Fretting corrosion on plain bearing

[yurilevi]

Hello,
What is the recommended way to deal with fretting corrosion in plain bearing? Lubricant? Coating? The bearing and the housing are made of cast iron.

 

[flexibox]

fretting suggests vibtration taking place. Cast Iron as a bearing? Never heard of that and I think you need to provide more information.

 

[yurilevi]

I have attached a sketch of the application.
This is not usual bearing. It is sleeve for roller bearing. The sleeve has to be able to move axially about 0.5 [mm] in order to compensate for the shaft thermal expansion during work. Without compansation of the shaft expansion, the bearings axial load increase and reducing the bearing life.
Fretting corrosion increase the friction between the housing and the sleeve and causes sticking of the sleeve in the housing during shaft expansion.
The sleeve is made from cast iron. Using of coating or lubricant must solve the problem of fretting corrosion. The question is what coating and lubricant?

 

[MikeHalloran]

So you want to plate or coat the sleeve with something that will slide on the housing without a lot of fuss.

Gorilla Snot, or EP Roller Grease, is the first thing that came to mind, but I'm guessing that lube oil is present, and that might eventually dissolve the grease.

The second thing that came to mind is tin.



Mike Halloran
Pembroke Pines, FL, USA

 

[yurilevi]

There isn't used lube oil. The system is running in vacuum. Only grease can be used like lubricant. I also have been thinking of tin or lead or harder plating like chromium plating, but I has to use some material that wouldn't make damage to the housing, because the housing existing allready and cannot be changed. The housing material is cast iron (hardness of ~160HB). I have heard that PTFE coatings are good with fretting corrosion, but I'm afraid that the coating will be removed too easly due to vibrations and the sliding, even if the sliding itself is little(0.5mm).

 

[dhengr]

That type of movement shouldn’t even be taking place at that interface. Isn’t that supposed to be a light press fit and/or retained with a ring? Both longitudinal expansion movement and any shaft vibration, harmonic, out-of-balance, etc. tends to be forced to occur there with roller bearings. Aren’t there cylindrical and spherical bearing systems which will tolerate some of these kinds of movements within the races and in the bearing itself, rather than force it out to the outer housing.

 

[Artisi]

Can you change to a roller bearing?

 

[yurilevi]

I can't change the bearing. The system is vertical and the speed is high (6500rpm). So I have only spindle bearings. There are springs on the sleeve that give preload for the bearing. The sleeve is in close fit with the housing. That little axial movement of the sleeve is very important because there isn't another way to take the shaft thermal expansion.

 

[arunmrao]

Cast iron has bearing properties due to the presence of free graphite on the surface. It is commonly used for sliding surfaces. Perhaps you may consider hot tinning too. Large bearings in tube mills have the babbitising done.

 

[dinjin]

I think a light coating of lead would help
or even a light coat of nickel.

 

[Tmoose]

Vertical is tough. Even tiny unbalances are the "indeterminate load direction" that would normally require a tight fit to prevent creep or worse.

How are you lubricating the spindle bearing?
Grease, air-oil, ??
Is this a new design?

I saw a press release from FAG recently for a spindle bearing with an integral housing (and maybe even preload springs) to provide a decent length/diameter ratio, unlike the common method of asking 1 or 2 disagreeable bearings to slide directly in the housing bore.
A search of the Schaeffler site came up empty.

Dan T

 

[yurilevi]

The design is new.The bearings are grease lubricated. The system is in vacuum, so oil lubricating is difficult mission.

 

[RolMec]

Hi yurilevi,
If the housing's the only one (8.8.) to be absolutely fixed, why not change the sleeve? Manufacture it from some just sufficiently strong yet "friction-friendly" material? Add grease, and maybe change it every xxx days during planned maintenance?
Or else, look at the sketch attached here: Change the contour of the sleeve (Seems to me there's still some reserve in the outer ring thickness..) if possible and add a bushing made from a suitable material. There's bronze doted up with greasing components out there, or other hightec things you maybe know already.
Regards
R.

P.S.: (Can't upload anything, so : Just approx. invert the inner contour of the sleeve (for outer ring of bearing) to the outside & add bushing into the now free space + leave some gap towards the housings inner surface. Should be able to be assembled OK.)

 

[tbuelna]

yurilevi,

If the fretting is occurring between the bearing sleeve OD and housing bore, it is due to relative movement between the two. You should install the sleeve with as much interference fit as the ball bearing will tolerate, or you should axially clamp the sleeve. The goal with either approach is to produce enough friction force to prevent the sleeve from moving under any circumstances. Fretting is a micro-level phenomena, thus it will occur with even the smallest amount of relative movement at the interface.

If you can't provide enough clamping force or interference fit to prevent any movement by friction, then the two interface surfaces must be made hard enough such that they are beyond the fretting limit for your loads. Or you must make one of the surfaces from a dissimilar material that will not diffusion bond to the other (like steel and a high strength polymer/plastic). Unfortunately, either of these approaches are difficult if the bearing loads are high.

Good luck.
Terry

 

[Tmoose]

Parts can fret in a serious vacuum? I guess the micro welding can still occur, I just think of fretting as needing oxygen to form that lovely red iron oxide.

I'd expect dissimilar materials to do the trick.
Kluber has assembly pastes that are mighty tough, but I don't know if they are suitable for your level of vacuum.

I'd think about mounting the upper, floating bearing housing in a pair of flexible diaphragms to allow the required thermal expansion without any mechanical sliding.

Or, apply some consistent significant side force with an idler and belt, or just a spring loaded teflon plunger or 2 pushing the housing sideways to simulate gravity.

 

[yurilevi]

Thanks to all replies. It's very helpfull to me.

I'm checking now for the most suitable combination of material, coating and lubricant for the sleeve. And also I'm checking the suggestion of Tmoose about simulating gravity.

 

[tbuelna]

Tmoose,

"Parts can fret in a serious vacuum?"

Fretting is actually much worse in a vacuum. The basic mechanism behind fretting is cyclic diffusion bonding/fracturing between metallic surface asperities. And the ideal conditions for diffusion bonding are two very clean, un-oxidized metallic surfaces contacting under pressure in a vacuum. The presence of surface "contaminants", such as oxide films, plating, or lubricants will inhibit diffusion bonding effects, and thus somewhat fretting.

Diffusion bonding between stainless steel surfaces in a vacuum is a serious problem with spacecraft components, such as fasteners. Unless the mating stainless steel thread surfaces are coated with a dry film or plating, they may never come apart after being exposed to a vacuum.

http://en.wikipedia.org/wiki/Cold_welding

 

[Tmoose]

Hi Tbuelna,

I was somewhat aware of the seizure/galling problems in a vacuum.

Would there still be dark red deposits I equate with fretting?

Dan T

 

[tbuelna]

Tmoose,

The reddish residue you sometimes see at a fretted steel surface are indeed iron oxide (rust)particles. But if there is no oxygen present, then obviously the abraded particles would not oxidize.

You won't commonly see iron oxide on fretted steel surfaces, since bare steel is normally only used within an environment where rust inhibiting grease or oil films are present. The most commonly seen indicator of mild fretting is material build up on one surface and mating pits on the faying surface. This material transfer is due to the continuous micro-welding and fracturing events that occur between surface asperities during each load cycle.

Regards,
Terry