HELP PLEASE - Bearing types, applications and failure modes

[NickT]

Guys I need your help please.

I have to gather some information on bearings quickly for an important project I am working on. I have a general understanding on the bearings used in automotive engines but I need more details on the types of bearings used, their construction, the materials used and why. I also need to know about failure modes and how to overcome them etc I have some info on DLC coatings but anything you have on reducing friction would be most welcome.

Thanks in advance

Nick

[Carlos]

The quickest over view would be to do a survey of Goggle search terms I used.

A general article from Wikipedia
http://en.wikipedia.org/wiki/Bearing_%28mechanical%29
Searches
'machine bearing design'
'machine bearing design and coatings'
'studies of machine bearing types'
'studies of machine bearing failure'
'advanced coatings for automotive bearings'
'metallic machine bearing coatings'
'polymer machine bearing coatings'
'composite machine bearing coatings'

Sift through and save the articles that seem of value.

[Belatti]

send me your e-mail address in a PM, I think I have got some Federal Mogul bearing powerpoint file that can help you

[NickT]

Carlos many thanks for the extra suggestions, I had most of them but there were quite a few I didn't think of that have been very useful

Belatti thanks for the presentation it is great and I managed to translate most of it using http://www.babelfish.yahoo.com However there are still a few words I couldn't find! Like I said though it is well illustrated which is very useful.

Thanks guys

[checkered]

A site well

worth visiting when investigating bearings (and other related wear and tear stuff) is Tribology-abc. It's quite a resource of basic calculators, general information and links to academic institutions and companies. NickT might be after a bit more specific information (I read that much into the formulation of the request, especially in light of your profession) but at the "bearingest" minimum the site seems to have the direct (or at least indirect) capacity to put you into contact with people who can provide you with sufficient and definitive resources in your quest. Certainly for the generally inquisitive F1T member "Tribology-abc" might prove to be rather enlightening.

[Ciro Pabón]

Well, I think Nick needed a "quick fix" but, if you're into bearings, there is always the huge "Bearing Manual", three volumes thick, that has been around for ages (at least since the 70's), I don't know if that's what Nick's looking for.

I googled for it and you can get it at http://www.bearingheadquarters.com or Amazon, if you have 200 devaluated dollars and you'll get some spare change.

BTW, if you need a bearing, or a spare for one, or you need to interchange bearing parts, I know no better website.

[NickT]

Thanks guys, have some excellent info here.

However, I need more about additional coatings such as PTFE for the bearing themselves or DLC (Diamond Like Carbon) for the journal. Also anything you might have on different types of finish used on the journal. I am looking a lowering friction at start up, extending life, retaining lubrication and improving hydrodynamic performance.

[Carlos]

A reference:
Three coatings were. tested: two carbon coatings and one chemical nickel PTFE coating used as a bearing material.
http://www.diva-portal.org/diva/getDocu ... lltext.pdf

A general article from Wikipedia that has some good references and links on 'diamond like coatings'
http://en.wikipedia.org/wiki/Diamond-like_carbon

[riff_raff]

NickT,

The subject of bearings is vast and your post is a little vague.

Most automotive engines use journal bearings. That is the bearing element itself is an elastohydrodynamic (EHD) oil film. The EHD oil film is generated by the presence of a fluid (oil,) trapped between two close proximity moving surfaces.

EHD oil films are characterized by their film thickness, which is only a few microns. A well designed journal bearing can have mechanical losses close to rolling element bearings, with the benefits of less cost, fewer parts and lower weight.

EHD oil film pressures can be very high (>50,000 psi), and that is why a very small journal bearing area can support huge loads without metal-to-metal contact. Since there is no metal-to-metal contact in a properly designed journal bearing (except during stopping and starting), low friction coatings (like DLC) have no benefit. The only mechanical losses are incurred through the shearing of the oil film itself, and are quite small. Of course, you may also wish to include the losses that are produced by the oil pump feeding the bearings. The flow rate of the pump is based on the amount of oil flow necessary to keep the temperature of the journal bearing materials within their design limits. And these pumping losses are usually greater than the oil film shear losses in the bearing itself.

The only coatings used with journal bearings are soft metal overlays (tin or lead) to provide a small measure of embedability, should the oil become contaminated with hard particles.

As for transmissions, they typically use rolling element bearings (ie. ball or roller bearings). As with journal bearings, a well designed rolling element bearing operates with the rolling elements supported by an EHD oil film. The effectiveness of this EHD operating regime is characterized by the bearing's lamda number. The life of rolling element bearings is defined by the number and magnitude of the contact (Hertz) stress cycles each section of the bearing races see. The efficiency of a rolling element bearing is usually very high, but can be adversely affected by roller/ball skidding, oil churning or race misalignment. Since a properly designed rolling element bearing operates with EHD contact, hard coatings (like DLC) provide no benefit.

The only instance I can think of where a low Mu coating, like DLC's, titanium nitride, MoS2 or TFE would be beneficial, is where you are operating in a boundary lubrication regime. But I don't know why someone would intentionally design for that condition. You would need to limit the contact pressures, the friction loss would be high, and the amount of power being transmitted through the joint would have to be small, or the interface would rapidly overheat and fail.

You said: "I am looking a lowering friction at start up, extending life, retaining lubrication and improving hydrodynamic performance."

With a journal bearing, you do not want to retain lubricant. The most important function of the lubricant is to provide cooling for the bearing materials. You want to design and manufacture the journal and bearing clearance such that you have have just enough oil flow leaking out of the bearing to provide adequate cooling under all operating conditions.

You can maximize journal bearing life by keeping the oil clean and at the correct temperature. And also by selecting bearing/journal materials that have adequate compressive fatigue life with regards to the peak oil film pressures they will see. Journal bearing failure modes are scoring (oil contamination), fatigue (spalling due to compressive/subsurface shear stresses), overheating (inadequate oil flow), or corrosion (oil contamination).

As for improving hydrodynamic performance, keep the diameter of the journal as small as possible while still achieving adequate bending stiffness. The L/D ratio of the bearing should be .30 or greater, as a rule of thumb. I would also highly recommend a journal orbit analysis to determine the optimum location for the oil feed hole.

Good luck.

[Abdul]

Hello,

You do not want to apply teflon coatings to any shaft journal operating in a hydrodynamic bearing such as sleeve bearings. Why? Because every sleeve bearing operates as a small hydraulic pump; the shaft rotation in the bearing pulls a film of oil during each revolution. This film is then forced into a small tapered gap. The oil film pressure suddenly rises because the gap is getting smaller, and the fluid is incompressible (no air bubbles please) reaching above 1000 psig in normal loaded bearings, but can rise as high as 20,000 psi in some applications.

Now, going back, the ability of the shaft to pull the oil film into the gap is dependant on the molecular surface attraction between the shaft surface metal and the lube oil film. If you apply Teflon to this shaft surface, adhesion drops by more than 75%, and you now have a very poor quality hydraulic pump, which generates a much reduced hydrodynamic pressure zone under the shaft. Result, very low load capacity and failure under normal service loads.

In the process industry, with centrifugal pumps/compressors/steam turbines, several incidents occurred in the 1970's, with users accidently applying a film of teflon to the journal through mishandling in the machine shop, led to immediate radial bearing failures.

In conclusion: For the bearing metal itself, yes, its important to reduce the frictional coefficient of the bearing surface to the minimum possible. For the shaft journal; no, its only important to achieve the best possible circularity of journal, the best possible alignment of journal to bearing centerline, and the best possible surface finish. One last point for those new to hydrodynamic bearings: The pressurised feed oil does not contribute to the oil film pressure that is separating the loaded shaft from the bearing.

Thanks,

Abdul

[NickT]

Thanks for all this great information, I never thought something as humble as a journal bearing could be interesting?

Another question: does any one have any details on BABBIT journal bearings? I was reading about them yesterday but the information was very limited, as far as I could gather this is a tri-metal bearing that can take hi loadings with minimal lubrication.

[Conceptual]

Thanks for all this great information, I never thought something as humble as a journal bearing could be interesting?

Another question: does any one have any details on BABBIT journal bearings? I was reading about them yesterday but the information was very limited, as far as I could gather this is a tri-metal bearing that can take hi loadings with minimal lubrication.

An engineering friend and I did the CAD work on a 3 peice bearing, but never had a prototype manufactured. The premise was to use a teflon inner moving piece, and 2 teflon outer pieces. The tolerance between inner and outer carriage was made up by powdered graphite.

We supposed that the teflon-on-teflon with a graphite lubricant would have generated a large amount of local static electricity, and helped to bear the suspended load.

We never had one made, but I believe that I have the quote of $350 for the prototype in my email still.

I should check the catalog of the quoting manufacturer... They seemed pretty interested in the concept, but not enough to eat the cost of making the first one to do any testing.

The story of my life!

Chris

[riff_raff]

NickT,

Babbit is a generic term for a soft metal alloy composed mostly of tin and lead. It was used many years ago as a journal bearing material, before fluid hydrodynamic theory was well understood. The soft babbit material was deemed necessary to protect the crankshaft journal, since they frequently came into contact.

A properly designed, modern journal bearing never experiences metal to metal contact, so using a soft babbit material is not necessary. In fact, you will find that a modern journal bearing shell is made of a hard, strong material like steel, in order to provide adequate surface compressive fatigue strength. Babbit material has very low compressive fatigue strength and thus would be unable to support high bearing loads. The only babbit used in modern journal bearings is a micro-thin flash plated finish to provide some scuff resistance under loss of oil conditions.

The term "tri-metal" refers to the thin layers of nickel and copper alloys that are applied to the steel backing to provide improved heat transfer away from the bearing surface. As I mentioned in my previous post, the oil in a journal bearing is not used for "lubrication". Since the EHD oil film is only a few microns thick, this would only require a few drops of oil. 99% of the oil flow required by a journal bearing is used for cooling the bearing itself. Without cooling the bearing would quickly overheat and melt.

To give you an idea of the bearing cooling oil flow required by an F1 engine: If the engine makes, just for example, 800 bhp and the main/rod journal bearings have friction losses equal to 3% of that, that means you must have 61,080 Btu/hr of oil cooling just for those bearings. Assuming a 40degF temp rise in the oil and an oil specific heat of .50 Btu/lb-degF, that equates to an oil cooling flow requirement of about 7.3 gallons/min. (please excuse me if I got the math wrong!)

Best regards.