A Journal Bearing comprises a solid shaft that rotates inside a cylindrical sleeve with a small oil filled gap between them. You made a simple analysis of this problem in CW1 where we assumed that the centrelines of the shaft and sleeve were aligned. By “unwrapping” the annulus you can show that the imposed velocity profile of the oil is linear, (Fig. 1).
Figure 1. Flow in the annular space between two concentric cylinders.
In practice the Journal bearing must support the weight of the inner shaft. In CW1 you showed that the pressure is constant and the viscous stress acts circumferentially, which implies a concentric arrangement cannot generate a vertical force to support the shaft. One way of overcoming this is to offset the centrelines of the shaft and sleeve slightly.
To see how this provides a supporting force consider the simpler slider bearing arrangement shown in Fig.2. This comprises a slider separated from a fixed plate by a thin film of oil. Notice that the gap between the slider and plate varies.
Figure 2. Slider Bearing.
Assuming that the flow is steady, 2-d, and incompressible, the
continuity and Navier-Stokes equations governing the flow are:
To non-dimensionalise these equations put:
̅ , ̅
where is characteristic of the gap width and and
are to be determined. is very small compared to so / is a small parameter.
(a) By considering the order of magnitude of terms in the
(b) By considering the order of magnitude of the terms in the first N-S equation show that we must put and that
to leading order the equation becomes:
̅ ̅ .
(c) Now non-dimensionlise the second N-S equation and by considering the order of magnitude of the terms, deduce
(d) Integrate the equation, and, stating the b/cs you use, show
that when written in dimensional units:
1 ,1 2
(where you should remember that .
(e) Use the fact that the volumetric flow rate, , is constant to show that
that to leading order it becomes
̅ can only depend on ̅ and the equation in part (c) is
0. (This means that
Appropriate b/cs for this equation are that 0 where is the constant (small) pressure away from the slider. By integrating it can be shown we obtain
And by noting that 0 it can be shown we obtain
the solution is:
Thus once the gap width, , is specified we can integrate to obtain the pressure and hence find the total upward force on
the bearing which is given by . For the
case where the gap width is a linear function:
(If you are brave and like complicated integrals you might like to show this!)
(f) Assuming that 0 ≪ min, deduce that a very large upward pressure force, ,
results. (Hint: Write as a small parameter and
expand the terms in square brackets as Taylor series as
far as .) [4 marks]
(g) What happens if the direction of travel of the slider, , is in the opposite direction? Discuss qualitatively the problematic implications of this for the load bearing capability of a Journal Bearing where the shaft axis is vertically displaced from the axis of the sleeve. Suggest some design changes to overcome this difficulty.
To help you may wish to look up Slider Bearings and Journal Bearings in appropriate textbooks. There is also a wealth of information on both available online. (You will notice that in discussing the vertical force we considered only the pressure. If can be shown that the normal viscous stress component, , is indeed very small compared to the pressure term.)
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