Offset Section Journal Bearing

[Slagathor]

As an alternative to a 3 lobe type journal bearing, has any research or development been done on a journal bearing with offset sections? The principle would be the same as a 3 lobe, except the areas of high pressure at 120 increments would not be on the same bearing...because each section has a circular section. Rather...for ease of mfg, individual simple journal bearings would be offset at 120 degree intervals to provide stabilization.

The two issues with a bearing like this would be the lower stiffness and film strength...relative to length, and the requirement for the shaft to be very stiff. Other than that...it seems like a feasible design, that would be cheap to mfg.

The initial application would be for a grinding spindle. I would put a herringbone pattern on the middle of the shaft, and relief grooves on the end to create oil flow. Middle diameter would be about 2.5". RPM range from 3500 RPM to 25000 RPM (very wide range).

See attached image, with the offsets multiplied about 250 times for clarity.

Using the methods described in Fuller, this would be a pretty easy bearing to design...because you can look at it a the design of 3 different bearing. Iterations would take a while...but I think that you could converge on a very stable, VERY stiff design...perfect for a grinding spindle.

 

[MikeHalloran]

I'm not detecting a question.

If the design you're bragging about is that easy, you should have it ready for test in a couple of days. Let us know how it turns out.



Mike Halloran
Pembroke Pines, FL, USA

 

[Slagathor]

What is with the attitude?

The first sentence is pretty clearly a question. I have not done fluid film bearing calcs in 10+ years...and I am sure there are people here who are very familiar with this stuff. Rather than spend a ton of time re-inventing the wheel, and doing a lot of calcs.... I throw out a question.

So someone chimes in..."oh yeah they tried that years ago...and it does not work because you did not take X into consideration". Great....saves me hours and hours of doing iterative calcs. Isn't that the point of this whole forum?

So let me rephrase this for you: "As a general concept, do those of you experienced with fluid film bearing design think this is a viable concept"?

 

[electricpete]

I have seen it described in books, so I'm sure it works. One of many ways to achieve multipe converging wedges to improve stability.

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(2B)+(2B)' ?

 

[electricpete]

Actually the version I have seen described is 2 bearing halves offset from each other. I think you're describing three third-bearing sections (?).

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(2B)+(2B)' ?

 

[MikeHalloran]

Sorry about the attitude. I just sort of skipped over the question mark without seeing it.

I think you're talking about axially stacking three full-circle bearing shells with slightly offset bores in a circular housing to get the effect of a trilobular bore.

I'm thinking, given that the offset is so small, that you'd need some indexing means, like a key, to avoid confusion while stacking. And you'd need pretty good manufacturing precision to make the shells stack up in a common housing bore with a controlled net clearance to the shaft.

I'm also thinking you'd need six, not three, shells, to effectively support both ends of the shaft, and not wind up with a 'corkscrew' bore, which I'm guessing might have stability problems.

I'm also thinking you might still have stability problems, because every revolution of the shaft would apply three different bending moment sets to the shaft once the bearings got pressurized.

I.e., I'd expect it to vibrate itself to death. But I could be wrong; it's happened often enough. And I admit to not being a wizard WRT hydrodynamic bearings.



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

I've seen only one. I've only seen the concept in drawings and pictures.

The only segmented bearing I've seen was plain Bobbitt bearing that had a rather large area in the middle of the bearing removed. If I recall this bearing was used for reciprocating motion.

Someone has look at this rather closely.

http://people.rit.edu/sxbeme/chaos conference paper.pdf

 

[Slagathor]

Awesome responses guys....thanks so much. You have validated some of the thinking I have done on this...

The bearing sections themselves would be cylindrical/conical ...and 'easy' to make (compared to a three lobe...or even a lemon bore)...but your are exactly right that the assembly would require some VERY precise indexing (+/- .0002" from the 1st reference center section)

Also...the shaft would indeed have to be stiff enough that the bending moments noted would be inconsequential. Since the bearings are adjacent...I think the moments can be handled. The L/D on the shaft would be pretty low compared to most shafts. Luckily that is not a hard calc or worst case...static FEA with separate loading regions. A 2"-2.5" steel shaft will be pretty darned stiff...

Of course...the moments will be dependent on eccentricity at each section, oil viscosity, speed. The more the eccentricity (more offset)..the more forces you have working against each other. Same goes for speed. More speed = more bending on the shaft.

My gut tells me that as long as the shaft is stiff enough that it does not bend 'enough', the faster you go...the more stable it will get.

A lot of iterations would be required. Just designing one bearing can require a number of iterations to converge. Having 3 different journals areas loaded against each other could present quite an analysis challenge.

That paper was a great find...and yet pretty hilarious. It is obvious that only having two offsets (at 180 degrees!) is inherently unstable. There is a reason that trusses (triangles) predominate in structures.... In fact...with a two offset design, the faster you go or the greater eccentricity you have at each section..the more unstable it will get.... Lots of pretty math though...