The journal bearing

Journal bearings are usually used in pairs to support a shaft that might be an axle for, say, the armature of an electric motor or perhaps a large fan that it drives. The shaft has two coaxial journals that are accurately turned with a good finish. These journals rotate in a bearing bushes housed in blocks. The bushes are made of some alloy that has suitable properties for letting the shaft rotate freely when suitably lubricated and for resisting wear during the instant of starting. It may be that the bushes are lined with expensive alloys having particularly good mechanical properties.

 

The bearing, as a device, must have diametral clearance between the shaft and the bearing bush to accommodate the lubricant and this diametral clearance will have a minimum value at which the bearing is made and some maximum value at which the bearing is regarded as being worn out and must be refurbished. A typical ratio of diameter to clearance is 500. So a bearing of 50 mm diameter will run on a bush with a diametral clearance of about 0.1 mm.

 

Bearings, generally, lose lubricant by leakage and, in the case of the journal bearing, this results from axial flow of lubricant. This loss must be made good and a journal bearing can be fed with lubricant from a cup in the cap that must be refilled at intervals or by a ring resting on a part of the shaft exposed by a cutaway in the top brass and lifting lubricant from a well in the bearing block. It can also be fed by continuous pumping, that is, by forced lubrication.

 

We need to know what happens during starting and when running. Obviously any diagram that one might draw must grossly exaggerate the clearance.  Figure 16-14 shows three states for a journal bearing in which I have exaggerated the clearance.

 

In the first state the shaft is at rest and in metal-to-metal contact. The space between the shaft and the bush is, or should be, filled with lubricant as I have shown in red. There will be a gap in the lubricant at the lowest point but once solid contact is made, there will still be tiny spaces in the region of contact that are filled with lubricant.

 

When the shaft starts to move the film quickly becomes continuous and then the shaft tends to “climb” up the right hand side of the bearing because of the viscous drag. I very much doubt whether it moves through the angle I show but, again, I need to distort to carry the message. This offset to the right reaches some maximum and then, as the speed of rotation increases, lubricant is dragged faster and faster by viscosity into the tapering space on the right hand side and the pressure of the lubricant in that space increases. This pushes the shaft to the left and, as it moves the high-pressure region creeps under the shaft and lifts it until ultimately, when the speed becomes steady, it runs in the position shown in the third diagram supporting the load on the lubricant with the solid surfaces not in contact.

 

Here we have the fundamental mode of operation of all of our successful plain bearings. It is this action of lubricant being dragged by viscosity into the reducing space between two solid surfaces set at a small angle and creating a high pressure in the film of lubricant to separate the moving and stationary surfaces. It is called wedge action.

 

In figure 16-15 I have drawn a typical measured pressure distribution in the lubricant when the shaft is rotating. The radial arrows represent the magnitude of the pressure acting on the inside of the bush. Equal and opposite pressures act on the shaft to push it to the left and, more especially, upwards.

 

These high pressures lead to axial flow in both directions and the greater the clearance the greater the loss of lubricant sideways. There is another incentive here to keep the clearance to a minimum.

 

There is yet another incentive and that is to do with the “stiffness” of the bearing but we need more information and I will deal with it later.