Full Cylindrical Air Journal Bearing - Radial Load Capacity

[rudiCORG]

Hi All,

I would like to know if there is some difference in radial loading capability at different angular positions for a full cylindrical air journal bearing. This type of bearing is used to support shafts in air-bearing spindles. I am trying to understand in context of this figure (see link) which shows the difference in the air film thickness (lubricant in this case) at different angular locations. What would be the relationship between film thickness at a particular angular location and its radial load capacity?

Thank you,

 

[electricpete]

Let me talk about oil journal bearings.

The direction of the load determines the position of the shaft and the narrow / thick pattern around the circumference that you're talking about. In the particular photo, the load is straight down, the converging wedge (highest pressure) is roughly at the bottom, and pushes up on the shaft. If you took this entire thing and considered a vertical bearing, assuming the bearing is symmetrical, then the entire pattern shifts if the load shifts (to always keep the high pressure converging wedge in position that will oppose load). By the way, bearings on horizontal machine are not symmetrical... there are features such as distribution grooves, but they have a predictable direction of load (usually downward on horizontal machine).

Air journal bearings? I have no idea. Compressible fluid...wouldn't think it would work very well.






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(2B)+(2B)' ?

 

[electricpete]

I am trying to understand in context of this figure (see link) which shows the difference in the air film thickness (lubricant in this case) at different angular locations. What would be the relationship between film thickness at a particular angular location and its radial load capacity?

Setting air aside, if you want to understand the figure, google hydrodynamic lubrication
Here's a good overview:

http://nptel.iitm.ac.in/courses/IIT-MADRAS/Machine_Design_II/pdf/5_2.pdf

Figure 2.10 shows pressure distribution. Somewhat makes sense considering the shaft is pumping the fluid CCW through the annulus.... shaft has to find a position where force from pressure is in equilibrium with applied force. The narrow part forms the major pressure drop. With the narrowest part around 7:00 position, then the highest pressure are just upstream of that, around the bottom of the bearing (of course doesn't have be be bottom of bearing, the pressure contributions to force are integrated).

This is the type of theory that supports the figure you posted. I'm pretty sure the theory in that link is not applicable to air journal bearings ... would require modification to accomodate compressibility.

You may want to post more specific link to the type of bearing you're talking about.

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(2B)+(2B)' ?

 

[electricpete]

Correction

Somewhat makes sense considering the shaft is pumping the fluid CCW through the annulus

shold be

Somewhat makes sense considering the shaft is pumping the fluid CW through the annulus

=====================================
(2B)+(2B)' ?

 

[rudiCORG]

Hi electricpete,

Thank you for the prompt reply and the link to calculation for a hydrodynamic journal bearing.
You are correct when you say that the angular distribution of the thickness of air-film (or oil) depends on the direction of load (shaft weight).

In continuation to this, suppose, the shaft is in rotation and an air-film profile like above (image) is created. At this point, suppose I introduce a small static load (non-contact loading) on the tip of the shaft (far left). Below is a section view of one such spindle consisting of two journal bearings (brown in color) with shaft (grey) which is rotated by an electric motor (pink).

Now my question is: Does the stiffness of shaft (while in rotation) will vary with the angular location to counter the torque applied by the small static load at the tip of the shaft (far left)?
My argument: Since the air-film has a unique profile due to the rotation of the shaft, its load carrying capacity might not be same at each angular location. For instance: the angular position where the film-thickness is minimum is a high pressure point. If I try to apply a load in the direction which further tries to compress the air-film, I may not be able to do due to high stiffness. Does this makes sense?

Thank you,

 

[sreid]

For the type of air bearing spindle you show, air is introduced into the bearing through a number of "Jets." These are usually small diameter restrictors. The interaction between the restrictors and the shaft gap keep the shaft centered and also determine the stiffness of the bearings.