Raceway surface and subsurface failure in ball brgs.

[Gerry45]

Generally it is accepted that if one maintains an adequate ehl film in a rolling brg, the eventual failure will be one of subsurface fatigue due to crack propagation from partical contamination in the steel. Conventional thinking also says that if the ehl is reduced, or contaminents introduced into the lubricant, rolling surface interaction takes place and the failure becomes one of 'surface distress'. Most brgs would be removed from service when either of these two events occur.

It it is logical to assume that even though a brg may be removed because of unacceptable surface damage, some subsurface stressing must have also taken place. However, I have seen many 'abused' rolling brgs in the past,(ie damaged by vibration or small movements whilst idle etc), but not once can I recall seeing one that exhibited both types of failure mode together. Are there good technical reasons for this, or is it simply that the brgs weren't left in service long enough to allow time for both types of damage to develop ?

The reason I ask is this. I have a slewing ring application where 'surface distress' (due to the slow and occasional nature of the rotation) is normal and tolerated. (It is not noise or smoothness sensitive, but because of the operating conditions there is no alternative but to accept gradual with wear and deterioration of the raceways.) However, in the future, the frequency of slewing movements is likely to increase dramatically on my machines so I am wondering if I can expect to see not only surface damage, but some subsurface fatigue developing too after a period of time. My concern is that if this happened this could take the deterioration beyond the norm and up to unacceptable levels !


Does anyone have any thoughts or experience in this field ? My own feelings are this. Although increasing frequency of angular oscillation will contribute to more sub-surface stress cycling, it will also increases the surface wear rate. This may have the effect of wiping out the fatigue. For example, I can imagine that if I had the potential to create a 'false brinell' pits of say 10% of the ball diameter, this would be far deeper than the initial maximum shear stress point from where fatigue would normally originate. So the progression of the wear would mean that the max shear stress cycling point is constantly being moved lower down to fresh material and the onset of fatigue is delayed. I look forward to your comments.

Gerry

 

[rnd2]

A bellows type static seal as EdDanzer suggested and able to cope with a 90 deg. flex would have to be a winner.
I'm sure there are silicone, synthetic rubber (EPDM?) and maybe a low durometer polyurethane with sufficient
S T R E T C H and weather resistance. (All of the above have excellent weather resistance and there are silicone elastomers that can be stretched 300%).

 

[rnd2]

Add in Hippo 41's suggestions and the bearings would last forever.

 

[Gerry45]

Interesting point about the gear lubes. The auto cvj industry also knows that oils work well, and since their tribology is similar to mine, a gear oil could have many benefits in a slew ring. Unfortunately trying to keep a liquid in a rubber boot seems to have defeated the whole of the cvj industry for years, so I'm not that confident that the slew ring industry would do any better here.

Double seals have been used on pitch brgs in the past with poor results. One problem seems to be that they are too delicate for use in very large brgs and get easily damaged during brg assembly. Also its difficult to check the integrity of the inner seal after brg assembly. Perhaps the biggest problem is one of field replaceability. This is generally impossible with double seal designs, and since the suppliers won't warrant for even a fraction of the required 20yr life, then I have to assume that seals will need to be changed in-situ several times in the turbines life.

Some older pitch brg designs did have double seals and continuous grease injection between the seals (as well as into the brg). I looked at my first one of these designs the other week. It had operated for 5yrs. Guess what.... it was full of water !

As for the grease manufacturers, while they may be good in some areas of industry, in wind energy applications they all lack experience and seem to have to rely totally on the turbine maker for feedback. Since most turbine makers will only generally work with a couple or so of greases, there isnt really much information available in terms of lubricant performance comparisons. Standard laboratory tests have been shown to be unrepresentative of real operating conditions and dont help us much. As a previous correspondent said, turbines are regarded as 'tribology experiments on sticks'. This will be the case for many years to come.

Now then, the big 'twistable' boot is an idea Ive been thinking about for a little while. The biggest problem is the size. For a 3m dia brg for example, a one piece moulding is almost certainly out of the question, so a jointed component looks likely but this does introduce a weakness. Its all down to finding a durable material and one that is reliably 'joinable'. But certainly this is something that warrants continued investigation. However I would think of it more as protection for the greased and sealed pitch brg, rather than the only barrier between an oil filled brg and the outside world.....for the present time at least !
Gerry

 

[rnd2]

Gerry,
Why not form a seamless flexible seal by moulding it insitu? It seems do-able.

 

[Gerry45]

rnd2, can you explain this one a bit more please ? Are you talking about moulding after the blade is bolted to the brg and the brg bolted to the hub?

 

[Boomerang]

Yes.
Conceptually, some sort vacuum bag or low pressure injection whereby resin could be induced to flow into the mould. Conceptually, for easy flexure, something like the Michellin man.

 

[rnd2]

Gerry,
The seal has to be formed insitu. For the seal to be to twist effortlessly, part of it would have to suspend relative to the twist direction and not be in direct contact with the moving part.
To form the seal, an internal mould as well as an external mould would be needed, with possibly air used on the internal mould to form a bulge where required.

 

[EdDanzer]

As to sealing the slew ring, a loaded face seal seems to work better than a radial seal in our experimentations. There is some axial deflection in the bearing design that causes radial seals to fart, allowing oil out and contamination in. It does require a relatively high force against the face seal and 10 rms contact faces. We used a urethane o-ring for preload and UHMW for the seal material running against the steel the bearing is bolted to.

 

[EdDanzer]

The December NASA Tech Briefs has an article about a bearing seal system that might be what you need. The company, Randolph Research Company, Akron, Ohio provided the information and is based of patent # 6,524,007. Their phone No. 330-666-1667.

 

[diamondjim]

Gerry,
If indeed water is getting in, there
should be a design that would allow it
to escape via a drain hole or such.
Nice to know this. Again thanks for
sharing. In tunnel boring equipment
they have holes near the bottom of the
bearing to drain off the oil and it is
recirculated and filtered. Kind of a
gravity system but it works.

 

[MASSEY]

http://www.uniquetechnologies.com/

http://www.micropolylubricants.com/

Gerry these are links to two types of solid lubricants. Both are recommended for washdown conditions. The lubricants are 'baked' into the bearing prior to installation.

They both eliminate the need for supplemental sealing, however additional conventional seals will only help.

The roller elements can also be surface treated with applications similar to Timken's AquaSpexx bearing treatment.

These surface and lube treatments would at least remove your sealing problem very effectively. The solid lube stays in contact with the rolling elements so it would not be affected by the lube settling to bottom of bearing races that may occur during oscillations.

It's a different approach, let me know what you think.