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silver plate over super finished gear profiles (6)
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The subject is
super finished gear teeth which then require silver plate over gear teeth profiles, including the major diameter, roots, & end faces.
the issue I am having is I have customers that wants a 4 micro
on their tooth profiles. similar process to ISF Isotropic Super finish Process. However my suppliers complain that the plating process will not adhere to the parent material. It will peel off after the post embrittlement bake operation. the bake can only be 275 Deg F because these are Carburized parts.
what the suppliers normally do is an abrasive (glass bead peening) which roughs out the surfaces to make the plate stick.
which of course this is not acceptable because it violates the 4 micro.
what is this boards recommendations, I looking for constructive suggestions. or is there a process which will make the plating stick to the super finished gears.
can the super finish be 15-20 micro. so the plating will adhere.
I am at a loss with this scenario.
Thanks in Advance
Mfgenggear
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Why a silver plate at all? it will not last during the gear working due to the high surface contact stresses. Are those gears run dry?
I do not believe that the risk of hydrogen embrittlement cracks can be avoided for carburized surfaces. Therefore, the plating if any should be mechanical or a process that doesn't produce hydrogen at all. |
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Israelkk
to my understanding the silver is only there for the break in.
The gears are not run dry.
I need a rational to advise my customer it's better to not plate.
since this gears are super finish does it need the silver?
will the super finish be enough?
Thanks
Mfgenggear |
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I am more worried of the hydrogen embrittlement using electroplated coating. To my best knowledge and familiarity with aerospace and military specifications this should be avoided. There are other plating processes such as mechanical or vacuum deposited or low hydrogen processes but I will avoided any hydrogen producing process at all. I do not see any advantage for the silver over the carburized surfaces if the gear are lubricated. The silver will not last and the risk of hydrogen embrittlement cracking and tooth breakage is by far more important. More than that, the plating process needs to be checked for hydrogen removal. Therefore, carburized tensile test samples should be plated too at the same time. Then they should endure 200 hours of constant stress level (bending/tensile) as the gear tooth without developing cracks and breakage. |
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Israelkk
The plating spec. is AMS2412
The coatings are post embrittlement baked w/i (4) hours of plate
@ 275 deg F +/_ 25 deg F for (8) hours.
I can google the information & the leads that you have recommended. but do you have any data or spec #
that you can give .
"Then they should endure 200 hours of constant stress level (bending/tensile) as the gear tooth without developing cracks and breakage".
Is there a spec for the above test?
Thanks
Mfgenggear |
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The hydrogen embrittlement process verification usually is specified in the plating process. Here is an example from MIL-DTL-16232G PHOSPHATE COATING, HEAVY, MANGANESE OR ZINC BASE.
4.7.2 Hydrogen embrittlement testing.
Unless otherwise specified, testing to determine the adequacy of the hydrogen embrittlement relief treatment shall be performed in accordance with the following:
a. For parts that are surface or through hardened at Rockwell C 39 and above, testing shall be performed in accordance with ASTM F519 using Type 1a cylindrical specimens to represent the parts. Phosphated specimens shall be subjected to a sustained tensile load equal to 75 percent of the ultimate notched tensile strength of the material. Loading of the specimen shall be accomplished within one hour after completing the hydrogen embrittlement relief treatment stated in the approved procedure. The steel, 4340 at Rockwell C51-54, is acceptable for worse case situations unless otherwise specified on the drawing or in the contract.
b. Unless otherwise specified (see 6.2) the specimens shall be held under the load for a minimum of 200 hours and then examined visually under lox magnification and an illumination of
1100 lux (lx) for cracks. The production parts covered by the test period shall be rejected if any coated specimen develops any crack or breaks as a result of the test.
The tensile specimens for the tests should be prepared according to "ASTM F519 Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating Processes and Service Environments".
The heat treatment for the specimen in ASTM F519 is only to Rockwell C51-54 unless otherwise specified in the drawing. In your case the carburized surfaces are probably more than C60. Therefore, they are more sensitive and to my understanding and as used to be specified in the past in military specifications the specimens should have the same heat treatment and same base metal.
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Israelkk
thanks for the info.
This plating issue has been done by GM Allison's since the 1950's
I don't understand why it's been done so long.
with out issues.
I would really be happy if the silver was eliminated but unfortunately it's my customer decision. and not mine.
I will discuss this with our customers & address the testing with our suppliers.
but back to my original question?
Silver over super finished gears. |
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ANS 2412 was adopted by the DOD only in 1994. It didn't exist in the 1950's. Before that the Silver plating spec was "QQ-S-365 Silver plating electrodeposited, general requirements for". The last version was version "D" which was canceled for new designs on Feb 2, 2001 and was partially replaced by "ASTM B 700 Standard Specification for Electrodeposited Coatings of Silver for Engineering Use". The QQ-S-365 spec doesn't say about surface roughness before plating except surface cleanliness. The hydrogen embrittlement relief temperature in the QQ-S-365 spec is 375F but it is too high for carburized parts. There is also a demand for stress relieve treatment at 375F prior to plating. |
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Israelkk
this is very very good information & I really appreciate your response, I am more informed than before.
It is well known the stress or post bake should not exceed the tempering temperature.
the embrittlement test is for sure a good test to make sure parts are not defective. all good stuff.
but this does not resolve my issue with the super finish & the silver plating.
Take Care
Mfgenggear |
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TVP (Materials) |
10 May 12 9:12 |
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tbuelna (Aerospace) |
10 May 12 20:01 |
I don't personally see that silver plating of gear flanks would be of much real benefit with gears designed to operate under hydrodynamic contacts. A better approach is to use a lubricant with an EP additive. However, I do think silver plating parts can be helpful where there is boundary contact conditions (ie. splines) or the possibility of fretting (ie. clamped flanges). With conventional case hardened & ground gears and proper oil lubrication the surface finish will actually improve slightly during the initial green run, if the green run is conducted properly. The green run produces a similar effect to ISF on the active gear flank surfaces. ISF is a chemical/mechanical process that modifies the microsurface topology by removing only the asperity tips. It leaves behind plateaus but does not affect the valleys, so the while the microsurface may not be "smoother" in the conventional sense, the max height from valley to peak (or plateau) is greatly reduced . Thus ISF of flanks helps by improving the lambda ratio (film thickness/surface roughness) of the contact lube film, which in turn extends the mesh scoring limit, all other things being equal. Since the topology of the valleys is not improved, I don't think ISF would help with fatigue life. And unless a lubricant with lower viscosity is used, I also don't think ISF would improve efficiency (assuming hydrodynamic conditions). Hydrogen embrittlement due to silver electroplating is primarily an issue with steels at higher tensile strengths. An excellent reference for hydrogen embrittlement relief is AMS 2759/9. As israelkk notes, you need to be careful of the temperatures involved so that you do not affect the temper of the steel part. If you still feel silver electroplating poses a problem, a good alternative might be a spray-on dry film like moly disulfide. Hope that helps.
Terry |
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Terry
To my understanding it's like a car motor that was just re-build.
mechanics must remove the distributer ( older engine versions)
insert a connecting tool to the oil pump then prime the engine with oil.
this prevents the engine from running dry at first startup. once this was done
it never needs priming again.
To my understanding that is what the silver plate is used for. breaking in until all the moving parts
where lubed.once the system is running the the silver is spent.
Is there any documents that reinforce the above idea. I don't know.
This may have been an 50 year old O.E.M. design.
Mfgenggear |
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TVP
I believe your right. but I need more ammunition.
so I am doing more research, so that I can present to my customer.
Mfgenggear |
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to add to my above comment, where as in the old days it was a simple task to silver plate over Carburize gear teeth , now all the customers are adding super finish on their parts, because of the benefits. well it now complicates the silver plating.
Mfgenggear |
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Here is another good reference. Too smooth is not such a good thing. http://www.cryogenicsuperfinish.com/pdf/Gear%20Tech%20-%20Part%201%20Nov-Dec%202003%20User%20Friendly.pdf Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
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tbuelna (Aerospace) |
12 May 12 20:15 |
mfgenggear,
You've got the basic premise of how something like silver plating would help prevent scuffing/adhesive wear during the very brief start-up period where there will be boundary contact conditions. The silver plate creates a surface barrier that inhibits local diffusion bonding. But the drawback is that the silver plate quickly shears away and only provides protection for a few load cycles. [link http://www.machinerylubrication.com/Read/1406/extreme-pressure-additives]EP oil additives[/link] create a surface barrier that functions similar to silver plating, but the EP oil additive surface barrier constantly regenerates itself.
ISF will not prevent the adhesive wear that occurs during the brief start up period where there is boundary contact, it will only reduce it. If all you think you need is a smoother surface finish, you can also get a 4 microinch surface finish with honing. If cost is an issue, I personally would choose honing over ISF. ISF only improves the microsurface topology, while rotary honing also can correct some profile, lead and index errors.
Interesting discussion.
Terry |
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Gearcutter
Thank you for the link. Info is always welcomed.
your experience & constructive comments are always welcomed.
good information on that link.
Terry
Your comments & wealth of information is also always welcomed.
If your meaning the fasler honing I would agree, if it's the older method of
gear honing I may not agree. never the less there may be some data I am missing.
good information about lubrication.
Mfgenggear
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I found this publication from the US Army corps of engineers EM 1110-2-1424
I uploaded a partial page but it can be uploaded from the web.
look at third page, para "C", protect the gears during the running in.
it specifies to coat with iron-manganese phosphate, or plating them with copper or silver.
Mfgenggear |
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here is the uploaded file Mfgenggear |
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TVP (Materials) |
15 May 12 14:33 |
Manganese phosphate would be a lot easier to deal with than silver plating on gears that have been finished using ISF. |
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TVP
thanks for the reply
but could you please elaborate more on the Manganese phosphate.
You have star
Thanks & respectfully
Mfgenggear |
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Manganese phosphate produces hydrogen too, See "MIL-DTL-16232G Phosphate coating, heavy, manganeese or zinc base" for instructions. |
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tbuelna (Aerospace) |
15 May 12 19:39 |
Nice catch israelkk. Attached are the relevant pages from MIL-DTL-16232 regarding hydrogen embrittlement relief requirements. |
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Terry & Israelkk
true OK post embrittlement is required but would it stick (adhere) to the super finish.
that is what I was inquiring to TVP
seems no way out of some type of plating. did you gentlemen see my post above about
the Army core of engineers, data.
Respectfully
Mfgenggear |
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spigor (Mechanical) |
16 May 12 1:53 |
This maybe sounds funny to some of you, but what does the term "baking" (used in MIL-DTL-16232) mean? Is this some kind of a heat treatment? Many thanks for a hint. |
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TVP (Materials) |
17 May 12 10:45 |
spigor,
Yes, baking means heating to an elevated temperature, usually in the range of 100-220 C.
mfgenggear,
Phosphating adhesion is improved somewhat with surface roughness, but since this is a chemical process in an acidic bath, there is a degree of etching that takes place, which is why adhesion should be better than with an electroplated metal coating. This acid etching is also why embrittlement is a concern, albeit a small one given that inhibitors are used in the bath. Just follow the relief requirements in the appropriate specification. |
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TVP
thanks for your reply
Terry & Israelkk
If selective Carburizing is required, then masking with copper plate in accordance with AMS 2418 type 2 is normally used.
prior stress relief & post embrittlement is required.
Just saying that since I have been making gears for selective gears copper plating has been used even before my time.
what are your thoughts on this.
Mfgenggear |
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Copper plating for shielding is done prior to Carburizing. Therefore, there is no problem of hydrogen embrittlement during Carburizing. However, if the base metal hardness is 35RC and more there is a need for a stress releive prior to copper plating and hydrogen embrittlement relief after the copper plating (prior to Carburizing). See attached MIL-C-14450B Copper plating that was replaced by the AMS 2418. Note that the 44RC in para. 3.7 was changed later on to 35RC in amendment 3 of MIL-C-14450B. |
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Terry
My Parts are normally Normalized, Subcritical anneal, then Harden to 27-36 prior to carburize.
it allows for better control of broaching & gear cutting.
Mfgenggear |
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israelkk
Sorry that above comment was for you too.
thanks for the explanation
Take Care
Mfgenggear |
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tbuelna (Aerospace) |
18 May 12 20:16 |
mfgenggear,
First, yes I did read the document you linked from the US Army Corp of Engineers. The document recommends some useful practices for running-in new gear drives. Unfortunately, I think the recommendation of applying silver plate to the tooth flanks as scuffing protection during break-in is a bit of an old-wive's tale legacy. Even under part load operation, the soft silver plating on the tooth flank surfaces would be largely displaced within only a few load cycles. So any scuffing protection it provided would be extremely short term.
Second, I don't see why electroplated copper or silver would have trouble adhering to a 4 microinch surface roughness. I don't have a copy of AMS 2412 (silver) handy, but I read thru AMS 2418 (copper) and there was no mention of minimum surface roughness on the basis metal. However, AMS 2418 section 3.4.3 does briefly discuss acceptance test criteria for plating adhesion.
The only reasons I can speculate on for lack of plating adhesion with your ISF gear surfaces is either due to residue from the ISF process contaminating the surface, or possibly that the plating is being applied too thick. Sometimes a nickle strike can also promote better adhesion of the silver or copper plate with certain alloys.
Good luck to you.
Terry |
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Terry
I am gratefully for the comments You and Israelkk
are willing to take time and share with us.
Respectfully
Mfgenggear |
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