Stainless steel gear help (galling conserns) 2

[titansfk]

Any advise on preventing galling on stainless steel gears running dry (no lube). Spur gears are running about 120 rpm and dp24. I was thinking of using gall-tough, but it is not so easy to source. So I am thinking of DLC (diamond like coating), or some other coating.

Thanks for any advice.

Steve Krause

 

[TVP]

Are both mating gears stainless steel? Why is stainless steel being used? DLC should reduce the friction, but if the stress intensity is too high, the coating will break down over time and galling will merely be postponed, not eliminated.

 

[titansfk]

It is a medical instrument, so I need to use approved classes of materials. One other material that I could possibly use is cobalt chrome CoCr.

I read a thread about molykote 106 from Dow and I am looking into this also.

 

[MikeHalloran]

Consider coating just one gear of each pair, so only dissimilar materials contact in the mesh.



Mike Halloran
Pembroke Pines, FL, USA

 

[titansfk]

Good point about disimilar metals. I still get galling when using 17-4 h900 and custom 455 h900 on a different application. Contact stresses just too high.

I just sent a sent a set of gears out for TiN coating using vapor deposition. The thickness is about 1.6 micron, so I shouldn't have dimensional issues.

I will let you know the results.

 

[Occupant]

HI,
Does it have to be austenitic stainless? Heat treating is out? You'll have to watch this TIN. Not all Coaters do a good job and it tends to flake off.

 

[TVP]

Some engineers from ZF wrote a good article in Gear Technology 2004 about coating of gears. They achieved good results with a PVD-applied WC/C (Balzers) on FZG-A and FZG-C steel gears. I am not familiar with the suitability of PVD/CVD coatings like WC/C, TiN, DLC, etc. with stainless steel, meaning do the coatings have good adhesion to stainless steel. I would contact Balzers to discuss your applications in more detail.

 

[tbuelna]

titansfk,

You say this is a medical device application. Is contamination an issue? If so, then any DFL or hard coating will present a problem because they ALL will eventually flake off and create debris.

You also said that you experienced galling in a similar application with Custom 455 at an H900 condition (Rc 48/230 ksi UTS), so it is unlikely that you would fare any better with a "coated" part. Any coating you apply would be very thin and will only be capable of supporting loads within the limits of the underlying gear material itself.

Your best bet would be to enclose the gears and seal/lubricate them with autoclave compatible materials.

 

[titansfk]

Here is more clarity on the application/problem.

Currently the device works well with a moly lubricant, but this is a prototype. I need to get the gears to run smoothly with no lubricant. We don't want to seal the unit.

My understanding is the TiN coating is HRc 70. I need it for wear resistance and lubricity purposes, not for load capacity. The 455 H900 application was not a gear, it was a shaft in a bore. The loads I calculate create minimal deflections in the gear teeth, so maybe I can "get away" with the coating.

The device with be used approximately 100 rotations per surgery, so I don't think it will wear away that quickly. It will also be disassembled after every surgery and flushed with hot soapy water, then reassembled and autoclaved.

 

[dimjim]

It would be interesting to see your gear
parameters too. Can't you use mineral oil?
Would be better than nothing and non toxic.

 

[unclesyd]

One process that may work is the QPQ (quench polish quench) from Kolene, now Nitromet. I've had great success with this process with both Austenitic and the PH grades of SS.

http://www.nitromet.com/A55B6F/nitromet/nitromet.nsf/b0da3a2f63208410852571930048a8da/Home

There is a fairly new process for specifically surface hardening Austenitic SS from Germany that was to be offered in the US by Swagelock. My information came from Industrial Heating Magazine.

I would also look at Nitronic 60 for both gears, self mated, or as a mate for something like 17/4 PH that has been liquid nitrided. Another material is Gall-Tough that has similar properties as Nitronic 60.

Nitronic 60 Check the full brochure

http://www.hpalloy.com/DataSheets/ni60.htm

Gall-Tough

http://crswnew.cartech.com/wnew/techarticles/TA00030.html

 

[strokersix]

Do the gears have to be metallic? How about one or both gears in a polymer? If the polymer material can't withstand autoclave them maybe it's a disposable item?

 

[tripleZ]

The WC/C (I think it's referred to as Hardlube) will lower your mu value some, but when used on tooling members under abrading conditions, it does start to wear off. I've used it on core rods for compacting SS powder. I can generally get a couple thousand parts off of one core before the SS has started to really abrade the Hardlube substrate off of the core. If your gear assembly though works anything like some of my medical customers' assemblies do, then that coating should hold up well enough under your normal OR conditions.

One of our medical customers does use a sodium stearate solution that can be applied wet and then dried on. It works well as a lubricant for single piece use, but likely won't work well if the gear is to be used for multiple patients w/ intermittent washing & baking.

 

[ExRanger]

I don't know how well it applies to stainless steels, but you could look into ferritic nitrocarburizing, which applies a very shallow hard case and a super hard, thin surface "white" layer of nitrides and carbides (I think) that resists wear well in many applications.

How is your surface finish?

 

[titansfk]

Thanks ExRanger.
There are some many coatings out there it boggles my mind.
I am getting protos of custom 465 H900 gears and sending them out for TiN coating. The surface finish is Ra=.8 or 32 microinch.
I really wanted to get a DLC coating, but I could not find a vendor to do a proto quick turn around.
Steve

 

[plasgears]

I have used stainless PM gears with oil quench after sinter. Life performance with limited lube and high loads and cycles was very satisfactory.

 

[tripleZ]

Plasgears, what was the purpose of the oil quench? Modified impregnation, etc.? Just curious (and sorry to threadjack here).

 

[plasgears]

ZZZ,
We used PM output gears on high production runs for fractional gearmotor drives. In one demanding application showing premature failures under test, we opted for oil quench after sintering by recommendation of mfr. Not only did it serve as a quenching medium, it provided internal lube. PM being porous to some extent provides space for lube in the matrix. Highly recommended.

 

[tripleZ]

The oil impregnation makes sense. I don't have a drop tank on my furnaces to test out oil quenching any of the stainless PM parts I make, so I was curious to see how it worked out for you. Normally we sinter in an atmosphere of straight hydrogen or dissociated ammonia, then impregnate with the desired liquid under vacuum as a secondary operation. Although I couldn't tell you the last time we quoted an oil impregnation job.

Just interesting to see how the competition is doing things. Thanks for the update!