Gear Corrosion 1

[TugboatEng]

Are there any tips or guides for qualifying gears for use after corrosion? I have a palloid gear set that spent a short time immersed in salt water and has bloomed a bit.

 

[SwinnyGG]

'a bit' may not be a problem.. I'd expect you would want to disassemble the gearset, remove any corrosion present, refinish the gears as necessary to provide the right tooth surface finish, characterize any dimensional losses as a result of that process, and then determine if they are suitable for continued service.

This of course may not be possible, if you can't afford the downtime, can't disassemble the gearset, or whatever other issues may arise.

If this is a set which is very low load, you may be able to just run them and let the gears 'descale' themselves on mating surfaces. Much depends on your application.

 

[TugboatEng]

We are hand buffing to clean with nylon mesh abrasive. This pic is before any cleaning. Pic is not the best due to back light. To me, the corrosion is similar to corrective pitting but I don't know if corrosion causes other issues. All bearings are being replaced, they were much worse off.

Some specifics about the application are 2500 horsepower at 250 rpm through ~3.5:1.

 

[mfgenggear]

tugboateng
I recommended to have a gear shop use a gear checker analyze the gears before touching them (to late), analyze(measure) the surface finish before and after.
have them do alight abrasive blast, vapor blast preferred, , (do not use aluminum oxide) do not go over 70 PSI pressure)
then apply super finish (16 micro)if possible then re-chart the gears, then post hydrogen embrittlement bake gears, since I don't know
the steel and heat treat, stick to 300 deg, see AMS 2759/9

 

[TugboatEng]

The steel is 18crnimo7-6.

Hydrogen embrittlement crossed my mind which is why I came here. The immersion lasted several days, the gear was never loaded in that condition.

Can you explain the aluminum oxide issue? The nylon mesh abrasive, Scotch-Brite, is aluminum oxide based.

 

[mfgenggear]

tugboateng

the blasters will try to do a good job blasting with aluminum oxide blasting and don't realize they are removing to much material changing size and configuration.
aluminum oxide blasting is a very aggressive and works well , but for gears it can scrap them out.
the involute and lead, toe in and toe out will and can change drastically. same with super finish, need to tell the suppliers to remove very little stock, just a slight clean up.
that is why I recommended to analyze the gears before and after. spiral bevels is all about center distance and back lash. to obtain the correct toe in and toe out.

 

[TugboatEng]

I really appreciate the knowledge given here. My powers that be are not familiar with these invisible problems. It will be a while before I learn the consequences of our choice (if any) but I have 10 of these units in operation and if this one fails prematurely I'd like to not make assumptions.

 

[mfgenggear]

Tugboat

I would normally suggest to touch up by means of a re gear grind, but spiral bevels require tooling , and the correct wheel geometry, either a master or cnc gear program, for inspection.
and once the center distance is set then a the locating face needs to be reground to obtain the correct center distance. real PITA.
the corrosion kills the the surface finish, so easy to super finish in a tumbler, and removes very very small amount of stock.

 

[TugboatEng]

Fortunately, most of the surface was unaffected other than the pits. This is the worst of it here.

It's quite a large gear and permanently assembled on its shaft.

 

[mfgenggear]

wow, yes no clearance for hob/mill rotary cutter or grinding, pitting is bad news, but it's a beefy gear

 

[spigor]

It looks like a cyclo-palloid gear, so it can't be ground. It can be hard-finished though. It all depends on how deep these pittings are and if there are any sharp corners that can serve as stress concentrators. In such case I would have them re-finished by manufacturer until these marks are 100% gone. If too much stock would have to be removed and the case gets thin - they are scrap. Otherwise these gears can fail anytime. I wish all the news was good, but this is what I think.

May I ask what that nice red gear marking compound that you are using is?

 

[TugboatEng]

Spot check, dye penetrant. Checking for cracks as this gear experienced some load transients that resulted in the sea water exposure (boat went aground).

 

[mfgenggear]

actually a spiral bevel gear can be ground from solid, hobbed, milled, it's the processing of after hardening that dictates the post secondary machining process, a hobbed spiral has to be finished hard hobbed, or finishing lap. a milled spiral bevel can be post ground after heat treat. if ground from solid, it can be ground from after heat treat as well. it's the process used that dictates the method.
problem with tugboateng gears it has a shaft with will not allow clearance. need to look up the processed used to mfg these gears.
here is a great article on the subject
By Dr. Hermann J. Stadtfeld

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.573.2003&rep=rep1&type=pdf