Wedge action and the thrust pad

Figure 16-22 is figure 16-18 brought forward to use again. If we consider one pad when the whole assembly is in use we can see that lubricant will flow into the space between the pad and the thrust collar and immediately the pressure will start to rise and also immediately the lubricant will start to flow radially in both directions to leak out. We might suppose that there will be one circular path through the space between the pad and the collar where the flow is actually along this circular path. Then we can treat this as one-dimensional flow and we can apply equation:-

                               to this path to find the pressure distribution and go on to find a position of the thrust so that the position of the pivot can be determined.

In figure 16-19 I have drawn part of a Michell thrust pad bearing. The pads have been simplified to sectors of an annulus. I have also shown dotted a possible path to which we might apply our physics.

 

In the lower diagram I have drawn the section through a pad where this notional path lies. The pad is at some angle  and its leading edge has a gap with the moving face of . The length of the path is .

 

Ignoring the small difference between the length of the arc and the

length  this is identical to the diagram for the journal bearing but this time we shall require a Cartesian plot and not a polar plot..

Again we can explore this system if we let  be constant and vary  and vice versa for fixed values of the other relevant quantities.

 

Figures 16-24 and 25 give the details of the Mathcad programmes for graphs 16-11 and 12. The pad might fit in a bearing assembly for a collar of about 750 mm overall diameter and turning at 120 rpm. The figures for the inlet gap appear to be realistic and there is not much scope for change. The pressures are quite significant and it is easy to see why this design is successful especially on ships where the shaft may turn non-stop for several days at a time.

 

This pad has to be mounted on a pivot and part of the design will be the location of the pivot. We can go one more step and extend the programme to find the force on the pad per unit width and to find the point of action of the force from, say, the inlet edge. In figure 16-23 the first integral will find the force and the second the distance from the leading edge.

 

There is no point to plotting graphs so I give typical values in table 16-1

 

Angle  = 0.000075 radians			Inlet gap z =0.04 mm
z	Force			Force
0.03 mm	90 kN/m	0.114 m	0.0001 rad	50 kN/m	0.114 m
0.04	29	0.109	0.00008	3.2	0.11
0.06	6.7	0.106	0.00006	2	0.107
0.08	2.5	0.104		1.1	0.104.

 

These forces are trivial by engineering standards and large forces will be associated with smaller values of  and . It is worth looking at these. However it is the value of  that is of interest and that varies very little for any combination of  and . This means that the position of the pivot is not far away from the radial centreline of the pad and would, no doubt be found by testing.

 

This analysis has given a better understanding of the way in which the Michell thrust bearing works. One thing that is clear is that the bearing must be made very accurately if the eight pads are to share the load.