Scuffing of Gear Surfaces 2

[gearcutter]

I'm in the middle of an investigation and thought I might share some of the issues that I'm coming across.
This image is the best example that I have which shows an adhesion phenomenon known as scuffing, sometimes incorrectly called scoring.
Shown is a tooth tip and a metric ruler for scale, increments are in 0.5mm.
The gear from which the image was taken is operating under misaligned conditions, so the contact is non-uniform with heavy bearing over 50% or less of the facewidth.
The whole transmission has had little use so there is quite some concern amongst all those involved with the project.

AGMA 1010-F14 says this about scuffing;

Scuffing is severe adhesion that causes transfer of metal from one tooth surface to another due to welding and tearing. The damage typically occurs in the addendum, dedendum, or both, away from the operating pitch line, in narrow or broad bands that are oriented in the direction of sliding. Scuffing may occur in localized patches if it is due to load concentrations. The scuffed area appears to have a rough or matte texture. Under magnification, the scuffed surface appears rough, torn and plastically deformed.

Scuffing is not a fatigue phenomenon and it may occur instantaneously.


Scuffing by ronvol, on Flickr


Sequence of events leading to scuffing by ronvol, on Flickr

 

[tbuelna]

gearcutter-

Great post! I especially like the chart describing the failure process of scuffing.

I would just add that the mechanical adhesion and material transfer/smearing are the product of a scuffing condition. The root cause of scuffing is a lube oil film that for whatever reason(s) has degraded to boundary contact conditions. While scuffing damage does not actually occur "instantaneously", in some cases it may only take a few seconds to produce permanent damage.

The worst thing about scuffing is that the failure process is self-perpetuating. It starts with the lube oil film degrading close to boundary contact conditions. This can be caused by excess contact sliding at the mesh or high oil feed temps that reduce the lubricant viscosity within the lube oil film. Excessively high contact forces due to mesh misalignment or overload operating conditions can also reduce the thickness of the lube oil film. When the contact initially transitions to boundary conditions the friction heat generated at the contact location significantly increases. The added heat input to the lube oil film raises its temperature and reduces its viscosity. The lower oil viscosity results in a thinner film thickness, which in turn results in more surface contact at the asperity tips and increased friction. The cycle repeats itself until the surface pressure and temperature at the asperity contacts is sufficient to produce mechanical bonding. The repeated mechanical bonding and shearing of the asperity tips results in material building up on one surface and pitting of the mating surface. The increased surface roughness of the gear flank surfaces from material transfer degrades the oil film contact conditions, resulting in greater friction, which just adds to the problem.

Relative surface roughness, contact sliding, oil viscosity, flash temps within the oil film, etc. all have an influence on scuffing potential.

 

[tbuelna]

gearcutter-

Do you have any additional information or photos of this gear? From the photo provided it appears this is a spur gear(?) and the most serious galling damage occurs above the PD and extends to the tooth tip. You mentioned there was a known misalignment issue. But since the worst of the scuffing damage is located fairly close to the tooth tip you might also check to see if there was adequate tip relief used.

One significant factor with scuffing is flash temperature in the oil fluid film. Excessive contact sliding can raise the oil film flash temps quite a bit. So it would be worthwhile to run a quick analysis to check the degree of mesh contact sliding, and also whether the relative sliding is approach or recess in nature.

Very interesting topic and I'd like to hear more about your investigation of this problem.

Regards,
Terry

 

[3DDave]

I've seen a similar process slide the case hardening off the teeth. The worst was a group that decided that annealed 17-4 was a good choice for pinion material in a preloaded (for no-backlash) non-lubricated (Customer wanted there to be no 'regular' service). It took all of 5 minutes to look worse than that.

 

[romke]

i can see the reason for concern when this happens. as you noticed misalignment i guess there is not much you can do to prevent a thing like this to happen - it will lead to localized overload and subsequent failure. if careful alignment is not possible, it will happen again when you replace the gears.

as you state that the gears have not run for many hours it may well be that they are still rather rough, which in turn may help to get a initial overload. if possible i would suggest running the gears in with a light load with a low viscosity oil for a couple of hours to wear off initial asperities. after that, when the gears are put into service, a gear oil of the correct (thicker) viscosity should be used with a appropriate amount of EP additives.

 

[tbuelna]

romke-

Your comment about the change in surface roughness after a run-in period is excellent. Most of the gear analyses I have seen use the flank surface roughness after run-in when calculating scuffing probability. And newly assembled high-performance transmissions are often given a "green run" similar to what you described. Knock off the peak asperity tips and then flush the lube oil to remove the metallic debris from the system.

As for mesh misalignments, these can be compensated for somewhat by crowning or lead modification if they can be quantified during design of the gearbox. But there is no free lunch, and the amount of face crown applied must be a careful compromise between reduced contact load capability for a given face width and improved misalignment tolerance.

 

[Tmoose]

"the amount of face crown applied must be a careful compromise between reduced contact load capability for a given face width and improved misalignment tolerance."

SKF and others hawk logarithmic roller bearing profiles as improving edge loading conditions compared to even extremely well aligned cylindrical rollers.

http://s1072948.instanturl.net/digitalmetrology.com/wp-content/uploads/2014/06/profile-2.jpg

Maybe this would be only comparable at the gear pitch line, but I'm thinking load is load, either for EHD or when hoping for HD lubrication.

 

[gearcutter]

Here's more info related to this investigation.

I've been able to approximate the amount of angular misalignment and model the mesh accordingly. The results are comparable to what we're seeing in-situ.


Angular Displacement, 0.50mm by ronvol, on Flickr



Model, 0.50mm displacement by ronvol, on Flickr

 

[gearcutter]

Image showing one of the gears.
Apologies for the very poor use of the flash.

Spur Profile by ronvol, on Flickr

 

[gearcutter]

Contact pattern at initial assembly, before commissioning.
Strangely enough; even at this point the chief engineer doesn't seem to think that there's an issue.


Contact Pattern @ assembly by ronvol, on Flickr

 

[gearcutter]

I've found this to be very interesting.
The manufacturer has rough machined the full tooth profile in a CNC Milling machine.
If you look carefully at the root profile, you can see the tool passes created by the 'endmill' type of cutting tool.
After case carburizing; the part is then finish ground on a 'profile' type gear grinder.
Also notice the tip interference below the SAP.

Tip Contact Interference by ronvol, on Flickr

 

[spigor]

gearcutter,
this is very interesting and well done, thank you for sharing.
Could you please explain, what is the interference zone, where are the black lines along the flanks comming from and why they are not present in the intereference zone?

 

[tbuelna]

gearcutter-

Thanks for the detailed results of your investigation so far. I know how hard it can be to convince people of the importance of paying attention to every little detail in gear design and manufacturing. The initial contact pattern shown in the 2nd to last photo should have been enough to stop the test and thoroughly investigate the source of the problem.

The last photo is also interesting. The scuffing at the transition from the active profile to the root is localized, so this would seem to indicate interference with just the tip section of the mating tooth flank. Maybe it could be resolved with some profile tip relief.

Lastly, your shaft misalignment study image shows +/-.50mm of displacement at each end of the green shaft, and the photos appear to show a spur gear that is around 200mm wide. That's a substantial amount of misalignment. In fact, with that amount of shaft misalignment I'd think there would also be edge loading problems with the bearings, unless something like a spherical roller bearing was used.

I hope you'll continue posting more details as your investigation progresses.

 

[geesamand]

Impressive example. What circumstances allowed that machine to be tested and operated without sending local residents into their earthquake emergency plans?

David