal contact near the remaining flank tips of this 38T gear, I'd say that the problem is more related to contact geometry.

I would agree with dinjin that both the pinion and gear would benefit from using a full radius root fillet. I would also suggest that both the gear and pinion would benefit from some tip relief. But if root fillet fatigue was the actual cause of this failure I would have expected it to occur first in the much more highly stressed 9T pinion.

 

[thetank8]

Thanks to everyone for the responses so far. We are still leaning toward changing the material to a low-alloy steel (SCM415) and carburizing or carbonitriding the teeth. Will this help at all? I've included a few more photos of a second sample of the same gear that also failed the same vibration test (see Photos 1 and 2).

To answer some questions:

Gearcutter, the gear geometry was chosen based on reverse engineering another part that experiences similar operating conditions and has been in the field for a while. The heat discoloration seems appears to be a remnant of the heat treatment. See Photo 3 showing a new gear with the same discoloration.

tbuelna, Photos 4 and 5 show the pinion that the gear was meshed with during the vibration test. Photo 4 shows where the tooth flank wore down but there is no obvious damage near the root. Photo 5 shows that the tip of the pinion was also deformed during the test (the pinion has a sharp tip due to a profile shift of 1.1). Does this provide more evidence that we are experiencing hard meshing and that is the reason for the gear tooth failure?

Photo 1:

Photo 2:

Photo 3:

Photo 4:

Photo 5:

 

[mfgenggear]

thetank

there is some fundamental questions that need to be asked.
I am surprised no metallurgist have jumped in on this one.

what was the heat treat procedure?
was the hardness verified/tested?
is this the correct material?

there appears to be plastic flow deformation.
to me the material is to soft. & the material needs a harder case.
did you do a full metlab report on the gears that where reversed engineered. Very Important.
was there a full lab report the size, involute, & the AGMA quality of gears. very Important
what is the AGMA quality of the gears. where the gears test for Total composite error.
was the measurement over wires, correct. if so what is the required backlash?
if there is interference the Total composite test will verify this.
what is the RPM ?
what is the load applications vs the stress analysis of the material?
what type of bearings vs the AGMA quality.
it could be two simple solutions.

The quality of the gear could be deficient. better gear quality.
was the gear hobbed? does it require grinding or honing. the AGMA quality will dictate this.
the material needs to induction harden or case harden.
be advised that both of these will require post machining of the gears which = more cost $$$



Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[tbuelna]

thetank8-

Thanks for the additional photos. However, based on the photo in your OP I still think your gears had a tooth-to-tooth meshing/load share conflict. I don't think changing the heat treatment process used for the gears will resolve the issue. Instead, I would recommend modifying the profile geometry of both gears to ensure better load sharing. With a 9T pinion it is critical to apply sufficient tip relief. And as others noted, your gears might benefit from improved accuracy provided by a finish grind operation.

Good luck.
Terry

 

[mfgenggear]

TheTank

I have much respect for Terry. he is clearly a design expert.
But I will stick to my guns and say the gear is not hard enough.
i clearly see plastic deformation on the gear tooth profile.

this could be a simple tooth bending & deformation issue then chipping & catastrophe failure.
the fillet radius appears to be correct.

I forgot to add ask about lubrication.
was there any?

what is the RPM & load?

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[gearcutter]

As a manufacturer; let me start off by saying that the gear cutting finish on those parts is atrocious!
The material that you've used normally doesn't leave tear and gouge marks as can be seen in your pictures.
Using that material normally results in a smooth and continuous surface finish, even straight off a hobbing machine.
You really need to tell your supplier that the quality isn't near good enough as it's going to affect the performance of your gear sets.

The fracture zone clearly shows the classic signs of a brittle failure. It is flat and faceted. If it were a fatigue failure, the surface would be concave and smooth.
A brittle failure suggests a sudden impact type of overload has occurred.
I don't see how increasing the hardness would help in this situation as it's an increase in ductility and toughness that is required to help reduce the risk of a similar failure occurring again.

As tbuelna suggests; with such a small pitch, tooth to tooth spacing accuracy is critical. Putting the parts through a severe process like induction hardening will add profile & geometry errors due to the quenching process.
I'm quite sure that this would affect the load sharing between successive teeth.

A much better result would be achieved if material such as 4140 is used and then GAS nitrited to an appropriate depth.
Ensure that the material is through hardened to around 32-35HRC before machining, gear cutting and then nitriding.



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

Consider the images below and compare them to yours.
The first images shows the fracture surface of a ductile fatigue failure.
The second image is of a fracture surface as a result of a brittle failure.



Ductile Fracture Surface by ronvol, on Flickr



Brittle Fracture Surface by ronvol, on Flickr

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

attached is for testing for tooth bending

if gear cutter & terry are correct a simple total composite gear test will verify that.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[mfgenggear]

G. Gasparini

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[mfgenggear]

failure analysis examples.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[mfgenggear]

so yes it could of being shock overload, but also from the chunks of teeth jamming the gear teeth.
I still stick with hardness. 32 HRc in very rare in my business. Since the OP has not disclosed the function it's hard to say.
most of the gear I manufacture usually have 60 min HRc case hardness. for high stress applications. with a 30 Min Hrc Core.
a gear in my business will not function at that 40 HRc gear profile hardness.

I want add this is a very fine pitch gear. it would be nice to know the gear tooth thickness.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.