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Superfinishing 4

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geesamand

geesamand

Mechanical
Jun 2, 2006
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I've read some recent articles on superfinishing of gears and bearings. It seems the academic studies are showing substantial improvements and it's not a gimmick. I have a few questions for those of you who have used it:
1) Does superfinishing benefit parts that have already broken-in to a decent polish, or is it mostly of value for OEMs who need to get maximum efficiency with minimal break-in?
2) Can superfinishing be used in conjuction with shot peening and not causing excessive damage to the gear profile?
3) It would seem that chemically accelerated vibratory finishing is being used for these types of parts. Is that the state-of-the art in superfinishing for a reasonable cost? (My intended use is racing, but my budget is more value-driven)
 
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Did you ever watch an old machinist make a part in a lathe?

The last step is always burnishing the diameters with a bit of crocus cloth, and then some worn crocus cloth, or the back of the new crocus cloth.

That's what superfinishing is; automated machine, fancy strip of high tech sandpaper, similar result.

Not equivalent to shaking the part in a bucket of rocks, not even with magic chemicals.



Mike Halloran
Pembroke Pines, FL, USA
 
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I've seen it done, certainly. The commercial processes I'm considering using light abrasives and chemical accelerant to achieve .025 micrometer Ra. (1 microinch Ra)

It looks like mechanical polishing can get that good, taken to the limit. But will it have the same surface characteristics? Will the manual tendency ruin the gear profile or cause low spots? This is why I didn't necessarily assume polishing would be as effective.

I'm more than happy to try, considering that I want to start with a couple of gears and bearings.
 
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If you're going to GO racing, there's probably no benefit in paying anyone to superfinish stuff on your behalf.

If you're going to SELL stuff to racers, anything that sounds zoomy and doesn't noticeably cut power or driveability will sell.



Mike Halloran
Pembroke Pines, FL, USA
 
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Well I'm in both categories. But I'd like to mess around with it in small scale before sending parts out and spending bucks.

Thanks,

Dave
 
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There are articles out there on the benefits/drawback of finishes and those are application specific. In bearings (or any Hertzian contact with relative motion), the surface finish cannot be too good or the ability of the oil to form a EHL film is impaired. In roller rocker (the follower od) the finish is critical. The roughness of the part must be such that lubricant can be retained and a "superfinish" would remove such capability.

In gears, (as in bearings) the direction of the grinding striation/furrows has a more critical impact than the surface texture value itself. Specular finishes are not particularly great in the highly stressed contact patches in gear flanks. In helicopter transmissions, surface finish is an highly sensitive topic. Military application require survivability of gearboxes (I seem to remember 30') at full power with complete loss of lubricant. This is accomplished using several techniques, (with lubricant retention being one of them and this is accomplished by increasing the contact area while increasing the retaining volume of the lubricant). The surface in such cases is defined by the DIN symbol Sk (I believe it is still part of a development standard).

Any good tribology books will give you the acceptable finish and texture shape limits to reduce friction and wear while still be able to retain/build complete metal surface separation.
 
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"The roughness of the part must be such that lubricant can be retained and a "superfinish" would remove such capability."

This idea is being reevaluated.

While I am not a tribologist, I have to say the studies mentioned in this article:
(800kB download) are of considerable interest.

I don't have any of the helicopter requirements - I'm in an automotive application where the NASCAR rear axle studies are relevant and interesting.

Dave
 
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geesamand,

"Can superfinishing be used in conjuction with shot peening and not causing excessive damage to the gear profile?"

It's my understanding that shot peening and superfinishing of case hardened gear flanks and roots both have essentially the same effect. They improve gear life by removing minute surface imperfections where fractures/surface spalls tend to propagate from. When performed properly, neither will be detrimental to the gear tooth profile.

If you have designed, manufactured and assembled your gears and lube system properly, they should always be operating within hydrodynamic film conditions. That means low probability of scoring and a high lambda value. As long as you operate in full hydrodynamic conditions, having a super finished surface won't be of benefit. Most gears (and bearings) tend to ultimately suffer spalling failures that initiate at pits due to surface corrosion. If there is no corrosion present, a well designed gear or bearing will have essentially unlimited life.

Superfinishing and/or shot peening of gear tooth flanks and roots removes the tiny corrosion pits in those highly stressed surfaces, that are an inevitable result of the manufacturing process. I tend to prefer shot peening instead of superfinishing. They both accomplish the same thing, but I have more experience with shot peening.

Regards,
Terry
 
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I have a different understanding:
- shot peening: induce compressive stresses in the surfaces to improve fatigue/bending strength
- isotropic superfinishing: extremely fine surface finish to boost the lambda value and reduce friction. In loss-sensitive applications where the original lambda was sufficient, this can allow lower viscosity lubricants for still lower losses.

I'm mostly interested in the superfinishing aspects. I see the REM has a chemical/vibratory superfinishing process available. There are a couple of shops I've found that advertise the service. I have yet to find a shop offering a superfinishing service other than the REM ISF method.
 
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I agree with geesamand; "shot peening: induces compressive stresses in the surfaces to improve fatigue/bending strength”.

Without actually removing material, I don't see how shot peening can "remove the tiny corrosion pits in those highly stressed surfaces" as mentioned above. I've never heard of shot peening being used for this purpose.

Below is a link to an excellent article from NASA on the subject of super finishing.



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
 
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Went to a seminar this past week, and one topic was new stuff for the Rolls Royce ( formally Allison ) model 250 turboshaft engine; seems like there is a service bulletin reworking the accessory drive, and the new style gears are "superfinished". I asked what that meant, and what were the benefits, ect, and well "he would have to get back to me......next question?"
 
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To all:
I have personally and actively been involved over the past four years working to qualify the REM ISF process for use on Army helicopter platforms. Bottom line: this process works.

In every test performed to date, and there have been many, each with hundreds of hours of very demanding tests, ISF has proven to improve our transmissions' performance. We have seen lower operating temperatures (20F at the sump), gear surface finish of 2-4 Ra was achieved without a significant amount of material lost… less than 0.0002 inches, reduced scoring, reduced pitting, reduced gauling, etc. All of these with no negative effects noted. There are no tribology issues, no issues at all in fact. Even the old gray beards that originally had their doubts have finally come around and admitted that the transmissons they originally designed are made better as a result of ISF application.


And FYI. I do not work for REM nor do I represent them in any way. But over these past years I have become an ardent believer in the ISF process...because a ton of testing has proven it.



 
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Lowlife, thanks for the response.

Our gearing vendors, when asked about REM ISF, immediately responded with "we can grind to 4Ra if you want, what's wrong with that?". Obviously they have a ground finish and not isotropic, but if the roughness is the same they ask a valid question.

Have you seen tests between ground surfaces and REM Isotropic surfaces of the same Ra? Was there still a difference?

Dave
 
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I stand by my previous comments about superfinishing vs. shot peening. My comments were conditional and assumed a properly designed, manufactured and lubricated gear mesh. What this means is that tooth root bending stresses and flank contact stresses are kept within a limit that allows unlimited life. Besides root stresses and contact stresses, the only other design consideration is scoring.

If a gear mesh has any probability of scoring, it doesn't matter what type of finish operations you perform on the gear. It's going to fail, and it will fail fast. Superfinishing may provide a small margin over conventional grinding/honing with regards to lambda, but it's not much. The proper approach is to design your gear mesh action so that it does not produce a situation where scoring is likely to occur in the first place.

If you've designed your gears correctly with regards to bending, contact and scoring, they will theoretically have unlimited life. In practice, corrosion will eventually occur on the gears, and it will cause small surface pits. These surface pits will eventually cause spalls in the case, and lead ultimately to fracture propagation.

The reason shot peening helps is that it eliminates/consolidates the surface irregularities that result from the rough machining and heat-treating processes. Stock removal during finish grinding of case hardened gears is normally controlled very closely. So it is very likely that some surface imperfections may not be fully removed if the surface is not improved beforehand by shot peening.

Shot peening can induce beneficial residual compressive stresses in the peened surfaces, but these are somewhat relieved during the finish grinding operations on the gear tooth flanks and roots. Of course, finish grinding of root fillets is a controversial issue for this reason.

Superfinishing can give an extra margin of safety to a good gear design, but it won't help a poorly designed gear mesh. Superfinishing of gears helps with scoring, but nowhere near as much as a properly designed lube system and lubricant will.

Regards,
Terry
 
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Geesamand your gear cutters may well be right. They probably can grind, hone and polish to an Ra of 4, but why? This requires they to do it tooth by tooth, gear by gear. To achieve this surface finish conventionally is both time consuming and expensive. superfinishing is a batch process where multiple gears can be processed simulataneously, thus lowering overall costs. Shot peening prior to superfinishing is the way we have elected to go.

And Terry you are also right. Superfinishing will not make a fundamentally bad gear good. what we have found is that it makes a good gear better. and it can be used to remeove small pits from the gear if found early enough, and restore the gear to a serviceable condition. Penn State conducted testing for the Navy under a REPTECH effort that indicted that used gears processed using ISF out performed new virgin gears in single tooth bending fatigue tests.

ISF will certainly not solve world hunger or aids, but if you have a gear application where high torque and low speeds are involved, ISF I believe is certainly worth at least trial processing some test gears for evaluation. then, when you see the test results, you can decide for yourself if it was advantageous or not.
 
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Lowlife,

I agree. ISF will give a well designed gear mesh a more comfortable scoring margin. But it will not magically make a bad gear mesh "good".

I haven't seen the data you note from Penn State/NAVAIR, but from my personal experience (I currently design military aircraft power transmissions), the gears I design are analysed for infinite life in both bending and contact, and for a scoring probability of less than 1%. We use carburized/ground VIM-VAR (double vacuum melt) 9310 for the gears with shot peened roots and flanks. To get unlimited gear life, we keep the unidirectional tooth bending root stresses below about 55 ksi and the face contact stresses below about 185 ksi. In theory, these particular gears would not benefit from ISF in normal service, as your article might suggest.

The only in-service failure mode for my particular gears would be scoring due to loss of lube, overloads experienced during an OEI condition, or fractures propagating from surface corrosion pits. We design the gears and bearings to survive for a certain period of time under loss of lube conditions, we design the gears to have adequate fatigue life under any OEI condition, and we design the lube system to be factory sealed, maintenance free, and with a dessicant breather system to eliminate moisture induced corrosion.

thruthefence,

Most rotorcraft MRGB's are designed to meet a 30 minute loss-of-lube condition. The power transmitted by the MRGB during this loss-of-lube condition must be adequate to maintain level flight of the aircraft. The way MRGB gears and bearings fail in a loss of lube condition is through material mechanical failures, due to de-tempering of the bearing or gear steels brought on by overheating due to increased friction under the marginal lubrication conditions and the reduced heat transfer with no oil flow.

To meet the 30 minute loss of lube requirement, most rotorcraft MRGB's employ high temperature steels like X-53 for the critical gears and M-50NiL for the critical bearings. They also use (total loss) supplemental lube systems to provide a minimal amount of oil to critical gears and bearings.

Regards,
Terry
 
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Terry- very informative post, even to me (electrical engineer)- whose only mechanical engineering efforts are for hobbies.

When you say "X-53 high temperature steels" are you referring to 153/253/353-M alloys from AvestaPolarit?

Your use of 9310 alloy gears in "unlimited life" transmissions intrigues me. Are you familiar with so-called "Pro" ring/pinion gearsets commonly used in mega-power drag race vehicles- top fuel dragster, funny car, etc.? They are 9310 and have the stipulation of "not for highway use". I've queried some manufacturers (Richmond and U.S.Gear) about expected life if a person DID use them on the street, but the only answer was "that's not their intended usage". Any comments?
 
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pontiacjack,

X-53 (trade name Pyrowear) is a carburizing alloy steel that can be tempered above 600degF, so it will maintain its hardness up to that temp in service. 9310 alloy steel is a very good gear material, but if exposed to temps above about 350degF, it will de-temper and lose its strength. The only benefit to using X-53 over 9310 is that X-53 will survive longer at the high temps experienced under something like a loss-of-lube condition.

A hypoid R&P gear set is normally limited by scoring, more so than bending or contact. So using a good gear lube with the proper EP additives and making sure the gears are set up (shimmed) correctly is the most important thing. As for the benefit of using 9310 steel, it's all dependent upon the quality of the raw material and the processing the gear undergoes during manufacture. Aerospace quality, double vacuum melt 9310 is fantastic stuff, but is very expensive. If you compare it to a commercial air-melt material of the same chemical composition, the difference in mechanical properties is like night and day.

X-53 data:
Regards,
Terry
 
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