Splined Shaft Wear 9

[triumph993]

Folks,

Here's a photo album of the splined shaft from my hydraulic pump. The interior splines of the mating PTO are identically worn.

I'm in the process of identifying the steel, but I believe it to be a high tensile steel (4140 or 4340). My apologies for not being able to narrow this down just yet.

Wondering if these wear patterns tell a story to the metallurgists out there?

https://picasaweb.google.com/dmb2000/PTOPumpSplines?authkey=Gv1sRgCNSloqW9qonCaQ

Thanks,
David

 

[swall]

Tmoose--if you hadn't just mentioned the BMW splines, I was about to. Always a PITA to pull the tranny just to grease the input shaft splines. I have never seen any data that suggested that it was effective.

 

[gearcutter]

There is evidence in the OP's photos of a substantial amount of fretting that has been occurring; the presence of red iron oxide debris confirms this.
But I believe that the large amount of fretting that has taken place is not the root cause.
The fretting appears to have occurred as a result of initial wear.
A decrease in tooth thickness, as a result of wear, causes an increase in backlash. Increased backlash in a system that is experiencing torsional vibration will increase the risk of fretting due to the impacting nature of the contact.

"Adhesive wear occurs on sliding surfaces"
While I agree with this statement, the statement isn't really adequate to help explain what it is that's going on in a splined joint.
Keep in mind that, in a properly aligned splined joint that is fixed, there would be no sliding contact. The contact is compressive in nature only.
Sliding contact in a splined joint will only occur if there is any misalignment between the mating components.
Angular misalignment generates a small oscillatory movement, between mating spline teeth, which traces the path of a figure of 8.
In the initial stage of damage; these small oscillatory movements generally result in some kind of fretting taking place but the amount of fretting, at this stage, rarely results in any form of destructive damage. A good example of this can be seen on the crowned/barrelled teeth of a gear coupling.



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[tbuelna]

gearcutter,

I agree that this spline has suffered from severe fretting erosion, and you give a fairly good explanation of the situation. However, for fretting damage to occur there does not necessarily need to be sliding contact. Fretting damage can also occur from excessive cyclic contact loading, where two faying surfaces come in and out of contact. This type of fretting damage is common on the OD of rolling element bearing races that don't have a proper bore fit. I can imagine a similar situation occurring at the spline tooth flank contacts, where extreme torsional oscillations cause the flanks to come in and out of contact.

The mechanism behind fretting is basically a repeated process of diffusion bonding and breaking between contacting metal surface asperity tips. Over numerous cycles, this results in a transfer of material from one surface to the other. The surface that loses material has the typical pitting seen with fretting. The metal diffusion bonding effect that causes fretting can be inhibited by providing a surface film that is non-reactive, such as oil, grease, plating, dry films, etc. (picture trying to MIG weld two pieces of steel that are greasy or oily). Unless these materials are continually replenished at the contact interface, they no longer provide protection once displaced.

Interesting topic of discussion.

Terry

 

[mrfailure]

The red oxide surrounding the contact damage indicates fretting occurred at some point during the degradation process. The actual tooth damage at the time of removal is galling based on the pictures. (Fretting damage is caused by relatively small vibrational movements between contact surfaces; galling is larger displacement wear damage). Fretted surfaces are rough and show evidence of transfer between contact damage. Small displacement from fretting means one usually does not observe parallel wear marks at contact as was observed on these gear teeth. Also, fretting does not displace the volume of material observed on these teeth. The main issue under pure fretting is usually the generation of fretting debris. However, fretting loading can also initiate fatigue cracking leading to tooth fracture (called fretting fatigue.)

I still think the mechanism of damage at the time the spline was removed was adhesive wear. But I also see the scenario where problems started under fretting. Roughening of contact surfaces and the presence of fretting debris could have provided the asperities to then created nonuniform loading across the contact surface, starting the adhesive wear process outlined previously. This mechanism then predominated from the relatively large displacement contact.

Under this scenario, fretting actually would be the primary failure mechanism (adhesive wear/galling were secondary since they would not have happened without fretting first occurring). We really do not know the relative direction of movement between surfaces during the fretting phase because subsequent radially-oriented galling obliterated that evidence. Using this scenario, issues which could cause fretting contact during service (including alignment and design) would need to be examined. However, fretting situations are not corrected through lubrication because the lubricant film simply is displaced by contact unless lubricant can be continuously applied. Fretting can be reduced though by applying a hardening carburizing or nitriding surface treatment.

Finally, you may want to consider having metallurgical analysis performed of the worn spline to confirm failure mechanism(s). Microscopic examination of both the damaged contact surface both directly and in cross-section can better illuminate whatever happened.

Aaron Tanzer

http://www.lehightesting.com

 

[mfgenggear]

mrfailure

Please explain what in the metallurgical analysis what evidence one would be expecting to visually find?


Gearcutter

"if the correct size spline coupling is installed" & if periodic maintenance is done like greasing with with Zerk fittings, I believe this would help the situation. this is done for automotive drive shaft splined connections. as long as the connections are sealed correctly. High velocity shafts are sealed units with grease.

I can see if the the unit are open to the environment, adding grease is like adding an attraction for dust, dirt debris, which is like adding sand or lapping compound.

Would it not depend on the design of the unit.

at first I thought maybe this post was getting a little long but now I realize there's more to this post. it's a sleeper issue.
nice going every one, it's a very Interesting & great responces by all.

Mfgenggear

 

[mrfailure]

Mfgenggear,

First one should look at the loaded tooth faces using a stereobinocular microscope. All of our opinions are based on macro photos but binocular views may tell a different story and in any case will provide more detailed information on wear mode as well as some confirmation that discoloration around the damage is in fact consistent with fretting debris. It is also easier to differentiate fretting from galling wear at magnification.

A cross-section through a tooth will also be useful. Microstructure will be able to show differences between the contact surface and the matrix. One indication of adhesive wear involves plastic deformation at the contact surface. You will also be able to see if the contact surface shows a rehardened layer (such as untempered martensite) and adjacent plastic deformation of the structures, both consistent with adhesive wear. Fretting usually does not affect surface structure because the particles abrasively machine away the surface. Etched structure will also show whether the spline had been given a carburizing or nitride hardening treatment.

The cross-section should also be used for microhardness testing. Specifically, hardness should be measured by the surface both in the damaged area and in a non-contact area (such as a tooth root), in the middle of a contacted tooth, and in base metal away from the teeth. This information will shed light on the failure mechanism by identifying work hardening (if any) associated with wear, surface hardening of the as-manufactured spline (for example, from carburizing), and possible softening that may be found in the teeth that may have also occurred from wear.





Aaron Tanzer

http://www.lehightesting.com

 

[tbuelna]

mrfailure,

Excellent comments. I don't have any formal training in metallurgy or tribology, so I'm learning something from your posts.

Just one comment about the presence of red iron oxides. The pictures of the internal spline seem to show significant rust outside the area of the spline flanks. It looks like the whole spline joint was exposed to moisture, and moisture penetration may have been the cause of the iron oxide on the flank surfaces.

With grease lubed splines and bearings, moisture can be a problem even with a sealed environment. If there is sufficient air volume and humidity within the housing, repeated heating and cooling cycles can cause the moisture to condense out onto surfaces and wick into gaps like those between the spline teeth. Ideally, grease lubed splines should have some sort of dam or shield that keeps the grease tightly packed around the spline joint to minimize such moisture intrusion.

Terry

 

[gearcutter]

Aaron & Terry..........stars for you.
Thanks to both of you for the excellent information and descriptions.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[mrfailure]

Terry,

Fretting wear contact generates small metal particles. In the case of steels and irons, these particles oxidize instantly to form hematite, Fe2O3. This is the same oxide associated primarily with low temperature corrosion/oxidation of steel. This is also why some people call this fretting corrosion. I think that is a misleading term since the oxide is generated from a wear mechanism. The red-brown discoloration surrounding the wear contact areas is consistent with steel fretting debris and the basis for the opinion expressed by many (myself included) in this thread that there is evidence of fretting. There are a couple of clues in the first set of pictures indicating this discoloration is from fretting contact rather than corrosion. First, the red oxide is concentrated around the worn contact areas and disappears away from it. In addition, you will note that discolored non-contact areas still clearly show original tooling marks, which means the surfaces are not yet corroding.

Aaron Tanzer

http://www.lehightesting.com

 

[tbuelna]

mrfailure-

Thanks again for taking some time to elaborate on the subject!

Terry