Journal Bearing Bushing dilema

[tgallifet]

Hi,

This is my first post here. I was directed here several times after searching Google and found several answers, so I decided to try this.

Here is my dilema. I am designing a journal bearing with rotating housing. The shaft is static and made of steel and the housing is made of steel with a pressed-in Be-Cu bushing. We want to be sure that the bushing will stay static with the housing.

As the housing rotates, there is a "wave" effect at the shaft and bushing sliding surface due to the contact deformation between the two cylinders. The deformation of the compliant material (bushing) is related to its thickness; the thinner it is, the smaller the "wave" is. However, the thinner the bushing is, the less "clamping" (shear)can be achieved through press-fit. There must be a happy middle. Do you have any idea on how to aproach this problem?


An other question. I tried to do an analysis of the contact between two cylinder using hertzian equations. What are the assumptions/limits of these equations. With the common equations, it is possible to find a contact width larger that the cylinders diameter !!! Something must be up... I do not have the derivations of these equations, so I am clueless...

Thanks.

 

[GregLocock]

Well, I must admit I've never heard of this wave effect. I can sort of see where it comes from, and certainly a Hertzian approach would give a good estimate of the amplitude.

For a derivation of the Hertzian approach see one of Timoshenko's books. Are you suing the bearing only, or the whole assembly, as your outer surface? It should be the latter. What is E for the bearing material?

For your press fit use Lame's equation (it's in the FAQ for this forum).

Failing that you'll need a reasonably good FEA model.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[eromlignod]

If the bearing wall is not excessively thin, it is a common practice to drill a hole through the housing and bushing and install a dog-point setscrew to prevent rotation. Crude, but effective.

Don
Kansas City

 

[electricpete]

Setscrew mentioned seems worth looking at.

"As the housing rotates, there is a "wave" effect at the shaft and bushing sliding surface due to the contact deformation between the two cylinders. The deformation of the compliant material (bushing) is related to its thickness; the thinner it is, the smaller the "wave" is. However, the thinner the bushing is, the less "clamping" (shear)can be achieved through press-fit. There must be a happy middle. Do you have any idea on how to aproach this problem?"

I would think that the more the interference and the thicker the bushing, the less probability of relative movement. I don't see how a too thick of a bushing could result in relative movement.

"An other question. I tried to do an analysis of the contact between two cylinder using hertzian equations. What are the assumptions/limits of these equations. With the common equations, it is possible to find a contact width larger that the cylinders diameter !!! Something must be up... I do not have the derivations of these equations, so I am clueless..."

I didn't realize the word "Hertzian" applied to this situation. There are standard formulas for contact pressure given the radii and material strength (and if very high speed or high diameter also need to consider centrifugal force).


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[tgallifet]

Thanks for the answers.

What I call the "wave" is the propagation/rotation of the contact pressure inside the rotating housing. I assimilate this to a wave effect, I might be wrong... This sliding contact create some shear, greater than pure friction. I am trying to read a book on contact problem, I am discovering all this...

Concerning the thickness of the bushing, is there a thickness where the bushing can be considered infinite? I am confused here. With hertzian equations applied to cylinder I can find the stress distribution and the contact width, but I'm not sure I can get the strain directly, even less the deformation... which i am really interested in.

By the way, this is for a high weight low speed application. ~10,000 lbs on a 2" journal, .600" long @ <350 rpm.

If anyone has some more ideas, they are welcome. I am still reading a lot of books and if I find something I'll try to let everybody know.

Thanks again.

 

[zekeman]

Keying the bushing to the housing should work but if you are doing this correctly, i.e. drilling and reaming the housing hole and following design practice on press fits, your deformation should be uniform resulting in a circular bearing hole, not an elliptical one, which I suspect is the case.

 

[ccw]

This seems like a terrible waste of berrylium copper and steel. Why not use a real journal bearing with lubrication, or a roller bearing. Even a SST roller or ball bearing running dry would beat this. The BeCu and Steel are going to grind the crap out of one another at 10,000 lbs 350 RPM. What about a sintered powdered metal BeCU bushing and steel shaft.

 

[ccw]

Ooops, my bad! You did not say you were going to run dry so now I assume this is a properly lubricated journal. Is the 10,000 lbs a rotating vector or a static vector? In other words is it an eccentric rotating mass or is it like a wheel rotating on a stationary shaft?

Watch your lube system carefully. A failed lube applicator/retainer will result in friction, heat, fire.

 

[tlee123]

Since this is a new design, I recommend you investigate other materials for the bushing. If your BeCu bushing generates dust, the beryllium poses a serious health hazard. I have also seen other threads here discussing the possiblity that beryllium alloys could be banned in the future by the EU (who knows what they'll do).

If you're running it dry, it surely will create dust (as ccw said). If you're providing lubrication why do you need this material with its low friction coefficient? Either way it doesn't make sense to use BeCu in a new application where dust could be generated.

 

[tgallifet]

The bearing is not run dry. We use grease; the bearing stop and start quiet a lot and there is a lot of vibrations... The shaft is static with the load and the housing / bushing rotate with the load. This is where the "wave" (propagation of the load) comes from.

The regular operation of the bearing is lubricated, but as it stop and start again the bearing is dry for a fraction of time...

What I was trying to determine is how to design the bushing so that it does not spin under such a high load. I was not able to find how the bushing thickness influence the tangential load transmitted by the bearing. I finally decided to use a simple Coulomb friction to obtain a transmitted torque. I compared this to the torque sustainable by the press-fit bushing. As a conservative approach I assumed that the friction coefficient between the bushing and the shaft and the one between the bushing and its housing are the same, this way they cancel out.

The next step was to get the thickness. For this, I determined the maximum interference that would allow easy installation by thermal slip fit. Then I determined the tickness that gave me the safety factor I wanted, while being sure that the compressive hoop stress in the bushing was under the yield stress.

I'm sure this has been done by number of people before me but when you have to start from scratch it is not necessarily obvious.

Of course I could use loctite and be done with it, but my boss and manufacturing does not like this...

Thanks again.

 

[ccw]

There are many ways to secure the bushing to the housing to keep the bushing from turning. Look at journal inserts for piston crank / crank shaft joint, how these are prevented from turning. Another method would be to key between the housing and bushing. Another method would be to use a bushing shape like a weld neck flange and use cap screws to bolt the flange to the housing.

Use predicted dry sliding friction between the steel and BeCU to determine starting torque, plus a safety factor. This should be resisted by the key, cap screws, tangs, etc. in shear.

 

[tgallifet]

The key idea is good, but I think it is preferable when the bushing is static with the load. Otherwise it creates a stress riser when the key cavity is loaded.

Also I should have mentioned that this is a small journal. The Shaft Diameter is around 2" and the journal length is limited to aroun .650" and it still has to sustain over 16,000 lbs... That might give you an idea of the size requirement.

 

[ccw]

It seems to me, now that you have revealed more, that you have other problems:

Projected bearing area based on [not-conservative] half circumference:

A = 1/2 x .650 x PI x 2 = 2.042 sq. in.

Pressure = F / A = 16,000 / 2.042 = 7835 psi.

This is off the scale as far as "current" practices in Journal Bearing design pressures. Ref. "Design Analysis of Journal Bearings", Machine Design, 28, Feb. 9, 199 (1956). I wish I had something more current.

You did not indicate if this is a rotating eccentric that generates 10,000 lb centrifugal force, or if this is a track or road wheel, where the vector is static relative to the shaft stub. Also, you did not indicate if there was any thrust requirements on the journal.

Now you speak of frequent start/stops. Journals are not good for frequent start stops. Portion of stop start vs. running at speed / at load should be only a small fraction of a percent.

I think I would begin to prepare my tormenters for a redesign, if I were you. Or, could this be a miracle design where bearing considerations were included early on?

 

[tgallifet]

This is a design for a wheel type arangment. The is thrust but it is taken by an other component and is not considered here.

Sart and stop are generally every 30 minutes. And the size is fixed, no more space available - these dimensions are already 30% bigger than our previous design!!!

I realized that this is off the scale... Sometime I tell myself, "this is not physicaly possible". But experience shows it is, especially the competition...

 

[EdDanzer]

When designing a plain bearing you need to be more concerned about P/V and film strength than turning in the housing. In start stop applications or high load slow speed applications the projected area times the film strength (about 6000 psi) should be the maximum load. The only other consideration is shaft deflection, which can cause point loading, removing the lubrication and promoting galling. A press fit of .001”-.0015”/inch of bore typically will not turn in the housing if the pin and bushing don’t weld together.

 

[ccw]

This is my parting shot on the subject. You can make the fit between bushing and housing a pressed spline fit. That should take care of turning. You can get the BeCu as strong or stronger than steel. Use generous filleting in the splines to assure minimum stress risers. EdDanzer beat me over here. I was going to recommend a thread in the bearing design group that he was engaged in called plain bearing limits or something like that:

thread:
thread 821-162268

Since this is a wheel design, the load vector is static relative to the steel axle stub, which is good. At least you don't have to worry about the stub fatiguing off. Limber shafts that undergo large bending deflections under load are problematic to precision journals as Ed points out.