Journal Bearing Bushing dilema
Journal Bearing Bushing dilema
(OP)
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.
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.





RE: Journal Bearing Bushing dilema
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.
RE: Journal Bearing Bushing dilema
Don
Kansas City
RE: Journal Bearing Bushing dilema
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.
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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RE: Journal Bearing Bushing dilema
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.
RE: Journal Bearing Bushing dilema
RE: Journal Bearing Bushing dilema
RE: Journal Bearing Bushing dilema
Watch your lube system carefully. A failed lube applicator/retainer will result in friction, heat, fire.
RE: Journal Bearing Bushing dilema
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.
RE: Journal Bearing Bushing dilema
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.
RE: Journal Bearing Bushing dilema
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.
RE: Journal Bearing Bushing dilema
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.
RE: Journal Bearing Bushing dilema
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?
RE: Journal Bearing Bushing dilema
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...
RE: Journal Bearing Bushing dilema
RE: Journal Bearing Bushing dilema
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.